Determination of the methanogenic potential of an apple processing wastewater treatment system
Thesis (Msc Food Sc (Food Science))--University of Stellenbosch, 2006.
The food and beverage industry generates large volumes of wastewater annually. The disposal of factory effluent from the fruit processing industry has always been a cause of concern to both the fruit processors and controlling bodies responsible for effluent management. Traditional disposal of wastewater into sewerage works has become undesirable due to its economical and environmental impacts. Therefore, on-site anaerobic treatment of wastewater has received considerable interest due to lower capital outlays and energy recovery possibilities. Thus, the aim of this study was to establish an operational treatment profile for an anaerobic pond system treating fruit-processing wastewater. The specific activity of the microbial populations was also monitored to determine the effect of the fruit processing seasons (peak and off-peak season). The biogas production potential at various temperatures was also assessed to determine the viability of methane recovery. The influence of the processing and environmental conditions on the ponds’ performance was established by monitoring various process parameters. The results showed that the chemical oxygen demand (COD) levels decreased during the off-peak season but the pond pH remained relatively stable between 6.0 and 6.4 during the entire year. Pond alkalinity was found to be dependent on the regular lime dosing to maintain the necessary alkalinity. The volatile fatty acid (VFA) concentrations indicated that the microbial populations of the pond were functioning well. However, a decrease in microbial activity and VFA concentrations were observed at the lower temperatures during the winter months. The temperature profile of the pond showed that the pond temperature was impacted by the fluctuations in the ambient air temperature. The general trend established by the operational treatment profile clearly showed the impact of the peak and off-peak season. The sludge activity of the anaerobic pond was evaluated to determine the effect of the apple-processing peak and off-peak season on the specific activity of the acidogenic and methanogenic populations within the sludge. An activity test using four different test media was used during the activity assays. Sludge samples were taken at four different sampling positions across the pond’s sludge bed. The sludge was also subjected to a biogas formation study, which was designed to simulate pond conditions on laboratory scale in order to evaluate the biogas production potential of the anaerobic pond. The cumulative biogas volume and total CH4 composition showed little or no difference between the four sludge sampling sites. A major difference was found between the activity of the microbial populations during the peak and off-peak seasons. The overall trend regarding the biogas production rate (SB) and the methane production rate (SM) values showed an increased activity during peak-season and a decreased activity during off-peak season. For the biogas formation test the highest incubation temperature (25°C) resulted in the most biogas being produced, followed by 18°C, and with 10°C resulting in the lowest biogas volume. The biogas formation tests indicated that microbial activity and therefore biogas production was dependent on especially favourable temperature conditions. The pond and activity of the microbial populations are therefore influenced by factors like environmental changes such as decreased air temperatures and substrate changes such as decreased COD concentrations during the off-peak season. This in turn influences the rate of biogas production as well as the methane production rate. The theoretical CH4 calculations and estimates based on the results obtained during the biogas formation tests indicated that CH4 recovery from the anaerobic pond would definitely be a worthwhile consideration. If it were assumed that the estimated CH4 volumes (based on only 15% of the pond volume for practical reasons) obtained could be applied as an energy source, the minimum yearly savings in coal usage would amount to about R 665 000. This study was valuable in evaluating the factors such as pond conditions, pond activity and air temperatures and the effect on the biogas production potential as well as more importantly, CH4 production for the purpose of energy recovery.