Optimization of Anaerobic Co-Digestion of Sewege Sludge and Brewery Yeast Waste

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Addis Ababa University


A batch anaerobic digestion experiment was carried out to determine the optimum methane produced from digestion of sewage sludge and brewery yeast waste in different mix, and the fertilizer potential of the sludge after digestion. The maximum methane production from anaerobic digestion of sewage sludge and brewery yeast waste was assessed in batch tests using armfield glass made anaerobic digester in chemical engineering laboratory. The volume of biogas produced and methane composition of different mix of the feed stock were determined by water displacement and geotechnical land fill gas analyzer model GA 45, respectively. It was observed that the independent variable – mix ratio significantly affect (p-value 0.0027) the dependent variable – amount of methane produced since the p-value is less than the chosen α-level usually 0.05. The calculated volume of methane in the biogas produced per gram COD removed were 0.218L, 0.103L, and 0.075L and per gram volatile solid removed were 0.271L, 0.172L and 0.136L for sewage sludge alone, 80:20% mix, and 60:40% mix of sewage sludge to brewery yeast waste respectively. In general From the digestion of sewage sludge alone: 0.44 m3/d/m3 biogas with 57.75% methane was produced; 0.74 m3/d/m3, and 0.91 m3/d/m3 biogas with 19.84% and 19.34% methane produced from 80:20% mix and 60:40% mix of sewage sludge to brewery yeast waste, respectively. The mix ratio significantly affects the biogas production (p-value 1.65*10-9). Totally 83,850 m3 CH4/year can be captured and hence same amount of methane emission into the atmosphere is prevented from the sewage sludge currently managed in Addis Ababa waste water treatment plant if anaerobic digester is used. With regard to the fertilizer potential of the digestate; heavy metal content is not a limiting factor in the use of sewage sludge managed in Addis Ababa waste water treatment as a soil conditioner and fertilizer. Moreover, the nitrogen, phosphorus and potassium content of the sludge satisfy specific plant uptake requirements in many land application systems. Sun drying on drying beds after anaerobically digested sludge is helpful in the viability of land application of treated sludge by completely inactivating the pathogen and reducing the potential risk. Key words: Biogas, Methane, Organic fertilizer, Anaerobic digestion, Digester, Sewage sludge, Brewery yeast waste



Biogas; Methane;Organic fertilizer; Anaerobic digestion; Digester; Sewage sludge; Brewery yeast waste