Short notes on Secondary Treatment of Liquid Wastes

Primary treatment removes most of the solids and particulate materials. Waste waters now have plenty of dissolved organic matter. It may also contain a little amount of very fine organic debris and particulate materials. It is now subjected to secondary treatment which is in fact the biological treatment. There are two main kinds of secondary treatments:

A. Aerobic treatment B. Anaerobic treatment.

A. Aerobic Treatment

Aerobic treatment involves bio-degradation in presence of oxygen. Conditions suitable for rapid growth and multiplication of micro-organisms are created in especially designed filters or tanks. Proper temperature and availability of oxygen are the two important requirement of the treatment. Low temperatures slow down growth and activity of the microbes. Lack of oxygen inhibits aerobic activity. Aerobic treatment is usually carried out in :

(a) Trickling filters. (b) Oxidation ponds.

(a) Trickling filters:

The sewage is gently sprayed over the filter beds and allowed to trickle down through a thick layer of sand and gravel, prepared over a perforated plate. As it moves down, the microbial populations which occur in different zones of the filter bed cause rapid oxidation or mineralization of organic matter content of the liquid. These giant filters are called the Trickling filters. Many such filters are equipped with means to force compressed air through pipes under the filter bed. The air provides oxygen to the microbial population.

The microbial populations grow and multiply in-between the sand and gravel particles. These include various bacteria, green and blue green algae, diatoms in upper zones where plenty of light is available. In the lower zone various fungi, bacteria, protozoans and other small animals which live and thrive on dead and decaying organic matter are found. Passage through biologically active layers oxidizes much of the organic matter present in the waste waters. After treatment in trickling filters, waste waters are usually subjected to further oxidation in a series of tanks which are referred to as

(b) Oxidation ponds:

Treatment in an oxidation pond, also known as aeration tanks, involves slow passage of waste waters, through a large tank in which a large number and variety of micro­organisms are added. Waste waters are mixed “with activated sludge in this tank. Activated sludges are solid or semi-solid sediment derived from the settlement of particulate matter in waste waters. It comprises of a complex mixture of microbes present in the sewage.

It also includes pathogenic organisms. When dry it forms a dark coloured powder, one gram of which may contain as many as 10¹⁴ bacteria. The specific composition of microbes in sludge depends chiefly on the composition of waste water. These microbes rapidly decompose most of the organic matter present in water. Compressed air is blown through the bottom of the tank which provides oxygen necessary for the oxidative decomposition.

The liquid from the oxidation tank is then taken out to the settling tanks where activated sludge and other particulate material is allowed to settle down and the clear supernatant is taken out for disinfection and final disposal or subjected to the tertiary treatment.

B. Anaerobic Treatment :

Anaerobic treatment involves degradation of organic wastes under conditions of oxygen deficit or its total absence. Many of the common heterotrophs of the soil, water and intestinal track are anaerobes or facultative anaerobs. These microbes acting together possess a remarkable capacity to decompose proteins, fats, carbohydrates, cellulose, woody materials, phenols and many other complex substances. The products of anaerobic degradation are reduced substances such as hydrogen sulphide, ammonia, methane and other simpler compounds.

A number of products of anaerobic degradation are useful. Many organic compounds like organic acids, alcohols etc. can be obtained by the activity of microbes under anaerobic conditions on suitable substrates. However, sewage effluents are a complex mixture of organic substances, dissolved salts, particulate material, pathogenic organisms, etc.

It is not possible to obtain useful products with the present state of our knowledge and technology. Methane is a notable exception. It is a common product of anaerobic degradation of organic matter. It can be used as a cheap fuel gas as is done at present in many domestic establishments. So is the solid material left after anaerobic decomposition which is exceedingly rich in plant nutrients? It can be used as manure to fertilize our fields.

Septic tanks are employed for anaerobic treatment of waste waters. A septic tank is usually a large elongate reservoir with a curved or funnel-like bottom. Sewage is allowed to flow slowly through the tank. Sludge precipitates and settles at the bottom. Lighter part of it rises to the surface to form a continuous floating layer. Rising gases trap sedimented particles and bring it to the surface where a thick crust develops.

The oxygen supply from the atmosphere is cut off due to this crust of solid and semi-solid material. The mass of waste water moving in between the upper crust and bottom sludge undergoes rapid decomposition under anaerobic conditions. The products of microbial activity accumulate in the middle layers.

Much of the gaseous material produced is methane, ammonia, hydrogen sulphide etc. which slowly escape to the atmosphere through the upper crust and the smell turns foul. After passage through the septic tank which may take few days to a week, the liquid is drawn out from the middle layers of the tank. This leaves back the upper crust and the bottom sediment in the tank. Waste waters from a septic tank are subjected to further treatment in the tertiary phase.

The semi-solid and solid sludge is left in the septic tank. It is retained in the tank for long durations. During this period much of it is decomposed and its volume is greatly reduced. In order to ensure satisfactory mineralization of organic matter the entire decomposing mass should be periodically stirred and diluted with fresh sewage.

Quite a large number of pathogenic organisms, spores, cysts, eggs of helminths etc. are present in the bottom sludge. Though quite rich in plant nutrients it cannot be used as fertilizer. It is usually spread out on waste land. Microbial activity continues while drying occurs. As there is little organic matter left now pathogenic organisms are slowly reduced and the dried scum becomes fit for application in agricultural fields as manure.

A slight variation of the simple septic tank discussed above is the conventional covered septic tank used in many households which lack sewer lines. Faecal material and waste waters are dumped into a covered chamber. Microbes decompose it under anaerobic conditions. Semi-solid faecal matter being light collects in the chamber while liquids pass through an aperture near the bottom to equalize the level of the tank.

It takes a long time for the matter in the first two chambers to attain the height of aperture to reach the third tank. By the time it reaches this chamber most of the organic matter has been decomposed. Most of the gases collect in the first chamber as it is in this chamber that much of the faecal matter decomposes.

These foul smelling gases escape through a water seal and pipes provided for the purpose. The liquid which collects in the third chamber flows out when it attains the level of the last outlet. This water is odorless, rich in mineral nutrients and disease causing organisms. It is further subjected to tertiary treatment. In practice, however, it is allowed to flow through municipal drainage system.

Liquids and semi-solids with high organic matter content may be used to obtain methane, a fuel gas. Wastes are dumped in a specially designed compartment. A large jar is inverted over it in such a way that its side walls dip deep into the liquid wastes along the walls of the chamber. As decomposition precedes gases rise and are trapped over the liquids under the top of the inverted jar.

Methane forms the bulk of these gases which is taken out through a tube to be used as fuel gas. A system of pullies and weights is attached to the jar to adjust the pressure under the jar. The scum left in the chamber after decomposition of organic matter is rich in plant nutrients. It is dried and finally applied to the fields as fertilizer.