Formation and speciation of haloacetic acids in seawater desalination using chlorine dioxide as disinfectant

Evaluation and strategy for improving the quality of desalinated water

Seawater desalination is practiced in many coastal countries, which is accepted as clean water by the general populations. The untreated seawater reported high concentrations of bromide (50,000 - 80,000 µg/L) and iodide (21 - 60 µg/L) ions, which are reduced to non-detectable levels during thermal desalination while the concentrations of bromide and iodide ions were reduced to 250-600 µg/L and < 4-16 µg/L, respectively during reverse osmosis processes. During the treatment and/or disinfection, many brominated and iodinated disinfection byproducts (Br-DBPs and I-DBPs) are formed in desalinated water, some of which are genotoxic and cytotoxic to the mammalian cells and possible/probable human carcinogens. In this paper, DBPs' formation in desalinated and blended water from source to tap, toxicity to the mammalian cells, their risks to humans and the strategies to control DBPs were investigated. The lifetime excess cancer risks from groundwater, and desalinated and blended water sourced DBPs were 4.15 × 10^-6 (4.72 × 10^-7 - 1.30 × 10^-5), 1.75 × 10^-5 (2.58 × 10^-6 - 5.25 × 10^-5) and 2.59 × 10^-5 (4.02 × 10^-6 - 8.35 × 10^-5) respectively, indicating higher risks from desalinated and blended water (2.56 and 4.51 times respectively) than groundwater systems. Few emerging DBPs in desalinated/blended water showed higher cyto- and genotoxicity in the mammalian cells. The findings were compared with safe drinking water standards and strategies to produce cleaner desalinated water were demonstrated.

Reverse Osmosis Concentrate: Physicochemical Characteristics, Environmental Impact, and Technologies

This study's aim is to generate a complete profile of reverse osmosis concentrate (ROC), including physicochemical characteristics, environmental impact, and technologies for ROC treatment, alongside element recovery with potential valorization. A systematic literature review was used to compile and analyze scientific information about ROC, and systematic identification and evaluation of the data/evidence in the articles were conducted using the methodological principles of grounded data theory. The literature analysis revealed that two actions are imperative: (1) countries should impose strict regulations to avoid the contamination of receiving water bodies and (2) desalination plants should apply circular economies. Currently, synergizing conventional and emerging technologies is the most efficient method to mitigate the environmental impact of desalination processes. However, constructed wetlands are an emerging technology that promise to be a viable multi-benefit solution, as they can provide simultaneous treatment of nutrients, metals, and trace organic contaminants at a relatively low cost, and are socially accepted; therefore, they are a sustainable solution.

Effects of changing supply water quality on drinking water distribution networks: Changes in NOM optical properties, disinfection byproduct formation, and Mn deposition and release

Field studies were conducted in a Croatian city supplied by two distinct groundwater sources (referred to as A and B) to investigate both the effects of changing water source on the water quality in the drinking water supply system, as well as to further understand discoloration events that occurred in city locations that switched water from source A to B. The water treatment processes at site A were found to alter organic matter (OM) characteristics, removing humic substances while enhancing protein-derived (tryptophan) content. Although the humic-like component predominated in raw waters, microbially/protein-derived components were found to increase throughout the distribution networks of both systems. Disinfection byproducts (DBPs) such as total trihalomethane (TTHM) and total haloacetic acid (THAA) were prevalent in water distribution system (WDS)-A, which correlated with elevated OM content as well as re-chlorination with hypochlorite (NaOCl). Our field study revealed that THMs were more readily formed than HAAs during ClO₂ treatment. Unsurprisingly, chlorite concentrations were generally higher than chlorate concentrations during ClO₂ treatment, whereas (secondary) NaOCl disinfection contributed to higher chlorate production. Principal component analysis indicated that variable pH values and humic-like OM could affect Mn, As and Al concentrations at the consumer's tap. Our results suggested that although Mn concentrations complied with regulations at WDS-B and were below 50 μg/L after disinfection, Mn was oxidized and formed particulate Mn oxides capable of causing discoloration events depending on prevailing network physico-chemical and hydraulic conditions. Aluminium also appears to be released during hydraulic disturbances from extensive deposits within the network. Thermodynamic calculations showed that Mn-oxidation was strongly dependent upon the ORP, and to lesser extent the pH value. Collectively, our results confirm that ensuring the provision of safe drinking waters to consumers requires an understanding of water quality across entire distribution networks in addition to any routine post-treatment monitoring.

The effect of operational conditions on the disinfection by-products formation potential of exopolymeric substances from biofilms in drinking water

Biofilms are ubiquitous in drinking water systems due to their external matrix of exopolymeric substances (EPS) that provide them protection and adaptability. They are even more common in low flow conditions where hydraulics favor their growth. EPS are organic substances (i.e., proteins, carbohydrates and humic substances) that can react with disinfectant, forming disinfection byproducts (DBP), some of which are controlled by water regulation. However, there is little information available on biofilm-disinfectant interaction and the effect of operational conditions such as biofilm age, water velocity, chlorine and pipeline length on the DBP formation potential of EPS (DBP_fpEPS). Using experimental setup and studies of two different biofilms: Biofilm 1 (2.6 ± 0.8 mg Cl/L) and Biofilm 2 (0.7 ± 0.2 mg Cl/L), the DBP_fpEPS was studied and compared to the DBP_fp of filtered water (FW). The DBP studied were trihalomethanes (THM), haloacetic acids (HAA), haloacetonitriles (HAN), chloropropanones (CP) and chloropicrin (CPK). The DBP concentration trend in both EPS and FW was HAA > THM > CP > HAN > CPK. Biofilm age only increased chloroform (CF)_fpEPS in Biofilm 1, while other DBP_fpEPS decreased. A direct relationship between water velocity and CF_fp in Biofilm 1 was found, probably related to higher chlorine diffusion and the production of a more reactive matrix. Chlorine positively affected DBP_fpEPS, increasing Cl-HAA, Cl-THM, CPK and Br-HAN. Biofilm 2 produced higher quantities of EPS per meter of pipeline, this constituting a precursor of intermediary DBP 1,1 dichloropropanone (1,1, DCP). The study compared DBP in chlorinated water in contact with biofilm (BCW) and without (CW). Biofilm 1 increased levels of Cl-HAA, Cl-CP and dichloro-acetonitrile, while Biofilm 2 diminished Cl-HAA and Cl-HAN. Biofilm 1 reduced some Br-HAA in BCW, whereas Biofilm 2 promoted Br-HAA and 1,1, DCP in BCW. EPS and biofilms were significant in terms of their effect on DBP formation.

Environmental impact of desalination technologies: A review

Due to the limited availability of freshwater supplies, desalination has become an increasingly reliable process for water supply worldwide, with proved technical and economic feasibility and advantages. Recently, desalination capacity significantly increased from approximately 35 million m³ daily (MCM/day) in 2005 to about 95 MCM/day in 2018. Seawater desalination accounts for about 61% of global desalination capacity, while brackish water desalination accounts for 30%. Membrane desalination, mainly using reverse osmosis (RO), accounts for ¾ of global desalination capacity, with the rest mostly used for thermal desalination using multi-stage flash distillation (MSF), and multi-effect distillation (MED). Despite the undeniable role of desalination for securing water supply in areas where natural freshwater supplies are scarce, desalination impacts the natural environment at different aspects. Environmental impacts (EIs) of the desalination process are different and vary significantly according to the nature of the utilized feedwater, the desalination technology in use, and the management of waste brine generated. In this work, the EIs of each desalination technology were thoroughly investigated, with careful consideration given to different feedwater qualities, and various brine management techniques. Although the different aspects of desalination EIs have been extensively studied in the literature, the literature lacks comprehensive reviews and summaries of all the associated EIs. This article compiles the different EIs associated with the whole desalination process in one-hub, applying an intake-to-outfall approach. The leading desalination technologies of RO, MSF, and MED were analyzed, along with different feedwaters. This article provides a mapping of the different technologies involving feedwater and brine management techniques and a detailed description of their impact on the environment. Finally, recommendations and conclusions were given to minimize the negative impacts of desalination on both the local and global environments.

Environmental impact of desalination processes: Mitigation and control strategies

Freshwater supplies are in shortage relative to the high demand for different human activities, making desalination of saline water a must. Desalination to extract water from saline water has been well established as a reliable non-conventional water supply. However, desalination as any human-based process has resulted in many impacts on the environment. Brine loaded with chemicals being discharged back to the environment, along with greenhouse gases (GHGs) emissions being released to the atmosphere, are the most significant impacts, which has been extensively studied, with some efforts given to its mitigation and control. The current work discusses the mitigation and control strategies (M&CS) to the different environmental impacts (EIs) of desalination processes. The article compiles the M&CS in one work, instead of the distributed and separate treatment of the EIs of each desalination step and its respective M&CS as currently present in literature. The article tracks the water flow in an intake-to-outfall approach exploring how to minimize the impacts at each step and as a whole process. This starts from intake, pretreatment processes, desalination technology, and finally, brine discharge. The EIs associated with each desalination process element is thoroughly discussed with proposed M&CS. The work shows clearly that many EIs can be eliminated or minimized by incorporating specific design criteria and process improvements. The feedwater source has shown to have a great effect on EIs. Similarly, desalination technology has shown a considerable effect on the EIs related to brine characteristics and energy consumption. Hybrid and emerging desalination systems have shown reduced EIs relative to conventional thermal and membrane desalination technologies, while the utilization of renewable and waste energy sources has shown a significant reduction in EIs related to energy consumption.

Long-term Changes of Disinfection Byproducts in Treatment of Simulated Ballast Water

This study investigated the changes in concentrations of haloacetic acids (HAAs) and haloacetonitriles (HANs) as disinfection byproducts (DBPs) for different storage times (as long as 20 days) and temperatures (5 to 20°C). A ship's voyage after treatment of its ballast water with active substances was considered. The HAA showed a clear trend of increasing concentration only with storage time, especially for dibromoacetic acid (DBAA). Dissolved organic nitrogen concentration was increased by the decomposition of dead organisms at 10 days, and then reacted with the remaining total residual oxidants, resulting in increased concentration of DBPs. An environmental risk assessment indicated that DBAN and monochloroacetic acid (MCAA) could have a negative impact on the marine environment. This study suggests that, because all international vessels must have a ballast water management system installed by September, 2024, the conc e ntra tio ns of DBPs, especially DBAN, MCAA, and DBAA, should be monitored in the waters at major international ports.

Adsorption of low concentrations of bromide ions from water by cellulose-based beads modified with TEMPO-mediated oxidation and Fe(III) complexation

Due to strong activity, it is very difficult to remove low concentrations of bromide in medical wastewater by traditional method, thus highly effective and greener adsorbents should be utilized to design. In this work, the cellulose beads (CBs) were modified by the TEMPO-mediated oxidation and then bonded with Fe³⁺ to fabricate Fe(III)-complexed carboxylated cellulose beads (Fe-CCBs) adsorbents. Structure and properties of Fe-CCBs were analyzed using Energy dispersive spectrum (EDS), Scanning electron microscopy (SEM), Fourier transform infrared spectrum (FT-IR), total acidity and basicity groups, X-ray diffraction (XRD), N₂ adsorption-desorption and Thermogravimetric (TGA). Moreover, batch adsorption experiments showed that the adsorption of Br^- was better consistent with general-order kinetic model and Liu isotherm model, which could also further clarify the adsorption process mechanism. Meanwhile, the results revealed that removal of Br- was a spontaneous exothermic process and was more suitable to be carried out under neutral or acidic conditions. Furthermore, the mechanism of adsorption behavior of bromide ions on Fe-CCBs was based on a combination of electrostatic attraction and outer-sphere complexation. The results of this study can provide guidance for the design of novel material adsorbents and the removal of harmful anions from aqueous solutions.

Sunlight-induced phototransformation of transphilic and hydrophobic fractions of Suwannee River dissolved organic matter

Sunlight-induced chemical changes of both transphilic (SWR-TPH) and hydrophobic (SWR-HPO) fractions of Suwannee River dissolved organic matter (DOM) were followed by ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Irradiated SWR-TPH exhibited increase of chemodiversity, loss of some aromatic compounds, and almost no change in terms of average values of m/z, O/C and double bond equivalents (DBE). Irradiated SWR-HPO showed decrease of chemodiversity, average values of m/z, O/C and DBE. Irradiation of SWR-HPO produced oxygenated (O/C > 0.7) and aliphatic new compounds and removed some aromatics and carboxyl-rich alicyclic molecules (CRAM). Comparatively, CHO-compounds of SWR-TPH were relatively stable with a minor class of aromatic compounds disappeared under sunlight irradiation. Photochemical processing of SWR-HPO generated highly oxygenated new compounds that were readily present in SWR-TPH, implying that sunlight changes the hydrophobicity of DOM and that SWR-HPO is a photochemical precursor for SWR-TPH. This study contributed to the developing knowledge on organic matter phototransformation, particularly the transformation pattern of SWR-TPH that was never described previously; it also demonstrated the role of sunlight in producing SWR-TPH compounds from SWR-HPO and consequently driving the transformation of organic matter.

Formation, distribution, and speciation of DBPs (THMs, HAAs, ClO2-,andClO3-) during treatment of different source water with chlorine and chlorine dioxide

Formation potential and speciation characteristics of two important groups of disinfection byproducts (DBPs), namely, trihalomethanes (THMs) and haloacetic acids (HAA_S), during Cl₂ and ClO₂ treatment of water samples collected from three different sources, namely, sea, river, and reservoir, were investigated with reference to key controlling parameters. Formation of inorganic DBPs such as chlorate and chlorite was evaluated. Dissolved organic carbon (DOC) and UV absorbance (UV₂₅₄) of the sea, river, and reservoir samples were 3.35 ± 0.05, 3.12 ± 0.05, and 3.23 ± 0.05 mg/L and 0.062 ± 0.01, 0.074 ± 0.01, and 0.055 ± 0.01 cm^-1, respectively. For Cl₂ and ClO₂ treatments, the respective formation potential of THMs and HAAs from the three water sources studied exhibited unidentical trend suggesting that higher THM formation was not necessarily associated with higher HAA formation. On chlorination, the concentrations of total HAAs formed were 9.8 μg/L (sea), 12.8 μg/L (river), and 20.6 μg/L (reservoir) and total THM yields were 38.3 μg/L (sea), 18.8 μg/L (river), and 21.5 μg/L (reservoir) for a Cl₂ dose of 1 mg/L and 30 min reaction time. The trend of formation of THMs and HAAs for Cl₂ treatment was similar to that for ClO₂ treatment. However, the amount of HAAs (3.5 μg/L (sea), 1.8 μg/L (river), and 1.9 μg/L (reservoir)) and THMs (not detected) formed was much lower than that formed during chlorination. Regardless of source water type, di-HAAs were the most favored HAAs, followed by tri-HAAs with a small amount of mono-HAAs formed for both Cl₂ and ClO₂ treatment. Chlorination yielded more THMs than HAAs, whereas it was reverse for chlorine dioxide treatment. Irrespective of treatment with ClO₂ or Cl₂, seawater samples showed the highest bromine incorporation percentage (BIP) in both THMs and HAAs followed by that for river and reservoir water samples. HAAs were found to be always associated with lower amount of BIP than THMs.

Comparison between OCl−-Injection and In Situ Electrochlorination in the Formation of Chlorate and Perchlorate in Seawater

To prevent biofouling from occurring in the cooling systems of coastal power plants, chlorine is often added to the cooling water. In this study, we have evaluated the fate of the total residual oxidants and the formation of inorganic chlorination byproducts including ClO3− and ClO4− during in situ electrochlorination with seawater. Then, the results were compared with those during direct OCl−-injection to seawater. The in situ electrochlorination method based on Ti/RuO2 electrodes produced much less ClO3−, while a similar level of total residual oxidants could be achieved with a reaction time of 5 min. Moreover, no ClO4− was observed, while the direct OCl−-injection system could still result in the production of ClO4−. The less or no production of ClO3− or ClO4− by the electrochlorination of seawater was mainly attributed to two reasons. First, during the electrolysis, the less amount of OCl− is available for ClO3− formation. Secondly, the formation of ClO3− or ClO4− is affected by the electrode material. In other words, if the electrode material is carefully chosen, the production of harmful reaction byproducts can be prevented or minimized. In short, based on the results from our study, electrochlorination technology proves to be a marine environmentally friendly method for controlling biofouling in the pipes of the cooling system in a coastal power plant.

Control of halophenol formation in seawater during chlorination using pre-ozonation treatment

The reverse osmosis process is widely used for seawater desalination, whereas the pre-chlorination step for controlling membrane biofouling results in undesirable disinfection by-products, such as halophenols (HPs) which are not yet regulated but of increasing concerns. The formation and speciation of HPs during chlorination of three filtered seawater samples (SA, SB, and SC) with various phenol concentrations (0.25, 0.5, 1.0 mg/L) were evaluated. 4-Bromophenol (4-BrP), 2,4,6-trichlorophenol (2,4,6-TClP), 2,4-dibromophenol (2,4-DBrP), and 2,4,6-tribromophenol (2,4,6-TBrP) were identified during chlorination, with 2,4,6-TBrP as the predominant HP. Ozone as a common oxidant in water and wastewater treatment was subsequently applied to assess its effect in dissolved organic matter (DOM) and its ability of reducing HP precursors in the seawater samples. An initial ozone dose of 5 mg O₃/L was capable of reducing dissolved organic carbon (DOC) in SA, and UV absorbance at 254 nm (UV₂₅₄) in SB, whereas it induced an elevation of UV₂₅₄ in SC. When ozone dose increased to 10 mg O₃/L, the DOC and UV₂₅₄ levels in all seawater samples were reduced. Ozone was more powerful on degrading DOM with molecular weight (MW) of near 1000 Da than those with MW of 20-100 Da, both of which composed the majority of DOM in the seawater samples. As determined by excitation emission matrix fluorescence spectroscopy, the most ozone-susceptible fraction of DOM was soluble microbial by-product-like substances, while the least was tryptophan-like aromatic proteins. Despite that the initial ozone of 5 mg O₃/L was less effective in DOM degradation than the higher dose, it successfully degraded HP precursors. By pre-ozonation at 5 mg O₃/L, no chlorophenol was detected during chlorination, and the mean reductions of the three bromophnols formed were above 92% in all seawater samples, with the reduction of 2,4,6-TBrP being the highest of 99.7, 99.6, and 99.1% in SA, SB, and SC, respectively.

Characterization of trihalomethane, haloacetic acid, and haloacetonitrile precursors in a seawater reverse osmosis system

Seawater reverse osmosis (SWRO) has been extensively applied to replenish the limited freshwater resources. One concern of such technology is the formation of disinfection by-products (DBPs) during the pre-chlorination process. For the SWRO tested in this study, the concentrations of trihalomethanes (THMs), haloacetic acids (HAAs), and haloacetonitriles (HANs) increased by 35.1, 23.7 and 4.9μg/L, respectively, after a seawater sample (with UV₂₅₄/DOC of 3.7L/mg·m and Br^- of 50.9mg/L) was pre-chlorinated (1-2mg-Cl₂/L). The dissolved organic matter (DOM) with molecular weight (MW) <1kDa dominated the formation of total THMs, HAAs and HANs. To further investigate DBPs precursors in the seawater, the DOM with MW<1kDa was fractionated to hydrophobic acids (HOA), hydrophobic bases (HOB), hydrophobic neutrals (HON), and hydrophilic substances (HIS). The excitation emission matrix fluorescence spectra analysis showed that most aromatic protein and fulvic acid of the DOM with MW<1kDa were present in the HON and HIS fractions. The HON fraction was the dominant precursor to form THMs and HAAs, while HIS controlled the formation of HANs. Furthermore, bromo - DBPs dominated the total DBPs yields after the chlorination of HIS fraction.

Oxidative Dissolution of Silver Nanoparticles by Chlorine: Implications to Silver Nanoparticle Fate and Toxicity

The kinetics of oxidative dissolution of silver nanoparticles (AgNPs) by chlorine is investigated in this work, with results showing that AgNPs are oxidized in the presence of chlorine at a much faster rate than observed in the presence of dioxygen and/or hydrogen peroxide. The oxidation of AgNPs by chlorine occurs in air-saturated solution in stoichiometric amounts with 2 mol of AgNPs oxidized for each mole of chlorine added. Dioxygen plays an important role in OCl(-)-mediated AgNP oxidation, especially at lower OCl(-) concentrations, with the mechanism shifting from stoichiometric oxidation of AgNPs by OCl(-) in the presence of dioxygen to catalytic removal of OCl(-) by AgNPs in the absence of dioxygen. These results suggest that the presence of chlorine will mitigate AgNP toxicity by forming less-reactive AgCl(s) following AgNP oxidation, although the disinfection efficiency of OCl(-) may not be significantly impacted by the presence of AgNPs because a chlorine-containing species is formed on OCl(-) decay that has significant oxidizing capacity. Our results further suggest that the antibacterial efficacy of nanosilver particles embedded on fabrics may be negated when treated with detergents containing strong oxidants, such as chlorine.

Disinfection by-product formation during seawater desalination: A review

Due to increased freshwater demand across the globe, seawater desalination has become the technology of choice in augmenting water supplies in many parts of the world. The use of chemical disinfection is necessary in desalination plants for pre-treatment to control both biofouling as well as the post-disinfection of desalinated water. Although chlorine is the most commonly used disinfectant in desalination plants, its reaction with organic matter produces various disinfection by-products (DBPs) (e.g., trihalomethanes [THMs], haloacetic acids [HAAs], and haloacetonitriles [HANs]), and some DBPs are regulated in many countries due to their potential risks to public health. To reduce the formation of chlorinated DBPs, alternative oxidants (disinfectants) such as chloramines, chlorine dioxide, and ozone can be considered, but they also produce other types of DBPs. In addition, due to high levels of bromide and iodide concentrations in seawater, highly cytotoxic and genotoxic DBP species (i.e., brominated and iodinated DBPs) may form in distribution systems, especially when desalinated water is blended with other source waters having higher levels of organic matter. This article reviews the knowledge accumulated in the last few decades on DBP formation during seawater desalination, and summarizes in detail, the occurrence of DBPs in various thermal and membrane plants involving different desalination processes. The review also identifies the current challenges and future research needs for controlling DBP formation in seawater desalination plants and to reduce the potential toxicity of desalinated water.