Sewage Treatment Work Sheet

Example - 1

A grit chamber is designed to remove particles with a diameter of 0.2mm, specific gravity 2.65.
Settling velocity for these particles has been found to range from 0.016 to 0.022m/sec, depending
on their shape factor. A flow through velocity of 0.3m/sec will be maintained by proportioning
weir. Determine the channel dimensions for a maximum wastewater flow of 10,000cu m/day.
Solution Let us provide a rectangular channel section, since a proportional flow weir is provided
for controlling velocity of flow.

Example -2

Design a suitable grit chamber cum Detritus tank for a sewage treatment plant getting a dry
weather flow from a separate sewerage system @4001/s. Assume the flow velocity through the
tank as 0.2m/sec and detention period of 2 minutes. The maximum flow may be assumed to be
three times of dry weather flow.
Example – 3
Each arm length can thus be made of three sections, i.e. first 6m from center to be 130mm
diameter, next 6m of 124mm diameter, and the last 7m of 103mm diameter. If economy is not
much affected, and or if different sized pipes are difficult to join, then the entire arm length may
be kept of 130mm diameter; or the first 12m may be kept of 130mm diameter, and the last 7m of
103mm diameter.
Design of Under-drainage System Total discharge through each filter unit at peak flow =
0.065m3/sec. Let us design the under-drainage system with a central rectangular channel, fed by
radial laterals discharging into the channel. The under-drain block lengths, containing
semielliptical openings, can be used as laterals.

The size and slope of the rectangular effluent channel should be such as to allow, say a velocity
of 1m/sec through it (min. = 0.9m/sec).
Discharge through the filter
Example -4

The sewage is flowing @ 4.5Million liters per day from a primary clarifier to a standard rate trickling
filter. The 5-day BOD of the influent is 160mg/l. The value of the adopted organic loading is to be 160
gm/m3/day, and surface loading 2000 l/m2/day. Determine the volume of the filter and its depth. Also
calculate the efficiency of this filter unit. Solution Total 5-day BOD present in sewage
Example 5 - design of activated sludge treatment

GIVEN

The population to be served = 120, 000

Per capita sewage generation rate = 200 lpcd

BOD5 of the raw sewage = 300 mg/l

30 % of the BOD is removed in the primary sedimentation tank and the allowable effluent BOD
=20 mg/l

Sludge Volume Index (SVI) = 120 ml/g

Food to microorganism ratio (Fm) in the aeration tank and microorganism concentration (Xr) is
0.3 and 3.3 gm/l respectively

SOLUTION

The design of activated sludge unit consists

 Aeration tank
 Return sludge
 Oxygen supply
 Final clarifier
 Removal of excess sludge
DESIGN AERATION TANK
Daily BOD load is the difference between BOD after primary sedimentation and the expected
effluent standard

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If infiltration is 100 % of Qd = 24, 000 m3/day = 1000 m3/hr

Total design discharge for dry weather = Qpeak + Qinfiltration = 1715 m3/hr + 1000 m3/hr = 2715 m3/hr

Total design discharge for wet weather = 2*Qpeak + Qinfiltration = 2* 1715 m3/hr + 1000 m3/hr = 4430 m3/hr

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to be provided the length of the tank,

DESIGN OXYGEN SUPPLY SYSTEM

For aeration tank the recommended value is 1.5 kg O2/kg BOD

Then the percentage of 0xygen in air in the area should be checked

If 21% the 1m3 of air = 0.26 kg O2,

If the efficiency of the aeration system which will be dependent on altitude is 70% then the total
volume of air required will be

Design 2 blowers with capacity of 1200 m3/hr and 1500 m3/hr

DESIGN OF THE FINAL CLARIFIER

Available data: - Xr = 3.3 gm/l = 3.3 kg/m3 and SVI = 120 ml/gm

RSV = Xr * SVI = 3.3 gm/l * 120 ml/gm =396 ml/l

Solids loading (qsl) = 0.3 – 0.4 m3/m2 hr, let’s use qsl=0.3

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RETURN SLUDGE

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Choose two sludge pumps with a capacity of 700 m3/hr

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Check if it is within the allowable range

Check the area of the final clarifier for wet weather condition

Example – 6 Design of Sewage Treatment Plant Design

Sewage flow
Source – the site considered for provision of wastewater treatment system has 39 condominium buildings
with 1130 household units. Taking the average family size of Ethiopia [5/HH] the total population
equivalent is determined as follows:-

Based on GTP II plan of Ethiopia water supply service the town is categorized as ‘‘Category B’’ and the
corresponding per capita water supply is found to be 80LPCD. Assume 85% of water consumption
converted to wastewater [between 70 – 90%] the per capita water consumption will be determined:-
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Maximum hourly flow

Minimum hourly flow

Typical Raw Sewage Quality and Desired Effluent Standard

Table 1 Specific Wastewater Loads for WWTP Design

Table 2 Proposed Effluent Criteria

Technology Option One- Design of Membrane Biological Reactor (MBR) System
Flow Sheet of MBR System

Components in MBR System include:-

 Screen  Membrane filter unit

 Grit chamber  Sludge drying beds
 Imhoff tank (primary clarifier and  Inlet, excess sludge and return sludge
sludge digestion) pumping stations
 Aeration tank

Sizing of MBR System Components

Preliminary Treatment Units

1. Screen: - though the sewage will be collected from communal septic tanks provision of
screen is mandatory to protect clogging of downstream components. The screen will be fitted
in qa rectangular channel cross section with a maximum water depth of 0.1m and peak flow
velocity of 0.6m/s.

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Taking 8mm bar width and 10mm open spacing between bars the number of bars and total width
occupied by the bar will be determined as follows:-

Assuming a clogging rate of 40 %, the total space will be reduced to 7.8cm. Corresponding the
velocity is:-

Taking 7l/capita/year (5 -15 l/capiat/year) screenings, the total volume of screenings is estimated
to be:-

2. Grit Chamber – second part of the preliminary treatment of wastewater in removal of sand
and other heavy materials. The design considers the following conditions i.e.
I. Elimination rates:-
 100 % for grains with a diameter of 0.2 mm
 90% for 0.16 mm
II. Settling velocity: 0.3 m/s
III. maximum surface load is
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The sand trap will have two compartments, each 0.254 m wide and 6.354 m long, with a water
depth of 0.1 m. While one compartment is in operation the other one is locked for sand removal.
The sand trap will be emptied manually. The area is being served by separate sewerage system.
Therefore, lower value of sand disposal per capita can be taken i.e. 3l/P.E.yr (2-5 l/P.E.yr)

Imhoff Tank (Primary Clarifier and Sludge Digestion)

The Imhoff Tank combines the following two different objectives within the wastewater
treatment process:-

 Pre-treatment of the wastewater by sedimentation

 Anaerobic digestion of sludge

Undissolved organic matter with a specific higher weight than water is settling down in the upper
part of the Imhoff Tank. Further on, the primary sludge, originated from the Imhoff Tank itself
as well as the secondary sludge of the Final Sedimentation Tank is digesting in the Imhoff
Tank’s lower part
 Figure 1 Principle of Imhoff Tank

The dimensioning of the sedimentation part of the Imhoff Tank is basing on the following
conditions:-

 Average Hydraulic Retention: 0.75 hours

 Surface Load: 2.5 m/h

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Table 3 Loads and Removal in Primary Sedimentation

Parameters Specific Load Total Load of Raw % Removal of PS Load on Aeration Tank
BOD5 51g/cap/day 288.15 kg/day 25 216.1125 kg/day
COD 102g/cap/day 576.3 kg/day 25 432.225 kg/day
SS 60 g/cap/day 339 kg/day 50 169.5 kg/day
TKN 9.4 g/cap/day 53.11 kg/day 9 48.3301 kg/day
TP 1.6 g/cap/day 9.04 kg/day 11 8.0456 kg/day

Biological Treatment

The biological treatment stage will comprise the following basic components parts:-

 Aeration Tank
 Membrane Filtration Tank
 Machine Room

Aeration Tank

Figure 2: Longitudinal rectangular aeration tank

Two aeration tanks will be constructed for the biological treatment of wastewater. The tank will
be designed as rectangular basins with circular flow, in which aerated areas function as carbon
removal zones, whereas unaerated areas function as nitrogen removal zones.
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Taking the depth of aeration tank 4.5m (recommended between 3 - 4.5m)

However the method of determination of dimensions of aeration tank using organic loading rate
results large volume and area especially for rectangular shape. Therefore, hydraulic retention
time will be considered for the design of the dimensions of the aeration tank.

Recommended hydraulic retention time for aeration tank is between 4 – 8 hours. To be effective
area for the provision of aeration tank HRT = 8hours is considered.

Taking the depth of aeration tank 3m (recommended between 3 - 4.5m)

Aeration will be done using surface aerators. The oxygen supply to the aeration tanks will be
controlled by oxygen measurement devices. Submerged mixers keep the water body in a circular
motion through the aeration tank.

The (aerobic) dimensioning sludge age to be maintained for nitrification is:-

The value of 3.4 is made up from the reciprocal of the maximum (net) growth rate of the
ammonium oxidants (nitrosomonas) at 15° C (2.13 d) and a factor of 1.6. Through the latter it is
ensured that, with sufficient oxygen transfer and no other negative influence factors, enough
nitrificants can be developed or held in the activated sludge. With a sludge age of 2.13 d (15° C),
nitrificants cannot accumulate.

Using the safety factor (SF) the following are taken into account:-

 Variations of the maximum growth rate caused by certain substances in the wastewater,
short-term temperature variations or/and pH shifts.
 The mean effluent concentration of the ammonium.
 The effect of variations of the influent nitrogen loads on the variations of the effluent
ammonia concentration. The recommended safety factor is recommended to SF = 1.8.

The mean annual temperature of Adama City is between 19oC and 22oC. In this design at
temperature of 19°C was chosen.

For calculation of the total required sludge age, the nitrogen balance is set up

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Nitrogen incorporated in the Biomass: - 0.05 kgN/kgBOD5

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Total nitrogen in the influent of the aeration tank:

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  Org. Nitrogen (Average) in the effluent: SorgN, SST= 2.0 mgN/l
 Ammonium-Nitrogen (average) in the effluent: SNH4, SST= 7.0 mgN/l
 Nitrate Concentration in the effluent: SNO3,SST= 10.0 mgN/l

Nitrogen to be denitrified:

Ratio nitrate nitrogen / BOD5 @T = 19°C:

Denitrification to total reactor volume can be determined using interpolation

Table 4Standard Values for the Dimensioning of Denitrification

Pre-anoxic zone denitrification and comparable Simultaneous and intermittent

processes denitrification
0.2 0.11 0.06
0.3 0.13 0.09
0.4 0.14 0.12
0.5 0.15 0.15

The total sludge age is:

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The sludge concentration in the aeration tank is usually between 3.0 and 3.8 g/l. The chosen
concentration is SSAT = 3.15 kg/m³.

Aeration and mixing

To supply bacteria with oxygen, aerators are required. Mechanical surface aerators are foreseen
because of easy and convenient technology. The oxygen demand of the activated sludge is as
follows:

Oxygen demand for carbon removal (TOD)

Specific oxygen demand for carbon removal: = 1.20 kgO2/kgBOD5 (at 20 °C)

Oxygen demand for nitrogen removal

 Nitrate for denitrification: SNO3, D = 57 kg/d

 Nitrate influent aeration: SNO3, AT = 8 kg/d
 Nitrate effluent clarifier: SNO3, EAT = 10.0 kg/d
 Nitrogen for nitrification: SN, N = 58.5 kg/d
Specific oxygen demand for nitrification (SON):

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Denitrificated nitrogen: SSNO3, DD = 29.8 kg/d
Recovery of oxygen by denitrification: ROD = 86.4 kg/d
Oxygen demand:
Peak factor for carbon: FC = 1.00
Peak factor for nitrogen: FN = 2.50
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O2-concentration (OC) in aeration tanks with surface aerators and simultaneous denitrification:

 OC = 0.50 mg/l
 O2-solubility at 20 °C, at 2,000m a.s.l. (OS) = 6.95 mgO2/l
 Alpha-factor (transfer factor wastewater/clear water): α = 1.0
Required oxygen transfer (ROT)

Number of surface aerators: 2

Specific power consumption: 2kgO2/kW
Power consumption of each surface aerator: 9 kW
Total power demand, peak: 18 kW
Total power demand, average: 11 kW
For intermittent denitrification, the aerators shall be completely switched off. To avoid
settlement of activated sludge, electrical mixers are required. Depending of the type of mixers
about 2 to 3 Watts per m³ tank volume are required:

Chosen: 2 mixers, each 1.1 kW

Kubota MBR Design

Kubota membranes are package filtration plate membrane available at different size

Design inputs

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We May Select:
Model Number of cartridges(X) Units required(854/X)
 6 units of ES150 ES150 150 5.69 or 6
 5 units of ES200
ES200 200 4.27 or 5
 3 units of EK400
EK400 400 2.135 or 3
Figure 3: Typical layout of ES200 and EK400

Technology Option Two - UASB Reactor with High Rate Trickling Filter

UASB involves a reactor in which untreated wastewater enters at the bottom and flows upward through a
biological sludge blanket. The key feature of the UASB process is that it can handle high BOD loadings
compared to other anaerobic processes due to the formation of dense granulated sludge.
UASB is an anaerobic process that produces biogas as part of its treatment process which could be used
as an additional energy source. The gas is collected at the top of the reactor and provides circulation and
assists with the formation of the granules within the reactor. Biogas attaches to particles causing them to
rise until they strike the degassing baffles releasing the gas.
At the top of the UASB is a phase separator which collects solids rising with the liquid. The solids
accumulate and finally fall back down to accumulate as the sludge blanket. Suspension of the sludge is
achieved by maintaining up flow velocities between 0.6 to 0.9 m/h.
The UASB process itself requires relatively little land area compared to other technologies, however,
additional treatment processes are required to achieve a suitable effluent. Therefore, a trickling filter has
been included to polish the treated effluent from the UASB.
 Figure 4: Typical Configuration of UASB with Trickling Filter
UASB systems produce less sludge in comparison to some of the other systems; however, the trickling
filter produces a quantity of active volatile sludge. This trickling filter sludge can be recycled to the
UASB for digestion, thereby eliminating the need for sludge digesters. The UASB process produces a
very good quality granular sludge which can be sold as seed sludge for other reactors or can readily be
dried and used as fertilizer.
A moderate level of operator skill is required to ensure proper operation of the UASB. On the other hand,
both the UASB and trickling filter processes have low energy requirements compared to the other options
which can save operating costs.
One drawback to the UASB process is that it is quite sensitive to changes in flow, BOD load and toxicity.
If the bacteria responsible for providing treatment are strongly affected, it can take weeks to months to
regenerate the colonies, depending on the availability of good seed sludge.
Design of UASB Reactor

The preliminary system components in USAB with high rate trickling filter (HRTF) are the same
as those used in membrane bioreactor. Therefore, the dimensions will be adopted from it.

Table 5: UASB Influent Load

Parameters Specific Load Total Load of Raw Load (mg/l)
BOD5 51g/cap/day 288.15 kg/day 750
COD 102g/cap/day 576.3 kg/day 1500
SS 60 g/cap/day 339 kg/day 882.3529
TKN 9.4 g/cap/day 53.11 kg/day 138.2353
TP 1.6 g/cap/day 9.04 kg/day 23.52941

Table 6 Recommended loading on UASB reactor based on the COD concentration

Design Inputs

 Qavg. = 384.2m3/d
 COD = 1500mg/l
 OLR = 5 kg COD/m3/d
 Vup = 0.5 m/h
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Design of High Rate Trickling Filter

Design parameters
 Hydraulic loading rate(HLR) = between 110 – 330 [220 M.l/ha.d]
 Organic loading rate(OLR) = between 6000 – 18000 kg BOD5/ha-m
Design inputs
 Qavg. = 384.2m3/day
 Taking 90% BOD5 removal by UASB, BOD5 influent @HRTF = 0.1*750mg/l=75mg/l
 Required effluent BOD5 = 15mg/l

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Dimensions of trickling filter unit

Taking the effective depth to be 2m