Wastewater to renewable energy at a tapioca factory in Vietnam
Abstract: Anaerobic digestion is a cost-effective way of treating high organic content wastewater, as it efficiently removes large amounts of organics at the same time as biogas is produced. The production of starch from cassava roots, named tapioca, creates large amounts of high organic content wastewater and uses large amounts of thermal energy for drying the starch. At Wusons tapioca factory in southern Vietnam the wastewater is treated in an anaerobic covered pond, a low technology system for anaerobic digestion which has become popular at tapioca factories in Vietnam. The performance of the digester is however not optimal and process failures occur regularly which can partly be ascribed to the fact that the understanding of the system is poor and other than pH no measurements has previously been done on the wastewater. In this project the influent and effluent wastewater from the digester has therefore been characterised and the conditions at the factory investigated with the aim of coming up with in-situ adapted suggestions of how the performance of the digester could be improved. The results from the characterization showed that the influent wastewater was close to optimal for anaerobic digestion, with a ratio between organics and nutrients (nitrogen and phosphorus) close to the theoretically optimal value commonly found in literature. Furthermore the results showed that the concentration of nitrogen and phosphorus were kept constant through the digestion process, but the ammonia concentrations doubled, according to the expected. Solids removal showed to be high, as did the removal of organics, but the discharge standards were still not met for any of these parameters. Cyanide, present in the wastewater because of the cassava roots special composition, was found to be removed very well and the levels met the discharge standards. The results indicate that the digester had problem with low pH, which is normal for this kind of wastewater. This problem could be addressed by for example adding calcium carbonate in the influent to increase the alkalinity (instead of sodium hydroxide which is used at the factory today), using fungi as primary treatment for increasing alkalinity or separating the acidifying phase of the digestion from the biogas-forming phase. Furthermore it was found that sodium hydroxide, which is used both for cleaning the equipment in the factory and regulating pH, might have an inhibitory effect on the digester and alternatives should be sought for. It was also found that improvements to the digestion process could be done by regulating the flow to the digester. During the project an additional anaerobic covered pond was built at the factory but as the results of the wastewater characterization performed showed that there was not much biodegradable matter left in the effluent from the first digester and that ammonia levels were high it is suggested that the flow is regulated for better use of both anaerobic ponds. For example the ponds could be used to separate the phases of the digestion process as suggested above. Further suggestions consider the removal of big solids in the influent wastewater, better usage of primary pond, and keeping the outlet from the digester clean. In addition to the above it was found that the factory should keep better track of what is happening in the digester by starting to monitor some parameters and in this way introduce some forward planning and thus avoid process failures. First of all alkalinity should be measured instead of pH to be able to regulate the alkalinity before pH drops and causes process failure as was observed. The flow should also be monitored as the big variations, that are normal in the factory, can cause problems for the digester.
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