Evaluating a Large-Scale Batch Chlorination Method for Household Cistern Water Treatment in the US Virgin Islands

In the US Virgin Islands (USVI), roof-harvested rainwater is a primary source of domestic water and is collected in large cisterns and pumped throughout the household premise plumbing system. However, previous literature suggests that roof-harvest rainwater is prone to microbial contamination. A common local water treatment practice is direct batch chlorination of this water ($>\mathrm{20,000}$ L). Provided this local habit, Love City Strong (LCS) established a pilot program to evaluate if direct batch chlorination of water in cisterns was effective and economically feasible to produce consistently adequate levels of free chlorine residual (FCR) at the kitchen tap. A trained field team utilized nine actively used cisterns to conduct a series of chlorine dosing trials where the cistern water was dosed, and, subsequently, monitored every three days. Water quality data were collected throughout, and new trials were initiated once FCR values reached $\le 0.05\mathrm{ppm}$. Evaluation criteria included: (1) the variability in duration between a dosing event and an FCR measurement $<0.2\mathrm{ppm}$, (2) the variability of achieving a target FCR level of 1.5 ppm, (3) the variability in the first-order chlorine decay rate, and (4) the costs of the method. Results suggested that the variability in duration, achieving a target FCR, and the decay rate were prohibitively high for consistent use of the method. The duration ranged from 3 to 33 days (mean: 11.5; $\mathrm{n}=34$). This large range may have been influenced by the low probability (18%) of achieving an initial target FCR between 1.25 and 1.75 ppm. In addition, the chlorine decay rate ranged from 0.095 to $0.482{\mathrm{d}}^{-1}$ (mean: $0.251{\mathrm{d}}^{-1}$), resulting in an estimated duration ranging between 5 and 22 days. Finally, the first-year cost was $622 ($244 excluding person-time costs), while annual costs thereafter were $567 ($204 excluding person-time costs). Given these data, we do not recommend the use of direct batch chlorination for treating cistern water for microbial contamination in USVI households.