ANAEROBIC DIGESTION OF WATER HYACINTH

Abstract

As the scope of man's activities has increased, so has his need for energy. The present-energy resources i.e. oil, coal, and natural gas etc. are finite and supplies of these fossils fuels will become scarce in near future. Anaerobic digestion 6f renewable biomass produce a 'concentration' and easily utilisable energy i.e. BIOGAS. The anaerobic digestion of organic substances consists of four stages: hydrolysis of complex organic molecules, acid production, hydrogenogenesis,from acids and methane formation. However,.'Heavy Metals' are inhibitory, to most microbial processes. Water hyacinth has been considered a problematic aquatic weed. However, the highly prolific water hyacinth when put to work, under controlled conditions can produce high quality vegetable protein, vitamins, minerals, fertilizer, chemicals, fibre, energy (biogas) etc. Water hyacinth harvested from a toxic waste cannot be used as a feed'or fertilizer. Such a toxicated water hyacinth biomass can possibly be digested-to produce biogas. The heavy metals present in such biomass may/may not affect the anaerobic digestion-. A few preliminary experiments particularly to study the effect of cadmium ion on water hyacinth anaerobic digestion were. carried out in- this laboratory. The results clearly. indicate the increase in methane percentage as well as failure of digestion process at different levels of cadmium ion. It therefore, necessitated to find out optimum level and lethal dose of cadmium salt in water hyacinth anaerobic digester. The present project aims at the - i) estimation of optimum and lethal dose of cadmium salt, and ii) effect of intermittent feeding of raw material i.e. rate of loading on biogas production The later aspect shall be very useful for field applications. Nine similar digesters were fed daily with water hyacinth and stabilized under identical conditions for 35 days. Different • concentrations of cadmium were maintained in five digesters at constant temperature and their performance, after stabilization, were compared with the digester fed with water hyacinth only and no toxic material. Intermittent feeding at regular interval was done in two digesters, kept at room temperature, and the performance was compared with the digester fed daily. This study revealed that in stable condition of diges-ters, the daily yield of. biogas was 291.07, 223.21, 227.50, 239.29, 243.57, and 245.36 litres/kg of volatile solids added and their methane fraction was 71.55, 87.71, 88.00,'89.00, 89.40,an< 89.90 percent for digester no. I(blank-no toxic matter) and II to VI (containing different concentrations of -cadmium)respectively. M1 i i i • Although the total yield of biogas from contaminated digesters • decreased, its methane content increased by as much as 25.65 • percent in comparison to the methane produced in uncontaminated digester. Increase in biogas production as well as in its methar fraction with the increase in cadmium concentration was also noti Also, from the results of the present project it appears that • the cadmium salt, when present in lower concentration (e.g. 5 mg/l00 gm wet weight water hyacinth) is more toxic than the higher concentration. Toxicity has been measured as gasification • activity. The calorific value of gas from cadmium contaminated digesters was found as much as 33.99 percent more compared to uncontaminated digester. In batch feed digest-ers, the rate of loading affects.` the biogas production and daily feeding has been observed to • give maximum gas production.