Water supply systems in residential buildings, public utilities or industrial plants are made of various materials [1
]. The type of material and the quality of tap water are the most important factors affecting the risk of losing water safety reaching the consumer [2
]. Therefore, the assessment of risks related to internal water supply installations, including hazards related to products and materials in contact with drinking water is extremely important [3
]. A particular health risk is caused by the emergence of biofilm and increased colonization of organisms on the surface of water pipes. The intensity of biofilm growth in a water distribution system depends on numerous factors such as content of biogenic compounds in water injected into a network, amount of disinfectant, temperature, hydrodynamic conditions and the type of material from which the water conduits are made [4
]. The chemical composition of the water supply system material, as well as its properties, that is, porousness and susceptibility to corrosion, are regarded as one of the main causes of increased colonization of microorganisms. The multitude of factors influencing the formation and development of biofilm makes prevention of the phenomenon very complex [9
Plastics and polyester resins in tap water distribution networks are replacing cast-iron and galvanized steel, however; pipes and elements made of copper or chromium-nickel steel are used as well. These materials are characterized by high resistance to corrosion and low surface porousness. Due to their ever-reducing price and valuable properties, they can become genuine competition for traditional materials used in water supply systems (such as polyethylene (PE), and polyvinyl chloride (PVC)). Currently, cast-iron and concrete are used decidedly less often to build water-pipe networks than just a few years ago, however; numerous sections of networks made of these materials are still in exploitation [12
Synthetic materials characterized by low porousness were expected to eliminate corrosion and decrease the risk of secondary water pollution. The research conducted thus far shows that biofilm forms on all water supply system materials, but each of them creates different conditions for duplication and adhesion of microorganisms in the form of biofilm [13
According to the prevailing literature on the issue, plastics can support the formation of biofilm, however, the growth of microorganisms on the surface of plastic pipes is usually the same or lower than in the case of iron, steel, or concrete [4
Contact between water and the internal surface of the water conduits may lead to, i.e., corrosion or ageing of materials, eluting chemical substances, and to the formation of biofilm on their surfaces. Metals from brass elements, e.g., joints, can also be transferred into the water. This applies particularly to lead, which presents a serious risk to pregnant women and children below six years of age [1
Phosphorus and carbon, which are nutrients for microorganisms, can transfer to water when materials come into contact with them in the form of microbiologically available phosphorus (MAP) [5
] or available organic carbon (AOC) [14
], speeding up biofilm formation.
Despite the possible influence of material on biofilm formation [15
], in research conducted on actual conduit sections after a year of contact with tap water, this dependency has not been noted [1
]. It was determined, however, that the biofilm formation process was influenced by water temperature and flow conditions [1
Some researchers state that type of material influences biofilm formation, but only at its initial stage. According to the research, the small difference in the number of microorganisms populating internal surfaces of PE and copper pipes after 21 and 200 days shows the lack of dependency between material type and intensity of biofilm growth after a longer time period in contact with the medium [5
None of the so far examined materials allows the complete elimination of biofilm formation in tap water distribution networks. It is, therefore, noteworthy that, due to their structure, both plastics and corroding materials create different opportunities for the formation of biofilm [5
The research so far has focused on specific materials and selected different conditions of biofilm formation on material surfaces [1
], experiment time duration [1
], biofilm detachment method for analysis [4
] and biofilm quantification methods [1
]. Therefore, this article presents an evaluation method of the susceptibility of water supply system materials to biofilm formation in conditions close to those of actual water supply systems.
An additional possibility of analyzing biofilm morphology is provided by the use of fractal analysis together with the appropriate tools of fractal geometry [21
] used to quantify the roughness of any structure. The biofilm research in such a wide range has not been conducted so far.
The aim of the study was to assess changes in water quality and biological stability depending on the time spent in the distribution system and to determine the susceptibility of materials to the adhesion of microorganisms.
In earlier research, Pietrucha et al. confirmed that tap water supplying the experimental system is chemically unstable water with a tendency to solve solids with the possibility of occurrence of slight corrosion [30
Corrosion encourages biofilm development [31
] and presents an additional risk of loss of physical and biological stability of tap water [10
]. Striving to achieve biological stability of water directed to distribution networks is connected to the necessity of ensuring extremely low content of nutrients for microorganisms developing on the surfaces of the water-pipe network. This is a very difficult task, especially in the case of water treated in conventional systems (chemical oxidation, coagulation, filtration, disinfection). For the purpose of ensuring biological stability of water, effective elimination of organic substances and biogenic elements, nitrogen and phosphorus, is necessary. In order to maintain stability and maximally limit the risk of secondary biological water pollution, two out of three biogenes determining microorganism growth should be removed [10
]. Zamorska in her research, confirmed the lack of biological stability of water injected into the water-pipe network [32
In the case of water containing natural organic matter and inorganic nitrogen, phosphorus ions are crucial [2
]. Too low content hinders microorganism development at a significantly higher degree than in the case of other biogenes [33
]. Lehtola et al. suggest that due to the lowest required phosphorus content, it is this element that limits microorganism growth [34
]. It should be noted that phosphorus and other nutrients may, in the first days of exploitation of water systems made of plastics, be eluting from these materials, causing quicker development of biofilm, which is what may have occurred in the described case [5
Thresholds of parameters limiting redevelopment of microorganisms in distribution networks should be lower than 0.25 mg C/L BDOC, 0.2 mg Nnorg
/L and 0.03 mg PO43−
]. In the first few days after launching the system, concentration of PO43−
ions increased systematically from 0.05 to 0.15 mg PO43−
/L. No further increase of the analyzed parameter was noted after 26 days (Figure 12
). The phosphorus eluted into the water in lower quantities could be used by increased numbers of bacteria developed in the system. Similar research results were noted by [5
]. Nonetheless, in conduits made of plastics, small amounts of phosphorus may be present for as long as 200 days [5
]. A similar dependency was noted in the case of total organic carbon (TOC), the content of which, after passing through the system, increased from 2.03 mg C/L to 2.93 mg C/L.
Decrease of biofilm growth depends on water temperature, time duration of presence in the system, as well as type and concentration of disinfectant [32
Due to the fact that the research was conducted in winter and spring, water temperature had a strong influence on microorganism development. A strong positive correlation between total number of psychrophilic bacteria and temperature was determined [37
]. The temperature of water supplying the system changed in the range between 14.6 and 20.3 °C. Additionally, after passing through the system, the water temperature increased maximally up to 24 °C. According to the literary data, microorganism metabolic activity at 7 °C is lower by 50% than at 17 °C [38
]. Total and elementary chlorine content in the water leaving the experimental system in comparison to water supplying the system decreased to 0.029 and 0.014 mg Cl2
/L, respectively. Chlorine is a disinfectant and a guarantee of the microbiological safety of water, which in this case, was clearly put at risk.
In Poland, the required concentration of elementary chlorine in water injected into the water-pipe network should equal 0.2–0.5 mg Cl2
/L, and in water at the ends of the network, it can be no lower than 0.05 g Cl2
/L. Gillespie et al. proved that systems distributing water with elementary chlorine concentrations below 0.5 mg Cl2
/L were associated with a larger number of bacteria cells in water [39
]. Francisque et al. also shows that the number of heterotrophic bacteria was much higher in water samples with chlorine content < 0.3 mg Cl2
The proposed new method of assessing the adhesion of microorganisms based on the fractographic analysis of the material surface has proved to be a useful tool in the quantitative description of the biofilm structure.
The obtained values of the fractal dimension D (Table 5
) indicate a greater surface roughness of the PVC material than galvanized steel material, which may result in a greater possibility of bacteria deposition in the depressions of this material. As shown by the results of microbiological tests and fractographical analyses, the dominant factor that indicates the possibility of biofilm formation is the type of material, not the roughness of its surface.
The reduction in the fractal dimension of the surface of galvanized steel with biofilm (D = 1.18) compared to the fractal dimension of the surface of this reference material (D = 1.23) indicates the deposition of biological material on this surface. The biofilm stratification is visible on an exemplary scanned fragment of material surface (Figure 4
). In addition, the analysis of the shape of the profile lines and the definite change in total height of the roughness profiles indicate a significant occurrence of the biofilm on galvanized steel (Table 5
, Figure 5
). On the basis of total height of the roughness profile, the thickness of the biofilm layer can be estimated to be around 300 μm (the difference between the total height of the roughness profile of the biofilm material and the reference material).
A similar biofilm formation mechanism was found for chromium-nickel steel. The reduction in the fractal dimension of the surface of chromium-nickel steel with biofilm (D = 1.35) compared to the fractal dimension of the surface of this reference material (D = 1.43) indicates the deposition of biological material on this surface (Table 5
, Figure 8
). On the basis of total height of the roughness profile, the thickness of the biofilm layer can be estimated to be around 60 μm (Table 5
, Figure 9
In the case of PVC material, the fractal dimension does not show statistically significant (on the significance level of 0.05), changes in the roughness of the profile line. The changes are seen on exemplary fragments of the surface of the material (Figure 10
) and in the total height of the roughness profile (Table 5
, Figure 11
). The reduction of the total height of the roughness profile indicates the formation of a biofilm in the material cavities. As can be seen in Figure 11
, the structure is clearly orientated in the direction of the water flow.
No statistically significant difference was also found for PE material. In this case, an increase in total height of the roughness profile was demonstrated (Table 5
, Figure 6
). The increase in the total height of the roughness profile indicates the deposition of the biofilm not only in the cavities of the material, but also in the creation of new peaks (Figure 7
). On the basis of total height of the roughness profile, the thickness of the biofilm layer can be estimated to be around 50 μm (the difference between the total height of the roughness profile of the biofilm material and the reference material).
Water flowing into the installation did not meet criteria of biological stability. The availability of nutritional substances and an increase in temperature during water residence in the installation resulted in the increase in the number of microorganisms in water and the biofilm formation on internal surfaces of pipes. An increase in turbidity (an average of 1.01 NTU) and decrease in the concentration of chlorine (an average of 74% total chlorine and 63% free chlorine) in water leaving the installation indicated the increase in the risk of the loss of water biological stability and secondary contamination danger.
The development of effective treatment technology, allowing the maintenance of the stability of tap water, and proper selection of installation materials is the key issue in the context of ensuring the health security of consumers. This analysis of obtained results of microbiological tests (ATP, flow cytometry, HTC methods) confirmed that regardless of the material from which the water supply system is made, it is still at risk of formation of biofilm.
Galvanized steel and PE were most susceptible to microorganism adhesion, while PVC and chromium-nickel steel created the least suitable conditions for the development of biofilm (galvanized steel > PE > chromium-nickel steel > PVC).
The most similar amounts of microorganisms occurred on galvanized steel and PE. The biggest differences in the settlement of the surface of materials by microorganisms were found between galvanized steel and PVC. Such a dependence was obtained for all the methods used for the quantitative determination of biofilms. However, due to the high dependence of microbiological determinations on external factors, these conclusions should be confirmed with time-consuming and primary statistical research (ongoing).