Chemically induced alteration in PAC characteristics and its influences on PAC/UF water treatment: Implications for on-line membrane cleaning with NaClO

The integrated PAC/UF technique has gained increasing applications in water treatment systems. However, the fouling and substantial PAC deposition on membrane surfaces remain perennial hurdles. To tackle these issues, on-line membrane cleaning with NaClO should be frequently performed to sustain the membrane permeability, leading to the inevitable contact between NaClO and PAC particles, which may significantly change the PAC properties, hence the entire performance of water treatment during the next round of operation. This study simulated the two reactions between PACs and NaClO (i.e. deposited PAC with high-concentration NaClO and suspended PAC with low-concentration NaClO) based on the current practice of on-line membrane cleaning, and further investigated the impacts of NaClO-induced PAC variations on PAC/UF performance. The results showed that (i) the adsorption capacities of altered PACs for humic acid (HA), bovine serum albumin (BSA) as well as contaminants in surface water were seriously weakened because of the decreased specific surface area and pore volume; (ii) The altered PAC surface became more hydrophilic, presumably leading to the higher deposition on membrane surface. Besides, (iii) the PAC deposition amounts were all enhanced by the presence of BSA, while substantially decreased by HA. Further addition of Ca²⁺ with HA dramatically increased PAC deposition probably by forming the cross-linked molecular chains to connect PAC-PAC or PAC-membrane. As a result, membrane fouling in PAC/UF water treatment system was remarkedly aggravated when the altered PACs were applied due to NaClO exposure. Meanwhile, the surface water treatment performances in terms of COD_Mn removal, UV₂₅₄ removal and turbidity removal significantly decreased, implying the adverse impacts of employing such altered PAC in the practical water treatment system. Given on-line chemical cleaning is frequently conducted during a long-term operation, the current study raised serious concerns on membrane cleaning induced PAC variations and potentially negative influences on the real-life PAC/UF water treatment process.