Anaerobic Treatment of Agricultural Residues and Wastewater - Application of High-Rate Reactors

Abstract: The production of methane via anaerobic digestion of agricultural residues and industrial wastewater would benefit society by providing a clean fuel from renewable feedstocks. This would reduce the use of fossil-fuel-derived energy and reduce environmental impact, including global warming and pollution. Limitation of carbon dioxide and other emissions through emission regulations, carbon taxes, and subsidies on biomass energy is making anaerobic digestion a more attractive and competitive technology for waste(water) management. This thesis is concerned with some important aspects of anaerobic digestion of solid potato waste, sugar beet leaves and opaque beer brewery wastewater. Studies were performed using batch, one-stage and two-stage processes using laboratory-, pilot- and full-scale anaerobic reactors. For improved understanding of the anaerobic digestion of solid potato waste, some of the aspects investigated in this work were the profiles of hydrolytic enzymes, the distribution of the major volatile fatty acids produced in the acidification stage, the organic matter degradation, methane yield and the effect of co-digestion. During the hydrolysis of solid potato waste, both free and cell-bound hydrolytic enzyme activity was observed; amylase activity was found to be the highest followed by carboxylmethyl cellulase and filter paper cellulase. The fermentation products during batch anaerobic digestion of solid potato waste were chiefly acetic, butyric, propionic and lactic acid. The concentration and proportions of individual volatile fatty acids in the acidogenic stage are important in the overall performance of the anaerobic digestion system since acetic and butyric acids are the preferred precursors in methane formation. The performance of two-stage anaerobic digestion systems under mesophilic and thermophilic conditions showed that the digestion period for solid potato waste was shorter under thermophilic conditions than under mesophilic conditions; the concentrations of volatile fatty acids in the effluent of the second-stage thermophilic upflow anaerobic sludge blanket (UASB) reactor were no different from those in the effluent from the mesophilic UASB reactor. High rate reactors (packed-bed reactors with plastic and straw as biofilm carriers and a UASB reactor) were found to perform well during methanogenesis in two-stage anaerobic digestion of solid potato waste. The UASB performed better than the packed-bed reactor with plastic carriers. Straw, a common agricultural by-product, was confirmed to work well as a biofilm carrier. Employing efficient but low-cost technology is important for increased utilisation of anaerobic digestion, and the possibility of doing this has been demonstrated by a simple pilot-scale, two-stage anaerobic digestion system for the treatment of solid potato waste and sugar beet leaves, alone and combined, with the recovery of biogas. Co-digestion of solid potato waste and sugar beet leaves improved the methane yield by 60% compared with that from digestion of the separate substrates in both batch and pilot-scale studies. Results from this work suggest that potato waste and sugar beet leaves are suitable substrates for anaerobic digestion giving high biogas yields, and could provide additional benefits to farmers. The performance of a full-scale UASB reactor treating opaque beer brewery wastewater investigated over a period of two years enabled the brewery to meet the requirements for wastewater discharged into the municipal sewage system of Harare, Zimbabwe. The installation of a high-rate reactor by the brewery is an attractive economic and environmental alternative considering that an era of critical energy shortage, substantially higher energy prices and high demand on environmental protection lies ahead.