Sewage Plants' Potential to Produce Biogas for Electricity Generation-Case Study of the Three Zimbabwe Municipalities.

Abstract

Treatment of municipal sewage sludge is a problem in Zimbabwe. However, if the appropriate waste to energy technologies are applied, sewage plants can generate their own electricity thus minimizing municipalities’ reliance on the already strained national grid. An experimental study was therefore conducted on the Chitungwiza, Firle and Crowborough sewage plants, assessing the potential to harness biogas for electricity generation. These plants have sewage treatment capacities of 19.6 ML/day, 140 ML/day and 54 ML/day, respectively. Plant tours and inspections of the plants were conducted and an understanding of the plant designs as well as the current process flow was attained. Particular emphasis was placed on establishing the availability and state of infrastructure available for the production, handling and storage of biogas. All three plants have bio-digesters on site, however, the Chitungwiza digesters are open at the top thus releasing gas to the atmosphere. Firle and Crowborough plants are equipped with the basic infrastructure for biogas generation and storage; however, major refurbishments are required. Samples of sewage sludge were collected from the plants and placed under conditions that mimic a typical digester, the resultant biogas was analysed. The biogas was predominantly composed of methane (53-65%), CO2 (22-27%), trace gases such as H2S, N2 and H2 accounted for the balance. Experimental results revealed that the use of 50 g/m3 Actizyme as a bio-catalyst increases the quantity of methane produced to 72-78%. Based on the experimental results and the design capacities of the plants, the estimated power generation potential was 0.57-1.20 MW, 4.2-8.1 MW and 1.53-4.56 MW for the Chitungwiza, Firle and Crowborough, respectively. These capacities vary depending on whether it is the wet or dry season. An economic assessment indicated the viability of harnessing biogas from the three plants especially after incorporating Acti-zyme as the digestion catalyst to actively increase the electricity generated.