Two-phase anaerobic digestion of water hyacinth or urban garbage

Diphasic fermentation of water hyacinth or garbage to biogas was tried by coupling a solid-phase acidogenic system (TS 20-30%) to an upflow, anaerobic packed-bed, methanogenic digester. A daily sprinkling of the biomass bed with an aqueous suspension of biodegrading bacteria resulted in accumulation of intermediate volatile fatty acids (VFA) in the sprinkling liquid which gradually lowered the pH to about 5 and suppressed methanogenesis in the biomass bed. A part of the VFA-bearing leachate (following sprinkling) from the acidogenic reactor was fed to the methanogenic reactor for methane production. Clear phase separation was obtained only in the digester fed with garbage. Phase separation occurred when the total VFA levels were 5-6 g $litre^{-1}$ (pH 5). In the case of fresh- and dried-water hyacinth significant levels of gas production occurred from the decomposing biomass bed and phase separation could be achieved only for short periods (25 days). Low rates of acidogenesis (therefore moderate pH levels) from these substrates encouraged methanogenesis within the biomass bed. Results indicated that the biomass bed may be used alone as a packed-bed digester to obtain high biogas production rates. In the present study specific gas yields between 146 and 181 litres $kg^{-1}$ TS were obtained for the substrates tested. Solid-phase fermentation is feasible with these substrates and could reduce digester volume and costs of operation. This process overcomes problems associated with feeding, control of floating scum and continuous/semi-continuous operation normally encountered when untreated-biomass feeds are used in conventional biogas digesters