This article presents the results of studies of changes in water biostability levels in water treatment systems. In order to evaluate the potential of microorganism regrowth, both the organic and non-organic nutrient substrate content was taken into account. Pre-treatment in the analyzed water treatment plants ensured high phosphate ion removal effectiveness but a significantly worse effectiveness in removing biodegradable dissolved organic carbon (BDOC). Lowering nutrient substrate content during the main treatment stage was only possible in water treatment systems that incorporated biological processes. Conversely, final water treatment processes only influenced BDOC content in the treated water. Irrespective of the water type and unit treatment process, the limiting factors for microorganism regrowth in the distribution system were the phosphate ion content and BDOC content. However, none of the analyzed treatment systems ensured a reduction in non-organic nitrogen content that would ensure biological stability of the water.

INTRODUCTION

The problem of secondary water contamination in the water distribution system, especially by microorganisms, concerns most water distribution networks, especially extensive ones. The main cause of the worsening of water quality during transport from the water treatment plant (WTP) to the consumer is a lack of biological and chemical stability at the point of entry into the water distribution network (Srinivasan & Herrington 2007). Water is considered to be biologically stable if it is not only free of microorganisms or their dormant forms, but if it also does not contain nutrient substrates necessary for heterotrophic organism development, i.e. biodegradable dissolved organic carbon (BDOC), non-organic nitrogen compounds (Ninorg = NO3 + NH4+ + NO2) and phosphates (Becheli-Witschel et al. 2012). The effectiveness of removing organic compounds in a unit process is well known (Bolto et al. 2002, Kłos & Gumińska 2009, Ødegaard et al. 2010), yet these studies usually limit themselves to determining the effectiveness of chlorinated organic compound precursor removal, which are organic compounds of medium or large molecular weight. Despite an effective elimination of organic compounds in conventional water treatment systems (Zhan et al. 2010), the concentration of compounds of a low molecular mass is too large to limit microorganism growth (Stackelberg et al. 2007).

There is, however, a lack of unambiguous and sufficient information concerning the effectiveness of water treatment systems in removing non-organic nutrient substrates, especially non-organic nitrogen compounds.

Sometimes the phosphate content may be a factor that could limit microorganism regrowth in the water distribution system (Jianga et al. 2011).

The goal of the study presented in this paper was to determine the effectiveness of water treatment systems in removing bio-generating substances in full scale and to determine the changes in biostability levels during water treatment.

METHODS

Studies of changes in biostability levels were conducted in full scale at three WTPs, with the following types of water: surface water, infiltration water (surface water that had been filtered by a layer of soil) and mixed surface and groundwater. The average flow rate during the study amounted to 2,525, 2,977 and 1,030 m3/h for WTP1, WTP2 and WTP3 respectively. In all WTPs, three water treatment stages were used: pre-treatment (I°), main treatment (II°) and final treatment (III°). For surface (WTP1) and mixed (WTP3) water, pre-treatment encompassed coagulation and sedimentation processes, while for infiltration water (WTP) it included aeration processes with sedimentation in a reaction tank underneath the aeration hall and filtration through a sand bed. At WTP 1, coagulation was volumetric using aluminum polychloride (dose range 1.16–4.12 gAl/m3), while at WTP 3 contact coagulation occurred with iron sulfate (dose range 13.6–20.3 gFe/m3).

The main surface water treatment encompassed filtration through a sand bed, ozonation and filtration through a biologically active bed made of granulated activated carbon (BAF). For treating infiltration water, ozonation and BAF filtration was used. Conversely, mixed water was chemically oxidized by a mixture of chlorine and chlorine dioxide and filtered with a two-stage filtration process using first a sand and then a dolomite bed.

Final treatment at all three plants encompassed disinfection and alkalization, which ensured chemical stabilization of the water. For water disinfection at WTP1 and WTP2, a mixture of chlorine and chlorine dioxide was used, while mixed water was disinfected only with chlorine dioxide. Chemical stabilization was conducted using sodium hydroxide (WTP1 and WTP2) or sodium carbonate (WTP3).

During the study period, all three plants operated at variable outputs, and therefore with variable parameters of the individual water treatment processes. During the study period, the doses of the individual reagents also varied, depending on input water quality. At all three plants, due to reduced water demand, unit processes were conducted with very long times of water holding in individual treatment devices.

The objects of study, whose goal was to determine the effects of individual water treatment stages on the water biostability level, were raw water samples as well as samples taken after each treatment stage. The samples were taken regularly once a month, taking into account the time for water to flow through the subsequent treatment processes.

In water samples, the organic substance content (TOC) and its fractions, i.e. dissolved organic carbon (DOC) and BDOC were determined. TOC and DOC analysis was performed using a TOC analyzer. The BDOC concentration was calculated as the reduction in DOC content after incubation with microorganisms – in accordance with Standard Methods. The non-BDOC content was taken to be the difference between DOC and BDOC contents.

Apart from organic substance content, the content of non-organic nutrient substrates that are necessary for heterotrophic microorganism growth was determined for the samples, i.e. the concentrations of the following ions NH4+, NO3, NO2, PO4−3. These concentrations were determined by colorimetric methods.

Based on the literature (Percival et al. 1998, LeChevallier 1999, Niquette et al. 2001) the boundary concentrations that limit microorganism growth in disinfected water were taken to be: BDOC = 0.25 gC/m3, Ninorg = 0.2 g/m3 (N = NH4+ + NO3 + NO2) and PO4−3 = 0.03 gPO4−3)/m3.

RESULTS AND DISCUSSION

This study evaluated the effectiveness of each water treatment stage in eliminating nutrient substrates and changing the water biostability level at introduction to the distribution network. It allowed for a determination of the utility and validity of each treatment stage in terms of preventing microorganism regrowth in the water network and lowering the probability of secondary water contamination during its transport to the consumer.

Raw waters

Each of the input waters during the study period was characterized by a large variability in biogenerative substance content (Table 1). All of the raw water samples contained concentrations of phosphates and non-organic nitrogen higer than the boundary values, while only the BDOC concentration in some of the samples were at a low enough level that they could be the limiting factor in heterotrophic microorganism development (Table 2). The characteristic trait of all the raw waters was a very large fraction of dissolved organic substances in TOC, which on average amounted to 85.9%, 84.4% and 89.9% for WTP1, WTP2 and WTP 3 respectively. Conversely, the biodegradable fraction, which is the one that is directly responsible for microorganism growth, amounted to 4.4–19.3%, 3.5–15.5% and 3.1–24.1% DOC respectively and was in the range usually encountered in natural waters (Hrubec 1995). Among non-organic nitrogen compounds, nitrates were the dominating fraction in all of the waters (on average 91.1%, 73.6%, and 78.7% respectively). As was to be expected, the nitrate fraction was the largest in surface water (Table 1).

Table 1

Values of water quality measures of raw water and water after subsequent treatment stages

   TOC DOC BDOC NBDOC NH4+ NO3 NO2 Ninorg PO4−3 
   gC/m3 gC/m3 gC/m3 gC/m3 gN/m3 gN/m3 gN/m3 gN/m3 gPO4−3/m3 
 WTP1 min 2.61 2.12 0.20 1.80 0.02 0.95 0.00 1.10 0.04 
  max 11.11 9.55 0.88 8.96 0.40 4.70 0.02 4.78 0.23 
Fresh water WTP2 min 1.44 1.21 0.11 1.02 0.11 0.26 0.00 0.45 0.05 
  max 5.16 4.83 0.39 4.47 0.31 0.95 0.01 1.24 0.37 
 WTP3 min 4.50 3.27 0.18 2.92 0.07 0.26 0.00 0.33 0.05 
  max 9.71 9.54 1.23 9.01 0.47 3.79 0.01 4.06 0.31 
After pre-treatment WTP1 min 1.62 1.45 0.11 1.27 0.02 0.62 0.00 0.67 0.00 
 max 6.91 5.88 0.44 5.48 0.24 4.00 0.02 4.23 0.05 
WTP2 min 1.09 0.88 0.10 0.71 0.02 0.24 0.00 0.29 0.01 
 max 4.66 4.55 0.38 4.21 0.16 0.91 0.93 0.10 
WTP3 min 2.02 1.34 0.11 1.19 0.01 0.23 0.00 0.23 0.00 
 max 5.40 5.01 0.86 4.83 0.15 3.77 3.92 0.15 
After main treatment WTP1 min 0.80 0.75 0.10 0.62 0.01 0.48 0.00 0.51 0.00 
 max 4.32 3.82 0.35 3.70 0.17 3.69 3.84 0.03 
WTP2 min 0.80 0.41 0.04 0.23 0.01 0.16 0.00 0.21 0.01 
 max 3.49 3.42 0.31 3.16 0.11 0.68 0.70 0.03 
WTP3 min 1.98 1.30 0.08 1.10 0.01 0.16 0.00 0.17 0.00 
 max 4.50 4.25 0.97 3.31 0.07 3.14 3.20 0.15 
Treated water WTP1 min 0.70 0.65 0.10 0.44 0.01 0.45 0.00 0.51 0.00 
 max 3.94 3.45 0.34 3.34 0.16 3.68 3.83 0.03 
WTP2 min 0.75 0.38 0.05 0.21 0.01 0.16 0.00 0.21 0.01 
 max 3.30 3.30 0.24 3.06 0.11 0.68 0.70 0.03 
WTP3 min 1.84 1.25 0.11 1.07 0.01 0.15 0.00 0.15 0.00 
 max 3.95 3.90 0.88 3.03 0.05 2.91 2.96 0.15 
   TOC DOC BDOC NBDOC NH4+ NO3 NO2 Ninorg PO4−3 
   gC/m3 gC/m3 gC/m3 gC/m3 gN/m3 gN/m3 gN/m3 gN/m3 gPO4−3/m3 
 WTP1 min 2.61 2.12 0.20 1.80 0.02 0.95 0.00 1.10 0.04 
  max 11.11 9.55 0.88 8.96 0.40 4.70 0.02 4.78 0.23 
Fresh water WTP2 min 1.44 1.21 0.11 1.02 0.11 0.26 0.00 0.45 0.05 
  max 5.16 4.83 0.39 4.47 0.31 0.95 0.01 1.24 0.37 
 WTP3 min 4.50 3.27 0.18 2.92 0.07 0.26 0.00 0.33 0.05 
  max 9.71 9.54 1.23 9.01 0.47 3.79 0.01 4.06 0.31 
After pre-treatment WTP1 min 1.62 1.45 0.11 1.27 0.02 0.62 0.00 0.67 0.00 
 max 6.91 5.88 0.44 5.48 0.24 4.00 0.02 4.23 0.05 
WTP2 min 1.09 0.88 0.10 0.71 0.02 0.24 0.00 0.29 0.01 
 max 4.66 4.55 0.38 4.21 0.16 0.91 0.93 0.10 
WTP3 min 2.02 1.34 0.11 1.19 0.01 0.23 0.00 0.23 0.00 
 max 5.40 5.01 0.86 4.83 0.15 3.77 3.92 0.15 
After main treatment WTP1 min 0.80 0.75 0.10 0.62 0.01 0.48 0.00 0.51 0.00 
 max 4.32 3.82 0.35 3.70 0.17 3.69 3.84 0.03 
WTP2 min 0.80 0.41 0.04 0.23 0.01 0.16 0.00 0.21 0.01 
 max 3.49 3.42 0.31 3.16 0.11 0.68 0.70 0.03 
WTP3 min 1.98 1.30 0.08 1.10 0.01 0.16 0.00 0.17 0.00 
 max 4.50 4.25 0.97 3.31 0.07 3.14 3.20 0.15 
Treated water WTP1 min 0.70 0.65 0.10 0.44 0.01 0.45 0.00 0.51 0.00 
 max 3.94 3.45 0.34 3.34 0.16 3.68 3.83 0.03 
WTP2 min 0.75 0.38 0.05 0.21 0.01 0.16 0.00 0.21 0.01 
 max 3.30 3.30 0.24 3.06 0.11 0.68 0.70 0.03 
WTP3 min 1.84 1.25 0.11 1.07 0.01 0.15 0.00 0.15 0.00 
 max 3.95 3.90 0.88 3.03 0.05 2.91 2.96 0.15 
Table 2

Percentages of biostable samples with respect to individual nutrient substrate content and average efficiencies of treating steps in nutrient removal

  Percentages of biostable samples Average efficiency of nutrient removal 
  Fresh water After pre-treatment After main treatment Treated water Pre-treatment Main treatment Final treatment 
WTP1 BDOC 20.8 37.5 87.5 95.8 23.2 33.1 5.5 
 PO4−3 0.0 62.5 100.0 100.0 76.2 24.9 0.0 
 Ninorg 0.0 0.0 0.0 0.0 24.6 25.6 0.0 
WTP2 BDOC 37.5 45.8 91.7 100.0 6.1 20.0 9.1 
 PO4−3 0.0 37.5 100.0 100.0 75.3 48.4 0.0 
 Ninorg 0.0 0.0 0.0 0.0 28.1 24.6 0.0 
WTP3 BDOC 7.7 53.8 38.5 61.5 27.7 −4.3 4.7 
 PO4−3 0.0 69.2 92.3 92.3 74.0 44.3 0.0 
 Ninorg 0.0 0.0 0.0 0.0 23.3 27.8 0.0 
  Percentages of biostable samples Average efficiency of nutrient removal 
  Fresh water After pre-treatment After main treatment Treated water Pre-treatment Main treatment Final treatment 
WTP1 BDOC 20.8 37.5 87.5 95.8 23.2 33.1 5.5 
 PO4−3 0.0 62.5 100.0 100.0 76.2 24.9 0.0 
 Ninorg 0.0 0.0 0.0 0.0 24.6 25.6 0.0 
WTP2 BDOC 37.5 45.8 91.7 100.0 6.1 20.0 9.1 
 PO4−3 0.0 37.5 100.0 100.0 75.3 48.4 0.0 
 Ninorg 0.0 0.0 0.0 0.0 28.1 24.6 0.0 
WTP3 BDOC 7.7 53.8 38.5 61.5 27.7 −4.3 4.7 
 PO4−3 0.0 69.2 92.3 92.3 74.0 44.3 0.0 
 Ninorg 0.0 0.0 0.0 0.0 23.3 27.8 0.0 

Infiltration and mixed waters were characterized by similar values in phosphate ion concentrations, which were greater than those for surface waters (Table 1).

Pre-treatment

After pre-treatment, there was a decrease in the total organic carbon concentration and the concentrations of all its fractions. The average decrease in TOC content was 1.51. 0.56 and 2.44 gC/m3 for surface, infiltration and mixed waters respectively. The varied effectiveness in eliminating organic substances was connected with the kinds of water treatment processes and the contamination levels in the raw water. The dissolved fraction was dominant among the removed fractions, which are made up of 51.4–94.7%. 20.9–97.9% and 56.8–99.2% of removed organic content respectively. Unfortunately, most of removed DOC was made up of substances with a high molecular mass. Therefore, the reductions in BDOC concentrations were small (Table 2), and in 8.3%, 70.8% and 15.4% of water samples respectively did not go beyond the limits of experimental error. The lowest removable effectiveness BDOC was found for infiltration water, which was characterized by a low content of these substances in raw water, and a lack of a unit process which would ensure BDOC removal.

More effective removal of BDOC from surface and mixed waters as compared with infiltration water was caused by BDOC elimination during coagulation and/or adsorption of particles onto the surface of post-coagulation precipitates. As Lin and colleagues have shown, the effectiveness of removing BDOC during the coagulation process depends on the structure of these substances, and, above all, whether they are hydrophobic or not, which may explain the different removal effectiveness that was observed for surface and mixed waters (Lin et al. 2006). The effectiveness of removing these nutrient substrates depends on the coagulant used, which was not unambiguously proven by the results of this study (Lee et al. 2003). Higher efficiency of BDOC removal from mixed water can be explained by higher doses of coagulant at WTP3.

In general, the use of coagulation and sedimentation for pre-treatment ensured a greater effectiveness of BDOC removal from water than aeration and filtration processes. Despite the removal of organic substances of low molecular mass from surface and mixed waters, the percentage of biostable samples with respect to this nutrient substrate increased insignificantly as compared with that observed for infiltration water (Table 2).

At all three WTPs, pre-treatment also ensured a reduction in non-organic nitrogen, which was contained in the ranges of 0.01–1.43 gN/m3, 0.09–0.51 gN/m3 and 0.03–0.97 gN/m3 respectively. Among the nitrogen compounds removed from surface and mixed waters, nitrate ions dominated (Figure 1), while in infiltration waters ammonia ions were dominant, which is a consequence of the different concentrations of nitrogen forms in different raw waters. Ammonia ions may have been removed via absorption (Eturki et al. 2012) on the surface of solid particles that were removed during sedimentation, which occurs after coagulation or aeration.

Figure 1

The percentage of different nitrogen forms in removed non-organic nitrogen.

Figure 1

The percentage of different nitrogen forms in removed non-organic nitrogen.

However, the elimination of nitrate ions that was found is surprising. This is because there is a lack of information about the possibility of NO3 removal in coagulation, sedimentation, filtration or aeration processes. Since all processes were performed with a long time of water hydraulic retention in treatment devices, it is probable that nitrate ions were assimilated by microorganisms that grew in the sediment accumulated in the sedimentation tanks.

In contrast, phosphate ion elimination effectiveness was very large, irrespective of water type and used unit process (Table 2). Phosphate ions were removed in the form of insoluble iron phosphates in the case of infiltration and mixed water and in the form of aluminum phosphates in the case of surface water.

Effective PO4−3 removal made phosphate content the limiting factor in microorganism development in most water samples, even after only pre-treatment (Table 2). In addition, irrespective of water type, phosphate elimination increased with raw water phosphate content. This fact is confirmed by the following linear correlations found for a confidence level of α = 0.05: ΔPO4−3 = 0.957*[PO4−3]o-0.02 (WTP1); ΔPO4−3 = 0.856*[PO4−3]o-0.02 (WTP2); ΔPO4−3 = 0.973*[PO4−3]o-0.04 (WTP3).

Main treatment

The second stage of water treatment also ensured an elimination of organic substances (TOC) from treating water, which was contained in the ranges of 0.62–3.50, 0.29–1.17 and 0.04–2.09 gC/m3 for WTP1, WTP2 and WTP3 respectively. Mainly high-molecular-weight substances were removed from these waters, which made up on average 91.7%, 91.6% and 86.4% of removed TOC respectively. In the majority of surface and infiltration water samples, reductions in BDOC content up to 0.231 gC/m3 and 0.101 gC/m3 respectively were observed. On the other hand in 8.3%, 8.3% and 61.5% of water samples for WTP1, WTP2 and WTP3 respectively, an increase in organic substrate content was observed, which was caused by the formation of low-molecular weight substances during chemical oxidation. These substances were then removed with too low an effectiveness in subsequent main treatment processes. In surface and infiltration water treatment systems, during the BAF filtration process, BDOC was removed by biodegradation and assimilation by microorganisms that inhabited these beds (Wert et al. 2008).

The values of BDOC removal effectiveness during biofiltration that were achieved were lower than those encountered in the literature, probably due to an excessively low concentration of phosphate ions in water that had undergone main treatment. A decrease in the bioactivity of microorganisms that inhabit beds due to a limiting of non-organic nutrient substrates in water introduced into the beds was also noted by Simpson (Simpson 2008).

In contrast, the increase in BDOC concentration in most of the mixed water samples may be explained by a lack of a process that would ensure a biodegradation of low-molecular-mass organic substances.

None of the main treatment systems ensured effective BDOC removal, which was connected with a limited bioactivity of the granulated activated carbon beds, and a lack of biological treatment processes at WTP3.

Main treatment in all plants ensured a reduction of non-organic nitrogen compounds in the ranges of 0.12–1.83, 0.02–0.31 and 0.05–0.71 gN/m3 for WTP1, WTP2 and WTP3 respectively. In general, the mean non-organic nitrogen elimination effectiveness was similar irrespective of water type and the unit processes used. As in pre-treatment for surface and mixed water, mostly nitrate ions were removed, and from infiltration water it was ammonia ions (Figure 1). The elimination of non-organic forms of nitrogen from surface and infiltration water was caused by their assimilation by microorganisms that inhabited the absorption beds (Simpson 2008). In the case of mixed water Ninorg could have been removed by assimilation by organisms that develop on the surface of filtration beds, or in the case of NH4+, by absorption onto the surface of solid particles. The average efficiencies of non-organic nitrogen compounds were insignificant in three WTPs (Table 2). In consequence for all of the samples taken after main water treatment, the nitrogen concentrations were higher than the limit value with respect to microorganism regrowth. Efficiencies of nitrogen removal depended on its concentration at the beginning of the process.

The effectiveness of phosphate removal was lower than that obtained for pre-treatment and on average amounted to 0.01, 0.02 and 0.03 gPO4−3/m3 for WTP1, WTP2 and WTP3 respectively. The larger effectiveness of phosphate removal for infiltration and mixed water was probably caused by the larger content in water that entered main treatment (Table 1). Despite a low average reduction in PO4−3 concentration, this was enough to ensure biostability with respect to this nutrient substrate for all samples of surface and infiltration water and 92.6% of mixed water samples (Table 2). Due to the used filtration processes in each of the water treatment systems, phosphate ions were removed in the form of insoluble phosphates or assimilated by microorganisms that inhabited the filtration beds.

Final treatment

The last treatment stage resulted in only low reductions of DOC, on average amounting to 0.16, 0.11 and 0.17 gC/m3 for WTP1, WTP2 and WTP3 respectively. The disinfection processes in the analyzed water treatment systems caused an increase in BDOC content, which is evidence of organic substance transformation, in only one, two and three water samples from WTP1, WTP2 and WTP3 respectively. On the other hand, Lethola and colleagues (Lehtola et al. 2002) observed an increase in BDOC during disinfection with a simultaneous very small decrease in DOC. A limited transformation of organic compounds in the analyzed water treatment systems could be explained by the use of intermediate oxidation in each of them, where a portion of organic substances is broken down into smaller ones.

In most of the surface (87.5%) and infiltration (54.2%) water samples, the BDOC reduction values did not exceed analysis error. For mixed water, which was characterized by the greatest organic substance content, this reduction was greater and reached a maximum of 0.185 gC/m3. Consequently, there was an increase in the proportion of samples with a BDOC < 0.25 gC/m3.

In all of the plants, final treatment had no effect on the non-organic substrate content (Tables 1 and 2). Due to this, there was no change in the amount of biostable samples taken at this treatment stage with respect to phosphate and nitrate ions (Table 2).

Treated water

The treated waters were characterized by an organic content acceptable for water for human consumption in Poland (5 gC/m3). Despite differing levels of contamination of raw waters, TOC concentrations in treated waters did not differ greatly (Table 1). The organic nutrient substrate fraction in DOC in all water types varied within the wide ranges of 3.2–31.8%, 2.7–44.7% and 3.7–24.0% for WTP1, WTP2 and WTP3 respectively. For mixed water it was comparable with that found in raw mixed water. On the other hand, the percentage of BDOC in DOC for treated surface and infiltration water was larger than that found in respective raw waters. This can be explained by the use of ozonation during main treatment, which leads to the generation of a greater amount of low-molecular-weight organic substances than using chlorine for chemical oxidation. This hypothesis is confirmed by the results of studies (Yavich et al. 2004, Black & Bérubé 2014).

The concentrations of BDOC, like those of PO4−3, are the limiting factors in treated waters that inhibit secondary microorganism development, irrespective of the type and quality of raw water.

Unfortunately, the non-organic nitrogen content in all treated water was too large to ensure biostability with respect to this nutrient substrate.

CONCLUSION

The organic nutrient substrate content in water was decided by all three water treatment stages, while non-organic nutrient substrate content was determined only by the first two treatment stages.

Pre-treatment, due to the greatest contamination being found in raw water, ensured the greatest elimination of nutrient substrates in each water treatment system.

The effectiveness of each treatment stage depended on the level of contamination of water that entered the stage and on the types of processes used.

The final BDOC content was a function of the removal of these substances in water treatment processes and the amount of low-molecular-mass organic substances generated during chemical oxidation – especially indirect oxidation.

In the water treatment processes studied, the lack of a process that would ensure a biodegradation of BDOC is a cause of a lower percentage of biostable samples with respect to this nutrient substrate.

Irrespective of the coagulant used, the coagulation process achieved comparable values of BDOC removal effectiveness.

Phosphate ions were effectively removed in the form of insoluble iron or aluminum phosphates from all types of water in the first stage of water treatment. Small concentrations of these ions in water flowing onto the absorption beds probably contributed to the limited bioactivity of these beds and limited BDOC biodegradation.

None of the water treatment stages in any of the WTPs ensured a level of non-organic nitrogen removal sufficient to ensure biostability.

A greater biostability of filter beds would ensure a better biodegradation of organic compounds. Conversely, more effective non-organic nitrogen removal can be achieved by incorporating ion exchange or membrane separation processes into the water treatment systems.

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