Metagenomics reveal the effect mechanism of zero-valent iron with different sizes on the mesophilic and thermophilic anaerobic digestion of sludge

The anaerobic digestion (AD) performance can be enhanced by zero-valent iron (ZVI), however, the response mechanisms of AD of waste activated sludge (WAS) to ZVI with different sizes, and their differences under different digestion temperatures remain unclear. The size effects of zero-valent iron (ZVI) on the mesophilic and thermophilic anaerobic digestion (AD) of waste activated sludge (WAS) were investigated with a focus on metabolic products, functional microbial community, and methanogenic pathway. Experiments were conducted with nano-ZVI, micron-ZVI, and iron scrap (IS) of 100 mg ZVI/g total solid. Results showed that ZVI in these three sizes could boost the cumulative methane production regardless of digestion temperature, and the smaller the size, the better the promoting performance. Meanwhile, for the same size of ZVI, the better enhancement was always obtained under thermophilic condition.