One of the major problems with waste water treatment methods is that none of the available technologies has a direct economic return. The available technologies are unaffordable due to high capital and maintenance costs. With no economic return, local authorities are generally not interested in taking up treatment of waste water, thereby causing severe health hazards and environmental pollution. In India out of about 4700 towns / cities, only 232 have the sewerage system, and that too only partial. Most of the untreated waste water is, therefore, discharged into rivers or other water bodies. In rural areas it is a common practice to discharge waste water/sullage without collection. There is no question of treatment/recycle or even reuse of waste water/sullage as people are not aware of this technology.
The Sulabh has successfully developed demonstration projects on duckweed-based cost-effective waste water treatment in rural and urban areas with direct economic returns from pisciculture. Although duckweed is found in ponds and ditches, due to almost complete absence of any know-how of this technology in the country, the potential of duckweed for the waste water treatment, its nutrient value and economic benefits have not been fully exploited.
Duckweed – a small free-floating and fast growth aquatic plant-has great ability to reduce the BOD, COD, suspended solids, bacterial and other pathogens from waste water. It is a complete feed for fish, and due to the high content of proteins and vitamins A & C, it is also a highly nutritious feed for to 3 times when fed with duckweed, than with other conventional feeds in ponds. Reduction of BOD, COD in effluents varies from 80-90% at the retention time of 7-8 days. The first project funded by the Ministry of Environment & Forests, Govt. of India, was successfully completed in collaboration with the Central Pollution Control Board, New Delhi. Based on the R & D outputs of the project, the CPCB has made guidelines on the use of duckweed for waste water treatment.