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9.3.3 Case Study 3 - Duckweed based wastewater treatment and reuse

PRISM, a non-government organization (NGO) in Bangladesh, has carried out a research and development programme with duckweed based, wastewater treatment and reuse through fish culture. There are systems fed with conventional wastewater or sewage in three districts, the largest being at the Kumudini Hospital Complex (KHC), Mirzapur, Tangail district (Photo 12). These are located in different parts of the country and serve also as demonstration and training centres. PRISM has also developed a village level sanitation system which stimulates the installation of latrines connected to small derelict ponds which are used to cultivate duckweed (Photo 13). Harvested duckweed is again used for fish culture.

Photo 12: A duckweed based wastewater treatment system at Mirzapur, Bangladesh

Conventional wastewater

The development of the first duckweed, conventional wastewater treatment system began, in 1989 at the KHC in Mirzapur. The facility consists of one duckweed covered, 0.7 ha plug flow lagoon constructed as a 500 m long serpentine channel with seven bends. It is fed with a mixture of hospital, school and domestic wastewater from some 2,350 people with per caput production of wastewater estimated at 100 l/day. The plug flow wastewater-fed duckweed pond is preceeded by a 0.2 ha anaerobic pond with a hydraulic retention time (HRT) of 2-4 days. HRT in the plug flow pond is estimated at 21-23 days. Duckweed harvested from the 0.7 ha wastewater treatment pond is fed daily to three adjacent fish ponds, each 0.2 ha.

Photo 13: Demonstration of a nightsoil - fed duckweed system for rural households at Mirzapur, Bangladesh

Wastewater treatment efficiency is excellent. The duckweed remove nutrients and the plant cover suppresses phytoplankton growth. Average removal efficiencies for BOD5, N and P, and faecal coliforms are 90-97%, 74-77%, and 99.9%, respectively. Effluent turbidity is always below 12 NTU. The effluent is used to top up the water level of the adjacent fish ponds. The quality of the effluent is so high (< 102 FC/100 ml) it could be used for unrestricted irrigation of vegetables according to WHO standards for wastewater reuse.

The wastewater treatment system produces from 220 to 400 tonnes freshweight duckweed/ha/year (about 17 to 31 tonnes dry weight/ha/year). The fish ponds are stocked with a polyculture of Indian major carps (rohu, mrigal and catla), Chinese carps (grass carp and silver carp), and common carp. Tilapia is not stocked but fingerlings enter the ponds incidentally. Fish production varies from 10 to 15 tonnes/ha/year, about 40% of which is tilapia. Fish yields are relatively high because of frequent harvesting and addition of other feed besides duckweed such as oil cake and rice bran. Research elsewhere has demonstrated a food conversion ratio of duckweed to fish of 3.4 (duckweed dry matter and fish freshweight bases).

Over the last two years the wastewater-fed duckweed-fish system has generated a net profit of almost US$3,000/ha/year. This is about three times that of the major agricultural crop of the area, rice. The internal rate of return was 25.9%. The system could be optimized further in terms of pond design and operation because it is overdimensioned; a high degree of treatment efficiency is already attained at about 60% of the length of the plug flow pond. This case study demonstrates that it is possible to develop a wastewater treatment system incorporating fish culture that not only achieves cost recovery but derives a net profit.

Nightsoil

PRISM has also implemented a sanitation - aquaculture programme in the rural areas of Bangladesh. Over 1,000 villagers belong to almost 150 small-scale enterprises which they own and operate collectively. As there is no conventional wastewater in rural villages, latrines have been constructed around derelict or unused ponds to cultivate duckweed, which is harvested and fed to fish in separate, nearby ponds.

The latrines consist of a moulded concrete slab connected to a pipe to convey nightsoil directly to the duckweed pond. The nightsoil is discharged in the water inside a retaining basket made of woven bamboo slats. The duckweed is cultivated in derelict or unused ponds and ditches. As most are shallow and shaded, they are unsuitable for fish culture.
Most of the ponds used to culture fish have multiple ownership which often constrains productive use. Aquaculture is being promoted through the programme leading also to the construction of new fish ponds on unproductive land. Poorer pond owners now consume more fish from their own ponds and earn more income. Furthermore, the availability of fish in local markets has increased, particularly fish smaller than 0.5 kg as encouraged by PRISM which provides greater access of the poor to cheaper fish. Prior to the introduction of duckweed as fish feed by the programme, it was totally unused. Now there is an informal market for duckweed as the poor collect it from floodplains in the rainy season to sell to fish pond operators.

There are technical and social constraints to the programme. Most household ponds are seasonal and cannot be used year round for duckweed cultivation. Duckweed may not comprise the major input to feed the fish, requiring purchase of fertilizer and feed. During the dry season, the latrines may present a health hazard and require redesigning. Some of the enterprises are losing concerns, frequently due to a credit overburden because of a large initial investment for pond re-excavation or new pond construction on low-lying land. Nevertheless quite a number of the enterprises are making an annual net profit from their duckweed-based, fish culture systems. This indicates that the system, which is still relatively new, may have potential for more widespread dissemination in Bangladesh and elsewhere.

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