Fe-doped hydrochar facilitating simultaneous methane production and pharmaceutical and personal care products (PPCPs) degradation in co-anaerobic digestion of municipal sludge and food waste

Anerobic co-digestion (co-AD) of food waste and municipal sludge (MS) has the challenges in volatile fatty acids (VFAs) accumulation and uncertainty removal of pharmaceuticals and personal care products (PPCPs), which would impact methane production and safe disposal of digestates. In this study, Fe-doped hydrochar (Fe-HC) derived from biogas residues was added to co-AD system to enhance ten types of PPCPs removal, energy recovery, and digestates safety simultaneously. Results suggested Fe-HC was porous and had a good electrochemical activity due to the presence of ---Fe2+/---Fe3+, C--O, pyridinic-N and graphitic-N that were benefit for inducing interspecies electron transfer. Adding Fe-HC to co-AD system was effective to enhance hydrolysis, acidogenesis, acetogenesis and methanogenesis, consequently facilitating PPCPs removal and methane production. At optimal Fe-HC loading of 3.0 g L-1, lag phase was shortened by 3 days and the cumulative methane yield was increased by 1.8 times. The presence of Fe-HC in co-AD system resulted in the shift of microbial community and increase of its alpha diversity. Methanogens including phylum Halobacterota and Euryarchaeota were enhanced by Fe-HC. The highest genus Methanosarcina proportion at 3.0 g L-1 of Fe-HC might account for the maximal daily methane production. The syntrophic relationship between genus Syntrophaceticus and Methanoculleus was enhanced by Fe-HC addition. The metabolisms activities of substances utilization and energy production were enhanced with Fe-HC addition via function prediction of microbial communities. Overall, this study demonstrated that redox-active porous Fe-HC was conductive to promote both PPCPs removal and methane production by enhancing the metabolic activities of microbial communities.