Safety Analysis of Tap Water Biostability

In water distribution systems more than 95% of the microorganisms is on the internal surfaces of water pipes in the form of biofilm. Biofilm is a group of microorganisms that are linked by trophic dependence from autotrophic and heterotrophic bacteria and fungal toxins and their parallel developing consumers, such as flagellates, ciliates, and creepers [8–10]. Bio-stable water is considered to be nonmicrobial and does not sustain the growth of microorganisms in the water supply network. The epidemiological risk caused by the presence of biological membrane in the water supply network is related both to the quality and type of water taken by the water treatment plants and to the efficiency of treatment and disinfection processes [11–14].

Based on the conducted studies, it was found that the intensity of microorganisms growth in the water supply network is mainly determined by the presence of organic compounds, in particular biodegradable dissolved organic carbon (BDOC) and assimilable organic carbon (AOC). The values of these parameters indicate the content of nutrients that can be easily assimilated by water-borne microorganisms. The AOC concentration in the water entering the network is usually 0.1–9% of dissolved organic carbon (DOC) and about 15–25% of biodegradable dissolved organic carbon (BDOC) [5]. The conventional water purification system based on coagulation, sedimentation, filtration and disinfection eliminates the AOC fraction in less than 30% and total organic carbon (TOC) in 22%, while the system extended with an additional biofiltration by activated carbon deposit removes these fractions up to 60 and even 74% [15–20].

For the risk analysis and assessment, the following threshold values for the biological stability parameters of water were adopted: BDOC < 0.25 gC/m³, ∑N_inorganic < 0.2 gN/m³, PO₄ ^3-< 0.03 gPO₄ ^3-/m³, according to the metodology described by Wolska [5].

The risk of failure of not meeting certain (required) water quality parameters in the water supply network that may adversely affect the physicochemical parameters and the bacteriological quality of the water reaching the consumer is a measure of lack of biostability [22]. The purpose of risk analysis is to determine risk value using the appropriate method. The value of the risk of biological instability of water in the water supply network describes the so-called risk function f(r), defined as the expected value of losses under certain system operating conditions that influence the system’s vulnerability (resistance) to threat [3, 23]. In the risk analysis, the priority is to identify potential threats. Risk assessment is a comparison of the obtained risk value with the assumed criteria values.

For the analysis of the risk of loss of water biological stability, the following risk definition was adopted: (1) r = E ( C ≥ C g r )

where E(C ≥ C_gr ) – the expected value of losses C greater than the accepted limit losses C_gr .

and: (2) C g r = f ( S 1 , S 2 , S 3 )

where S₁ is the criterion corresponding to the content of BDOC, gC/m³, S₂ is the criterion corresponding to inorganic content ∑N_inorganic, gN/m³, S₃ is the criterion corresponding to PO₄ ^3-, gPO₄ ^3-/m³.

The limit values were proposed for the probability of exceeding the parameters BDOC, ∑N_inorganic and PO₄ ^3- based on the criteria contained in the work [5]. The following threshold values for the biological stability parameters were adopted for the risk analysis and assessment: BDOC < 0.25 gC/m³, ∑N_inorganic < 0.2 gN/m³, PO₄ ^3- < 0.03 gPO₄ ^3-/m³.

Assuming that P_S is the probability of not exceeding the water quality parameters (BDOC, ∑N_inorganic and PO₄ ^3-), P_i is the probability of not exceeding the water quality parameters (BDOC, ∑N_inorganic and PO₄ ^3-) in a given water sample, P_p is the probability of exceeding the water quality parameters (BDOC, ∑N_inorganic and PO₄ ^3-) and P_pi is the probability of exceeding the water quality parameters (BDOC, ∑N_inorganic and PO₄ ^3-) in a given water sample.

where: (4) P S = ∑ i ∈ E + P i (5) P i = ∏ j = 1 M P i 0 ⋅ ( 1 - P i 0 ) (6) P p = ∑ i ∈ E _ P p i

Table 1 presents the criteria for the probability of exceeding BDOC, ∑N_inorganic and PO₄ ^3- parameters, based on the formulas 4-6 and the work [5].

In addition, the controlled risk should be adopted if one of the six criteria is met. For risk analysis, the following values of limit losses E(C<C_gr ), related to maintaining the biological stability of tap water, were assumed:

The parameters defining the tolerable risk area indicate that the parameters of water provide the required biological stability in the water supply network (acceptable safety level (SLT)), whereas the controlled risk means that the water quality parameters indicate the possibility of changes in the chemical stability of water in the water supply network (safety level which requires control and reduction of (SL_C)). The unacceptable risk means the occurrence of such water quality parameters that cause biological instability of water in the water supply network, which may result in the secondary contamination (unacceptable safety level (SL_UNA)).

The analysis of the underground water treatment system is performed for the city of 80 thousand inhabitants. Water devices for the water intake are SI pumps consisting of 5 drilling wells (Q_emax 183 m³/h) and SII consisting of 22 drilling wells (Q_emax = 715 m³/h). The designed maximum capacity of the water treatment plant (WTP) is 715 m³/h. The annual average, for example in 2012/2013, was about 265 m³/h.

The treated water is discharged from the clean water tank to the two water mains through 3 pumps. Two of them have a capacity of 440 m³/h and the third one the capacity of 280 m³/h. Their lifting capacity is 58 m. The city's water supply network consists of the 16.8 km main network, 176.6 km of the distributional network. The household connections network consists of 3958 connections and has length of 98.40 km. Approximately 42.70% of the entire water supply network is made of pipes less than 20 years old but more than 11 years old. 90.2 km of the network is made of materials less than 10 years old.

The intake water does not meet sanitary requirements in terms of turbidity, colour, permanganate index, ammonium, iron and manganese. The physicochemical composition of the intake water indicates that there may be difficulties in its treatment. The physical and chemical parameters of the intake water were presented in Table 2.

The intake water is directed to the collection well (stop time 2-4 hours, depending on the current water production). Then the water is directed to the oxygenation cascade. Just beneath the cascade lies the dosage of chemical oxidant – potassium permanganate and coagulant PAX-18. The next step in water treatment are vertical coagulatory and sedimentation chambers with a contact time of 6-8 hours. The water after the chambers, which is directed to the horizontal settlers, is dosed with calcium milk to raise the pH. The settling time in the settlers is several hours. The next step is the filtration of water in the filters at a rate of 1.5–3 m/h and the final step – disinfection with sodium hypochlorite.

Samples of water were taken twice a month from November 2015 to November 2016. The treated water was taken from the clean water tank prior to the final disinfection process. The modified Kaplan and Newbold method was used to determine the BDOC content by using for this purpose colonized by autochthonous bacteria bioreactor with granular activated carbon.

The physicochemical parameters were determined according to the applicable research procedures presented in Table 3.

The existing water treatment station very effectively removes iron and manganese. In the treated water there are traces of these impurities. On the other hand, the colour parameter is maintained at 10-15 Hazen, with an acceptable Hazen value of 15. In the discussed example the colour was affected by organic substances - the content of TOC in the treated water was 7–12 mg C/dm³, exceeding the value recommended by the WHO – 5 mg C/dm³. Other parameters of the treated water are as follows: BDOC 2.27–11.20 g/m³, ∑N_inorganic 0.16–1.20 g/m³ and PO₄ ^3-0–0.007 g/m³. The detailed analysis of BDOC, N_inorganic and PO₄ ^3- parameters was performed to analyse the risk of lack of biological stability.

Figure 1 shows the parameters of the treated water and the safety criteria established in Table 1.

Figure 1.

All tested samples are in the range of controlled risk (R_C ).

Of the 22 tested water samples 21 corresponds to variant IV of controlled risk (R_C ):

One sample of water corresponds to variant III of controlled risk (R_C ):

The controlled risk level (R_C ) means that the nature of the water samples determined by BDOC, ∑N_inorganic and PO₄ ^3- corresponds to a safety level that requires control and reduction (SL_C).