What Is Decentralized Wastewater Treatment System (DEWATS) ?


The most appropriate technology is that which provides the most socially and environmentally acceptable level of service at the least economic cost. But to the amusement of many DEWATS is an approach, rather than just a technical hardware package. A lot of Decentralized Wastewater treatment systems (DEWATS) have been constructed in India over the past, with the intention of providing treatment to wastewater generated from various sectors like hospitals, hotels, institutions, small and medium scale enterprises, community-based sanitation complexes, individual houses and housing colonies.

There has always been a need for a system like decentralized wastewater treatment in Developing Countries. The use of low maintenance technology is the only realistic approach for the time being. In most of the DEWATS units, reuse infrastructure is provided to ensure a holistic approach to environmental sanitation. It’s based on a set of treatment principles the selection of which has been determined by their reliability, longevity, tolerance towards inflow fluctuation, and its control and maintenance.

In general, DEWATS are locally organized and people‐driven systems that typically comprise a settler, anaerobic baffled tanks, filter beds of gravel and sand, and an open pond. It provides treatment for wastewater flows from 1 – 500 m3 per day, from both domestic and industrial sources. DEWATS is  based on four treatment systems :

  1. Sedimentation and primary treatment in sedimentation ponds, septic tanks.
  2. Secondary anaerobic treatment in fixed bed filters or baffled septic tanks (baffled reactors )
  3. Secondary and tertiary aerobic/ anaerobic treatment in constructed wetlands (subsurface flow filters )
  4. Secondary and tertiary aerobic/anaerobic treatment in ponds.


The septic tank is the most widely used in decentralized wastewater treatment. It was invented by French engineers around the turn of the last century. But this system reduces pollution load by only 30 percent. The baffled septic tank (or the baffled tank reactor) is far more efficient; it reduces pollution load by up to 90 percent. Open ponds have been used for treating wastewater since ancient times.


The open pond or the polishing tank recreates a living environment for the wastewater to clean itself, naturally. The system operates without mechanical means and sewage flows by gravity through the different components of the system. This system can treat up to 1,000 cubic meters of domestic and non-toxic industrial wastewater. DEWATS applications are based on the principle of low‐maintenance since the essential parts of the system work without technical electrical energy inputs and cannot be switched off intentionally.

It guarantees permanent and continuous operation. However, fluctuation in effluent quality may occur temporarily. DEWATS is not everywhere the best solution. However, where skilled and responsible operation and maintenance cannot be guaranteed, DEWATS technologies are undoubtedly the best choice available.

DEWATS approach is a practical, efficient and affordable wastewater treatment solution for not only small and medium-sized enterprises but also for the un‐served (rural and urban) households in developing countries, especially South Asia. For instance, DEWATS can operate in individual houses, at the neighborhood level and even in small and big factories not connected to sewage lines. DEWATS can also treat municipal waste. The recycled water is used for irrigation or growing plants and is safe for human use.


Source: BORDA

The need for decentralized initiatives in wastewater treatment

In India, about 12 million (7.87 %) urban households do not have access to restrooms and defecate in the open. Approximately 5 million (8.13 %) urban homes use community latrines and 13.4 million households (19.49 %) use shared baths. About 12 million (18.5 %) families do not have access to a drainage network while approximately 27 million (39.8 %) households are connected to open drains. The status in respect of the urban poor is even worse. The percentage of notified and non‐notified slums without latrines is 17 percent and 51 percent respectively. More than 37% of the total human excreta generated in urban India, is unsafely disposed of. This imposes significant public health and environmental costs to urban areas that contribute more than 60% of the country’s GDP. Impacts of poor sanitation are especially significant for the urban poor (22% of the total urban population), women, children and the elderly. Inadequate discharge of untreated domestic/ municipal wastewater has resulted in contamination of 75 percent of all surface water across India.

Specifically, in India, domestic wastewater, including sewage that is often not even collected, is a significant source of pollution of surface water. This contributes to contamination of groundwater  ‐  an essential or only source of drinking water for many urban and peri‐urban areas. Besides, the economies of scale required for using conventional technologies would not be achieved in all settlements for various reasons, including i) different climatic conditions; ii) topography; iii) geological conditions and water tables; iv) levels of urbanization; and v) population densities and size of settlements.

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