The role of coagulation on the fate of PFAS, brominated flame retardants and other trace contaminants in tertiary wastewater treatment for phosphorus control.

Coagulant dosing to achieve low phosphorus concentrations in wastewater effluents may favour the removal of trace organics such as pharmaceuticals, plasticisers and flame retardants. Nevertheless, the behaviour of trace organics in coagulation processes is currently poorly understood because of the complex interactions between these compounds, the coagulants and dissolved organic matter (DOM). This study assessed the coagulation removal from synthetic secondary effluent of twenty-four compounds including ten PFAS and four brominated flame retardants. Testing involved two coagulants (alum, ferric chloride) and five DOM surrogates (resorcinol, benzoic acid, citric acid, tannic acid, humic acid); DOM surrogates had assorted molecular weights, structures, charges, and hydrophobicity. With coagulant doses of 14 mg Fe/L and 4 mg Al/L, ten trace organics were removed by >30 % in the presence of at least one DOM surrogate. Humic acid effected the highest removals owing to complexation of trace organics and subsequent co-removal by adsorption or sweep floc. For instance, removal extents for three brominated diphenyl ethers were 60 to 75 % with Al and 50 to 88 % with Fe (initial concentration 0.4 to 0.8 ng/L); PFTDA, a long-chain PFAS, was removed by 87 and 91 % with Fe in the presence of tannic or humic acid, respectively (initial concentration 0.03 μg/L). The varying coagulation performance of different treatment works in terms of trace substance removal can be explained because of the site-specific DOM characteristics. Addition of humic acids as complexing agents has the potential to improve the removal of hydrophobic trace substances, including some long-chain PFAS and brominated flame retardants.