Boosting methane production: the effect of sand, activated carbon, and nano zero-valent iron in anaerobic digestion

Introduction: Methane production (MP) efficiency depends on the nature of the waste and the activity of hydrolytic, acidogenic, acetogenic and methanogenic microorganisms. Materials, like activated carbon (AC), are widely recognized for boosting MP from different types of waste. In contrast, more sustainable materials, like sand, received less attention. To better understand the effect of different materials on MP efficiency, we compared the effect of AC, nano zero-valent iron (nZVI) and sand during the degradation of different types of substrates by complex microbial communities. The direct effect of these materials on hydrogenotrophic methanogenesis was also investigated. Methodology: The effect of 0.5 g/L of nZVI, AC and sand on MP kinetic parameters during the degradation of acetate, butyrate, ethanol, glucose, starch and mixed sludge (4 g/L in COD) was investigated. An automatic methane potential test system (AMPTS II) was used, allowing real-time data logging of accumulated methane volume (batch experiments). The inoculum was a suspended anaerobic sludge from Águas do Norte WWTP. Batch incubations with these materials and Methanobacterium formicicum (H2/CO2 as substrate) were also performed. Results: The accumulated methane volume from the 6 substrates tested was higher in the incubations with materials compared with incubations without materials. For instance, mixed sludge (4 g/L COD) only yielded 236±12 mL of methane (around 560 mL would be expected), while MP with sand was higher (283±17 mL). nZVI and AC significantly decreased the lag phase duration during butyrate and ethanol degradation, around 24 % and 12 %, respectively (p < 0.05). AC and sand increased the methane production rate (MPR) for all substrates, except for ethanol and butyrate degradation, respectively, while nZVI only enhanced MPR during starch and mixed sludge degradation. Overall, the MPR were accelerated up to 1.4 times with materials. The effect of sand was similar to that of AC in terms of MPR accelerating and methane yield, depending on the type of substrate. MP by M. formicicum was also affected by materials, showing their direct effect on methanogenesis. Conclusions: nZVI, AC and sand enhanced the conversion of simpler (e.g., butyrate) and more complex (e.g., mixed sludge) substrates to methane. Particularly, energy recovery capacity from mixed sludge (usually exhibiting low MP efficiencies) was improved with sand. These materials may benefit bacteria involved in the first steps of the anaerobic digestion process, enhancing their syntrophic relationships. Still, the materials also had a direct role in methanogenesis, since MP from H2/CO2 was also improved.