Design considerations for aerated lagoons

DESIGN CONSIDERATIONS

• The following factors  should be kept in mind when designing an aerated lagoon

1. The desired quality of effluent is based on concentration of solids to be maintained in the effluent
2. BOD5 or COD removal gives the efficiency of substrate conversion
3. Quantity of oxygen to be supplied
4. Power needed for mixing and supply of oxygen
5. If the temperature variations between summer and winter are significant, the effect of temperature should be considered.
6. The design criteria required for designing an aeration system are:
1. Detention time: Suspended growth aerated lagoons are designed on the basis of Hydraulic Retention Time (HRT) and Mean.Cell Residence Time (mCRT) or Solids Retention Time,𝜃c (SRT). Typical design value of SRT or HRT for suspended growth aerated lagoon is ~ 3 to 6 days while for facultative aerated lagoon is ~ 4 to 10 days.
2. Oxygen requirement: The oxygen required for oxidising organic solids varies from 0.7 to 1.4 kg of oxygen per kg of BOD5 removed. Oxygenation capacity of aerators varies from 1.85 to 2.0 kg O2 per kW of power delivered under standard conditions. Oxygenation capacity of aerator at field conditions is given by the equation:                                               N = [Ns (Cs - CL) * 1.024^(T-20a)] / 9.2
3. Power requirement: Power required for mixing the contents of the aerated basin varies from 0.8 to 1.0 kW/1000 cu.m of basin volume. The power required to keep bio-solids in suspension is 1.5 to 1.75 kW/1000 cu.m and the power required to keep ALL solids under suspension is 15 to 18 kW/1000 kW/1000 cu.m
7. The assumptions for the design of an aeration system are:
1. Oxygen content = 2302%
2. Diffuser efficiency = 30 to 50% (or as per manufacturer specifications) 
3. Field Oxygen Transfer Efficiency = 50% (or as per manufacturer specifications) 
4. Air weight (density) = 1.201 kg/cu.m for aerobic reactors
8. Criteria adopted in design of aerated lagoons:
1. mCRT = 3 to 6 days without recycle for domestic wastewaters and 10 to 30 days with recycle for domestic wastewater
2. Oxygen requirement = 0.7 to 1.4 * BOD5 removed, kg/d
3. Solids concentration in lagoon  = 30 to 300 mg/L for aerobic flow through type                                                              = 30 - 150 mg/L for facultative type and                                                                        = 4000 to 5000 mg/L for extended aeration type
4. Hydraulic detention time = 2 to 10 days for aerobic flow through type  

                                                                   = 3 to 20 days for for facultative type and                              
                                                                   = 0.2 to 7 days for extended aeration type
                           5. Depth of lagoon = 2 to 5m
                           6. Power required for oxygen supply = 1 to 8 HP/1000 cu.m of basin volume
                           7. Oxygen transfer capacity of surface aerators = 1.85 to 2.0 kg O2/kW h at standard     
                                                                                                      conditions
                9. Lagoon surface area is assumed rectangular in the ratio 1.75 - 1.95:1 (Length:Width)

               10.Dispersion number (D/UL) = 0.2 to 1.0 for rectangular or long lagoons and 2 to 4 and above 
                                                                 for squarish lagoons
               11. BOD5 removal efficiency = k * t  where k = 0.776
               12. %BOD removal efficiency is determined using the graph showing wehner-wilhelm equationc   
                      for substrate removal efficiency based on dispersed flow model
               13. Sludge accumulated is determined using a cleaning interval of 3 years
               14. Dispersion number (D/UL) generally lies between 0.1 and 0.4
                       if (D/UL) ~ 0.2 or less the reactor is a plug flow reactor
                       if (D/UL) ~ 3.0 to 4.0 the reactor is well mixed or is approaching complete mixing
 USEFUL FORMULAE:

Power = Work/Time and the units of power is W (watt)

Watt = Joules/second

Horse Power (HP) = 750 W

Power = Force * Displacement / Time = Force * Velocity

V = Q * t

Hydraulic loading or surface loading rate or Overflow rate = flow (m3/d) / surface area (m2)

Weir overflow rate = flow rate / total weir length

Organic loading = Applied kg of BOD per day / Volume of tank

Mean Cell Residence Time (mCRT) or Solids Retention Time (SRT) = 

Biomass in reactor / Biomas removed from reactor

N = [Ns (Cs - CL) * 1.024^(T-20a)] / 9.2

N = Oxygen transferred under field conditions in kg O2/h

Ns = Oxygen transfer capacity under standard conditions in kg O2/h

Cs = Dissolved Oxygen saturation value for sewage under operating temperature

CL = Operating Dissolved Oxygen level in aeration tank 1 to 2 mg/l

T = Temperature in oC

a = Correction factor for oxygen transfer for sewage (0.8 to 0.85)