Environmental Engineering : Treatment of Sewage

By Aditya Kumar|Updated : September 14th, 2021

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Treatment of Sewage


Sedimentation Tank

Settling Velocity


 byjusexamprep for d < 0.1 mm


 byjusexamprep The velocity of the settlement of particle or settling velocity in m/sec.

 byjusexamprep The diameter of the particle in the meter.

 byjusexamprep Specific gravity of the particle.

 byjusexamprep Kinematic viscosity of water in m2/sec.

 byjusexamprep byjusexamprep Dynamic viscosity

 byjusexamprep Density




  For laminar flow

 byjusexamprep Reynolds number  byjusexamprep 

 byjusexamprep for transition flow.

 byjusexamprep for turbulent flow.



(iv) Newtons Equation for Turbulent Settling


(v) Modified Hazen’s Equation for Transition Zone

 (a) byjusexamprep 

 Where T = Temperature in oC.

 (b) Putting G = 2.65 for Inorganic Solids


 (c) Putting G = 1.2 for Organic Solids


Critical Scour Velocity in Constant Velocity Horizontal Flow Grit Chamber byjusexamprep 








 byjusexamprep Width of the channel.

 byjusexamprep Horizontal flow velocity.

 byjusexamprep Coefficient of discharge.

 x and y are coordinates on weir profile.


Parabolically or V-Shaped Grit Chamber Provided with a Parshall Flume

(i) Parshall Flume



 byjusexamprep Width of the throat in the meter.

 byjusexamprep Flow in (m3/sec) through Parshall flume.

 byjusexamprep Depth of flow in the upstream leg of a flume of one-third portion in the meter.



(ii) Parabolic Grit Channel



 n = Discharge coefficients of the control section.

 = 1.5 for partial flume.

 = 1 for proportional flow weir.

·                         Aerated Grit Channels


·                         Detritus Tank


Skimming Tank

 (i) Detention Period = 3 to 5 minutes.

 (ii) Amount of compressed air required = 300 to 6000 m3 per million liters of sewage.

 (iii) Surface Area,



 byjusexamprep Rate of flow of sewage in m3/day.

 byjusexamprep Min. rising velocity of greasy material to be removed in m/min

 = 0.25 m/min mostly.


 Vacuum Pressure = 0 to 25 cm of Hg

 For 10 to 15 minutes.

·   Sedimentation Tank

 (i) Overflow rate

 = 40000 to 50000 lit/m2 day for plain sedimentation.

 = 50000 to 60000 lit/m2 day for sedimentation with coagulation.

 = 25000 to 35000 lit/m2 day for secondary sedimentation tank

 (ii) Depth ~ 2.4 to 3.6 m.

 (iii) Detention time = 1 to 2 hour.

 (iv) width = 6.0 m

 (v) Length = 4 to 5 times width.

 (vi) Velocity of flow Vf = 0.3 m/min.



 V = Flow velocity

 B = Width of the Basin

 H = Depth of sewage in the tank.

 (viii) byjusexamprep 

 (ix) byjusexamprep 


·   Detention Time

(a) byjusexamprep For rectangular Tank

(b) byjusexamprep for circular tank


d = Dia of the tank

H = Vertical depth of wall or side depth

·   Displacement Efficiency byjusexamprep 


Trickling Filter

 (a) Conventional Trickling Filter or Low Rate Trickling Filter



 byjusexamprep The efficiency of the filter and its secondary clarifier, in terms of % of applied BOD

 byjusexamprep Organic loading in kg/ha-m/day applied to the filter (called unit organic loading)

 (b) High Rate Trickling Filter

 (i) byjusexamprep 

 Where, F = Recirculation factor

 Recirculation ratio

 (ii) byjusexamprep 


 Y = Total organic loading in kg/day applied to the filter i.e. the total BOD in kg.

 byjusexamprep Unit organic loading in kg/Ha-m/day

 V = Filter volume in Ha-m.

 % efficiency of single-stage high rate trickling filter.

 (iii) byjusexamprep


 byjusexamprep Final efficiency in the two-stage filter.

 byjusexamprep Total BOD in the effluent from the first stage in kg/day.

 byjusexamprep Recirculation factor for second stage filter

 byjusexamprep Volume in second stage filter in ha-m.




 Dunbar Filter

 Surface loading = 25000 MI/m2/day.

 BOD removed = 85%

 Sludge and its Moisture Content


byjusexamprep The volume of sludge at moisture content P1%

byjusexamprep The volume of sludge at moisture content P%

Sludge Digestion Tank

(i) When the change during digestion is linear.

 (a) byjusexamprep 


 byjusexamprep The volume of digestion in m3.

 byjusexamprep Raw sludge added per day (m3/day)

Equivalent digested sludge produced per day on completion of digestion, m3/day.

Digestion period in the day.

(b) byjusexamprepwith monsoon storage


T = Number of days for which digested sludge byjusexamprep is stored (monsoon) storage)

(ii) When the change during digestion is parabolic

(a) byjusexamprepwithout monsoon storage

 (b) byjusexamprep without monsoon storage




Destruction and Removal Efficiency (DRE)



 byjusexamprep The mass fill rate of one POHC (Principal organic Hazardous constituent) in the waste stream.

 byjusexamprep Mass emission rate of the same POHC present in the exhaust emission prior to release to the atmosphere.

Aeration Tank (ASP)





(i) Detention period, byjusexamprep 


 V = Volume of the tank in m3.

 Q = Quantity of wastewater flow into the aeration tank excluding the quantity of recycled sludge (m3/day)

(ii) Volumetric BOD Loading or Organic Loading, (U) byjusexamprep 


 byjusexamprep Mass of BOD applied per day to the aeration tank through influent sewage in gm.

 byjusexamprep The volume of the aeration tank in m3.

 byjusexamprep Sewage flows into the aeration tank in m3.

 byjusexamprep BOD5 in mg/lit (or gm/m3) of the influent sewage.

(iii) byjusexamprep 


byjusexamprep Food (F) to Microorganism (M) ratio byjusexamprep Daily BOD applied to the aeration system in gm.

byjusexamprep 5 day BOD of the influent sewage in mg/lit.

byjusexamprep The flow of influent sewage in m3/day.

byjusexamprep MLSS (Mixed liquor suspended solids) in mg/lit.

byjusexamprep The volume of the Aeration Tank (lit).

byjusexamprep Total microbial mass in the system in gm.

(iv) Sludge Age byjusexamprep 

 (a) byjusexamprep 

 (b) byjusexamprep 


byjusexamprep The concentration of solids in the influent of the Aeration Tank called the MLSS i.e. mixed liquor suspended solids in mg/lit.

V = Volume of Aerator

byjusexamprep The volume of waste sludge per day

byjusexamprep The concentration of solids in the returned sludge or in the wasted sludge (both being equal) in mg/lit.

byjusexamprep Sewage inflow per day.

byjusexamprep The concentration of solids in the effluent in mg/lit.

(v) Sludge Volume Index (S.V.I)



byjusexamprep Concentration of suspended solids in the mixed liquor in mg/lit.

byjusexamprep Settled sludge volume in ml/lit.

byjusexamprep Sludge volume index in ml/gm.

(vi) Sludge Recycle and Rate of Return Sludge




byjusexamprep Sludge recirculation rate in m3/day.

byjusexamprep MLSS in the aeration tank in mg/lit.

byjusexamprep MLSS in the returned or wasted sludge in mg/lit.


 S.V.I = Sludge volume index in ml/gm.

·   Specific substrate utilization rate


 byjusexamprep for MLSS and 0.6 for MLVSS, byjusexamprep 

·  Oxygen Requirement of the Aeration Tank




· Oxygen Transfer Capacity (N)



 byjusexamprep Oxygen transferred under field conditions in kg O2/k.wh (Or MJ)

 byjusexamprep Oxygen transfer capacity under standard conditions in kg O2/kwh (or MJ)

 byjusexamprep Dissolved oxygen-saturation value for sewage at operating temperature.

 byjusexamprep Operation D.O level in Areation tank usually 1 to 2 mg/lit.

 byjusexamprep Temperature in oC

 byjusexamprep Correction factor for oxygen transfer for sewage usually 0.8 to 0.85.

Oxidation Ponds

· Depth  1.0 to 1.8 m.

·  Detention period 2 to 6 weeks.

·  Organic loading 150 to 300 kg/ha/day.

 Under hot condition 60 to 90 kg/ha/day.

 Under cold conditions.

·   Length to width ratio = 2

·  Sludge Accumulation = 2 to 5 cm/year

·  Minimum depth to be kept = 0.3 m.

For Inlet Pipe Design

 Assume V = 0.9 m/s

 Assume flow for 8 hrs.

For Outlet Pipe Design

 Dia of outlet = 1.5 dia of the inlet pipe


Septic Tank

·  Detention time = 12 to 36 hr.

·   Sludge accumulation rate = 30 lit/cap/year.

·   Sewage flow = 90 to 150 lit/capita/day.

·   Cleaning period = 6 to 12 months

·   Length to width ratio = 2 to 3 m.

· Depth = 1.2 to 1.8 m

· Width 0.9 m.

·   Free board = 0.3 m.                 byjusexamprep 



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