Sedimentation is carried out with the object of removing such suspended mineral and organic matter from sewage. Sedimentation tanks are units in which sedimentation is brought about. The lighter organic sewage solids which settle in the sedimentation tanks are termed as sludge. The sewage that has been partially classified by the settling out of the solids is known as the effluent.
In a general set up the disposal of sludge can be done by adopting any one of the following methods:
- Dumping into waste bodies
- Shallow Burial
- Lagooning
- Mechanical dewatering and
- Drying in Beds
If such a general disposal system is not available one has to go in for a sedimentation tank. Generally sedimentation tank adopted is a septic tank which is a horizontal continuous flow sedimentation tank. A description of septic tank is explained in next section. The putrescible and highly odorous efficient from the septic tank requires to be properly treated and disposed of sub-surface irrigation field method or discharging into other soil absorption system such as soak pit and leading cell pool which is described later.
1. Septic Tank
A septic tank is a horizontal continuous flow sedimentation tank. Here the sewage is allowed to move very slowly so as to retain for a period sufficient to develop 60–70% of suspended matter to settle in the form of sludge. Lighter solids float to the surface and combine with grease and fat and form floating scum. The scum and sewage are allowed to stay for a period of 7 months during which period complete decomposition takes place through a process called sludge digestion. The volume of sludge also gets reduced for easy disposal. The effluent from septic tank has very bad colour. It is dark in colour with fine solid particles and has to be disposed with utmost case. A schematic layout of a septic tank is shown in Fig. 27.6.
Figure 27.6 Septic tank
A septic tank is constructed in such a way so as to prevent direct current between the inlet and the outlet. This provides a better sedimentation and is achieved by using T-pipes with submerged ends as inlet and outlet. As an alternative, baffle walls may be provided. The T-pipe or the baffle at the outlet also helps in retaining the scum in the tank. Certain quantity of scum is needed to hold-back odours and to create a form of heat insulations which in turn aids the bacterial action. Usually a manhole is fixed on the RCC cover slabs. The tank cover allows to keep the sewage warm, lessening odour, etc. Gases are separately removed through a vent pipe. Sludge is removed periodically.
Septic tanks are to be located at a place which is exposed to sky and accessible for cleaning. The sewage in a septic tank can be taken as only a primary treatment effect. Before disposal the effluent needs a secondary treatment. The septic tank has only a restricted use in practice, such as schools, hospitals, small residential colonies and other public institutions where sewers are not yet laid.
2. Soak Pit and Leaching Cess Pool
The putrescible and highly odorous effluent from the septic tank requires to be properly treated and disposed. The methods of disposal are:
- Sub-surface irrigation employing absorption field method.
- Discharge into other soil absorption system as soak pit or seepage pits and leaching cess-pools.
Sub-surface irrigation is also termed as land infiltration. It is the application of sewage or its effluent into the land through a system of open-jointed pipes or drains placed near the surface of the ground and thereby enabling the effluent to percolate into the surrounding soil.
A soak pit is a covered pit through which the effluent is allowed to be soaked or absorbed into the surrounding soil. The pit may be empty or filled up with brick or stone aggregates (Fig. 27.7).
1. Brick lining with dry joints
2. Outer casing with coarse aggregates (7.5 cm thick min)
Figure 27.7 Soak pit
A leaching cesspool is a pool such that the top portion acts as an absorption field and the bottom as a septic tank. This is done by providing open-jointed lining at the upper portion which enables the sewage effluent to the easily dispersed to the surrounding soil. The bottom portion is of solid wall which functions as a septic tank (Fig. 27.8).
1. Outer casing with coarse sand (30 cm thick).
2. Lining with coarse aggregate (15 cm thick).
3. Brick work with mortar joints.
Figure 27.8 Leaching cesspool
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