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160 <br /> 36 SEPTIC TANK. SYSTEt EFFECTS ON GROUND WATER QUALITY <br /> Table 13: Setback Requirements for Drainfields (Cotteral and Norris, <br /> 1969) <br /> ■ <br /> S•tbaek keqult.eent. (feet) <br /> Ped.ral Onlfora San Sant, Santa Contra hsrtn <br /> Douala Plumbing hates) Cfln Clara Coot. County <br /> authority Code County County County County <br /> j <br /> Dr.lnfleld. To: <br /> ~ building, S e S S <br /> 10 10 10 <br /> Property lines 5 S 10 <br /> S 10 5 5 <br /> i; w11, 100 50 75 <br /> 100 100 50 100 <br /> iv Creek, or Stream. 20 100 50 25 <br /> - ! SO .. <br /> Cuts or embankments ■ - 20 15 « <br /> so 25 <br /> Pool. <br /> . - 25 - 25 <br /> Mater lines lD s - 5 - 10 - <br /> Valk. and drive, <br /> Large er.e, - • 10 - - <br /> 10 <br /> considered` prior to'designing the soil absorption system. Three types of <br /> drainfield loading can be utilized, including continuous ponding, dosing and <br /> resting, and uniform application without ponding. In the continuous ponding <br /> method the infiltrative surface is covered at all times with wastewater. <br /> This method has the advantage of increasing the effective infiltrative area <br /> f by submerging the sidewalls of the drainfield trenches. It also increases the <br /> hydraulic gradient across the infiltrative surface and this in turn may <br /> increase the infiltration rate. However, since clogging occurs at the <br /> infiltrative surface there will be no aeration at the surface and this may <br /> cause subsequent problems in terms of both hydraulic flow and biological <br /> decomposition. <br /> • <br /> To overcome some of the concerns related to continuous ponding, <br /> dosing and resting can be utilized. This approach provides reaeration in that <br /> periods of loading are followed by periods of resting. The resting phase <br /> allows the soil to drain and reaerate, thus encouraging degradation of the <br /> }' clogging mat which may build up at the infiltrative surface. <br /> . s The process of alternate dosing and resting of a drainfield can <br /> therefore markedly prolong the effective life of the system. For practical <br /> purposes the resting period required for restoration appears to be on the <br /> • <br /> order of several months (Cotteral and Norris, 1969). In uniform application <br /> without ponding the liquid is distributed •uniformly over the -entire <br /> infiltrative surface at a rate lower than that which the soil can accept <br /> liquid. Therefore, the soil always remains unsaturated and aerobic <br /> ..: iconditions prevail at the infiltrative surface. When these aerobic conditions <br /> prevail, the resistance of the-clogging mat is minimized. <br /> • j <br /> i <br /> • <br /> • <br />