Aerated lagoons are basins where wastewater can be treated in a flow-through only manner or without solids recycling. Lagoon depths vary from 1 to 4 m (Ramalho 1983). Oxygenation of the wastewater in lagoons is usually accomplished by surface, turbine, or diffused aeration. The turbulence created by aeration keeps lagoon contents suspended. Depending on the retention time, the aerated lagoon effluent contains approximately one-third to one-half the value of the incoming BOD in the form of cellular mass. Wastewater treatment facilities can use a settling basin (see Figure 8.1.1 in Chapter 8) or tank for solids removal, by settling, from the effluent prior to discharge.
In designing an aerated lagoon, environmental engineers must incorporate the following parameters: (1) BOD removal, (2) effluent characteristics, (3) temperature effects, and (4) oxygen requirements. The design basis for a lagoon can be the mean cell residence time since the aerated lagoon is a completely mixed reactor without recycling. Selected mean cell residence time should ensure that the suspended biomass has good settlement properties, and be high enough to prevent cell wash-out. Typical design mean cell residence time for lagoons treating domestic waste varies from 3 to 6 days.
From the mean cell residence time, environmental engineers can estimate soluble substrate concentration in the effluent and determine the removal efficiency from substrate utilization equations used in activated-sludge process design. Alternatively, they can assume a first-order removal function for the observed BOD5 removal in a single aerated lagoon (Metcalf and Eddy, Inc. 1991) as follows:
S = effluent BOD5 concentration, mg/l So = influent BOD5 concentration, mg/l k = overall, first-order, BOD5, removal-rate constant, day—1 V = volume, l Q = flow rate, l/day
The values for the removal-rate constant k vary from 0.25 to 1.0. Effluent characteristics of significance are the BOD5 and the SS concentration. Environmental engineers can estimate both of these characteristics using the equations presented in Section 7.25 for calculating similar parameters in an activated-sludge effluent.
The effect of temperature on biological activity is described in Section 7.22. When influent wastewater temperature, ambient temperature, lagoon surface area, and wastewater flow rate are known, environmental engineers can estimate the resulting temperature in the lagoon using the following equation (Metcalf and Eddy, Inc. 1991):
Ti = influent wastewater temperature Tw = lagoon wastewater temperature Ta = ambient temperature f = a proportionality factor that incorporates heat transfer coefficients and the effects of surface area increase due to aeration, wind, and humidity (typical value for the eastern United States is 0.5 in SI units)
A = lagoon surface area Q = wastewater flow rate
Oxygen requirements are computed as outlined in the design calculations for aeration in the activated-sludge process (see Section 7.25).
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