Assessment of Free Chlorine Residual and Microbiological Parameters in Supply Water of Centralized Supply Water Companies in Vietnam

"In the study, the free chlorine residual and microbiological parameters in supply water of centralized supply water companies were assessed with a total of 1,612 samples in the period of 2020 and 2022. The results showed that 334 samples did not meet the permitted limit QCVN 01-1:2018/BYT for free chlorine residual (0.2-1.0 mg/L), of which, about 80% of sample have concentration less than the lower limit. The reason is due to the inaccurately used chloramine dose, mainly quantified without adjusting for actual output quality and distribution system. Some companies even periodically disinfect water 1-3 times/week. In many cases, the water samples at the companies’ storage tanks have high free chlorine residual, about 2 times greater than upper limit (>2.0 mg/L). Regarding microbiological parameters, for total coliforms about 40-50% surveyed provinces have water samples that were non-compliance with the acceptable quality standard for QCVN 01-1:2018/BYT with total of 102, 57 and 105 samples. For those water samples, free chlorine residual found also lower than required limit (<0.06 mg/L). Apart from total coliforms and E. coli, Pseudomonas aeruginosa (Ps. aeruginosa) was also detected in water samples in some provinces and cities with levels exceeding the allowable standards from several times to dozens of times. In addition, the number of provinces/cities where Ps. aeruginosa was detected in supply water also increased over the years but insignificantly. Water samples that did not meet the standard are mostly from water supply companies with a capacity of less than 1,000 m3/day. Correlation analysis by regression coefficient R2 and Pearson correlation coefficient showed that there was a negative correlation between free chlorine residual and microbiological parameters (total coliforms, E.coli), but the correlation was low and non-linear."