S =

effluent substrate concentration, mg/l

So =

influent substrate concentration, mg/l

a =

(1 + 4ktd)1/2

d =

dispersion factor (D/uL)

D =

axial dispersion coefficient, (m2/h)

u =

fluid velocity (m/h)

L =

characteristic length (m)

k =

first-order reaction constant (h-1)

t =

retention time (h)

The term kt in Equation 7.27(1) can be plotted as a function of S/So for various dispersion factors—varying from zero for PF reactors to infinity for completely mixed reactors—to yield a graph that facilitates the use of the equation in designing ponds (Metcalf and Eddy, Inc. 1991). The dispersion factor ranges from 0.1 to 2.0 for most stabilization ponds. For aerobic ponds, the dispersion factor is approximately 1.0 since completely mixed conditions usually prevail in these ponds for high performance. Depending on the operational and hydraulic characteristics of the pond, typical values for the overall firstorder BOD5 removal-rate constant k range from 0.05 to 1.0 per day (Metcalf and Eddy, Inc. 1991).

Although aerobic pond efficiency is high—up to 95%— and most soluble BOD5 is removed from influent waste-water, bacteria and algae in the effluent can exert a BOD5 higher than that of the original waste. Hence, wastewater treatment facilities must apply methods of removing biomass from the effluent.

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