Abstract

In municipal wastewater treatment, inorganic coagulants (IC), e.g. polyaluminium chloride (PAC), are normally used to remove pollutants such as dissolved and particulate nutrients, in a process called coagulation/flocculation. However, IC use has been linked to issues e.g. in effluent water post-treatment, sludge management and disposal (IC increase sludge volume and metal concentrations in sludge), etc., raising uncertainties about their overall cost-efficiency and environmental benefits. In this study, the suitability of organic coagulants (OC) as sole precipitation agents to replace IC (PAC) was investigated. A total of 10 synthetic (i.e. polyDADMACs and polyamines) and semi-natural (chitosan, starch, and tannin-based) OC products were tested in treatment of samples from primary sedimentation and secondary sedimentation stages of municipal wastewater treatment, and their performance was compared with that of PAC. The study was conducted using the jar test methodology. The coagulants were tested for their ability to remove target pollutants (e.g. BOD₇, COD, SS, tot-P, PO₄-P, tot-N) and form rapidly settling flocs. In general, higher (up to 60%) coagulant doses were needed in treatment of secondary wastewater samples than primary samples. In comparison with the OC doses required for effective treatment, the PAC doses were higher (up to 80%). In treatment of secondary wastewater samples, OC with high molecular weight (MW) and high charge density (CD) (e.g. pAmine1) achieved best removal of target pollutants (e.g. 72% SS, 87% PO₄-P, 88% BOD₇), followed by PAC. In treatment of primary wastewater, PAC performed best (removing e.g. 96% SS, 96% PO₄-P), closely followed by chitosan and polyamine products. Based on these results, polyamine products with high MW and (very) high CDs have the potential to act as the sole precipitation agent in both primary and secondary stages of municipal wastewater treatment. Further research is needed to determine the effect of residual coagulant on downstream water and sludge treatment processes (e.g. activated sludge process, sludge dewatering, etc.).