Uso de materiais condutores no tratamento anaeróbio de água residuária de suinocultura

Abstract

Pig farming is of utmost importance for the global and Brazilian economies. However, this activity has an environmental liability for generating swine wastewater (SW), which has a high pollutant load, making its treatment essential before its final disposal. In this context, anaerobic digestion (AD) is a bioprocess that allows the adequate treatment of this effluent combined with the recovery of methane (CH4), a valuable energy resource. Applying insoluble redox mediators (IRMs), also called electron conductive materials (ECMs), in the AD of organic waste is a powerful strategy to increase CH4 production, as it stimulates microbial syntrophy and the processes of direct and indirect interspecies electron transfer. However, the use of these materials in the anaerobic treatment of SW, aiming to accelerate and increase CH4 production, has been little explored in scientific research. Therefore, this work evaluated the use of ECMs, graphene oxide functionalized with nano-magnetite (GO-Fe3O4), granular activated carbon (GAC), zero-valent iron (ZVI), nano-magnetite (Fe3O4), and the associations Fe3O4+GAC and ZVI+GAC in the anaerobic treatment of SW, evaluating kinetic parameters that govern CH4 production, analyzing changes in microbial communities, and discussing the main electron transfer mechanisms promoted by these materials. The Biochemical Methane Potential (BMP) tests were conducted in glass reactors with 300 mL of total volume operating in batch mode at a mesophilic temperature of 37 °C or thermophilic 55 °C and orbital agitation of 150 rpm. SW was used as a substrate, and biomasses not adapted to the anaerobic treatment of this effluent were used as inocula. ECMs were added at the following concentrations: 3.0 and 150.0 mg L-1 for GO-Fe3O4 and 0.4, 2.0, and 4.0 g L-1

for GAC, ZVI, and Fe3O4. ECMs doses were limited to the inoculum concentration (volatile suspended solids) applied to the reactors. Regarding the impact of GO-Fe3O4 on the anaerobic treatment of SW, GO-Fe3O4 (150.0 mg L-1

) made DA more efficient, increasing the BMP and the organic matter removal efficiency by 17.2% and 12.6%, respectively. This additive also accelerated the CH4 production rate (k) by 31.9%, decreased the adaptation time of microorganisms to the substrate (lag phase - λ) by 25.6%, and increased the maximum CH4 generation rate (μ) by 32.3%. Chloroflexi and Methanobacterium beijingense were the main microorganisms enriched by this additive. The potential Direct Interspecies Electron Transfer (DIET) partners were bacteria belonging to the phylum Chloroflexi and the bacteria Clostridium sensu stricto 1 (phylum Firmicutes) with the methanogenic archaea Methanobacterium beijingense and Methanothrix soehngenii.Regarding the application of GAC, ZVI, and Fe3O4 in the SW methanization, GAC (4.0 g L-1 ) was the material that most benefited the methanogenic kinetics, promoting the absence of a lag phase and increasing k and μ by 52.9% and 63.6%, respectively. ZVI (4.0 g L-1

) was the material that most increased BMP (+ 15.2%) and CH4 content in biogas (+ 13.5%). Fe3O4 (4.0 g L-1 ) also improved methanogenic kinetics, increasing μ by 52.6% and k by 37.7%. However, doses higher than 0.4 gFe3O4 L -1 did not produce additional positive impacts on BMP. The bacterial genera Clostridium sensu stricto 1, Mesotoga, Longilinea, and JGI-0000079-D21 were enriched in the presence of ECMs and were classified as potential DIET partners of the archaea Methanothrix soehngenii. Regarding the influence of EMCs on the AD of SW, it appears that the performance of mesophilic and thermophilic digesters was improved, even at different inoculum substrate ratios (SIR). In mesophilic anaerobic digestion (MAD) at SIR 0.5, GAC was more successful in optimizing methanization kinetics, increasing k by 29.9% and reducing technical digestion time (T80) by 15.9%. Meanwhile, ZVI was the material that most increased BMP (+19.4%), μ (+40.5%), and CH4 content in biogas (+22.1%). At SIR 2.0, GAC stood out among the other materials, promoting increases of 11%, 10.2%, and 26.3% in BMP, k, and μ, respectively, and reducing T80 by 7.0%. In thermophilic tests at SIR 0.5, ZVI increased BMP, Y1, CH4 content, and μ by 22.6%, 23.6%, 20.0%, and 35.5%, in that order, while GAC and the combinations provided a better kinetic performance. In thermophilic anaerobic digestion (TAD) at SIR 2.0, Fe3O4, ZVI, and the combinations Fe3O4+CAG and ZVI+GAC obtained the best performance. Fe3O4, for example, increased BMP (+32.2%), Y1 (+30.1%), CH4 content (+1.9%), μ (+38.9%), and reduced T80 by 4.3%. The additives enriched bacteria and archaea participating in DIET, increasing, and accelerating CH4 production. Methanothrix soehngenii dominated the MAD, and Methanolinea tarda and Methanosarcina thermophila prevailed in the TDA. In MAD, acetoclastic methanogenesis must have been the main route of CH4 synthesis, while in TAD, hydrogenotrophic methanogenesis became predominant. Methylotrophic methanogenesis, associated with the presence of Candidatus Methanomethylicus, was stimulated in TAD. It is concluded that the addition of ECMs at relatively low doses compared to other studies reported in the technical literature and at doses limited to the biomass concentration present in the reactors was sufficient to optimize the anaerobic treatment of SW from inocula not adapted to the degradation of this substrate.