Evaluation of a tannin-based cationic polymer as a coagulant for coloured humic water

The performance of a new commercial tannin-based cationic polymer (TBP) has been investigated at bench-scale in terms of its capability to coagulate colour and humic material. The TBP is characterised as a moderate-to-high molecular weight polymer (~600,000 g mol⁻¹) with a cationic charge (~3 meq g⁻¹) arising from a single tertiary amine group per monomer. The TBP is substantially affected by hydrolysis/hydration processes which lead to changes in its charge density and solubility with solution time and pH. Laboratory experiments were carried out to evaluate the optimal dose, pH and treatment performance of TBP with simulated and real coloured waters. With simulated water TBP was able to achieve a high dissolved organic carbon (DOC) reduction over a wide pH range (4–9), but the optimal dose increased with pH as a consequence of the loss of cationic charge and solubility. At pH 7, the coagulation performance of TBP was comparable to that of alum and polyDADMAC at optimum coagulant doses. With real coloured water, jar and flotation tests showed that the TBP was able to match the performance of a polymeric iron sulphate in terms of treated water colour and turbidity, but residual DOC was significantly higher with TBP.