Determination of the methanogenic potential of an apple processing wastewater treatment system

[摘要] The food and beverage industry generates large volumes of wastewater annually. Thedisposal of factory effluent from the fruit processing industry has always been a cause ofconcern to both the fruit processors and controlling bodies responsible for effluentmanagement. Traditional disposal of wastewater into sewerage works has becomeundesirable due to its economical and environmental impacts. Therefore, on-siteanaerobic treatment of wastewater has received considerable interest due to lower capitaloutlays and energy recovery possibilities. Thus, the aim of this study was to establish anoperational treatment profile for an anaerobic pond system treating fruit-processingwastewater. The specific activity of the microbial populations was also monitored todetermine the effect of the fruit processing seasons (peak and off-peak season). Thebiogas production potential at various temperatures was also assessed to determine theviability of methane recovery.The influence of the processing and environmental conditions on the ponds'performance was established by monitoring various process parameters. The resultsshowed that the chemical oxygen demand (COD) levels decreased during the off-peakseason but the pond pH remained relatively stable between 6.0 and 6.4 during the entireyear. Pond alkalinity was found to be dependent on the regular lime dosing to maintainthe necessary alkalinity. The volatile fatty acid (VFA) concentrations indicated that themicrobial populations of the pond were functioning well. However, a decrease in microbialactivity and VFA concentrations were observed at the lower temperatures during the wintermonths. The temperature profile of the pond showed that the pond temperature wasimpacted by the fluctuations in the ambient air temperature. The general trend establishedby the operational treatment profile clearly showed the impact of the peak and off-peakseason.The sludge activity of the anaerobic pond was evaluated to determine the effect ofthe apple-processing peak and off-peak season on the specific activity of the acidogenicand methanogenic populations within the sludge. An activity test using four different testmedia was used during the activity assays. Sludge samples were taken at four differentsampling positions across the pond's sludge bed. The sludge was also subjected to abiogas formation study, which was designed to simulate pond conditions on laboratory scale in order to evaluate the biogas production potential of the anaerobic pond. Thecumulative biogas volume and total CH4 composition showed little or no differencebetween the four sludge sampling sites. A major difference was found between the activityof the microbial populations during the peak and off-peak seasons. The overall trendregarding the biogas production rate (SB) and the methane production rate (SM) valuesshowed an increased activity during peak-season and a decreased activity during off-peakseason. For the biogas formation test the highest incubation temperature (25°C) resultedin the most biogas being produced, followed by 18°C, and with 10°C resulting in the lowestbiogas volume. The biogas formation tests indicated that microbial activity and thereforebiogas production was dependent on especially favourable temperature conditions. Thepond and activity of the microbial populations are therefore influenced by factors likeenvironmental changes such as decreased air temperatures and substrate changes suchas decreased COD concentrations during the off-peak season. This in turn influences therate of biogas production as well as the methane production rate.The theoretical CH4 calculations and estimates based on the results obtained duringthe biogas formation tests indicated that CH4 recovery from the anaerobic pond woulddefinitely be a worthwhile consideration. If it were assumed that the estimated CH4volumes (based on only 15% of the pond volume for practical reasons) obtained could beapplied as an energy source, the minimum yearly savings in coal usage would amount toabout R 665 000.This study was valuable in evaluating the factors such as pond conditions, pondactivity and air temperatures and the effect on the biogas production potential as well asmore importantly, CH4 production for the purpose of energy recovery.

[发布机构] Stellenbosch University