Free Chlorine Dose Research Articles

Comparative analysis of chlorinated disinfection byproducts formation from 4-nitrophenol and 2-amino-4-nitrophenol during UV/post-chlorination

Nitrophenol compounds (NCs) are widely distributed in water environments and regarded as important precursors of disinfection byproducts (DBPs). Herein, 4-nitrophenol and 2-amino-4-nitrophenol were selected as representative NCs to explore chlorinated DBPs (Cl-DBPs) formation during UV/post-chlorination. Dichloronitromethane (DCNM), trichloronitromethane (TCNM), dichloroacetonitrile (DCAN), and trichloromethane (TCM) were formed from 4-nitrophenol and 2-amino-4-nitrophenol during UV/post-chlorination, and the yields of individual Cl-DBPs from 2-amino-4-nitrophenol were higher than those from 4-nitrophenol. Meantime, increasing chlorine contact time, UV fluence, and free chlorine dose could enhance Cl-DBPs formation, while much higher values of the three factors might decrease the yields of Cl-DBPs. Besides, alkaline pH could decrease the yields of halonitromethane (HNMs) and DCAN but increase the yields of TCM. Also, higher concentrations of 4-nitrophenol and 2-amino-4-nitrophenol would induce more Cl-DBPs formation. Subsequently, the possible formation pathways of DCNM, TCNM, DCAN, and TCM form 4-nitrophenol and 2-amino-4-nitrophenol during UV/post-chlorination were proposed according to transformation products (TPs) and density functional theory (DFT) calculation. Notably, Cl-DBPs formed from 2-amino-4-nitrophenol presented higher toxicity than those from 4-nitrophenol. Among these generated Cl-DBPs, DCAN and TCNM posed higher cytotoxicity and genotoxicity, respectively. Furthermore, 4-nitrophenol, 2-amino-4-nitrophenol, and their TPs exhibited ecotoxicity. Finally, 4-nitrophenol and 2-amino-4-nitrophenol presented a high potential to produce DCNM, TCNM, DCAN, and TCM in actual waters during UV/post-chlorination, but the Cl-DBPs yields were markedly different from those in simulated waters. This work can help better understand Cl-DBPs formation from different NCs during UV/post-chlorination and is conducive to controlling Cl-DBPs formation.

Kinetics of chlorine and chloramine reactions in reverse osmosis permeate and their impact on radical formation during UV/chlorine advanced oxidation for potable reuse

Knowledge of the speciation of chlorine and chloramines in reverse osmosis (RO) permeate is needed to estimate the performance (i.e., pollutant log reduction) of subsequent UV/chlorine advanced oxidation processes (AOPs). To accurately predict the speciation, a previously reported breakpoint chlorination kinetic model was experimentally validated for pH 5.5 and reaction times < 3 min and used to predict the kinetics of breakpoint chlorination in RO permeate. The predictions showed that eliminating chloramines by adding chlorine at a dose beyond the chlorine-to-nitrogen (Cl/N) breakpoint ratio is not practical due to the high breakpoint Cl/N ratio for RO permeate (∼3.0 molar ratio) and an estimated > 40 min reaction time. The conversion from monochloramine (NH2Cl) to dichloramine (NHCl2) is the major process involved, and either or both free chlorine and chloramines may be the major species present, depending on the Cl/N ratio. Model simulations showed that increasing the oxidant dose may not always enhance the performance of UV/chlor(am)ine in RO permeate, due to the need for a low free chlorine dose for optimal •OH exposure in RO permeate. Further UV/AOPs modelling showed that it is important to control the NH2Cl concentration to improve the UV/AOP performance in RO permeate, which may be achieved by extending the reaction time after chlorine is added or increasing the applied Cl/N ratio (e.g., increasing chlorine dose). However, these measures only enhance the pollutant percentage removal by about 5 % under the conditions modelled. A simulation tool was developed and is provided to predict the speciation of chlor(am)ine in RO permeate.

Photochemical Transformation of Free Chlorine Induced by Triplet State Dissolved Organic Matter.

Photolysis of free chlorine is an increasingly recognized approach for effectively inactivating microorganisms and eliminating trace organic contaminants. However, the impact of dissolved organic matter (DOM), which is ubiquitous in engineered water systems, on free chlorine photolysis is not yet well understood. In this study, triplet state DOM (3DOM*) was found to cause the decay of free chlorine for the first time. By using laser flash photolysis, the scavenging rate constants of triplet state model photosensitizers by free chlorine at pH 7.0 were determined to be in the range of (0.26-3.33) × 109 M-1 s-1. 3DOM*, acting as a reductant, reacted with free chlorine at an estimated reaction rate constant of 1.22(±0.22) × 109 M-1 s-1 at pH 7.0. This study revealed an overlooked pathway of free chlorine decay during UV irradiation in the presence of DOM. Besides the DOM's light screening ability and scavenging of radicals or free chlorine, 3DOM* played an important role in the decay of free chlorine. This reaction pathway accounted for a significant proportion of the decay of free chlorine, ranging from 23 to 45%, even when DOM concentrations were below 3 mgC L-1 and a free chlorine dose of 70 μM was present during UV irradiation at 254 nm. The generation of HO• and Cl• from the oxidation of 3DOM* by free chlorine was confirmed by electron paramagnetic resonance and quantified by chemical probes. By inputting the newly observed pathway in the kinetics model, the decay of free chlorine in UV254-irradiated DOM solution can be well predicted.

Chloramine Prevents Manganese Accumulation in Drinking Water Pipes Compared to Free Chlorine by Simultaneously Inhibiting Abiotic and Biotic Mn(II) Oxidation

The oxidation of residual Mn(II) in finished water can lead to MnOx deposit formation in drinking water pipes. Previous work has illustrated that microbes readily cause Mn deposit build-up in nondisinfected pipes. Here, we investigated how disinfectant type and dose affected Mn(II) oxidation and MnOx accumulation through long-term pipe experiments using water produced by a full-scale water treatment plant. The results showed that Mn(II) oxidation initiated quickly in the new pipes chlorinated with 1.0 mg/L free chlorine. After 130 days of MnOx accumulation, 100 μg/L Mn(II) in water could drop to 1.0 μg/L within 1.5 h, resulting from autocatalytic Mn(II) oxidation and Mn(II) adsorption by MnOx deposits accumulated on pipe walls. In contrast to chlorination, chloramination (1.0 mg/L Cl2) caused almost no MnOx accumulation during the entire study period. The underlying mechanism was probably that monochloramine inhibited microbial Mn(II) oxidation without causing significant abiotic Mn(II) oxidation like free chlorine. A low free chlorine dose (0.3 mg/L) also reduced Mn deposit formation by mass but to a lesser extent than chloramination. After disinfection (chlorination or chloramination) was discontinued for days, biotic Mn(II) oxidation occurred, and this process was inhibited again once disinfection was resumed. In addition, Fe(III) of 200 μg/L enhanced the stability of MnOx accumulated on pipe surfaces, while humic acid induced MnOx deposit resuspension. Overall, this study highlighted the regulating role of disinfectants in MnOx formation and provided insights into developing appropriate disinfection strategies for Mn deposit control.

Genotoxic effects of chlorinated disinfection by-products of 1,3-diphenylguanidine (DPG): Cell-based in-vitro testing and formation potential during water disinfection

1,3-diphenylguanidine (DPG) is a commonly used rubber and polymer additive, that has been found to be one of the main leachate products of tire wear particles and from HDPE pipes. Its introduction to aquatic environments and potentially water supplies lead to further questions regarding the effects of disinfection by-products potentially formed. Using different bioassay approaches and NGS RNA-sequencing, we show that some of the chlorinated by-products of DPG exert significant toxicity. DPG and its chlorinated by-products also can alter cell bioenergetic processes, affecting cellular basal respiration rates and ATP production, moreover, DPG and its two chlorination products, 1,3-bis-(4-chlorophenyl)guanidine (CC04) and 1-(4-chlorophenyl)−3-(2,4-dichlorophenyl)guanidine (CC11), have an impact on mitochondrial proton leak, which is an indicator of mitochondria damage. Evidence of genotoxic effects in the form of DNA double strand breaks (DSBs) was suggested by RNA-sequencing results and further validated by an increased expression of genes associated with DNA damage response (DDR), specifically the canonical non-homologous end joining (c-NHEJ) pathway, as determined by qPCR analysis of different pathway specific genes (XRCC6, PRKDC, LIG4 and XRCC4). Immunofluorescence analysis of phosphorylated histone H2AX, another DSB biomarker, also confirmed the potential genotoxic effects observed for the chlorinated products. In addition, chlorination of DPG leads to the formation of different chlorinated products (CC04, CC05 and CC15), with analysed compounds representing up to 42% of formed products, monochloramine is not able to effectively react with DPG. These findings indicate that DPG reaction with free chlorine doses commonly applied during drinking water treatment or in water distribution networks (0.2–0.5 mg/L) can lead to the formation of toxic and genotoxic chlorinated products.

Activation of hydrogen peroxide, persulfate, and free chlorine by steel anode for treatment of municipal and livestock wastewater: Unravelling the role of oxidants speciation

Despite the extensive application of electrochemical advanced oxidation processes (EAOPs) in wastewater treatment, the exact speciation of oxidants and their effects on pollutants removal efficiency, by-products formation, and effluent toxicity are largely unknown. In this study, galvanostatic steel anodes were used to drive the electrochemical activation of hydrogen peroxide (EAHP), persulfate (EAP), and free chlorine (EAFC), for industrial-scale treatment of municipal and livestock wastewater with a focus on micropollutants and transformation products (MTPs) and effluent toxicity. Response surface methodology determined the optimized conditions for each treatment towards total organic carbon ([TOC]0 = 180 mg/L) removal at pH 3.0: persulfate dose = 0.12 mmol/min, 26.5 mA/cm2; free chlorine dose = 0.29 mmol/min, 37.4 mA/cm2; H2O2 dose = 0.20 mmol/min, 45 mA/cm2. Probe-compound degradation revealed that HO•, SO4•− and FeIVO2+ species were simultaneously generated in EAP, whereas HO• and FeIVO2+ were the principal oxidants in EAHP and EAFC, respectively. Samples were analyzed via liquid and gas chromatography in non-target screening (NTS) mode to monitor the generation or removal of MTPs and by-products including compounds that have not been reported previously. The speciation of oxidants, shifted in presence of halide ions (Cl−, Br−) in real wastewater samples, significantly affected the mineralization efficiency and by-product formation. The production of halogenated by-products in EAFC and EAP substantially increased the effluent toxicity, whereas EAHP provided non-toxic effluent and the highest mineralization efficiency (75 − 80%) to be nominated as the best strategy.

Identification and Modelling of Chlorine Decay Mechanisms in Reclaimed Water Containing Ammonia

Keeping an effective disinfectant residual concentration in reclaimed water is still a challenge, due to its high levels of ammonia and organic matter when compared with those in drinking water. This research proposes the integration of the reaction schemes of monochloramine auto-decomposition with an empirical kinetic mechanism accounting for reactive chlorine species decay in the presence of organic matter, for which three mechanisms were hypothesized and tested. A parallel second order mechanism, where monochloramine reacts both with fast and slow organic matter reactive fractions, was identified as the most suitable. The model, comprising two rate constants and two fictive concentrations of organic matter as parameters, was further successfully calibrated with real reclaimed waters with two initial free chlorine doses of 8.01×10−5 M (≈5 mg/L) and 2.67×10−4 M (≈20 mg/L). The proposed model is believed to support future studies aiming to predict and manage chlorine decay in reclaimed water distribution systems.

Impact of sodium hypochlorite on rejection of non-steroidal anti-inflammatory drugs by biomimetic forward osmosis membranes

Biomimetic forward osmosis membranes with a polyamide-based active layer containing polymersomes incorporating aquaporins were exposed to various free chlorine doses (free chlorine concentration x exposure time) and pH. Membrane chlorination was found to decrease the apolar/polar balance of the membrane surface energy and to increase its negative surface charge density. A substantial modification of the hydrogen-bond network in the membrane active layer was also highlighted. The separation performance of biomimetic membranes was also impacted by chlorine-induced chemical ageing. For the lowest free chlorine doses (100 ppm h), the reverse salt flux was found to increase with respect to the pristine membrane while the water flux dropped by 10–20%. For higher free chlorine doses (1000 and 10 000 ppm h), both the reverse salt flux and the water flux were found higher than for the pristine membrane. These behaviors were explained by the combination of several mechanisms with possible antagonist effects, namely a reduced internal concentration polarization due to ageing of the membrane support material, and modifications of the hydrogen-bond network and surface charge density of the membrane active layer. Although the degradation caused by sodium hypochlorite increased the draw-solute loss per unit of water passed, the biomimetic membranes maintained their rejection ability towards naproxen and diclofenac as rejections higher than 95.9% were measured irrespective of the ageing conditions.

Bioassay- and QSAR-based screening of toxic transformation products and their formation under chlorination treatment on levofloxacin

Levofloxacin (LEV) is a broad-spectrum quinolone antibiotic and widely used for human and veterinary treatment. Overuse of LEV leads to its frequent occurrence in the water environment. In this study, the transformation characteristics of LEV in water during the simulated chlorination disinfection treatment were explored. Fifteen major transformation products (TPs) of LEV were identified, and their plausible formation pathways were proposed. The reaction pathways were strongly dependent on pH condition, and LEV removal was relevant to free available chlorine (FAC) dose. Antibacterial activity of chlorination system was dramatically declined when FAC was more than 3-equivalent (eq) due to the elimination of antibacterial related functional groups. Genotoxicity of chlorination system increased more than 3 times at 0.5-eq of FAC and then decreased with increasing FAC dose, which were in accordance with the relative concentration of toxic TPs estimated by QSAR model. These results implied that the combination of bioassay, QSAR computation and chemical analysis would be an efficient method to screen toxic TPs under chlorination treatment. It is anticipated that the results of this study can provide reference for optimizing operational parameters for water disinfection treatment, and for scientifically evaluating the potential risk of quinolone antibiotics.

Efficient degradation of ethanolamine by UV/chlorine process via organic chloramine photolysis: Kinetics, products, and implications for ethanolamine wastewater treatment

The degradation of ethanolamine (ETA), a recalcitrant pollutant containing a primary amine moiety, can be significantly enhanced in a water treatment with chlorination followed by UV (UV/chlorine) via photolysis of the N–Cl bond of monochloro- (N-Cl ETA) and dichloro-ethanolamine (N-Cl2 ETA) which are formed from the reaction of chlorine with ETA. This study investigated the reaction kinetics, products, and mechanisms of ETA degradation in UV254/chlorine and the effects of process variables on ETA’s treatment efficiency. Fluence-based, photolysis rate constant (kUV) was 1.0 × 10−3 cm2/mJ for N-Cl ETA and 1.3 × 10−3 cm2/mJ for N-Cl2 ETA. UV/chlorine was found effective at treating ETA under wide-ranging conditions for the free available chlorine (FAC) dose over ETA concentration (i.e., [FAC]0/[ETA]0 ≥ 1), pre-chlorination duration (i.e., >a few minutes), and solution pH (i.e., 2–11), due to the rapid formation of N-Cl ETA from the reaction of HOCl with the amine of ETA (k of 1.2 × 108 M−1 s−1). Increasing the FAC dose was capable of enhancing the degradation rate of ETA so that it attained a kUV of 1.5 × 10−3 cm2/mJ at [FAC]0/[ETA]0 of 3, mainly due to the formation of N-Cl2 ETA (≥75%) and to a lesser extent from the involved OH reaction. The energy consumption of UV/chlorine to achieve a 90% ETA abatement ranged from 0.22 kWh/m3 ([ETA]0 = 0.025 mM) to 3.59 kWh/m3 ([ETA]0 = 10 mM). Ammonia, formaldehyde, and glycolaldehyde were the major degradation products of the UV/chlorine treatment of ETA, which could have formed through the photolysis of N–Cl bonds forming aminyl radicals and their transformation via a 1,2-hydrogen shift and β-scission. Overall, this study provides useful kinetic and mechanistic information for designing an effective UV/chlorine process for treating ETA wastewater.

UV/chlorination process of algal-laden water: Algal inactivation and disinfection byproducts attenuation

The disinfection of algal-impacted surface waters results in high concentrations of disinfection byproducts (DBPs). Here, the formation of DBPs when water containing Pseudanabaena limnetica (P. limnetica) was disinfected combining chlorination and ultraviolet irradiation (UV) was investigated. Because of UV/chlorination to inhibit algal growth and reduce DBP formation effectively, the effects of UV doses, free chlorine doses, reaction time and pH on the formation of DBPs during UV/chlorination were studied. The results showed that higher UV dose, longer oxidation time, neutral and acidic pH were beneficial in controlling DBPs. Moreover, UV/chlorination could enhance the formation of brominated DBPs, and reduce the formation of chlorinated DBPs in algal solutions containing bromide. In addition, the DBP formation from intracellular and extracellular organic matter (IOM and EOM, respectively) derived from algae was compared. Finally, the economic efficiency of our system using the electrical energy per order (EE/O) was evaluated, which was calculated to be 18.68 kWh/m3 during the reaction. This information indicated that UV/chlorination was a feasible treatment method for algal-rich water.

Peroxymonosulfate/chloride disinfection versus sodium hypochlorite disinfection in terms of the formation and estimated cytotoxicity of CX3R-type disinfection by-products under the same dose of free chlorine

• CX 3 R-type DBP formation is compared between PMS/Cl − and NaClO disinfection. • CX 3 R-type DBP level in PMS/Cl − is lower than that in NaClO disinfection. • DBP formation pathway during PMS/Cl − disinfection is proposed. • The cytotoxicity from DBPs in PMS/Cl − is lower than that in NaClO disinfection. Peroxymonosulfate (PMS) is receiving increasing interest as an oxidant used during the disinfection of drinking water. However, it does not have continuous disinfection ability as sodium hypochlorite (NaClO) does. PMS/Cl − can produce reactive chlorine species and thus provide a persistent disinfectant residual in distribution systems. This study investigated PMS/Cl − disinfection versus NaClO disinfection in terms of the formation and estimated cytotoxicity of CX 3 R-type disinfection by-products (DBPs) under the same dose of free chorine. Results showed that free chlorine produced by PMS/Cl − reaction can react with organic compounds and the productions of CX 3 R-type DBPs were dependent on the free chlorine dose, pH and reaction time. Specifically, chloroform (CF) and chloral hydrate (CH) generally increased with increasing of free chlorine dose and reaction time. CF increased steadily with increasing pH, while CH firstly increased and then decreased with increasing pH. Haloacetonitriles and haloacetamides firstly increased and then decreased with increasing of free chlorine dose and reaction time, and decreased continuously with increasing pH. The formation pathway of CX 3 R-type DBPs during chlorination (PMS/Cl − ) of tyrosine was proposed, based on the detected intermediates (i.e., Benzyl cyanide, 4-hydroxy-benzyl-cyanide, 4-chlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol). The estimated cytotoxicity of CX 3 R-type DBPs in chlorinated water firstly increased and then decreased with the increasing of reaction time, free chlorine dose and pH, which highly related to the changes of DCAN (the most toxic CX 3 R-type DBP) levels. Comparing to NaClO disinfection, PMS/Cl − disinfection can better control the formation and cytotoxicity of CX 3 R-type DBPs, which was attributed to the degradation of natural organic matter, CX 3 R-type DBPs intermediates and CX 3 R-type DBPs by the residual PMS. This study demonstrated the superiority of PMS/Cl − as an alternative disinfectant comparing to NaClO.

Conversion of chlorine/nitrogen species and formation of nitrogenous disinfection by-products in the pre-chlorination/post-UV treatment of sulfamethoxazole

Pre-chlorination and UV disinfection are two common processes in drinking water treatment plants. Sulfamethoxazole (SMX), an antibiotic widely detected in source water, was selected as a precursor to study the conversion of chlorine/nitrogen species and DBP formation in pre-chlorination/post-UV process. The combined chlorine (mainly organic chloramines) produced in pre-chlorination of SMX can self-degrade and release free chlorine back again as pre-chlorination time goes on. With free chlorine dose increasing, the self-degradation rate of combined chlorine increased obviously. But the combined chlorine stopped self-degrading and remained stable around 1 mg-Cl2/L after adding 0.30 mM chlorine for 30 min. Post-UV treatment after pre-chlorination can enhance the degradation and achieve a complete removal of combined chlorine (including organic chloramines). Deamination occurred during pre-chlorination/post-UV process and deamination amount (-NH2) per SMX concentration was 0.19 M/M. Radicals in this process had no obvious influence on chlorine/nitrogen species conversion. Direct chlorination of SMX had the lowest DBP formation potentials while the application of pre-chlorination and UV enhanced them. Compared with UV treatment only, dichloroacetonitrile formation potential of SMX reduced by 1.58 × 10−3 mol/mol-SMX (17.37 μg/l) after pre-chlorination/post-UV treatment. During pre-chlorination/post-UV/final-chlorination treatment of SMX, Br− and natural organic matter can enhance DBP formation and toxicity-weighted values. Acid conditions showed a very high DBP risk, while alkaline conditions could cut this risk obviously, especially for the toxicity-weighted values of these DBPs.

Combining solar irradiation with chlorination enhances the photochemical decomposition of microcystin-LR

Microcystin-LR (MC-LR) generated by cyanobacteria is a potent toxin threatening human health. In this study the kinetics and mechanisms of MC-LR elimination from drinking water under solar irradiation with free chlorine (the solar/chlorine process) was evaluated. The rate of MC-LR degradation was dramatically enhanced in the solar/chlorine process (1.1 × 10−2 s−1) compared with chlorination alone (2.6 × 10−3 s−1) or solar irradiation alone (1.2 × 10−4 s−1) with a free chlorine dose of 42 μM (3.0 mg L−1 as Cl2). The enhancement was due to the presence of hydroxyl radicals, reactive chlorine species (RCS), and ozone during free chlorine photolysis. The second-order rate constants of Cl • and Cl2•- reacting with MC-LR were determined to be (2.25 ± 0.07) × 1010 and (5.58 ± 0.42) × 107 M−1s−1, respectively. Cl • was the major RCS contributing to MC-LR elimination. The highest MC-LR degradation rate was observed at pH 8.0. Free chlorine, HO •, Cl • and O3 together accounted for almost 95% of the MC-LR elimination. Hydroxyl- and chloro-MC-LR were generated in the process, followed by dechlorination, dehydration and cleavage of cyclic structures in MC-LR. Aldehyde- and ketone-MC-LR byproducts were also observed. The destruction of dienes led to a great reduction in MC-LR's toxicity. MC-LR removal in natural water samples under natural sunlight irradiation with free chlorine was demonstrated with limited formation of disinfection byproducts. The solar/chlorine process is an energy-efficient approach for MC-LR control, especially suitable for rural areas or where algal blooming threatens.

Operational limits of sodium hypochlorite for different fresh produce wash water based on microbial inactivation and disinfection by-products (DBPs)

The goal of the present study was to establish free chlorine operational limits for the disinfection of different types of fresh-cut produce wash water using sodium hypochlorite (NaClO). The proposed operational limits in the wash water are based on both the effectiveness of microbial inactivation and the occurrence of disinfection by-products (DBPs). For this purpose, lab-scale dynamic experiments were performed simulating the conditions of commercial fresh-cut processing lines. Different free chlorine (FC) concentrations (≈0, ≈10, ≈20, and ≈30 mg/L) were maintained in process wash water (PWW) of shredded iceberg lettuce, baby leaves (rocket, red oak leaf, and red swiss chard), shredded red cabbage and diced onions by means of the constant addition of the different doses of NaClO. Physicochemical characteristics of PWWs were monitored during the experiments including FC, total chlorine (TC), pH, temperature, oxidation-reduction potential (ORP), electrical conductivity (EC), chemical oxygen demand (COD), absorbance at 254 nm (UV254), and the presence of DBPs such as trihalomethanes (THMs), haloacetic acids (HAAs) and chlorate. The antimicrobial capacity of FC doses was evaluated by the inactivation of indigenous total aerobic bacteria (TAB). In all cases, FC had a significant antimicrobial effect (p < 0.05), but the operation limit differed depending on the type of PWW. A lower FC operational limit based on TAB counts of 10 mg/L was effective for onion PWW, while levels up to 20–25 mg/L were established for lettuce and cabbage PWWs. However, the levels of TAB in PWW of baby leaves were not controlled even at the highest FC concentration of 30 mg/L. The establishment of a maximum operational limit for FC based on the concentration of DBPs in PWW was not possible as even the lowest FC concentration (10 mg/L) would potentially lead to unacceptable levels of chlorate in the different types of product washed including lettuce, baby leaves, red cabbage and onion.

Inhibition of regrowth of planktonic and biofilm bacteria after peracetic acid disinfection

Peracetic acid (PAA) is a promising alternative to chlorine for disinfection; however, bacterial regrowth after PAA disinfection is poorly understood. This study compared the regrowth of bacteria (Gram-negative Pseudomonas aeruginosa PAO1 and Gram-positive Bacillus sp.) after disinfection with PAA or free chlorine. In the absence of organic matter, PAA and free chlorine prevented the regrowth of planktonic cells of P. aeruginosa PAO1 at C·t (= disinfectant concentration × contact time) doses of (28.5 ± 9.8) mg PAA·min·L−1 and (22.5 ± 10.6) mg Cl2·min·L−1, respectively, suggesting that they had comparable efficiencies in preventing the regrowth of planktonic bacteria. For comparison, the minimum C·t doses of PAA and free chlorine to prevent the regrowth of P. aeruginosa PAO1 biofilm cells in the absence of organic matter were (14,000 ± 1,732) mg PAA·min·L−1 and (6,500 ± 2,291) mg Cl2·min·L−1, respectively. PAA was less effective than free chlorine in killing bacteria within biofilms in the absence of organic matter most likely because PAA reacts with biofilm matrix constituents slower than free chlorine. In the presence of organic matter, although the bactericidal efficiencies of both disinfectants significantly decreased, PAA was less affected due to its slower reaction with organic matter and/or slower self-decomposition. For instance, in a dilute Lysogeny broth-Miller, the minimum concentrations of PAA and free chlorine to prevent the regrowth of planktonic P. aeruginosa PAO1 were 20 mg PAA·L−1 and 300 mg Cl2·L−1, respectively. While both disinfectants are strong oxidants disrupting cell membrane, environmental scanning electron microscopy (ESEM) revealed that PAA made holes in the center of the cells, whereas free chlorine desiccated the cells. Overall, this study shows that PAA is a powerful disinfectant to prevent bacterial regrowth even in the presence of organic matter.

Effect of sodium hypochlorite exposure on polysulfone recycled UF membranes and their surface characterization

The present work aims to investigate the effect of chlorine on the surface of the polysulfone (PSF) layer uncovered in the transformed ultrafiltration (UF) membranes (recycled from end-of-life reverse osmosis (RO) membranes). With this purpose, 6 end-of-life RO membranes used previously during their useful life to treat brackish water or seawater were transformed using two higher exposure doses of free chlorine (50,000 and 300,000 ppm h). On one hand, the end-of-life membranes were first characterized in terms of the type of fouling identified by TGA, ICP and bacteria detection. On the other hand, to determine the stability of the PSF layer, the transformed PSF UF membranes were then characterized by means of its permeability and molecular weight cut-off (MWCO). In addition, membrane surface characterization was performed by ATR-FTIR, SEM and AFM. The results show that all the end-of-life RO membranes with organic and inorganic fouling were effectively transformed to PSF UF membranes at the ppm∙h values studied. However, one of the analysed sea water membrane model (HSWC3) showed more resistance to be transformed and the pores of the uncovered PSF layer only appeared at 300,000 ppm h. This membrane showed different surface appearance, with bigger pores after 300,000 ppm h exposure dose that might have affected the PSF UF membrane performance. This study indicates that a complete characterization of transformed PSF UF membrane surface is crucial for a deep understanding of the recycling process of end-of-life membranes.

Inactivation of various bacteriophages in wastewater by chlorination; Development of more reliable bacteriophage indicator systems for water reuse

There has been an accelerating increase in water reuse due to growing world population, rapid urbanization, and increasing scarcity of water resources. However, it is well recognized that water reuse practice is associated with many human health and ecological risks due to numerous chemicals and pathogenic microorganisms. Especially, the potential transmission of infectious disease by hundreds of pathogenic viruses in wastewater is one of the most serious human health risks associated with water reuse. In this study, we determined the response of different bacteriophages representing various bacteriophage groups to chlorination in real wastewater in order to identify a more reliable bacteriophage indicator system for chlorination in wastewater. Different bacteriophages were spiked into secondary effluents from wastewater plants from three different geographic areas, and then subjected to various doses of free chlorine and contact time at in a bench-scale batch disinfection system. The inactivation of was relatively rapid and reached ~4 log10 with a CT value of 5 mg/L*min. On the other hand, the inactivation of bacteriophage PRD1 and MS2 were much slower than the one for and only ~1 log10 inactivation was achieved by a CT value of 10 mg/L*min. Overall, the results of this study suggest that bacteriophage both MS2 and PRD1 could be a reliable indicator for human pathogenic viruses for chlorination in wastewater treatment processes and water reuse practice.

Removal of C.I. Reactive Red 2 by low pressure UV/chlorine advanced oxidation

Azo dyes are commonly found as pollutants in wastewater from the textile industry, and can cause environmental problems because of their color and toxicity. The removal of a typical azo dye named C.I. Reactive Red 2 (RR2) during low pressure ultraviolet (UV)/chlorine oxidation was investigated in this study. UV irradiation at 254nm and addition of free chlorine provided much higher removal rates of RR2 and color than UV irradiation or chlorination alone. Increasing the free chlorine dose enhanced the removal efficiency of RR2 and color by UV/chlorine oxidation. Experiments performed with nitrobenzene (NB) or benzoic acid (BA) as scavengers showed that radicals (especially OH) formed during UV/chlorine oxidation are important in the RR2 removal. Addition of HCO3− and Cl− to the RR2 solution did not inhibit the removal of RR2 during UV/chlorine oxidation.

Effects of assimilable organic carbon and free chlorine on bacterial growth in drinking water.

Assimilable organic carbon (AOC) is one of the most important factors affecting the re-growth of microorganisms in drinking water. High AOC concentrations result in biological instability, but disinfection kills microbes to ensure the safety of drinking water. Free chlorine is an important oxidizing agent used during the disinfection process. Therefore, we explored the combined effects of AOC and free chlorine on bacterial growth in drinking water using flow cytometry (FCM). The initial AOC concentration was 168 μg.L-1 in all water samples. Without free chlorine, the concentrations of intact bacteria increased but the level of AOC decreased. The addition of sodium hypochlorite caused an increase and fluctuation in AOC due to the oxidation of organic carbon. The concentrations of intact bacteria decreased from 1.1×105 cells.mL-1 to 2.6×104 cells.mL-1 at an initial free chlorine dose of 0.6 mg.L-1 to 4.8×104 cells.mL-1 at an initial free chlorine dose of 0.3 mg.L-1 due to free chlorine originating from sodium hypochlorite. Additionally, free chlorine might be more obviously affected AOC concentrations than microbial growth did. These results suggested that AOC and free chlorine might have combined effects on microbial growth. In this study, our results showed concentrations determined by FCM were higher than those by HPC, which indicated that some E. coli detected by FCM might not be detected using HPC in drinking water. The level of free chlorine might restrain the consumption of AOC by inhibiting the growth of E. coli; on the other hand, chlorination might increase the level of AOC, thereby increase the potential for microbial growth in the drinking water network.