Using cerium chloride to control soluble orthophosphate concentration and improve the dewaterability of sludge: Part...

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.