Critical Review on Bromate Formation during Ozonation and Control Options for Its Minimization.

Ozone is a commonly applied disinfectant and oxidant in drinking water and has more recently been implemented for enhanced municipal wastewater treatment for potable reuse and ecosystem protection. One drawback is the potential formation of bromate, a possible human carcinogen with a strict drinking water standard of 10 µg/L. The formation of bromate from bromide during ozonation is complex and involves reactions with both ozone and secondary oxidants formed from ozone decomposition, i.e., hydroxyl radical. The underlying mechanism has been elucidated over the past several decades, and the extent of many parallel reactions occurring with either ozone or hydroxyl radicals depends strongly on the concentration, type of dissolved organic matter (DOM), and carbonate. On the basis of mechanistic considerations, several approaches minimizing bromate formation during ozonation can be applied. Removal of bromate after ozonation is less feasible. We recommend that bromate control strategies be prioritized in the following order: (1) control bromide discharge at the source and ensure optimal ozone mass-transfer design to minimize bromate formation, (2) minimize bromate formation during ozonation by chemical control strategies, such as ammonium with or without chlorine addition or hydrogen peroxide addition, which interfere with specific bromate formation steps and/or mask bromide, (3) implement a pretreatment strategy to reduce bromide and/or DOM prior to ozonation, and (4) assess the suitability of ozonation altogether or utilize a downstream treatment process that may already be in place, such as reverse osmosis, for post-ozone bromate abatement. A one-size-fits-all approach to bromate control does not exist, and treatment objectives, such as disinfection and micropollutant abatement, must also be considered.

Reliability of pathogen control in direct potable reuse: Performance evaluation and QMRA of a full-scale 1 MGD advanced treatment train.

To safely progress toward direct potable reuse (DPR), it is essential to ensure that DPR systems can provide public health protection equivalent to or greater than that of conventional drinking water sources. This study collected data over a one-year period from a full-scale DPR demonstration facility, and used both performance distribution functions (PDFs) and quantitative microbial risk assessment (QMRA) to define and evaluate the reliability of the advanced water treatment facility (AWTF). The AWTF's ability to control enterovirus, Giardia, and Cryptosporidium was characterized using online monitoring of surrogates in a treatment train consisting of ozone, biological activated carbon, microfiltration, reverse osmosis, and ultraviolet light with an advanced oxidation process. This process train was selected to improve reliability by providing redundancy, defined as the provision of treatment beyond the minimum needed to meet regulatory requirements. The PDFs demonstrated treatment that consistently exceeded the 12/10/10-log thresholds for virus, Giardia, and Cryptosporidium, as currently required for potable reuse in California (via groundwater recharge and surface water augmentation). Because no critical process failures impacted pathogen removal performance during the yearlong testing, hypothetical failures were incorporated into the analysis to understand the benefit of treatment redundancy on performance. Each unit process was modeled with a single failure per year lasting four different failure durations: 15 min, 60 min, 8 h, and 24 h. QMRA was used to quantify the impact of failures on pathogen risk. The median annual risk of infection for Cryptosporidium was 4.9 × 10-11 in the absence of failures, and reached a maximum of 1.1 × 10-5 assuming one 24-h failure per process per year. With the inclusion of free chlorine disinfection as part of the treatment process, enterovirus had a median annual infection risk of 1.5 × 10-14 (no failures) and a maximum annual value of 2.1 × 10-5 (assuming one 24-h failure per year). Even with conservative failure assumptions, pathogen risk from this treatment train remains below the risk targets for both the U.S. (10-4 infections/person/year) and the WHO (approximately 10-3 infections/person/year, equivalent to 10-6 DALY/person/year), demonstrating the value of a failure prevention strategy based on treatment redundancy.

Nitrosamines in pilot-scale and full-scale wastewater treatment plants with ozonation.

Ozone-based treatment trains offer a sustainable option for potable reuse applications, but nitrosamine formation during ozonation poses a challenge for municipalities seeking to avoid reverse osmosis and high-dose ultraviolet (UV) irradiation. Six nitrosamines were monitored in full-scale and pilot-scale wastewater treatment trains. The primary focus was on eight treatment trains employing ozonation of secondary or tertiary wastewater effluents, but two treatment trains with chlorination or UV disinfection of tertiary wastewater effluent and another with full advanced treatment (i.e., reverse osmosis and advanced oxidation) were also included for comparison. N-nitrosodimethylamine (NDMA) and N-nitrosomorpholine (NMOR) were the most prevalent nitrosamines in untreated (up to 89 ng/L and 67 ng/L, respectively) and treated wastewater. N-nitrosomethylethylamine (NMEA) and N-nitrosodiethylamine (NDEA) were detected at one facility each, while N-nitrosodipropylamine (NDPrA) and N-nitrosodibutylamine (NDBA) were less than their method reporting limits (MRLs) in all samples. Ozone-induced NDMA formation ranging from <10 to 143 ng/L was observed at all but one site, but the reasons for the variation in formation remain unclear. Activated sludge, biological activated carbon (BAC), and UV photolysis were effective for NDMA mitigation. NMOR was also removed with activated sludge but did not form during ozonation.

N-nitrosodimethylamine (NDMA) formation from the ozonation of model compounds.

Nitrosamines are a class of toxic disinfection byproducts commonly associated with chloramination, of which several were included on the most recent U.S. EPA Contaminant Candidate List. Nitrosamine formation may be a significant barrier to ozonation in water reuse applications, particularly for direct or indirect potable reuse, since recent studies show direct formation during ozonation of natural water and treated wastewaters. Only a few studies have identified precursors which react with ozone to form N-nitrosodimethylamine (NDMA). In this study, several precursor compound solutions, prepared in ultrapure water and treated wastewater, were subjected to a 10 M excess of ozone. In parallel experiments, the precursor solutions in ultrapure water were exposed to gamma radiation to determine NDMA formation as a byproduct of reactions of precursor compounds with hydroxyl radicals. The results show six new NDMA precursor compounds that have not been previously reported in the literature, including compounds with hydrazone and carbamate moieties. Molar yields in deionized water were 61-78% for 3 precursors, 12-23% for 5 precursors and <4% for 2 precursors. Bromide concentration was important for three compounds (1,1-dimethylhydrazine, acetone dimethylhydrazone and dimethylsulfamide), but did not enhance NDMA formation for the other precursors. NDMA formation due to chloramination was minimal compared to formation due to ozonation, suggesting distinct groups of precursor compounds for these two oxidants. Hydroxyl radical reactions with the precursors will produce NDMA, but formation is much greater in the presence of molecular ozone. Also, hydroxyl radical scavenging during ozonation leads to increased NDMA formation. Molar conversion yields were higher for several precursors in wastewater as compared to deionized water, which could be due to catalyzed reactions with constituents found in wastewater or hydroxyl radical scavenging.

Effects of ozone and ozone/peroxide on trace organic contaminants and NDMA in drinking water and water reuse applications.

An ozone and ozone/peroxide oxidation process was evaluated at pilot scale for trace organic contaminant (TOrC) mitigation and NDMA formation in both drinking water and water reuse applications. A reverse osmosis (RO) pilot was also evaluated as part of the water reuse treatment train. Ozone/peroxide showed lower electrical energy per order of removal (EEO) values for TOrCs in surface water treatment, but the addition of hydrogen peroxide increased EEO values during wastewater treatment. TOrC oxidation was correlated to changes in UV(254) absorbance and fluorescence offering a surrogate model for predicting contaminant removal. A decrease in N-nitrosodimethylamine (NDMA) formation potential (after chloramination) was observed after treatment with ozone and ozone/peroxide. However, during spiking experiments with surface water, ozone/peroxide achieved limited destruction of NDMA, while in wastewaters net direct formation of NDMA of 6-33 ng/L was observed after either ozone or ozone/peroxide treatment. Once formed during ozonation, NDMA passed through the subsequent RO membranes, which highlights the significance of the potential for direct NDMA formation during oxidation in reuse applications.

Two new methods of synthesis for the perbromate ion: chemistry and determination by LC-MS/MS.

Historically, the synthesis of perbromate ion through conventional oxidation routes has proven elusive. Herein, we report perbromate ion formation through the reaction of hypobromite and bromate ions in an alkaline sodium hypobromite solution. Formation was established via LC-MS/MS analysis of the bromate and perbromate ions in the reaction solutions over a 13-day period. Furthermore, it was discovered that the perbromate ion was also formed as a result of the electrospray ionization process. Selective reduction of the bromate ion prior to analysis was used to confirm the two formation pathways.

Rapid analysis of perchlorate, chlorate and bromate ions in concentrated sodium hypochlorite solutions.

A sensitive, rapid, and rugged liquid chromatography with tandem mass spectrometry (LC-MS/MS) method for measuring concentrations of perchlorate, chlorate, and bromate ions in concentrated sodium hypochlorite solutions is presented. The LC-MS/MS method offers a practical quantitation limit (PQL) of 0.05 microg L(-1) for ClO(4)(-), 0.2 microg L(-1) for BrO(3)(-), and 0.7 microg L(-1) for ClO(3)(-) and a sample analysis time of only 10 min. Additionally, an iodometric titration technique was compared with the LC-MS/MS method for measurement of chlorate ion at high concentration. The LC-MS/MS method was the most reproducible for chlorate concentrations below 0.025 M while the iodometric titration method employed was the most reproducible above 0.025 M. By using both methods, concentrations of chlorate can be measured over a wide range, from 0.7 microg L(-1) to 210 g L(-1) in hypochlorite ion solutions. Seven quenching agents were also evaluated for their ability to neutralize hypochlorite ion, thereby stopping formation of perchlorate ion in solution, without adversely impacting the other oxyhalide ions. Malonic acid was chosen as the quenching agent of choice, meeting all evaluation criteria outlined in this manuscript.

Detection of ozone gas using gold nanoislands and surface plasmon resonance.

Gold nanoislands interact with gaseous ozone to produce a surface plasmon resonance shift, similarly to the interaction of ozone and gold nanoparticles in water. Gold nanoislands are produced by sputtering, which significantly simplifies the synthesis and produces controlled size for the gold nanoislands. The shift of surface plasmon resonance peak was monitored while gold nanoislands were exposed to variable concentration of gaseous ozone. The shift was then correlated with ozone concentration. Our current results indicate sensing gaseous ozone at concentration of as low as 20 microg/L is achievable. Gold nanoislands were reversed to their original wavelength and were able to cycle between the wavelengths as ozone was introduced and removed. Potentially, this system can be useful as a sensor that identifies the presence of ozone at low part-per-billion concentrations of ozone in gaseous media.