Recirculation of reject water in deep-dewatering process to influent of wastewater treatment plant and dewaterability of sludge conditioned with Fe2+/H2O2, Fe2+/Ca(ClO)2, and Fe2+/Na2S2O8: From bench to pilot-scale study

Deep dewatering of sewage sludge pretreated with advanced oxidation processes (AOPs) is a strategy for efficient sludge reduction and subsequent disposal. The pretreatment and dewatering performance of sludge conditioned with three types of AOPs (Fe2+/H2O2, Fe2+/Ca(ClO)(2), and Fe2+/Na2S2O8), compared with sludge conditioned with traditional conditioner (Fe3+/CaO), were investigated in both bench and pilot-scale tests. All of those conditioner systems could reduce the water content of dewatered sludge cake to below 60 wt% in bench-scale (about 16 kg raw sludge per round) and pilot-scale (approximate 800 kg raw sludge per round) diaphragm fil-ter press dewatering. Compared with raw sludge, the deep-dewatering filtrate after different conditioning and dewatering processes had higher ammonia nitrogen (NH4+-N) and chemical oxygen demand (COD) contents due to the degradation of organic matter, and much lower total phosphorus (TP) content due to the formation of iron phosphate precipitate. A better biodegradability (i.e. higher BOD5/COD ratio) was found in the deep-dewatering filtrate of sludge conditioned with Fe2+/H2O2 (25.2 %) and Fe2+/Ca(ClO)(2) (17.4 %). Most of the heavy metals (Cr, Cu, Ni, and Pb) (>79 wt%) have remained in the dewatered sludge cake, and most of the Cl element (>90 wt %) in the sludge pretreated by Fe2+/Ca(ClO)(2) and Fe3+/CaO was kept in the filtrate, rather than the dewatered sludge cake. Based on the pilot-scale experimental results, if all the filtrate in the deep-dewatering process returned to the influent of WWTP, the loading ratios of TP, NH4+-N, COD in the four conditioner systems were less than 3 wt%. The above results proved that the AOPs conditioned sludge could achieve deep-dewatering in pilot-scale and the direct recirculation of deep-dewatering filtrate to the influent of wastewater treatment plant was feasible.