Influence of kinetics of hydrolysis, acidogenesis and methanogenesis on enhancement of the production of biogas from animal excreta

Abstract

Experiments were undertaken with the aim of increasing the biogas production obtained from animal wastes. In order to determine the rate-limiting step of the anaerobic process, kinetics of some metabolic steps have been considered: polysaccharide hydrolysis, acidogenesis and methanogenesis. To a 12-litre digester, fed daily with pig sludges, known quantities of substrates, microcrystalline cellulose (MC), carboxymethylcellulose (CMC), hemicellulose (HC), glucose (G) and acetate (A), were added. In the following hours yield and composition of the biogas produced were measured together with microbial flora, adenosine triphosphate (ATP), glucose and volatile fatty acids (VFA). The results showed that the rate of biogas production from addition of glucose and acetate is very high. In the case of glucose the rate was 140 litres/kg VS/h at the 8th hour; the biogas yield in 8h was 72% of the maximum theoretical value. HC, CMC and MC gave lower biogas productions. The addition of a readily fermentable substance gave a temporary decrease in the microbial biomass. Work has shown that acidogenesis and methanogenesis are not rate-limiting steps in the process of biogas production. The hydrolysis rates of pure polysaccharides (HC, CMC, MC) are slower than those of the acidogenic and methanogenic steps. However, the pure polysaccharides are almost completely converted to biogas and it would seem that the presence of lignin limits the conversion of polysaccharides in natural fibres.