Water hyacinth
Practical Action
Eden (1994) considers the requirements for large-scale production of charcoal briquettes
from water hyacinth. He states that with an energy density of 8.3 GJ/m3 this would be
comparable with the energy density of charcoal at 9.6 GJ/m3.
However, for a plant to produce 40 tonnes per day of briquettes an area of 12 hectares would be
required for drying the water hyacinth, 1,300 tonnes of wet hyacinth would be required daily and
the climate would need to be one of low humidity and relatively high temperature.
Biogas production. The possibility of converting water hyacinth to biogas has been an
area of major interest for many years. Conversion of other organic matter, usually animal
or human waste, is a well established small and medium scale technology in a number of
developing countries, notably in China and India. The process is one of anaerobic
digestion which takes place in a reactor or digester (an air tight container usually sited
below ground) and the usable product is methane gas which can be used as a fuel for
cooking, lighting or for powering an engine to provide shaft power. The residue from the
digestion process provides a fertiliser rich in nutrients.
The use of water hyacinth for digestion in a traditional digester presents some problems.
Water hyacinth has a very high water content and therefore harvesting effort yields a low
reward in terms of organic matter for conversion to biogas. The digester size has to be
large compared with that of a traditional type due to the low gas production to plant
volume ratio and this can in turn present problems for obtaining an airtight seal. Water
hyacinth has to be pre-treated before entering the digester (macerated, chopped or
beaten) to promote digestion and to remove air entrapped in the tissue of the plant which
would cause it to float.
To reduce the need for large volume digesters high rate digestion techniques have been
employed. One such design has been tested in Bangladesh by a team from Warwick
University, UK and the Housing and Building Research Institute, Dhaka, Bangladesh.
The design was for a small (8.3 cubic metre) baffled reactor which was fed with juiced
water hyacinth. The throughflow of the reactor was 1.2 cubic metres per day. Some cow
dung and rumen (taken from a cow’s stomach) was added to the water hyacinth juice to
promote digestion. Gas was produced in reasonable quantities but some problems were
experienced with throughflow and further development is still required.
Other studies have been carried out, primarily in India with quantities of up to 4000 litres of gas
per tonne of semi dried water hyacinth being produced with a methane content of up to 64%
(Gopal 1987). Most of the experiments have used a mixture of animal waste and water hyacinth.
There is still no firm consensus on the design of an appropriate water hyacinth biogas digester.
Water purification. Water hyacinth can be used to aid the process of water purification
either for drinking water or for liquid effluent from sewage systems. In a drinking water
treatment plant water hyacinth have been used as part of the pretreatment purification
step. Clean, healthy plants have been incorporated into water clarifiers and help with the
removal of small flocs that remain after initial coagulation and floc removal or settling.
(Haider 1989). The result is a significant decrease in turbidity due to the removal of
flocs and also slight reduction in organic matter in the water.
Figure 4: Sewage system using water
hyacinth. ©Paul Calvert/Practical Action
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