Chlorine Decay Study in a Proto-type Distribution Network

Abstract

Chlorine, when used as a secondary disinfectant in the distribution network (DN) undergoes many concurrent reactions on way to consumer’s tap. These reactions results in chlorine residual decrease, commonly termed as chlorine decay, resulting in failure of this barrier against microbial contamination. This chlorine decay poses a serious threat to community health. The major part of this chlorine decay is attributed to generation of total Trihalomethanes (TTHMs) which is reported as a function of various environmental factors including organic precursor type like humic acid (HA) or fulvic acid (FA), bromide ion, applied chlorine, pH and reaction time. Therefore aim of this study was to investigate effect of various environmental factors on TTHMs formation leading to chlorine decay in a DN. For this, a proto-type DN was established and design expert software (Trial version 9, Stat-Ease Inc., MN) coupled with central composite design (CCD) was used to model chlorine decay along with monitoring of TTHMs speciation as a function of various precursors, their ratio and other environmental factors. Results showed that TTHMs formation and speciation specially brominated species are dependent on HA, Br-, water pH and time. Multifactorial optimization showed that the pH regions between range of 6.5-7.5 and HA: Br- ratio between the range of 3.0 - 6.0 was satisfactory for maintaining TTHMs below recommended concentration of 80μg/L in the proto-type DN. It was also analyzed that their respective concentration may be minimized by changing precursor’s individual concentration or possible combinations of precursor’s molecules/ions like HA and Cl-HA and Br- and Br- & Cl-. Furthermore efficiency of chlorine residuals to inactivate an accidental microbial intrusion entering DN and optimization of time required for combating this microbial intrusion were also analyzed. For this purpose, a known amount of chlorine residuals were maintained in proto-type DN and a required number of bacteria, Escherichia coli K-12 were introduced in pipe loop system. It was observed that in combatting accidental microbial episode, efficiency of chlorine residuals is governed by microbes’ concentration entering DN, time period, pipe length, concentration of chlorine residuals and time. Meanwhile morphology and biodiversity of biofilm was also observed in DN by scanning electron microscopy (SEM) and polymerase chain reaction (PCR)