DEWATS-Decentralized wastewater treatment systemsNovember 23, 2008
DEWATS to the rescue
|A look at an efficient method of treating wastewater|
Over 80 per cent of water consumed in flats and buildings comes out as wastewater. In un-sewered areas, the conventional practice has been to use a septic tank as the recipient of wastewater flows and the liquid effluents then emerging being led into soak pits or leaching trenches. This form of treatment is insufficient to render the outgoing effluents pollution free; in high water table areas, septic tanks can cause contamination of groundwater and surface water.
Cleaning septic tanks too is a cumbersome and unpleasant affair. There has been continuous work to find better systems of decentralised treatment of sewage. Domestic wastewater has a high percentage of nitrogen and carbonaceous materials as well as bacteria but is relatively simple to treat as compared to industrial wastewater. The world over, focus is on shifting to decentralised methods of treating wastewater which are simple to operate and economical too.
Low on maintenance
One approach being tried globally is called DEWATS or Decentralised Wastewater Treatment Systems. It aims to use local materials in design while following rigorous technical norms. It tries to be as low in energy intensity as possible and in favourable circumstances the whole treatment process of wastewater can be completely gravity driven without any energy requirement at all. This means that power cuts and load shedding or even accidental switching off of motors or pumps does not come into the picture at all, something which has been the bane of traditional wastewater treatment systems. Wastewater flows as low as 100 litres or 1 cubic metre to as high as 1,000 cubic metres can be handled by DEWATS systems. There is very little or no maintenance though the performance has to be monitored regularly.
A typical system for a domestic household consists of a primary treatment system consisting of a settling and floating tank, a secondary treatment system of an up-flow type baffled reactor which digests wastewater anaerobically, a tertiary treatment in subsurface horizontal flow sand filters with reed beds, and, finally, a polishing pond for oxygenation and UV disinfection from the sun’s rays.
The treatment of wastewater is highly effective and consistently meets pollution norms. Since the baffled reactors work very well, there is complete digestion of solids and usually there are no emptying or cleaning requirements unlike a septic tank. The quality of treated wastewater that emerges into the polishing pond is good enough for landscape applications. The reed bed system in the filter part can be a very good landscape feature with plants like canna offering a colourful and verdant look.
The DEWATS approach reports a 80 to 85 per cent reduction in BOD and COD, a 80 per cent reduction in phosphates and a 60 per cent reduction in ammonia from the input wastewater.
The Bremen Overseas Research Development Association (BORDA) ( www.borda.de) has been at the forefront of DEWATS research and outreach globally and has installed thousands of systems. More information is available on the website www.bord-sa.org where the Centre for Dewats Dissemination (CDD) is working.
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