Optimization of the use of pickling waste from steel industry as an enhancer in biogas production

Abstract

Anaerobic digestion enables valorization of organic waste into bio-methane for cooking and lighting. However, low productivity and eventual collapse attributable to ammonia inhibition and process instability limit the spread of this technology. To contribute to solving the problem, this study investigated the effect of employing iron oxide from steel industry waste as an enhancer in the production of bio-methane. Fourier Transformation Infra-Red and sieve analyses of the waste revealed properties that are relevant to improve anaerobic digestion. Specific surface area of 9.73 m²/g indicated availability of sufficient sites for bacteria attachment and activity. The central composite design in response surface methodology was employed to maximize methane yield and gas quality from 300ml of digestate by varying iron oxide loading (0.5-5g), inoculum to substrate ratio (0.5-2), and retention time (15-30 days). Cumulative gas yield, methane composition, carbon-dioxide composition values ranged from 2584 to 11876 ml, 31.4 to 73.9% and 21.1 to 42.3%, respectively. Optimal conditions were determined as iron oxide loading 0.728g, inoculum to substrate ratio of 2, retention time of 16 days with predicted gas yield, methane composition and carbon dioxide composition of 183,937 ml, 74.9 %, and 20.6% respectively. The gas yield increased by 1.5 times when optimum quantities of iron oxide were added. Methane concentration increased by 11%, whereas CO2 concentration dropped by 13.5%. Positive NPV of USD 773.7 and Internal rate of return of 24.5% were obtained. Therefore, utilization of the waste was found to be technically and economically viable in anaerobic conversion of organic waste.