Tubular anaerobic digesters are used in developing countries to produce biogas from livestock waste. In this research, field measurements and physical and biological process modeling studies were used to investigate transport and transformation mechanisms for particulate and soluble organic matter in household-scale tubular digesters in the Monteverde region of Costa Rica. Greater than 75% removal of volatile solids and biochemical oxygen demand (BOD 5) were observed. The high effluent quality was attributed to the formation of a biologically active floccular sludge layer, which allowed for separation of hydraulic and mean cell residence times (HRT and MCRT). A reduced order transport model was developed and validated using field tracer study data. Key assumptions of the reduced order model were verified via computational fluid dynamics (CFD) analysis. The mean HRT predicted by the reduced order model was 23 days and was in good agreement with the tracer experiment. A simplified floccular sludge biological process model was developed and used to estimate an average MCRT of 115 days. The results showed that household-scale tubular anaerobic digesters can provide enough biogas to meet households' cooking energy needs, which was consistent with field results. This is the first study to combine mathematical modeling with field studies of tubular anaerobic digester performance.
The objective of this study is to compare the removal of Taenia eggs to the removal of Ascaris eggs in a wastewater stabilization pond system consisting of three ponds in series, where the hydraulic residence time distribution has been characterized via a tracer study supported by computational fluid dynamics modeling. Despite a theoretical hydraulic retention time of 30 days, the peak dye concentration was measured in the effluent of the first pond after only 26 hours. The smaller-sized Taenia eggs were detected in higher concentrations than Ascaris eggs in the raw wastewater. Ascaris eggs were not detected in the pond system effluent, but 45 Taenia eggs/L were detected in the system effluent. If some of these eggs were of the species Taenia solium, and if the treated wastewater were used for the irrigation of crops for human consumption, farmers and consumers could potentially be at risk for neurocysticercosis. Thus, limits for Taenia eggs in irrigation water should be established, and precautions should be taken in regions where pig taeniasis is endemic. The results of this study indicate that the theoretical hydraulic retention time (volume/flow) of a pond is not always a good surrogate for helminth egg removal.
numerical approximations of model equations. His current research interests include sustainable water treatment system; computational fluid dynamics (CFD) applications to water and wastewater treatment, including ozone disinfection, digestion, and stabilization pond system; and integrated modeling framework of CFD and sustainability assessment models.
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