Increase of Biogas Production at Wastewater Treatment Plants Addition of urban organic waste and pre-treatment of sludge

University dissertation from Water and Environmental Engineering Department of Chemical Engineering Lund University P.O. Box 124 SE-221 00 Lund Sweden

Abstract: Anaerobic digestion of organic matter results in the production of biogas, which can be used to replace fossil fuels. There is potential for increased biogas production at wastewater treatment plants by digesting urban organic waste either separately or together with the sludge. Biogas production could also be increased by sludge pre-treatment. The objective of this work was to evaluate the potential for increased biogas production at wastewater treatment plants by: 1) digestion of urban organic waste either separately or together with sludge, and 2) sludge pre-treatment/enhanced hydrolysis by enzyme addition, thermal treatment or ultrasonic treatment. Source-sorted organic household waste constitutes a large potential for anaerobic digestion. The amount of waste generated is high, and it can be separately digested yielding 300-400 Nm3 CH4/ton VSin. Co-digestion with sludge (20:80 on volatile solids basis) is advantageous since the combination leads to higher methane yield than would be expected from separate digestion of sludge and waste. Smaller amounts of grease trap sludge and fruit & vegetable waste are generated in the urban area but these types of waste are still interesting for anaerobic digestion, since their organic contents are high (>90%). Digestion experiments showed that the biogas potential of these types of waste is high, and they are best utilised when they are co-digested with sludge. Separate digestion of grease trap sludge could not be achieved in continuous digestion. Fruit & vegetable waste gave a higher methane content and better methane yield when co-digested with sludge than in separate digestion. Enzyme addition, at different doses, was seen to increase the methane yield from mixed sludge (primary and waste activated sludge) in mesophilic continuous digestion. Different strategies for the addition of enzymes were also studied. Direct feeding of enzymes to the digester results in about the same increase in methane yield as separate enzyme pre-treatment of sludge before digestion. Pre-treatment of biosludge by ultrasonication or thermal treatment was seen to increase the methane production in batch digestion tests. When the methods were followed by digestion with enzyme addition, additional effects were observed on methane production. Thermal treatment (70ºC for 1 h) was seen to increase the methane yield by 20% in continuous digestion of mixed primary and waste activated sludge. Thermophilic digestion of sludge resulted in a significantly higher methane yield than mesophilic digestion. Thermophilic digestion of source-sorted household waste showed only a minor increase compared with mesophilic digestion.

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