Use of Ca- and Mg-type layered double hydroxide adsorbent to reduce phosphate concentration in secondary effluent of domestic wastewater treatment plant

World widely, the excess loads of phosphorus (P) is one of the most common chemical contamination in freshwater bodies (e.g. lakes and rivers), which is one of the major causes of eutrophication. Effluents from wastewater treatment plant (WWTP) are considered to contribute up to 60%-80% of the P in rivers due to discharge of treated effluents often containing residual P concentrations of 1-2 mg/L. In this study, Ca and Mg incorporated layered double hydroxide (LDH) adsorbents were synthesized and assessed for phosphate removal from both P spiked synthetic solution and real effluent sample of a WWTP to provide a comparative overview of using such LDHs as a P removal treatment option in WWTPs. The phosphate removal with Ca-type LDHs were 85%-99%, whereas the removal with Mg-type LDHs were <50% under the same operating conditions, suggesting a significant influence of the type of precursor metals in synthesizing LDH compounds for removing phosphate. However, the Mg-type LDH up to 2 g/L can provide a similar P removal as Ca-type LDHs at the dose of 0.3 g/L to reduce P concentration to 0.1 mg/L from an initial concentration of 10 mg/L. The effluent from WWTP's secondary treatment process with P concentration of 1.4. 5.6 mg/L can be treated by both types of LDH to meet future stringent discharge limit at the concentration of 0.1 mg/L. The Mg-type LDH showed mineral stability and regeneration efficiency with ligand exchange and anion exchange as the main mechanism for P removal, while the Ca-type LDH removed P as calcium-phosphate precipitation due to dissolution with broken layered structure in aqueous phase. Overall, both of Ca- and Mg-type LDHs can be potentially applied for tertiary treatment step with the WWTP's conventional treatment process to further polishing P discharge level. But future studies are required to investigate the effect of practical wastewater conditions (medium-term) and estimate costs of application (long-term) of using LDHs for phosphorous removal in real wastewater treatment.