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One surface aerator manufacturer's rule of thumb suggests that the HP/mg power density required for mixing with up-draft, direct-drive aerators is up to 1 HP/1,000 ft3 or about 133 HP/MG.


In practice both aerated lagoon approaches are frequently sized using simple complete mix hydraulic model and first order reactor kinetics. Considered relatively low mass or low rate technologies, removal is predicted basically as a function of detention time, temperature and wastewater characteristics using different sets of reaction rate coefficients. Industrial wastewater however, may not be amenable to easy formulation.

ESTIMATING OXYGEN UPTAKE IN AERATED LAGOONS

The following approach can be used to estimate oxygen requirements

Ro = (1 - b * Yg ) * Rs  + b * d * X ,     mg O2/L-hr          
           
 where: 

Rs = rate of COD conversion, mg COD/L-hr (usually COD load * removal efficiency), 
Ro = rate of oxygen uptake, mg/L-hr,
X = microorganism concentration, mgVSS/L
Yg = biomass yield coefficient, mass VSS/mg COD removed (usually 0.3 kg VSS/kg COD removed)
b         = 1.42
d = endogenous decay rate (usually 0.1/day)

     So for 42 mg COD/L-hr maximum COD loading rate ( = 1 g COD/L-d) and an assumed 2,000 mg/L VSS in the mixed liquor and an endogenous decay rate of 0.00417/hr (= 0.1/day), the oxygen uptake rate would be:

Ro = (1 - 1.42*0.3) * (42 * 90%) + 1.42 * 0.00417 * 2000 mg/L = 34 mg O2/L/hr 

     The aeration equipment would have to equal or exceed this rate to insure positive DO in the aeration basin. Of course, you will need to make sure you calculate the loading rates correctly in the zone of interest. For example, for plug flow type processes, the COD loading rate would be that in the first section of the basin. 

     Most manufacturers and equipment vendors have readily available software for preliminary designs and will generally assist you with proper equipment/unit selection including recommended/minimum/maximum depth, oxygen dispersion diameter, complete mix diameter and so on.

MIXING

     Adequate contact must be provided between organic wastewater constituents and the microorganisms. Effluent recycle can also aid in mixing.  Mechanical aerator manufacturers often provide sizing charts or layout guidelines including recommended water depth, oxygen dispersion diameter and complete mix diameter estimates, the following being sample formulas for low-speed, floating surface mechanical aerators:
Mixing diameter (feet)                  =  2 x ((  646 x H.P.)/DEPTH)^0.5
Oxygen dispersion diameter (feet) = 2 x ((6490 x H.P.)/DEPTH)^0.5

One surface aerator manufacturer's rule of thumb suggests that the HP/mg power density required for mixing with up-draft, direct-drive aerators is up to 1 HP/1,000 ft3 or about 133 HP/mg.

ACTUAL IN-WASTE/FIELD OXYGEN TRANSFER RATES

Rightly or wrongly, AOR/SOR ratios of about 0.7 are frequently used for quoting or budgeting direct-drive, mechanical surface aerators. However, until alpha, beta, theta, elevation and required residual D.O. are known the 0.7 factor is nothing but a glorified guess. It should be obvious you don't want this type of guessing especially in the light of present day availability of highly qualified treatability assessments. 
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