Nitrite in drinking water is a potential health hazard and monitoring its concentrations in distributed water is of paramount importance. When monochloramine is used in secondary disinfection in drinking water distribution systems (DWDSs), nitrite is often formed by nitrification in the biofilm on the inner surface of distribution pipes. This article attempts to identify areas with a risk of increased nitrite concentrations as well as the main reasons leading to nitrite occurrence in a large urban DWDS in Finland using spatial inspection of obligatory monitoring data. Nitrification was found to occur throughout the study area, though nitrite was not increased everywhere. Instead, nitrite was increased close to the water treatment plants (WTPs) and was connected to fresh drinking water than stagnant drinking water. Temperature effects on nitrite concentrations were surprisingly insignificant, even though it is well known that nitrification reactions are affected by temperature. The temperature dependence of ammonium and total residual chlorine was more significant than the dependence of nitrite. The findings of this study emphasize the need to monitor nitrite concentrations close to WTPs.
Today, the microbiological quality of drinking water is often ensured by using monochloramine (NH2Cl) as a secondary disinfectant in the drinking water distribution systems (DWDSs). Monochloramine is less reactive than hypochlorous acid or chlorine dioxide, which makes it a good option for disinfection in distribution pipes, especially with high water ages. Monochloramine also forms less trihalomethanes (THMs) and other disinfection by-products (DBPs). However, one disadvantage is the formation of nitrite in the biochemical nitrification of ammonia (Equation (1)), which is formed in the autodecomposition and consumption of monochloramine.
As a result, some water works struggle with nitrification episodes that increase the nitrite concentrations of the distributed drinking water. Concentrations as high as 1 mg N l−1 of nitrite have been observed during nitrification episodes in drinking water [
Monochloramine was first introduced to secondary disinfection in 1917 [
Nitrite concentrations can decrease in several reactions, the most important of which is the biochemical oxidation of nitrite into nitrate (Equation (2)) [
This article attempts to utilize the obligatory monitoring data collected from a large drinking water distribution system to study the spatial formation of nitrite in relation to the locations of water treatment plants (WTPs), substrate concentration, disinfection, water age, and temperature. When the spatial dynamics of nitrite formation are understood more deeply, we will have a better chance to avoid the risks of increased nitrite concentrations.
The obligatory monitoring data from a chloraminated DWDS was studied. Nitrification had earlier been observed in the DWDS [
mains with larger than 200 mm diameter in Helsinki, and 255.7 km of similar mains in Vantaa [
The drinking water in Helsinki and Vantaa is supplied by the Helsinki Region Environmental Services Authority (HSY). The raw water was received from Lake Päijänne, via a 120 km tunnel. One percent of the annual flow of the lake was used for drinking water production.
The treatment processes of both WTP1 and WTP2 consisted of coagulation-flocculation, sedimentation, filtration, lime addition to raise the pH, ozonation, carbon dioxide addition to increase alkalinity, filtration with granular activated carbon (GAC), and UV disinfection. As a final step, ammonium was added into the drinking water with hypochlorite to form monochloramine. The target of the addition was 0.35 - 0.4 mg Cl2 l−1 as total residual chlorine and the mass ratio of chlorine and ammonium was approx. 4. The purpose of monochloramine was to stabilize the distributed water hygienically.
The analyses were made at a commercial laboratory, Metropolilab Ltd. [
In Helsinki, drinking water samples were taken only once or very few times from each tap, but several taps close to each other were sampled, leading to geo-
Analysis | Number of analyses |
---|---|
2151 | |
1189 | |
55 | |
Total residual chlorine Cl2 | 1850 |
Temperature | 2176 |
Escherichia coli | 1767 |
Clostridium perfringenss | 954 |
Coliforms | 1755 |
THM | 61 |
graphically dense data (830 sampling locations,
A skeleton hydraulic model of the DWDS was devised by a consulting company (Pöyry Finland Ltd., [
The monitoring data was saved from pdf files into spreadsheets (in MS Excel) and the sampling locations were geocoded. All concentrations below the LoQs were also included in the statistical analysis. The numerical value of LoQ divided by two was used in calculations. Background maps of the area were retrieved from a local internet service portal of the area [
Contour maps were drawn of the interpolated descriptive statistics in Matlab. Areas where the median of the nitrite was 50 µg N l−1 or higher were highlighted with dotted lines and numbered in all maps. Areas with consistently high nitrite concentration and groups of smaller areas with high nitrite concentration were highlighted. Later in this text these areas are referred as Nitrite Formation Areas (NFAs), followed by a number (NFA1‒NFA10).
The mean, median, minimum, and maximum of nitrite, ammonium and the total residual chlorine concentrations in the drinking water were calculated for the cities of Helsinki and Vantaa. The Student’s t-test was used to compare differences in nitrite, ammonium and total residual chlorine concentrations between the cities.
The spatial inspection revealed that nitrite was formed close to the WTPs, which can be observed in both minimum, median and maximum concentrations of nitrite (Figures 2(a)-(c)). Nitrite concentrations formed distinct areas where ni-
trite occurred (defined as nitrite concentration above 50 µg N l−1 in the medians map,
Two large Nitrite Formation Areas (NFAs) were located close to WTP1 (Figures 2(a)-(c)), one of them (NFA1) in the main distribution area of WTP1 and another (NFA2) in the area in which waters from WTP1 and WTP2 were mixed (mixing zone MZ1). It is notable that nitrite occurred in a small area closer than 1 km to WTP1 (NFA3). The highest nitrite concentrations occurred in NFA1, which was the largest consistent area of nitrite occurrence in the study area. Farther away from WTP1, nitrite occurred in NFAs 4 and 5. The pattern of the concentrations and the level of the concentrations was rather similar in the distribution area of WTP1 and the mixing area in all maps, Figures 2(a)-(c), reflecting the sampling strategy in which few drinking water samples were taken from each tap (see also Section 2.3 in Materials and Methods).
In the distribution area of WTP2, nitrite occurred in four sites (NFAs 6 - 9), all of which were at some distance from WTP2. The minimum and maximum maps differed considerably from each other. In the city of Vantaa, which formed most of the distribution area of WTP2, several drinking water samples were taken from each tap, giving more variation to each geographical location. This suggested that there were underlying reasons for nitrite occurrence resulting in a variation in nitrite concentrations within the same locations. For example, at Myyrmäki Sports hall, in NFA6, 21 drinking water samples were taken and nitrite concentrations varied between 30 and 100 µg N l−1.
The nitrite concentrations decreased from the highest values towards the northern and southern extremities of the DWDS. In the northern extremity, the lowest nitrite concentrations were below the limit of quantification. For example, at Katriina hospital, 9.2 km north of WTP2, 34 drinking water samples were taken, and all nitrite concentrations were below the LoQ (<3 µg N l−1). In the northeast corner of the study area, low nitrite concentrations occurred in the distribution area of the small groundwater plant. A large area of low nitrite concentrations, consisting mostly of the northern part of the mixing zone (MZ2), occurred between WTP1 and WTP2. In addition, nitrite was low in a few scattered spots that were clearly smaller than the nitrite formation areas (Figures 2(a)-(c)).
NFA10 was situated inside the mixing zone (MZ1) (
The nitrite concentrations differed slightly, but significantly, between the two cities (difference 4 µg N l−1, t = 3.17, p = 0.002, see also
Mean | Median | Range | ||
---|---|---|---|---|
NO2− | All data | 45 | 46 | <3 - 128 |
µg N l−1 | Helsinki | 46 | 46 | <3 - 100 |
Vantaa | 41 | 43 | <3 - 128 | |
NH4+ | All data | 43 | 38 | <8 - 148 |
µg N l−1 | Helsinki | 44 | 40 | <8 - 132 |
Vantaa | 37 | 29 | <8 - 148 | |
Cl2 | All data | 0.12 | 0.11 | <0.03 - 0.54 |
mg Cl2 l−1 | Helsinki | 0.12 | 0.11 | <0.03 - 0.54 |
Vantaa | 0.10 | 0.06 | <0.03 - 0.31 |
When the maps of nitrite concentrations and maximum water age were compared, it was noted that nitrite occurrence was more often related to fresh than stagnant water (Figures 2(a)-(c) and
Ammonium and nitrite concentrations were above 30 µg N l−1 in areas that were geographically remarkably similar (Figures 2(a)-(c) and
While the spread of nitrite was similar to that of ammonium, it was spread over a slightly larger area, indicating that, even though ammonium was below 30 µg N l−1, nitrite was still forming from it, reducing the ammonium concentrations even further towards the ends of the mains. In the southern half of the study area, the highest concentrations of ammonium and nitrite were in different spatial locations, which can be explained by ammonium being the substrate for nitrite formation.
Nitrite occurrence did not overlap consistently with high residual total chlorine concentrations nor with low residual total chlorine concentrations, but nitrite occurred on the edges of high and low residual total chlorine concentrations. Presumably, the monochloramine concentrations were so low that they did not prevent nitrite formation. The medium-level nitrite concentrations in
the WTP1 distribution area, where residual total chlorine was high, indicate that some nitrification occurred, thus it was not prevented.
Ammonium and total residual chlorine concentrations in the obligatory monitoring data differed significantly between the cities of Helsinki and Vantaa (the difference of ammonium: 7 µg N l−1, t = 3.22, p = 0.001 and the difference of total residual chlorine: 21 µg Cl2 l−1, t = 3.54, p < 0.001, see also
The median of nitrate concentrations was 340 µg N l−1 and range 270 - 450 µg N l−1. The nitrate analyses were, however, too few to enable statistical analysis (
The monochloramine concentration (initially 0.35 - 0.4 mg Cl2 l−1 in freshly produced drinking water) was able to keep the microbiological quality of the distributed water: the maximum of E. coli was <1 mpn (100 ml)−1, coliforms were on one occasion 74 mpn (100 ml)−1, the maximum on other occasions was 4 mpn (100 ml)−1, and Clostridium perfringens maximum was 1 cfu (100 ml)−1. The medians were <1 mpn (100 ml)−1, <1 mpn (100 ml)−1 and <1 cfu (100 ml)−1, respectively.
According to this data, low temperature did not prevent nitrite formation (
The behavior of ammonium and residual total chlorine in relation to temperature was very different from nitrite; they both occurred in significantly diminished areas above 9˚C compared to below 9˚C (Figures 4(c)-(f)). Both ammonium and residual total chlorine formed areas that showed high and low concentrations. It is well known that monochloramine dissociates or reacts faster at higher temperatures so it is clear why there is less total residual chlorine at higher temperatures. In addition, ammonia is nitrified faster at higher temperatures via Monod kinetics.
Compared to the water supplied from the WTPs, the nitrite concentrations in the DWDS were elevated in the study area. In the minimum concentrations map approx. half of the study area had a concentration of nitrite above 30 μg N l−1, in
the median map it was above 40 μg N l−1 and in the maximum map it was above 50 μg N l−1 (Figures 2(a)-(c), respectively). The median nitrite concentration of water supplied from WTP1 was 9 μg N l−1 (range: 2 - 23 µg N l−1) and from WTP2 it was 19 μg N l−1 (range: 5 - 39 μg N l−1). As the freshly produced drinking water contains all the nitrite from raw water and the treatment process, we can conclude that nitrite was also formed in the distribution network.
It was dubious why the nitrite concentrations were so high in the freshly produced drinking water supplied from the WTPs. In 2015, it was observed that nitrite concentrations were already raised in the sampling line leading to the onsite laboratory at WTP2 [
The raw water from Lake Päijänne contained ammonium 5.0 μg N l−1as median and 230 μg N l−1 of nitrite plus nitrate between 2000 and 2013 [
No nitrite concentrations exceeded the current Finnish statutory limit of 150 mg∙l−1 (0.5 µg N l−1 of nitrite, [
All in all the observed concentrations of nitrite in the study area were low compared to the highest concentrations in nitrification episodes (1 mg N l−1) that were noted by Wilczak et al. [
Wilczak et al. [
Vahala et al. [
The problem with nitrite in drinking water is not nitrification itself, but nitrite formation as a result of the two nitrification reactions: ammonia oxidation into nitrite (Equation (1)) and nitrite oxidation into nitrate (Equation (2)). The observed nitrite concentrations were a result of these two reactions and the relative balance between them. Harrington et al. [
Nitrifying bacteria and nitrification activity have been found in Finnish DWDSs in spite of low water temperature [
Wolfe et al. [
Speitel et al. [
The research revealed that nitrification occurred close to the WTPs and increased nitrite concentrations were related to fresh water rather than stagnant water. The low initial monochloramine concentrations were not high enough to prevent nitrification, but limited the maximum nitrite concentrations. Low temperature did not significantly inhibit nitrification or nitrite formation. The temperature dependence of ammonium and total residual chlorine was more significant than the temperature dependence of nitrite. The findings of this study emphasize that nitrite concentrations should be monitored close to the WTPs; focusing on stagnant water is not sufficient.
We thank the foundation of Maa- ja vesitekniikan tuki ry., for financing the research. We also thank HSY (Helsinki Region Environmental Services Authority) for their cooperation and for providing the obligatory monitoring data, as well as their internal monitoring data for our use. We are grateful to Ritva Laitala from Pöyry Finland Ltd. for providing the water age modeling results. We express our gratitude to Dr. Joseph Guillaume from Aalto University for his invaluable comments.
Rantanen, P.-L., Keinänen-Toivola, M.M., Ahonen, M., Mellin, I., Zhang, D.Y., Laakso, T. and Vahala, R. (2017) The Spatial Distribution of Nitrite Concentrations in a Large Drinking Water Distribution System in Finland. Journal of Water Resource and Protection, 9, 1026- 1042. https://doi.org/10.4236/jwarp.2017.98068