Challenging the conductive paradigm: The unexpected role of sand in anaerobic digestion

The application of conductive materials (CM) has emerged as a strategy to enhance methane production (MP) in anaerobic digestion (AD) systems. This study highlights the potential of sand as an alternative to CM for biomethane production during AD, utilizing complex microbial communities and pure cultures of methanogens. Sand (non-CM) was compared with activated carbon (AC) and nano-zero valent iron (nZVI), both CM, regarding MP kinetic parameters, including lag phase duration, methane production rate (MPR), and maximum MP. Biomethane potential tests revealed that all tested materials improved methane potential, with sand showing significant advancements in the degradation of various substrates, including sewage sludge, glucose, butyrate, and ethanol. Notably, sand led to an increase of up to 16.8 % in the maximum MP from sludge degradation compared to controls without materials, outperforming AC and nZVI. In assays with pure cultures, sand increased the MPR from the hydrogenotrophic Methanobacterium formicicum culture from 9.87 ± 1.97 mM·d^-1 to 12.45 ± 2.63 mM·d^-1. No significant effect was observed on MPR from acetoclastic Methanothrix harundinacea. Our findings emphasize the remarkable potential of sand as a low-cost and environmentally friendly alternative to traditional CM to enhance AD efficiency.