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Guiding principles in the design and operation of a wastewater sludge digestion plant with biogas and power generation
Expanded Title:The current study uses City of Johannesburg’s full scale CHP installation as a case study to • provide a practical guideline for the design and operation of a sludge treatment plant, with enhanced CHP generation; and • identify and quantify the opportunities to replicate this approach across the South African industry, at municipal WWTWs which already incorporate anaerobic digestion. In this regard, design principles of the various process units that are involved in sludge processing (sludge thickening, cell lysis, anaerobic digestion, and biogas to electrical energy (CHP), struvite (MAP) recovery, solar drying beds, sludge composting and offsetting of the final biosolids product) were identified, while operational data was collected from the Johannesburg Northern plant, and Olifantsvlei WWTW, for each process step involved in producing, thickening, and conditioning, treating and digesting sludge related to the biogas production for the CHP system. Performance was analysed for each process unit by considering the sludge quality input to the process unit, the outflow from the unit and the expected design performance of each particular process unit. The hydraulic retention in the anaerobic digesters was identified as an important operating parameter with regard to biogas production and a direct correlation between reduced hydraulic retention and reduced electrical power made. In addition, the lower temperature has a negative knock-on effect on digester efficiency and biogas production and this result in reduced efficiencies. Mapping of anaerobic digestion in SA indicated that 46 Water Services Authorities (WSAs) (out of 152) have approximately 420 anaerobic digesters, which are spread across 108 WWTWs (out of 824) across 9 Provinces, with a corresponding design volume of 1 367 Ml. A tool designed to calculate the potential electrical energy at each of the 108 WWTWs with anaerobic digesters estimated to achieve a total biogas production of 282 671 m3/day, translating to electrical energy of 657 765 kWh/day, with a monetary value of R 144 million per annum at 60 cents per kWh electricity. The study further offered a model to give practical guidance on the minimum requirements to economically and sustainably develop biogas to energy at a given plant. For the purposes of this study, the “minimum feasibility requirement” is defined as ‘a CHP project with an assumed lifespan of 15 years that will pay back the investment including financing cost over the project life cycle of 15 years. In this regard, any loading or condition better than the model curve will result in the generation of a positive cash flow over the project life cycle.
Date Published:01/09/2016
Document Type:Research Report
Document Subjects:Wastewater Management - Sludge management, Wastewater Management - Sewers
Document Format:Report
Document File Type:pdf
Research Report Type:Technical
WRC Report No:TT 681/16
Authors:van der Merwe-Botha M; Juncker K; Visser A; Boyd R
Project Leader:Marlene vd Merwe-Botha
Originator:WRC
Document Size:2 871 KB
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