VALORISATION OF KITCHEN AND ANIMAL WASTE THROUGH A PORTABLE ANAEROBIC-DIGESTION UNIT: DESIGN, FABRICATION, AND EXPERIMENTAL EVALUATION OF BIOGAS YIELD

Abstract

Untreated kitchen and animal waste remains a major source of unpleasant odours, pathogen transmission, and greenhouse-gas emissions in rapidly urbanising South Asian cities. At the same time, seasonal fluctuations in Pakistan’s natural-gas supply highlight the need for affordable and decentralized renewable-energy alternatives. This study presents the design, fabrication, and bench-to-pilot-scale evaluation of a low-cost, movable anaerobic digestion system for converting household kitchen and animal waste into combustible biogas. The system consisted of a 208-L polymer digester equipped with a mechanical shredder, a three-stage gas purification unit for desulfurisation, carbon-dioxide removal, and moisture reduction, a flexible-membrane gas-storage reservoir, and an Arduino-based gas-leak detection system. Seven controlled bottle- and drum-scale experiments were conducted to investigate the effects of collection-vessel elasticity, feedstock composition, particle size, ambient exposure, digester filling level, and feedstock loading on biogas production. These experiments were followed by a 12-day pilot-scale trial using approximately 13–60 kg of mixed vegetable and kitchen waste. Gas production commenced after a three-day lag phase in all experimental trials and subsequently increased approximately monotonically. Greater feedstock loading, smaller particle size, and warmer, sun-exposed conditions resulted in higher gas production. During the pilot trial, the complete system continuously generated combustible biogas over 12 days, with the storage tube reaching a diameter of approximately 23 inches, corresponding to an estimated storage volume of 67 L. Subsequent combustion tests demonstrated successful flame ignition, although flame stability varied. Overall, the findings demonstrate that a low-cost household-scale anaerobic digester constructed from readily available materials can effectively convert mixed organic waste into combustible biogas. The proposed system offers a practical decentralized approach to renewable cooking-fuel generation and organic-waste management for urban and peri-urban households, particularly in regions experiencing unreliable natural-gas supplies