Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
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
Photo 12: A duckweed based wastewater treatment system at Mirzapur, Bangladesh
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.
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.