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.
High concentration of orthophosphate ion (OP) in anaerobically digested sludge can lead to struvite crystallization, deterioration of sludge dewaterability, and elevated mainstream OP loading through centrate recirculation. The Upper Occoquan Service Authority (UOSA) has observed seasonally high OP levels in its dewatering blend tank, which was found in this study to be a consequence of unwanted biological phosphorus accumulation during the intensified winter denitrification operation and the subsequent OP release in the course of anaerobic digestion. In order to control the nuisance struvite scaling issues, a bench study was conducted and cerium chloride (CeCl3) was dosed as an effective OP precipitant. The results of this study demonstrated that CeCl3 dosing showed higher OP removal efficiency than other commonly used OP precipitants. In addition, bench‐scale simulations indicated sludge dewaterability improvements which were used to predict lower polymer and dewatering energy demands at the full scale. The economic analysis conducted in this case study showed that the seasonal dosing of CeCl3 at UOSA has the potential to provide a net annual saving of US $47,000.
Practitioner points
Biological phosphorus accumulation during the intensified denitrification operation caused seasonally high sludge OP and struvite scaling issues at UOSA.
CeCl3 was evaluated as an effective OP precipitant for struvite control and dewaterability improvement when aluminum and iron were determined to be unfavorable.
Seasonal dosing of CeCl3 at UOSA projected a net annual saving of US $47,000.
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