Simple systems available to treat sewage

INSTEAD of energy-consuming systems, simple sewage treatment methods such as oxidation ponds could be utilised in Indian cities and towns. Oxidation ponds are lagoons or basins in which waste water is cleansed through sedimentation and the action of microorganisms.

These ponds require land -- and land is expensive and difficult to acquire in urban areas. Another constraint of an oxidation pond is that does not purify effluents adequately before discharge into rivers. Nevertheless, water from these ponds can be used for irrigation and so not using oxidation ponds means a significant loss, especially as they are more reliable than conventional mechanical plants.

Oxidation ponds are mistakenly labelled "second class treatment plants" because they lack machines, even though efficient ponds have been built in such locations as the Bhilai steel plant and Ahmedabad city. But some of these ponds have since been replaced by conventional, trickle-filter plants that do not work as well. The Jordanian capital, Amman, with a population of one million, has the world's largest oxidation ponds.

In recent years, the use in oxidation ponds of anaerobic processes, which involve microorganisms such as certain bacteria that do not require oxygen, in conjunction with aerobic processes (using microorganisms, such as algae, that need oxygen) has reduced land requirements and lowered power consumption to negligible levels.

Anaerobic treatment is ideal for India's tropical climate. The near-zero power requirement for the anaerobic step in which 80 per cent water purification takes place, is the main feature. However, some post-anaerobic treatment is required to reduce toxicity and meet effluent discharge standards. This can be achieved by simple aeration, wherein carbon dioxide is charged through the water so that effervescence takes place. Natural aeration can be achieved in long, open, delivery channels used in irrigation or, in some cases, by using short-detention aeration equipment.

For many reasons, a combination of anaerobic and aerobic treatment, followed by irrigation, would appear to be ideal for larger towns. Land requirement is manageable and no mechanical equipment is needed, which means no plant repairs or maintenance. Power requirement is minimal and, in fact, electricity can even be produced. Biogas produced in the anaerobic step also can be collected for domestic consumption.

Sewage treatment and disposal practices in India should include one of the following: oxidation ponds and irrigation or anaerobic treatment, which is a combination of natural aeration or short-detention ponds and irrigation. Planners should be made aware that anaerobic processes can now be used in conjunction with aerobic processes.

In small towns and on the outskirts of cities, where the population density is not high enough to make a sewage system viable, the use of low-cost sanitation methods, such as pit latrines and soak pits, would be appropriate as these, too, do not require electricity.

In these post-Rio days, the use of such simple methods makes sense both economically and ecologically and so funding agencies must insist natural methods are given priority, with power-intensive methods being tried only if essential.

---Soli J Arceivala is managing director of Associated Industrial Consultants (India) Pvt Ltd, Bombay, and former chief of environmental health for WHO in southeast Asia. This piece was written in response to an article on low-cost water treatment alternatives by Janusz Niemczynowicz in Down To Earth, October 15, 1992.

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