In developing countries, the organic load of river system is a major problem. In India, the contribution of local bodies, e.g., Nagar Mahapalika, Nagar Palika, Town Area, etc., through sewage is about 90 percent, while 3 percent is contributed by small-scale and 7 per cent by medium and large-scale industries.
Unfortunately, control of pollution due to sewage cannot be self supporting, e.g., it is not economically viable. However, considering the social responsibility and other indirect advantages of pollution control, it is essential to have pollution control for sewage.
Even in developed countries, pollution control measures are slower in case of domestic effluent than in that of trade effluent. With the establishment of the River Ganga Authority, pollution control, due to sewage, is receiving greater attention.
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It is expected that even those local bodies, which are not contributing directly to the pollution of rivers Ganga and Yamuna, will go in for pollution control in the near future.
Pollution control for sewage has been described with Kanpur, one of the largest industrialized cities of North India, as a typical example.
The people of Kanpur generate 1800 cubic meters, i.e., approximately 900 tonnes of refuse and 1700 cubic meters of night soil, every day. Only about 25 percent of the refuse is transported by the Municipality and removed, the remaining 75 percent is carried away in open drains, creating a great health hazard.
As a result of excessive discharge of untreated sewage, sullage and effluents, the river Ganga, noted for its high natural self-purification capability has become extremely polluted at Kanpur and its water is unfit for use.
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At present, all 17 aqueducts in Kanpur are discharging sewage/sullage into the river Ganga as many sewers are undersized, silted and damaged. The drains also discharge effluents from various industrial units.
The volume and BOD load transported dally by the waster falls are nearly 225 million liters and 75 tones, respectively. The sewage contributes about 50-55 percent of the total load on the stream.
As the sullage drains from Kanpur start pouring into the river Ganga, the quality of water steadily deteriorates. At various ghats, viz., Bhaironghat, Sarsaiya ghat, etc., the quality of water ranges between classes ‘C’ and ‘E’, i.e., it is unfit for any beneficial use
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In view of the deteriorating situation and the significant contribution of organic load, estimated to be about 50-55 percent, through sewage, it is considered essential to suitably treat sewage before introduction into the river Ganga.
Disposal of Sewage:
The following methods are considered to be of potential importance in sewage treatment. Broad Irrigation
In this method, sewage is disposed off on land. The land area acts as a crude filter and stabilizes sewage by aerobic filtration. The sewage is usually given primary treatment before disposal. This method is similar to intermittent sand filtration.
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Sewage Farming:
In this method, the water and fertilizing elements, viz., nitrogen, phosphorous and potash are utilized to raise crops. It has the special advantage of fertilizing the land and also being beneficial from the economic point of view.
Dilution:
In this method, the sewage is discharged into large bodies of water such as rivers, lakes or the sea. The sewage is treated to ensure that the condition of the receiving water stream does not deteriorate to the extent of impairing its normal utility.
In Kanpur, the latter method is mainly used for sewage disposal. Its working is considered satisfactory during the rainy season when the dilution factor is extremely high, but not suitable for the period November through June when the volume of flow is low.
It requires either preliminary or primary Dilution may usually need full or, at least, treatment of sewage partial treatment. This involves extra cost and operational troubles. But when treatment is not required, it offers the simplest solution to the sewage disposal problem.
It saves the inland streams from getting it causes stream pollution and unavoidable health polluted by the sewage and returns to hazards. Towns situated downstream, therefore, the land the fertilizing elements present cannot use the water in the stream for their water in the sewage. supply purpose without adequate treatment.
The pollution also travels long distances downstream and the stream may lose its utility for different human activities over a considerable distance. It requires high head pumping, the opera- It usually requires no or low head pumping be- optional cost of which is, therefore, high cause streams pass through the lowest contours and valleys.
Treatment of Sewage:
The general operations in a sewage treatment plant and the units employed for the purpose are given in. The main features are as follows:
Primary Treatment:
The primary treatment consists of physical separation of floating and suspended (settle able) solids from the sewage. Screens, grit chambers, detritus tanks, skimming tanks and settling tanks are used for this purpose.
Operation no. 5 is practiced only under special circumstances, i.e., where the sewage is disposed of by dilution and the outfall is very near to water intake of some other town.
Secondary Treatment:
The secondary treatment includes flocculation and precipitation of the remaining material in the sewage with the help of biological agencies, and their physical separation in secondary settling tanks. A layout for a typical sewage treatment shows schematic diagram for bio-gas production from sewage.
For simplicity and economy of operation, the treatment processes are usually carried out in the sequence indicated the efficiencies normally encountered in practice, in the various units of a treatment plant.
The by-products of the treatment process are screenings, grit and sludges of different kinds, of which the first two can be easily disposed of by burial or burning. Sludge contains unstable volatile organic materials and may be treated by the method of digestion, in which gases are produced with high calorific values which can be successfully utilized for heating or power generation.
The solid material left, behind, i.e., the digested sludge, contains good manurial elements and is useful as a manure or soil buildings.
Sludge Digestion:
The process of sludge digestion is the liquefaction of organic material by anaerobic bacteria which produce an alkaline reaction. When sludge is first placed in a tank, acid digestion with the production of noxious gases results, but eventually alkaline digestion prevails and, once established, the alkaline condition remains for an indefinite period of time.
The gases given off by the sludge during digestion are mainly methane with some C02 and smaller quantities of other gases6. Digestion takes place in three stages:
1. A short period of rapid digestion, during which the pH value of the sludge decreases.
2. An extended period of slow digestion during which the pH value rises slowly with low gas production.
3. During the third stage, it attains a pH of 7.0 and gas production is maximum. Two-stage digestion is usually preferred because it simplifies gas collection and assists the removal of supernatant water.
A major part of the gas is produced in primary tanks, which is collected for utilization. Supernatant water is removed from secondary tanks. Primary tanks have to be treated for this purpose but there is no need to heat the secondary tanks2.
In primary digestion tanks at a temperature of 32°C, about 40.8 percent digestion of volatile solids takes place, giving 0.96 cubic meter gas/kg of solids destroyed or 29.25 per, head of population per day.
However, this depends mainly on the animal and vegetable oil content of the sludge; the digestion period depends on the temperature and can be reduced to 15 days at 52° C. At all temperatures, the total amount of gas produced during digestion remains the same.’
Digestion tanks:
As the gas is collected in the tanks, it is desirable to use floating covers which rise and fall with the level of the sludge as the tanks are filled and emptied. For the sake of economy of cost and to reduce heat losses, digestion tanks should be deep and circular. Different types of tanks with their special features.
Feeding method:
To prevent several risks, e.g., the production of an explosive mixture of gas and air inside the tank and the sucking in of air by negative pressure, the following methods should be adopted.
In the feeding method, sludge is delivered into the tank, causing digested sludge to pass over an adjustable weir, such as a telescopic weir, and on to secondary digestion or disposal. By adopting this method, the sludge can be maintained within safe limits. In case there are many tanks, the sludge should be fed to them in equal quantities, preferably once a day, when the primary sedimentation tanks are sludge.
Seeding and Mixing:
Raw sludge is mixed with about 20 percent of actively digested sludge before it is passed into the primary tank. For stirring, mechanical stirrers can be used.
Heating methods:
Any of the following methods, singly or in convenient combinations, can be used to heat the digestion tanks:
1. By circulating hot water through fixed coils of pipes of about 4 cm diameter, located near the walls of the tank.
2. By rotating coils or grids of pipes in which hot water is circulated. The pipes serve the double purpose of heating and stirring the sludge.
3. By drawing sludge from the primary tank, heating it in a heat exchanger and passing it back into the digestion tank.
4. By heating alkaline supernatant water drawn off from the secondary tank and returning it into the primary tank.
5. By injecting steam into the bottom of the tank.
Sludge control:
The sludge is pumped (usually daily) into the digester and passes into the tank through the recirculation return part at the bottom of the sludge heater chamber, which is specially designed, so that the direction and velocity, at which the sludge enters the digester, give a rotator motion to the tank contents.
The sludge feed displaces an equal volume of digested sludge, which passes through the sludge draw-off pipe, from the central discharge point, at the bottom of the cone, to the sludge draw-off chamber.
It, then, passes over an adjustable weir penstock into the discharge chamber before being delivered to the secondary digester or sludge drying beds. By adjustment of the weir-penstock, the sludge level in the tank can be controlled.
Removal of supernatant water:
The secondary digestion, tanks serve the purpose of completing digestion to an economic limit, and providing quiescent conditions to permit separation of supernatant water.
The common arrangement for the removal of supernatant water is the installation of several small-diameter valves, connected at various levels in the tank. The operation is to open a valve at a time, to find where the supernatant water is, then, to draw it off until sludge appears. The percentage of dewatering varies between 10-50 percent, depending upon the nature of the sludge.
Sludge Gas and its Utilization:
Sludge gas consists of about 67 percent methane, 33 percent CO2 and small quantities of other gases. As methane has about 8000 kcal/m3 calorific value, the calorific value of sludge gas can be taken to be approximately 5334 kcal/m3.
The sludge gas occupies 0.95 m3/kg at normal day temperature and atmospheric pressure. The weight of sludge gas produced is about the same as in the digestion process.
On the basis of the amount of sewage discharged from Kanpur city, it is estimated that about 2.5 x 104 cubic metre gases could be generated per day with 1.6 x 108 kcal calorific value. As reported by the Indian Oil Corporation, the consumption of Indian gas at Kanpur is about 50,000 cylinders per month, each containing 15 kg of LPG.
The total consumption works out 750,000 kg of LPG per month, having a calorific value of 5,370 kcal/kg. On the basis of this calorific value of Indian gas, the total calories consumed in the city of Kanpur, at present, is approximately 1.33 x 108 kcal.
The main use of sludge gas, apart from heating the digestion tanks, is for power. The power generated is sufficient to pump the entire sewage. At activated sludge works, the power can be used for aeration purposes. Elsewhere, it finds use in recirculation pumping in the alternating double filtration scheme. It has also been used for driving road vehicles.
For this purpose, the gas has to be purified, particularly its CO2 content. After which it is compressed and stored in high-pressure cylinders. It can also be used for cooking purposes.
Dried digested sludge can be used as a fertilizer as it contains 0.8-3.5 percent nitrogen, 1.6 percent phosphorous, and 0.35 percent potash on the basis of the dry weight of solids. It may also be used for land filling, dumped in large bodies of water, or incinerated to produce heat and power.