Wastewater flow characterization at selected points of the Harare sewerage system
M.P. Meki* and Z. Hoko
University of Zimbabwe, Department of Civil Engineering, Box MP 167, Mt Pleasant Harare ,
Zimbabwe
ABSTRACT
Some models and theories on the flow variation of sewage in sewers as well as infiltration appear to
exist in some developed countries. There is little published information on the variation of flow in
sewers as well as the amount of infiltration for most developing countries including Zimbabwe. These
aspects are critical for the proper design of sewerage reticulations, sewage pump stations, flow
equalization facilities and sewage treatment works. A study was carried out in the period February to
May 2009 to investigate the flow characteristics and infiltration at selected points on the sewerage
network of Harare, Zimbabwe’s capital. Sewage flow was measured at 2 selected points on the
network at pre-determined time intervals. The depth of flow was measured by a calibrated plate and
flows computed using the modified Colebrook-White formula taking a roughness coefficient of 1.5
mm as suggested in most literature. It was found out that for a 225 mm diameter pipe the ratio of the
minimum flow to average flow ranged from 0.4 - 0.7 during the study period while the ratio of
maximum flow to average flow was 1.4 - 1.9. The ratios of minimum to average and maximum to
average were 0.5-0.7 and 1.5-1.7 for a 525 mm pipe. Infiltration estimated from the minimum night
flows was found to be 51% to 70% for all points studied and appeared independent of the pipe
diameter when expressed as a percentage of the average flow. It was concluded that flow variation
was highest in the smaller pipe (225mm) compared to the larger pipe (525mm) and that infiltration
expressed as a percentage of the average flow appeared independent of the pipe size. The effect of the
pipe age and reticulation length on flow variation and infiltration need to be investigated in future
studies. Strategies for reduction of infiltration such as lining of pipes and replacement of old pipes
should be considered.
Keywords: flow variation, infiltration, minimum night flows, sewage, sewerage
1
Introduction
An estimated 40% of the population lacks access to sanitation facilities with approximately
6000 children, most of them from developing countries, dying every day of diseases related to
inadequate sanitation and a lack of access to drinking water (Rosenquist, 2005). Furthermore 14% of
the sewage in Latin America is treated before being discharged in waterways and in Europe merely 79
major cities out of 542 are estimated to have full treatment of their sewage. In Europe merely 30% of
all the waterborne sewage functions are in a sustainable manner (Rosenquist, 2005). Municipal
sanitary sewage collection systems (pumping stations, force mains, manholes and treatment plants)
play a critical role in protecting our cities and towns and the performance of these systems can
significantly influence the performance of the wastewater treatment plants (EPA, 2008). Accurate
reliable measurement of flows in municipal sewer systems is essential for effective routine wastewater
operations as well as infrastructure planning and regulatory compliance Trivedi and Walkowiak,
(2005). Flows important for the designs of sanitary sewers are daily minimum and maximum, daily
average and peak flow (ASCE, 1982). Peak flows estimated for the end of the design period usually
determine the desired hydraulic capacities of sanitary sewers, pumps and some treatment conduits
*
corresponding author : muxias.meki7@gmail.com / mukiluc@yahoo.co.uk : telephone;+263 11 77 47 67
(ASCE, 1982). Therefore reliable information on existing and projected flows must be available if
these facilities are to be designed properly (Escritt, 1960). . However reliable information is scarce
and data available reflect a wide range which is not easily rationalised in practical terms thus typical
sewage flows are estimated (IWPC, 1973). According to Hoko (2005) Harare has problems of
accurately quantifying sewage flows, thus lack of data in Zimbabwe makes the selection of the
appropriate factor difficult. Relatively little research has been undertaken on the quantity and quality
of wastewater discharges from domestic households Butler et al. (1995). The majority of the data
collected has been collected at the inlet of wastewater treatment plants (Metcalf and Eddy Inc., 1991).
Knowledge of the quantity and quality of sewage at both ends of the sewer network might give an
indication of the effects on sewer such as dilution, sedimentation, erosion and biochemical
transformations (Butler et al., 1995). In general Southern African sewerage systems are on a separate
basis on measures being taken to prevent as far as possible the inflow of stormwater into the foul
sewers (IWPC, 1973). The two documents currently used in Zimbabwe for the design of sewerage
reticulations are silent on the details of flow variation and infiltration1.
Infiltration and inflows are a major source of sanitary overflows that release sewage into
lakes, streams, streets and basements Chamberline, (2008). However historic data from several
sources suggest that under peak wastewater conditions as much as 75% of wastewater flow is
generated from infiltration and inflow as some surveys indicated that 50% to 70% of these infiltration
and inflow is from private property sources (WEF, 1999). Infiltration and inflow on water treatment
facilities results in increased operating expenses (GWI, 2007). According to Nhapi et al. (2002) the
eutrophication in Lake Chivero is mainly due to discharge of poorly treated effluent into rivers.
Furthermore Lake Chivero the major potable water source for Harare, Zimbabwe’s capital is seriously
polluted as a result mainly of poor wastewater management in the catchments. The deterioration of
the water quality of the inflow rivers into Lake Chivero greatly affects water treatment rendering it
more sophisticated and expensive (Nhapi and Tirivarombo, 2004). Furthermore overloading of the
reticulation and pumping stations, which is partly attributed to inaccurate information on flows and
levels of service, is also a major problem which has resulted in spillages of raw sewage into streams
and consequently pollution of river sources (Hoko, 2005). The study was carried out in Newlands and
Belvedere, which are low-density suburbs in Harare. The study investigated the variation of sewage
flow and the level of infiltration at two selected points (one in each of the suburbs).
2
Background theory
2.1 Wastewater generation
Sewage is generated from such water sources as the water closet, water from the kitchen sink,
bath water (Butler et al., 1995). About 70 % of domestic consumption may be expected to reach the
sewer but this percentage will be higher where from dwellings where there are no gardens (IWPC,
1973). Based on a research from Mufakose (Hoko 1999, Gumbo 2000) a typical high density suburb
in Harare, 85 % of water consumption is retained as sewage. According to O’Brien and Gere (2004),
water consumption which returns to sewer ranges from 80% to 90%. Some 85% of average daily
water demand returns to the sewer generally in Zimbabwe (SALA, 1990). Santosh (1996) and Metcalf
and Eddy (1991) suggest that water consumption is affected by metered services, cost and availability
of water, quality of water supplied, climate, population, and economics, density of community and
water conservation. Flow is estimated using average wastewater generation figures to establish
1.
1
2.
BCHOD(Brian Colquhoun, O’Donnell and Partners), HDS(Housing Development Services), 1982.
Design Approach to Water and Sewerage Problems related to Urban (Predominantly High Density) and
Rural Communities in Zimbabwe. Ministry of Local Government and Housing. Government of
Zimbabwe. Harare.
Swedish Association of Local Authorities, 1990. Sanitation Manual Design Procedures (Manual
No.5) SALA
2
average flow and peak factors related to position in the network using population served or the
average flow rate at a particular point (WFPC, 1970) therefore the peak factor varying with the size of
flow.
2.2 Flow variation
Hourly variations in water consumption affect wastewater flows, with the first peak generally
occurring in the late morning and other peaks during lunch time and late in the evening depending on
the distance from the point of generation (Metcalf and Eddy Inc, 1991). These fluctuations are less
pronounced downstream as they are damped because of storage space and time required to reach the
gauging point, as such peak factors (expressed as the number of times of the average values) will be
much greater in lateral (collector) sewers compared to main sewers (Santosh, 1996). According to
literature (Metcalf and Eddy Inc, 1991; DWSD, 2003; Raju, 2003) factors are expressed as a ratio of
maximum flow to average flow. Santosh (1996) and Metcalf and Eddy (1991) suggest that the ratio of
peak hour flow to average daily flow is 1.5. Minimum flows, both for initial and final conditions are
critical directly to the design of sewers to insure proper cleansing velocity and protection against
sulphide corrosion (WPCF, 1970).
2.3
Flow computation and measurement
Largely in America and Great Britain the Manning’s formula is used but the coefficient of
roughness should be accurately chosen (Escritt, 1960). However studies by hydraulicians
recommended that the most accurate basis for hydraulic design which is shown by most experiments
to be applicable to virtually any commercial surface and fluid over a wide range is the ColebrookWhite equation (Ackers, 1958). In addition SALA (1990) suggest that the Colebrook-White formula
should be used for the calculation of velocity and discharge in sewers. Three methods can be used to
measure flows, hydraulic structures, slope hydraulic radius and Area velocity Trivedi and Walkowiak,
(2005). According to Hammer and Hammer, (1996) flow measurement in pipes can be done by the
installation of flumes, measuring the flow depth. However Hoko, (2005) suggest that measurement at
manholes can be done using a calibrated aluminium plate to measure the depth of flow and using the
Colebrook White formula to calculate flow. This approach gives a deviation of 5% (Hoko, 2005)
compared to the range for most meters of up to 21%.
2.4 Infiltration and inflow
Infiltration is groundwater that enters sanitary sewer systems through joints and cracks in the
sanitary sewer pipes (GWI, 2007). Exfiltration can occur when the sewer liquid level is above
groundwater level introducing a serious environmental and health hazard (Stephenson and Barta,
2005).The problems of infiltration and inflow range from environmental and public health to
increased costs to convey and treat peak flows of sewage (EPA,2002). According to GDSDS (2005)
infiltration is calculated as a percentage of dry weather flow expressed as the ratio of minimum night
flows to average dry weather flow. However (O’Brien and Gere, 2004) infiltration is also calculated
from water consumption data where
Average dry weather flow (ADWF) = Sanitary Base flow + Groundwater infiltration (GIW)
Where ADWF = average flow from sanitary sewer on daily basis without reaction to rainfall; Sanitary
Base flow = the amount of water consumption returned to sewer, GIW = allowance that is added to
sanitary base flow to obtain dry weather flow.
Karpf, (2007) suggest the use of Darcy’s law to calculate infiltration.
Where Qin is
the infiltration, k is pipe permeability (m3.m-2.s-1), dl is the mean distance, dh is the hydraulic slope
and Agw is the wetted area.
3
Study area
Figure 1 below shows the study area map of Harare, Zimbabwe
3
Fig. 1: Study area with black fills in the areas studied map
The study area Harare has a population of 1 900 000 (CSO, 2002) with 106 560 houses in 15 high
density suburbs. Reports from the Harare Municipality show that the sewerage reticulation’s pipes are
predominantly concrete, asbestos cement and earthenware. According to Municipal by laws it is
illegal to discharge stormwater into the sewer system (SALA, 1990) suggesting that Harare has a
separate sewer system. Water consumption in Harare is 430,000 m3/d and 304 000m3/d (70% of it) is
collected as wastewater with 23 800 m3/d of wastewater is treated on site and at two main wastewater
treatment plants, Firle (capacity 144 000 m3/d) and Crowborough (45 000m3/d) (Nhapi, 2004). About
55% of the wastewater is treated in systems that are over loaded (Nhapi et al. 2006). Furthermore the
current challenge in Harare is what to do with the increasing volumes of wastewater. According to the
Harare Municipality Crowborough currently receives around 139 000 m3/d. Onsite systems like the
bucket systems and pit latrines are not allowed in Zimbabwe (Taylor and Mugede, 1997). Smaller
urban centers use mostly use septic tanks but few towns have vacuum cleaners and around 92 % of
urban households are connected to the sewerage system (Nhapi, 2004).About 70% of the effluent is
reused for pasture and irrigation whilst the rest is discharged into the Mukuvisi and Marimba Rivers
(JICA, 1996). Because of poor management of water treatment plants effluent discharges runoff and
seepage intrusions from pasture irrigation and other upstream point and non point sources of pollution
in Lake Chivero (Mugadza, 1997). According to Hoko, (2005) Chitungwiza a town 20 km from
Harare has problems of surcharging sewer lines and break down of pump stations. Chitungwiza
Cholera outbreak was due to poor sewer reticulation systems and personal hygiene (OCHA, 2008).
Newlands and Belvedere are low density residential areas in Harare, Zimbabwe as shown in figure 1.
Newlands 525mm sewer pipeline serves 669 stands with a population of 8 697 and Belvedere 225mm
sewer pipeline serving 130 stands with a population of 1 690 taking occupancy rates of 13 persons
per stand according to (JICA, 1996) and design maps from the Harare City Council Department of
Works.
4
4
Materials and methods
4.1 Study Design
The study area locations Belvedere shown below in fig. 1 and Newlands, fig.2 where lines with proper
performance of sewage where there is smooth laminar flow Trivedi and Walkowiak, (2005). The
selection of points of measurement was a straight channel with no connections to avoid hydraulic
disturbances. The Colebrook White formula is regarded as the most basis for hydraulic design (Ackers
1958) which is recommended by most hydraulicians and SALA (1990) suggest the use of the
Colebrook White formula. Novonty et al. (1989) suggest that use of coefficient of roughness, k = 1.5,
regardless of the material since the coefficient of roughness is more a function of the sewer alignment
Fig.2 and 3 below shows the location of points of measurement
Fig 2: Study area location and catchment for Belvedere
5
Fig 3.: Study area location and catchment for Newlands
4.2 Methods of flow measurement and computation
According to Hoko (2005) flow measurements at manholes can be conducted using a calibrated
aluminium plate. The slope of the pipe line before and after the selected manhole can be determined
using actual levels and distances as measured on site. The modified Colebrook-white formula given
below (Casey, 1992) was used to calculate the velocity of flow.
 k
2.51v
v  2 2DS . log 

 3.7D D 2 gDS

 ----------- (1)

Where D – diameter of the sewer in (m)
Fig. 4: Sewage flow in pipe.
S – Slope (dimensionless or m/m)
k – Roughness of pipe (m)
6
 – Kinematic viscosity (m2/s)

  sin 

------------------------------------ (2)
  2 cos (1  2 d D)
1
 is the angle subtended by the water surface at the centre of the pipe.
d is taken as the depth of flow.
A roughness (k) value of 1.5 can be used as suggested by Butler and Pinkerton, (1987) and Novonty
et al. (1989).
4.3 Peak factors
The most common method of determining peak factors is from the analysis of flow rate data (Metcalf
and Eddy Inc., 1991).The resulting peaking factors for any average flow is plotted against the average
daily flow to find a correlation so that the peaking factor for any average flow can be calculated
(DWSD, 2003) and (Metcalf and Eddy Inc, 1991).
4.4 Infiltration
According to GDSDS (2005) infiltration is calculated as a percentage of dry weather flow expressed
as the ratio of minimum night flows to average dry weather flow. Furthermore Kratch, (2007) suggest
that expressing infiltration to average wastewater discharge yields comparative values.
5
Results and discussion
5.1 Results
5.1.1 Results of flow measurement for Newlands
The results for the flow measurements for Newlands are presented in Fig. 5 to Fig. 7.
Fig 5 New lands flow Measurements manhole 1 08/04/2009-09/04/2009
7
Fig 5: New lands flow measurement results 29/04/2009-30/04/2009
Fig 6: New lands flow measurement results manhole 1 18/05/2009-19/05/2009
8
Fig 7.New lands flow measurement results manhole 1 21/05/2009-22/05/2009 (No water supply).
Table 1: Summary of results for Newlands from 8 April to 21 May 2001
Day 1
Day 2
Day 3
Day4
08//04/200909/04/2009
29/04/200930/04/2009
18/05/200919/05/2009
21/05/200922/05/2009
Maximum flow 41.91
(l/s)
47.70
47.70
36.41
Minimum flow 13.96
(l/s)
21.83
21.83
13.96
Average
(l/s)
32.23
32.45
23.26
0.7
0.7
0.6
Min/ave
flow 24.98
0.6
9
Max/ave
1.7
1.5
1.5
1.6
% < 0.6m/s2
63
30
23
82
% Infiltration
56
68
67
60
5.1.2 Results of flow measurements for Belvedere
The results of the flow measurements for Belvedere are as in Fig. 8 to Fig. 11.
Fig 8: Belvedere flow measurement results manhole 1 10/04/2009-11/04/2009
2
Cleansing velocity for trunk sewers according to Swedish Association of Local Authorities, 1990. Sanitation
Manual Design Procedures (Manual No.5) SALA
10
Fig 9: Belvedere flow measurement results 01/05/2009-11/05/2009 (public holiday).
Fig 10: Belvedere flow measurement results manhole 2 15/05/2009-16/05/2009
Fig 11: Belvedere flow measurements results manhole 2 20/05/2009-21/05/2009
Table 2: Summary of results for Belvedere
Day 1 10/04/2009- Day 2 01/05/2009- Day 3 15/05/2009- Day 4 20/05/200911/04/2009
02/05/2009
16/05/2009
21/05/2009
Maximum
flow (l/s)
4.23
4.23
4.23
4.23
11
Minimum
flow (l/s)
2.03
2.03
1.42
1.42
Average flow 3.02
(l/s)
2.91
2.78
2.78
Min/ave
0.7
0.7
0.4
0.4
Max/ave
1.4
1.5
1.9
1.9
% < 0.75m/s3
100
100
100
100
% Infiltration
67
70
51
51
5.2 Discussion
5.2.1 Flow variation
The peak flows for Newlands 525mm pipeline were generally between 7.30am and 8.30am ranging
from 36.41(l/s) – 47.7(l/s) and between 4pm and 5pm whilst minimum flows were between 11pm and
2am ranging from 13.96(l/s)-21.83(l/s). The peak flows for Belvedere 225 mm is generally between
6am -7am, 12.30pm-1pm,6pm and 8pm-9pm ranging 3.45 l/s to 4.23l/s whilst minimum flows are
between 1am-3am and 10pm - 11.30pm ranging from 0.91 l/s to 2.03l/s. O’Brien and Brien, (2004)
found peaks flows during the period 7am to 8am and 8pm to 9pm with low flows at early hours of
the morning from 2am to 3am which is comparable to findings of this current study. Metcalf and
Eddy suggest that generally three peaks appear in the morning, mid day or lunchtime and in the
evenings. On 21 May 2009, Fig. 7, low flows were experienced due to lack of water supply and the
variations had low flows twice in the late hours of the morning and in late hours of the morning
(11pm to 12 pm and 4am to 5am) compared to other days. In Belvedere higher flows where
experience on a public holiday with low flows between 9pm and 10pm.The peak flows for Belvedere
are earlier than for Newlands because the point of gauging is nearer to households, 20 to 30 metres
and 5 to 10 km respectively. However the 225mm pipeline has larger variations of flow than the 525
mm pipeline with the standard deviation of the flows ranging from 21% to 29% compared to 16% to
26% respectively expressed as a ratio of the average flow.
5.2.2 Velocity of flow
The requirements for cleansing velocities for sewers in Zimbabwe are 0.6 m/s and 0.75 m/s for
collector and, trunk and main sewers (SALA, 1990). The range of velocity flow for Newlands is
between 0.52m/s and 0.70m/s. The 525mm trunk sewer pipeline for Newlands achieves cleansing
velocity 20% to 80% over a 24 hrs. The Belvedere 225mm sewage pipeline does not achieve
cleansing velocity in all measurements with velocity ranging from 0.15m/s to 0.25 m/s. According to
Hammer and Hammer (1986) low flows in pipes a result in anaerobic reactions which corrode the
crown of the pipe resulting in cracking of the pipe. The absence of vermines on opening the manhole
and wearing and corrosion of the steel steps in the manhole shows the production of these reactions.
According to Arthur et al. (2008) majority of pipe blockages are from diameter 225 mm or less and
failure to meet self-cleansing criteria. However manholes in this study do not show any signs of
overflows because there are several factors that contribute to blockages according to (Arthur et al.,
2008).
3
Cleansing velocity for lateral sewers according to Swedish Association of Local Authorities, 1990. Sanitation
Manual Design Procedures (Manual No.5) SALA
12
5.2.3 Infiltration
Based on a method by White et al. (1996) and Ainger et al, (1998) for calculating infiltration a range
of 56% to 68 % for Newlands and 51% to 70% for Belvedere was found and appear to be independent
of pipe size. The percentage of infiltration is high compared to studies done in developed countries
where GDSDS (2005) reported infiltration to be 30% of dry weather flow. In the UK infiltration ratios
have been found to range from 15% to 50% of dry weather flow White et al. (1997). There is a
decrease in infiltration as the average flow decreases which may be as a result of settling particles due
to low flows thus settling solids blocking some points of leakage.
5.2.4 Peak factors
The maximum to average flow range for Newlands was 1.5 to 1.7 and minimum to average flow was
0.6 to 0.7 and Belvedere had a ratio of 1.4 to 1.9 of maximum to average and 0.4 to 0.7 minimum to
average flow. The range of peak factors for the 225mm (Belvedere) pipe was higher than that of the
525mm pipe (Newlands) due to a larger variation in water supply. The peak factors obtained for both
areas studied were lower than the peak factor of 2 generally used for sewerage reticulation in
Zimbabwe based on SALA, (1990). However the peak hour factors are close to a value suggested by
Metcalf and Eddy Inc. (1991) of 1.5. Table 3 below shows the comparison of peak factors obtained
from measured flow to estimated peak factors based on the French Model
Harmon’s formula
and
(Little, 2004).
Table 3: Comparison of estimated and actual peak factors.
Area
Day
average
of
flow
measurement [l/s]
Belvedere 1
2
3
4
Newlands 1
2
3
4
3.02
2.91
2.78
2.78
24.98
32.23
32.45
23.26
Actual
Peak
factors
1.4
1.5
1.9
1.9
1.7
1.5
1.5
1.6
Estimated
Factors
(French
Model)
2.9
3.0
3.0
3.0
2.0
1.9
1.9
2.0
Deviation Estimated Deviation
[%]
Factors
[%]
Harmon's
Formula
110
1.3
-6
98
1.3
-13
58
1.3
-31
58
1.3
-31
18
1.1
-33
29
1.1
-24
29
1.1
-24
26
1.1
-29
The deviation from the actual peak factors (based on measured flow) of the French model is high and
positive compared to that of the Harmon formula. However the deviation for the large pipe of the
French Model reduces whilst the deviation of the Harmon Formula increases. Little (2004) suggest
that the Harmon formula is more accurate for populations greater than 7000, however in this study it
is more accurate for populations less than 7000. It was concluded that the Harmon formula is more
accurate than the French model.
6 Conclusions.
Belvedere has risk of blockage and backflows due its low flows that do not achieve cleansing
velocity. Infiltration is generally too high in both (RC) pipe (AC) pipe and is independent of pipe size
and age. The Harmon formula can be used estimate peak factors for flow range (2-35 l/s) found in this
study.
13
7
Recommendations
There is need for extensive collection of flow data on the Harare sewerage collection system over a
number of years to determine an appropriate model for Zimbabwe. Further studies need to be done to
relate pipe age to infiltration, soil texture and pipe material. Sealing deformed joints and open cut
repairs are also required. New lining might be also required to prevent migration of infiltration to
minor or non structural defects. This information is useful in management and decision making in
wastewater management.
Acknowledgements
This paper presents part of the research results of a BSc study by Munyaradzi Meki at the University
of Zimbabwe, Department of Civil Engineering.
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