Decentralised Wastewater Treatment (DWWT)

What is decentralised wastewater treatment system?  

There are several kinds of wastewater treatment systems which are in use. They range from conventional low-cost options and decentralized small systems to large, expensive, centralized systems.

Decentralised systems are small, individual or cluster type wastewater facilities to provide wastewater treatment services to residents. In the decentralised wastewater treatment systems, wastewater can be treated onsite through aerobic and anaerobic techniques. The anaerobic modules comprise of settlers, baffle reactors and anaerobic filters. The aerobic modules have horizontal planted gravel filters and polishing ponds. This approach is based on different natural treatment techniques, put together in different combinations according to need. It is used for recycling both “grey” and “black” domestic wastewater. This systems include:

  • Primary treatment, which includes pre-treatment and sedimentation in settlement tank or septic tank;

  • Secondary anaerobic treatment in baffled reactors;

  • Tertiary aerobic/anaerobic treatment in reed bed system; and polishing in Ponds

Primary Treatment      
Pre-treatment is used for the screening and separation of the floating oil and grease in a grease trap.
Fig. 1 Separation of floating oil & grease in a grease trap
Primary treatment takes place in a settler or a septic tank in which the liquid part is separated from the solid matter through sedimentation process. The settled sludge is stabilised by anaerobic digestion.
Secondary Treatment      
Secondary treatment of the wastewater takes place in the Baffled Reactors which consist of a series of chambers, in which the wastewater flows up-stream. On the bottom of each chamber activated sludge is retained. During inflow into the chamber wastewater is intensively mixed up with the sludge whereby it is inoculated with wastewater organisms, which decompose the contained pollutants.
Tertiary treatment      
Tertiary treatment takes place in the planted Gravel Filter through rootzone system. . The main removal mechanisms are biological conversion, physical filtration and chemical adsorption. The treated water at this stage meets the requirement for recycling and reuse of water for horticulture/irrigation purposes.


Fig. 3 Rootzone treatment system for tertiary treatment
Polishing Ponds      
Here, both aerobic degradation and pathogen removal takes place. It is simple in construction, reliable in performance if properly designed, high pathogen removal, can be used to create an almost natural environment, fish farming s possible in large and low-loaded polishing ponds.


Fig. 4 Polishing pond for post treatment