Pilot-scale coagulation of organic and inorganic impurities: Mechanisms, role of particle concentration and scale effects

The intrusion of natural organic matter in the surface waters has impeded the dosing approaches in the realm of coagulation-flocculation. Scaling up bench-scale coagulant doses to pilot or full-scale systems generally results in decreased system efficiency. In this study, the pilot-scale reactors of Sludge blanket clarifier (SBC) and Conventional clariflocculator (CC), as well as jar test data, are compared for the treatment of synthetic waters (prepared by kaolin and humic acid) at high alkalinity by using poly aluminium chloride (PACl). The turbidity removal for CC and SBC was 43-84 % and 20-81 %; the TOC removal was 20-83 % and 11-84 %; the DOC removal was 27-87 % and 38-92 %; and the UV254 removal was 72-89 % and 75-91 % respectively. pH, alkalinity and zeta potential were analysed to understand the process mechanisms. Adsorption models of Langmuir, Freundlich, BET and Temkin isotherms were compared to further validate removal mechanisms. For SBC, the R-2 and RMSE (Root mean square error) values suggested BET model at lower turbidity and Temkin model at higher turbidities, indicating physical adsorption at lower turbidity followed by significant humic acid-kaolin interactions at high turbidity. For CC, the dominant mechanism was physical and chemical adsorption which was monolayer at lower turbidity and multilayer at higher turbidity. The charge neutralization was dominant in CC, unlike SBC. The sludge collected in the pilot plants was analysed by FTIR for linking chemical interactions to the mechanisms of removal. The FTIR data revealed the presence of Al(OH)(3) and aromatics suggesting dominant removal by adsorption and entrapment.