Waste water from human habitations is harmful to the environment from several points of view:
1. Spread of disease caused by bacteria, viruses and parasitic organisms.
2 Removal of oxygen from the water of rivers or lakes which receive the waste-water
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3. Poisoning of the lakes or rivers by chemicals and metals originating from industry.
4. Enrichment of the natural waters by salts such as nitrates and phosphates which, then, cause abnormal growths of algae and other plants, with undesirable results.
If untreated waste-water is allowed to enter a river, problems from most of these categories can be expected. The severity of the problem will, of course, depend on the volume of effluent and the size of the receiving river.
In an extreme case, where the receiving body of water is the sea, waste-water from quite large cities, can be disposed of completely untreated. Surprisingly, few problems are encountered where this is done due to the large dilution factor involved and the tendency of sea water to kill most organisms which cause human disease.
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Some people do not entirely agree to this statement but, in view of economic considerations, waste-water, including sewage, continues to be disposed off in this way by coastal cities in most parts of the world.
Inland towns and cities, of course, do not always have the opportunity to dispose off their waste in a large body of water. An interesting case is that of Calcutta where a large proportion of the sewage is passed along an open ditch for many miles before eventually reaching the sea. The authorities there do not regard this as an ideal solution but alternatives are likely to be very costly.
Further inland, the treatment procedures vary from complete (primary and secondary) measures to no treatment at all. Where there is little or no treatment, the harm done to the environment is bound to depend on the size of the population centres and the size of the river which is receiving the effluent.
Of course, it is the size of the river, especially in the dry season, which is important. This goes against the opinion, sometimes, voiced that a flood- plain river Ganga can receive large amounts of pollution because it is all washed out each rainy season.
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This point of view may apply, to some extent, to inorganic pollutants such as some metals which require years to build up to toxic levels. One could, then, expect the flooding to slow down their accumulation in the river’s environment.
The situation is quite different, however, with biological pollutants. Here, we are dealing with infectious agents which can produce their effects (in the spread of disease) within hours of entering the drinking water.
Though the danger will be reduced, in the rainy season, due to dilution, this could be more than offset by the danger of contamination of drinking water by flood water.
One of the most obvious and immediate effects of pollution from a town or city sewage is the removal of oxygen from the waters of the receiving stream. Oxygen measurements upstream and, at some distance, from any sewage, inlet downstream, will demonstrate this quite clearly.
In fact, city effluents, which can be described as a suspension or solution or organic materials, and living organisms contribute to a heavy organic load. The organic material provides an unlimited supply of food for micro-organisms existing in the river.
Unfortunately, in order to consume the organic material, the majority of these micro-organisms require oxygen which they consume at a faster rate than its rate of diffusion into the river from the atmosphere.
Thus, the oxygen content is depleted, rendering it unsuitable for most forms of aquatic life. However, if no further organically rich effluents enter the river, it will gradually recover and its oxygen content will return to normal through the natural fortification mechanism for dissolved oxygen, i.e., diffusion. The ability to recover is, of course, related to such factors as the size of the river, compared with the volume of effluent.
Waste Water Treatment:
The foregoing discussion has concentrated mainly on dissolved organic matter and its consequences for dissolved oxygen in the aquatic environment, which receives the effluent.
Many other materials notably bacteria and dissolved inorganic salts may also be present which can cause problems in the environment. Methods are known, which can completely purify waste water, even sewage, to such an extent that it can be safe for drinking.
Such procedures are, however, only justifiable in the case of a large city with a limited supply of water, because the cost of the installation is extremely high. Also, expensive is the process for primary and secondary treatment, including activated sludge treatment and slack digestion.
This involves the removal of nearly all particles and organic matter present but excludes the dissolved salts, many of which are plant nutrients. Bacteria, however, survive this treatment in enough numbers to constitute a possible health hazard in effluent. The same applies to viruses, in particular, the hepatitis virus.
Once again, however, capital costs are considerable and an required to make the plant function. Exceptions to this statement are plants where used as fuel to power the machinery.
The trickling filter is a surprisingly effective machine for treating waste water and sewage which has first passed through a primary settling tank. If properly designed, it required no power input and the rotating arms of the sprinkler are moved by momentum of the water.
This requires a pressure of about 60 cms. The gravel bed, beneath the sprinkler, becomes colonized by a large number of organisms which effectively remove most of the organic matter. The effluent, however, after this process, will still contain large amounts of plant nutrients. Capital costs are also significant and the capacity of the system is not large enough for very large communities.
The stabilization pond or oxidation pond is another method of treatment. Compared with the methods describe above, these ponds have, somewhat, lower BOD loadings.
They rely on the activity of micro-organisms in a large body of water, about 1 m deep. Particular use is j made of microscopic plants. The organic matter is first consumed by aerobic bacteria and oxygen is supplied from both the atmosphere and plants during the day.
The stabilization I pond works best where temperatures are high and there is abundance of sunshine. Hence, their obvious advantage lies in India. The effluent can be quite low in organic matter but bacteria and parasite eggs remain a problem. This is a pity in view of the potential use of the nutrient-rich water for irrigation.
The hazard, still present, in the stabilization pond-effluent, can be greatly reduced by growing crop, not used as food, cotton or crops grown on trees, including coco-1 nuts and citrus fruits. Much work of this kind has been carried out by N.E.E.R.I. at Nagpur.
A good account of this has been given by Sundaresan. The details of the rearing of edible fish, in the effluent from stabilization ponds, are also included. Other criteria for evaluating and comparing the performance of stabilization ponds in India are given by Bopardikar and Dave and Jain.
The general opinion seems to be that their low cost, ease of maintenance and possible I integration with some types of agriculture and fish production, makes the stabilization pond a practical choice.
However, extreme care will be necessary to make sure that the potential I disease causing organisms and parasites are kept in control. The complete treatment of waste water is usually divided into three stages.
These are called primary, secondary and tertiary treatments. The final level of purity depends on the economics of the situation. It would naturally cost more to refine the water after the second and third stages so as to make the water fit for drinking.
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Primary Treatment:
Waste water usually contains large quantities of floating rubbish such as cans, cloth and wood. These larger objects are removed by metal bars which act like strainers as the water j moves beneath them in an open channel.
The velocity of the water is, then, reduced in a grit- settling chamber of a larger size than the previous channel. This results in most of the grit and sand settling at the bottom. Periodically, this settling chamber is disconnected from the main system and the grit is removed manually.
The smaller and lighter particles take longer time to settle under gravity. This slower process takes place in the primary sedimentation tanks. Such tanks are quite large and are usually circular with a bottom shaped like a shallow cone.
At the bottom of the cone, the sediment or sludge is drained away by a horizontal pipe. Rotating scrapers slowly scrape the sludge from the sloping sides, moving it gradually from the perimeter to the centre where it is drained off as described above.
The disposal of this primary sludge can be a problem because of its offensive smell. It also has a water content of over 95 percent. It must be dried before it can be disposed of on land.
Alternatively, it may be digested by the method described below under the heading “sludge digestion”. The liquid effluent from the primary sedimentation tanks has now lost over half of its original suspended matter. This effluent may be drained directly into a river or the sea if other considerations permit this.
Secondary Treatment:
The suspended particles, remaining after primary treatment, are too fine to be removed by a simple sedimentation process. This would take far too long and much of the material is colloidal and would never settle down. Most of this material is, however, organic and this means that certain micro-organisms can consume it as food.
The carbon chains of the large organic molecules are, thus, broken down to smaller molecules such as carbon dioxide. This can be accomplished in a trickling filter or in an activated sludge digester.
A large area of land is required for trickling filters and they, sometimes, cause problems because they attract insects and emit an undesirable smell. They have the advantage; however, of needing very little maintenance once they are working. They also operate without electricity.
Incoming waste water (after primary treatment) is sprayed over circular filter beds by a rotating boom. This is a pipe with holes along its length and the water emerges in horizontal jets.
The reaction to these jets causes the boom to rotate; thus, no additional power is required. The water is kept at the required pressure head by a type of siphon at the inlet.
The filter bed is usually about 1.5 m deep and surrounded by a circular brick wall. The filter consists of various combinations of stony material or even plastic material. The stones should, if possible, be round and between 3 and 10 cms in diameter.
Stones of irregular shape can, however, are used. One of the problems which could be faced, if the organisms, which grow on the stones, eventually block the filters is too much effluent of high B.O.D. is applied. If the B.O.D. is too high at certain times in the days it can be diluted by recalculating some of the low B.O.D. effluent which emerges from the bottom of the filter.
The bottom of the filter must allow free entry of air, and, for this reason, it is usually provided with a floor of perforated tiles under which the incoming air passes. This air makes its way up through the porous filter-bed to the upper surface.
It is important to note that trickling filters, like other forms of secondary treatment, require aerobic conditions. If there is lack of oxygen, the wrong types of organisms would develop and toxic materials would find their way into the final effluent.
Under ideal conditions, the filter 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.