Waste water contains a lot of chemical pollutants as well as dissolved solids and suspended solids. The major contributor of pollutants is industrial waste water.
Waste water can be purified by the following ways:
1. Primary treatment
2. Secondary treatment
1. Primary Treatment :
The purpose of primary treatment is to remove pollutants like large, floating and suspended solid materials, grit, oil and grease, etc.
The large floating rubbish like sticks, rags, cans, cloth and various other objects are removed by screening. The screens are generally made up of steel bars spaced very few centimeters apart, followed by a wire mesh screen with smaller openings compared to the opening of steel bar assemblage. The collected materials are now disposed off.
However, if a comminute is used, it will cut the large materials into very small pieces and at a later stage can be settled in the grit chamber.
After screening, the waste water is passed into a grit chamber and time just sufficient for the heavy materials, like sand, grit, etc., to settle down is allowed. The settled materials can be disposed off in the land fill.
From the grit chamber, the waste water is now allowed to pass through primary settling tank or ‘primary clarifier’. The flow rate is now reduced with a detention time of about two to three hours and most of the lighter suspended solids settle down because of gravity.
The settled solids are generally circular in shape and the waste water flows horizontally. The settled suspended materials can then be removed by mechanical scrapping into hoppers and are pumped out subsequently. In the overall process, about 60 per cent of suspended solids and 40 per cent BOD can be removed.
However, finely divided suspended solids and colloidal particles cannot be removed by this process. Under this condition, mechanical flocculation and chemical coagulation can be employed.
In mechanical flocculation, the finely divided suspended solids are allowed to coalesce by rotating paddles with sufficient detention time. Later on, they get separated with settlement due to gravity.
In coagulation process, chemical coagulate like alum or hydrated lime is added and for making the process more efficient, coagulant aids like activated silica or polyelectrolyte can be used and colloidal suspension can easily be removed.
2. Secondary Treatment:
The purpose of the secondary treatment is to remove the finely suspended solid, dissolved solid and BOD which still remain after the primary treatment. The BOD is removed by biological processes using bacteria and other microorganisms. The processes may be aerobic or anaerobic.
Thus, in the secondary treatment, oil, grease and organic matter (BOD) are removed to the largest extent. The complex cell tissues thus, formed can be removed by agglomerating them and finally settling out.
The effluent from the primary sedimentation tank is first subjected to aerobic oxidation in systems like trickling filters, rotating biological contractor, active sludge units and oxidation ponds. The sludge obtained in the primary sedimentation tank, is allowed to undergo anaerobic oxidation in the sludge digesters.
Anaerobic decomposition is generally employed for digestion of sludge. However, organic wastes from dairies, slaughter houses, etc., can also be treated by this method. The efficiency of both aerobic and anaerobic decomposition depends on many factors such as:
1. Temperature 2. pH 3. Concentration of wastes 4. Nature of waste.
In the secondary treatment, the decomposition is done under controlled conditions in the treatment plant itself and about 85 per cent BOD and suspended solids can be removed efficiently.
However, neither the primary nor the secondary treatments are effective in removal of nutrients, dissolved materials and biologically resistant materials. Under such circumstances, tertiary treatment is required.
(a) Trickling Filters:
The trickling filter consists of circular filter beds usually about 1.5 m deep and is surrounded by circular brick wall. The beds are made up of materials like rocks or plastic materials.
The diameter of the material varies from 3 to 10 cm. The spaces between the rocks or plastic materials are sufficient to allow air to circulate and aerobic conditions are thus maintained.
The incoming waste water after the primary treatment now can be sprayed over these circular filter beds by a rotating distributor.
The rotating distributor is a metallic pipe with holes along its length and the waste water emerges in horizontal jets. The bottom of the filter is provided with a floor of perforated tiles through which incoming air passes.
The individual rocks in the bed are covered with a gelatinous film consisting of bacteria, algae, fungi, protozoa, worms, insects, etc. The BOD in the incoming waste water is absorbed in the bed surface and undergoes aerobic decomposition by these bacteria or microorganism.
With increase in the thickness of the film, a part of it gets attached and carried away along with the treated waste water to a secondary settling tank and is removed finally. The sludge so obtained is now pumped to the sludge digestion unit.
The efficiency of trickling filter depends on temperature, pH, nature of the waste, uniformity of loading of the waste matter and air supply.
If there is lack of oxygen in the trickling filter, then wrong type of organisms will develop and many toxic materials will be included in the final effluent.
Thus, the bed should provide a home for a wide variety of organisms including bacteria, protozoa, fungi, worms and insects. The upper surface will also be colonized by algae.
Trickling filters are very simple to operate and BOD can be removed to the extent of 65 to 85 per cent. However, it is costlier and efficiency decreases if effluent of high BOD is passed continuously. Such problem can, however, be overcome if high BOD is replaced by low BOD occasionally.
(b) Rotating Biological Contractor (RBC):
An RBC consists of a series of closely spaced circular plastic disks (diameter, 3.6m) and is attached to a rotating horizontal shaft.
The arrangement is so designed that about 40 per cent of each of the circular plastic disk is submerged in the tank containing waste water which needs treatment. The circular disks are covered with gelatinous film consisting of bacteria, fungi, algae, worm and insects.
As the RBC rotates, the submerged portion adsorbs organics (BOD) and when they come out they are exposed to atmospheric oxygen.
Under such condition, there is no scarcity of oxygen and the process becomes very efficient. The device is easy to handle and can be operated under high load conditions not possible by trickling filter. Compared to trickling filters, higher rate of BOD removal is achieved in RBC.
(c) Activated Sludge Process:
In the activated sludge process, the effluents from the primary sedimentation tank and recycled bacteria and other microorganisms (bio-organisms) from the secondary settling tank (the activated sludge), are collected in a tank called aeration tank.
The object of the process is to provide sufficient amount of oxygen for complete aerobic decomposition of the wastes. To do so, air with oxygen is pumped into the mixture and thoroughly agitated by mechanical stirrers. The water is kept in constant turbulent motion in the aerated tank (concrete tank).
After about 3 to 6 hours of aeration (for sewage) and 6 to 24 hours of aeration (for industrial waste), the agitated effluent water again flows to the secondary tank and the effluent is separated from the sludge by settling and is discharged.
Some part of the sludge is recycled again to the aeration tank, so as to maintain effective microbial population for fresh treatment, the remaining sludge can be processed and disposed of. The BOD removal from the waste water can be as high as 95 per cent. Sludge Treatment
In the activated sludge process, the BOD and suspended and dissolved solids are removed to a greater extend from the waste water and then that water is released to be used as a good water source.
The remaining matter to be disposed off is only sludge (mixture of solids and water). The sludge can be dried up and used as fertilizer or it can be dumped into the sea. Whatever be the way of disposal, it must be environment friendly.
Another effective measure of sludge disposal is the digestion of the sludge under anaerobic condition at 35°C. Under anaerobic condition, the microorganisms convert the organics into simpler carbon dioxide, methane gas and many other stable products.
The methane gas has fuel value and can be utilized to warm digestion tank. It is to be noted that methane forming bacteria are extremely sensitive to temperature, pH and oxygen and thus, proper condition should be maintained for sludge treatment.
The digested sludge still contains lot of water. The water can be removed by pumping sludge into drying beds or by the process of incinerators. The digested and dewatered sludge is inoffensive. It can be used as soil conditioner or disposed of in a land fill.