In this article we will discuss about the treatment and disposal of septic tank effluent.
Although sewage under goes treatment in a septic tank, the effluent may still contain pathogenic organism and hence septic tank effluents cannot be considered safe. The effluent coming from the tank will be malodorous and hence more objectionable than the liquid that goes into the tank. This does not mean that the tank has not purpose or value.
Its primary purpose is to condition the sewage so that it will less clog the dispersal field or make more amenable to other treatment. Final purification of the effluent and the removal and death of pathogens is effected by percolation of effluent through the soil or other media.
Usually the disposal of effluent may be either underground or over ground. Normally underground disposal either in the form of soak pit or dispersion trenches is practised. Both the methods are designed to achieve subsurface percolation characteristics of the soil.
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Other factors such as level of sub-soil water table, climatic conditions, presence of vegetation, deration of soil and concentration of suspended solids in the effluent also influence the application of these methods. Soak pits and dispersion trenches can be used in all the porous soils where percolation rate is below 25 minutes per cm and the depth of water table is 180 cm or more from the ground level.
Dispersion trenches are preferred in soils with percolation rates between 12-25 minutes, if sufficient land is available. In the areas with higher water table, the dispersion trenches should be loaded partly or fully above ground level, in a maund.
The total subsurface area required for soak pits or dispersion trenches is given by the following empirical formula:
Q = 130 t
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Where Q = maximum rate of effluent application in lpd/m2 of leaching surface
t = standard percolation rate for the soil in minutes.
While calculating effective leading area, only area of trench bottom in case of dispersion trench and effective side wall area below the inlet level for soak pits should be taken into account.
Tank effluent of very large tanks is only given secondary treatment by passing it through biological filters. Effluent of small septic tank is not given any type of treatment before its disposal.
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The septic tank effluent can be disposed off in any one of the following ways:
(i) By subsurface irrigation.
(ii) By surface irrigation.
(iii) By discharging into nearby water courses.
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(iv) By soil absorption system.
The soil absorption system has the following method for the absorption of effluent:
(a) Soakage Pit:
This is also known as seepage pit. These are circular pits more than one metre in diameter and 1 m in depth below the invert of the inlet pipe. These pits are lined with dry bricks or stone and are filled with brick-bats or coarse aggregate more than 7.5 cm, size. In the case of large pits the top portion is reduced in size for the reduction in the size of the R.C.C. cover Fig. 21.8 shows the section through a soakage pit.
(b) Leaching Cesspool:
The same cesspool can be used for soaking the effluent of septic tanks. In this cesspool the bottom is made water-tight to retain the effluent get dispersed into the surrounding soil. The open joined lining is surrounded by 15 cm coarse aggregate of 4-5 cm in size and an outer casing of 30 cm thick sand for the better distribution of supernatant effluent in the soil also provided. Fig.21.9. shows the section through a Leaching cesspool.
(c) Dispersion Trench:
These are also called soakage trenches. Fig. 21.10 shows the plan and section through dispersion trenches. The dispersion trenches should be 50 to 100 cm deep and 30 to 100 cm wide excavated to a light gradient and are tilled with 15-25 cm of washed gravel and cursed stones. Open jointed pipe of unglazed earthenware or concrete are laid down inside the trenches.
The maximum internal diameter of these pipes should not be more than 75-100 min. Each dispersion trench should not be longer than 30 m. The dispersion trenches should not be placed closer than 1.8 m. These pipes should be covered by 15 cm coarse aggregate over which graded aggregate is filled upto 15 cm above the general ground level as shown in Figs. 21.10 (a) and (b).
The performance and capacity if the soakage-trenches can be improved by keeping them away from the shady trees, and planting pararie to grass which have a high rate of evapotranspiration.
