ENVIRONMENTAL ENGINEERING : Treatment of Sewage Notes

By Deepanshu Rastogi|Updated : April 19th, 2021

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     

 Treatment of Sewage

 Sedimentation Tank

Settling Velocity

(i) byjusexamprep

  for d < 0.1 mm

 Where,

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

 d= The diameter of the particle in the meter.

 G= Specific gravity of the particle.

 v= Kinematic viscosity of water in m2/sec.

 byjusexamprep

byjusexamprep  Dynamic viscosity

 byjusexamprep Density

(ii)byjusexamprep

byjusexamprep

where, 

 → For laminar flow

byjusexamprep

 byjusexamprep Reynolds number   

  for transition flow.

byjusexamprep(when flow is transition)

  for turbulent flow.

byjusexamprep(when flow is turbulent)

(iii)

 byjusexamprep

(iv) Newtons Equation for Turbulent Settling

 byjusexamprep

(v) Modified Hazen’s Equation for Transition Zone

 (a)  

byjusexamprep

 Where T = Temperature in oC.

 (b) Putting G = 2.65 for Inorganic Solids

  byjusexamprep

 (c) Putting G = 1.2 for Organic Solids

  byjusexamprep

Critical Scour Velocity in Constant Velocity Horizontal Flow Grit Chamberbyjusexamprep  

  byjusexamprep

PROPORTIONAL FLOW WEIR

 byjusexamprep

byjusexamprep

  

 Where,

  B= Width of the channel.

Vh = Horizontal flow velocity.

 CD =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

 Where,

  W=Width of the throat in the meter.

  Q=Flow in (m3/sec) through Parshall flume.

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

byjusexamprep 

(ii) Parabolic Grit Channel

  byjusexamprep

 Where,

 n = Discharge coefficients of the control section.

 = 1.5 for partial flume.

 = 1 for proportional flow weir.

  • Aerated Grit Channels

  byjusexamprep

  • Detritus Tank

 byjusexamprep

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

 Where,

 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.

Vacuators

 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.

 (vii) byjusexamprep

 Where,

 V = Flow velocity

 B = Width of the Basin

 H = Depth of sewage in the tank.

 (viii)  byjusexamprep

 (ix)  byjusexamprep

byjusexamprep

 

  • Detention Time

(a)  For rectangular Tank

byjusexamprep

(b)  for circular tank

byjusexamprep

Where

d = Dia of the tank

H = Vertical depth of wall or side depth

  • Displacement Efficiencybyjusexamprep

  byjusexamprep

Trickling Filter

 (a) Conventional Trickling Filter or Low Rate Trickling Filter

  byjusexamprep

 Where,

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

 Where,

 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

 Where,

 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.

 byjusexamprep

byjusexamprep

 Dunbar Filter

 Surface loading = 25000 MI/m2/day.

 BOD removed = 85%

 Sludge and its Moisture Content

  byjusexamprep

 byjusexamprepThe 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

 Where,

 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)byjusexamprep with monsoon storage

Where,

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

(ii) When the change during digestion is parabolic

(a) without monsoon storage

byjusexamprep

 (b)  without monsoon storage

byjusexamprep

 byjusexamprep

 Destruction and Removal Efficiency (DRE)

byjusexamprep  

 Where,

  byjusexamprepThe 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)

byjusexamprep 

(i) Detention period,  

byjusexamprep

 Where

 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

 Where,

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

  byjusexamprepThe volume of the aeration tank in m3.

 Q= Sewage flows into the aeration tank in m3.

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

(iii)  byjusexamprep

 Where,

 (F/M) Food (F) to Microorganism (M) ratio

byjusexamprep Daily BOD applied to the aeration system in gm.

 byjusexamprep5 day BOD of the influent sewage in mg/lit.

 Q=The flow of influent sewage in m3/day.

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

 V=The volume of the Aeration Tank (lit).

 byjusexamprepTotal microbial mass in the system in gm.

(iv) Sludge Age byjusexamprep 

 (a)  byjusexamprep

 (b)  byjusexamprep

 Where,

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

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

 Q=Sewage inflow per day.

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

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

  byjusexamprep

 Where,

 byjusexamprepConcentration of suspended solids in the mixed liquor in mg/lit.

byjusexamprep Settled sludge volume in ml/lit.

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

(vi) Sludge Recycle and Rate of Return Sludge

  byjusexamprep

byjusexamprep

 Where,

 byjusexamprepSludge 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.

 byjusexamprep

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

  • Specific substrate utilization rate

  byjusexamprep

 byjusexamprep for MLSS and 0.6 for MLVSS,  

byjusexamprep

  • Oxygen Requirement of the Aeration Tank

 byjusexamprep

 Where,

  byjusexamprep

  • Oxygen Transfer Capacity (N)

  byjusexamprep

 Where,

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

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

  Ds =Dissolved oxygen-saturation value for sewage at operating temperature.

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

 T= Temperature in oC

  byjusexamprepCorrection 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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