Avaliação da co-digestão anaeróbia em estágio sólido de resíduos agroindustriais: estratégias de aclimatação do inóculo e condições do processo

Abstract

Solid-state anaerobic digestion (SSAD) is an efficient and sustainable alternative for the treatment of organic solid waste, offering advantages such as lower water demand and greater tolerance to substrates with high solids content. This study investigated the solid-state digestion process through four experimental trials distributed across two chapters. The focus was on optimizing methane production (BMP) from different combinations of substrate and inoculum, identifying operational limitations. In experimental assay 1, extruded cattle manure (ECM) and digested cattle manure (DCM) were used as inoculum and sugarcane bagasse (SCB) was used as the substrate under different hydrothermal pretreatment conditions. The highlight was trial BPT9, which achieved a cumulative methane production of 1.6 NL kgSV⁻¹, indicating that pretreatment at room temperature combined with prolonged contact time resulted in better performance. However, despite this result, overall efficiency was still considered low, demonstrating that the strategy was insufficient to significantly enhance methane production. Additionally, the total alkalinity (TA) of the inoculum (1,000 mg CaCO£ L⁻¹) did not ensure effective buffering, reflected in low methane contents (5.9% to 21.6%). Experimental assay 2 evaluated the co-digestion of SCB with fruit and vegetable waste (FVW), using digestate from a pilot-scale reactor (PSI) and fresh bovine manure (FBM) as inoculum. Results showed superior performance in trial E9 (56 NLCH¤ kgSV⁻¹; 58% CH¤) under conditions of S/I 0.5 and TS 14.9%. Although TA increased to 1,583 mg CaCO£ L⁻¹, it was still insufficient to prevent acidification, compromising the stability of some trials. In experimental assay 3, FVW remained as the substrate, but with a new inoculum formulation composed of liquid digestate (PSI), solid digestate (RSI) and fresh cattle manure (FCM), achieving high TA (3,500 mg CaCO£ L⁻¹). The control with FVW presented the best performance (297.7 NLCH¤ kgSV⁻¹; 56.2% CH¤), while trials with higher S/I and TS, such as E12 (S/I 2; TS 15%), showed significant drops in production and methane content, attributed to acidification and accumulation of volatile fatty acids. The second chapter addressed the co-digestion of SCB with FBM, adjusting S/I ratios (0.5–2.0), TS (8.9%–17.2%), and C/N (16/1–32/1). The best results were obtained in trials T4, T8, and T12 (12.6–13.9 LCH¤/Lreactor), with volatile solids removal of up to 36% and a C/N reduction to 6/1. Process stability was observed at TS between 12.5% and 11.6% (T4 and T12). Overall, the four experimental trials demonstrated that the efficiency of solid-state anaerobic digestion critically depends on inoculum composition and fine-tuning of S/I, TS, and C/N ratio.