The Bactericide Effects of Chitosan When Used as an Indicator of Chlorine Demand

Abstract

The Miradero Water Treatment Plant (MWTP) in Mayagüez, Puerto Rico, uses sodium hypochlorite (SH) as a disinfectant. However, SH reacts with humic substances present in surface water, forming disinfection by-products (DBPs) regulated by the U.S. EPA. This study evaluated whether chitosan, a biopolymer with known bactericidal properties, could reduce chlorine demand and improve disinfection. Chitosan, with a 75% degree of deacetylation and a molecular weight of 460 kDa, was solubilized in four acids (acetic, citric, hydrochloric, and L-ascorbic) and tested under two turbidity ranges (236.0 and 2556 NTU). Chlorine demand curves were generated, and coliform presence–absence (P–A) tests were performed to assess bactericidal effects. Results showed that chitosan-treated samples achieved disinfection at the breakpoint with lower SH doses. For water with a turbidity of 236.0 NTU, all chitosan-acid solutions reached the breakpoint at 3.60 mg/L of SH. At 2556 NTU, three solutions required 4.20 mg/L SH, while hydrochloric acid–chitosan required only 3.60 mg/L. All chitosan-treated samples tested negative for coliforms, while controls treated with SH alone tested positive. These findings demonstrate that chitosan enhances bacterial removal and reduces chlorine demand, potentially lowering DBP formation in water treatment.