Liu S, Gunawan C, Barraud N, Rice SA, Harry EJ, Amal R. Understanding, Monitoring and Controlling Biofilm Growth in Drinking Water Distribution Systems. Environ Sci Techno. 2016 Aug 1.
In drinking water distribution systems (DWDS), biofilms are the predominant mode of microbial growth with the presence of extracellular polymeric substance (EPS) protecting the biomass from environmental and shear stresses. Biofilm formation poses a significant problem to the drinking water industry as a potential source of bacterial contamination, including pathogens and in many cases also affecting the taste and odor of drinking water and promotes corrosion of pipes. This article critically reviews important research findings on biofilm growth in DWDS, examining the factors affecting their formation and characteristics, as well as the various technologies to characterize, monitor and ultimately, to control their growth. Research indicates that temperature fluctuations potentially affect not only the initial bacteria-to-surface attachment but also the growth rates of biofilms. For the latter, the effect is unique for each type of biofilm-forming bacteria – ammonia oxidizing bacteria for example, grow more developed biofilms at typical summer temperature of 22○C compared to 12○C in fall , while the opposite occurs for the pathogenic V. cholera. Recent investigations have found formation of thinner yet denser biofilms under high and turbulent flow regimes of drinking water, in comparison to the more porous and loosely attached biofilms at low flow rates. Further, in addition to the rather well-known tendency of significant biofilm growth on corrosion-prone metal pipes, research efforts also found leaching of growth-promoting organic compounds from the increasingly popular use of polymer-based pipes. Knowledge of the unique microbial members of drinking water biofilms and importantly, the influence of water characteristics and operational conditions on their growth, can be applied to optimize various operational parameters to minimize biofilm accumulation. More detailed characterizations of the biofilm population size and structure are now feasible with fluorescence microscopy (epifluorescence and CLSM imaging with DNA, RNA, EPS, protein and lipid stains) and electron microscopy imaging (ESEM). Importantly, thorough identification of microbial fingerprints in drinking water biofilms is achievable with DNA sequencing techniques (the 16S rRNA gene-based identification), which have revealed prevalence of previously undetected bacterial members. Technologies are now moving toward in situ monitoring of biomass growth in distribution networks, including the development of optical fibres capable of differentiating biomass from chemical deposits. Taken together, management of biofilm growth in water distribution systems requires an integrated approach, starting from treatment of water prior to entering the networks, to potential implementation of ‘biofilm-limiting’ operational conditions and finally, to the careful selection of available technologies for biofilm monitoring and control. For the latter, conventional practices, including chlorine – chloramine disinfection, flushing of DWDS as well as nutrient removal, and emerging technologies are discussed with their associated challenges.