Improving swine wastewater anaerobic digestion via granular activated carbon, zero-valent iron, and nano-magnetite

Using electron-conducting materials (ECMs) is a strategy to boost anaerobic digestion (AD). However, research commonly investigates high doses of ECMs, often higher than the sludge concentration in the reactors, making AD more expensive and economically unviable. In contrast, this study evaluated the effect of granular activated carbon (GAC), zero-valent iron (ZVI), and nano-magnetite (Fe₃O₄) on the AD of swine wastewater (SW) at concentrations equal to or lower than the sludge content in the reactors. The biochemical methane potential (BMP) test was conducted in reactors with 100 mL of reaction volume inoculated with brewery sludge, operating in fed-batch mode in two treatment cycles, each with 35 days. The reactors were maintained at 37 ± 0.1 °C under orbital agitation of 150 rpm. ECMs were evaluated individually at concentrations of 0.4 g L⁻¹ (0.1 rECM/VSS), 2.0 g L⁻¹ (0.5 rECM/VSS), and 4.0 g L⁻¹ (1.0 rECM/VSS). CH₄ production kinetics were improved with 4.0 g L⁻¹ of additive. GAC promoted the absence of lag phase and increased by 52.9 % and 63.6 % the speed and maximum rate of CH₄ production, respectively. ZVI was the additive that most increased BMP (15.2 %) and CH₄ content in biogas (13.5 %). Doses higher than 0.4 g L⁻¹ of Fe₃O₄ did not produce additional positive impacts on BMP. ECMs enriched volatile fatty acid-oxidizing syntrophic bacteria and methanogenic archaea (Methanothrix soehngenii and Midas s 4938 - genus Methanolinea). ECMs (0.1, 0.5, and 1.0 rECM/VSS) improved the AD of SW from an inoculum not adapted to the evaluated substrate.