Treatment of Sewage Notes | Study Environmental Engineering - Civil Engineering (CE)

Sedimentation Tank

Settling Velocity

1. 
   
   for d < 0.1 mm
   Where,
   V_s = 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.
    Y = Dynamic viscosity
   δ = Density
2. 
   
   where,
   
   → For laminar flow
   R_e = Reynolds number
   
    for transition flow.
    for turbulent flow.
3. 
   
4. Newtons Equation for Turbulent Settling
   
5. Modified Hazen’s Equation for Transition Zone
   (i)
   
   Where T = Temperature in ^oC.
   (ii) Putting G = 2.65 for Inorganic Solids
   
   (iii) Putting G = 1.2 for Organic Solids
   

Critical Scour Velocity in Constant Velocity Horizontal Flow

Grit Chamber (V_H)

Proportional Flow Weir

Where,
B = Width of the channel.
V_h = Horizontal flow velocity.
C_d = Coefficient of discharge.
x and y are coordinates on weir profile.

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

1. Parshall Flume
   
   Where,
   W = Width of the throat in the meter.
   Flow in (m³/sec) through Parshall flume.
   H_a = Depth of flow in the upstream leg of a flume of one-third portion in the meter.
   
2. Parabolic Grit Channel
   
   Where,
   n = Discharge coefficients of the control section.
   = 1.5 for partial flume.
   = 1 for proportional flow weir.
   (i) Aerated Grit Channels
   
   (ii) Detritus Tank
   

Skimming Tank

1. Detention Period = 3 to 5 minutes.
2. Amount of compressed air required = 300 to 6000 m³ per million liters of sewage.
3. Surface Area,
   
   Where,
   q = Rate of flow of sewage in m3/day.
   V_r = 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.

1. 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)
   Where,V = Flow velocity
   B = Width of the Basin
   H = Depth of sewage in the tank.
   (viii)
   
   (ix)
   
   
2. Detention Time
   (i)
   
   For rectangular Tank
   (ii)
   
   for circular tank
   Where
   d = Dia of the tank
   H = Vertical depth of wall or side depth
3.  Displacement Efficiency (η)
   

Trickling Filter

1. Conventional Trickling Filter or Low Rate Trickling Filter
   
   Where,
   η = The efficiency of the filter and its secondary clarifier, in terms of % of applied BOD
   u = Organic loading in kg/ha-m/day applied to the filter (called unit organic loading)
2. High Rate Trickling Filter
   (i)
   
   Where, F = Recirculation factor
   Recirculation ratio
   (ii)
   
   Where,
   Y = Total organic loading in kg/day applied to the filter i.e. the total BOD in kg.
   Y/VF = Unit organic loading in kg/Ha-m/day
   V = Filter volume in Ha-m.
   % efficiency of single-stage high rate trickling filter.
   (iii)
   
   Where,
   n' = Final efficiency in the two-stage filter.
   Y' = Total BOD in the effluent from the first stage in kg/day.
   F' = Recirculation factor for second stage filter
   V' = Volume in second stage filter in ha-m.
   
   
   Dunbar Filter
   Surface loading = 25000 MI/m²/day.
   BOD removed = 85%
   Sludge and its Moisture Content
   
   The volume of sludge at moisture content P₁%
   The volume of sludge at moisture content P%

Sludge Digestion Tank

1. When the change during digestion is linear.
   (i)
   
   Where,
   The volume of digestion in m³.
   Raw sludge added per day (m./day)
   Equivalent digested sludge produced per day on completion of digestion, m³/day.
   Digestion period in the day.
   (ii)
   
   with monsoon storage
   Where,
   T = Number of days for which digested sludge (V₂) is stored (monsoon) storage)
2. When the change during digestion is parabolic.
   (i)
   
   without monsoon storage
   (ii)
   
   without monsoon storage
   

Destruction and Removal Efficiency (DRE)

Where,
W_in = The mass fill rate of one POHC (Principal organic Hazardous constituent) in the waste stream.
W_out = Mass emission rate of the same POHC present in the exhaust emission prior to release to the atmosphere.

Aeration Tank (ASP)

1. Detention period,
   
   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)
2. Volumetric BOD Loading or Organic Loading, (U)
   
   Where,
   QY_o = Mass of BOD applied per day to the aeration tank through influent sewage in gm.
   V = The volume of the aeration tank in m³.
   Q = Sewage flows into the aeration tank in m³.
   BOD₅ in mg/lit (or gm/m³) of the influent sewage.
3. 
   
   Where,
   F/M = Food (F) to Microorganism (M) ratio QY_o = Daily BOD applied to the aeration system in gm.
   Y_o = 5 day BOD of the influent sewage in mg/lit.
   Q = The flow of influent sewage in m3/day.
   MLSS (Mixed liquor suspended solids) in mg/lit.
   V = The volume of the Aeration Tank (lit).
   M = X_tV = Total microbial mass in the system in gm.
4. Sludge Age (θ_c)
   
   Where,
   X_T = 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
   Q_w = The volume of waste sludge per day
   The concentration of solids in the returned sludge or in the wasted sludge (both being equal) in mg/lit.
   Q = Sewage inflow per day.
   X_E = The concentration of solids in the effluent in mg/lit.
5. Sludge Volume Index (S.V.I)
   
   Where,
   X_ab = Concentration of suspended solids in the mixed liquor in mg/lit.
   V_ab = Settled sludge volume in ml/lit.
   S.V.I = Sludge volume index in ml/gm.
6. Sludge Recycle and Rate of Return Sludge
   Q_R·X_R = (Q + Q_R) x R
   
   Where,
   Q_R = Sludge recirculation rate in m³/day.
   X_t = MLSS in the aeration tank in mg/lit.
   X_R = MLSS in the returned or wasted sludge in mg/lit.
   
   S.V.I = Sludge volume index in ml/gm.
   (i) Specific substrate utilization rate
   
   α_y = 1 for MLSS and 0.6 for MLVSS,  k_e = 0.66
   (ii) Oxygen Requirement of the Aeration Tank
   
   Where,
   
   (iii) Oxygen Transfer Capacity (N)
   
   Where,
   N = Oxygen transferred under field conditions in kg O₂/k.wh (Or MJ)
   N_s = Oxygen transfer capacity under standard conditions in kg O₂/kwh (or MJ)
   D_s = Dissolved oxygen-saturation value for sewage at operating temperature.
   D_L = Operation D.O level in Areation tank usually 1 to 2 mg/lit.
   T = Temperature in ^oC
   α = 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
• 
• Free board = 0.3 m.
  Volume of Septic Tank = (Sewage flow x Detention time) + (Sludge accumulation rate) x Clearning rate