OPTIMISATION OF ANAEROBIC DIGESTION USING MICROALGAL-BASED CARBON-ENCAPSULATED IRON NANOPARTICLES

The study explores the use of microalgal-based carbon-encapsulated iron nanoparticles to enhance the anaerobic digestion process. These nanoparticles are synthesised through hydrothermal carbonisation (225°C, 3h) by combining iron nitrate with algal biomass grown on urban wastewater (Mantovani et al., 2022; Peng et al., 2014). The nanoparticles have been characterised through ICP-OES, TEM, SEM-EDX, and XRD to understand their morphology, properties, presence and abundance of oxides, and crystalline phases. They were integrated into semi-continuous lab-scale reactors fed with municipal sludges, where different concentrations of nanoparticles (100 mg/L and 500 mg/L) were tested. At 100 mg/L, the nanoparticles significantly enhanced biomethane production by 24%, achieving 180 ± 50 NmL/gVS, compared to 145 ± 23 NmL/gVS in the control reactor. Biogas production also increased by 30% to 248 ± 58 NmL/gVS, compared to the control (191 ± 31 NmL/gVS). Metagenomic analysis of the microbial community shows changes in the microbial community structure. The presence of nanoparticles at 100 mg/L promoted the growth of methanogenic archaea by 46%, consistent with previous findings (He et al., 2023; de Jong et al., 2020). The addition of nanoparticles also improved the digestion process, as evidenced by a 15% reduction in total solids (TS) to 17 ± 5 g/L compared to 20 ± 4 g/L in the control. Soluble COD levels decreased by 11%, suggesting enhanced organic matter degradation, with values of 236 ± 39 mg/L in the test reactor compared to 266 ± 98 mg/L in the control. Additionally, volatile fatty acids (VFA) concentrations were reduced by 7%, measuring 158 ± 50 mg/L in the test reactor versus 170 ± 54 mg/L in the control, probably attributable to the increase in methanogens revealed by metagenomic analyses. Increasing the nanoparticle concentration to 500 mg/L did not further improve performance; biomethane production slightly decreased to 168 ± 19 NmL/gVS. Despite this, the higher concentration significantly reduced hydrogen sulfide levels in biogas from 685 ± 98 ppm in the control reactor to 312 ± 154 ppm in the test reactor, likely due to metal sulfide precipitation (Gran et al., 2022).