Investigation of the Interrelationships between Orthophosphate Corrosion Inhibitors, Monochloramine Residual, Biofilm Development, and Nitrification in Chloraminated Drinking Water Distribution Systems

Abstract

Lead contamination in drinking water distribution systems (DWDS) due to pipe corrosion is a human health concern. Orthophosphate, used to control corrosion, creates passive films to eliminate lead release. At the same time, it may enhance biofilm growth, monochloramine decay, and nitrification potential since phosphorus is an essential nutrient for microorganisms. However, there is limited and contradictory information on these effects in the previous studies, which may be attributed to variations in nutrient limitations in the water used across these studies. Specifically, the addition of phosphate may enhance microbiological growth in phosphorus-limited water. However, most previous studies did not examine phosphorus limitations in the water employed in their experiments. Biofilm growth and monochloramine breakdown have not been tracked concurrently in most of the previous studies. This could be the key to understanding how orthophosphate affects monochloramine decay. Furthermore, there is a lack of research on the effect of phosphate on nitrification in real-world DWDS; hence, more research needs to be conducted. The main goal of this thesis was to investigate the effect of orthophosphate on biofilm development by assessing microbiological growth, biofilm formation potential, and metabolic activity, in addition to monitoring the effects of orthophosphate on monochloramine decay and nitrogenous compounds. These parameters were monitored simultaneously in both the presence and absence of orthophosphate to facilitate a more comprehensive understanding of its effects. This objective was achieved primarily through experiments with bench-scale flow through model distribution systems (MDS) and additional laboratory batch tests using treated water from a Great Lakes utility. In the first phase of this study, initial batch tests indicated that the test water used throughout the thesis is phosphorus-limited. Subsequently, in a 3-month experiment with 4 MDS fed with chloraminated water (2 mg Cl2/L) and orthophosphate doses of 0 to 4 mg PO43-/L, it was found that increasing the dose of orthophosphate enhanced the biofilm growth and monochloramine decay (measured as total chlorine) in the MDS, with the highest increases between 1 and 2 mg PO43-/L. A positive relationship between biofilm microbiological growth and the total chlorine decay coefficients indicates that the higher monochloramine decay due to orthophosphate addition are attributed to increased microbial activity. In the second phase, the impacts of monochloramine doses of 2 and 3 mg Cl2/L were explored with and without 2 mg PO43-/L of orthophosphate over 108 days. The presence of orthophosphate enhanced both the growth and development of the biofilm and the rates of monochloramine degradation, as observed in the first phase. Increasing the monochloramine dose from 2 to 3 mg Cl2/L slightly reduced microbiological growth and noticeably decreased first-order monochloramine coefficients (measured as total chlorine). Despite this reduction, free ammonia levels increased with the higher monochloramine dose due to a greater total ammonia presence. A strong correlation was also noted between total chlorine decay coefficients and biofilm microbiological parameters. Additionally, orthophosphate increased the genetic diversity within biofilm communities, whereas increasing the monochloramine dose resulted in a noticeable reduction in genetic diversity. In the third phase, the effect of residence time (6 days and 12 days) on monochloramine decay in the presence and absence of orthophosphate (2 mg PO43-/L) was studied using two MDSs. It was found that the longer residence time of 12 days led to higher microbial activity, monochloramine decay coefficients (measured as total chlorine), and nitrite formation compared to the shorter residence time of 6 days. Additionally, orthophosphate enhanced microbiological growth, monochloramine decomposition, and nitrite formation at the 12-day site, whereas its impact was less pronounced and became only evident after day 62 at the 6-day site. First-order total chlorine decay coefficients and nitrite concentration remained stable throughout the experiment in both MDSs at the 6-day residence time. However, at the 12-day residence time, monochloramine decay progressively increased over time, accompanied by a rise in nitrite formation by the end of the experiment. The links between monochloramine decay and biofilm microbiological parameters were also noted. These correlations suggest that the increase in monochloramine decomposition, which may have resulted from increased residence time and/or the addition of orthophosphate, was largely driven by microbiological growth and activity. In the fourth phase, the results from previous phases were evaluated on another phosphorus-limited water source with different water chemistry. A selected water source from batch-testing of different water sources and reference water from earlier phases were chloraminated at 2 mg Cl2/L and tested in four MDSs, two fed with the reference water (one control and one with 2 mg PO43-/L) and two fed with the selected water source (one control and one with 2 mg PO43-/L). The effects of orthophosphate on the two water sources concerning the growth and development of biofilm and the decomposition of monochloramine were similar. The similar impacts of orthophosphate on both water sources indicate that the results obtained in the previous phases may be valid for other phosphorus-limited water sources, even with different chemical compositions. In the final phase, a batch test study was conducted on full-scale DWDS samples that employ monochloramine and orthophosphate to assess monochloramine decay and nitrification potential. This study was compared to an earlier study conducted before the introduction of orthophosphate, which utilized samples from the same DWDS sampling sites and identical batch-testing procedures. Monochloramine decomposition due to microbiological processes was found to be higher at further points in the DWDS with longer residence time after adding orthophosphate. Also, nitrite formation during batch tests using samples collected from locations far from the distribution system's entrance was greater after adding orthophosphate, indicating a higher potential for nitrification. Monochloramine decay due to chemical processes was similar before and after orthophosphate addition. In conclusion, orthophosphate promoted biofilm formation, genetic diversity, and nitrification potential, which, in turn, increased monochloramine decay. To mitigate these effects, the thesis recommends some strategies that can be adopted, including decreasing orthophosphate dosages, increasing monochloramine dosages, and shortening the residence time while closely monitoring water quality parameters, especially nitrification indicators.