Design of Biogas Septic Tanks for Treating Domestic Sewage

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This study is to design a novel septic tank, named Anaerobic Upflow Domestic Septic Tank (AUDST) to recover biogas as energy and treat domestic sewage. The green technology proposes alternate options to existing Domestic Septic Tanks (DST), encourages anaerobically pre-treatment to reduce bacteria, pollutants, Total Suspended Solids (TSS), Chemical oxygen demand (COD) and Biological oxygen demand (BOD) before the effluent is discharged or is removed by cesspit trucks. Studies have shown that DST in homes partially treat or just store sewage. Again, these DST have to be emptied from time to time because it lack features that will sustain anaerobic activity and usually the sludge is disposed of directly into the sea, water bodies and even into open places such as “Lavender Hills’’ without any treatment or disinfection. These practices cause severe public health and environmental problems. To tackle the challenge at household level, DST are redesigned to treat domestic sewage with less management, low operating cost, low secondary discharge of pollutants. The proposed new design concept is operated through three (3) units: such as desilting, anaerobic digestion and facultative filtration units. The anaerobic digestion stage is made up of baffle and anaerobic filter for accommodating sludge and providing a more intimate contact between anaerobic biomass and sewage which improves treatment performance. The anaerobic unit is fitted with locally woven baskets prefilled with packing materials. The aim is to strengthen the biological treatment process at this stage. The Facultative Filtration unit of the model is also packed with filtering media such as gravels (3-6mm in diameter) that is low in cost, and has a high durability to produce effluent with lower pollutants and suspended solids content to meet Ghana’s Environmental Protection Agency (EPA) standards for the discharge of domestic effluents.
A thesis submitted to the Department of Mechanical Engineering, Kwame Nkrumah University of Science and Technology in partial fulfillment of the requirements for the degree of Master of Science in Renewable Energy Technologies, 2014