Avaliação da produção de biogás através da co-digestão anaeróbia de resíduos de hortifrutigranjeiros e resíduos de lodo ativado

Abstract

Anaerobic digestion (AD) is an alternative for the treatment of organic waste, where organic matter is converted through a series of biochemical reactions in biogas, with methane as the main component. The possibility of increasing the conversion of organic matter to biogas by co-digestion of different residues, parallel to the improvement in the process stability, is a primary issue mentioned in this study. Fruit and vegetable wastes (FVW) present high biogas production potential. However, due to their characteristics, they tend to acidify the AD process. Waste activated sludge (WAS) presents a low biogas production rate, due to the difficulty of the anaerobic microorganisms to access the intracellular organic content that makes up the ASR. This study sought to establish the best relationship between WAS and FVW, which promotes higher biogas production, together with operational stability. The useful volume of the biodigester used was 40L, the reactor temperature was kept fixed at 35ºC, as well as the rotation of the mixing system at 29 rpm. Was opted for the batch operation regime. Five different batches were performed. The ratio between substrate/inoculum (S/I) of 1/3, in terms of mass of volatile solids (VS), was defined as a fixed parameter in all batch. The batch number 1 operated only with WAS and the batch 5 only with FVW. Batch 2, 3 and 4 operated with WAS/FVW ratios, respectively of 3/1, 1/1 and 1/3. The WAS/FVW ratios were defined based on the mass of VS characteristic of each residue. The process monitoring was carried out through a series of solids, total alkalinity, volatile fatty acids, pH, methane volume and biogas characterization. For the characterization of the substrates, inoculum and digestate, total and ammoniacal nitrogen and phosphorus tests were added. Batch 4 and 5 presented acidification of the process, characterized by the pH drop of 5.66 and 5.43, and by the VFA concentration, from 4116.87 mgHAc/L and 6501.54 mgHAc/L, respectively, for reactors 4 and 5. The methane yield for both residues used when co-digested was higher than the yield obtained individually for each residue, yielding 306.58 NLCH4/kgVS and 146.83 NLCH4/kgVS, for WAS and FVW , respectively. The batch 3 presented the highest methane yield in relation to the others, with a production of 643.56 NLCH4/kgVS and methane content of 79.50%.