Present and future solutions of waste management in a candied fruit – jam factory: Optimized anaerobic digestion for on site energy production

Abstract

This paper presents a cost-benefit analysis of a new solution for the management of waste and wastewater in a candied fruit – jam factory. The solid waste products are presently treated via composting, while wastewaters are sent to a conventional biological treatment, after a chemical pre-treatment. This management modality involves an economic cost of 15 k€/y and a direct emission of 435 t CO2/y.Due to the high potential of this kind of waste products to generate methane under anaerobic conditions, a Waste-to-Energy solution that considers an optimized anaerobic digestion (AD) process was investigated. Digestibility tests carried out at a lab-scale demonstrated that the solid waste had an average methane specific yield of 0.276 Nm3/kgVSadded and the liquid waste, that make up the wastewater, had methane specific yields ranging from 0.250 to 0.330 Nm3/kgVSadded. Moreover, tests demonstrated that an accurate regulation of the food vs. microorganism ratio (F/M), and the addition of nitrogen and buffering resources, were compulsory for a steady development of the AD process.The whole amount of waste generated in the factory can be digested in a 320 m3 reactor coupled with a 40 kW combined heat and power unit. The AD process can produce approximately 30% of the electrical needs of the plant and supply one part of the heat necessary for the industrial processes, thus saving fossil fuels. Net CO2 emissions could decrease by approximately a half compared with the present solution.