Enhanced phosphate removal by coral reef-like flocs: Coagulation performance and mechanisms

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In order to meet stricter phosphorus discharge standards for wastewater treatment plant effluent and better protect natural aquatic environments, this study investigated the coagulation performance and mechanisms of coral reef-like flocs formed in a continuous dosing coagulation (CDC) process. The CDC process exhibited enhanced phosphorus removal compared to the conventional coagulation (CC) process. To meet the 0.3 mg/L total phosphorus discharge standard, the CDC dosage (2 mg Al/L) was half the CC dosage (4 mg Al/L). Furthermore, at 4 mg Al/L, the CDC process obtained a very low total phosphorus concentration (0.035 mg/L). The CDC process had better phosphate removal performance when the initial pH was 6 or 5, and exhibited steady phosphate removal in the presence of organics and various ions. The CDC process enhanced phosphate removal in the following two ways. First, the “low pH and low Al3+ concentration” environment in the early stage benefited the co-precipitation of monomeric Al species and phosphate to form insoluble AlPO4 particles. Second, the coral reef-like flocs formed in the continuous dosing process had a stronger positive charge (2.3 mV) and larger specific surface area (264.11 m2/g). The enhanced absorption ability towards undissolved AlPO4 particles and dissolved phosphate anions promoted the overall phosphate removal of the CDC process. The coral reef-like flocs formed by a continuous dosing method provided new insights into the advanced treatment of dissolved contaminants in wastewater.