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The results of this analysis indicate that the kinetic in the discharge permit, the mass of pollutant dis- relationships for BOD and TSS removal are valid for charged will be smaller than in conventional waste- overland flow systems receiving primary or raw waste- water treatment systems. As explained earlier, less water only. The lowest applied concentration of BOD wastewater is discharged from overland flow systems and TSS where these relationships hold is estimated than is applied. to be 45 mg L-'. Different kinetic relationships need The fraction of wastewater that reaches the runoff to be developed for overland flow systems receiving collection ditch can be estimated from local evapo- pond effluent. transpiration and percolation rates. Typically, the runoff fraction will range from 0.6 to 0.9 depending on Iotal climatic conditions and soil characteristics. DESIGN PROCEDURE The detention time used for design should be the longest time determined from Figures 6, 8,and 9. The primary purpose of the proposed design For equal removal percentages, the controlling design ' procedure is to properly size the overland flow sys- parameter is ammonia removal followed by BOD 'i tem so that it meets the quality requirements of the and TSS removal. For example, if 90016 ammonia, discharge permit. The three basic steps in this pro- BOD and TSS removal is required, the detention cedure are times needed are 68,57 and 40 minutes respectively: j 1. Determine the detention times required to In this case, the design should be based on ammonia remove pollutants specified in the discharge removal since it requires the longest detention time. permit. However, in most cases ammonia removal is not re- 2. Calculate the application rate needed to quired and BOD removal will be the controlling de- satisfy the longest or most critical detention sign parameter. time. 3. Calculate the land area required from the Step 2: Calculate the application rate application rate and system design flow. The annual application rate which will satisfy the detention time requirements determined in Step 1 ( Step 1: Determine the detention time can be calculated as follows: j The detention time required to achieve the de- sired removal of BOD,TSS and NH3-Ncan be de- Q =9- y (15) termined from Figures 6, 8,and 9. The phosphorus ' r { removal vs detention time relationship shown in Figure 10 is not used in this procedure because over- where Q.=annual application rate, m' yr' m' land flow is not considered to be an efficient process r=overland flow coefficient for removing this pollutant. As noted earlier, phos- 1'= operating time,hr yr r. phorus removal can be improved by chemical pre- cipitation with alum. The average overland flow rate q can be calculated To use Figures 6,8,and 9, the percentage removal by rearranging eq 13 so that of BOD,TSS and NH3-N must be calculated on a mass basis. Information needed for this calculation0.078 L 0.2 includes the design flow,an estimate of the applied q_S113 jr where q <S1 1313 (16 wastewater concentrations, desired runoff concen- trations and the runoff fraction. The values for L and S are selected by the designer Concentrations of BOD,TSS and NH3-N in the based on the topography of the potential site. Natural applied wastewater will depend on the degree of pre- terrain contours should be used to the maximum ex. treatment. In most cases it is advisable to use less tent possible to minimize cut and fill operations. pretreatment in order to reduce costs and take ad- However,careful surface preparation will still be vantage of the excellent treatment capabilities of, needed to ensure even flow distribution. overland flow. If the design includes a storage pond, The upper limit on q in eq 16 is necessary to the diluting effect of this effluent when mixed with avoid a situation where the application rate is so primary or raw wastewater should also be considered. high that it causes scouring. Typically this could The desired runoff concentrations can be deter- occur when a design calls for a short detention time mined from the discharge permit. It should be noted on a long terrace. The limitation placed on q was that by satisfying the concentration limits specified based on calculations of the scour velocity shown in Appendix C. 14