Abstract

Urban solid wastes (USW) pose a major threat in terms of disposal in the developing countries. By virtue of having >53% putrescibles, it has been considered as a suitable feedstock for biogas plants for generating renewable energy (Kayhanian, 1995). In this study, 11 feedstocks of USW origin (mixed fruit wastes, waste paper, agro residue used in packaging, aquatic weeds from urban water bodies) were subjected to anaerobic digestion and their decomposition properties were studied. The gas production pattern and potential problems of fermentation were monitored using a modified biological methane potential (BMP) technique. Results showed that most feedstocks studied had a good biogas production potential except water hyacinth roots. They also had high CH₄ content suggesting amongst others, a balanced fermentation. Paddy straw (600mL/gTS), water hyacinth (625L/kgTS) and sugarcane trash (600mL/gTS) were better among the feedstocks tried. Many of these feedstocks with high BMP showed lower daily gas production and volumetric efficiencies under SSB mode of fermentation in laboratory trials. A fed batch approach to anaerobic digestion in solid-state stratified bed reactor (SSB) was followed for a 70d-fermentation period. More than 50% TS degradation has been achieved with green feedstocks compared to the dry feedstocks. In the presence of a rapid VS reduction, lower gas production was often achieved. (i.e. the gas production observed was well below levels of VS destruction). These low gas production rates in all the feedstocks tried appear to be due to inadequate colonization of and acid conversion of older biomass by methanogens. This therefore requires a further study to determine the reasons for such a low gas yield with respect to rapid VS reduction occurring without resulting in gas production. The anaerobically digested residue was used as methanogenic support to digest liquid effluents. The suitability of these anaerobically digested biomass feedstocks as biofilm support for re-use of digested material in wastewater treatment examined in down flow fixed bed reactor (DFFBR) mode. For this part of the study three reactors packed with bagasse, mixture of bagasse and biomass and reticulated PVC spirals were studied for their property to decompose the large quantities of soluble organic matter for a 100-day operation period. Rapid fluctuations in the gas production and VFA build up suggest the requirement of proper start up. In spite of these abnormalities, packed bed reactor with mixture of bagasse and biomass possess 52.8% COD reduction of synthetic wastewater. Bagasse and PVC reticulated spirals showed 49.5% and 49.2% COD reduction levels, respectively.