CVNG 3016
DESIGN OF ENVIRONMENTAL
ENGINEERING
Year 2018/2019
Semester I
WASTE WATER TREATMENT PLANT FOR
TOWN OF ALEXANDRIA
GROUP MEMBERS
ID NUMBER
DANIELLE BRUCE
815006573
OSAWAI JOHN
815007434
SHIKIEL BRATHWAITE
815008417
MKAMA PETERS
815117301
COURSE COORDINATOR: Dr. Vincent Cooper
MODULE LECTURER: Wayne Williams
SUBMISSION DATE: 23/11/2018
Table of Contents
ABSTRACT .................................................................................................................................... 4
OBJECTIVES ................................................................................................................................. 4
INTRODUCTION .......................................................................................................................... 5
TOWN DESCRIPTION.............................................................................................................. 5
SOIL CONDITIONS &TOPOGRAPHY ............................................................................... 6
CLIMATE ............................................................................................................................... 6
HYDROLOGY ....................................................................................................................... 7
WASTE WATER TREATMENT PLANT DESCRIPTION ..................................................... 7
REASON FOR NEW WASTE WATER TREATMENT PLANT ........................................ 7
DESIGN OF WASTE WATER TREATMENT PLANT ............................................................... 8
RIVER CHANNEL (INLET) ..................................................................................................... 8
CALCULATION: ................................................................................................................... 8
DESIGN CHECK OF CHANNEL: ........................................................................................ 8
PUMPSTATION....................................................................................................................... 10
CALCULATION: ................................................................................................................. 10
RACKS AND SCREENS ......................................................................................................... 14
CALCULATION: ................................................................................................................. 14
GRIT CHAMBER .................................................................................................................... 17
CALCULATION: ................................................................................................................. 17
PRIMARY SEDIMENTATION TANK................................................................................... 18
CALCULATIONS: ............................................................................................................... 18
BIO REACTOR ........................................................................................................................ 20
CALCULATION: ................................................................................................................. 21
CLARIFIER .............................................................................................................................. 25
CALCULATION: ................................................................................................................. 25
GRAVITY THICKENER ......................................................................................................... 28
CALCULATION: ................................................................................................................. 29
AEROBIC DIGESTOR ............................................................................................................ 31
CALCULATION: ................................................................................................................. 31
UV DISINFECTION ................................................................................................................ 34
CALCULATION: ................................................................................................................. 35
SLUDGE DRYING BED ......................................................................................................... 36
CALCULATION: ................................................................................................................. 37
SECURITY ............................................................................................................................... 38
WORKER ACCOMMODATION ............................................................................................ 39
LABORATORY ....................................................................................................................... 40
LAB OCCUPATIONAL HEALTH OF SAFETY ............................................................... 40
TESTS ................................................................................................................................... 42
TREATMENT PROCESSES ....................................................................................................... 46
SCREENING ............................................................................................................................ 46
AERATED GRIT CHAMBER ................................................................................................. 46
PRIMARY TREATMENT ....................................................................................................... 47
PRIMARY SEDIMENTATION TANK............................................................................... 47
SECONDARY TREATMENT ................................................................................................. 47
BIOREACTOR ..................................................................................................................... 47
CLARIFIER .......................................................................................................................... 47
AEROBIC DIGESTER ............................................................................................................. 48
DISINFECTION ....................................................................................................................... 48
SLUDGE DRYING BED ......................................................................................................... 48
LAYOUT OF WWTP ................................................................................................................... 50
CONCLUSION ............................................................................................................................. 51
REFERENCES ............................................................................................................................. 52
APPENDICES .............................................................................................................................. 54
ABSTRACT
This report entails the design of a wastewater treatment plant in the town of Alexandria. This
treatment facility utilizes the extended aeration for the treatment of wastewater and the design is
broken up into each of the unit processes to be carried out at the plant. Within each unit process,
step by step design calculations are shown and drawings of the respective tanks are provided.
OBJECTIVES
1. To design a waste water treatment facility that is capable of treating waste for an entire
town.
2. To be aware and utilize the different standards that exist for designing a waste water
treatment facility.
3. To understand the role of each unit process in the treatment of waste water.
INTRODUCTION
TOWN DESCRIPTION
Alexandria is a town located in North Trinidad and is located near the equator. Figure 2 below
shows Alexandria town which houses a population of 35250 persons. The town contains many
facilities including, recreational parks and a mall, however there is no airport in this town due to
its dense population. Placing an airport in this town will result in many residents being displaced
or increase in deforestation to make space for such a structure. Therefore, the airport is located
outside the of the town. On the Eastern side of the town there is the Cancun forest which is home
to capuchin monkeys and macaws. The Alexandrian mall is also located to the East of the town
along with the church, nightclub and the hotel, and facilitates many different of activities, such as
shopping and cinema. The town’s water treatment plant is located in the North Eastern side of the
town which provides the community with potable water. The hospital, fire services, police station
and health care centre are all located in the centre of the town which is the most densely populated
area and thus allows for a shorter travel distance to these facilities in the event of an emergency.
The schools and elderly home are also located in this region and are near to the different emergency
services. The Western side of the town has a less densely populated area and is where the waste
water treatment plant is to be located. The selected site of the wastewater treatment plant was
downstream of the water treatment facility to ensure that there would be no contamination of the
surface water that goes into the water treatment plant.
Figure 1 Site Map showing Location of Alexandria in Trinidad
Figure 2 Town of Alexandria
SOIL CONDITIONS &TOPOGRAPHY
Alexandria falls within the Tunapuna/Piarco Regional Corporation. The topography of the land is
relatively flat, with elevations ranging from 5 m to 49 m. The land slopes to the Western side of
the town. The soil in Alexandria belongs to the group of Terrace soil with restricted internal
drainage. The town has a soil composition of fine sandy loam. Sandy loam soils have high
concentration of sand within the soil that allows relatively quick drainage of water. The natural
land found within the town comprises of recreational grounds that are utilized by members of the
Alexandria, community. Around some of the residential developments, there is landscape
horticulture to add to the aesthetic appeal of the neighbourhoods as well as foster a green
environment.
CLIMATE
The country of Trinidad is located near to the equator, and thus, the town of Alexandria,
experiences a tropical climate which is separated into two (2) distinct seasons. These are the dry
season and the wet/rainy season. The dry season begins in the month of January and ends in the
month of May. Precipitation does not occur often at that time of the year and the daytime is
considerably warm. In the rainy season however, precipitation is more frequent and intense. This
season begins in late May and ends in early January. During this season the day time is warmer
and more humid than the dry season, however, the nights are very cool compared to the nights in
the dry season and have stronger winds.
HYDROLOGY
The town consists of one (1) river. This is the Alexandrian river which flows from the North
Eastern side of the town and ends on the South Western side of the town. Apart from the river,
there are also two (2) aquifers. The first aquifer is located near to the water treatment facility while
the second aquifer is located nearer to the densely populated areas of the town. These aquifers are
used for obtaining potable water for the town when the water level in the river is low. Since the
land is sloped to the West, when precipitation occurs, the water flows in the Western direction.
The town is not located near the coastline and has a generally low water table, however, as
previously mentioned, the land is relatively flat and therefore, there is some risk of flood
occurrence on the land near to the river
WASTE WATER TREATMENT PLANT DESCRIPTION
REASON FOR NEW WASTE WATER TREATMENT PLANT
Currently, Alexandria does not have a waste water treatment plant. Instead, each structure has at
least one (1) septic tank and soakaway that assists in the treatment of the waste water before it is
released into the environment. As previously mentioned, the town does not usually have a high
water table, however during the rainy season, excessive rainfall causes an increase in the water
table. Recently, the town’s water treatment plant found evidence of coliform in the water and made
a report. Upon investigations it was revealed that the waste water released from the soakaway was
seeping into the ground water causing contamination of the water source in the densely populated
area in the town. There have also been reports concerning the failure of soakaways on the North
Western side of the town. For this reason, a new waste water treatment plant is required to prevent
further contamination of the water source.
DESIGN OF WASTE WATER TREATMENT PLANT
RIVER CHANNEL (INLET)
CALCULATION:
Channel Dimensions:
Using Q = 68235 m3/d = 0.79 m3/s and va = 0.7 m/s
Q = Ava
𝑄
A=𝑣=
0.79
0.7
A = 1.13 m2
𝐵
Assuming Width to Depth Ratio, 𝐷 = 2
B = 2D
A = B x D = 2D x D
A = 2D2
2𝐷 2 = 1.13 𝑚2
01.13
D=√
2
= 0.751 m
∴ B = 2D = 2(0.751) = 1.50 m
Using freeboard, f = 0.6 m
Dt = 1.351 m
DESIGN CHECK OF CHANNEL:
Net cross-sectional area:
An = B x D = (1.5) x (0.751)
An = 1.128 m2
Wetted perimeter:
Pw = 2D + B = 2(0.751) + 1.50
Pw = 3 m
Hydraulic radius:
𝐴
R = 𝑃𝑛 =
𝑤
1.128
3
R = 0.376 m
Slope:
1
S = 1000
S = 0.001 m
Flow through velocity:
Using roughness coefficient (Concrete lined), n = 0.013 and Manning’s equation
1
v = 𝑛 𝑅32 𝑆21
1
2
1
1
v = ( ) (𝑅 3 𝑆 2 ) = (
𝑛
2
1
) ((0.376)3 (0.001)2 )
0.013
v = 1.266 m/s
Since 1.266 ms-1 > 0.7 ms-1, the design is OK!
Table 1 River Channel Design
RIVER CHANNEL DESIGN
Parameter
Value
Peak flow, Q
68235
Cross-sectional area, A
1.13
Net cross-sectional area, An
1.128
Width of river channel, B
1.5
Depth of river channel, D
0.751
Total depth of channel, Dt
1.351
Wetted perimeter, Pw
3
Hydraulic mean radius, R
0.376
Channel slope, S
0.001
Flow through velocity, v
1.266
Assumed Parameters
Assumed velocity, va
0.7
Manning’s coefficient of roughness, n
0.013
Width to depth ratio, B/D
2
Freeboard, f
0.6
Units
m3/d
m2
m2
m
m
m
m
m
m
m/s
m/s
m/s
m/s
PUMPSTATION
The well is to be constructed with sulphate resistant reinforced concrete to withstand chemical
reactions of the wastewater. Pipes for wastewater should be greater than 4 inch to maintain velocity
above 0.6 m/s. Design is to have adequate space for pumps, to facilitate cleaning, and to have
sufficient volume. Spacing between pumps should correspond to the diameter of the pumps; the
distance of the pumps near walls is to be half the diameter of the pump. Distance from the wall to
the centre of the pump impeller should be 1.5 times the pump diameter. The wet well would be
designed to have 4 pumps.
CALCULATION:
Wet Well Active Volume:
Using minimum cycle time, T = 10 mins and q = 68235 m3/d = 47.385 m3/min
𝑉𝑤𝑤 = 𝑇𝑞
𝑉𝑤𝑤 = (10)(47.385)
Vww = 473.85 m3
Dimensions of wet well:
Using Vww = 473.85 m3 and an assumed well depth, Dww = 4.5 m
𝑉
Aww = 𝐷𝑤𝑤
𝑤𝑤
Aww =
473.85
4.5
Aww = 105.3 m2
Conditions of the incoming and discharge sewers:
1. Size of incoming sewer: 609.6 mm (24”) SDR 41
2. Bottom Elevation of incoming sewer= -8.55 m
3. Pressure Pipe Bottom Level= 3 m
4. Size of inner diameter: 577.80 mm (22.748”)
5. Length: 800 m
Gravity Flow Pipe:
1. Bottom Level: 3m
2. Diameter: 609.6 mm (24”)
Pump Diameter:
Pump diameter for pump #1, dp = 14.25 in
Pump diameter for pump #2, dp =
Water Head:
1. Suction water level = -8.55 m
2. Pressure Pipe Bottom Level = 3 m
Discharge Water Level = Pipe Bottom Level + Diameter
Discharge Water Level = 3 + 0.578
Discharge Water Level = 3.578 m
3. Actual Head, H = 3.578 – (-8.55)
H = 12.13 m
4. Total head loss by pump equipment
Table 2 Pump Station Design
PUMP STATION DESIGN
Parameter
Peak flow, Q
Pumping rate for a single pump, q
Active volume of wetwell, Vww
Area of wetwell, Aww
Actual head, H
Discharge water level
Length of wetwell, Lww
Width of wetwell, Www
Assumed Parameters
Minimum cycle time between pump starts, T
Design volume of wetwell, Vdes
Depth of wetwell, Dww
dp (Pump #1)
dp (Pump #2)
Value
68235
47.385
473.85
105.3
12.13
3.578
11.92
5.52
Units
m3/d
m3/min
m3
m2
m
m
m
m
10
140
4.5
14.25
min
m3
m
in
m
Figure 4 Showing Horizontal Section of Wet Well
Figure 3 Showing Vertical Section of Wet Well
Table 3 Selected Pump Specifications
Pump Specifications
Flygt N-Technology N 3301 LT 3 phase 8p SmartRun 60hz Pump
Pumping Rate (m3/h)
1368
Max Head, h (m)
9.65
Motor Output (kW)
44.7
Efficiency (%)
79.5
Height (m)
1.71
Width (m)
1.44
RACKS AND SCREENS
Screening is one of the first operations at the wastewater treatment plant. The screening device’s
main function is to remove large object present in the wastewater. The performance of screens
depends on the bar spacing. Screens can be cleaned by removing them manually or by the process
of backwashing. It must be noted that the frequency of cleaning of the screens all depends on the
quality of wastewater entering the plant.
-
Coarse Screens
-
Fine Screens
Provide two identical barracks, each capable of handling max flow. One screen chamber could be
taken out of service for routine maintenance without interrupting the normal plant operations.
CALCULATION:
Clear areas through rack opening:
Using Q = 68235 m3/d = 0.79 m3/s and va = 0.7 m/s
𝐴𝑐𝑙 =
𝐴𝑐𝑙 =
𝑄
𝑣𝑎
0.79
0.7
𝐴𝑐𝑙 = 0.658 𝑚2
Clear width of opening at rack:
𝑊𝑐𝑙 =
𝑊𝑐𝑙 =
𝐴𝑐𝑙
𝐷
0.658
= 0.876 𝑚
0.751
Number of spacings:
Using Wcl = 0.876 m = 876 mm and assuming bar width of 1 cm =10 mm and clear spacing is
2.5 cm = 25 mm
𝑁𝑠 =
𝑊𝑐𝑙
= 35 𝑠𝑝𝑎𝑐𝑖𝑛𝑔𝑠
𝑆𝑐𝑙
𝑁𝑠 =
876
= 35 𝑠𝑝𝑎𝑐𝑖𝑛𝑔𝑠
25
Total number of bars:
Nb = Ns – 1
Nb = 35 – 1 = 34 bars
Width of chamber:
Using bar width, Wb = 10 mm = 0.01 m
𝑊𝑐ℎ = 𝑊𝑐𝑙 + 𝑁𝑠 𝑊𝑏
Wch = (0.876) + (35)(0.01) =1.226 m
Bar efficiency:
E=
E=(
𝑆𝑐𝑙
× 100
𝑊𝑐ℎ
25 × 35
1226
) × 100 = 71 %
Head Loss:
Case 1: Bars are unclogged
Using va= 0.7 m/s; vb = 1.266 m/s and c1 = 0.7
ℎ𝐿 =
ℎ𝐿 =
1 (𝑣𝑏2 −𝑣𝑎2 )
∗
𝑐
2𝑔
1 (1.266 2 − 0.7 2 )
[
] = 0.081 𝑚
0.7
2(9.81)
Case 2: Bars are clogged
Using v1= 0.7 m/s; v2 = 1.266 m/s and c2 = 0.6
ℎ𝐿 =
ℎ𝐿 =
1 (𝑣𝑏2 −𝑣𝑎2 )
∗
𝑐
2𝑔
1 (1.266 2 − 0.7 2 )
∗
= 0.095 𝑚
0.6
2(9.81)
Table 4 Racks and Screens Design
RACKS AND SCREENS
Parameter
Value
Peak flow, Q
68235
Clear areas through rack space, Acl
0.658
Clear width of opening at rack, Wcl
0.876
Depth of river channel, D
0.751
Number of spacings, Ns
35
Number of bars, Nb
34
Width of chamber, Wch
1.226
Efficiency, E
71
Head loss, hL (Unclogged)
0.081
Head loss, hL (Clogged)
0.095
Assumed Parameters
Assumed velocity, va
0.7
Clear spacing, Scl
0.025
Bar width, Wb
0.01
Acceleration due to gravity, g
9.81
Approach velocity, v1
0.7
Velocity through bar openings, v2
1.266
Head loss coefficient, c
0.7
(Unclogged)
Head loss coefficient, c (Clogged)
0.6
Units
m3/d
m2
m
m
m
%
m
m
m/s
m
m
m/s2
m/s
m/s
-
GRIT CHAMBER
Providing two identical basins, each capable of handling max flow, one could be taken out of
service for routine maintenance without interrupting the normal plant operations.
CALCULATION:
Volume of aerated grit chamber:
Using Q = 68235 m3/d = 0.79 m3/s and DT = 3 min = 180 seconds
𝐷𝑇
𝑉𝑎𝑔 = 𝑄 ( )
2
Vag = (0.79) (
180
2
)
Vag = 71.1 m3
Grit Chamber Geometry:
Width-depth ratio for aerated grit chambers range from 1:1 to 5:1. Depths range from 2.1 to 4.87
m. Using a width-depth ratio of 1.2 and a depth, Dg = 2.5 m
Wg = 1.2Dg
Wg =1.2(2.5) = 3 m
𝑉𝑔
Lg = 𝑊
𝑔 𝐷𝑔
𝐿𝑔 =
71.1
= 9.5 𝑚
3 × 2.5
Length check:
Assuming a freeboard, f = 0.4 m
Length-width ratios range from 3:1 to 5:1. Length to width ratio for the aerated grit chamber sized
9.5
above is: 3 = 3.17:1, which is acceptable.
Required air supply:
The air supply requirement for an aerated grit chamber ranges from 0.185 to 0.46 m3/min·m. of
chamber length. Using 0.35 m3/min·m
𝐴𝑖𝑟 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 = 𝑎𝑖𝑟 𝑠𝑢𝑝𝑝𝑙𝑦 𝑟𝑒𝑞𝑢𝑖𝑟𝑚𝑒𝑛𝑡 × 𝐿𝑔
Air required = (0.35)(9.5)
Air required = 3.33 m3/min
Total air supply required = (3.33)(2)
Total air supply required = 6.66 m3/min
Quantity of grit expected:
Using grit collection rate of 0.10x10-3 m3
𝑉𝑔 = 𝑄 × 𝑔𝑟𝑖𝑡 𝑐𝑜𝑙𝑙𝑒𝑐𝑡𝑖𝑜𝑛 𝑟𝑎𝑡𝑒
𝑉𝑔 = (0.79)(0.10 x 10-3)
Vg = 7.9 x 10-5 m3/s = 6.83 m3/d
PRIMARY SEDIMENTATION TANK
CALCULATIONS:
Number of tanks = 4
Number of trains = 2
Flow per tank:
Using Q = 68235 m3/d and assuming that each train will be able to take the design flow
𝑄𝑡 =
𝑄
𝑚3 /𝑑
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡𝑟𝑎𝑖𝑛𝑠
68235 3
𝑚 /𝑑
2
𝑄𝑡 =
Qt = 34118 m3/d
Volume of tank:
Using an assumed detention time, DT = 2 hours = 0.08 days and Q = 34118𝑚3 /𝑑
V = QtDT
V = (0.08)(34118)
V = 2843 m3
Area of tank:
Assuming a side water depth, Dsw = 4.5 m and freeboard, f = 0.5 m
A=𝐷
𝑉
𝑠𝑤 +𝑓
=
2843
4.5+0.5
A = 569 m2 ≈ 570 𝑚2
Tank diameter:
2𝜋𝑟(𝑑𝑐 + 𝑓𝑐 ) + 𝜋𝑟 2
570 = (2)(3.142)𝑟(4.5 + 0.5) + (3.142)𝑟 2
Solving the above equation quadratically
r = 9.34 m ≈ 10 𝑚
∴ 𝑑 = 2𝑟 = 2(10)
d = 20 m
Surface overflow rate:
𝑆𝑂𝑅 =
𝑄𝑡
34118
=
𝐴
570
SOR = 60 m3/m2/d
Recommended overflow rates range from 60 to 120 m3/d/m2 ∴ 60 𝑚3 /𝑚2 /𝑑 is okay
Table 5 Primary Sedimentation Tank Design
PRIMARY SEDIMENTATION TANK DESIGN
Design Parameters
Value
Units
Peak flow rate, Q
68235
m3/d
Flow rate per clarifier, Q
34118
m3/d
Volume per clarifier, V
2843
m3
Area of clarifier, A
569
m2
3
Surface overflow rate, SOR
60
m / m2/d
Assumed Parameters
Number of primary clarifiers
4
Number of trains
2
Detention time, DT
2
hours
Side water depth D
4.5
m
Freeboard, f
0.5
m
BIO REACTOR
The bio-reactor is also commonly known as the aeration tank and its purpose is to provide the
waste water with sufficient oxygen to ensure that aerobic bacteria do not die and cause the water
to become septic. There are many different systems that may be used for the aeration process such
as the sequence batch reactor, oxidation ditch, rotating biological contactor along with a few
others. However, the membrane bio-reactor is the most common system used for aeration
purposes. In terms of aeration there are also different methods of introducing oxygen into the tank
in the activated sludge process such as diffused air aeration, mechanized aeration and combined
aeration. In diffused air aeration there is the option of selecting fine bubble diffuser and coarse
bubble diffuser. The fine bubble diffuser operates by introducing air into the tank through pipelines
at the base which then go through the water and appear as bubbles on the water surface. It is called
the fine bubble diffuser since the bubbles on the surface are very small compared to the coarse
bubble diffuser which are larger. In terms of efficiency, fine bubble diffusers are the better
selection since it introduces more oxygen into the tank due to slow rising of the air bubbles and
the system is easier to maintain than coarse bubble diffusers. Therefore, for the design of the
bioreactor for Alexandria a fine bubble diffusion system was selected. The air is pumped into the
aeration tank using one or more blowers. The water entering the tank is mixed with sludge that has
been taken from the secondary clarifier to be reused. It is important to maintain the required sludge
mass in the aeration tank since the sludge helps to remove the bacteria from the water. However,
too much sludge in the aeration tank may result in air not being efficiently distributed throughout
the water which is extremely important in the aeration process.
In Alexandria, the aeration tank is used for the extended aeration treatment process. As, previously
mentioned a fine bubble diffuser will be used to supply the wastewater with sufficient air needed
for the survival of the aerobic bacteria in the tank. A total of four (4) four tanks were selected to
be designed in two (2) trains where each train has the capacity to carry the full flow of the
wastewater to ensure that efficiency of the plant is not reduced in the even that one of the trains
may need to be shut down and serviced. The following calculations were carried out to produce
the final design of the aeration tanks and the selection of a suitable diffuser to be used for aerating
the wastewater.
CALCULATION:
Number of aeration tanks = 8
Number of trains = 2
Number of tanks per train = 4
Design flow per aeration tank:
Using a design flow, Q = 68235m3/d
𝑄𝑖 =
𝑄𝑑
68235
=
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡𝑟𝑎𝑖𝑛𝑠
2
Qi = 34118 m3/d = 34118000 L/d
Flow per aeration tank:
𝑄𝑡 =
𝑄𝑖
34118
=
4
4
Qt = 8529 m3/d
Volume of wastewater per train:
Assuming X = 4000 mg/L, BOD5 = 200 mg/L and F/M = 0.10
𝐹
𝑄𝐵𝑂𝐷5
=
𝑀
𝑉𝑡 𝑋
𝑉𝑡 =
(34118000)(200)
𝑄𝐵𝑂𝐷5
=
𝐹
(0.10)(4000)
(𝑀 ) 𝑋
𝑉𝑡 = 17058750 𝐿 = 17059 𝑚3
Volume of sludge returned to aeration tank:
Assuming 25% additional volume due to returned sludge
Using 25% = 0.25
Vsr = % returned x Vt = 0.25 x 17059
Vsr = 4265 m3
Total volume of aeration tank:
V = Vt +Vsr
V = 17059 + 4265
V = 21323 m3
Total volume per tank:
𝑉𝑡𝑎𝑛𝑘 =
𝑉
21323
=
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡𝑎𝑛𝑘𝑠 𝑝𝑒𝑟 𝑡𝑟𝑎𝑖𝑛
4
Vtank = 5331 m3
Hydraulic retention time:
𝜃=
𝑉𝑡𝑎𝑛𝑘
5331
=
𝑄𝑖
34118
θ = 0.16 days
Tank dimensions:
Assuming a freeboard, f = 0.5 m and depth, d = 4.5 m
𝐴𝑖 =
𝑉𝑡𝑎𝑛𝑘
5331
=
𝑑+𝑓
4.5 + 0.5
Ai = 1066 m2
Using a width to depth ratio of 2.5
W = 2.5dt = (2.5)(5)
W = 13 m
∴𝐿=
𝐴𝑖
1066
=
𝑊
13
L = 85 m
Air Requirements:
Using BOD5 = 200mg/L = 0.2 kg/m3
BOD5 Loading = BOD5Qt = (0.2)(8529)
BOD5 Loading = 1705.9 kg/m3
Oxygen transfer rate:
Assuming 2kg of oxygen is required to remove 1kg of BOD5
OTR = BOD5 Loading x 2 = 1705.9 x 2
OTR = 3412 kg/day
Using a specific oxygen transfer efficiency, SOTE = 2 %/m
OTE = SOTE x water depth = (2) (4.5)
OTE = 9%
𝑂𝑇𝑅
Oxygen required = 𝑂𝑇𝐸 =
(3412)
(
9
)
100
Oxygen required = 37909 kg/day
Calculating oxygen transfer correction factor:
𝑋
2
3
2
α = 1 − 0.16 (1000) = 1 −
4000 3
0.16 (1000)
α = 0.56
Applying the transfer correction factor for wastewater:
Oxygen required for waste = α x oxygen required
Oxygen required for waste = (0.56)(37909) kg/day
Oxygen required for waste = 21229 kg/day
Using percentage of oxygen in the air = 20.95% ≈ 21%
𝑜𝑥𝑦𝑔𝑒𝑛 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑
Air required per tank = 𝑝𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝑜𝑓 𝑜𝑥𝑦𝑔𝑒𝑛 𝑖𝑛 𝑎𝑖𝑟 =
(37909)
(
21
)
100
Air required per tank = 180516 kg/m3
Air to be supplied =
𝐴𝑖𝑟 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑝𝑒𝑟 𝑡𝑎𝑛𝑘
𝑆𝐺𝑎𝑖𝑟,20
=
(180516)
1
Air to be supplied = 180516 m3/ min
Table 6 Bio-Reactor Design
BIOREACTOR DESIGN
Parameter
Peak design flow, Q
Flow per train, Q
Flow per aeration tank, Q
Mixed liquor suspended solids, X
Volume per train, V
Volume of sludeg returned, V
Total Volume, V
Total volume per tank, V
Hydraulic retention time, θ
Area of aeration tank, A
Width of tank, W
Length of tank, L
Total depth of tank, d
Value
68235
34118
8529
4000
17059
4265
21323
5331
0.16
1066
13
85
5
Units
m3/d
m3/d
m3/d
mg/L
m3
m3
m3
m3
day
m2
m
m
m
BOD Loading
Oxygen transfer rate, OTR
Oxygen transfer efficiency, OTE
1705.875
3412
9
kg/m3
kg/day
%
Table 7 Bioreactor Design Cont'd
BIOREACTOR DESIGN
Parameter
Value
Oxygen required
37909
Oxygen transfer correction factor,
0.56
Oxygen required for wastewater
21229
Percentage of oxygen in air
21
Air required per tank
180516
Air to be supplied
180516
Assumed Parameters
Number of aeration tanks
8
Number of trains
2
Number of aeration tanks per train
4
Food to mass ratio, F/M
0.1
Depth of tank, d
4.5
Freeboard, f
0.5
Width to depth ratio, W/d
2.5
Specific oxygen transfer efficiency, SOTE
2
BOD5
200
Percentage of sludge returned
25
Oxygen required to remove 1kg BOD
2
Specific gravity of air, SG
1
Units
kg/day
kg/day
%
kg/m3
m3/min
m
m
%/m
mg/L
%
kg
-
CLARIFIER
The purpose of a clarifier is to effectively separate the solid waste, also know as sludge, from the
water so that the water can be further treated and eventually released into the river channel.
Circular clarifiers are commonly used in waste water treatment systems; however, they can also
be rectangular in shape. Water can enter the clarifier either at the centre or around the periphery of
the tank so as to not disturb the sludge that is already settling. The water that has entered the tank
is detained for approximately 2 – 3 hours before it is allowed to leave the tank via overflow weirs
located along the inside perimeter of the clarifier. This detention time may vary depending on
parameters such as type of processes, surface loading rate, side water depth, design flow and
recirculation rate. The base of the clarifier has an inverted cone shaped with a sludge hopper to
allow the sludge to accumulate in one area where the it can then be easily removed from the tank.
Rake arms slowly circulate at the base of the tank guiding the sludge to the hopper. The sludge
accumulated at the cone shaped base is periodically emptied to prevent the sludge from floating to
the surface due to lack of oxygen supply for extended periods of time. When the sludge is pumped
out of the tank, a fraction of the sludge is wasted while the remainder is then reintroduced to the
aeration tank. This treatment process minimizes the excess sludge that leaves the treatment facility.
The wasted sludge is taken to an aerobic digestor for treatment before it can be removed from the
system.
In the design of the clarifiers for the town of Alexandria, a circular clarifier was chosen. Circular
tanks are favoured because they require less maintenance, the drive bearings are not under
wastewater, and the construction cost is generally lower than that for rectangular tanks, hence a
circular tank was selected.
CALCULATION:
Number of clarifiers = 4
Flow into each clarifier:
𝑄𝑐,𝑖 =
𝑄
68235
=
# 𝑜𝑓 𝑐𝑙𝑎𝑟𝑖𝑓𝑖𝑒𝑟𝑠
4
Qc,i = 17058.8 m3/d
Using a maximum detention time, DTmax = 4 hours and a minimum detention time, DTmin = 2
hours
Maximum volume for each clarifier:
DTmax = 4 hours = 0.17days
𝑉𝑐,𝑚𝑎𝑥 = 𝐷𝑇𝑚𝑎𝑥 𝑄𝑐,𝑖 = (0.17)(17058.8 )
Vc,max = 2843.1 ≈ 2843 m3
Minimum Volume for each clarifier:
DTmin = 2 hours = 0.08 days
𝑉𝑐,𝑚𝑖𝑛 = 𝐷𝑇𝑚𝑖𝑛 𝑄𝑐,𝑖 = (0.08)(68235)
Vc,min = 1421.6 ≈ 1422 m3
Total Solids into each Clarifier:
Assuming a re-circulation rate of 100% and X = 4000mg/L = 4,000,000 m3
TS = 2𝑄𝑐,𝑖 𝑋 = 2(17058.8)(4000000)
TS= 136,470 kg/m3
Minimum Surface Area Required:
Using a solid loading rate, SLR = 90kg/m2/d
SAmin =
𝑇𝑜𝑡𝑎𝑙 𝑆𝑜𝑙𝑖𝑑𝑠
𝑆𝐿𝑅
=
136470
90
SAmin = 1516.3 ≈ 1517 𝑚2
Checking Discharge Overflow Rate:
𝑄
DOR = 𝑆𝐴 𝑐,𝑖 =
𝑚𝑖𝑛
17058.8
3033
𝑚3
DOR = 11.25 ≈ 11.3 𝑚2 ≤ 16
𝑚3
𝑚2
∴ Discharge overflow rate okay
Clarifier dimensions:
Assuming a water depth, dc = 4.5 m and a freeboard, fc = 0.5 m
Ac =
𝑉𝑐,𝑚𝑎𝑥
𝑑𝑐 +𝑓𝑐
=
2843
4.5+5
Ac = 568.6 m2 ≈ 570 m2
Ac = 2𝜋𝑟(𝑑𝑐 + 𝑓𝑐 ) + 𝜋𝑟 2
570 = (2)(3.142)𝑟(4.5 + 0.5) + (3.142)𝑟 2
Solving the above equation quadratically
r = 9.34 m ≈ 10 𝑚
∴ 𝑑 = 2𝑟 = 2(10)
d = 20 m
Table 8 Clarifier Design Parameters
CLARIFIER DESIGN
Design Parameters
Value
Peak flow rate, Q
68235
Flow rate per clarifier, Qc,i
17059
Maximum volume per clarifier, Vc,max
2843
Minimum volume per clarifier, Vc,min
1422
Total Solids, TS
136470
Minimum surface area required, SAmin
1517
Discharge overflow rate, DOR
11.3
Area of clarifier, Ac
570
Tank radius, r
10
Tank diameter, d
20
Assumed Parameters
Number of clarifiers
4
Maximum detention time, DT
4
Minimum detention time, DT
2
Mixed liquor suspended solids, X
4000
Solid loading rate, SLR
90
Re-circulation rate
100
Water depth, dc
4.5
Freeboard, fc
0.5
Units
m3/d
m3/d
m3
m3
kg/m3
m2
m3/ m2
m2
m
m
hours
hours
mg/L
kg/m2/d
%
m
m
Figure 5 Drawing Showing Section through the Clarifier
GRAVITY THICKENER
This process applies gravity forces to separate the solids from the sludge, it reduces the sludge
volume for handling and also reduces the operating cost of other processes. The sludge is pumped
to a circular tank that comprises of a slow rotating rake, that breaks the junction between the sludge
particles and increases settling and compaction, once thickened the volume of the sludge is reduced
to approximately 90% of the original volume.
Alexandria uses a gravity thickener for the volume reduction of sludge since the process consumes
little energy and is one of the easiest methods for thickening.
CALCULATION:
To determine the daily primary solids produced:
𝐴𝑣𝑔. 𝑑𝑎𝑖𝑙𝑦 𝑓𝑙𝑜𝑤 𝑄𝑑 𝑥 𝑃𝑟𝑖𝑚𝑎𝑟𝑦 𝑖𝑛𝑓𝑙𝑢𝑒𝑛𝑡 𝑥 % 𝑜𝑓 𝑇𝑆𝑆 𝑟𝑒𝑚𝑜𝑣𝑒𝑑
22745 ∗ 240 ∗ 0.65
=
𝑔
1000
1000
𝑘𝑔
Ps = 3548.22 kg/d
To determine Primary Sludge Volume:
𝑝𝑟𝑖𝑚𝑎𝑟𝑦 𝑠𝑜𝑙𝑖𝑑𝑠
3548.22
=
1000𝑘𝑔 1.01𝑥 0.05 𝑥 1000
𝑆. 𝐺 𝑥 % 𝑠𝑜𝑙𝑖𝑑𝑠 𝑥
𝑚3
Vp = 71 m3/ d
To determine Gravity thickener tank volume:
𝑚3
𝑉𝑜𝑙. 𝑠𝑙𝑢𝑑𝑔𝑒 ( ) 𝑥 𝑟𝑒𝑡𝑒𝑛𝑠𝑖𝑜𝑛 𝑡𝑖𝑚𝑒 (ℎ𝑟)
71 𝑥 20
𝑑
=
24ℎ𝑟 𝑥 # 𝑜𝑓 𝑡𝑎𝑛𝑘𝑠
24 𝑥 1
VT = 60 m3
To determine Tank Area:
𝑉𝑜𝑙𝑢𝑚𝑒 (𝑉𝑡)
60
=
𝐻𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑊𝑎𝑡𝑒𝑟
3.5
A = 17m2
To determine Tank Diameter:
√
4 𝑥 𝐴𝑟𝑒𝑎
4 𝑥 17
=
𝜋
3.14
D = 4.64 m
Check Retention time:
Conditions: Retention time > 18 hrs
𝐷𝑖𝑎𝑚𝑒𝑡𝑒𝑟 2 𝑥 𝜋 𝑥 (𝐷𝑒𝑝𝑡ℎ 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑥 # 𝑜𝑓 𝑡𝑎𝑛𝑘)
4.642 𝑥 𝜋 𝑥 3.5
=
71
𝑠𝑙𝑢𝑑𝑔𝑒 𝑣𝑜𝑙𝑢𝑚𝑒
(4 )𝑥
4 𝑥 (24)
24ℎ𝑟
R = 20 hrs
 Conditions Satisfied
Gravity Tank Dimensions: 4.64 m Diameter x 4.0 m Depth
Table 9 : Gravity Thickener Design
GRAVITY THICKENER DESIGN
Design Parameter
Value
Average Daily Flow, (Qd)
22745.10
Primary Influent TSS (Residential)
100 - 350
Primary settling tank % TSS removed
40 – 65
Primary Effluent % of Solids
3–7
Water Depth, (H)
3-4
Freeboard, f
0.5
Retention time, (R)
R > 18
Specific Gravity, (S.G)
1.01 – 1.06
Assumed Parameter
Primary Influent TSS
250
Primary Settling Tank % TSS removed
65
Primary Effluent % of Solids
5
Water Depth, (H)
3.5
Retention Time, (R)
20
Specific Gravity, (S.G)
1.01
Units
m3/d
mg/L
%
%
m
m
Hrs
mg/l
%
%
m
hrs
AEROBIC DIGESTOR
The aerobic digester stabilizes the primary and secondary sludge by converting organic sludge to
carbon dioxide, ammonia, and water by use of aerobic bacteria. It reduces volatile solid, pathogens
and offensive odour in a process called aeration. In the digestion process the contents are well
mixed and requires air supply to maintain the dissolved oxygen (DO) surplus in the digester, in
the extended aeration the microorganisms enter the endogenous stage which then reduces the
biological degradable organic matter, the food supply are then depleted and the microorganisms
begin consuming their own protoplasm, this oxidizes it to CO2, H2O and ammonia (NPTEL 2018,
2). As the digestion continues, the ammonia is then converted to nitrates and the bio-solids matter
is reduced to organic matter and stable volatile solids.
This method of stabilization is used for the Alexandria Wastewater treatment plant, as the aerobic
digester can reduce the water content to about 90%.
CALCULATION:
Waste Activated Sludge (W.A.S) was found to be = 1909kg/d
To determine Quantity of Sludge:
𝑊. 𝐴. 𝑆
1909
=
𝑠. 𝑔 𝑥 % 𝑠𝑜𝑙𝑖𝑑 𝑥 1000 𝑘𝑔/𝑚3
1.02 𝑥 0.025 𝑥 1000
Quantity of sludge = 121.9 m3/d
To determine Sludge Age:
Winter Conditions/ Rainy season = 900o.d /15oC = 60 days ………………….Controlling Factor
Summer Condition/ Dry season = 6000 . d / 300 C = 20 days
To determine the Digester volume using Controlling factor (winter):
Influent suspended solids concentration (Xi): 1.5 % = 15000 mg/L
Digester Suspended Solid concentration (X): Xi/ 0.75 = 15000/ 0.75 = 11250 mg/L
Aerobic Tank Volume (V):
𝑉=
𝑄𝑖(𝑋𝑖 + 𝑌𝑆𝑖)
121.19 (11250 + 175)
=
1
1
𝑋 (𝐾𝑑𝑃𝑣 + 𝜃𝑐 ) 11250 ((0.05 ∗ 0.8) + θc)
V = 2852 m3
Where:
Qi – Quantity of Sludge (m3/d)
X – Digester SS conc. (mg/L)
Xi - influent suspended solids concentration
Kd – reaction rate constant (d-1) = 5%
(mg/L)
Pv – Volatile fraction of digester SS
Y – Fraction of influent BOD5 (decimal)
(decimal) = 0.8
Si – influent BOD5 (mg/L)
θc – Solid retention time (d) = 60d
To determine Dimensions for the digester:
Number of Aerobic Digester Tank = 2
Volume of Aerobic Digester = 2852 m3
Volume for 1 Tank =2852/ 2 = 1426 m3 per tank
Assuming a height of water = 4.5 m
Assuming Freeboard = 0.5 m
4 𝑥 𝐴𝑟𝑒𝑎
D =√
𝜋
= √
4 𝑥 285.16
𝜋
D = 19.05 m
Dimension of Aerobic Digester Tank: 19.05 m Diameter x 5 m Height (x 2 Tanks)
Table 10 Aerobic Digestor Design
AEROBIC DIGESTER DESIGN
Design Parameter
Specific Gravity
Secondary Sludge Solid Content
Volatile Solid Reduction
Reaction rate Constant, (Kd)
Minimum winter/ rainy temperature
Maximum summer/ dry temperature
Residence Time (Land Application)
9000 @
Residence Time (Land Application)
6000 @
Volatile Fraction of digester SS, (Pv)
Influent Suspended Solid concentration, (Xi)
SS concentration in digester % of influent SS
Retention Time
Value
1.02 – 1.06
0.5 – 2.5
40 – 50
0.5 - 0.14
15
25
15
30
0.8
0.5 - 2
75
40 – 60
Units
%
%
d
0
C
0
C
0
C
0
C
%
%
d
Table 11 Aerobic Digestor Design
AEROBIC DIGESTER DESIGN
Assumed Parameter
Specific Gravity
Secondary Sludge Solid Content
Volatile Solid Reduction
Reaction rate Constant, (Kd)
Minimum winter/ rainy temperature
Maximum summer/ dry temperature
Residence Time (Land Application)
Residence Time (Land Application)
Volatile Fraction of digester SS, (Pv)
Influent Solid Concentration
9000 @
6000 @
Value
1.02
2.5
45
0.5
15
30
15
30
0.8
1.5
Units
%
%
d
0
C
0
C
0
C
0
C
%
Oxygen and Air requirement:
To determine the VSS reduction:
Total Mass of VSS: W.A.S x (VSS/TSS) = 1909/0.8 = 1527.02 kg/d
VSS reduction: Total Mass of VSS * Volatile Solid reduction = 1909 * 0.8 = 858.95 kg/d
To determine the oxygen required:
Oxygen required = 2.3 kg O2/kg x VSS reduction = 858.95 x 2.3
Oxygen required = 1975.58 kg/d
To determine the Volume of air required under standard condition:
𝑂𝑥𝑦𝑔𝑒𝑛
Volume of Air Required = 𝑀𝑎𝑠𝑠 𝑜𝑓 𝑎𝑖𝑟 𝑥 % 𝑂2 =
1975.58
1.2 𝑥 23.2%
Volume of Air Required = 7096.18 m3/d
To determine Total air required:
Total Air Required =
𝐴𝑖𝑟 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑
𝑜𝑥𝑦𝑔𝑒𝑛 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦
=
7096.18
0.06
Total Air Required = 118269.62 m3/d = 82.13 m3 / min
To check diffused Air requirement:
𝑇𝑜𝑡𝑎𝑙 𝐴𝑖𝑟
Diffused Air Requirement = 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑑𝑖𝑔𝑒𝑠𝑡𝑒𝑟 =
82.13
2851.57
Diffused Air Requirement = 28.8 L/ min. m3
Check regulations requirement:
Condition: 25 – 35 L/ min. m3
 Condition Satisfied
Total Air Required for Digester: 28.8 L/ min. m3
Table 12 Aerobic Digestor Design
AEROBIC DIGESTER DESIGN
Design Parameter
Value
VSS/TSS
0.8
Percent of Oxygen
23.2
Mass of Air
1.2
Oxygen Requirement
VSS Destroyed
2.3
Diffused Air requirement for W.A.S
23 -35
Assumed Parameter
Oxygen Transfer Efficiency
6
Units
%
Kg/m3
O2/kg
L/min. m3
%
UV DISINFECTION
Disinfection is a process that kills or inactivate pathogenic organisms such as bacteria, protozoans
and viruses so that the effluent discharge does not cause harm to public health or the environment
(Lin 2001, 688). Chlorine leaves a residual in the effluent after the disinfection process, this
residual causes a chemical reactions with organic matter and forms disinfection by products (DBP)
such as Trihalomethanes (THMs) which has been proven to be cancer causing agents (Pedersen
1997).
Alexandria’s wastewater facility uses ultraviolet light for disinfection. UV has good germicidal
qualities and is very effective in destroying microorganisms, the ultraviolet light disinfects water
by making the pathogenic organism incapable of reproducing by disrupting the genetic material in
the cell (Davis and Cornwell 2011, 304). In the process the effluent is exposed to the UV light for
a period of time so that disinfection can occur and the wastewater must contain less than or equal
to 30 mg/L of BOD and TSS (Spellman 2003, 615).
Design Criteria:
1. Three –Bank System to be operated in series, with a capacity to meet average daily flow.
2. The UV units will be configured such that space for maintenance is adequate.
3. Maintenance Schedule will be implemented to ensure functionality of units
4. UV units will be horizontally configured.
5. Sensors will be installed to monitor the efficiency of the units.
6. Safety control system will be implemented to event of high lamp temperatures.
CALCULATION:
To determine Flow per channel Qc:
Average daily Flow Qd (L/min) 15795.21
=
# of channels
2
Qc = 7898 L/min
To determine Number of lamps per channel:
Number of Lamps =
𝐹𝑙𝑜𝑤 𝑝𝑒𝑟 𝑐ℎ𝑎𝑛𝑛𝑒𝑙 𝑄𝑐
𝑙𝑜𝑎𝑑𝑖𝑛𝑔 𝑟𝑎𝑡𝑒
=
7898
30
Number of Lamps = 263 lamps per channel
To determine Number of lamps per bank:
𝑛𝑜.𝑜𝑓 𝑙𝑎𝑚𝑝𝑠
Number of Lamps = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑏𝑎𝑛𝑘𝑠 =
263
3
Number of Lamps = 88 lamps per bank
Check for System configuration:
Condition: hydraulic loading rate 20 – 80 L/lamp.min
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑑𝑎𝑖𝑙𝑦 𝑓𝑙𝑜𝑤 𝑄𝑑
7898
Hydraulic Loading Rate = 𝑚𝑜𝑑𝑖𝑓𝑖𝑒𝑑 𝑙𝑎𝑚𝑝 𝑥 𝑏𝑎𝑛𝑘 = 96 𝑥 3
Hydraulic Loading Rate = 27 L/ lamp. Min
 Condition Satisfied
Table 13 Disinfection System Design
DISINFECTION SYSTEM DESIGN
Design Parameter
Value
Average Daily Flow (Qd)
22745.10
15795.21
Minimum transmittance
55
System head loss coefficient
1.8
Lamp / Sleeve diameter
23
Area of Quartz Sleeve
4.2 x 10-4
Lamp Spacing (center to center)
75
Minimum Design Dose
100
Hydraulic loading rate
20 – 80
Number of banks
3
Acceleration due to gravity
9.81
Assumed Parameter
Number of Channels
2
Flow per channel
7898
Number of lamps per channel
263
Minimum number of lamps per bank
88
Lamp per module
16
Number of modules
6
Corrected Lamp per bank
96
Manufacturer check
27
Units
m3/d
L/min
%
mm
m2
mm
mJ/cm2
L/min.lamp
Only
m/s2
Only
L/min
Only
Only
Only
Only
Only
L/lamp.min
SLUDGE DRYING BED
This uses a method that removes the water from the sludge naturally through percolation, gravity
and evaporation, it requires land space and uses less operating energy than mechanical dewatering
systems. The dewatering process removes moisture from the sludge after the sludge has been
stabilized, it helps to reduce the overall cost of transportation to the disposal site since the
dewatered sludge is easier to handle than the thick sludge or liquid sludge (Lin 2001, 786) .
Paved decanting drying beds was selected for the Alexandria Town, despite the fact that it requires
land space, it is less complex than mechanical systems and produces higher solid cake, and is
advantageous in warm climates. The paved bed consists of a concrete or asphalt pavement above
porous gravel sub-base with a slope of at least 15%.
CALCULATION:
Volume of Digested Sludge = 1909 kg/ d
To determine Wet bio-solids = 1909 x 365 = 696785 kg/yr
To determine Dry Solids = Wet bio-solids x % solid = 696785 x 0.06 = 41807.1 kg/ year
Free water pan evaporation Re = 10 x ke x Ep = 10 x 0.6 x 120 = 720
Area:
1.04𝑆 [
𝐴=
103 𝑘𝑔
1 − 𝑆𝐷 1 − 𝑆𝑒
−
]+(
) 𝑥 (𝑃)(𝐴)
𝑆𝐷
𝑠𝑒
𝑚3
𝑅𝑒
103 𝑘𝑔
1 − 0.08 1 − 0.35
1.04 𝑥 41807 [ 0.08 −
]+(
)
0.35
𝑚3
𝐴=
720
A = 583.70 / (1 – (103 x 0.6)/ 720)
A = 3502.20 m3 = 0.35 ha
Area of bed = 0.10 ha
Number of Bed = 0.35 / 0.10 = 3.5
Number of Sludge Drying bed: 4 (four)
Table 14 Sludge Drying Bed Design
SLUDGE DRYING BED DESIGN
Design Parameter
Value
Thickened Sludge
1909
Solids
6
Wet Bio-Solids
696785
Dry Bio- Solids
41807.1
Assumed Parameter
Dry Solids in applied bio-solid (before decanting)
6
Dry solids in bio-solids (after Decanting)
8
Dry Solids for final disposal
35
Free water pan evaporation, Re
120
ke
0.6
Evaporation rate, Re
720
Area, A
0.35
Number of Beds to be used
4
Units
kg/d
%
kg/yr
Kg/yr
%
%
%
Cm/yr
ha
-
SECURITY
An important element in security systems is the use of closed-circuit television as a tool to assess
the alarms triggered by exterior or interior intrusion sensors. CCTV systems can also provide
recorded evidence for use in criminal courts or administrative procedures against employees.
Closed-circuit television (CCTV) coverage on the area over the intake can help prove or disprove
whether anyone attempted to place contaminants at that location. Cameras will be employed at the
perimeter, at access control points, at critical operation locations, and on the interior of buildings.
The resolution of the camera will dictate the quality of the image captured. The higher the
resolution, the clearer the picture and the greater the ability to recognize or identify an intruder as
a specific person so the tapes can be reviewed to disprove an allegation of wrong doings on the
compound, thus helping to maintain the public's confidence in the utility.
The first line of defence for a facility is at the perimeter. Barriers define the utility's borders and
create a buffer zone with neighbouring property. The most common perimeter barrier is a chainlink fence. The fence delineates the boundary of the facility and whether it is a water treatment
plant, finished water storage tank, or a booster pumping station. However, fences will keep out
only the casual passer-by. A sturdy, well-maintained 7- to 8-ft galvanized chain-link fence will
serve two purposes. First, a well-maintained, secure fence, topped by three strands of barbwire
with at least a 20-ft clear zone on the outside of the fence line, may give an indication of the type
of security an adversary may face. (American Water Works Association, 2005)
The entrance of the Wastewater Treatment Plant is outfitted with a pedestrian gate to service the
pedestrian traffic coming in to the facility; in addition to which there is a double swing gate to
allow for vehicular traffic for vehicles as large as backhoes that may enter the compound to
conduction maintenance on the facility.
WORKER ACCOMMODATION
Health, safety and welfare are basic requirements of any work place. Worker accommodation is
important to ensure high moral and increase in good working conditions which directly improves
productivity. Not providing adequate working conditions/correct health and safety practices is in
breach of the General Duty Clause of the OSH Act. Upholding labour rights is not a costly
procedure if done correctly from the beginning with sufficient staff time and documentation.
However, when labour rights are not upheld the costs for resolving breaches can be considerable
in the form of compensation, redoing work, hiring equipment for longer than anticipated, delays
in schedule completions, and additional monitoring and due diligence. The workers area is
equipped with a bathroom, kitchen, sickbay and a safety equipment storage room.
Figure 6 Showing Plan view of Worker Accommodation Area
LABORATORY
LAB OCCUPATIONAL HEALTH OF SAFETY
Adapted from Standard Methods for the Examination of Water and Wastewater

Promptly flush eyes with water if chemicals get into them for a minimum of 15 minutes
and seek immediate medical attention.

Promptly flush affected area with water for approximately 15 minutes and remove any
contaminated clothing. If symptoms persist after washing, seek medical attention.

Promptly clean up spills, using appropriate protective apparel and equipment and proper
disposal procedures.

Do not smell or taste chemicals. Vent any apparatus that may discharge toxic chemicals
into local exhaust devices.

DO NOT eat, drink, smoke, chew gum, or apply cosmetics in areas where laboratory
chemicals are present. Always wash hands before conducting these activities.

Handle and store laboratory glassware with care to avoid damage.

Use equipment for its designed purpose only.

Wash areas of exposed skin well before leaving the laboratory.

Do not use mouth suction for pipetting or starting a siphon.

Remove laboratory coats immediately on significant contamination with hazardous
materials.

Confine long hair and loose clothing. Wear shoes at all times in the laboratory but do not
wear sandals or perforated shoes.

Keep work area clean and uncluttered, with chemicals and equipment properly labelled and
stored. Clean up work area on completion of an operation or at the end of each day.

Wear appropriate eye protection where chemicals are stored or handled.

Wear appropriate gloves when the potential for contact with toxic chemicals exists.
Inspect gloves before each use, wash them before removal, and replace periodically.

Use appropriate respiratory equipment when engineering controls are unable to maintain
air contaminant concentrations below the action levels.
Water quality is very important as water is needed every day to sustain life as we know it on earth. It
is absolutely necessary to study the physical, biological, chemical and microbiological characteristics
of water when testing the water quality. It is of utmost importance to test the quality of water at different
components of the waste water treatment plant to determine whether the plant is functioning as it
should.
Table 15 Testing Scheme
COMPONENT
Influent
Effluent
PARAMETER
TESTED
Flow
Biochemical Oxygen
Demand
Total Suspended
Solids
Biochemical Oxygen
Demand
Total Suspended
Solids
Ammonia
Phosphorus
Dissolved Oxygen
pH
Faecal and Total
Coliform
SAMPLING
LOCATION
At Wet Well
FREQUENCY OF
MONITORING
Daily
Daily
Daily
Daily
Daily
Daily
Daily
Daily
Daily
Weekly
Bioreactor
Clarifier
Backwash Filters
Aerobic Digester
UV Disinfection
Total Suspended
Solids
Biochemical Oxygen
Demand
Total Suspended
Solids
Total Suspended
Solids
Total Volatile
Suspended Solids
Settleable Solids
Flow
Temperature
Daily
Weekly
Daily
Daily
Daily
Daily
TESTS
Total Suspended Solids (TSS)
Total Suspended Solids measures the wastewater treatment plant’s efficiency with respect to
effluent quality. Total suspended solids can be referred to as particles that do not dissolve in water
and cannot be filtered during the wastewater treatment process. Suspended solids may account for
the unpleasant odour as organic solids may be degrading anaerobically and releasing a smell.
Table 16 Total Suspended Solids Test
EQUIPMENT
Analytical Balance
Beaker
Vacuum Pump
Aluminium Dishes
Drying Oven
Desiccator
Filter Paper
Pipettes
Graduated Cylinders
CHEMICALS
Distilled Water
PROCEDURE
1. Weigh dry filter paper and Aluminium dishes.
2. Place filter disk on filtration apparatus and rinse with distilled water and
suction to dryness.
3. Pour sample into beaker and slowly stir with magnetic stirrer.
4. Rinse sides of apparatus with distilled water and suction to dryness.
5. Transfer filter to aluminium dish and dry dish and filter in oven.
6. Remove after 1 hour and put to cool in desiccator for 20 minutes.
7. Record weight of dried filter and sample.
Total Volatile Solids (TVS)
Total volatile solids normally represents the amount of organic solids in water. It observes the
quality of the sludge.
Table 17 Total Volatile Solids Test
EQUIPMENT
Analytical Balance
Drying Oven
Desiccator
Petri Dishes
Muffle Furnace
CHEMICALS
PROCEDURE
1. Place sample and dry dish in the muffle furnace.
2. Burn samples for 1 hour.
3. Cool samples inside furnace until temperature is 100oC.
4. Transfer samples to desiccator and cool.
5.Weigh cooled samples.
Settleable Solids (SS)
Settleable Solids measures the volume of solids settling from the wastewater for a given time
period. It measures the influent quality, show the settling behaviour of the wastewater and allows
for estimation of sludge volume accumulation. Settleable solids consist of both organic and
inorganic matter.
Table 18 Settleable Solids Test
EQUIPMENT
Stirring Rod
Stopwatch
Imhoff cone & rack
CHEMICALS
PROCEDURE
1. Gentle agitate the sample.
2. Fill the Imhoff cone with the sample up to a sufficient mark.
3. Allow the sample to settle for 45mins them agitate by lifting and rotating
the Imhoff cone.
4. Allow the sample to settle for a further 15 minutes. 5. Record the volume
of solids in the Imhoff cone.
Biochemical Oxygen Demand (BOD5)
The Biochemical Oxygen Demand (BOD) is an estimate of the strength of industrial or domestic
wastes in terms of the oxygen consumed by microorganisms and bacteria to decompose the organic
matter present in the waste over a period of time. The BOD test is most commonly used to measure
waste loading at treatment plants and in evaluating the efficiency of wastewater treatment. The
higher the BOD, the more oxygen will be demanded from the waste to break down the organics.
Table 19 Biochemical Oxygen Demand Test
EQUIPMENT
Pipettes
Volumetric Flasks
Cylinders
Incubator
Dissolved oxygen probe & meter
Dissolved Oxygen bottles and
sealing caps
CHEMICALS
Dilution Water
PROCEDURE
1. Prepare dilution water.
2. Prepare seed source.
3. Add an appropriate volume of sample to the BOD bottle,
add seed and fill with dilution water.
4. Measure initial dissolved oxygen.
5. Incubate for 5 days at 20 oC.
6. Measure final dissolved oxygen.
Dissolved Oxygen (DO)
Dissolved Oxygen (DO) is an indicator for the health of a water body as it measures the amount
of dissolved oxygen in water and evaluates pollution concentration. If the temperature is too high,
there will not be enough oxygen present in water. Also, excessive amounts of microorganism use
dissolve oxygen and deplete it from water starving aquatic animals. Dissolved oxygen levels must
be kept above 4.2 mg/L to sustain aquatic life.
Table 20 Dissolved Oxygen Test
EQUIPMENT
Burette
Pipettes
Beakers
Erlenmeyer Flasks
BOD Bottles
CHEMICALS
Manganous
Sulphate
Alkaline Azide
Iodide Starch
Solution
Sodium
Thiosulphate
Sulphuric Acid
PROCEDURE
1. Fill BOD bottle with sample, remove any air bubbles and rinse bottle.
2. Add Manganous Sulphate to BOD bottle from the pipette.
3. Add Alkaline Azide Iodide to BOD bottle from the pipette.
4. Invert bottle to mix sample and reagents.
5. Allow sample to settle to half the volume, invert sample again and allow to
settle again.
6. Add Sulphuric Acid to BOD bottle, cover and invert until floc disappears.
7. Transfer an appropriate amount of sample to a flask.
8. Use Sodium Thiosulphate to titrate the sample until the yellow colour
disappears.
9. Add starch solution to sample, mix and continue titrating to colourless
endpoint then record final burette volume.
Total and Faecal Coliform
Total coliforms are bacteria that are usually found in the environment. Faecal coliforms is a subgroup of bacteria of total coliforms that are considered to be present primarily in the intestines and
faeces of warm-blooded animals. Origins of faecal coliforms are more identifiable than the origins
of total coliform. Faecal coliforms are considered a more accurate indication of animal or human
waste than the total coliforms. Faecal coliform colonies tend to be less than total coliform as it is
diluted.
Table 21 Total and Faecal Coliform Test
EQUIPMENT
Membrane Filters
Pipettes
Graduated Cylinders
Incubator
Forceps
Funnels
Water Bath
Alcohol Burner
Petri Dishes
Filtration Apparatus
Dilution Bottles
CHEMICALS
Dilution Water
PROCEDURE
1. Place membrane filter on suction bed of filter funnel.
2. Add an appropriate volume of sample to measuring cylinder and
top up with sterile water.
3. Pour contents into filter funnel to be suctioned.
4. Rinse cylinder and funnel with sterile water.
5. Suction membrane filter until dry.
6. Transfer dry filter to petri dish with forceps.
7. Incubate inverted petri dish for 16-24 hours.
pH:
pH is a measure of hydrogen ions present in a solution and is expressed as the negative log of H
plus ions concentration. The pH scale runs from 1-14. Solutions that have a pH of less than 7 is
considered to be acidic. Solutions that have a pH greater than 7 is considered to be alkaline.
Solutions that have a pH of 7 is considered to be neutral.
Table 22 pH Test
EQUIPMENT
pH metre with electrodes
pH paper
Beakers
Thermometer
Magnet
CHEMICALS
PROCEDURE
Method 1 (Visual using pH paper):
1. Immerse pH paper into sample.
2. Match colour on paper to with scale of colours provided.
3. Record matched pH value.
Distilled Water Method 2 (Electrometric using pH meter):
1. Rinse electrodes with distilled water.
2. Rinse beaker with some of sample and fill to a suitable volume.
3. Immerse electrode into sample.
4. Use magnet to slowly agitate sample and record pH after 5 minutes.
Ammonia-Nitrogen
In wastewater, phosphorous removal and nitrogen removal do not occur simultaneously. Nitrates
appears to interfere with phosphorous uptake, and phosphorus will not be removed until the
nitrate is gone.
Table 23 Ammonia-Nitrogen Test
EQUIPMENT
CHEMICALS
Pipettes
Retort Stand
Volumetric Flasks
Sodium Hydroxide
Sulphuric Acid
PROCEDURE
1. Pour sample into Volumetric flask and dilute if necessary.
2. Add borate buffer to sample.
3. Adjust pH by adding sodium hydroxide.
4. Attach flask to distillation apparatus and distil.
5. Collect distillate in sulphuric acid.
Phosphorus
Phosphate present in water at a wastewater treatment plant is responsible for the growth of algae.
The source of phosphates is detergents and excessive agricultural application of fertilizers.
Phosphate removal requires true anaerobic conditions, which occur only when there is no other
oxygen donor.
Table 24 Phosphorus Test
EQUIPMENT
Spectrophotometer
Filter photometer
Acid-washed glassware
CHEMICALS
Sulphuric Acid
Potassium antimonyl tartrate solution
Ammonium molybdate solution
Ascorbic acid
Stock phosphate solution
Standard phosphate solution
Phenolphthalein indicator
PROCEDURE
1. Pipet sample into volumetric flask.
2. Add combination of potassium antimonyl tartrate
solution,
ammonium molybdate solution, and ascorbic acid and
mix thoroughly.
3. Measure absorbance after 30 min, using reagent
blank as reference sample.
TREATMENT PROCESSES
Figure 7 Flow Chart Showing Wastewater Treatment Process
SCREENING
Screening is one of the first operations at the wastewater treatment plant. The screening device’s
main function is to remove large object present in the wastewater. The performance of screens
depends on the bar spacing. Screens can be cleaned by removing them manually or by the process
of backwashing. It must be noted that the frequency of cleaning of the screens all depends on the
quality of wastewater entering the plant. The screeners can be classified as follows:
-
Coarse Screens
-
Medium Screens
-
Fine Screens
AERATED GRIT CHAMBER
Grit Chamber is the next stage of the treatment process, this also forms the preliminary stage of
treatment, it removes heavy inorganic material with specific gravity of 2.4 -2.7 such as sand, this
process depends on gravity for sedimentation to occur. This stage of the treatment process is
important as it prevents damage to the pumps and prevent inorganic particles from accumulating
in the sludge digester. The aerated grit chamber consists of a spiral flow aeration tank, the grit
settles to the bottom of the tank and the lighter organic particles are carried with the roll of the
spiral motion (NPTEL 2018).
PRIMARY TREATMENT
PRIMARY SEDIMENTATION TANK
This is the only primary treatment stage in the treatment process. The water is usually introduced
peripherally into the tank. This stage involves the removal of particles from the water by means
of settlement. The water in this stage may be detained for about 2 – 3 hours before it is taken to
the secondary treatment process.
SECONDARY TREATMENT
BIOREACTOR
This is the beginning of the secondary treatment process and is where aeration takes place. The
water is pumped into the bioreactor/aeration tank where air is forced into the system through
different aeration methods. In extended aeration treatment, sludge is measured, and a fraction is
pumped back into the aeration tank to be reused in the treatment. The water in the tank may stay
approximately 6 – 8 hours in the conventional treatment plant, but in the extended aeration process
the water may stay in the tank at least 24 hours before it is then taken to the clarifier for sludge
separation.
CLARIFIER
Water enters the secondary clarifier after it leaves the aeration tank so that the sludge can be
separated from the water which is then further treated before it is discharged into the river channel.
The clarifier mainly consists of a hopper, inlet, rake arm, skimming arm and effluent weir. The
hopper is located to the base of the clarifier and is where the sludge is collected so that it can either
be removed from the system or returned to the aeration tank for reuse. The inlet is located usually
located to the centre of the clarifier so that the influent does not disturb the sludge that is already
settling in the tank. The rake arm assists in guiding the sludge to the sludge hopper to be removed
and the skimming arm removes sludge from the water surface. The effluent weir allows the treated
water to be removed front the tank so that it can be further treated before discharge into the
environment.
AEROBIC DIGESTER
This is the first stage of the secondary treatment process. The aerobic digester main purpose is to
produce stable end product suitable for disposal, the process converts organic sludge solids to
carbon dioxide, ammonia, and water by aerobic bacteria. In this process the sludge is introduced
to the digester on a batch, semi batch or continuous basis, the design standard varies based on the
sludge characteristics and method of disposal. The sludge is usually thickened before this process
to aid in volume reduction of water content.
DISINFECTION
Disinfection is last process in wastewater treatment. Disinfection of water is done to kill any
disease causing organisms present in the water after it undergoes primary and secondary treatment.
Outbreaks that can occur as a result of untreated water are acute gastroenteritis, hepatitis
shigellosis, ciardiasis, chemical poisoning, typhoid fever, and salmonellosis (Cheresmisinoff,
2002, 446). The use of ultraviolet light disinfects effluent by instantaneously neutralizing
microorganism as they pass the submerged UV light, when microorganism are exposed to
germicidal wavelengths the are rendered incapable of reproducing. This disinfection process is
considered safer than the use of chlorination, it is cost effective and does not react with organic
matter to form carcinogenic by products due to the by-products that are produced from chemical
reactions with organic matter. UV is one of the safest cost effective method that does not react to
form carcinogenic by-products (Trojan Technologies 2018).
SLUDGE DRYING BED
Sludge drying beds are used for sludge dewatering; sludge dewatering is separation of liquid and
sludge solids. Sludge drying beds can be of two type, either open or covered. The Wastewater
Treatment Plant of Alexandria Town utilizes the open sludge drying bed approach. Sludge from
the wastewater treatment process is removed and spread as evenly as possible on the drying beds.
The sludge is allowed to dry by a combination of evaporation and gravity filtration through the
layered sand on which the sludge rests. The sludge beds consist of a bottom layer of graded gravel
integrated with perforated pipes which supports a layer of coarse sand. The sludge takes 8-21 days
to dry, this can vary with climatic conditions. Sludge that has been dried appears on the bed surface
as cracked, it is bagged to be transported and sold as fertilizer for crops. Using dried sludge for
agricultural purposes can be deem as recycling, therefore it is sustainable as it prevents pollution
and generates funds that can do toward maintenance of the plant and facilities.
LAYOUT OF WWTP
Figure 8: Layout of WWTP for Alexandria
CONCLUSION
The wastewater treatment plant of Alexandria Town has proven to be successful as it allows
disease-causing bacteria and viruses to be destroy, allowing the population of Alexandria Town to
remain healthy citizens of Trinidad. The wastewater treatment plant is sustainable as it reduces
pollutants from being emitted into the environment, and it is easier to maintain than conventional
septic tanks systems as well as drainage field systems. The design wastewater treatment plant
operates at a 90% efficiency and would continue to do so as the employees of the Alexandria Town
operate with a strategic PLAN, DO, CHECK AND ACT action plan.
REFERENCES

Davis, Mackenzie L., and David A. Cornwell. 2011. Introduction to Environmental
Engineering United States of America The McGraw- Hill Companies

Dokuz University. 2018. “Wastewater Engineering”. Accessed November 18, 2018.
http://web.deu.edu.tr/atiksu/ana52/abdtre08.html

“CVNG 3007: Environmental Engineering I Laboratory Manual-The Analysis of Water
and

Wastewater.” Accessed October 26, 2018.
http://myelearning.sta.uwi.edu/pluginfile.php/406452/mod_resource/content/1/Envi%20L
aboratory%20Manual%2017-18.pdf

Lagoon Online. 2015. “Fine Bubble Aeration”. Accessed November 21, 2018.
http://www.lagoonsonline.com/fine.htm

Lin, Shun Dar. 2001. Water and Wastewater Calculations Manual Edited by C.C Lee,
Handbook of Environmental Engineering Calculations United States of America
McGraw-Hill

Meteorological Office of Trinidad and Tobago. 2018. “Climate”. Accessed November
20, 2018.

https://www.metoffice.gov.tt/Climate

NPTEL. 2018. "Sludge Management ". NPTEL IIT Kharagpur accessed November 20,
2018. https://nptel.ac.in/courses/105105048/M22L37.pdf.

Pedersen, T.L. 1997. "The Effects of Chlorination and Chlorine on Drinking Water ",
accessed November 21, 2018. http://extoxnet.orst.edu/faqs/safedrink/chlor.htm

Qasim, Syed R., Wastewater Treatment Plant: Planning, Design and Operation. 2nd ed.

USA: CRS Press, 1999.

Red Valve Company. 2002. “Aeration System design for loading peak loadings”

Accessed November 23, 2018. http://web.deu.edu.tr/atiksu/ana52/redvalve.pdf

Rice, E.W., R.B. Baird, A.D. Eaton, L.S. Clesceri, “Standard Methods for the
Examination of

Water and Wastewater”. 22nd ed. USA: American Public Health Association, American
Water Works Association, Water Environment Federation, 2012.


Spellman, Frank R. 2003. Handbook of Water & Wastewater Treatment Plant
Operations. United States of America Lewis Publishers

Water Quality Solution. 2018. “Fine Bubble vs Coarse Bubble Diffusion”.

Accessed November 22, 2018. https://waterqualitysolutions.com.au/fine-bubble-vscoarse-bubble-diffusion/
APPENDICES
Table 25 Average Domestic Dry Weather Flows for Alexandria
DOMESTIC FLOWS
Facility
Residential
Houses
Apartments
No. of
Facilities
Description
No. of
Person per
Facility
Consumption
(lpd)
Total
Consumption
(lpd)
Average
Flow
(m3/d)
Domestic
Population
4000
Persons
4
280
4480000
4480
16000
1800
Persons
4
280
2016000
2016
7200
6496
23200
Total
Table 26 Average Commercial Dry Weather Flows for Alexandria
COMMERCIAL FLOWS
Facility
Hotel
Mall
Theatre
Post Office
Supermarket
Elderly Home
Guest House
Coffee Shop
Gas Station
Bank
Restaurant
Bar
Total
No. of
Facilities
Description
No. of Person
per facility
Consumption
(lpd)
Total
Consumption
(lpd)
Average
Flow
(m3/d)
Commercial
Population
2
2
2
Guest
Employee
Parking Spaces
200
450
300
190
4
4
76000
3600
2400
76
3.6
2.4
400
900
600
2
Employees
300
40
24000
24
600
2
2
Toilet Room
Seat
20
200
2000
12
80000
4800
80
4.8
40
400
1
2
2
Employee
Toilet Room
Employee
60
20
10
55
2000
40
3300
80000
800
3.3
80
0.8
60
40
20
2
Resident
55
421
46310
46.31
110
2
4
Employee
Person
70
120
48
200
6720
96000
6.72
96
140
480
2
2
2
2
3
6
4
Customer
Employee
Vehicles Served
Employee
Employee
Meal
Customer
100
15
300
20
100
150
50
24
48
40
50
55
10
8
4800
1440
24000
2000
16500
9000
1600
4.8
1.44
24
2
16.5
9
1.6
200
30
600
40
300
900
200
4
Employee
20
50
4000
4
80
487.27
6140
Table 27 Average Institutional Dry Weather Flows for Alexandria
INSTITUTIONAL FLOWS
Facility
No. of
Facilities
Description
No. of Person
per facility
Consumption
(lpd)
Total
Consumption
(lpd)
Average
Flow
(m3/d)
Institutional
Population
2
2
2
2
3
3
Students
Employees
Student
Employees
Student
Employees
200
275
250
60
600
100
72
55
72
55
96
55
28800
30250
36000
6600
172800
16500
28.8
30.25
36
6.6
172.8
16.5
400
550
500
120
1800
300
2
2
1
1
2
2
Employees
Employees
Bed
Employee
Employee
Seats
25
25
300
250
25
45
55
55
78.2
48
48
50
2750
2750
23460
12000
2400
4500
2.75
2.75
23.46
12
2.4
4.5
50
50
300
250
50
90
3
Seats
150
42
18900
18.9
450
2
Persons
500
144
144000
144
1000
501.71
5910
ECC
Primary School
Secondary
School
Police Station
Fire Station
Hospital
Health Centre
Church
Recreational
Area
Total
Table 28 Projected Design Flows for Alexandria Town for 2018 - 2038
PROJECTED FLOWS
Year
Dry Weather Flow
(m3/d)
Wet Weather Flow
(m3/d)
Total Average Flow
(m3/d)
Total Peak Flow
(m3/d)
2018
2023
2028
7485
7859
8252
11227
11789
12378
18712
19648
20630
56137
58944
61891
2033
8665
12997
21662
64986
2038
9098
13647
22745
68235
1.5
Growth Factor
Table 29 Population Distribution Percentage
Table 30 Population Growth for Year 2038
FLOW CATEGORY
POPULATION NUMBER
Domestic
23200
POPULATION
(%)
66
Commercial
6140
17
Institutional
5910
17
Total Population
35250
100
FUTURE POPULATION
Parameter
Value
Units
P
35250
-
r
5
%
t
20
years
Pi
89075
-
Figure 9: Water pollution rules parameters