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ABSTRACT

India continues research to find a use for one of its least coveted resources: the 15 million cubic meters of sewage it generates every day. An experimental plant that has been in operation since 1994 processes sewage with aquatic vegetation and fish, based on the concept that sewage is not only waste water but also a source of nutrients. A million gallons of primarily processed sewage is deposited daily in duckweed-infested ponds, followed by carp ponds. Water quality has improved to the point that it can be utilized for agriculture, but not for drinking, after five days.

Keywords: Duckweed, Denitrifying bacteria, Primary productivity, Nutrients value

 

Duckweeds are small, fragile, free-floating plants that help to absorb the nutrients load from sewage waste. They do, however, occasionally grow on dirt or water with a depth of a few mm to 3 meters. When nutrient levels are optimal, their vegetative reproduction can be quick. They grow slowly in areas where there are nutrient deficits or substantial nutrient imbalances. They take advantage of nutrient flushes and might have growth surges during these times. Nowadays it has been widely applied in genetics or biochemical research. The conspicuous potential of duckweed-fish-based systems has been prompted by the ICAR-Central Institute of Freshwater Aquaculture (ICAR-CIFA, Bhubaneswar) and Xavier Institute of Management, Bhubaneshwar (XIM-B) by coming together for demonstration and refinement of this technology for large-scale treatment of the sewage arising from Bhubaneshwar city. With financial assistance of the India-Canada Environment Facility (ICEF), New Delhi, a project named ‘Project WATER’ was initiated in October 2000 in Bhubaneshwar city.

 

 

The Central Institute of Freshwater Aquaculture (CIFA), Bhubaneshwar, India has developed and evaluated an aquaculture-based sewage treatment system with integration of duckweed and fish as biological components. This integrated system where wastewater passes through a series of duckweed ponds, followed by fish ponds was developed at Matagajpur, Cuttack, Odisha under a Project funded by the Ministry of Environment and Forests, New Delhi during 1992-97. The pilot plant has a capacity of treating sewage of 1 million litre/day. The duckweed-fish-based treatment system is an integrated system where wastewater passes through a series of duckweed ponds, followed by fish ponds (Leng, 1999). It is an innovative biological wastewater treatment system possessing the unique advantage of resource recovery in the form of fish and protein-rich duckweed utilizing nutrients from wastewaters. The system developed by CIFA mainly comprised three-component systems, viz. (i) duckweed ponds, (ii) fish ponds, and (iii) depuration ponds.

 

Treatment strategy using duckweed species

The treatment strategy included initial treatment of sewage in duckweed ponds in a retention time of two days, followed by second-stage treatment in fish ponds in a retention period of three days before release into the natural water bodies. The duckweed species, viz. Lemna sp, Spirodela sp, and Wolffia sp act as the nutrient sink absorbing nutrients from wastewater, thereby ensuring their permanent removal from the system through harvested weeds. Fish production levels of 3-4 tonnes/ha/year are achieved in fish ponds with the utilization of sewage as the only input. The technology is being replicated at two strategic points of the Bhubaneshwar city, viz. Vanivihar and Niccopark, under a collaborative project, the details of which are as follows. This one MLD treatment plant covering 0.76 ha area comprised 18 duckweed ponds (0.36 ha) of 25m x 8m x 1m each and two fish culture ponds (0.2 ha) of 50m x 20m x 2m each. In addition, two marketing reservoirs (0.16 ha) of 40m x 20m x 2m were also provided for the depuration before marketing the fish produced in the system. The treatment strategy included allowing the initial retention time of two days in duckweed ponds, followed by three days in fish ponds at the second stage of treatment before release into the natural water bodies.

 

 

The weeds, grown on ponds of the duckweed-fish-based treatment system; act as a nutrient pump, primarily absorbing nitrogen, phosphorus, calcium, sodium, potassium, magnesium, carbon, and chloride from the wastewater. The rapidly growing plants act as a nutrient sink, absorbing various nutrients from wastewater which are removed permanently from the system as the plants are harvested. The duckweed mat covering the pond in association with the bacterial flora jointly works for the purification of the wastewater. There are more than 2,000 species of bacteria and fungi present in the root zone bed. Degradation of organic matter takes place around the root. Most of the organic content in the wastewater is decomposed in the oxidized zones around the root to carbon dioxide and water. Further, ammonia is oxidized to nitrate by nitrification bacteria in these zones. Free oxygen is depleted in regions away from the root surface, thus the denitrifying bacteria present in the zone convert nitrate present in this anoxic zone to free nitrogen. Organic matter is decomposed anaerobically to carbon dioxide and methane by the fermentation process. Simultaneous interactions between different kinds of microbial degradation processes in aerobic and reduced zones decompose organic matter and efficiently remove the nutrients by the plant root systems. The second component of the duckweed-fish-based treatment system, i.e. fish pond further helps in the purification of the semi-treated water received from the duckweed ponds. The microbial population in the fish ponds helps in the treatment of water through both aerobic and anaerobic processes. Higher nutrient availability in the semi-treated water also ensures rich growth of plankton which is grazed continuously by the fish. Such simultaneous production of plankton and their grazing result in the removal of nutrient load from the system continuously. The high protein and fat contents make duckweed plants an attractive food source for animals and poultry.  

Table 1: - Some mineral compositions of duckweed and their potential to remove minerals from water bodies (calculated from the literature).  Source: FAO.org

Concentration in			Potential removal
	Culture medium	Duckweed tissue	at 10 ton DM/ha
Element	(mg/l)	(mg/kg DM)	(kg/ha/y)
N	0.75	60,000	600
P	0.33-3.0	5,000-14,000	56-140
K	100	40,000	400
Ca	360	10,000	100
Mg	72	6,000	60
Na	250	3,250	32
Fe	100	2,400	24

 

General fish species used

Five types of carp species, namely catla, rohu, mrigal, silver carp, and common carp were evaluated in the fish ponds at a stocking density of 10,000 fingerlings/hectare (Ayyappan et al., 2006). Without the provision of any supplement feed and fertilizers, fish production levels of 3-4 tonnes/ha/year could be achieved with utilization sewage as the only input. The fishes harvested from these ponds are kept in depuration ponds for at least one week before marketing for reducing the possible risks due to microbial contaminations. Such depuration processes, are not necessarily be located at the treatment site.  

 

Pond Productivity

Gross Primary Productivity (GPP) and Net Primary Productivity (NPP) are two important parameters indicating the condition of pond water for aquaculture activity. Nutrient enriched sewage water facilitates high production of plankton population which is considered to be the base of the trophic level of the food chain (Bhaumik et al., 1993; Boyd and Tucker, 1998). Phytoplankton production increase rapidly due to fertilization through sewage, constituting about 80% of the total plankton population, with a range recorded between 65,000-1,20,000 no’s/L and zooplankton being about 20%, with the range between 20,000-45,000 no’s/L. Diatoms represent the major constituent of the benthic community. Phytoplankton population in sewage-fed pond dominates over zooplankton during most of the culture period.

 

 

Growing Duckweed and its nutrient value

                                                                                                                          

The optimal circumstances for growing duckweed are a protected lagoon or a lagoon with surface partitions to prevent wind from moving the plants onto the banks, producing self-shading and nutrient competition. Nutrients must be made accessible at a pace proportional to growth, generated either from organic or mineral fertilizers applied daily, for high growth rates. Duckweed reproduces predominantly by vegetative reproduction. Before the original plant senesces, a solitary leaf may go through 10 divisions over 10 days to several weeks. Under ideal nutrition supply, sunshine, and water temperature, duckweeds can double their mass in 16 to 20 days. This is far faster than nearly any other higher plant. The growth pattern is more similar to that of unicellular algae than that of higher plants, which means it has a high potential for production as a livestock feed resource. A multitude of factors can slow the growth of duckweed colonies, including nutrient scarcity or imbalance, toxins, pH and temperature extremes, colony overcrowding, and competition for light and nutrients from other plants. When conditions are favourable, however, duckweed has significant amounts of protein, fat, carbohydrates, and minerals, which appear to be mobilized for biomass development when nutrient concentrations fall below critical levels.

 Conclusion

Proteinaceous feed resources are generally scarce, and they are also the most expensive components of most animals` diets in underdeveloped countries. Duckweed is a protein source with a lot of promise for feeding domestic animals, especially fish. The nutrients in the water where the duckweed is cultivated have a significant impact on its nutritional value, especially for mono-gastric animals for which the duckweed`s fiber and protein content are vital.

The well-developed system of concentrating minerals in duckweed allows them to grow under a wide range of conditions. As a result, an environmentally friendly approach to duckweed culture in nutrient-rich village ponds will not only aid in the free extraction of nutrients (which would otherwise pollute the water and go to waste) in the form of protein-rich duckweed but will also bio-remediate the ponds and make them more suitable water resources for aquaculture. Poly-culture of wastewater, duckweed, and carp is the ideal integrated solution for pollution management and nutrient reuse.

 Reference

Ayyappan, S., Jena, J. K., Gopalakrishnan, A., & Pandey, A. K. (2006). Handbook of fisheries and aquaculture.

Bhaumik, V., Pandit, P.K. & Chatterjee, J.G. (1991). Impact of epizootic ulcerative syndrome on the fish yield, consumption, and trade in West Bengal. Journal of Inland Fisheries Society of India, 23:45-51.

Boyd, C.E. & Tucker, C.S. (1998). Pond aquaculture water quality management. Kluwer Academic Publishers, London. 700p.

ICAR-CIFA. 2004. Annual Report, Central Institute of Freshwater Aquaculture, Bhubaneswar.

Leng, R. A. (1999). Duckweed: A tiny aquatic plant with enormous potential for agriculture and environment.

 

https://www.fao.org/ag/againfo/resources/documents/DW/dw2.htm.

 

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5.Integrated duckweed cum fish culture practice in CIFA_compressed.pdf