In this article we will discuss about the primary, secondary and tertiary treatment of waste water through plant.
Primary Treatment:
A series of settling chambers are designed to remove heavy grits including coarser organic materials. Then there is a sedimentation clarifier, where suspended particles are oxidised and sedimented at the bottom as sludge. In this process considerable amount of suspended solid and organic materials are removed; as such substantial lowering of total solids, BOD and COD are noticed.
Secondary Treatment:
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The organic matter and other impurities remaining after the primary treatment are in solution or in fine suspension that does not settle and cannot readily be filtered.
There are two ways by which finer particles and dissolved materials are sedimented out viz., activated sludge (aerobic) treatment and trickling filter treatment. In the activated sludge process, sewage, after primary treatment, is pumped into an aeration tank, where it is mixed for several hours with air and with bacteria-laden sludge.
The bacteria then decomposes organic waste along with oxidative sedimentation of suspended and dissolved materials, ultimately leads to sludge deposition at the bottom of the tanks. The effluent from the biological action is still laden with bacteria and is not fit for discharge into open waters.
An alternative technique is ‘trickling filter’ treatment. In this device, long pipes rotate slowly over a bed of stones, it offers its nutrients in the presence of air to an abundance of rather unappetizing forms of life. A fast-moving food chain is set in operation. Bacteria consume molecules of protein, fat and carbohydrates. Protists consume bacteria.
Further up the chain are worms, snails, flies and spiders. Each form of life plays its part in converting high energy chemicals to low-energy chemicals. All the oxygen consumed at this stage represents oxygen that will not be needed later when the sewage is discharged to open water. There, this process constitutes a very significant purification.
Thus each step in the biological consumption of this waterborne waste, from sewage nutrients to bacteria to protozoa and continuing to consumers of higher orders (such as worms), represents a degradation of energy, a consumption of oxygen and a reduction in the mass of pollutant matter.
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Finally, when the microorganisms die, their bodies stick together to form aggregates large enough to settle out air in a reasonably short period of time. Some agglomeration also occur in the metabolic processes of the protozoa, so that their excreta are usually larger than the particles of food that they ingest.
Thus the process of making big particles out of little ones is of prime importance in any system of waste water treatment. The mushy mixture of living and dead organisms and their waste products at the bottom of a treatment tank constitute the ‘biologically active sludge’.
Tertiary Treatment:
Although considerable purification is accomplished by the time that waste-water have passed through the primary and secondary stages, these treatments are still inadequate to deal with some complex aspects of water pollution. First, many pollutants in domestic sewage are not removed. Inorganic ions, such as nitrates and phosphates, remain in the treated waters.
These materials, as we have seen, serve as plant nutrients and are, therefore, agents of “eutrophication“. If there is excess chlorine, it may react with industrial wastes such as organic solvents and produce chlorinated products that are more objectionable than the original pollutants.
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Additionally, many pollutants originating from sources such as factories, mines, agricultural run-offs and even homes cannot be handled by municipal sewage treatment plants. Some synthetic organic chemicals from Industrial wastes are foreign to natural food-webs (non biodegradable).
They not only resist the bacteria of the purification system but may also poison them and thereby nullify the biological oxidation.
However, several troublesome pollutants which arise from different industrial units need special provision for treatment viz. oil separation, phenol recovery, lime treatment, and polyelectrolyte or alum treatment etc. A generalized outline of three stages of waste water treatment process is given in Fig. 11.7.
Waste-water treatment with aquatic macrophytes provided an option for selecting plants in low-cost treatment of municipal and domestic sewage waters.
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The capacity of ecosystems that are dominated by aquatic macrophytes to assimilate and decompose inputs of nutrients and organic matters have resulted in the extended use of such systems to different types of waste waters. In general, municipal and domestic sewage effluents are the rich source of organic and inorganic nutrients (organic carbon, nitrogen and phosphate etc.).
Due to higher organic loading the dissolved oxygen level is very poor. Under such anoxic environment several aquatic macrophytes can grow and help in the treatment of waste water by removal of BOD, COD, phosphate and nitrate. Thus constructed wetland can be developed for such kind of waste water treatment using selected macrophytes. This system was practiced in many European countries.
Currently an assessment was made for domestic waste water generated and subsequent treatment facilities in various States of India. The data shows that over 3/4 of the total waste water disposed off without treatment (Table 11.15).
i. Industrial Wastewater Treatment:
Before treatment, industrial waste water should be characterised fully and the biodegradability of waste water constituents should be determined. Then treatment option can be made.
This waste water can be treated by a variety of physical and chemical processes like sedimentation, filtration, flotation, osmosis, acid-base neutralization, oxidation-reduction, use of synthetic resin for coagulation and precipitation, ion exchange process etc. Biological treatment also help in removal of nitrogen, phosphorus and some other dissolved salts.
Sludge which generate in the treatment process require proper disinfection through chlorine dioxide or otherwise.
ii. Water Reuse and Recycling
With the passage of time, water becomes a precious resource and there is need for treatment of wastewater and then reuse it back. Of course, for drinking and food processing purpose, highest quality of potable freshwater is required.
But for a number of other uses treated water can easily be reused and recycled.
Some examples are:
i. Irrigation for cropland, golf courses and other applications requiring water for plant and grass growth. This is the largest potential application for reused water and one that can take advantage of plant nutrients, particularly nitrogen and phosphorus in water.
ii. For cooling and process water requirement of some industries can be made through treated wastewater.
iii. Ground water can be recharged with properly treated water either by direct injection into an aquifer or by applying the water to land, followed by percolation into the aquifer.
However the major concern with recycled water is the potential presence of harmful pollutants viz bacteria, persistent organic pollutants, parasites and some other chemical contaminants. During treatment’ appropriate monitoring is required before use of such water in ground water recharge.