Abstract：
Nitrification is a serious water-quality issue in chloraminated engineered water systems (EWSs). Nitrification is often remediated by a chlorine burn (i.e., a free‑chlorine conversion), a short-term switch from chloramination to chlorination in EWSs. Opportunistic pathogens (OPs) are the dominant infectious agents in EWSs. However, the responses of OPs to a chlorine burn are unknown. This study for the first time assessed how a chlorine burn affected OPs in a full-scale EWS. We determined the impact of a 1.5-month chlorine burn on four dominant OPs (Legionella, Mycobacterium, Pseudomonas, and Vermamoeba vermiformis) in a representative full-scale chloraminated EWS in the United States. Legionella and Mycobacterium were the most abundant OPs. In the water main, the summed concentration of the four OPs during the chlorine burn [3.27 ± 1.58 log10(GCN·L-1); GCN: genome or gene copy number] was lower (p ≤ 0.001) than before the burn [4.83 ± 0.50 log10(GCN·L-1)]. After the burn, the summed concentration increased to 4.27 ± 0.68 log10(GCN·L-1), comparable to before the burn (p > 0.05), indicating a transient effect of the chlorine burn in the water main. At the residential sites, the summed concentrations of the four OPs were comparable (p > 0.05) at 5.50 ± 0.84, 5.27 ± 1.44, and 5.08 ± 0.71 log10(GCN·L-1) before, during, and after the chlorine burn, respectively. Therefore, the chlorine burn was less effective in suppressing OP (re)growth in the premise plumbing. The low effectiveness might be due to more significant water stagnation and disinfectant residual decay in the premise plumbing. Indeed, for the entire sampling period, the total chlorine residual concentration in the premise plumbing (1.8 mg Cl2·L-1) was lower than in the water main (2.4 mg Cl2·L-1). Consequently, for the entire sampling period, the summed concentration of the four OPs in the premise plumbing [5.26 ± 1.08 log10(GCN·L-1)] was significantly higher (p < 0.001) than in the water main [4.04 ± 1.25 log10(GCN·L-1)]. In addition, the chlorine burn substantially increased the levels of disinfection by-products (DBPs) in the water main. Altogether, a chlorine burn is transient or even ineffective in suppressing OP (re)growth but raises DBP concentrations in chloraminated EWSs. Therefore, the practice of chlorine burns to control nitrification should be optimized, reconsidered, or even replaced.
摘要：
在氯化工程水系统（EWS）中，硝化是一个严重的水质问题。硝化通常通过氯燃烧来修复(即，游离氯转化)，EWS中从氯胺化到氯化的短期转变。机会性病原体（OP）是EWS中的主要感染因子。然而,OPs对氯燃烧的反应是未知的。这项研究首次评估了氯燃烧如何影响全尺寸EWS中的OPs。我们确定了1.5个月的氯燃烧对四种主要OPs（军团菌，分枝杆菌,假单胞菌,和Vermamoebavermiformis）在美国具有代表性的全尺寸氯化EWS中。军团菌和分枝杆菌是最丰富的OPs。在水总管，氯燃烧过程中四种OPs的总浓度[3.27±1.58log10（GCN·L-1）；GCN：基因组或基因拷贝数]低于烧伤前[4.83±0.50log10（GCN·L-1）]（p≤0.001）。烧伤后,总浓度增加到4.27±0.68log10(GCN·L-1)，与烧伤前相当(p>0.05)，表明氯在水总管中燃烧的瞬态效应。在住宅区，四种OPs的总浓度在5.50±0.84、5.27±1.44和5.08±0.71log10（GCN·L-1）之前具有可比性（p>0.05），during,氯气燃烧后，分别。因此,在前提管道中，氯燃烧在抑制OP（重新）生长方面效果较差。低效率可能是由于前提管道中更明显的水停滞和消毒剂残留腐烂。的确,在整个采样周期内，前提管道中的总氯残留浓度（1.8mgCl2·L-1）低于水总管（2.4mgCl2·L-1）。因此,在整个采样周期内，前提管道中四种OPs的总浓度[5.26±1.08log10（GCN·L-1）]显着高于（p<0.001），高于主要水[4.04±1.25log10（GCN·L-1）]。此外,氯燃烧大大增加了水总管中消毒副产物（DBPs）的水平。总之,氯燃烧在抑制OP（重新）生长方面是短暂的甚至无效的，但会提高氯胺化EWS中的DBP浓度。因此,应优化氯气燃烧控制硝化的做法，重新考虑,甚至被替换。
