A Rational Approach to Septic Tank Design

Abstract

A new approach to the design of septic tanks was developed based on a number of critical parameters, namely: residual detention time, minimum residual detention time, residual depth and minimum residual depth. This method involved rst specifying a desired desludging interval. This interval was then substituted in a septage accumulation model to obtain the volume of sludge accumulated in this time interval. Using a minimum detention time of 24 hours and a desired minimum residual depth, the plan area of the tank was then determined and hence, the depth of sludge volume. The total depth of the tank was obtained as the sum of the sludge depth, residual depth and depth of reserve space. The length and width of the tank were also obtained from the plan area by using a length to width ratio (L/W) ranging between 2 to 4 or any other range depending on land conguration. Design charts and a Microsoft Excel based design programme were produced to aid the design of septic tanks. A predetermined desludging interval ensures septic tanks are effcient and durable.