Correlation between oxidation-reduction potential values and sludge dewaterability during pre-oxidation

Pre-oxidation is effective in enhancing sludge dewaterability. Different types and doses of oxidants are used to improve sludge dewaterability in pre-oxidation. Rapid evaluation of the sludge dewaterability is vital for optimizing the type and dose of oxidants in pre-oxidation. It normally takes more time to evaluate sludge dewaterability by measuring typical indicators such as specific resistance to filtration (SRF), content of bound water, and composition of EPS. This study presented a rapid parameter, oxidation-reduction potential (ORP), to correlate it with the dewaterability of pre-oxidized sludge samples. An index of ΔORP (ΔORP = ORP_{t=0.5 min}-ORP_t=0 min) showed positive correlations with SRF (r = 0.89, p < 0.05), content of total organic carbon in soluble and loosely-bound EPS (r = 0.86 and 0.84, p < 0.05), zeta potential (r = 0.86, p < 0.05), and content of Fe(III) in the sludge cake (r = 0.92, p < 0.01). However, the ΔORP index showed negative correlations with tryptophan-like proteins, tyrosine-like proteins, microbial byproduct-like materials in tightly-bound EPS (r = -0.85, -0.90 and -0.90, p < 0.05), and sludge particle sizes (p < 0.01). A multiple linear regression model was developed to further reflect the linear correlation between the ΔORP values and the key factors reflecting sludge dewaterability. An optimal dose of oxone (0.4 mmol/g VS) for sludge pre-oxidation with the ΔORP value of 387 mV combined with Fe(III) coagulation conditioning system were verified by the results of dewatering experiments using a laboratory-scale diaphragm filter press. This study demonstrated the feasibility of using ΔORP as a potential rapid evaluation tool for sludge dewaterability.