They remove primarily suspended and organic matter. To a degree, they can also remove number of undesirable substances. However their ability to do so is very limited. These plants differ greatly in, their number and quality. Requirements for treating effluents have risen sharply in the last decades, as increase in the cost. In some cases sewer charges per unit already exceed fresh water costs.
Most countries are now accepting the principle of the polluter pays’ and trend is likely to continue. Therefore the elimination of problematic materials at the source should be aimed at whenever possible.
A complete treatment including screen, sedimentation, oxidation, sludge/ sullege, digestion, disinfection etc. is given to foul sewage so as to bring down its biochemical oxygen demand and concentration of other constituents to safer values before discharging into the rivers, sewage can also be used for irrigating crops.
There are two methods of disposing sewage effluents:
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(I) Disposal by Dilution:
Treated sewage or the effluent from the treatment plant is discharged into a river or lake or sea. The discharge sewage in due course of time is purified by self purification of natural water.
Conditions favouring disposal by dilution:
(a) When sewage is fresh, 4 to 5 hour old, free from floating and suitable solids.
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(b) When diluting water has high dissolved oxygen content
(c) When diluting water is not used for the purpose of navigation water supply.
(d) When flow currents of the deposition water are favourable causing no deposition, nuisance or destruction of aquatic life.
(e) When the outfall sewer of the city of the treatment plant is situated near some natural waters having large volume.
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(i) Dilution in Rivers and self purification of natural stream:
The various natural sources of purification are.
(i) Dilution and dispersion
(ii) Sedimentation
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(iii) Sun light
(iv) Oxidation
(v) Reduction
(ii) Dilution in sea water:
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Sea water contains 20% less oxygen than river water. Sea water contains dissolved matter. Capacity of sea water to absorb sewage solids is not as high as that of fresh water.
A sewage solid when thrown into sea water chemically is lower than stage then sewage lighter and warmer, sewage will rise up to surface. Spreading of the sewage at the top surface of sea in a thin film of sleek.
Sewage should be disposed off during low tides. Large sized tanks are connected to hold him sewage during high tides. Large sized sewer with non return valve is another solution.
Beach with the dissolved matter in a sea water resulting in precipitating some of the sewage solid, giving a milky aberrance to sea water and forming sludge bank-they produce offensive hydrogen sulphide gas by reacting with in sulphate rich water.
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Sewage is taken deep into the sea and away from the coast line. Specific gravity of sea water is greater than that of sewage,
(II) Disposal by Land Treatment/Disposal by Effluent Irrigation:
This method can, be used for irrigating crops. Percolating water may raise water table or collected below by a system of under drains. Increase crop yields 33%. Sewage contains fertilizing minerals and nitrogen, phosphate, potash.
When sewage is applied to the land, a part of it evaporates and the remaining portion percolates through the ground soil, suspended particles present in sewage are caught in the soil voids, get oxidized.
If the land is made of heavy, sticky and fine grained materials, then evolution of foul gases is possible.
Sewage effluents to farm:
(i) Broad irrigation (iii) Sprinkler irrigation
Sewage Treatment Processes:
The stages are
(i) Preliminary treatment.
(ii) Primary treatment
(iii) Secondary treatment
(iv) Complete final tertiary treatment.
(i) Preliminary treatment:
Separating the floating materials like dead animals, free branches, papers, pieces of rags, wood and heavy suitable inorganic solids, fats, oils and greases.
Screening:
Removing floating, Screening can be disposed easily either by burials or burning. Screens are coarse, medium, fine on the basis of size of opening.
Skimming tanks:
Removing fat oil, grease, Skimming tanks arc disposed oil stabilizing them in digestion tank by any aerobic process.
(ii) Primary treatment:
Removing large suspended organic solids.
Sedimentation in settling tank – Organic solids is stabilized by an aerobic decomposition in a degustation tank.
Residues are used for landfills or soil conditioning.
After treatment large amount of suspended organic materials and high B.O.D. 60% of original.
It involves further treatment of the effluent coming from the primary sedimentation tank. This is generally accomplished through biological decomposition of organic matter which can be carried out either under aerobic or anaerobic conditions. In these biological units, bacteria units will decompose into the fine organic matter to produce cleaner effluent.
The treatment reactors in which the organic matter is decomposed (oxidized) by aerobic bacteria are known as aerobic biological units and may consist of filter, aeration tank, iodations ponds, etc.
The treatment reactors in which the organic matter is destroyed and stabilized by anaerobic bacteria are known as anaerobic biological units and may consists of aerobic lagoon’, septic tanks, imh off tanks etc
The organic solids separated out in secondary settling tanks will be disposed of by stabilizing them under aerobic process in a sludge degustation tank.