Simulação do decaimento de cloro residual livre em reservatórios de distribuição de água Simulation of free residual chlorine decay in water distribution reservoirs

Disinfection process is used in the treatment of water for human supply to promote sanitary safety and provide users with drinking water that meets potability standards. Thus, it is necessary to sustain a minimal concentration of free residual chlorine (FRC) throughout the entire distribution system. The present study investigated the decay process of FRC concentration in water destined for human supply. The decay was evaluated in bench-scale testing, using sodium hypochlorite, calcium hypochlorite, sodium dichloroisocyanurate (organic chlorine) as disinfectant agents, and also an alternative disinfectant solution (ADS) produced in loco, with oxidizing and disinfectant properties, which is being used in Brazilian sanitation industry. To evaluate the decay, four models were fitted: first-order, nth-order, limited first-order and parallel first-order, hence determining the corresponding parameters which describe the decay speed of the FRC concentration in water. Achieved results demonstrated that all models were statistically significant and predictive. However, parallel first-order model produced the best fit. Regarding the evaluated disinfectants, it was noted the preeminence of ADS solution when compared to the others, since it imparted a higher FRC over time, a behavior indicated by lower values for reaction rate constant in all models and when compared to other disinfectants used in this study.

The objective of this study was to evaluate the effects of the transfer of the São Francisco River waters on the quality of the water produced by water treatment plant of Gravatá (WTP-Gravatá) using multivariate statistics. Monthly means of the variables pH, apparent color and turbidity of raw, decanted and treated waters were used, in addition to the volume accumulated by the Epitácio Pessoa reservoir, during the period from January 2016 to December 2017. The arrival of the transposition waters abruptly changed the apparent color and turbidity of the raw water. In spite of that, the treated water presented low variability of the mentioned parameters, indicating that the WTP-Gravatá was able to adapt its treatment. From the factor analysis/principal component analysis (FA/PCA) it was found that changes in reservoir volume alter the apparent color and turbidity of the raw water, requiring interventions in the coagulation/flocculation/decantation stages. The increasing of these parameters in the decanted water overloads the filtration step, raising the apparent color of the treated water. The cluster analysis distinguished the different phases experienced by the Epitácio Pessoa reservoir. The information obtained can help decision making in WTPs, considering changes in reservoir volumes.