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A family of curves describing BOD and TOC removal as functions of slope length and application rate have been developed for raw wastewater and primary �I ' effluent (see Figures 4-1 through 4-4) and serve as the basis for system design. Either parameter may be used as the independent design variable. When designing a system, the required or desired level of treatment is known. Thus, a value for ((C-c)/Co) can be established. If slope length is used as the independent variable, then a design value for slope length (z) is selected. The required value of application rate (q) is then determined by entering the appropriate curve with the known values of z and (C-c)/Co. Alternatively, if application rate (q) is the independent design variable, then a value for q is selected and the required I, value of slope length (z) is taken from the curves. Which parameter to use as r the independent design parameter depends on the individual case. Site conditions I� may dictate the allowable slope length, in which case slope length would be the independent parameter and application rate would be the determined parameter. If slope length is not restricted, which is usually the case, then application rate should be used as the independent parameter. I� To complete the design of a system, the total area required for slopes I� must be determined. Total area required may be computed using the following I equation, assuming an application frequency of 7 days/wk. Area = �( ) (�z) (q)(PY (5-2) where Q = design daily flow, m3/day z = slope length, m q = application rate, m3/hr-m P = daily application period, hr/day It was demonstrated in the Phase I report (Smith et al. 1980) that the required area could be minimized by selecting a maximum value for either q or z and for P within the valid region of the model. The model is valid for application -62-