Design Criteria

The principal design criteria for the OF process are application rate and slope length. Other design criteria include hydraulic loading rate, application period, and organic loading rate. The relationship between hydraulic loading rate and application rate is shown in Equation 8.8:


Lw =

Hydraulic loading rate (in./d; mm/d).

q =

Application rate per unit width of slope (gal/ft-min; m3/m-min)

P =

Application period (hr/d).

F =

Conversion factor (96.3).

Z =

Slope length (ft; m).

Application Rate

The application rate used for design of municipal OF systems depends on the limiting design factor (usually BOD), the preapplication treatment, limitations, and the climate. A range of suggested application rates is presented in Table 8.14. The lower end of the range shown in Table 8.14 should be used for cold climates and the upper end for warm climates. The relationship between application rate and slope length is shown in Figure 8.6. As mentioned previously, facultative ponds are not recommended as preapplication treatment for OF. If OF is used in conjunction with facultative ponds, however, the application rate should not exceed 0.12 gal/min-ft (0.10 m3/hr-m).

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