Behavior and Biochemical Mechanism of High Iron Attapulgite Dosages Affecting Sewage Sludge Composting

We first revealed the behavior and biochemical mechanism of high iron attapulgite (HIAP) and different dosages affecting sewage sludge (SS) composting. HS/TOC ratio increased, but HA/TOC and HA/FA ratios reduced with the increase in HIAP dose. High-dose HIAP promoted the formation of more HS by weak catalytic effect but could bind more FA than HA by strong adsorption effect to inhibit the polymerization of the adsorbed FA into HA. Mixing SS with HIAP and subsequent composting as two consecutive processes during HIAP-amended composting significantly influenced the species distribution of heavy metals (HMs) Cu, Zn, and Cr. Each process roughly contributed one-half to HMs passivation. The bioavailable fraction (BF) of HMs reduced with the increase of HIAP dose. HIAP dose greatly affected the microbial community. Both 1% and 5% HIAP treatments promoted Proteobacteria and Firmicutes, but 10% HIAP promoted Actinobacteriota and Bacteroidota. At the thermophilic phase, HIAP dose greatly affected core thermophilic microbial genera, which were significantly correlated to pile temperature and pH value. In the maturity stage, core microbial genera in different treatments were basically similar and closely correlated to the bioavailable fraction (BF) of HMs and HA, and the influence order was BF-Cr > BF-Cu > BF-Zn > HA. The optimal 5% HIAP dose was recommended.