Read this article to learn about the waste water treatment systems of individual residences.
The on-site sewage/waste water treatment systems for individual residences (or communities) with special reference to oxidation ditch, septic tanks and Imhoff tanks are briefly described.
1. Oxidation Ditch:
Oxidation ditch method, first developed in Netherlands, is a suitable method for the treatment of sewage in small communities. This is basically an aeration type of activated sludge process with a mechanical system of aeration. However, there is no primary sedimentation of sewage; consequently the problem of handling and treatment of primary sludge is eliminated.
Oxidation ditch consists of aeration units, namely ditch channels (2 or more) constructed side by side (Fig. 57.22). The sizes of the ditch channels are variable-length 150-1000 m, width 1-5 m and depth 1-5 m. They are constructed with brick or stone masonry. A special type of rotors (cage rotors) are fitted into each ditch channel for continuously agitating and circulating the sludge, besides supplying O2. The sludge is allowed to settle down in a separate sedimentation tank. The activated sludge is returned to ditch channels.
Instead of using a separate sedimentation tank, the sewage may be allowed to settle down in the ditch channels by stopping the rotors (usually during night). The supernatant from the ditch channels can be taken out. The excess sludge collected (in the sedimentation tank or at the bottom of ditch channels) can be stabilized.
2. Septic Tanks:
Septic tanks are recommended for individual houses and for small communities and institutions with a contributing population around 300. Septic tanks work on the principle of anaerobic digestion. This occurs as the solids of the sewage settle at the bottom of the tank. Under anaerobic conditions, the biodegradable organic matter is converted to gases (CH4, CO2, H2S etc.) and liquid compounds. This results in a drastic reduction in the volume of sludge.
A thick crust of scum formed on the surface of septic tank maintains anaerobic conditions. The effluent coming out of the septic tank contains some organic solids and pathogens. Therefore, disposal of the effluent from septic tank has to be dealt with very carefully. The septic tanks are dislodged and cleaned at regular intervals, usually once in 2-5 years (depending on the tank size and its use).
Construction and operation of septic tanks:
Septic tanks are usually constructed with bricks, or stone masonry. Thick-wall polythene and fibre glass tanks are also in use in recent years. Whatever may be the construction material used, the septic tank must be water-tight and must function efficiently. The size of the tank is variable, depending on the number of users. For a family of five members, a tank with a length of 1.5 m and a breadth of 0.75 m is recommended. For such a tank, the cleaning interval is 2-3 years.
A conventional septic tank has two compartments, the first compartment being twice the size of the second one (Fig. 57.23). For effective sedimentation of solids, the tank should be designed to prevent the short-circuiting at the top and the bottom of the tank.
Further, the location of inlet and outlet should be such that the contents of the septic tank are not disturbed while the sewage enters or effluent leaves. Septic tank should be provided with a ventilation pipe, the top of which should be covered with a mosquito proof wire mesh.
As the sewage enters the septic tank, the solids settle to the bottom while grease and other light materials float on the surface, and form a scum. The bottom-settled organic material undergoes facultative and anaerobic decomposition to form more stable compounds and gases (CH4, CO2, and H2S). In this way, there is a continuous reduction of solids of sewage entering the septic tank. However, sludge accumulates at the bottom of the tank. Desludging of septic tank has to be carried out periodically (once in 2-5 years).
3. Imhoff Tanks:
Imhoff tank is an improved septic tank. It basically consists of a two storey tank in which the sedimentation (settling) occurs in the upper tank while the digestion of the settled solids takes place in the lower compartment (Fig. 57.24).
As the sewage enters the sedimentation tank, the solids settle down to the bottom and the sewage flows into the digestion tank through slopes and slot of the sedimentation tank. Gas produced in the digestion process escapes through gas vent. The sedimentation tank is designed in such a way that the gases and the gas-buoyed sludge particles raising from the sludge layer do not enter into it. The sludge collection at the bottom can be withdrawn periodically.