VPE 511: Environment and Environmental Hygiene (2+1)

SMALL SCALE SEWAGE PURIFICATION AND DISPOSAL

• Where no public sewer is available, other methods such as the impervious cesspool or a septic tank installation must be adopted for the disposal of sewage.

Cesspool

• Cesspools of two kinds may be met with pervious and impervious.
• The pervious cesspool is a chamber built of concrete or brick or stone rendered in cement and surrounded if necessary by puddled clay, having the bottom open so that the liquid percolates into the soil, this type of cesspool is thus a soak-away pit and it is obvious that to be effective it must be situated in a sufficiently porous sub-soil.
• In the impervious cesspool the wall and the bottom is made of impervious material through which nothing can be percolate.
• In both types of cesspool decomposition of the sewage which is affected by bacteria is incomplete so that there is a continuous accumulation of partially decomposed evil smelling material which has to be removed periodically.
• During the process of decomposition objectionable gases including large volumes of hydrogen sulphide are given off.
• In the pervious cesspool comparatively little material accumulates; with the impervious type frequent emptying is necessary.
• In the case of singe houses or farms it is a common practice to provide an overflow drain, the liquid being disposed of by irrigation over land. When no overflow is provided the whole of the liquid requires to be pumped out.
• The cesspool is not placed in proximity to a well or other water supply

Septic tank Installation

• An installation suitable for the purification and disposal of sewage from isolated building usually comprises
  •  a single from of septic tank designed to retain the sewage for a period of time sufficient to permit anaerobic digestion of its organic solids and to allow insoluble matter to settle out as sludge and
  •  a percolating filter or a contact bed in which the effluent from the septic tank is purified by aerobic bacterial action. The final effluent should be fit for discharge into a stream or for disposal by subsoil irrigation without any potential risk arising. Rain and other surface water should not be allowed access to a septic tank. The sludge needs removal only at long intervals extending in some cases to as long as two years.