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Cerium chloride (CeCl3), being a superior orthophosphate (OP) precipitant, was found to be able to significantly improve sludge dewaterability in terms of sludge cake dryness and capillary suction time. In order to offer insights into the mechanism behind OP removal associated dewaterability improvement, the change in sludge specific resistance to filtration (SRF), compressibility (K), and bound water contents (Ub) in response to CeCl3 and CePO4 addition at the two cationic polymer doses was mathematically simulated. Results showed that 29.8 g/kg dry solid CePO4 addition was able to decrease the SRF by 52%, decrease the Ub by 42%, and reduce the K by 18%. Importantly, CeCl3 addition of equal cerium molarity showed even higher SRF and Ub reductions by 67% and 54%, respectively, but the same K reduction. A new theory depicting how the OP has outcompeted negatively charged sludge particles for cationic polymers is put forward in this study to interpret the effect of OP removal on sludge dewaterability improvement. Practitioner points Efficient orthophosphate (OP) removal and sludge dewaterability improvement were achieved with CeCl3 addition. Both CePO4 precipitate and OP removal contributed to the improved dewaterability. Competition between OP and sludge particles for cationic polymers was explained.

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