Pre-chlorination effects on drinking water treatment using tannin-based coagulant

ABSTRACT

Different techniques in the treatment of water intended for public supply such as pre-chlorination and use of organic coagulants have been reported as economic, technical and environmental viable alternatives. However, the result of the interaction between these processes should be studied for their safe application. Thus, this study aimed to evaluate the influence of pre-chlorination on coagulation and flocculation of water for public supply using a tannin-based coagulant. The assays were conducted in Jar Test using Tanfloc SG as a coagulant, varying the dosages from 2.5 to 45.0 mg L⁻¹, and sodium hypochlorite oxidant (NaClO) in the concentrations of free residual chlorine (HClO_(NaClO)) of 0.8 and 5.0 mg L⁻¹, in the water treatment of João Leite River. Optimal coagulant dosages were determined for the following treatment conditions: addition of tannin only; tannin and NaClO applied simultaneously; and, tannin added after 30 min of pre-chlorination. Under these conditions, the coagulation pH was analyzed from 6.0 to 8.5. Parameters such as turbidity, apparent color, pH and HClO (NaClO) were evaluated. The best treatment condition with pre-chlorination was the simultaneous addition of NaClO and coagulant at dosages of 0.8 mg L⁻¹(HClO_(NaClO)) and 10.0 mg L⁻¹ respectively, with turbidity removal of 71.6% and apparent color of 62.7%. In general, the results showed that pre-chlorination negatively affected the process being that the higher concentration of HClO_(NaClO) the higher tannin dosage, without presenting a significant increase in the efficiencies of turbidity and apparent color removal.

Keywords:
oxidation; sodium hypochlorite; coagulation; tannin