Estratégias operacionais para o incremento da metanização em estado sólido de resíduos orgânicos urbanos com vistas ao aproveitamento energético do biogás

Abstract

Regarding the urgency in the treatment of organic fraction of municipal solid waste (OFMSW) in Brazilian cities, solid-state anaerobic digestion (SSAD) stands out as an important treatment alternative, as it enables: mitigation of greenhouse gas emissions (GHG) and leachate pollution; reduction of disposal costs; biogas production, a renewable energy source; and biosolids production, to agricultural use. In order to define the best operational strategies for optimizing a SSAD process, a semi-industrial scale system was built and researched at the Caju’s Waste Transhipment Unit, in the City of Rio de Janeiro. This system is equipped with SSAD reactors integrated with an Inoculum Production Unit (IPU) and a cogeneration heat and power (CHP). In order to support the start-up and operation of this system, the following studies were carried out: (i) physicochemical characterization of a large generator of OFMSW and definition of fresh leachate management strategies; (ii) evaluation, through BMP assay and metagenomic sequencing, of the OFMSW methane yield and the microbiological composition of the inoculum used to start-up the SSAD process; (iii) evaluation of operational stability along OFMSW SSAD batches with leachate recirculation; (iv) estimation of the gains associated with different biogas energy recovery strategies (electricity or biomethane) in a OFMSW SSAD system, as well as the environmental and economic impacts. The research showed that OFMSW solid content was close to 20 %TS and a high biodegradation potential (VS/TS > 0.88), as well as a low carbon-waste sources (low C/N ratio) and no bulking materials (ex. : pruning). About 18% of OFMSW was composed by fresh leachate, an acidified effluent (pH < 4.22) with a high organic load (60 gVS.L-1; 112.5 gCOD.L-1) and high ammonia content (3.18 gN-NH4.L-1), which made its management even more complex. The metagenomic sequencing of the microorganisms in the inoculum was relevant to start-up the SSAD process and the highest OFMSW methane yield (285 Nm³CH4.t-1VS) was obtained by a more diverse anaerobic community and S/I ratio of 0.25 gVSsubstrate.gVSinoculum. The structuring of OFMSW with pruning (1:3 kg.kg-1OFMSW), associated to leachate recirculation (0.8 m³.t-1OFMSW), were key strategies to optimize the performance of SSAD, by improving the C/N ratio and the inoculum percolation through the substrate, in addition to increasing the CH4 content in SSAD reactors (CH4 ~ 30 %). Altogether, the operational strategies were not sufficient to promote methanogenic stability in SSAD reactors (CH4 > 30 %) and complete stabilization of the digested material (VS/TS < 0.60). However, these strategies enabled the leachate biological stabilization and the maintenance of methanogenic activity in the IPU (CH4 > 65 % and yield close to 400 m³CH4.t-1VS), corresponding to important environmental and economic gains related to the OFMSW treatment; waste transport and landfill reduction; GHG mitigation; biogas and renewable energy production (electricity and biomethane) - confirming the sustainability of the SSAD process.