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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, resid- ual 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 de-
tention 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 con guration. 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 ecient and durable.

1. Introduction

The septic tank system is the most widely used onsite system for wastewater treatment especially in developing countries where the cost of central
wastewater treatment facilities is prohibitive. In the United States only, over 50 million people use the septic system [1]. According to [2], over 46%
of the Nigerian population use the septic tank system. Given enough detention time, the septic tank can achieve as much as 81% total suspended
solids removal, 68% BOD removal, 65% phos- phate removal and 66% fecal coliform removal [3,4]. In Nigeria, septic tanks are rarely designed,
rather, most contractors resort to arbitrary sizing or adopt the speci cations of the Public Works Department [5] shown in Table 1 or other local
government speci cations. The speci cations on this table are based on 1 day detention time and a wastewater ow of 0.114m3/capita/day. This
speci cation is not realistic as a septic tank sized for 1 day detention time will need desludging fre- quently.
The code recommended that a septic tank serving 10 people should have a dimension of 2.032m (length), 0.457m (width) and 1.22m (depth) giving a total volume of 1.13m3. Even if the usual constant sludge accumulation rate of 0.04m3/capita/year [6,7] is assumed, in three years the tank will be over owing with sludge (1.2m3). This implies that the tank will need desludging about every two years. Compare this with the recommendations in Table 2 [8]. For instance, they recommended a tank of 2000 US gallons (7.57m3) for a four bedroom house (see Table 2). Obviously this is a very long shot from the meager 1.13m3 recommended by PWD for a septic tank serving 10 people.
The septic tank is a very vital aspect of waste management and public health that merits more than casual sizing. Every septic tank is unique
and must be designed to maintain minimum con- ditions. In this regard, [9] noted that the key to e ective sewage treatment is proper design, in-
stallation, periodic maintenance and responsible  operation. The most critical parameter in septic tank design and operation is the detention time.
At any point in time, the detention time must be sucient to allow solid particles to settle, other- wise, its performance will be impaired.

2. Development of Design Approach

The design approach developed in this study is based on critical parameters viz: residual depth which depends on the residual detention time, sludge depth which determines desludging inter- val and reserve space which helps accommodate intermittent overloading and malfunctioning.

 2.1. Residual depth

As sludge accumulates in the septic tank, the depth of the tank decreases. In the design of sep- tic tank, it is necessary to specify a minimum residual depth and a minimum residual detention time in order to maintain the eciency of the  tank above a threshold limit. These conditions should be the prevailing conditions at desludg- ing. Considering a septic tank of plan area A, receiving wastewater at the rate of Q m3/s per capita which is required to maintain a minimum residual detention time, re days and a minimum residual depth per capita hre (m), the following relationship holds.



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