DEWATS

DEWATS or DecentralisedWasteWaterTreatment System on the other hand looks attempts at smaller dispersed treatment systems, which are basically anaerobic, with low maintenance, low

cost. It is now well established that such Decentralised Anaerobic Treatment of Sewage andWaste water, is suitable for tropical climates like India. DEWATS is basically not a system to deal with excreta alone. It is part of a larger system to treat and re-cycle waste-water from all domestic and industrial and other sources. DEWATS also does not actually refer to a particular technical design or structure. DEWATS is an approach that uses several processes to treat wastewater, which is adapted to the local situation.

In Structure, DEWATS works like a sewer system, except the system is decentalised, which makes it simpler in process, technology, and operations and maintenance.

It has three characteristics or rather basic principles on which it is applied:

Decentralization: Responsibility, Capacity,Treatment.
Simplification: Process, Technology, O&M (operations&maintenance)
Conservation: Recycling Water, Nutrients, Energy

WASTE TREATMENT

Pre-treatment is done to eliminate solids by putting in an interceptor tank like a septic tank where all settle-able solids settle. The interceptor tank can be provided for every house or for a cluster depending on space availability The solid-free effluent is then let into the collection system. The sewers are laid at shallow depth as there is no solid component in the effluent, no regular manholes required.However a clean-out can be provided which can be used to push water to clean up in case of any clogging. Since solids are seperated out at the first stage, deep sewer lines are not required Treatment of this effluent waste is done in decentralised clusters through a variety of secondary and tertiary treatments systems such that the effluent conforms to discharge standards of the Pollution Control Boards. This treatment is based on four treatment systems:

Sedimentation and primary treatment in settlers, septic tanks or Imhoff tanks.
Secondary Anaerobic Treatment in Anaerobic Baffled Reactors or Anaerobic Filters.
Secondary and tertiary aerobic / anaerobic treatment in Planted Gravel Filters.
Tertiary anaerobic /aerobic treatment in ponds.

Further the water is such that it can be used for irrigation and other non-portable purposes.The various process are chosen such that the treatment process is does not need electricity and they are reliable and durable, requiring minimal maintenance
Since it is decentralised:

The design and nature of treatment can be specific to the nature of waste including domestic and industrialwaste.
The waste doesn't have to traverse long distances, distances thereby eliminating huge costs on pipes, pumps and appurtenances. The main disadvantage of the technology is that it requires space at the local level. Also like all decentralised and locally built options, quality is known to vary.

Model	Cost*	Capacity
Dry compost toilet	Rs 5000 -6000 per unit cost -usage lifetime	Individual
DEWATS-Ferro cement	Rs 0. 16 / user / day for 30 years (inclusive of maintenance)	500 users (125households)
DEWATS-Ferro cement	Rs 0. 19 / user / day for 30 years (inclusive of maintenance)	200 users (50 households)
DEWATS -FRP	Rs 0.33 / user / day for 30years (inclusive of maintenance)	1250 users (313 households)

[Taken from ECOSAN Costing. TNTRC Newsletter. www.tntrc.org ]
*Costs vary according to local conditions