Septic Tank

A well-designed septic tank must fix a minimum residual detention time at which it becomes necessary to dislodge the tank. Many standards usually specify 24 hours. A minimum residual depth per occupant corresponding to the chosen residual detention time should also be specified. The overall residual depth is a result of the product of the residual depth per occupant and the number of occupant’s.For good design and for practical purposes, the overall residual depth value should not be lesser than 10cm and not more than 75cm. A very low residual depth will help in washing out the sludge and also interfere with inlet and outlet fittings while too high residual depth will result in a tank that is of low length and depth ratio which will be inefficient. Narrow tanks are opted to provide quiescent hydraulic conditions which favour settling and thus solids removal determine the volume of sludge that will accumulate in that period of time. The depth of sludge in the tank at this time is then obtained by dividing the volume of accumulated sludge with the plan area obtained as described above. The total depth of the tank is considered as the sum of sludge depth. Dividing the volume of sludge by the plane area gives the depth of the sludge. Thus the total depth of the tank is the sum of sludge depth, overall residual depth and depth of reserve volume. The depth of the reserve space should be equal to the residual depth since it is based on 24 hours detention time. If the overall depth of the tank is much higher than the length, a lower overall residual depth should be chosen and the design repeated residual depth and the depth of the reserve space. The reserved space must be equivalent to the volume of 24 hours of detention time. At last, the ratio of length to width is chosen and thus the length and the width can be determined. The length must be longer than the width so as to provide for quiescent conditions. Every septic tank is unique and therefore should be designed taking cognizance of the number of users, desired desludging interval and expected wastewater flow which is a function of water availability. It is essential to know when to expect to dislodge their tanks. This should be an intrinsic aspect of the design. Septic tanks should have enough initial volume for long term storage of sludge to avoid frequent desludging. Tanks with small volumes will soon get silted up with sludge thus requiring frequent desludging. People should not wait for their septic tanks to be overflowing with sludge before desludging as this reduces the life span of the whole system and also reduces the efficiency of the drain field or soak pit.

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