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CE308
WATER AND PUBLIC HEALTH
ENGINEERING
PRESENTED BY
ENG. BERVERLY NYAKUTSIKWA
(BSc Civil Engineering MSc Urban Water And Sanitation)
LLEVEL 3.2
2020
DEPT OF CIVIL ENGINEERING
UNIVERSITY OF ZIMBABWE
2
• Water sources and intakes
• Types of intakes
• Water demands; factors influencing water use; design flows for water supply and
wastewater systems
• Water use systems
• Water demand projection
• Design horizon and design flows
• Water demand management (leak detection , strategies for water loss control)
• Basic principles of water distribution systems
• Water collection
• Service and storage reservoirs
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• Design of water distribution systems
• Pipes and pipe materials for water supply
• Design water distribution system using computer software
• Sewer systems
•
•
•
•
•
•
•
Determination of wastewater flows
Design period and flows
Elements of sewerage systems
design of sanitary sewers
Low cost sewerage systems
Construction of sewer systems
Management, Operation and Maintenance of water and sewer distribution system
WATER TRANSPORT AND DISTRIBUTION SYSTEMS
 Overview
 Storage
 Types of distributions systems
 Determination of wastewater flows
 Network Configurations
 Design period and flows
4
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Water transport and distribution networks supply water from the source and water
treatment plants to the consumers. It typically consists of
• Transport pipes
• Distribution pipes
• Pumps
• Storage tanks or balancing tanks
• Valves/meters/other fittings
• Fire hydrants
6
• Pipes for transporting water to storage reservoirs at distribution head are sized to handle
average load of the peak days
• The storage reservoirs are designed with total capacities of about 50% of the average
daily production, allowing them to function as output regulators, pressure regulators,
and emergency or fire fighting reserve
• Peak flows and firefighting requirements must be taken in to account while designing the
distribution system, while avoiding oversizing, which may lead to low velocities of the
flow in the pipes
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• In terms of supply type/energy
• Gravity system
• Pumping system
• Combined system
• Choice closely linked to topographical conditions
• In terms of duration of supply
• Continuous supply (over 24 hrs)
• Intermittent supply (1 or 2 times a day for a few hours )
• More than half of the urban water supply systems in the works operate intermittently
8
1. GRAVITY SYSTEM
• Source at higher elevation than distribution area
• Takes place without pumping at acceptable pressures
• Advantages
• No energy costs
• Simple operation ( fewer mech parts/no power needs)
• Low maintenance costs
• Slower pressure changes
• A buffer capacity for irregular situations
• Disadvantages
• Less flexible for extensions
• Require large pipe diameters to minimize pressure losses
• Capacity reduction due to air entrainment
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2. Direct pumping
• Operates without storage provision for demand balancing
• Entire demand pumped directly into network
• Pumping schedule follows variation in water demand
• Proper pump selection to optimize energy consumption
• Advantages
• Any pressures can be reached in system
• Disadvantages
• Complex operation and maintenance
• Dependent on reliable water supply
10
3. Combined system
• Operation of pumping stations with demand balancing reservoirs
• Part of area may be under direct pumping while another is under gravity
• Significant storage volumes required
• Pumps operate at lower capacities than in case of direct
• Common in hilly areas
• Pressure zones
• Help in savings as water at various elevations is supplied at lower pumping costs and using
lower class piping
• Help prevent too high pressures in lower parts of the network (pressure reducing valves)
• Provides sufficient pressures in higher parts (by pumping)
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12
1. Serial
2. Branched
3. Grid (looped)
4. Combined
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• Serial network is a network without branches or loops
• Has one source, one end and a couple of intermediate nodes (demand
points)
• Each intermediate node connect 2 pipes i.e. supply (upstream) and
distribution (downstream)
• Flow direction is fixed from source to end of system
• Common in small or rural distribution areas
• Cheap
• Unreliable system ( parallel pipes)
• Stagnation of water at end therefore water quality issues
• Large construction costs due to large diameters of pipes
14
• Branched network is a combination of serial networks
• Has one supply point and several ends
• Intermediate nodes connect to one upstream pipe and one or several downstream pipes
• Adequate for small communities bearing in mind acceptable investment
costs
• Disadvantages
• Low reliability,
• potential danger of contamination caused by large parts of the network being without
water during irregular situations,
• accumulation of sediments due to stagnation of the water at the system ends (‘dead’
ends), occasionally resulting in taste and odour problems,
• a fluctuating water demand producing rather high pressure oscillations.
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• Grid systems consist of nodes that can receive water from more than one side
• Looped structure of network eliminates disadvantages of branches ysystems
• the water in the system flows in more than one direction and a long lasting stagnation does not
easily occur any more,
• during the system maintenance, the area concerned will continue to be supplied by water
flowing from other directions; in the case of pumped systems, a pressure increase caused by a
restricted supply may even promote this,
• water demand fluctuations will produce less effect on pressure fluctuations.
• Hydraulically more complex than serial and branched networks
• Flow determined by layout and system operation
• Critical pressures vary with time
• More expensive in terms of investment and operation costs
• Offer more reliability esp. in larger towns and cities
16
• Combined network has a looped structure as the central part while outskirts of the
area are supplied through extended lines
• Commonly used in large urban areas
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• Clear water storage part of water supply system
• Placed at source, at end of transport system or any favorable place in distribution
system
• Serve following purposes
• Meeting variable supply to the network with constant water production
• Meeting variable demand in the network with its constant supply
• Providing a supply in emergency situations
• Maintaining stable pressure (if sufficiently elevated)
• Can help in savings
• Flow in trunk main would have to match demand in distribution area at moment therefore
higher flows and larger diameters
• Pipes convey average flow and peak met by additional volume in tank
18
• Selection of site depends on
Type of supply scheme
Topographical conditions
Pressure in system
Economic aspects
Climatic conditions
Security …..
• Can be constructed
Underground
• Used where safety or aesthetic issues arise or in preservation of water quality in tropical areas
Ground level
• Cheaper than underground and more easily accessible for maintenance
• Both types used for balancing demand and buffer storage
Elevated (water towers)
• Typically applied in flat terrains
• Primarily to balance shorter and smaller demand variations, preventing frequent on/off of pumps
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20
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22
1. If the level if the served area is in principal flat, and the consumption of the
water is quite similar over the area, the general rule is that 2/3 of the
consumption will be located between the treatment plant and the elevated
storage. Give best pressure conditions in reticulation net
2. Pressure depends on water level in reservoir and pressure from supply
pumps at the treatment plant. Highest pressure will occur when
consumption is low and reservoirs are filled up
3. Pressure in reticulation net is only dependent on the water level in the
reservoir
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• Required volume depends on
• Daily demand patterns
• Pump operation
• Stable consumption over 24 hours results in smaller volume requirements that in cases of wider
ranges between minimum and maximum hourly demands
• Storage volume between 20 - 50 % of max daily consumption within any particular design year
• Demand balancing volume
• A 24 hr demand balancing is usually considered by assuming constant (average) production feeding
the tanks and variable demand supplied from it (incl leakage)
24
• Calculation based on tank inflow/outflow balance for each hour
• Total balance volume compromise between two extremes:
• Max accumulated volume stored when demand drops below average
• Max accumulated volume available when demand is above average
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• The principles of calculating the capacity of a service or distribution reservoir are
almost the same as those for a storage reservoir.
• A service reservoir is meant to cater for the hourly fluctuation of demand.
• In general the feed to the reservoir, normally by pumping is by means of fixed speed
pumps, which tend to supply a constant flow rate.
• At certain times the demand is higher than the pump supply rate while at other
times the demand is lower than the feed.
• This therefore brings the need for a reservoir (service), which will store excess
water in times of low demand and supply the deficit during moments of high
demand.
• In other words the distribution reservoir is constructed to balance the difference in
patterns between supply and demand.
26
• Reservoir storage
• The capacity of the reservoir depends on the rate of the inflow, losses and demand or
outflow.
• Usually inflow and outflow (demand plus loses) are determined for at least 3 previous
consecutive years. The deficits and surpluses of water are calculated for each and the
minimum storage required is equal to the total deficit (cum surplus – cum deficit) in the
dry year.
• Alternatively by plotting the mass curve, the minimum storage can also be determined
graphically. Recent trends are that the demand or outflow must also include the flow
released from the reservoir or impoundment to meet environmental flow requirements
downstream. Environmental flow requirements is the flow required to maintain flora and
fauna downstream.
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Year
1
2
3
Cumm
surplus at
the
beginning
0
1.2359
1.6406
Cumm
surplus
at the
end
1.2359
1.6406
2.4990
Cumm
deficit at
the
beginning
0
0.6707
1.3829
Cumm
deficit
at the
end
0.6707
1.3829
2.1454
Surplus
of year
Deficit
of year
Surplus
minus
Deficit
1.2359
0.4047
0.8584
0.6707
0.7122
0.7625
+0.5652
-0.3075
+0.0959
• There is a carry over (surplus) of 0.5652 cubic meters from year one while year 2 has an overall
surplus or carryover of 0.0959 million cubic meters.
• The overall deficit for year 2 can thus be fully covered by the carryover from year 1.
• The minimum storage should therefore be 0.3075 million cubic meters arising from the overall
deficit of the worst year.
• Remember that the out flows of the reservoir or total demand should include the following:
a.
b.
c.
Actual demand of water (domestic + industrial +agricultural)
Loses due to evaporation (which must take care of the fact that increased area after reservoir construction will
increase the evaporation).
Seepage after dam construction
28
The hourly consumptions for a town for a single day are given in the table that follows. The two pumps
installed in parallel jointly discharge a uniform flow of 1.5m3/s, which is fed into a reservoir. Determine
the capacity of a distribution reservoir. Pumping rate in million litres per hour = 5.4 million l/hr. Fire flow
requirements are 190 l/s for a duration of 3 h for the high value district with 63 l/s from storage.
Time [hr] Consumption [106 litres/hour]
Time [hr] Consumption [106 litres/hour]
1
2.7
13
7.4
2
2.4
14
6.5
3
2.3
15
6.1
4
2.2
16
5.4
5
2.6
17
6.7
6
3.75
18
7.4
7
4.9
19
7.3
8
5.8
20
6.0
9
6.3
21
5.9
10
7.5
22
4.6
11
7.6
23
3.0
12
7.7
24
2.8
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• At each hour the following happens:
• If demand < pumping; then excess pumped water must go to storage.
• If demand > pumping, then additional water required to meet the demand is drawn from the reservoir.
• In mathematical terms the above can be expressed as follows:
Reservoir capacity required
• The reservoir storage capacity required is thus the maximum value of the difference between
cumulative pumping and cumulative demand.
• The maximum value is obtained from adding the maximum positive value of the cumulative pumping
minus the cumulative demand to the minimum (negative) value of cumulative pumping minus the
cumulative demand.
• In other words the maximum positive value gives the maximum amount to be stored during low
demand while the minimum value gives the amount to be drawn during periods of peak or maximum
demand.
30
Cumulative demand
(106 litres)
Cumulative pumping
(106 litres)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
2,7
5,1
7,4
9,6
12,2
15,95
20,85
26,65
32,95
40,45
48,05
55,75
63,15
69,65
75,75
81,15
87,85
95,25
102,55
108,55
114,45
119,05
122,05
124,85
5,4
10,8
16,2
21,6
27
32,4
37,8
43,2
48,6
54
59,4
64,8
70,2
75,6
81
86,4
91,8
97,2
102,6
108
113,4
118,8
124,2
129,6
2,7
2,4
2,3
2,2
2,6
3,75
4,9
5,8
6,3
7,5
7,6
7,7
7,4
6,5
6,1
5,4
6,7
7,4
7,3
6
5,9
4,6
3
2,8
2,7
3
3,1
3,2
2,8
1,65
0,5
-0,4
-0,9
-2,1
-2,2
-2,3
-2
-1,1
-0,7
0
-1,3
-2
-1,9
-0,6
-0,5
0,8
2,4
2,6
2,7
5,7
8,8
12
14,8
16,45
16,95
16,55
15,65
13,55
11,35
9,05
7,05
5,95
5,25
5,25
3,95
1,95
0,05
-0,55
-1,05
-0,25
2,15
4,75
PLOT OF HOURLY CONSUMPTION RATES AT
CONSTANT PUMPING RATES
Series1
Series4
9
8
7
Consumption, liters per hour
Cumulative
Time Consumption
Surplus/Deficiet
surplus/deficiet
[hr] [106 litres/hour] (106 litres)
(106 litres)
6
Reservoir emptying
Av pumping rate of 5.4 mill l/hr
5
Reservoir filling
Area under emptying or filing curve is the storage volume needed
to equalize demand at constant pumping rate of 5.4 million l/hr
4
3
2
1
0
1
2
3
4
5
6
7
8
9
10 11
Time
12
13
14
15
16
17
18
19
20
21
22
23
24
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Mass diagram of water consumption
140
120
Reservoir empty
100
80
av 24 h pumping rate = 5.4 mill l/hr
60
Accumulated demand line
40
20
Reservoir full
0
1
2
3
4
5
6
7
8
9
10
11
12
Series1
13
14
15
16
17
18
19
20
21
22
23
24
Series2
32
Cumulative consumption
Reservoir empty
Storage vol required =
19 for 24 hr pumping
av 24 h pumping rate = 5.4 mill l/hr
Accumulated demand line
Reservoir full
Time
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Reservoir empty
Storage vol required =
19 for 24 hr pumping
av 24 h pumping rate = 31 l/s
Cumulative consumption
av 8 h pumping rate = 93 l/s
Storage vol required =
100 for 8 hr pumping
Accumulated demand line
Reservoir full
Time
34
• Fire reserve equals flow rate during duration
=
⁄ ×
×
⁄
⁄
= 680
• Total storage for equalizing demand over 24 hr continuous pumping plus
fire flow requirement
19 MLD + 680 m3 = 19.68mld
• Total considering 8 hour pumping
100mld + 680 m3 = 100.68 mld
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ENERGY IN PIPE FLOW
LOSS OF HEAD IN PIPE FRICTION
PIPE NETWORK ANALYSIS
LOSSES IN PIPES
LOSSES IN PIPE FITTINGS/MINOR LOSSES
METHOD OF EQUIVALENT PIPES
36
Water flowing in a pipe may contain energy in various forms which are
• Kinetic energy
• Pressure energy and
• Potential energy
Energy head and head loss in a pipeline (Adapted from Houghtalen, Akan and Hwang)
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• In an ideal situation the sum of the above energy is the same at any two sections.
v1 2 P1
v2 2 P2
  z1 
  z2
2g 
2g 
Where
z:
v:
:
P: pressure (P1 pressure at point 1)
is elevation
velocity m/s
specific weight
• The above equation is the Bernoulli equation.
• Each term in the equation is the energy per unit weight of water or the energy head.
• In general the term
• v2/2g represents the kinetic energy,
• P/ the pressure energy while
• z is the potential energy.
38
• In practical or engineering terms, the algebraic sum of the kinetic head,
pressure head, and elevation head accounts for nearly all the energy per unit
weight of water flowing through a particular section.
• In reality the movement of water from point 1 to point 2 results in energy
loss due to friction and minor losses through fittings if any.
• The difference in the elevation of the total energy line at 1 and 2 represents
the head loss, hl between 1 and 2.
• Thus the Bernoulli's equation in hydraulic engineering is expressed as
follows:
v1 2 P1
v2 P
  z1  2  2  z2  hL
2g 
2g 
Where hl is the head loss from section 1 to 2
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• It should also be noted that the hydraulic grade line (HGL) is the same as the
pressure level or pressure head above the centerline of the pipe.
• For a uniform flow and uniform pipe size, v is the same at sections 1 and 2 in
such a way that the height of v2/2g is the same at both sections.
• This therefore means that
P2


P1

 ( z1  z2 )  hL
• Pressure head(2)= pressure head (1)+elevation difference –head loss
40
• In water supply the reference or datum is often taken as the sea level in such a
way that the values of z1 and z2 are the absolute ground levels referenced to a
trig beacon or a TBM.
• The HGL at each point is obtained by adding the pressure head to the ground
level (HGL=P/ + z).
• In pipeline design it is the HGL that is more important as it is gives an indication
of the residual head. The above equation can thus be re-arranged as follows
 P2
  P1

   z2      z1   hL

 

• HGL(2) =HGL(1) – head loss (1-2)
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1400.150
Static head
HGL
1392.00
PB
PA
Reservoir
1382.750
1380.050
B
A
The figure shows a layout of a water supply scheme from a reservoir. Calculate the
HGL level and residual head at points A and B shown in the figure above given the
following additional information.
• Pipe diameter is 100mm throughout
• Distance from the reservoir to A is 600m
• Distance from A to B is 500m
• Hydraulic gradient 0.1m per 100m (from charts)
42
Solution
HGL at reservoir =1400.150
Friction loss from reservoir to A =(0.1/100)600=0.16=0.6m
HGL at A=1400.150-0.6=1399.550
Residual head at A=1399.550-1380.050=19.5m
Friction loss from reservoir to b =(0.1/100)[600+500]=0.111=1.1m
HGL at B=1400.150-1.1=1399.050
Residual head at B=1399.050-1382.750=16.3m
Alternatively for B
Friction loss from A to B =(0.1/100)500=0.15=0.5m
HGL at B=HGL at a-losses A to B
HGL at B=1399.550-0.5=1399.050
Residual head at B=1399.050-1382.750=16.3m
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• It is important to note that the losses should include both friction and minor losses (discussed later).
• However under normal situations minor losses are very small (<1-2 m) as compared to friction
losses.
• When there is a fitting, the HGL makes a vertical drop equal to the minor loss due to the fitting then
it maintains the same grade as before the fitting (if there is no change in pipe size).
• The total energy line also drops by the same distance. The inclusion of the minor loss in the HGL is
shown
Total energy line
Hydraulic grade line
Minor loss
V
Valve
44
• Losses in pipes can be due to friction or through fittings in a pipe. Losses due to
friction are referred to as friction losses while those due to fittings are refereed to
as minor losses.
• The most common formula for computing friction in a pipe was derived by Henri Darcy
(1803-1858) and Julius Weisbach (1806-1871) and is known as the Darcy-Weisbach
formula. The Darcy-Weisbach equation is as follows:
hf  
L v2

D 2g
•  is also referred to as f in some text
Where
v is the velocity of flow [m/s]
 is the friction factor (dimensionless)
D is the pipe diameter [m]
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• It can also be seen that the Darcy-Weisbach equation is expressed in terms of the velocity
head in the pipe v2/2g.
• The friction factor  is a function of the Reynolds number and relative roughness (k/D). D
is the pipe diameter and k is the roughness height of the pipe.
• Having calculated the Reynolds number and the relative roughness (k/D), the value of 
can be obtained from the Chart developed by Lewis F. Moody in 1939 commonly referred
to as the Moody diagram. The value of  can also be estimated from the two equations by
Moody and Barr.
1
 
6
 3
k
10
  0.0055 1   20000 
 
D Re  
 


Where
:
Re:
k:
D:
friction factor
Reynolds number
linear roughness [mm]
pipe diameter [mm]
46
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• The above equation by Moody gives an error of 5% for Reynolds numbers
between 4 103 and 1107 and for k/D up to 0.01. Barr gave an equation with an
error of 0.04%. The Barr equation is as follows
5.1286 
 k
 2 log 

0.89 

 3.7 D Re

1
• Barr further refined the equation to obtain the following approximation.
 k
5.02 log(Re/ 4.518log(Re/ 7) 

 2 log 

 3.7 D Re 1  Re0.52 / 29( D / R )0.7  



1
48
Comparison of friction equations
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• These are losses as a result of fittings in the pipe. They are sometimes called local
losses. The general expression for calculating minor loses is as follows
hm  K 
v2
2g
Where K is the minor loss coefficient for a fitting. The other terms take their usual meanings.
• The term k is the sum of the coefficients for each of the fittings on the pipe
segment under consideration.
• If two fittings with coefficients of 0.18 and 0.2 respectively are within a segment of
a pipe then the term K is 0.38.
• Values of loss coefficients are available in many standard textbooks and manuals.
50
Examples of coefficients for different fittings (adapted from Hammer and Hammer)
Fitting or valve
Loss coefficient k
Tee (run)
0.6
Tee (branch)
1.8
900 bend Short radius
0.9
Medium radius
0.75
Long radius
0.60
450 bend 0.42
Gate valve (open)
0.48
Swing check valve (open)
3.7
Butterfly valve (open)
1.2
Equivalent length ( diameter of pipe)
20
60
32
27
20
15
17
135
40
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Fittings
The general expression for losses in a fitting: hm  K f 
v2
2g
where kf is the coefficient of loss due to a fitting
Valves
Losses in valves depend on the degree of closure of the valve and the general expression may be written
v2
as follows:
where kf is the coefficient of loss due to a valve
hm  K v 
2g
Contraction
• Loss dependent on the ratio of the diameters before and after
contraction (D1:D2), which also affects the velocities.
V1
• The angle of transition  also affects the value of the loss coefficient
due to contraction. The loss due to a contraction is obtained from
the equation that follows:
v2 2
hm  Kc 
V2

A2
A1
where kf is the coefficient of loss due to constriction
2g
52
Coefficients of contraction for different ratio of area (Adapted from Houghtalen, Akan and Hwang)
Figure 4: Pipe contraction
Bends
• These losses mainly a function of the ratio of the radius (R) of curvature to the pipe
diameter (D).
v2
h

K

• Losses are also proportional to the angle of bend.
m
b
2g
where kb is the coefficient of loss due to a bend
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Values of Kb for various values of R/D as determined by Beij* (Adapted from Houghtalen, Akan and Hwang)
Pipe manufacturers willing to supply prospective buyers with loss coefficients for bends, contractions,
confusors, expansions, and diffusors.
For more information refer “Fundamentals of Hydraulic Engineering Systems”
Equivalent length
Instead of computing the losses due to fittings, it is also possible to convert the fitting to an equivalent
pipe length.
Equating the minor loss coefficient to the friction loss coefficient does this.
In this case everything except L will be known. Solving the equation for L gives the equivalent length.

v 2   D 2g 
L K 
 

2
g    v2 

L
K D

54
• Valve manufacturers want to be able to supply the loss coefficients of their
products to prospective buyers. They usually perform experiments to
determine the coefficients. Determine the loss coefficient for a valve if water
flows through the 8-cm valve at the rate of 0.04 m3/s and produces a
pressure drop of 100 kPa
• A pipe goes through a sudden contraction to half its diameter and then a
sudden expansion back to its original size. Which loss is greater—the
expansion loss or the contraction loss? Prove your answer.
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55
HARDY CROSS SYSTEM
COMPUTERIZED DESIGN
56
• An urban water supply area is normally composed of a complex network of pipes making
a combination of loops and branches.
• Design of network based on the
• desired minimum flow rates and pressure heads (at peak or maximum demand)
• maximum pressure head (at low demand)
• The minimum flow and pressure requirements are to ensure adequate supply at peak
demand while the maximum pressure restriction is meant to avoid/minimize leakages
and potentially pipe failure (burst).
• The pressure and flow in the entire network is dependent on the general layout of the
pipes, sizes of pipes, type of pipes (if more than one type of pipes is used) and the location
(distribution) at out flows or tap offs. The change of size of a single pipe affects the flow
and pressure distribution of the entire network and for this reason the design process is
complex.
• The basic rational for pipe network design is based on continuity and energy
conservation principles.
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57
• Continuity principle: the sum of flows of pipes meeting at a junction is zero (including external
flows i.e. Flows into or out of a junction). In more simple terms this simply means the total in flows
must be totally equal to the total out flows.
Q = v1A1 = v2A2 = v3A3…
• Energy conservation: the sum of the head losses in the pipes (including heads from booster pumps
and losses in fittings) around a closed loop formed by the pipes is zero. In other words losses due to
flow in the clockwise direction must balance to losses due to flow in the anticlockwise direction.
• The “Hardy – Cross method” was developed by Professor Hardy Cross and is widely applicable to
closed loops. In this method outflows are assumed to occur at the nodes (junctions) for simplicity
and this results in uniform (same) flow in a pipe length (i.e. between two nodes). In practice this is
not the same in the case of stand connections in the reticulation. As a general rule a network with m
loops and n junctions results in a total of m + (n – 1) independent equations.
• The Hardy-cross method can best be described using an example problem. Even though commercial
computer software is available to solve these laborious calculations, the student should go through
the procedure a few times with small networks. By becoming familiar with the algorithms, a more
judicious and appreciative use of the computer software can be expected.
58
• An iterative procedure based on assigning estimated flows in the pipes.
• At each junction the total flow must be equal to zero (continuity).
• The head balance is based on the energy conservation principle.
• Knowing the diameter, length and roughness (related to material type), the head loss through a single
pipe can be expressed as follows:
L v2

D 2g
but v  Q / A
hf  
 hf   
L 1 Q2  L
1  2

 2    
Q
2
D 2g A
 D 2 gA 
 kQ 2
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59
Now consider the elementary loop in the pipe network system.
• Energy conservation: sum of clockwise losses = sum of anticlockwise losses.
• Total loses hft
h f t  h f  hm

K 
L
1
hf t      
  m 2   Q 2  kt Q 2
2
D
2
gA
2
gA 

•  Can be obtained from the Moody diagram using an initial value of say 1 m/s. Remember the
assumed value of velocity should be in the typical range of velocity for distribution mains (0.51.5m/s)
h f c    kc  Qc 2 
• Clockwise loses hfc
• It should be noted that kc is the sum of the friction coefficient and minor losses coefficient.
• Anticlockwise losses
h f a    ka  Qa 2 
60
• As the flows are only initial estimates, the clockwise and anticlockwise losses are
almost always not equal during the first trial. The difference between the two sums of
losses is as follows:
 h f c   h f a    kc  Qc 2     ka  Qa 2 
• Like in engineering survey, this difference is the closure error (or misclosure) of the
first trial.
• The flow has to be corrected by a correction flow q. To maintain continuity Q is
added to the anticlockwise flows and subtracted from the clockwise flows. This
operation (correction) must satisfy the following equation.
 hf c   hf a
 kc  (Qc  Q) 2   ka  (Qa  Q) 2
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• Expanding
 kc  (Qc 2  2Qc Q  Q 2 )   ka  (Qa 2  2Qa Q  Q 2 )
• Assuming Q to be relatively small as compared to both Qa and Qc then we can
neglect Q2 and we get
 kc  (Qc 2  2Qc Q)   ka  (Qa 2  2Qa Q)
 kc  Qc 2  2Q  kc Qc   ka  Qa 2  2Q  ka Qa
 kc  Qc 2   ka  Qa 2  2Q  ka Qa  2Q  kc Qc
 kc  Qc 2   ka  Qa 2  Q  2   ka Qa   kc Qc 
Q 
• From
hkQ2
• It can be demonstrated that
 kc  Qc 2   ka  Qa 2
2   ka Qa   kc Qc 
h
 kQ
Q
• Substituting this in the above equation for Q we get
Q 
 hc   ha
 hc
h 
2    a 
Qa 
 Qc
62
Taking care of the following
• The sum of all the head losses is the same as the difference between the sum of the clockwise head
losses and the sum of the anticlockwise head losses (after incorporating the sign convention, +ve for
clockwise and negative (-ve) for anticlockwise).
• The denominator in the preceding equation can be rephrased as follows:
 h
h 
h
2  c   a   2 
Q
Q
Q
c
a 

• The equation for q may also be expressed as follows:
h
Q 
2 h
Q
• The above equation may be derived assuming that the estimated flow has an error q in such a way
that in the first trial the flow used to calculate the head lost is q + q. The loss in a single pipe will
thus be
hl  k  (Q Q)2
• Expanding and neglecting Q2 and then summing up the losses in a loop as below. And simplifying.
hl   k  (Q 2  2QQ)
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A water-supply distribution system for an industrial
park is schematically shown in figure 4.9 (a). The
demands on the system are currently at junctions C,
G, and F with flow rates given in l/s. Water enters the
system at junction A from a water storage tank on a
hill. The water surface elevation in the tank is 50 m
above the elevation of point A in the industrial park.
All the junctions have the same elevation as point A.
All pipes are aged ductile iron (e = 0.26 mm) with
lengths and diameters provided in the table below.
Calculate the flow rate in each pipe. Also determine if
the pressure at junction F will be high enough to
satisfy the customer there. The required pressure is
185 kPa.
64
A table of pipe and system geometry is a convenient way to organize the available
information and make some preliminary calculations.
The first column identifies all of the pipes in the network.
Column 2 contains flow rates for each pipe, which were estimated to initiate the hardy-cross algorithm.
These estimated flow rates are shown in brackets on the system schematic in figure 4.9 (a). Note that
mass balance was maintained at each junction.
Flow direction in the table is indicated using the junction letters that define the pipe. For example, flow
in pipe AB is from junction A to junction B.
Friction factors (column 6) are found assuming complete turbulence and read from the Moody diagram
using e/D or, alternatively, from equation 3.23.
The “K” coefficient (column 7) is used later in the procedure to obtain the head loss in each pipe
according to equation 4.12.
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65
REFER TO EXCEL SHEET
66
Software examples
• EPANET (public domain from the U.S. Environmental protection agency)
• WATERCAD and WATERGEMS (proprietary from Haestad methods-Bentley)
• KYPIPE (proprietary from KYPIPE LLC)
• Syne.gee
• InfoWater
Factors to consider in selection of software
• Ease of use
• Hydraulic elements
• CAD, GIS interoperability
• Model building tools
• Advanced hydraulic modelling and optimisation features
• Technical Support + Training
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68
• Simulation in our case is the mathematical representation of the real system or
model
• Network simulations replicate the dynamics of an existing or proposed system
when its not practical for a real system to be experimented on
• To evaluate a system before it is actually built
• In situations when water quality is an issue
• To predict system responses to events under wide range of conditions
• Model based simulation can provide valuable information to assist an engineer in
making well informed decisions
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69
• Steady state simulations
• Represents a snapshot in time
• Used to determine operating behaviour of a system under static conditions
• Extended period simulations
• To evaluate system performance over time
• Graphical user interface (GUI)
• Makes it easier to create model
• Allows visualisation of results of simulations
TIMELINE OF DISTRIBUTION SYSTEM MODELING
1930’s
Hardy
Cross
Network
Flow
Analysis
1960’s
1970’s
Computer
Widely
Analysis
available
of
models for
Networks mainframes
and minis
1980’s
Dynamic
water
quality
models
PC-based
models
---Steady-state
water quality
models
1990’s
2000’s
Future
Integrated modeling Multi-platform
- mapping Models
database – GIS Critical
SCADA
Analysis
----Management
Contaminant
kinetics
Integration of
Transparent
-GIS
Optimization
Detailed
Water Quality
Modeling
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71
• Water distribution
• Long range master planning
• Raw water supply
• Rehabilitation
• Pressure irrigation
• Fire protection studies
• Fire protection
• Water quality investigations
• Sewage force mains
• Energy management
• Cooling water
• Daily operations
• Industrial applications
• Operator training
• Emergency response
• System troubleshooting
72
• Network layout
• Diameter
• Length
• elevations
• Hydraulic properties
• Demands
• Water consumption
• Operation data
• Levels in storage reservoirs
• Pumps/valves operation
• Calibration data
• Pumping station discharges
• Pump pressures
• Pressure gauge readings
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• Maps (paper or CAD)
• Metering
• As built drawings
• Aerial photos
• GIS system
• Census data
• Asset management system
• Work orders
• Field survey
• Manufacturer’s specifications
• Literature values
• Field inspections
• Interviews with operations
personnel
• Operation records and
manuals
• Flow monitoring program
74
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75
• Amount of time to perform an analysis can be greatly reduced
• Computer solutions can be more detailed than hand calculations
• Solution process may be less error prone
• Solution is easily documented and reproducible
• More comparisons and design trials can be performed due to the speed and accuracy of a
computer model.
• Therefore allows exploration of more design options therefore leading to better more efficient
designs
• Plan intelligently and deliver clean water safely
• Model water system operations accurately make reliable renewal decisions
• Reduce emergency response time
• Deliver high-quality design projects with minimal capital investments
• Improve team productivity with sustainable GIS- and CAD-integrated hydraulic models
76
• Loop programme
• Commonly used
• Static design based on a momentary peak factor
• Recently incorporated graphics etc.
• Suitable for both design and simulation
• EPANET
• Water flow (velocity, pressure etc.)
• Bacterial growth
• Chemicals (chlorine, trihalomethane [T.H.M.])
• Dynamic program, allows modeling at 1hour time steps for a selected period, which may be
a day, 10 days or any other duration, as compared to a single peak factor
• Most suitable for analysis of existing systems but can be used to design or check a design.
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HTTPS://WWW.EPA.GOV/WATER-RESEARCH/EPANET
USE EPANET
78
• Sewage refers to the liquid itself, which is composed of excretion (with flush water) and sullage. Sullage is
wastewater from bathing, washing clothes, kitchen etc., And is sometimes called “greywater” while the water
from the water closet (WC) is referred to as “blackwater”.
• domestic (originates from the sanitary facilities of dwellings , business houses or institutions),
• industrial (comes from liquid industrial waste produced as a result of industrial process).
• Sewerage: This is the system of sewer pipes collecting sewerage away from residential areas to a sewage
treatment plant. The reticulation may also comprise lift stations or pumping stations and other ancillaries
such as oil and grit traps.
• Effluent: This generally means liquid flowing out and this term is more commonly applicable to the final
treated liquid from a sewage works. However sometimes the term is loosely applied to wastewater (treated
or untreated) from an industry or treatment unit or step.
Collection works
• House connections: the pipeline connecting a stand. It is generally 100 mm in diameter and may also be
150mm where large flows are expected e.g. A conference center, church etc.
• Lateral: a sewer receiving sewage only from house connections and not from any other sewer. Lateral sewers
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PLANNING URBAN WASTEWATER COLLECTION
SYSTEMS
 Determination of wastewater
flows
 Design period and flows
79
80
• Branch sewer: 2 or more laterals meeting together form a branch sewer. Maybe 150 to 225 mm in diameter
• Sub-main sewer: 2 or more branches form a sub-main sewer. Predominantly some 225mm in diameter.
• Main sewer: more than one sub-main sewers meeting will form a main sewer. Main sewers are often greater
than 225mm and typical range of diameter is 300-525mm.
• Trunk sewer: when more than one mains meet to form a single sewer that conveys sewage to a treatment
unit. Trunk sewers vary in size. E.g. trunk sewer to Hatcliffe Sewage Works is 450mm, while that draining old
mabvuku (chizhanje) to the ponds is 225mm. Firle sewage works has two duplicate trunk sewers, which have diameters
ranging from 525 to 1,400mm.
• Sewage treatment: artificial process meant to remove or alter the objectionable constituents to render the
sewage less harmful and offensive.
• Sewage disposal: the act of disposing of sewage by any method to a body of water or land. This may be after
or without treatment
• Outfall sewer: the sewer carrying sewage to the disposal point. However in most instances trunk sewers
have been loosely termed outfall sewers. For example in Harare the trunk sewers along Mukuvisi and
marimba are popularly known as Mukuvisi outfall sewer and marimba outfall sewer respectively.
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• Design period is defined as the period through which the sewer capacity is adequate. In defining the design
period, care must be taken to estimate the population (flows).
• Design flows are function of population to be served at the design horizon. Estimate future population
(previous methods). Note: if the flows are under estimated then, sewers will be too small. Conversely if flows
are over estimated, then sewers will be too large and which may never achieve certain requirements.
• Return to sewer is the amount or proportion of the potable water supply that is converted to sewage
dependent on the nature of the development and size as well. In Zimbabwe applicable return to sewer factors
are as follows:
High-density stands
85%
Medium-density
70%
Low-density
50%
• High for high-density stand because there of little space for gardening/lawn thus water usage predominantly for
indoor purposes.
• Lower for low-density developments due to space for water consuming activities such as gardening, flowers lawn,
swimming pool.
• Generally, as stand size increases, water consumption increases while the return to sewer decreases as more
water is used for gardening purposes and other uses such as swimming pools and not returned to sewer.
82
• Typical sewage generation distribution
• Typical percentages of water uses for four countries in the table while a scheme showing the sources of
sewage is below
• Based on data from the table and research data from Mufakose (Hoko1999, Gumbo 2000) this scheme is
typical of a high-density suburb in Harare (85% of the total water consumption is disposed off as
sewage).
Bath
Component Water use l/c/d
USA
25%
Percentage of total
NL
UK
RSA
USA
NL
UK
RSA
Bath and
73
shower
Water
95
closet
Clothes and 32
washing
45
22
65
32
35
21
41
other
Total
Kitchen
10%
Potable water
supply 100%
Greywater
55%
Total
sewage
85%
Laundry
10%
36
37
48
42
28
34
30
23
12
23
14
18
11
14
WC
30%
27
24
36
24
12
19
34
15
227
128
107
160
100
100
100
100
Other
Garden etc
15%
Blackwate
r
30%
To
sewer
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83
Variation of sewage flows
• Sewage generation varies with water usage
• Morning peak (6-9am) due to preparation by people to go to work hence heavy usage of water for flushing,
bathing, and kitchen,
• In the afternoon preparation of lunch and break from business also causes a noticeable peak.
• Another peak is also observed at the end of the day when people return home and start preparing super.
• Variation of flows is also a function of the habits, culture and religion of society.
• E.g. in the Netherlands laundry is traditionally done on Mondays as most of the business starts at 1pm.
• Water is also used for anal cleansing on other cultures.
• Certain religions also do not do certain work on some days, all this has an impact on sewage generation.
• The presence of large industries or institutions can also affect the sewage flows.
• A heavy water using industry operating a 24-hour shift or an industry which discharges its sewage
intermittently will also affect the variation of sewage.
• UZ is an example of a large institution, which affects the sewage flow variation daily and seasonally in Gun hill.
84
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85
Average dry weather flow (ADWF)
• The average daily flow of sewage, together with infiltration if any, after several days (min 4) during which the
rainfall does not exceed 0.25mm. This can also be defined as the sewage flow including infiltration during a dry
period or weather.
Maximum daily flow (MDWF)
• Peak wet weather factors applicable in Zimbabwe are given in SALA(pp26) and range from 2.7 for flows above
2315l/s to 5.25 for flows of 0-23l/s.
• Peak factors are incorporated in design to cater for storm water entry into the sewer as well as other extraneous
flows. It should be noted that generally the discharge of storm water into sewers is actually “illegal” in terms of
municipal byelaws. However owing to lack of enforcement of byelaws, poor construction of sewers and manholes
as well as old age, entry of stormwater into the sewer is inevitable. The peak wet weather flow is thus obtained by
multiplying the dry weather flow by the peak factor.
Peak Wet Weather Flow=DWF Peak Factor
• In large and long reticulation, the time taken for the peak wet weather flows to come together are long and this has
an effect of attenuating (reducing) the peak flows or rather the peak factor applicable to the dry weather. So care
must be taken to choose the applicable factor. However in Zimbabwe these peak factors are minimum statutory
requirements if an authority is to avoid prosecution for discharge of sewage from sewers.
86
• Applicable formulae
• There are a number of formulae used in the design of sewers to compute the velocity and
discharge. Generally the formulae for computing the velocity are applicable for sewers
flowing full, although some of them are applicable to part full sewers.
• Velocity:
• Colebrook – White Formula
• Manning’s Formula
• Scobey Formula
• Stickler Formula
• Discharge
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87
Colebrook – White Formula
 k
2.51 
v  2 2 gDS  log 


 3.7 D D 2 gDS 
v – velocity (m/s)
D – diameter of the sewer in (m)
S – slope (dimensionless or m/m)
k – roughness of pipe (m)
 – Kinematic viscosity (m2/s), temperature dependent
• The Colebrook-white Formula is widely used in the United Kingdom and some of its
former colonies. Thus the most common formula with engineers in Zimbabwe.
• Kinematic viscosity is temperature dependent and may be calculated as follows:

1.31 106
0.72  0.028T
t is temperature in degrees.
• The temperature used is the mean temperature in the coldest month. In Harare for example a
temperature of 15oc is used this gives a kinematic viscosity,  of 1.1410-6 m2/s.
88
Manning’s formula
v
1 2 3 12
R S
n
n – manning’s coefficient
R – hydraulic radius i.e. Wetted area over wetted perimeter
for a circular pipe flowing full R=D/4.
S – slope (dimensionless)
• The Manning’s Formula is widely used in the
United States and also in some parts of Asia
such as Japan.
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SCOBEY FORMULA
v  3.0306Cs D 0.625 S 0.5
Cs is a coefficient taken as 0.39 for concrete pipes.
D is pipe diameter in meters
S is slope (m/m)
• The Hume Pipe Company, Zimbabwe commonly used this formula to rate the flow
capacity of their concrete pipes.
90
STICKLER FORMULA
2
v  kstr  R 3 S
1
2
R and S are as in the Manning’s Formula.
Kstr is strickler’s coefficient [m1/3/s]
• Strikler’s formula is popular in Europe in such countries as the Netherlands.
It is also used in South Africa.
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DISCHARGE
• The discharge in a sewer is calculated from the continuity equation as follows:
=
V – is as calculated using any of the applicable formula for the velocity.
A – is the wetted area of the duct or channel conveying the2sewage. The area of a circular pipe is as follows.
A
D
4
D is the pipe diameter [m].
• Note: sewers are not always circular. Eggs shaped sewers have been used in other countries as
they result in higher tractive force at the bottom of the pipe at low flows thereby enhancing the
self-cleansing capacity of the sewer. Egg shaped sewers are typically applicable or more suitable
for combined sewers where flows are usually very low (of the order of 1-2 % or so of capacity
during periods of no rainfall.). Other shapes are also common for sewers.
92
Colebrook-White vs. Manning’s formula
• The Colebrook – White Formula also known as the “Universal Formula” is
widely used for the following reasons.
• low sensitivity to inputs as compared to other formulae. For example a change of ‘n’ in
the Manning’s Formula from 0.011 to 0.015 (only 1.34 times increase) results in 27%
drop in velocity, while in the Colebrook – White Formula a change of k from 0.6 to 6 (10
times increase) only results in 19% drop in velocity.
• From the foregoing it can be concluded that the results of The Colebrook – White
Formulae are stable and do not rely heavily on the judgment, opinion and experience of
the designer in selecting the inputs.
• For these reasons the Colebrook – White Formula is used in all of the examples and
problems that follow.
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Recommended values of k
• k values in most literatures purely for new pipes and tested under factory conditions with clean water and
not sewerage. Also note the fact that construction of sewers does not have too much quality control.
• after installations, flows in the sewer continue to increase and at the same time k also increases with the old age of the pipe.
• use of k values in new pipes not rational; more practical to use k for used sewer pipes with normal quality.
Recommended Values For k
Pipe Material
k Value (mm)
Reinforced concrete with spigot and socket joint.
1.5
PVC
0.15
Earthenware
0.3
Asbestos Cement (A.C.)
0.6
• The above are derived from the Hydraulic Research Institute, based on extensive laboratory and field studies.
• Another school of thought is that after some years of operation, slimming takes place in sewers and the
hydraulic properties (flow) become independent of the material of construction (Novotny et al, 1989). Based
on this argument a design k value of 1.5mm is recommended for all sewers. Other sources also argue that the
resistance to flow or the flow characteristics in a sewer depends on the velocity on the pipe (Casey, 1992). In
this case different k values are recommended for different flow velocity ranges.
94
• The diameter of a pipe flowing full increases from 150mm to 200mm. The
flow rate is 20 l/s (0.020m3/s). Determine the mean velocity upstream and
downstream of the expansion.
• Upstream:
•
=
.
so
=
.
=
×
.
= 1.13m/s
• Downstream:
•
=
.
so
=
=
.
×
.
= 0.64m/s
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95
Calculate the velocity and discharge for the following sewer.
Slope – 1: 60
Diameter – 150mm
K – 1.5mm
T – 150C
Solution:


3
2.511.14 106
 1.5 10

 log 


60
 3.7  0.15 D 2  9.81 0.15  160 


To simplify the calculation evaluate the term under the square root first
 k
2.51 
v  2 2 gDS  log 


 3.7 D D 2 gDS 
2 gDS  2  9.81 0.15  1
60
☞
v  2 2  9.81 0.15  1
 0.22147
v  1.132m / s
Q  vA 
1.132    0.152
 0.020m3 / s (20l / s )
4
96
Tables and charts for calculating flow.
Tables
• The Colebrook-white Formula is a complex equation and the iterative approach in the
design of the sewer makes its use a tedious exercise.
• Various tables based on different roughness coefficients and different temperatures as well
as pipe sizes and slopes have been developed. These tables may be found in a number of
textbooks and manuals.
• The tables on the hydraulic design of sewers as in “Tables For The Hydraulic Design Of
Pipes, Sewers And Channels. Volume 1. 6th Edition, 1994” By Wallingford And Barr” can
also be used.
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Examples of tables
98
Tables and charts for calculating flow.
Charts
• There are also a number of charts in standard textbooks and manuals. Reference is given to the
charts prepared by the Hydraulic Research Station And those In Butler And Pinkerton, 1987
“Charts For The Design Of Gravity Pipelines”.
• It is important to note that the particular graph or table is valid for one single roughness value and
one single water temperature. Although the variation of these parameters has a smaller effect on
the friction loss than the variation of D, v or Q, this limits the application of the tables and graphs if
the values specifically for k differ substantially from those used in the creation of the table/graph.
• Key points on using the charts
• Ensure that the design flow lies in the range of flows on the right hand side of the chart selected
• The selection of k is governed by the recommended value of k for the pipe material and state (or age).
• Other tables based on the Colebrook White Formula also given in the Sanitation Manual, Design Procedures
(SALA, 1990).
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100
The chart shows an example of a flow rate
of 20 l/s (top axis) passing through a pipe
of diameter D = 200 mm (bottom axis).
From the intersection of the lines
connecting these two values it emerges that
the corresponding velocity (left axis) and
hydraulic gradient (right axis) would be
around 0.6 m/s and 2 m/km, respectively.
The same flow rate in a pipe D = 300 mm
yields much lower values: the velocity
would be below 0.3 m/s and the gradient
around 0.3 m/km.
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Computation of part full flow
Using an example. The sewer is currently receiving about 13l/s. Compute the part full
velocity and check whether the velocity will meet self-cleaning requirements.
Full solution bore velocity
= 1.13 m/s
Full bore discharge
= 20 l/s
Compute Qp /Qf
Qf is full-bore discharge while Qp is part full flow
Qp
Qf
For Qp /Qf
= 0.65

13
 0.65
20
vp/vf = 1.05
v p  vp v f

ratio
vf
v p  1.05 1.13  1.19m / s
102
Computation of part full flow
Using the chart “Relative Velocity and Flow in a Circular Pipe for any Depth of Flow”
k = 0.6mm
minimum velocity = 1.0m/sec.
Length of pipe = 1200m
Fall to outlet = 15m => gradient of 1:80
Design Discharge 0.1m³/sec
diameter = 300mm
What is the flow in the pipeline when 35% full?
Calculate depth of flow ratio = 35% = 0.35
From graph draw line along y axis value = 0.35
Read corresponding value of flow ( off the flow curve measured off x axis) = 0.27 m3/s when at 35% full
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RELATIVE VELOCITY AND FLOW IN CIRCULAR PIPE FOR ANY DEPTH OF FLOW
104
The modified Colebrook-White Formula
• Consider a pipe flowing partly full at a depth of d as shown in figure
D/2-d
D
D/2
/2

b
d
• Taking half of the triangle made by the water surface and the center we have
b  D 2  sin( 2 )
D
Cos ( / 2) 
• Employing the following
2
d
D
2
d

and   2 cos 1 1  2 
D

Area of a sector
=0.5 r2
Perimeter of a sector
= r
Area of a triangle
=1/2 base height
Sin 2A
=2SinA CosA
Aw 
1
2

D
D
8 D   2 sin( 2 )  ( 2  d )

(D  d ) 
D 2   sin( 2 )  2

D
2


 1 8 D 2   sin( 2 )  cos( 2 ) 
Aw 

1
1
8
Pw  D 2 
8
D 2   sin  
R
1   sin 
D
4

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• For a circular pipe the hydraulic diameter (Dh)=4R where R is the radius. The following
can be said generally
Dh  4 R
• Therefore for a pipe flowing partly full the following expression is true
Dh  4 R  D 
  sin 

Dh   D
where  
  sin 

• Therefore in The Colebrook White Formula the D can be replaced by D for a part-full pipe.
When the pipe is flowing full, =2 and =1.
• The modified Colebrook White Formula can then be stated as follows
 k
2.51

v  2 2 g DS  log 


 3.7 D  D 2 g DS 
106
• Minimum velocity in a sewer set so that sewer is able to clean, by hydraulic tractive force, the particles that
are likely to occur in it. Minimum velocities in table
Kind of material
Fine clay and silt
Fine sand
Round pebbles(up to 2mm)
Sharp edged particles
Velocity required [m/s]
0.075
0.15
0.5-0.6
0.9
Source: Chatterjee,1987
• Minimum cleansing velocity is dictated by the likely particle to enter the sewer.
• Areas where the soil nature is very fine and say predominantly silt, the cleansing velocity may be set at around 0.1-0.2
m/s.
• Points of entry of particles into the sewer include gullies , manholes, and poor pipe joints . sand is the most commonly
found particle in sewers especially in Zimbabwe, where its used to scour pots and for this reason the minimum cleansing
velocity is about 0.6m/s.
• Usual practice that the sewer should be designed to attain cleansing velocity at least once a day.
• Cleansing velocity generally checked at peak daily dry weather flow or maximum daily flow during the dry
period.
• Note cleansing velocity should be during the dry period as this is the critical period giving the low flows.
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• Peak flow generally occurs between 6-9am depending on the distance from the point of
generation to the point of flow measurement
• Sewage works far away from the city or community show much delayed peaks while those
very near community show peaks very early.
• Firle sewage works ~25 kilometers from the furthest areas of generation, Msasa industrial area (near
Zimphos) and gun hill has peak flows occurring between 8-9am.
• The presence of industrial wastewater and the mode and shift of operation of large wastewater producing
industries may affect the flow pattern considerably
• The peak flow is generally a function of the population being served.
• A sewer serving a large population tends to have a lower peak factor as compared to a small area.
• Crowborough sewage works flows for the period around the year 2000 indicated a peak factor of 1.6-1.8
while the ponds in Mabvuku have typical peak factors of 3-4.
• The peak factor therefore varies considerably with the size of flow. Lake of adequate data in Zimbabwe
makes the selection of the appropriate factor difficult.
108
• As a result in Zimbabwe the peak daily flow factor has been taken as 2 for design purposes. In South Africa
the peak factor is defined by the following function where p is population in thousands.
14
Peak Factor  1 
4 p
• In France the following formula is used to determine the peak flow where qave is the average flow in l/s.
2.5
Peak Factor  1.5 
Qave
• Recommended self –cleansing velocities in Zimbabwe are given below.
Reticulation and collector sewers
0.75m/s
Trunk and outfall sewers
0.6m/s
• These velocities primarily enable the sewer to erode compacted sand thus considered minimum as other
materials (which need higher values) find their way into the sewers. However the velocity should not
exceed 3.0m/s otherwise scour of the pipe will occur.
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• Limiting grades are essentially determined with the cleansing velocity in mind. As a rule
of thumb the minimum slope for any given pipe diameter is as follows
Min slope 
1
D
Where D is the pipe diameter in mm. Thus for a 150mm diameter pipe the minimum slope will be 1:150.
• Karl Imhoff's handbook (Germany) defines the minimum slope for large pipes as
follows
S min  1.7  105 D
Where smin is the minimum slope and D is the pipe diameter in mm.
• The recommended minimum slopes as well as the rule of thumb for the head of the
sewer for Zimbabwe outlined in the Sanitation Design Manual prepared by SALA(1990).
• The layout and general steps in locating the pipe route and minimum starting depths
are outlined in SALA, 1990.
110
Layouts
• Drawing must always have a scale and recommended scales are 1:2500 for medium and
low density and 1:1250 for high-density areas. In addition it is generally a good practice to
have scales compatible with drawings for already developed adjacent areas. Consultation
with the approval or governing authority is also essential.
• A site plan is generally necessary or alternatively there should be adequate existing
information such as named roads, streams, existing stands etc. Such information is key to
efficient response to problems such as sewer chokes when the sewer becomes operational.
• The North direction should be indicated and the drawing should have a legend or key
• Stand numbers for the proposed development and adjacent existing stands should be
conveniently shown.
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A sewerage system of sewer pipes including all
appurtenances for sewage conveyance and disposal. A
sewerage system consists of
House
connection
Lateral
• Collection works
• A stand connection which can be a 1m, 100mm diameter off
take from the manhole or a wye-junction forming part of the
pipeline
• A rodding eye which is usually for drainage within a stand
(internal drainage)
• Pipe lines and manholes
Trunk
• Interceptors
• Pump stations (lifting stations) and pumping mains
• Treatment plant
Branch
Sub-main
Main
To sewage works
• Disposal works
• Other structures
• such as grease and oil traps and siphons
112
Layouts ctd
• The drawing must be clearly labeled showing , name of development (including name of area or suburb),
name of consulting firm, name of developer, date, scale , drawing number, sheet number as is applicable.
• It is ideal that the size of any one drawing should not exceed A0 size as this may cause problems in filling
• Servitude size and location should be specified on the drawing. It should be noted that specifying the
servitude on the drawing without ensuring registration with the surveyor general renders the servitude
legally void and problems might arise in events of repairs or extension of existing services. Typical servitudes
are as below:
• 1.5 m on both sides of the stand boundary dividing two adjacent stands for high-density stands (i.e. Stand size less than
1000m2)
• 3.0 m on one side of the stand boundary dividing two adjacent stands for high-density stands (i.e. Stand size greater than
1000m2)
• All stands must have connections
• The sewerage reticulation should be located on the lower side for high density developments and water on
the higher side
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• Manhole sizes & spacing as recommended by SALA, 1990 page 40 e.g. not more than 75m for collector sewers.
• At head of public sewer (serving >1 stand) there must be a manhole and not rodding eye whose depth should be ideally 0.751.2m.
• Manholes to be located (where possible) at strategic points to serve functions as connection point, change of direction,
inspection point.
Longitudinal sections
• Levels should in absolute four figure national grid reference and only where there are no tsms or trigs within reasonable
distance shall the levels be based on a n arbitrary datum, of which this should be specified on the drawing. Arbitrary datums
may only be used for minor sewers. It is suggested that levels based on arbitrary datums should not be in four-figure format as
this may cause confusion; two or three figure system is suggested. E.G. 10.123 and 103.892.
• The following details should also be on the drawing:
• Datum
- Ground
• Invert level (IL)
- Depth
level (GL)
• Pipe slope (1:x)
- Distance
(GL minus IL)
and chainage
• Pipe material and bedding details (where applicable and necessary)
• Horizontal and vertical scales, which should be in the ratio of 1:10. For example if the horizontal scale is 1:1000 then the vertical
scale should be 1:100.
114
Manholes
• A manhole is an opening constructed in a sewer so that the sewer can be accessed for inspection,
cleaning, and maintenance.
• Opening is usually some 500-600mm in diameter for public sewers to allow a person to enter
(560mm acceptable in Zim).
• Apart from above purposes, manhole is also used for ventilation of the sewer (through a vent pipe
installed onto the manhole); to facilitate a bend, to allow for change of slope; for change of pipe
size and sometimes material; as well as being used for a connection.
• Manholes can be constructed from bricks (for construction of internal drainage) or pre-cast
concrete rings (public sewers).
• A public sewer is a sewer serving more than one stand and is usually located within servitude or a
road reserve. Its maintenance is also often the responsibility of the local authority.
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Manholes
• The major components of a manhole are
• Access hole
• Working chamber made of bricks or pre-cast rings
• Manhole cover
• Step irons
• Manhole sizes ranging from 900-1,350mm in diameter are common locally.
• Manhole covers are required to cover the opening of the access hole .Manhole covers and
frame are made of cast iron with different strengths to meet different conditions e.g. Those
under roadways are of a heavy-duty type.
116
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Drop manholes
• Drop manholes are constructed at the junction of two sewers
with different invert levels.
• This is necessary when the invert levels vary by more than
0.5m to avoid splashing of sewage in the manhole, which
could threaten the life of the walls.
• Splashing of the manholes also affect maintenance especially
if people have to go in.
• Two types of drop manholes are common depending on the
difference in invert levels. Type A for difference less or equal
to 1.5m and type B for more than 1.5m.
118
House connection
connection
• A house connection is usually 100mm in diameter. It
can be constructed in the following manner
• A wye-junction on a lateral sewer
• A take off from a manhole
• A vertical drop
•
• Typical layout details for a stand connection are
shown in figure
Lateral
sewer
Stand
boundary
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• Defined by collection:
• Combined – storm and wastewater together
• Separate – storm and wastewater apart
• Above ground/underground, “grey”/”green”
• Defined by mode of transport
• gravity
• pressure
• vacuum
120
• Combined sewers
• Separate sewers
• Simplified sewers
• Solid free sewers
• Pressurised sewers
• Vacuum sewers
• Open channel drains
• “Green infrastructure systems”
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• Large networks of underground pipes, mostly in urban areas.
• Collection of blackwater, greywater and stormwater.
• Do not require on-site pre-treatment or storage of the wastewater
• Discharge either into the WWTP or into a water body
122
• The construction costs can be higher than combined sewer system because separate networks required
• The replacement of a combined with a separate system is very costly
• When properly constructed, the sewage is transported in a closed system directly to the treatment plant
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• Types:
• Simplified/Condominial
• Solids free
124
• A sewer system that is constructed using smaller diameter pipes (e.g 100mm) laid at a
shallower depth and at a flatter gradient than conventional sewers
• Design Principles
• Sewers are laid within the property boundaries and side walks, rather than beneath the central road
• Every household should have a grease trap or an other appropriate pre-treatment facility.
• Semi-centralised treatment facility or transfer/ discharge station.
• Stormwater drainage system is still required
• Applicability
• More flexible design gives rise to lower costs (50 to 80% less expensive than conventional gravity sewerage),
sometimes even lower than those of onsite sanitation
• because of use of flatter gradients, shallow excavation depths, small diameter pipes, and simple inspection units
• Where the ground is rocky or the groundwater is high
• They can be installed in almost all types of settlements and are especially appropriate for dense urban settlements
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• Operation and maintenance
• No solids should enter the system
• Blockages need to be removed and the system needs to be flushed periodically
• The pipeline system components, such as cleanouts or ventilation points should be regularly checked and maintained
126
• These are similar to conventional sewer systems, except that the wastewater is pre-settled and
solids removed (in a settling or septic tank) before entering the system
• As solids are removed, sewer diameter can be smaller and can be constructed using less
conservative design criteria (lower gradients, fewer pumps, shallower pipe depths, etc) thus
significantly lower investment costs
• Due to the simplified design, solids-free sewers can be built at lower cost (20% to 50% less costs
than conventional sewerage).
• Design aspects
• A precondition is an efficient pre-treatment at household level. Interceptors remove settleable particles that
may clog the small pipes. If well used and maintained
• There is little risk of clogging; hence the sewers do not have to be self- cleansing and can be laid at shallow depths
• Can have fewer inspection points (manholes)
• Can follow the topography more closely and can even have adverse gradients (i.e. Negative slopes)
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• Operation and maintenance
• The pre-settling units must be maintained and emptied regularly to
• ensure optimal performance
• The risk of pipe clogging is low if the sewers are well operated and maintained, however, some
maintenance is required periodically
• Regardless of performance, the sewers should be flushed once a year
128
• Pressurised sewers use pumps instead of gravity to transport wastewater
• The primary effluent is delivered to the collection tank by gravity, where it is grinded (pressed) before being
transported into the pressurised system by pumps
• The system can be built with only shallow trenches and relatively small-diameter pipes
• An effective solution where conventional systems are impractical e.g. in rocky, hilly or densely
populated areas, or areas with a high g/w table
• Working principle
• A pit containing a grinder and pump or a settling unit (septic tank) connected to a holding tank with a pump
are installed close to the user
• When a certain level of effluent has been collected, it is pumped into the sewer, generating the pressure for
transportation
• Costs
• High capital costs, but still lower than gravity sewer systems
• The costs include the pumps, basins, controls, electrical services, and installation
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• Operation and maintenance
• All system components should be regularly serviced
• electricity needs to be available all the time
• the pumps should be checked regularly and the pipe connections should be controlled for leakages
• frequency of operation and maintenance depend on wastewater volume
130
The main stages in sewer construction are described below.
1. Location of the position of manholes and other appurtenances
• Work can proceed between manholes. Start with the lower end, which may be the trunk sewer, pumpstation or treatment works so that the portion, which is complete, can be readily used.
• This also avoids or minimizes problems of levels not tying in especially when there is poor level control.
2. Mark and align the center
• Mark the center and edge of the trench
3. Alignment and grade control
• The trench is divided into smaller segments between manholes which maybe 10-20m.
• Temporary bench marks (TBMs) are also established on fixed permanent objects which can be TSMs,
building floors, culvert headwall or even new pegs in concrete. Invert levels are then maintained using
the TBMs as reference. TBMs must be as near as possible to the points to be controlled. Grade control
can thus be done by use of a leveling machine or sight rails
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• Sewers should be tested for infiltration and exfiltration to prescribed limits.
• Infiltration will result in excess water getting into the sewer and this may result in surcharges and overloading
of the sewer and sewage works if not controlled.
• Initial test should be done when only partial backfilling has been done for ease of attending to faults that will
manifest during the initial test.
• Testing after complete backfill will detect faults as a result of damage resulting from backfilling. The final test
is done from manhole to manhole.
Initial test
2nd Test
Final test
(after partial backfill)
(After complete backfill
(Manhole to manhole)
• Methods of testing
• Commonly used methods for testing sewers are;
i.
Air test
ii. Water test
• Sewers carry liquids and therefore where possible the water test is more appropriate. However the air test is
quicker and thus economic under site conditions.
132
1. Air test
• Procedure for air test is specified in section 13.3 of BS 8005 part1, 1987 and has been adopted by Zimbabwean manuals.
• Both ends of section to be tested are plugged. A means of introducing air and a u-tube manometer are fixed to one of the
plugging end caps. Air is introduced by mouth or a hand pump to achieve a pressure of slightly more than 100mm in the utube. Five minutes is allowed for stabilization of air after which pressure is adjusted to 100mm.
• The pressure must not fall by more than 25mm in 5 minutes. If a fault exists i.e. a fall of over 25mm in 5 minutes, then a
smoke test can be used to identify the leaking point.
2.
Water test
• Procedure for water test is detailed in BS8005, part 1, 1987. In summary the test involves placing suitable plugs at the
upstream and downstream ends of the section under test. Water is then introduced through a hose from the upper end to a
pressure head of 1.2m above the crown at the top end and ensuring that the length maintains a pressure not exceeding 6m
at the lower end. Care must be taken to avoid air pockets and also propping and strutting is done where necessary to avoid
lateral movement of the pipe.
• The pipe is given 2 hours to saturate and topping of water may be necessary. After saturation the amount of water loss in
30minutes is determined as that used to top up at 5 minutes intervals to maintain the level. The water loss should be less
than 1 litre per hour per meter diameter per meter of pipe run.
total water for topping (l )
loss 
0.5(hrs )  Dia (m)  L(m)
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• Infiltration test
• After backfilling there is need to check for infiltration. The sewer is inspected from manholes after ensuring that all inlets
and junctions are plugged.
• Any flow observed in manholes indicates infiltration and should be investigated. Various ways to investigate.
• The infiltration is acceptable if it is less than 1l per hr per m diameter per m run.
• Straightness
• The straightness of a sewer can be checked using the following methods
i.
Surveyor’s level and staff (theodolite and target)
ii.
Sight rails and boring rods and travelers
iii. Laser beam with citing targets
iv. Lamp (torch) and mirror
• Test for ancillaries
• Test for ancillary works outlined in BS8005 part1, 1987. Ancillary works include manholes, oil interceptors, grease traps….
• With regard to manholes the test involves plugging the inlet and outlet of the manhole. For manholes less than 1.5m the
water is filled to the underside of the cover. For manholes more than 1.5m the test head should be at least 1.5m.
• An absorption period of 8 hours is then allowed during which topping up to maintain the level may be necessary. The
permissible water loss is still a subject of research and currently the engineer uses his opinion based on site conditions.
• For this reason visual inspection of the manhole is important and often when it is satisfactory, there is no further test.
134
Sewer bedding and trenches
• Trench widths are generally 500mm wider than the nominal diameter of the pipe for diameters of up to about
300mm.
• However for larger diameters the trench width may be determined taking into consideration the following
• Stability of the soil
• The depth of excavation
• Mode of pipe handling (crane or winch or by hand)
• Earth loads on a buried pipe increase with depth while the effect of wheel loads decrease with depth.
• It is necessary to take note of the fact that the loads on the pipe increases with the trench width as a wider
unsupported earth column will rest on the pipe.
• Young and Smith 1990 “Loads On Buried Pipes” is a good reference for external loads on pipes.
• Bedding at the trench bottom helps to protect the pipe from superimposed loads and also from local adverse
conditions such as rocky areas or excessive groundwater.
• A bedding from fine material such as sand is suitable to protect say PVC in ground with rock pebbles, while a
bedding incorporating porous media such as crushed stone may be useful for draining water away from the pipe in
water logged areas.
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Source: PVC Technical Manual
PIPING MATERIALS SELECTION
FACTORS IN PIPE MATERIALS SELECTION
COPPER
PIPES CATEGORIES
CONCRETE
PROPERTIES OF METALLIC AND NON METALLIC MATERIALS
POLYVINYL CHLORIDE
CAST IRON
POLYETHYLENE
DUCTILE IRON
GLASS REINFORCED POLYESTER
STEEL
ASBESTOS CEMENT
136
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137
1. Maximum and minimum depth of pipe cover,
2. Anticipated loading on ground surface,
3. Pipe embedment & support conditions,
4. Pipe flexibility to be laid in a curved trench,
5. Resistance to corrosion & chemical action,
6. Permissible longitudinal & diametric deflection,
7. Ring stress to withstand heavy backfill loads without pipe deformation,
138
8.
details of backfill material,
9.
length and weight for handling and storage,
10. ease of making repairs & future connections,
11. risk of damage from third parties,
12. in waterlogged ground, the weight of pipe in relation to the risk of flotation,
13. pipe length with respect to the number of joints required,
14. deformations caused by extreme temperatures, uneven subsidence, vegetation roots,
etc.
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Pipes can be categorized according to resistance to backfill and shock loads
1. Rigid:
• cast iron (CI),
• asbestos cement (AC),
• concrete,
2. Semi-rigid:
• ductile iron (DI),
• steel and
3. Flexible:
• polyvinyl chloride (PVC),
• polyethylene (PE),
• glass reinforced plastic (GRP).
• Pipes can also be categorized as metallic or non metallic
140
SOURCE: (NEMANJA 2015)
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142
• Used where external strength is the major requirement. Internal strength is also high.
Typical situations, which require cast iron pipes, are, under a railway, highways, buildings
(external strength) and components of a rising main (internal strength). Cast iron pipes
are
• Widely used for the piping and fittings of drainage systems (plumbing) of buildings
• Cast iron (CI) is one of the oldest pipe materials used for the conveyance of water under
pressure. The use of cast iron pipes has declined over the last few decades.
• The main disadvantage of CI pipes is generally low resistance to external and internal
corrosion. This reduces the pipe capacity and iron release causes the water quality to
deteriorate. Although the corrosion levels could stabilise after some time, the problems are
usually solved by applying an internal cement lining and/or an external bituminous
coating. The considerable weight and required wall thickness are additional disadvantages
of these pipes.
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• DI is a material that has been in use for almost 60 years. It is an alloy of iron, carbon, silicon, with
traces of manganese, sulphur and phosphorus.
• Unlike ordinary CI pipes made of grey iron where free carbon is present in flakes, the carbon in DI
pipes is present in the form of discrete nodules which increases material strength by 2–2.5 times.
• DI pipes are strong, durable and smooth pipes, usually laid in mid- range diameters (100–600 mm)
although they can be manufactured up to 1800 mm. They are suitable for almost all soil conditions
if corrosion protected. Compared to the CI pipes, DI pipes are lighter due to reduced wall thickness
as well as being less susceptible to external loadings.
• Despite their reduced weight, these pipes are still rather heavy and handling with appropriate
machinery is necessary even in smaller diameters. In aggressive soil conditions, DI may become
susceptible to corrosion. As a minimum, bituminous paint should be used as an external
protection. An internal cement lining is regularly applied to prevent corrosion resulting in
turbidity and colour that may appear in water.
144
• Steel pipes are manufactured in two ways: by welding steel plates or stripes either
longitudinally or in the form of a spiral (large diameters), or seamless from a steel billet
(small diameters). Compared to iron or pre-stressed concrete pipes of the same internal
diameter, steel pipes are stronger, more flexible and have thinner walls. Consequently, they
are lighter and easier for handling and laying.
• They are also available in longer units, which reduces the total number of joints required
• Second, repair of the pipes is relatively easy and can be conducted under space restriction.
Leaks would normally be localised to joints or pinholes resulting in an acceptable loss of
water. An additional safeguard in this respect is to weld the joints.
• Just as with other metal pipes, steel pipes are sensitive to corrosion. Hence, the internal
and external protection must be perfect for both pipes and joints.
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• Costs of steel pipes increases due to need for lining which can be
• bitumen,
galvanising or
epoxy
• Advantages
• Lightness
• Imperviousness
• Resistance to bursting pressure as in pumping mains
• Flexibility i.e. can resist shock movements, external pressures by deflection, buckling without failure.
• Typical diameters are between 100 and 1800 mm. Frequently used for
• the transport of large water quantities.
• Ultimate choice piping in pumping stations
• areas where the sewer is above the ground such as local depressions and stream crossings
• in circumstances where vandalism is likely
146
• A large number of service pipes and plumbing inside premises is made of copper.
• This material is popular as it is relatively cheap and reliable for large-scale
implementation.
• As they are not used for distribution pipes, copper tubes rarely exceed 50 mm in
diameter.
• The material is strong and durable, whilst at the same time sufficiently flexible to
create any kind of bend
• Low internal roughness helps to minimise the hydraulic losses resulting from long
pipe lengths.
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• AC pipes are rigid non-metallic pipes produced from a mixture of asbestos fibre, sand and cement.
The carcinogenic effect of asbestos-based materials used in water distribution has been studied
carefully in the last couple of decades. Although not dangerous when in drinking water, the fibres
can be harmful when inhaled. Therefore, the laying of new AC pipes has been prohibited by law in
many countries, due to possible hazards during manufacturing, maintenance and disposal of these
pipes.
• The main advantages of the AC pipes, compared to iron pipes, are:
•
•
•
•
freedom from internal corrosion and generally better resistance to soil corrosion,
a smooth inner surface,
Lighter weight and
lower production costs.
• The pipes can be drilled and tapped for service connections but are not as good (brittle) as iron
pipes; these locations are potential sources of leakages. Bursts and longitudinal deformations are
also more likely and AC pipes in aggressive soils tend to corrode.
• manufactured in 4m lengths, diameters from 100mm to 700mm. Jointing is by means of a flexible
rubber rings and a joint sleeve or collar.
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• Concrete pipes are rigid, cement-based pipes mainly used for sewerage. In drinking water supply, they will be more
frequently laid for water transport than distribution.
• Diameters of between 150 and 1500 mm. Can occasionally be larger, and are almost always reinforced and usually
pre-stressed to withstand internal pressure and external loads. Pre-stressed concrete pipes reinforced in two ways:
circumferentially pre-stressed with steel cylinder, or both longitudinally and circumferentially pre-stressed with
steel net.
• Joints usually spigot and socket and rubber ring
• As a material, concrete is lighter than iron or steel but concrete pipes are generally heavier than metal pipes of
corresponding inner diameter, due to thicker walls. E.g., a pipe of D = 600 mm has a mass of some 500 kg/m length,
which creates difficulties during transportation and handling.
• The advantage of heavy weight is it limits any risk of movement, specifically in waterlogged ground. Furthermore,
concrete pipes can carry heavy loads without damage or deformation and show good corrosion resistance; no
special precaution is needed for pipe bedding or backfilling. Low internal roughness enables good hydraulic
performance while conveying large water quantities. Concrete pipes are comparatively cheap in large diameters.
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• Laying a route with concrete pipes requires more time than with metal pipes and involves heavy
machinery. Operationally, a pipe repair also takes longer than with other types. If in the case of
failure an alter- native supply is not available, a larger part of the distribution area may be
excluded from service. As with AC pipes, aggressive soil or water can cause corrosion that is
normally restricted to cement mortar but will attack the steel reinforcement once it is exposed to
the environment; external coating is needed in such cases.
• They have a high crushing strength (min 10kN/m for small pipes) and exhibit considerable
flexural strength especially when they are reinforced with a steel mash. For these characteristics
concrete pipes are suitable for almost all conditions of construction. However they are easily
corroded especially with liquids such as sewage, which tend to be septic. Where chances of
corrosion are very high an additional 16mm or so layer “sacrificial layer” has been added in the
inside above the normal thickness in order to take up the corrosion.
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• PVC pipes are flexible pipes widely used in a range of diameters up to 600 mm (see figure 4.41). The properties of
this thermoplastic material give the following advantages:
• excellent corrosion characteristics,
• light weight,
• availability in long pieces,
• low production costs,
• reduced installation costs.
• The disadvantages lie in the reduction of its impact strength in extremely low temperatures. An incidence of pipe
bursts in wintertime has direct relation to the depth of the trench, which will be adopted based on the comparison
between the costs of excavation, pipe repairs and estimated water losses. Furthermore, the PVC pipes lose their
tensile strength in extremely high temperatures. As a result, careful handling, stacking and laying is crucial under
extreme temperatures. Pipe support must be as uniform as possible, free from stones or other hard objects, and the
filling material used must be well compacted.
• Despite radical improvement in the manufacturing process, PVC pipes are uncommon in diameters above 600 mm.
Larger diameters rarely withstand pressures much above 100 mwc and are therefore less suitable for highpressure flows, e.g. Pressure pipes in pumping stations, or trunk mains. Moreover, a fracture can develop into a
split along considerable lengths of the pipe, resulting in large water losses.
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• Another polymer used for pipe production is PE. There are three phases in the development of the
manufacturing technology resulting in the following types of PE pipes:
• low density PE (LDPE), manufactured previously exclusively for service connections in diameters up to 50 mm,
• medium density PE (MDPE), with improved performance and for diameters up to 200 mm,
• high density PE (HDPE), nowadays also manufactured in large diameters (exceptionally above 800 mm).
• Compared to PVC, PE pipes show the following enhanced characteristics:
• improved resistance to stress cracking,
better performance under extreme temperatures,
• extreme flexibility,
good welding compatibility,
• improved resistance to surge pressures.
Easier pipe handling
• Smaller diameters (up to 100 mm) rolled in coils up to 150 m while larger pipes are 10–12 m long.
• One of main disadvantages of PE vs PVC is higher price resulting cpz of thicker walls required for these pipes.
• Permeability of PE also an issue. Maintenance problems mostly relate to jointing of the pipes. The welding
technique is reliable but involves qualified personnel and electrical equipment. Special precautions to be
taken when using conventional mechanical fittings, due to creeping of PE. Water quality problems related to
this material result from bio-film formation on the pipe wall.
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• GRP pipes are composed of three main components: fibreglass, resin and sand. The strength of
the pipe is derived from bonding the fibreglass with resin. The purpose of adding sand is the
increase of wall thickness that improves stiffness of the pipe, which makes its handling easier.
• GRP pipes commonly manufactured in larger diameters and thus used for water transport.
Compared with concrete and steel pipes of the same size, they are:
•
•
•
•
lighter in weight,
more flexible
more resistant to corrosion
cheaper
• In relation to the technical lifetime of pipes, GRP is still a relatively new material as it has only
been in use since the 80s. No firm conclusions but life spans of > 50yrs are expected.
• Has excellent corrosion characteristics, which makes it predominantly suitable for industrial
and chemical sites where intensive corrosive conditions may occur. Applications of this material
frequently include the transport of waste water.
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• Excavation - trenches
• Embedment – bedding material
• Backfilling – covering up
• Anchorage
• Jointing
• Pipeline testing
http://media.amiantit.eu/Content/Flowtite/Technology/Manufacturing-brochure.pdf
O&M OF WATER DISTRIBUTION SYSTEMS
ADOPTED FROM SSWM AND TRIFUNOVIC
154
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• Generally, water and sanitation projects experience their most serious problems with operation and
maintenance and with cost recovery aspects.
• Hundreds of projects around the world demonstrate how the newly built infrastructure deteriorates after the
project’s termination.
• Therefore, it is imperative to plan for operation and maintenance, with a planned withdrawal of external
support as local ownership builds..
• Operation and maintenance is a crucial element of sustainability, and a frequent cause of failure of water
supply and sanitation service facilities in the past.
• Many failures are not technical ones. They may result from poor planning, inadequate cost recovery, or the
outreach inadequacies of centralised agencies (DFID 1998).
• Operation and maintenance has been neglected in the past, or been discussed and introduced only after a
project was completed. This neglect or delay in applying proper operation and maintenance has adversely
affected the credibility of the investments made, the functioning of the services, the well-being of
populations, and the development of further projects.
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• Advantages
•
•
•
•
O&M activities ensure that the project is sustainable in a long-term
O&M allow for the correct provision of services and benefit of end-users
O&M prevent the systems to collapse creating environmental and health hazards
Community can be involved in O&M
• Disadvantages
• O&M activities cost time and money, and therefore a provision for financing O&M has to be planned before the
project start
• Operation and maintenance (O&M) activities encompass
•
•
•
•
•
technical issues,
managerial,
social,
financial and
institutional issues,
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• Operation and maintenance refers to all of the activities needed to run a water supply and
sanitation scheme, except for the construction of new facilities.
• The overall aim of operation and maintenance is to ensure efficiency, effectiveness
and sustainability of water supply and sanitation facilities (CASTRO 2009).
• The two activities of “operation” and “maintenance” are very different in nature.
• Operation refers to the direct access to the system by the user (e.g. accessing water), to the activities of
any operational staff (e.g. Operators of motorised pumps), and to the rules or by-laws, which may be
devised to govern who may access the system, when, and under what conditions.
• Maintenance, on the other hand, is to do with the technical activities, planned or reactive, which are
needed to keep the system working. Maintenance requires skills, tools and spare parts (CARTER 2009).
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• Maintenance can be classified as follows (adapted from CASTRO 2009):
• Preventive maintenance: includes work that is planned and carried out on a regular basis to maintain and
keep the infrastructure in good condition, such as network inspection, flushing of the well, cleaning and
greasing of mechanical parts and replacement of items with a limited lifespan. It sometimes also includes
minor repairs and replacement as dictated by the routine examinations.
• Corrective maintenance: replacing or repairing something that was done incorrectly or that needs to be
changed; an example is the reallocation of a pipe route or replacement of a faulty pump.
• Reactive maintenance: a reaction to a crisis or public complaints; it normally occurs as a result of failures
and the malfunctioning or breakdown of equipment. In order to ensure the routine maintenance and
health of the system, the technician should adhere to a routine check-up. The project manager will need
to ensure that the technician is doing his/her job. If done correctly and on a regular schedule, preventive
measures can reduce the risk of costly repairs. The key to ensuring effective equipment maintenance is to
make certain that invalid link are clearly defined and maintenance personnel have the tools and skills to
do their job effectively. It is also essential to schedule preventive maintenance.
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• The consumer’s requirements will not be satisfied in a poorly operated network, even if it has been
well designed and constructed. Making errors in this phase amplifies the common problems and
their implications:
• low operating pressures causing inadequate supply,
• high operating pressures causing high leakage in the system,
• low velocities causing long retention of water in pipes and reservoirs, – frequent changes of flow direction
causing water turbidity.
• These problems can have a serious impact on public health and coping with them also influences
maintenance requirements and the overall exploitation costs.
• Successful operation and maintenance require following an “owner’s manual” prepared by the
contractor and engineer at the onset of the planning process. Includes
• schedule and procedures for maintenance
• methods to carry out tasks such as bookkeeping, paying employees, collecting bills (utility management),
inspection, refurbishments, replacement of parts, etc., giving an integral framework for operation and
maintenance (NETSSAF 2008).
160
• ORGANISING AND PLANNING OPERATION AND MAINTENANCE
• FACTSHEET BLOCK BODY
• ORGANISING FOR O&M DOES NOT REPRESENT A HUGE TASK, BUT IT DOES REQUIRE CERTAIN LEVEL OF
PLANNING, COMMITMENT AND MONITORING. THE ASPECTS TO BE ORGANISED ARE:
•
• WHAT: THE ACTIVITY WHICH IS TO BE CARRIED OUT
• WHEN: THE FREQUENCY OF THIS ACTIVITY
• WHO: THE HUMAN RESOURCES REQUIRED FOR THE TASK
• WITH WHAT: WHAT ARE THE MATERIALS, SPARE PARTS, TOOLS AND EQUIPMENTS NEEDED
•
• THE FOLLOWING TABLE GIVES AN IDEA OF THE TYPE OF TOOLS WHICH HAVE TO BE DEVELOPED TO SUPPORT
THE OPERATION AND MAINTENANCE OF A NEW INFRASTRUCTURE. THE EXAMPLE RELATES TO THE O&M OF A
SEPTIC TANK (ADAPTED FROM CASTRO 2009):
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• The operation of gravity systems is rather simple and deals with the balance between supply and
consumption, which can be controlled by operating valves. Pressure limitations in the gravity
systems that result from topographic conditions become even bigger in the case of a bad design. A
wrongly elevated tank, incorrect volume or badly sized pipe diameter will not guarantee optimal
supply, and errors will have to be corrected by what would otherwise be unnecessary pumping.
• In pumped systems, a more sophisticated operation has to be introduced to meet the demand
variations and keep the pressures within an acceptable range. Computer simulations are an
essential support in solving problems such as these. As well as pressures and flows, network
models can process additional results relevant to the optimisation of the operation, such as:
power consumption in pumping stations, demand deficit in the system, or decay/growth of
constituents in the network. These models are also able to describe the patterns developed
during irregular supply situations. Finally, the models can be linked with monitoring devices in
the system, which enables the whole operation to be conducted from one central place.
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• Monitoring of water distribution systems provides vital information while setting up their
operational regimes. It predominantly comprises:
• monitoring of pressure-, water level- and flow variations,
• monitoring of water quality parameters, such as temperature, pH, turbidity, chlorine concentration
• Pressure-, level- and flow variations can be observed periodically for specific analyses (e.g.
Leakage surveys or the determination of a consumption pattern). When monitored
continuously, they may indicate:
• operational problems that require urgent action(e.g. Pressure drop due to a pipe burst),
• need for change in the mode of operation,
• Monitoring of water quality parameters can also help to detect inappropriate operational
regimes. In addition, water quality parameters outside the normal range often indicate a
need for necessary maintenance
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• As with hydraulic measurements, the selection of sampling points should provide a good overview of the
whole system, preferably at the source, reservoirs and other easily accessible locations where long retention
times are expected.
• Decisions on the spatial distribution of measuring points depend on the configuration of the system.
Pressure- and flow- meters have to be installed in all the supply points and booster stations. Water levels in
the reservoirs should also be permanently recorded. The measurements in main pipelines may be registered
at critical points of the system (relevant junctions, extreme altitudes, pressure reducing valves, system ends,
etc.). All these data can be captured in one of the following ways:
• telemetry – there is a permanent online communication between the measuring device and control command centre where
the parameter can be monitored round the clock.
• data loggers – here a measuring device is permanently installed but the data for certain time intervals are captured
periodically and will be processed and analysed later. Hence, the results of the measurements are not directly visible.
• local reading – direct readings can be obtained from the measuring devices’ display and immediate action taken if required.
• sampling – the water sample will be occasionally taken and analysed in a laboratory.
164
EXAMPLE DATA CAPTURING METHODS AND POINTS
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• Operation and maintenance are often interrelated. Good operating systems that meet the
consumer’s requirements are also likely to reduce the level of maintenance. The maintenance
considered in this context is predominantly consequential, or so-called reactive maintenance (e.G.
Repair of pipe bursts).
• On the other hand, proper maintenance also contributes to the optimal operation of the system. As
such, it is more of a condition, or requirement, for good operation, and is therefore understood as
preventive maintenance.
• Just as with the operation, efficient maintenance relies largely on good monitoring of the system
• Planning of maintenance
• The selection of the type and level of maintenance follows a strategy based on the following
principles:
• standard of service to the consumer should be regarded as a primary objective
• within constraints set by the standards of service, decisions should be
• Made on economic grounds.
166
• Preference for either preventive or reactive maintenance is
derived from the strategy selected.
• Generally speaking, the annual costs of repairs and cleaning
operations responding to the consumers’ complaints are
smaller than the annuities of main rehabilitation and
replacement.
• However, this is only true for the standard frequency of
pipe bursts. The expected trend is that the future number
of ruptures will increase. Preventive maintenance can
extend the economic lifetime of the system, and therefore is
a ‘must’; only the level of maintenance is debatable.
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• The way in which operation and maintenance tasks are implemented has implications for
the organisational set-up of the water company.
• The availability of technical means and expertise in itself will not be enough if good
planning of activities and coordination between various services are missing.
• The entire management of the water distribution is usually taken care of by a department
that is part of a larger water supply company.
• Such a department has to function in line with the general policies of the company,
providing well-distinguished tasks and responsibilities among the employees.
168
The activities of the distribution department (company) are basically divided into office work and fieldwork, some of
these being centralised.
Hence, the global structure consists of the head office and a number of district centres. The most important tasks of
the central office are:
– collection of technical data (mapping),
– design of the main network,
– financial aspects of the network design and operation, – monitoring of the network operation (control centre), –
control of major consumers.
More practical responsibilities of district centres are:
– construction of the network and service connections, – preventive maintenance (repair and cleaning),
– failure service,
– installation and maintenance of water meters,
– leakage detection and repair,
– water quality control,
– control of indoor installations,
– connection and disconnection of the consumers,
– management of the stock of spare parts,
– measurements in the network,
– registration of technical data,
– administration of the activities.
Depending on the area supplied, some tasks can be reallocated between the district centre and the head office.
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•
As-built drawings: this information is computer processed. The detailed position and description of the valves, fittings, washouts, pits, crossings, etc. Is indicated on the
map layout, sufficient for all information necessary for the maintenance activities.
•
Pipe flushing: this is done at two-yearly intervals. The water for this purpose is drawn from the system.
•
Pipe replacement: the pipe is normally replaced in three instances: 1. when there is frequent leakage at the same segment, 2. when the route has to be diverted, 3 due to
increase in the capacity.
•
System monitoring: in selected points in the system (mostly the ends of the system), the pressure is monitored continuously but only during the seasonal peaks in
summer. Pressures and flows are measured automatically in all pumping facilities, during the whole year. All records are preserved in the head office.
•
Leakage: the leakage level in the system is about 5% of total production. It is predominantly (by number of leaks) in the service lines. Most of the breakages in the
distribution system are registered on AC pipes. No leakage detection programme exists due to the low leakage rate. Very often the consumers report the leaks. Precise
evidence about the breaks is recorded in the computer. Leak repairs normally take 3–4 hours and this service is available 24-hours a day. The team on duty has a
vehicle equipped with all the necessary tools. Outside regular working hours the vehicle is always with one of the workers, and when required they can drive directly to
the place of failure, or to the district centre if additional information is necessary.
•
Metering: all service connections in the system have water meters installed. The reading of domestic consumers’ meters is carried out once a year and for large
consumers four times a year. The meters are replaced every 7–10 years. The company has its own workshop for the maintenance of the meters. On average about
20,000 meters are replaced or rehabilitated per year. All records with respect to meters are computerized. The estimated meter under-registration is approximately
1.5% of the total delivery into the system.
•
Training and research: there are ‘on the job’ and ‘off the job’ training programmes. A compulsory two-year training for the technical staff is organised by VEWIN. The
company arranges different types of short training programmes. The company does not invest substantially in research and development, which is carried out in
cooperation with the KIWA institute.
170
• RWSN – rural water supply network
• IWA – international water association
• AWWA – American water works association
• EPA – environmental protection agency
• IHE delft
• Santa Clara Valley Water District
• Silicon Valley Advanced Water Purification Center
• KNYSNA Municipality water-and-sanitation/water conservation and water
demand management strategy 2013
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