Evaluating Anaerobic Digestion Technology in Reducing the Quantity of Solid Waste: a Case Study of Kigali Dumpsite

Abstract

Rwanda is a developing country where the waste generated from households continues to increase due to extensive urbanization and development. Biodegradable organic matter constitutes a great portion of the municipal solid waste from Kigali City. Biodegradable waste is composed of elements that can be degraded by bacteria such as paper, green waste (yard waste), general garbage and other common items containing organic elements like furniture, texture, footwear, as well as high-grade materials. Landfill disposal leads to the production of methane gas as the accumulated waste degrades. During anaerobic digestion, organic matter is decomposed by microbial aggregates in the absence of oxygen, resulting in the formation of digests and biogas, which consist primarily of methane gas and carbon dioxide. The digest is a decomposing substrate produced by biogas production and can be used as a biofertilizer. The research was focused on the evaluation of anaerobic digestion technology in reducing the quantity of solid waste: a case study of Kigali dumpsite. The research identified the quantity and characteristics of biodegradable municipal solids waste generated in Nyarugenge, the suburban part of Kigali. The survey was conducted every week for four weeks corresponding to a month to estimate the quantity of municipal solids waste discharge per a day. This survey was conducted between 24/June and 20/July. The sorting-and-weighing methodology was used in assessing the waste composition from each of the sample households every week. During the survey, the amount of solid waste generated and dumped in each of the survey areas (villages) was quantified. The total solids waste was found to be 686,000 kg, and the organic solids waste was about 500,000kg. This fraction formed 73% of entire municipal solids waste in Kigali city. The research analyzed the chemico-physical and biological characteristics of the municipal solids waste. The tests done, Dry matter (DM), Volatile Solids VS, Biological Oxygen demand (BOD) and Chemical Oxygen demand (COD) were 22.4, 910.228gVS/kgDM, 597.714gBOD/kg, and 1328.262g COD/kgDM. The results of laboratory experiments showed that the amount of biogas produced and the maximum cumulative production was about 79 L / kg DM, as the maximum cumulative volume of methane produced was noticed to be about 35.7 L / kg DM. The methane content is about 44% of the total dry biogas obtained. Due to the use of anaerobic digestion settings, biogas tank settings produce lower methane and biogas, but their physicochemical and biological characteristics indicate that this biowaste should produce the required biogas.

The research evaluated the viability of using anaerobic digestion technology. The results from the laboratory tests were used for calculating methane and the potential for electricity generation. The results indicated that the organic waste in Kigali City produces 457L/kg DM of methane. The overall assessed value of methane was 51,384,375 L. The electricity from derived methane was 180,873 KWh. The quantity of municipal waste generated in Kigali City was used for designing the biodigester required. The volume of the biodigester was found to be 58,065m3. A cost/benefit analysis on the anaerobic digestion technology was done based on the energy recovered, and it revealed that Kigali City will benefit from this project as the population accessing its electricity will be increasing. The electricity from derived methane was 180,873KWh, which is 54% of the daily demand in Kigali.