A septic tank or aqua-privy is simply a combined retention tank and digester; apart from losses through seepage and evaporation, the outflow from the tank equals the inflow. The effluent is anaerobic and may contain a large number of pathogenic organisms. Although the removal of suspended solids can be high in percentage terms, the effluent is still concentrated in absolute terms, and the need for safe disposal of septic tank effluents cannot be too strongly stressed.
The effluent from large tanks dealing with sewage from groups of houses or from institutions may be treated by conventional sewage treatment processes such as percolating filters. Effluent from septic tanks and aqua-privies serving individual houses is normally discharged to soakpits or drainage trenches for infiltration into the ground. The infiltration capacities of the soil given in Table 5.4 may be used to determine the required wall area of both soakpits and trenches.
Unfortunately it is not possible to predict the useful life of such disposal systems, which depend on the efficiency of the septic tank and the soil conditions. Pools of stagnant liquid often form when both toilet wastes and sullage are discharged to a septic tank and then to a drainage field which is too small or is clogged. This creates a potential health risk. Overloading of the drainage field may be avoided by allowing only toilet wastes to go to the septic tank. Sullage can be dealt with separately with fewer health risks than a mixture of partly treated toilet waste and sullage. Kalbermatten et al. (1980) proposed the use of a three-compartment septic tank, where sullage is introduced into the final compartment. It is suggested that the effluent infiltration rates may be double those for two-compartment tanks.
Pits used to dispose of effluent from septic tanks are commonly 2-5 m deep with a diameter of 1.0-2.5 m. The capacity should be not less than that of the septic tank.
Depending on the nature of the soil and the local cost of stone and other building material, soakpits may either be lined or filled with stones or broken bricks. Linings are generally made of bricks, blocks or masonry with honeycomb construction or open joints (Fig. 6.27), as for the linings of pit latrines which are described in Chapter 7. The infiltration capacity of the soil may be increased by filling any space behind the lining with sand or gravel (Cairncross & Feachem, 1983). Hard material such as broken rock or broken kiln-dried bricks not less than 50 mm in diameter may be used to fill an unlined pit (Fig. 6.28).
Fig. 6.27. Lined soakpit
Fig. 6.28. Unlined soakpit
Whether the main part of the pit is lined or filled, the top 500 mm should have a ring of blocks, bricks or masonry with full mortar joints to provide a firm support for the cover. The ring may be corbelled to reduce the size of the cover. Covers are usually made of reinforced concrete and may be buried by 200-300 mm of soil to keep out insects.
The area required for infiltration should be calculated from the data given in Chapter 5, as illustrated in Example 8.6 in Chapter 8. Increasing the diameter of the pit results in a disproportionate increase in the volume of excavation and in the cost of the cover slab compared with the increase of wall area. Therefore, if the required infiltration area is large, it may be more economical to provide drainage trenches.
The disposal of the large quantity of effluent from septic tanks is often effected in trenches which disperse the flow over a large area, reducing the risk of overloading at one place. The trenches make up a drainage field. The effluent is carried in pipes which are normally 100 mm in diameter with a gap of about 10 mm between each pipe. Unglazed stoneware pipes (tile drains) are often used, either with plain ends or with spigot and socket joints. The upper part of the gap between plain-end pipes may be covered with strips of tarred paper or plastic sheet to prevent entry of sand or silt. With spigot and socket pipes, a small stone or cement fillet can be placed on each socket to centre the adjoining spigot (Fig. 6.29).
Fig. 6.29. Open pipe joint in a drainage trench
Drainage trenches are usually dug with a width of 300-500 mm and a depth of 600-1000 mm below the top of the pipes. A common practice is to lay the pipes at a gradient of 0.2-0.3% on a bed of gravel, the stones with a diameter of 20-50 mm. Soil is returned to a depth of 300 500 mm above the stones, with a barrier of straw or building paper to prevent soil washing down (Fig. 6.30).
Fig. 6.30. Drainage trench
If more than one trench is needed it is recommended that the drains be laid in series (Cotteral & Norris, 1969). Drains in series are either full or empty, allowing the soil alongside empty drains to recover under aerobic conditions (Fig. 6.31). If drains are laid in parallel, there is a tendency for all trenches to contain some effluent. Trenches should be 2 m apart, or twice the trench depth if this is greater than 1 m.
Fig. 6.31. Drainage trenches laid in series in a drainage field. A-A indicates section shown in Fig. 6.30
The length of trench should be calculated by dividing the flow of effluent by the infiltration rate, allowing for the area of both sides of the trench, as illustrated by the examples given in Chapter 8.