Like the various water sources for the bottled water brands, municipal water system sources also contain naturally occurring microorganisms overwhelmingly dominated by the bacterial group (Casanovas-Massana & Blanch, 2012; Edberg & Allen, 2004). Depending on the source, either surface freshwater or
underground water, the bacteriological quality of drinking water is influenced by different sets of factors (Leclerc & Moreau, 2002; Senior & Dege, 2005). Municipal water sourced from groundwaters and distributed through pipe networks are typically inhabited by heterotrophic microorganisms. Compared with surface waters that are governed by suspended particles and their attached microflora, groundwaters are generally confined in high pressure interstitial spaces underground that have greater saturation resulting to decreased likelihood of subsurface contamination, low porosity (lesser interstitial space for microbes to thrive), and oligotrophic chemistry (low nutrient) (Leclerc & Moreau, 2002; Perk, 2006). Because of these factors, microbial growth are greatly limited in groundwaters in contrast to surface waters which are virtually open to the atmosphere and likely contamination. Hence, there is some degree of microbial exclusivity (only highly- tolerant organisms can survive that are usually non- pathogenic) in these systems that contribute to the natural protection of these groundwater habitats against pathogens compared with surface waters. Moreover, unlike packaged waters that acquire relative stationary residence in bottles, tap water flows in a continuous phase along a distribution network of pipelines. The pipeworks are usually made of metal materials, mostly iron and copper, as well as lead coming from solders that join copper pipes (Carter et al., 2000). In most typical scenarios, the delivery of water requires adequate amounts of pressure to reach treatment plants (if treated), reservoirs, substations or distribution loops, and ultimately to each single dead-end distribution loop. The water typically remains in the dead-end loop until extracted by a consumer through taps in private households, and commercial and industrial establishments (Percival et al., 2000). In these conditions the waters are
removed from their original source and transported to a different environment and accompanying microflora. More specifically, the type and levels of bacterial flora in municipal water systems are not the same as in bottled waters. The dominant bacterial types in municipal water supplies include many of the acid-fast bacilli, Gram-negatives and Gram-positives and spore formers, as described by Geldreich (1996).
Kumpel and Nelson (2014) studied the multiple mechanisms affecting water quality and contamination with coliforms and E. coli in the pipes in cases of water interruption and intermittent supply. They demonstrated that low water pressure in distribution pipes increases the levels of coliforms even in the presence of chlorine residuals. In contrast, high pressures, together with the presence of disinfection residuals, reduced the levels of coliforms and no E. coli was detected. Possible explanations for these contaminations during low water flow are the occurrence of external intrusion of contaminants into the pipes, internal backflow, internal pipe wall particulate release, and sloughing of bacteria from attached biofilms as a consequence of low flow (Kumpel & Nelson, 2014). This was hypothesised in the study of Carter et al. (2000) that when water pressure and residual chlorine are high in a municipal distribution system, bacterial counts seemed to decrease. However, the levels of bacteria along the pipe network varied depending on the location. Carter et al. (2000) showed that bacterial levels increased with distance from the treatment plant as a consequence of reduced effective chlorine residuals. In contrast, bacterial numbers decreased from booster pump stations to dead end supply loops which supply the household concessionaires (Carter et al., 2000).
It is also important to consider that bacterial contamination in distribution pipes is aggravated by the presence of biofilms, which are characterised by the presence of typical microorganisms, such as E. coli, Legionella, and Pseudomonas (Berry, Xi, & Raskin, 2006; Brettar & Höfle, 2008; Leclerc & Moreau, 2002). Problems with plumbing systems, ineffective maintenance of pipes, inefficient disinfection, low water pressure, and flow interruption are factors that could significantly contribute to the proliferation of biofilms at the periphery of long pipes, making treatment systems ineffective and unsustainable. This is in agreement with the studies of Carter et al. (2000), Edberg et al. (1997), and Kumpel and Nelson (2014), which support the idea that bacterial contamination in municipal tap waters is affected, not only by the microbiological properties of the sourced water, but also by defects in distribution, maintenance, and management. It has also been established that most disinfectants and sanitizers are ineffective against bacteria that form biofilms because of the relative protection and resilience of microorganisms capable of entering this matrix (Mah & O'Toole, 2001; Marshall, 1988; van der Merwe et al., 2013). It was also shown that the effective disinfection residuals of chlorine are greatly reduced by the chlorine-demanding organic matter found in soils and sewage that infiltrate the system (Edberg et. al., 2000). Furthermore, E. coli can survive in distribution pipes when chlorine residuals dissipate. Specifically, E. coli bacteria can survive for about 4 – 12 weeks in drinking water distribution systems (Edberg et. al., 2000). Similar to viruses, bacteria are sensitive to chemical oxidation treatments (for example, chlorine), but survive longer than viruses in the water. Hence, the presence of E. coli alone can provide information about the effectiveness of treatments applied. In addition, Edberg (2005) and Edberg et al. (2000) indicated that the
susceptibility of these organisms to the bactericidal effect of chlorine is markedly reduced because of the protective effect of faecal material and biofilms contaminating the water and pipes. Therefore, in addition to chlorination, proper application of sanitation and maintenance programs and engineering are major factors in the assurance of bacteriologically safe municipal water supply.
Many studies have shown the high disinfection power of chlorine, which yields effective microbicide residual in municipal water systems (Edberg et al., 1996; Edberg et al., 2000; Kumpel & Nelson, 2014; Senior & Dege, 2005). In contrast, treatments such as ozone, reverse osmosis, and distillation applied on bottled waters leave no effective disinfection residuals (Edberg et al., 1996; Kumpel & Nelson, 2014; Percival et al., 2000; Senior & Dege, 2005). Whilst most urban municipal waters require disinfection treatments, the application of source protection and monitoring is paramount to facilitate the efficiency of the water treatment regime applied (Edberg, 2005). It should be noted that once water enters the distribution system from the underground source, a new microecosystem is formed because of the effect of pipes and production materials on the original microflora of the water. One of the most influencing materials is the organic matter that serve as nutrients for heterotrophic bacteria, facilitating their colonization on the distribution pipe networks (Leclerc & Moreau, 2002; Senior & Dege, 2005).
As earlier mentioned, bacterial growth and survival is affected by the organic matter present originating not only from the raw water, but also from the distribution materials in pipes, such as sealants, lubricants, and joints (Geldreich, 1996). The colonisation of pipes with heterotrophic bacteria is facilitated by these
organic nutrients. More importantly, this material promotes the formation of biofilms that can contaminate pipe networks and edges where water stagnation occurs (Brettar & Höfle, 2008). Studies showed that the most widely used bacterial indicator, E. coli, is able to survive in water for a period of 4 – 12 weeks at a temperature range of 15 - 18°C (Edberg et al., 2000; Odonkor & Ampofo, 2013; Schets et al., 2005). Further, E. coli survives for up to 100 days in groundwater at a temperature of 10°C (Filip, Kaddumulindwa, & Milde, 1987). Thus, the transport of pathogens from these sources through municipal pipes is inevitable in the absence of source protection and effective treatment methods. Hence, these groundwater sources, and distribution networks with leaks in pipes and joints, can serve as intrusion points and ideal spots for the growth of enteric pathogens. Consequently, pathogen contaminated drinking water delivered from this distribution source poses potential health risks to consumers.
Therefore, various water disinfection methods are usually applied on drinking water originating from sources not guaranteed as being microbiologically pure and pathogen free (Edberg, 2005). For municipal water supplies, chlorination is the most appropriate treatment method because of chlorine’s effective disinfection residual consistently being present throughout the distribution network to ensure the destruction of pathogens (Kumpel & Nelson, 2014; Leclerc & Moreau, 2002). Nonetheless, as said above, the effectiveness of this method is affected by the pipe network reticulation and the type and levels of microorganisms present.
In the above discussion it was shown that there are fundamental differences between the bacteriological quality of municipal tap water supply and bottled
waters. However, both water types must not contain harmful bacteria or pathogens and must be safe (potable) for human consumption, regardless of the presence of a natural microflora (Senior & Dege, 2005). Therefore, microbiological testing methods are similar for both water types. Municipal water supplies usually achieve water safety by chlorine disinfection and the presence of chlorine residuals in the distribution system. This chemical treatment, however, cannot be applied to bottled water because of the organoleptic changes it causes to the bottle surface and the water itself.