Effects of ferric nitrate additions under different pH conditions on autothermal thermophilic aerobic digestion for sewage sludge

Abstract

The effects of ferric nitrate additions at different pH values on stabilization of sewage sludge and microbial communities were investigated in autothermal thermophilic aerobic digestion (ATAD). The lowest pH value but highest VS removal was achieved at optimal pH of 6.5 when Fe(NO₃)₃ was added, compared to other dosing groups, and the stabilization time was shortened by 7 days. The increased dosing pH reduced the effectiveness of Fe(NO₃)₃ on disinhibition of excessive volatile fatty acids, and even caused failure of stabilization. However, highly basic dosing conditions (such as pH 9.5) could restore the ATAD ability by promoting the processes of hydrolysis and acidification, and then take back control of the pH. Illumina high-throughput sequencing revealed the group at optimal dosing pH significantly enhanced the abundance of phylum Firmicutes (from 61.0% to 96.6%), while the rise of dosing pH decreased the richness of phylum Firmicutes. The structure of the microbial community was totally changed under strong basic conditions of Fe(NO₃)₃ addition.