Improving sewage sludge dewaterability with rapid and cost-effective in-situ generation of Fe2+ combined with oxidants

Abstract Zero-valent iron (ZVI) combined with oxidants is a promising conditioning strategy for improving sewage sludge dewaterability. However, challenges, such as a low dewatering rate and utilization efficiencies of ZVI, restrict further application of ZVI/oxidant treatments. This study employs, for the first time, rapid and cost-effective in-situ generation of Fe2+ combined with common oxidants, namely H2O2, Na2S2O8, and KMnO4, to effectively improve sludge dewaterability. A comparative investigation of a standalone ZVI/oxidants treatment and when combined with a thermal and acid-wash (TA) pretreatment (TA/ZVI/oxidants) was systematically conducted to determine the sludge dewatering efficiency. Under the optimal operational conditions, the average water content of dewatered cake (Wc) was 18.47% less with the TA/ZVI/oxidants treatment when compared with the ZVI/oxidants treatment. The Wc values decreased to 54.81, 58.06, and 59.32 wt%, after the TA/ZVI/H2O2, TA/ZVI/Na2S2O8, and TA/ZVI/KMnO4 treatments, respectively. The potential mechanism by which the TA/ZVI/oxidants treatment enhanced dewatering was investigated by determining the characteristics of both the sludge and the ZVI surface structure. Results indicate that the TA/ZVI/oxidants process caused more Fe2+ dissolution, more strong oxidizing radicals and more strong flocculating Fe3+. The large amount of strong oxidizing radicals and strong flocculating Fe3+ caused cell lysis and disrupted the protein-like and polysaccharide-like materials in the EPS, thereby decreasing the particle size, negative zeta potential, viscosity and colloidal forces, which then released the bound water trapped within the net-like floc and led to improved sludge dewaterability. In the ZVI reuse experiment, the TA/ZVI/oxidants treatment remained highly effective for sludge dewatering even after four cycles. Furthermore, economic analysis indicates that the TA/ZVI/H2O2 treatment can save approximately 52.32% per year compared with using the traditional polyacrylamide (PAM) treatment.