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The overland flow coefficient is the average area will be needed to handle wastewater flows fraction of applied wastewater flowing over the soil during harvest operations. Each terrace should be surface. The purpose of this coefficient is to convert harvested on a rotating basis. The length of the the average overland flow rate q to an application rate drying period before harvest will depend on local (Q,). It can be calculated from the relationship climatic conditions and should be long enough to allow harvesting machinery to drive over the terrace r= 1.0+f (17) without rutting the surface. In most cases,a week 2 should be adequate. An alternative to increasing the size of the wetted area would be to temporarily where f is the runoff fraction. increase the application time to the remaining terraces. The annual operating time Y will vary depending Additional land will also be needed for buildings, on the application cycle and season. The application access roads and buffer zones. Aly et al. (1979) cycle is the number of hours per day and days per indicated that this additional non-wetted area is week that wastewater is being applied to the terraces. usually 25% to 30%of the sum of wetted and non- Application cycles normally range from 6 to 8 hours wetted areas. The total area of the overland flow of continuous application per day,5 to 7 days per system should also include the land needed for a week. Obviously, the land area required for the storage pond if necessary. system can be reduced substantially by choosing a longer application cycle. No deleterious effects on performance were noted at Utica,Mississippi (Peters DESIGN EXAMPLE et al. 1981) when pond effluent was applied 24 hours per day, 7 days per week. However, shorter applica- The previously outlined design procedure can tion cycles of 8 to 10 hours per day are recommended best be explained with an example. In this example, if raw wastewater or primary effluent is applied, an overland flow facility is being considered for a Shorter application cycles will reduce the rate of small town in upstate New York. The design flow is solids accumulation at the top of the terrace and 3785 m3 day' (1.0 million gal.day'). Because of allow these solids to degrade more rapidly because its northern location the facility will have a holding of aeration during the off period. pond to store wastewater during the winter. The The application season is the number of weeks holding pond effluent and raw wastewater will be per year that the system can be expected to operate. mixed prior to application. The expected quality In southern areas the application season may extend of this mixture is 150 mg L-' BOD and 100 mg L' throughout the year because short periods of cold TSS. The discharge limits for this facility are 20 mg weather will normally not affect performance (Aly L' BOD and 20 mg L-' TSS. The soils in the area et al. 1979). In northern areas, the application season are slowly permeable so that the runoff fraction is usually coincides with the growing season. During expected to be only 0.6. The mass BOD and TSS the non-growing season, wastewater is stored in a removals required to meet the discharge limits are pond or lagoon. Martel et al. (1980) found that the EPA-1 computer program provided a good estimate (1.0 x 150-0.6 x 20) 100 of the number of storage days needed for overland %BOD removal = 1.0 x 150 flow systems. Step 3: Calculate the land area = 92% Since the length of terrace has already been spec- /o o T55 removal = (1.0 x 100-0.6 x 20) 100 ified, the only remaining dimension needed to cal- 1.0 x 100 culate the land area is width. Width can easily be determined by dividing the annual volume of waste- = 88%. water applied by the annual application rate Qa. If a storage pond is included in the design, the annual From Figure 6, the detention time needed to volume of wastewater applied should be adjusted to remove 92% of the BOD is 60 minutes. From Figure reflect the net volume of water entering or leaving 8, the detention time needed to remove 88%of the the pond due to precipitation and evaporation. TSS is 40 minutes. Since BOD removal is the limiting The land area calculated by this procedure is parameter, the design is based on a detention time of only part of the total wetted area. Additional wetted 60 minutes. 15