Decentralised wastewater treatment plant is a site-specific system. The different components of the system settler , anaerobic baffled reactor , planted filter and polishing pond are planned and designed according to the treatment requirement of the wastewater generated. Average water consumption for domestic usage in India is 135 Lpcd. 80% of the water which is used or consumed for domestic purposes comes out as a wastewater. An on-site wastewater treatment plant like DWWT can be installed to treat and recycle this wastewater in order to close the loop. The capacity of the system may vary from 1KLD to 100 KLD . For instance, DWWT system at CSE is designed for the capacity of 8 KLD to treat and reuse wastewater which is generated by the staff population of 150-200.
Broadly, planning and designing, implementation and operation & maintenance activities comprise of 25%, 60% and 15% respectively of the total cost incurred. Parameters which are to be considered while planning and designing DWWT system are land requirement, installation and operation & maintenance.
Land requirement
Depending on the total volume, total area of the land required to install different units of DWWT can be calculated. This is influenced by the nature of wastewater and depth of the unit tanks.
Settler 0.5 m2/m3 daily flow
Anaerobic baffled reactor : 1 m2/m3 daily flow
Constructed wetland: 30 m2/m3 daily flow
Anaerobic ponds: 4 m2/m3 daily flow
Facultative aerobic ponds:25 m2/m3 daily flow
These figures are approximate values, also the area requirement increases with the strength of the wastewater. Structures like settler and improved septic tank or anaerobic baffled reactor are underground hence leads to no wastage of open area.
Installation
This includes activities like excavation, plastering, brick work, plumbing , flooring etc along with the cost of construction material. The different items which are required for construction are walls of solid cement blocks with outside and inside plastering for water tightness, PCC base, PVC pipes , baffle walls, gravel filter media, RCC slab, perforated slabs, vent pipes and plants/reeds like cana, cattails, bulrushes etc. The cost component pertaining to the construction and installation varies from cities to cities. Typically , for installation of a plant of 8-10 KLD capacity would cost around Rs. 2.5-3 lakhs.
Operation & maintenance
Decentralised wastewater treatment systems are low cost on site treatment approach but continuous operation and maintenance is necessary for the sustainability and to maintain the desirable performance of the plant. The annual operation and maintenance cost for the plant is generally in the range of Rs. 3000-5000. Regular de-sludging of the settler and baffled reactor is required in the span of 1-3 years in order to meet the effluent standards. Replacement of filter media is also necessary when treatment efficiency goes down. The normal period of cleaning the gravel filter media is in the interval of 8 – 10 years.
As the capacity increases, up-to some level DEWATS is effective but for larger capacity multi- units of DWWT system is recommended of smaller manageable sizes limited to 100kld. Average cost of construction is Rs. 25,000-30,000 per KL flow per day (approximately).
| Components | Surface area requirement (sqm/KL) |
Capacity (Cum/KL) |
% of Total cost(approx) |
| Settler
|
0.5
|
1 | 10 |
| Reactor
|
1 | 1.73 (inner); 3 (outer) |
40 |
| Planted filter | 5 | 4 | 30 |
| Storage sump
|
- | 1 | 10 |
| Polishing Pond
|
1 | 0.88 | 5 |
| Misc
|
|
|
5 |
*Indicative only, the dimensions and rate may vary as per the inlet water quality and cities respectively. Current rates has to be taken for actual cost estimation.
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