Biofilm Detachment Significantly Affects Biological Stability of Drinking Water during Intermittent Water Supply in a Pilot-Scale Water Distribution System

Drinking water quality can deteriorate within piped distribution networks due to the detachment of biofilms. In continuous water supply (CWS) systems, pipes are continuously under fluid pressure and detachment is promoted by changes in hydraulic regime that increase shear stresses along the pipe walls. When services are intermittent, there are stagnant periods where water drains from the depressurized pipes, and biofilm detachment occurs as the supply is reconnected. This study compares how biofilm growth and detachment affect water quality under continuous and intermittent water supply (IWS) operating conditions. Here we investigate the microbial communities by comparing inlet and outlet bulk water conditions over three repeated sets of flushing experiments performed on 90 m long uPVC pipe sections in an above-ground testbed exposed to water containing low levels of the residual chemical disinfectant monochloramine. The CWS section is primed under conditions of laminar flow, while the IWS section is subjected to a 15 min daily supply window with 23.75 h of stagnation over a period of one month prior to each set of tests. Flow cytometry (FCM) reveals a substantial increase in total and live cell counts during initial flushing of the IWS section, which exhibits a higher biofilm detachment potential than CWS at similar flow rates. We find that the CWS section has much higher microbial -diversity (2D Hill number) than the IWS section based on both 16S rRNA gene metabarcoding and fingerprinting of flow cytometric data. Through statistical analyses we show FCM fingerprinting to be a robust method for monitoring and quickly diagnosing microbial water quality, although it lacks the taxonomical depth of 16S rRNA gene metabarcoding. Daily flushing of the IWS section revealed an increase in nitrate levels in the bulk water, together with a decrease in ammonia and nitrite concentrations, suggesting nitrifying activity in biofilms exposed to stagnant waters in the pipe. While both test sections show an increase in the relative abundance of ammonia oxidizing and nitrite oxidizing bacteria in biofilm samples at the end of the experimental test program, the effects are more pronounced in the CWS section. Hence, CWS hydraulic conditions are more conducive to the growth of nitrifiers in biofilms, while daily supply periods under IWS may slow the long-term development of nitrifying biofilms. We conclude that drinking water delivered through IWS presents significantly different microbial communities from CWS in the outlet bulk water and pipe biofilm.饮用水在管道输配管网中输送时，生物膜（biofilms）的脱落可能导致水质恶化。在连续供水（continuous water supply, CWS）系统中，管道始终承受流体压力，沿管壁剪切应力升高的水力工况变化会加剧生物膜脱落。当供水模式转为间歇时，管道会因卸压而排空，出现滞水期，待重新供水时便会发生生物膜脱落。本研究对比了连续供水与间歇供水（intermittent water supply, IWS）工况下，生物膜生长与脱落对水质的影响。 本研究针对搭建于地面的试验台中的90米长硬聚氯乙烯（uPVC）管段开展了三组重复冲洗试验，通过对比进水与出水的主体水微生物群落特征开展相关研究。试验用水中含有低浓度残留化学消毒剂一氯胺（monochloramine）。连续供水管段在层流条件下预启动，而间歇供水管段则在每组试验前的一个月内，每日仅供水15分钟，其余23.75小时处于滞水状态。 流式细胞术（flow cytometry, FCM）检测结果显示，间歇供水管段初次冲洗时，总细胞数与活细胞数均显著升高，其在相近流量下的生物膜脱落潜力高于连续供水管段。基于16S rRNA基因宏条形码测序（16S rRNA gene metabarcoding）与流式细胞术数据指纹分析结果，本研究发现连续供水管段的微生物α多样性（α-diversity，2D Hill number）显著高于间歇供水管段。通过统计学分析证实，尽管流式细胞术指纹分析在分类学分辨率上不及16S rRNA基因宏条形码测序，但该方法可作为一种可靠的微生物水质监测与快速诊断手段。 对间歇供水管段的每日冲洗结果显示，主体水中硝酸盐浓度升高，同时氨与亚硝酸盐浓度降低，表明管道内滞水条件下的生物膜发生了硝化活动。尽管在试验结束时，两个管段的生物膜样本中氨氧化细菌与亚硝酸盐氧化细菌的相对丰度均有所上升，但该现象在连续供水管段中更为显著。由此可见，连续供水的水力条件更利于生物膜中硝化菌的生长，而间歇供水的每日供水周期则可能延缓硝化生物膜的长期发育。 本研究最终得出结论：通过间歇供水输送的饮用水，其出水主体水与管道生物膜中的微生物群落与连续供水模式存在显著差异。