Influence of Water Treatment Technology on the Stability of Tap Water

: Control of water quality changes in the distribution system is an important issue, due to consumer health, and a threat to technical infrastructure. Annual monitoring of water quality allowed us to analyze the physical, chemical, and biological stability of water produced in two different technological systems. The article examines the extent to which water puriﬁcation technology affects the possibility of uncontrolled changes in water quality during its transport to the consumer (the risk of secondary water pollution has been estimated). Based on the obtained results, it was found that the groundwater treatment system based on the following processes—aeration, coagulation, sedimentation, ﬁltration


Introduction
Drinking water is delivered to customers through a network of pipelines called the drinking water distribution system (DWDS).As a result of the physical, chemical, and biochemical processes taking place in the DWDS, the quality of the transported water often deteriorates [1,2].The consequence of deterioration in water quality may be its turbidity and color, as well as an increased number of microorganisms that pose a threat to consumer health [3][4][5][6].According to the recommendations of the World Health Organization (WHO), water introduced into the distribution system should be characterized by physical, chemical, and microbiological stability, to limit the possibility of uncontrolled changes in water quality during its transport [7].
Physically stable water is water whose turbidity value does not exceed 0.8 NTU [8,9].Increased water turbidity is a major concern for water acceptability by consumers who raise concerns about visually assessed water quality.The increase in turbidity itself does not pose a direct threat to human health but, in some situations, it may indicate disturbances in the water treatment process.Among consumer complaints about water services, more than 40% are concerned about the deterioration of the aesthetic aspects of water [10].The distinct color of the water may result from the presence of iron and manganese compounds formed as a result of pipe biocorrosion or during the oxidation/reduction of these compounds by biofilm microorganisms [11][12][13].
Chemically stable water does not cause deposits on the internal surfaces of the installation materials and does not contribute to their destruction due to corrosion [6,14].The multitude of factors influencing the interactions between flowing water and water pipes makes it very difficult to fully define the requirements for the chemical stability of water.Although numerous indices can be used to assess the corrosiveness of water (e.g., the Langelier, Ryznar, and Strohecker index [15]), due to the complexity of the corrosion process, none of these provide a complete assessment of the corrosion rate in the water environment.
Biologically stable water does not contain microorganisms and does not support their development in the distribution system [16,17].All factors that enable the formation and development of biofilm on the internal surfaces of water pipes are not sufficiently understood.However, it is in no doubt that the main condition for the development of microorganisms indicating the lack of water stability is the presence of biogenic substances [18,19].Obtaining biologically stable water during treatment processes is not a simple task, because microorganisms can multiply even at low concentrations of nutrient substrates.The literature on the subject states that the threshold values of nutrients that limit secondary growth of microorganisms in distribution systems should be: biodegradable organic carbon (BDOC) < 0.25 g C/m 3 , nitrogen compounds < 0.2 g N/m 3, , and phosphorus compounds < 0.03 g PO 4 3− /m 3 [18,[20][21][22].The stable physicochemical and microbiological quality of water depends on many factors, among which the following should be mentioned: availability of nutrients [23,24], water temperature [11,25,26], presence of residual disinfectant [27][28][29], the condition of the network, and internal installations [14,30,31], hydraulic conditions in the network [32,33] and interactions between organisms [19].Obtaining biologically stable water requires not only the production of clean and safe water, but also limitation of changes during its distribution, which may lead to an uncontrolled increase in the number of microorganisms that pose a health risk.
The article analyzes and evaluates the risk of losing tap water's physicochemical and biological stability for the current water treatment system and the system supplemented with the biofiltration process on granulated activated carbons (GAC).To what extent the introduction of the biofiltration process will improve the parameters of tap water, and thus reduce the risk of uncontrolled changes in water quality in the distribution system, was assessed.Because secondary water pollution in the network is a threat to both the technical infrastructure and the health of consumers, the possibility of estimating changes in water quality during its transport is a very important issue.

Characteristics of the Research Object
The water treatment plant (WTP(I)) is supplied from an unconfined quaternary aquifer with a depth of approximately 15 m below ground level, using 27 wells.The physicochemical quality of the water being captured is presented in Table 1.
For research purposes, the current water treatment system WTP(I), including the aeration, coagulation, sedimentation, and disinfection processes, was supplemented with a second stage of filtration on granulated activated carbons WTP(II) (Figure 1).The model filters worked for 450 days.Water samples for analysis were collected twice a month after the first-degree filtration process (WTP(I)) and after the biofiltration process (WTP(II)).The results presented in the paper come from the period of 70-450 days of filter operation.During this period, the contaminants present in the water were removed through biological processes.

Determination of Stability of the Treated Water
The physical, chemical, and biological stability of the water before and after the biofiltration process on granulated activated carbons was assessed.The physical stability of the water was determined based on changes in the turbidity of the water; if the value of the turbidity was less than 0.8 NTU, the water was considered physically stable [8,9].Chemical stability was determined using the following indices: Langelier, Ryznar, and Strohecker (Table 2).
The biological stability of water was assessed based on the content of biogenic compounds: carbon, nitrogen, and phosphorus.The following threshold values were adopted to determine the biological stability of water: BDOC ≤ 0.25 g C/m 3 ; ∑N inorg ≤ 0.2 g N/m 3 ; PO 4 3− ≤ 0.03 g PO 4 3− /m 3 [18,24].It was assumed that the content of BDOC in groundwater is described by the following relation: BDOC = 9% DOC [18].Based on the test results obtained, the percentage number of samples that met the criteria of biological stability for waters before and after the biofiltration process was determined.A detailed list of the physicochemical parameters, methods, and equipment determined necessary to perform the analysis is presented in Table 3.

Method of Estimating the Risk of Instability of Tap Water
Technological safety is the ability of the water treatment system to avoid hazards that can result in: (1) failure to meet the quality parameters of the treated water (health risk) or (2) a threat to the technical infrastructure (technical risk).Threats resulting from undesirable events may occur during water intake, treatment, and distribution.The measure of the loss of technological security is the risk of failure to meet the required water quality parameters, which may adversely affect the change in the quality of water supplied to consumers.The risk of a lack of biological stability of water in the distribution system was determined based on the risk function f(r).The risk function was defined as the expected value of losses in specific operating conditions of the water supply network, which can determine the susceptibility of the system to the risk of loss of water biostability.The risk is defined as the probability of exceeding the assumed water quality parameters, i.e., BDOC ≤ 0.25 g C/m 3 , ∑N inorg ≤ 0.2 g N/m 3 , PO 4 3− ≤ 0.03 g PO 4 3− /m 3 [35,36].The measure of the risk of losing the physicochemical stability of water is the expected value associated with exceeding the water corrosiveness indices, taking into account the tendency to form protective layers, as well as the precipitation and dissolution of sediments.The assessment was made based on the following indices: Langelier, Ryznar, and Strohecker.
The risk of loss of biological stability water was determined on the basis of dependence [37]: where: -the expected value of losses C i greater the limit losses C gr , P i -the probability of losses On the assumption: PS-the probability of not exceeding the limit values S1, S2, S3, PSi-the probability of not exceeding the limit values S1, S2, S3 in a given water sample, PP-the probability of exceeding the limit values S1, S2, S3, PPi-the probability of exceeding the limit values S1, S2, S3 in a given water sample.
P S = ∑ P i and P p = ∑ P i (3) The PS and PP values were determined based on the percentage of water samples in which the limit values of biogenic substrates adopted for the assessment of water biostability were exceeded (BDOC ≤ 0.25 g C/m 3 , ∑N inorg.≤ 0.2 g N/m 3 , PO 4 3− ≤ 0.03 g PO 4 3− /m 3 ).The risk of loss of physicochemical stability and water was determined based on dependence [37]: where: I1-criterion corresponding to the value determined for the Langelier index, I2-criterion corresponding to the value determined for the Ryznar index, I3-criterion corresponding to the value determined for the Strohecker index.
To meet the given condition of maintaining chemical stability, one corrosivity parameter is sufficient; if all the determined parameters are available, the parameter with the most acceptable value is considered.
Water samples were taken before and after the biofiltration process and, depending on the parameters obtained, were assigned to three safety levels (Table 4) [37].

Tolerable risk
The parameters of tap water ensure the maintenance of the biological and physicochemical stability of water during its transport in the water supply network.

Controlled risk
The parameters of tap water requiring control and reduction-parameters of tap water indicate the possibility of changes in the stability of water in the distribution system and mild corrosion may occur (there are no protective CaCO 3 layers).
Biological stability Unacceptable risk Water quality does not ensure the stability of water in the water supply network; high probability of secondary water pollution and rapid corrosion.

Assessment of the Impact of Treatment Technology on the Formation of Biofilm on the Installation Material
Samples of polyethylene (PE) materials with dimensions of 1 × 1 cm were placed for 180 days in glass vessels filled with two types of water produced in the WTP(I) and WTP(II) systems.The tests were carried out in the form of a vascular test at room temperature, without access to light.The influence of the water purification method on the process of biofilm formation was assessed based on SEM images of the surface of the installation material, taken with a Joel SEM 5500-LV scanning microscope.

Physical and Chemical Stability
The physical stability of water can be determined by turbidity, which should not exceed 0.8 NTU [8,9].The water analysis of WTP(I) and WTP(II) showed that the critical turbidity value was not exceeded in all samples taken.The obtained values oscillated in the range of 0.21-0.60NTU (Figure 2).The mean values for WTP(I) and WTP(II) were 0.432 and 0.342 NTU, respectively (Table 5).
The statistical results of the parameters necessary to assess the physicochemical stability of water are presented in Table 5.To predict the corrosion of pipes in water supply networks, the corrosion trend was assessed using the following indices: Langelier (I L ), Ryznar (I R ), and Strohecker (I ST ) (Figure 3).Due to the large number of factors influencing the course of corrosion processes and their interactions, it is only possible to assess the quality of water in most cases.
-  The Langelier index values for the analyzed samples were always positive (0.3-0.77) and the Strohecker index negative (−0.1-−0.84),which indicated a tendency to precipitate CaCO 3 sediments (water non-aggressive).The Ryznar index values were in the range of 6.6 < I R < 7 (except for one sample for WTP(I)), indicating poor precipitation or incipient corrosion) (Figure 3).The obtained test results allow conclusions that the biofiltration process does not change the corrosive properties of water.The physicochemical parameters of water that determine the corrosion properties are comparable for WTP(I) and WTP(II).
According to the adopted risk assessment criteria, the quality of the water produced in WTP(I) and WTP(II) was at the level of tolerable risk, because, in all analyzed samples, at least one of the criteria was met, i.e., 0.5 < I L < 0.5, 6.2 < I R < 6.8 and I ST < 0.5 (75% of samples met two out of three criteria).The obtained values indicate that the chemical stability of the water will not be lost during the transport of the analyzed water in the distribution system.By analyzing each indicator separately, it can be concluded that the I R and I ST values are within the tolerable risk area for WTP(I) and WTP(II) (I R = 62.5% and I ST = 100%) (Table 6).In the case of I L values, most samples were assigned to the tolerable level (75-87.5% of samples).The analysis of the tested water showed that the pH value oscillated in the range of 7.62 to 8.35 (moderately alkaline water).Table 6.Percentage of samples that meet the water chemical stability criteria.

Biological Stability
The content of the biodegradable fraction in the waters analyzed ranged from 0.55 to 0.62 g C/m 3 for WTP(I) and from 0.05 to 0.38 g C/m 3 for WTP(II) (Figure 4).The test results obtained allow us to conclude that, in the water treated in the coagulation-sedimentation-filtration process, values ≤ 0.25 g C/m 3 were not achieved.For WTP(II), only 29.4% of BDOC values were higher than 0.25 g C/m 3 .The biofiltration process not only removed organic pollutants but also contributed to the reduction of the ammonium nitrogen content (on average by 56%).The content of this form of nitrogen in the water flowing to the biofilter ranged from 0 to 1.05 g N-NH 4 + /m 3 , while after biofiltration it was at the level of 0-0.5 g N-NH 4 + /m 3 .Nitrite nitrogen in water after WTP(I) reached values of 0-0.015 g N-NO 2 − /m 3 and after WTP(II) 0-0.12 N-NO 2 − /m 3 .On the other hand, nitrate nitrogen concentrations for the entire study period obtained comparable values for WTP(I) and WTP(II), ranging from 0.087 to 0.976 g N-NO 3 − /m 3 .Among the forms of nitrogen compounds analyzed, nitrates represented the largest share (in water after WTP(I) on average 69% and after WTP(II) 81%) (Table 7).The analysis of the test results obtained showed that the method is insufficient in removing inorganic nitrogen.In the case of WTP(I), 100% of the analyzed water samples exceeded the value of 0.2 g N inorg./m 3 (Table 6).The introduction of the biofiltration process into the technological treatment system allowed for a reduction in the value of inorganic nitrogen by an average of 22.7%.The content of phosphate ions changed to a small extent (from 0 to 0.002 g PO 4 3− /m 3 ); therefore, in none of the analyzed samples, the value of 0.03 g PO 4 3− /m 3 recommended for maintaining water biostability was not exceeded (Table 7).

--≤
Ensuring the required values of the physicochemical parameters of water, in particular biogenic compounds, reduces the risk of secondary water pollution in the distribution system.The introduction of the biofiltration process into the WTP(I) system contributed to the increase in the biological stability of water, as evidenced by the higher number of water samples reaching the values of BDOC ≤ 0.25 g C/m 3 , inorganic nitrogen ≤ 0.2 g N inorg./m 3 and phosphates ≤ 0.03 g PO 4 3− /m 3 (Table 8).Based on the data presented in Table 8, the probabilities of exceeding (PP) and the probability of not exceeding (PS) the limit values were determined based on the parameters analyzed (BDOC, N inorg., PO 4 3− ) and the method of groundwater treatment.Table 9 presents the PP and PS probabilities, as well as the calculated expected loss values for the analyzed risk areas for WTP(I) and WTP(II), i.e., (1) tolerable risk R Tb (indicating that the water will be biologically stable); (2) controlled risk R Kb (indicating the possibility of deterioration of the quality of tap water in the distribution system); and (3) the risk of unacceptable R NAb (indicating the lack of biological stability of water).
Based on the test results obtained, it was found that, in the case of water treated in the WTP(I) system, 100% of the samples are in the controlled risk area, which indicates the possibility of water quality degradation in the distribution system.Therefore, the quality of treated water must be controlled, and the content of nutrients present in the water should be reduced.Due to insufficient elimination of organic and nitrogen compounds in the processes of coagulation-sedimentation-filtration, it is necessary to introduce additional unit processes to the WTP(I) system.
The introduction of the biofiltration process on granulated activated carbon to the WTP(I) system increased the percentage of samples assigned to the tolerable risk category (i.e., 70.6%).The remaining 29.4% of water samples correspond to the controlled risk area (water quality requires control and reduction of the content of nutrients; however, there are reasons to maintain the biostability of water in the distribution system).The values of expected losses that determine the biological stability of tap water for specific risk areas were the following: (1) Tolerated risk: 0.019702-WTP(I) and 0.734118-WTP(II), (2) Controlled risk: 0.970497-WTP(I) and 0.263289-WTP(II), (3) Unacceptable risk: 0.009801-WTP(I) and 0.002593-WTP(II) (Table 9).
The obtained test results confirm that the introduction of the biofiltration process into the water treatment system ensures the production of water with better parameters and reduces the possibility of secondary water contamination in the distribution system.In the active carbon bed, impurities can be removed in the adsorption process, as well as in biochemical processes with the participation of microorganisms [38,39].As the time of the operation of biofilters increases, the sorption properties of the filter materials are exhausted, and the effectiveness of the removal of pollutants is determined by the intensity of the biodegradation processes.
Water treatment technology also influences the development and structure of biofilms.One of the methods of observing the formed biofilm is scanning electron microscopy (SEM), which makes it possible to determine the changes taking place on installation materials as a result of colonization by microorganisms.The primary polyethylene surface was completely colonized by microorganisms as a result of contact with water produced for WTP(I).On the surface of the PE material in contact with water after the biofiltration process, microcolonies were observed, but the original structure of the material was also preserved (part of the surface was unchanged due to the activity of microorganisms) (Figure 5).The formation of biofilm depends on the quality of the produced tap water and the physical structure of the installation material [40].As a result of their hydrophobic properties, polymeric materials may initially delay the biofilm formation phase (i.e., the adhesion of microorganisms).After colonizing the surface of the polymeric material, the subsequent stages of biofilm formation may proceed faster than in the case of pipes made of iron and steel materials.Plastic pipes may create a more favorable environment for potentially pathogenic taxa than cast iron [41].

Discussion
Water companies are obliged to supply water of appropriate quality continuously and reliably in the required quantity and at the appropriate pressure.Technological water treatment systems should ensure not only the removal of pollutants to the normative values, but also guarantee the constant quality of water during its transport to the consumer.The phenomenon of secondary contamination of tap water is observed in many distribution systems, and its main cause is the lack of physical, chemical, and biological stability of water [10,19,42].
Determining the requirements for the physicochemical stability of water is very difficult, due to the multitude of factors affecting the interactions between flowing water and water pipes.The result of the lack of physical and chemical stability of water may be the precipitation of sparingly soluble deposits on the internal surfaces of materials (mainly CaCO 3 ) and electrochemical corrosion of pipelines.The test results confirm that the corrosion resistance of carbon steel increases with increasing pH of the analyzed water [43].To prevent corroding of the copper pipes, it is necessary to ensure a pH above 7.5 [44].Precipitated deposits and corrosion products stimulate the formation of biofilms in the network.Corrosion tubercles provide potential niches in which microorganisms can be protected from bulk water and have increased surface area for survival and proliferation [45,46].This problem can result in reduced pipe life and create a serious public health problem [15].Both for the WTP(1) and WTP(II) systems, the analyzed water was characterized by physicochemical stability with a tendency to precipitate lime deposits (corrosive properties of water are weakened).The tested water was in the area of tolerable risk, which indicates the possibility of maintaining water stability during its transport through the distribution system.If the water was in an area of an unacceptable level of safety, the parameters of the treated water should be improved, and their correlation with hydraulic conditions analyzed.The right selection of materials for the construction of water supply systems and internal installations is also of key importance.When designing a water supply network, installation materials should be selected to ensure an appropriate level of water safety during its transport to the consumer.It should be noted that the installation material of the internal water supply network should be adapted to the parameters of the water, and not vice versa.Materials should not be susceptible to corrosion and should prevent water contamination [47].
The main factor determining the lack of biological stability of water in the distribution system is the presence of nutrients.The content of carbon, nitrogen, and phosphorus compounds provides information not only about the usefulness and effectiveness of a given process in water treatment, but also about the possibility of secondary water contamination in the water supply network [18,48].Conventional water treatment processes-i.e., coagulation, sedimentation, filtration, and disinfection-do not effectively remove nutrients, particularly biodegradable dissolved organic carbon (BDOC) and assimilation organic carbon (AOC) [49].It should be emphasized that drinking water production plants are not required to control the content of phosphorus compounds and the biodegradable organic fraction.Knowledge of this subject is crucial in assessing the biostability of water, as the introduction of unstable water into the distribution system poses a health risk to consumers [19,50].To maintain the stability of the water and reduce the risk of secondary contamination, two of the three nutrients that determine the growth of microorganisms must be removed.In studies [18], the content of the BDOC fraction in water introduced into the distribution system ranged from 0.05 to 0.88 g C/m 3 , depending on the type of treated water and the unit processes used.The efficiency of BDOC elimination was 30-84%, and the number of samples that met the conditions for biologically stable water reached 95.8% for surface water and 100% for infiltration water [18].In the work [51], the BDOC fraction was from 3 to 11% of DOC, and the value at the point of use was 0.185 g C/m 3 .Terry et al. [52] found that BDOC constituted 20% of the total organic carbon (TOC) in non-ozonated water.For ozonated waters, the biodegradable fraction values were higher and amounted to 30% TOC.The efficiency of the removal of the BDOC fraction in the biofiltration process is influenced by the type of water intake and the degree of its contamination; the contact time of the treated water with the bed; and the temperature at which the process is carried out.For lower temperatures (5 • C) with a 12-min contact time, the removal efficiency of BDOC is approximately 22%, while increasing the water temperature to 25 • C can increase the elimination of BDOC by up to 50% [53].The efficiency of TOC removal in the biofiltration process can range from 70% to 33% [48].The test results indicate that the introduction of biofiltration on granulated activated carbons into the system increased DOC removal efficiency by 68% on average.In Europe, the permissible content of TOC in water intended for consumption has been defined as "without incorrect changes", and water treatment plants producing water in quantities less than 10,000 m 3 per day are not required to monitor this parameter [54].However, water treatment systems should strive to reduce the content of organic compounds in the produced water as much as possible, to ensure the biostability of the transported water and reduce health risks.It should also be emphasized that organic pollutants can react with chemicals used during water disinfection.The reaction of organic matter with chlorine can produce chlorites, bromates, and disinfection byproducts (DPB).These compounds are harmful to the health of consumers and may be carcinogenic and mutagenic [55,56].Taking into account the growing demands of drinking water consumers and the constantly deteriorating quality of the intake water, many water treatment plants will be forced to modernize the current technological systems or improve the efficiency of unit processes.The production of stable water is a step forward in controlling water quality from the source to the tap.

Conclusions
The underground water treatment system based on the following processes does not ensure the effective elimination of biogenic substances that are crucial for sanitary safety: aeration coagulation, sedimentation, filtration, and disinfection.Despite meeting all the quality requirements for drinking water, the water was biologically unstable and the risk of secondary contamination was within the controlled risk area (the parameters of the water produced to indicate the possibility of changes in its quality in the distribution system).
The purification technology, extended with the process of biofiltration on granulated activated carbons (GAC), allowed reduction of the risk of losing water biostability.The values of BDOC and inorganic nitrogen decreased, on average, by 69% and 23% (BDOC ≤ 0.25 g C/m 3 was recorded in 70.6% of the analyzed samples; N < 0.2 g N inorg /m 3 in 11.8%).The water purification technology had an impact on the structure of the biofilms formed on the installation material.
The biofiltration process did not change the physical and chemical stability of the water.The tested water samples did not exceed the value of 0.8 NTU, and the calculated

Figure 1 .
Figure 1.Technological system for the treatment of groundwater.

Figure 3 .
Figure 3. Values of water corrosivity indices for the WTP(I) and WTP(II) systems.

Figure 4 .
Figure 4. BDOC content obtained in treated water for WTP(I) and WTP(II).

Figure 5 .
Figure 5.The structure of the formed biofilm on the polyethylene material depending on the treatment technology used: (a) WTP(I) and (b) WTP(II).

Table 1 .
Physicochemical quality of underground water (data from WTP(I)).

Table 3 .
Summary of analytical methods for laboratory experiments.

Table 4 .
Risk criteria determining the technological level of safety of the water treatment process.

Table 5 .
Summary of the physicochemical parameters necessary to assess the corrosive properties of WTP(I) and WTP(II) water.

Table 7 .
The content of biogenic compounds for WTP(I) and WTP(II).

Table 8 .
Percentage of samples meeting the water biostability criteria.