With the introduction of public water supply in rural areas and as a result of domestic activities a lot of sullage water is generated. This causes serious health hazards due to non-availability of proper drainage system. This wastewater discharged to ponds fouls the environment and results in unsanitary conditions, thereby jeopardizing the community health. The available technologies for the treatment of wastewater involve high capital as well as operational and maintenance costs and also offer no economic returns, hence, there is an urgent need for alternate technological options.

The duckweed based waste water treatment system in conjunction with pisciculture is one such technology that has the potential of offering effective waster water treatment besides providing economic returns as well as generating employment opportunities in the rural areas. The duckweed is a small free floating aquatic plant often seen growing in thick mats on nutrient rich fresh and brackish waters in tropical and semi tropical climate all over the world.

It belongs to botanical family Lamnaceae consisting of four genera namely; Lemma, Spirodela, Wolffia and Wolffiella of which first three genera are commonly found in India. The duckweed has the ability to bio accumulate up to 99% of the nutrients, dissolved solids and even heavy and toxic elements of wastewater up to certain extent. These are permanently removed from wastewater as plants are harvested. Hence, it reduces Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), suspended solids, bacterial and helminthic pathogens, some organic compounds, ions of potassium, nitrogen, phosphate and even heavy metals of wastewater to a level quite safe for disposal.

Further, the duckweed plants can double their mass in about two to three days under ideal conditions of nutrient availability, sunlight and temperature. The average daily yield of fresh duckweed from a pond of one hectare is about 0.5-1.5 tons. It is highly nutritious feed due to high content of proteins, fiber content and vitamins A & C. The prolific growth of duckweed can be harvested daily from the duckweed pond and can be utilized as fish feed. Hence, not only the duckweed treated wastewater

can be used for pisciculture but also the harvested duckweed can serve as complete feed for fish. Also interestingly, the duckweed species grow faster in warm weather, when fish need more feed and more slowly in cold weather when the fish also do not require as much feed. 
 

• To start a duckweed based sullage treatment unit in village pond, it is desirable to take up the duckweed project in villages where two separate ponds are available near by. The existing ponds are deepened, cut and dressed as per requirement. However, if the two separate ponds do not exist then a pond with a size of one hectare or more is partitioned into two equal parts (half hectare or more each) by construction of earthen bounds. Out of these two ponds, one is used for duckweed cultivation and other for pisciculture. For the treatment of sullage it is important to give it proper retention time (minimum 12-15 days) in the duckweed pond. Therefore, the depth of the duckweed pond from the invert level of incoming drain is maintained to give the sullage the minimum required retention time for treatment. 

• The minimum viable size of pond for implementation of this technology is one hectare (to be partitioned into two equal halves) as quantity of sullage discharged by village having population of 4000-5000 can successfully meet the nutritional requirements of duckweed in half-hectare pond and can thus be integrated with pisciculture. 

• Most of the village ponds are located in low-lying area and in rainy season, the area around the pond gets flooded. For successful execution of the project, bunds of the pond are raised so that the pond does not get flooded during the rains. Alternatively, bypass arrangement on the inlet drains along with a shutter is provided to check the flow of excess water into the ponds. 

• The over flow pipes encased with wire mesh are embedded in the raised earthen bunds of fishpond to allow over flow of water in case of heavy rains. The proper slopes and compaction of the bunds is very important for the development of ponds. The minimum slope required for preparation of the bund is kept 2:1 (2 horizontal and 1 Vertical) 

• The sullage brought by the main drain of the village after passing through the inlet screen (Where all the suspended debris are removed) is allowed to enter in the duckweed pond. About 100 Kg of duckweed is inoculated in duckweed pond. A floating bamboo grid (details given below) is used to check the drifting of duckweed towards one side by wind in the duckweed pond.

• For pisciculture, six varieties of fish viz. Catla, Rohu, Grass Carp, Common carp, Silver carp and Mrigal (suitable for flood water cultivation) cultivated in a separate fresh water village ponds. The sullage, after duckweed treatment is fit for pisciculture using the said varieties. 

• The treated sullage from the duckweed pond is allowed to flow to the pisciculture pond through culvert pipes encased with fine mesh as and when required and excess water if any is used for irrigation purposes.

• The duckweed grown in the sullage pond is fed to fish through open bottom fish feeding stations, which comprise 4m X 4m floating PVC grid. Four fish feeding stations are required in a fishpond of half-hectare size.

     Sites adopted under Duckweed project in various Districts of Punjab

Conclusion