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• The characteristics of mixed chlorine/chloramine species were summarized. • Analysis methods of mixed chlorine/chloramine species and limitation are reviewed. • Concomitant issues of chlorine/chloramine species conversion were identified. • Control strategies for alleviating mixed chlorine/chloramines were proposed. • Future research directions of mixed chlorine/chloramine species were recommended. Mixed chlorine/chloramines are commonly occurring in real drinking water distribution systems (DWDSs) but often overlooked. This review provides a comprehensive overview of the occurrences, characteristics, analysis methods, and control strategies of mixed chlorine/chloramines in DWDSs. The characteristics of mixed chlorine/chloramine species are summarized for treated water in drinking water treatment plants (DWTPs), secondary disinfection facilities, and DWDSs where different disinfectants could be blended. The key to differentiating and quantifying mixed chlorine/chloramine species is to separate organic chloramines (OCs) from di/tri-chloramines and overcome certain interferences. The complex interactions between water matrixes and chlorine/chloramine species could accelerate pipeline corrosions, enhance emerging disinfection by-products risks, lead to off-flavors in drinking water, and induce bio-instability issues (such as nitrification, microorganism regrowth, and promotion of horizontal gene-transfers). Three promising strategies for alleviating mixed chlorine/chloramine species are recommended, which include (i) removing precursors intensively and reconditioning the treated water, (ii) combining UV irradiation to eliminate undesired chlorine/chloramines species, and (iii) strengthening monitoring, operation, and maintenance management of DWDSs. Finally, the challenges for gaining insights into the mechanisms of mixed chlorine/chloramine species conversion are discussed and promising research directions are proposed. [Display omitted] [ABSTRACT FROM AUTHOR]

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-Cl 2 /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 (-NH 2) 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. Image 1 • Combined chlorine from SMX chlorination can self-degrade and release free chlorine. • UV can enhance the degradation of combined chlorine after pre-chlorination of SMX. • Pre-chlorination/post-UV treatment of SMX can lead to deamination. • Deamination in pre-chlorination/post-UV of SMX can reduce DCAN formation potential. • Alkaline condition can reduce DBPs in pre-chlorination/post-UV/final-chlorination. [ABSTRACT FROM AUTHOR]

Surface water are frequently subjected to problems of algal blooms and release of algae organic matter (AOM) from the algae cells, which cause many water quality issues. This study investigated the formation of organic chloramines and nitrogenous disinfection by-products (N-DBPs) during chlor(am)ination and UV/chlor(am)ination of AOM in drinking water. AOM caused higher organic chloramine formation than humic acid and fulvic acid during chlor(am)ination. The formation of organic chloramines increased first and then decreased with the increase of free chlorine dosage, but kept increasing with the increase of NH 2 Cl dosage. During AOM chlorination, the formation of organic chloramines kept decreasing as the reaction time went by, and the maximum organic chloramine proportion (79.1%) in total chlorine occurred at 8 h. However, during AOM chloramination, the formation of organic chloramines increased first, decreased in the following and then increased again as the reaction time went by, and the maximum organic chloramine proportion (22.1%) in total chlorine occurred at 24 h. UV irradiation pretreatment did not effectively influence organic chloramine formation during AOM chlor(am)ination, but accelerated the degradation of organic chloramines during chloramination. Besides, UV pretreatment enhanced the formation of N-DBPs during the subsequent chlor(am)ination of AOM, especially dichloroacetonitrile. [ABSTRACT FROM AUTHOR]

Iodinated trihalomethanes (I-THMs) are a group of emerging disinfection by-products with high toxicity, and iodide (I − ) as well as iodinated organic compounds are expected to be their iodine sources. Nevertheless, in this study, iodate (IO 3 − ) was proven to be a new iodine source of I-THM formation during UV/chloramination. In the iodate-containing waters (without any other iodine sources), I-THM formation increased with the increase of UV dose, IO 3 − and NH 2 Cl concentrations. With the increase of Br − /IO 3 − molar ratio, I-THM formation (especially for the brominated species) increased. Besides, NOM species could affect I-THM formation from IO 3 − during UV/chloramination. Fulvic acid could promote IO 3 − phototransformation to I − but humic acid impeded the production of I − during UV irradiation. Under realistic drinking water treatment conditions (DOC = 5.0 mg-C/L, IO 3 − = 12.7 μg-I/L, UV dose = 50 mJ/cm 2 , NH 2 Cl = 5 mg-Cl 2 /L), CHCl 2 I was detected as 0.17 μg/L using solid-phase microextraction method, and the production rate of I-THMs from IO 3 − was about 7% of that from I − . [ABSTRACT FROM AUTHOR]

Information is very limited on fate and transport of steroidal hormones in soils. In this study, the rainfall simulation tests were conducted with a soil slab reactor to investigate the transport of manure-borne testosterone in a silty-clay loam soil under six controllable operation conditions (i.e., three rainfall intensities and two tillage practices). The properties [e.g., rainwater volume, particle size distribution (PSD)] of the slurry samples collected in runoff and leachate at different time intervals were measured; their correlation with the distribution of testosterone among runoff, leachate and soil matrix was analyzed. The results indicated that more than 88% of the testosterone was held by the applied manure and/or soil matrix even under the rainfall intensity of 100-year return frequency. The runoff facilitated testosterone transport through both dissolved and particle-associated phases, with the corresponding mass ratio being ∼7 to 3. Soil particles collected through runoff were mainly silt-sized aggregates (STA) and clays, indicating the necessity of using partially-dispersed soil particles as testing materials to conduct batch tests (e.g., sorption/desorption). No testosterone was detected at the soil depth >20 cm or in the leachate samples, indicating that transport of testosterone through the soil is very slow when there is no preferential flow. Tillage practice could impede the transport of testosterone in runoff. For the first time, results and the methodologies of this study allow one to quantify the hormone distribution among runoff, leachate and soil matrix at the same time and to obtain a comprehensive picture of the F/T of manure-borne testosterone in soil-water environments. [ABSTRACT FROM AUTHOR]

Sorption of hormones to soil particles of different size fractions (DSFs) has been studied to understand their fate and transport (F/T) in soils. Conventional studies fractionated the soil particles into DSFs by using the high speed stirring method and/or adding surfactants to fully disperse the bulk soil. However, the natural processes (e.g., soil erosion, irrigation) often are relatively mild, and many soil particles may be still in the aggregate form. In this study, a method was developed for conducting the sorption test of a representative hormone (i.e., testosterone) to bulk soils first and then analyzing the results against DSFs. Results indicated the particle size distribution (PSD) of the two representative soils tested with partially-dispersed and fully-dispersed methods was significantly different due to the attachment of clay particles on sand and silt. Testosterone was sorbed mainly by the dominant aggregates even though they might have relatively lower sorption affinity than that of clays. However, the small particles (<2000 nm), even with ∼5% mass of the bulk soil, contributed more than 30% of sorbed testosterone in the “whole” soils. The partially-dispersed soil particles of DSFs should be used to understand the transport of hormone in runoff, because using the fully-dispersed soil particles will overestimate while the whole soil method will underestimate the transport potential. With the methodology developed in this study, the sorption tests will not compromise soil's original properties (e.g., aggregates) or the competition (e.g., sorption) among soil particles, and the contribution of DSFs (particularly the partially-dispersed aggregates) to the sorption of the “whole” soil can be determined. [ABSTRACT FROM AUTHOR]

This study compared the formation of iodinated trihalomethanes (I-THMs) from iodide-containing raw waters oxidized by chlorine, chlorine dioxide (ClO 2 ) and potassium permanganate (KMnO 4 ) at different oxidant concentrations, reaction times, pHs, initial iodide concentrations and bromide to iodide mass ratios. Among the six investigated I-THMs, iodoform was the major species formed during the oxidation using chlorine, ClO 2 and KMnO 4 . When oxidant concentration increased from 0.1 to 3.0 mg/L, the formation of I-THMs increased and then decreased for chlorine and ClO 2 , but kept increasing for KMnO 4 . As the reaction time went by, I-THM concentration increased to a plateau within 10 h (ClO 2 within only 1 h, especially) for all the three oxidants. I-THM formation gradually increased from pH 3.0 to 9.0 and remained stable at pH values higher than 7.5 for chlorine; however, for ClO 2 and KMnO 4 the highest I-THM formation showed at pH 7.0 and 7.5, respectively. As initial iodide concentration increased from 20 to 800 μg/L, the total amount and species of I-THMs increased for the three oxidants. Iodide contributed to I-THM formation much more significantly than bromide. [ABSTRACT FROM AUTHOR]

• Iodine species (e.g., I–, ICM, IO 3 −) and their occurrence in water are reviewed. • UV and UV-AOPs amend DOM and thus enhance I-DBP formation in post-disinfection. • UV deiodinates and partly mineralizes ICM, releases I–, but less efficiently than UV-AOPs. • UV and UV-AOPs can mineralize I-DBPs and liberate halides. • UV can reduce IO 3 − to I–, leading to I-DBP formation in subsequent disinfection. Iodinated disinfection byproducts (I-DBPs) formed in water treatment are of emerging concern due to their high toxicity and the tase-and-odor problems associated with iodinated trihalomethanes (I-THMs). Iodoacetic acid and dichloroiodomethane are currently regulated in Shenzhen, China and the Ministry of Health of the People's Republic of China has also been considering regulating I-DBPs. Iodide (I–), organoiodine compounds (e.g., iodinated X-ray contrast media [ICM]), and iodate (IO 3 –) are the three common iodine sources in aquatic environment that lead to I-DBP formation. While UV irradiation effectively inactivate a wide range of microorganisms in water, it induces the transformation of these iodine sources, enabling the formation of I-DBPs. This review focuses on the fate and transformation of these iodine sources in UV-based water treatment (i.e., UV irradiation and UV-based advanced oxidation processes [UV-AOPs]) and the formation of I-DBPs in post-disinfection. I– released in UV-based treatments of ICM and can be oxidized in subsequent disinfection to hypoiodous acid (HOI), which reacts with natural organic matter (NOM) to produce I-DBPs. Both UV and UV-AOPs are not able to fully mineralize ICM and completely oxidize the released I– to (except UV/O 3). Results reveal that UV and UV-AOPs are adequate for I-DBP degradation but require high UV doses. While the ideal I-DBP mitigation strategy awaits to be developed, understanding their sources and formation pathways aids in informed selections of water treatment processes, empowers water suppliers to meet drinking water standards, and minimizes consumers' exposure to I-DBPs. [Display omitted] [ABSTRACT FROM AUTHOR]

• Phosphonates showed weak effects on algal growth at the initial cultivation stage. • 10 d-EC 50 of HEDP and DTPMP on S. LX1 were 57.6 and 35.7 mg/L, respectively. • Phosphonates blocked algal assimilation of iron ions by complexation. • Iron deficiency hindered photosynthesis, leading to complete growth inhibition. • Intrinsic toxicity of the phosphonates toward algal cells was negligible. The effects of phosphonates, the heavily-used antiscalants in reverse osmosis systems, on microalgae are controversial, although they are harmless to most aquatic organisms. Herein, we assessed the inhibitory effects of etidronic acid (HEDP) and diethylenetriamine penta(methylene phosphonic acid) (DTPMP) on algal growth and revealed the mechanisms involved in both intrinsic toxicity and complexation. The phosphonates showed weak influences on Scenedesmus sp. LX1 in the first 4 d of cultivation. In contrast, a significant growth inhibition was observed subsequently with half maximal effective concentrations of 57.6 and 35.7 mg/L for HEDP and DTPMP, respectively, at 10 d. The phosphonates had little effect on cellular energy transfer and oxidative stress, quantified by adenosine triphosphate level and superoxide dismutase activity, respectively, demonstrating weak intrinsic toxicities to algal cells. Phosphonates blocked the algal assimilation of iron ions through complexation. Severe iron deficiency limited photosynthetic activity and caused chlorophyll decline, resulting in a functional loss of the photosystem followed by complete algal growth inhibition at the late cultivation stage. Our findings point to a potential ecological impact wherein harmful algal blooms are induced by the natural degradation of phosphonates due to the release of both iron and phosphate ions that stimulate algal regrowth after disinhibition. Image, graphical abstract [ABSTRACT FROM AUTHOR]

· Mn(VII)-Fe(III)/PMS process showed excellent coagulation and oxidation performance. · NOM in raw water can mediate the redox cycle of iron. · The synergistic effect of NOM, Fe and Mn can promote the activation of PMS. · SO 4 •− was the main active species in the Mn(VII)-Fe(III)/PMS process. To improve the performance of the conventional coagulation process, a permanganate (Mn(VII)) pre-oxidation combined with Fe(III)/peroxymonosulfate (PMS) coagulation process (Mn(VII)-Fe(III)/PMS) that can significantly improve the removal of dissolved organic carbon (DOC), turbidity, and micropollutants is proposed in this study. Compared with conventional Fe(III) coagulation, the Mn(VII)-Fe(III)/PMS process can also significantly enhance the removal of iohexol and sulfamethoxazole in raw water. During this process, the primary reduction product, Mn(IV), after Mn(VII) pre-oxidation was adsorbed on the floc surfaces and involved in the Fe(III)/PMS process. The natural organic matter (NOM) in raw water mediated the redox cycle of iron. The synergistic effect of NOM, Fe, and Mn facilitated the redox cycle of Mn(III)/Mn(IV) and Fe(III)/Fe(II) to promote the activation of PMS. The sulfate radical (SO 4 •−) played an important role in the degradation of micropollutants. The formation potential of the detected volatile disinfection by-product (DBP) during the subsequent chlorination was reduced by 21.9% after the Mn(VII)-Fe(III)/PMS process. This study demonstrated the promising application of the Mn(VII)-Fe(III)/PMS process for coagulation and micropollutant control and illustrated the reaction mechanism. This study provides guidance for improving conventional drinking water treatment processes. [Display omitted] [ABSTRACT FROM AUTHOR]

• Direct photolysis of iohexol had significant dependence of UV wavelength spectral. • Distinctive photogenerated intermediates were observed with different photosensitizers. • Reactive sites and transformation pathways of iohexol were proposed. • Iohexol elimination and DBP yields by UV-LED irradiation followed the order of 255 > 265 > 275 > 285 nm. Iodinated X-ray contrast media (ICM) are ubiquitously present in water sources and challenging to eliminate using conventional processes, posing a significant risk to aquatic ecosystems. Ultraviolet light-emitting diodes (UV-LED) emerge as a promising technology for transforming micropollutants in water, boasting advantages such as diverse wavelengths, elimination of chemical additives, and no induction of microorganisms' resistance to disinfectants. The research reveals that iohexol (IOX) degradation escalates as UV wavelength decreases, attributed to enhanced photon utilization efficiency. Pseudo-first-order rate constants (k obs) were determined as 3.70, 2.60, 1.31 and 0.65 cm2 J−1 at UV-LED wavelengths of 255, 265, 275 and 285 nm, respectively. The optical properties of dissolved organic matter (DOM) and anions undeniably influence the UV-LED photolysis process through photon competition and the generation of reactive substances. The influence of Cl− on IOX degradation was insignificant at UV-LED 255, but it promoted IOX degradation at 265, 275 and 285 nm. IOX degradation was accelerated by ClO 2 −, NO 3 −and HA due to the formation of various reactive species. In the presence of NO 3 −, the k obs of IOX followed the order: 265 > 255 > 275 > 285 nm. Photosensitizers altered the spectral dependence of IOX, and the intermediate photoactivity products were detected using electron spin resonance. The transformation pathways of IOX were determined through density functional theory calculations and experiments. Disinfection by-products (DBPs) yields of IOX during UV-LED irradiation decreased as the wavelength increased: 255 > 265 > 275 > 285 nm. The cytotoxicity index value decreased as the UV-LED wavelength increased from 255 to 285 nm. These findings are crucial for selecting the most efficient wavelength for UV-LED degradation of ICM and will benefit future water purification design. [Display omitted] [ABSTRACT FROM AUTHOR]

While reverse osmosis (RO) is a promising technology for wastewater reclamation, RO concentrate (ROC) treatment and disposal are important issues to consider. Conventional chemical and physical treatment methods for ROC present certain limitations, such as relatively low nitrogen and phosphorus removal efficiencies as well as the requirement of an extra process for hardness removal. This study proposes a novel biological approach for simultaneous removal of nitrogen, phosphorus, and calcium (Ca 2+ ) and magnesium (Mg 2+ ) ions from the ROC of municipal wastewater treatment plants by microalgal cultivation and algal biomass production. Two microalgae strains, Chlorella sp. ZTY4 and Scenedesmus sp. LX1, were used for batch cultivation of 14–16 days. Both strains grew well in ROC with average biomass production of 318.7 mg/L and lipid contents up to 30.6%, and nitrogen and phosphorus could be effectively removed with efficiencies of up to 89.8% and 92.7%, respectively. Approximately 55.9%–83.7% Ca 2+ could be removed from the system using the cultured strains. Mg 2+ removal began when Ca 2+ precipitation ceased, and the removal efficiency of the ion could reach up to 56.0%. The most decisive factor influencing Ca 2+ and Mg 2+ removal was chemical precipitation with increases in pH caused by algal growth. The results of this study provide a new biological approach for removing nitrogen, phosphorous, and hardness from ROC. The results suggest that microalgal cultivation presents new opportunities for applying an algal process to ROC treatment. The proposed approach serves dual purposes of nutrient and hardness reduction and production of lipid rich micro-algal biomass. [ABSTRACT FROM AUTHOR]

• Benzoquinone (BQ) can accelerate emerging organic contaminants degradation by Mn(VII). • Semiquinone radicals can act as ligands to stabilize Mn(III) to form Mn(III)-L. • The presence of BQ promoted the in-situ formation MnO 2. • 1O 2 , Mn(III)-L and MnO 2 contributed to the accelerated degradation of BPA in the Mn(VII)/BQ system. • Semiquinone radicals inhibited BPA self-coupling and promote mineralization. Permanganate (Mn(VII)) is widely used as a mild oxidant in water treatment. However, the reaction rates of some emerging contaminants with Mn(VII) are extremely low. In this study, benzoquinone (BQ), a redox mediator with the important component in dissolved organic matter (DOM), enhanced the oxidation of bisphenol A (BPA) by Mn(VII) in a wide pH range of 4.0-10.0. The redox cycle of BQ would produce semiquinone radicals, which could act as ligands to stabilize the formed Mn(III) in the system to promote the oxidation of BPA. Notably, the presence of BQ might promote the formation of MnO 2. A novel mechanism was proposed that singlet oxygen (1O 2), Mn(III)-ligands (Mn(III)-L) and in-situ formed MnO 2 were the main contributors to accelerate BPA degradation in the Mn(VII)/BQ system. Under acidic conditions, the in-situ formed MnO 2 involved in the redox reaction and part of the Mn(IV) was reduced to Mn(III), indicating that the electron transfer of BQ promoted the formation of active Mn species and enhanced the Mn(VII) oxidation performance. Semiquinone radicals generated by BQ transformation would couple with the hydrogen substitution products of BPA to inhibit BPA self-coupling and promote the ring-opening reactions of BPA. Mn(VII)/BQ had better effect in raw water than in pure water, indicating that the Mn(VII)/BQ system has high potential for practical application. This study provided insights into the role of DOM in enhancing the Mn(VII) oxidation in water treatment. [Display omitted] [ABSTRACT FROM AUTHOR]

• Entropy of image texture reveals densification, clarification, and sorting of sludge. • Dynamic entropy effectively differentiates various types of granular sludge. • Prompt sorting during the settling process indicates stable granulation. • Image-based monitoring correlate with respirogram indexes. Aerobic granular sludge is one of the most promising biological wastewater treatment technologies, yet maintaining its stability is still a challenge for its application, and predicting the state of the granules is essential in addressing this issue. This study explored the potential of dynamic texture entropy, derived from settling images, as a predictive tool for the state of granular sludge. Three processes, traditional thickening, often overlooked clarification, and innovative particle sorting, were used to capture the complexity and diversity of granules. It was found that rapid sorting during settling indicates stable granules, which helps to identify the state of granules. Furthermore, a relationship between sorting time and granule heterogeneity was identified, helping to adjust selection pressure. Features of the dynamic texture entropy well correlated with the respirogram, i.e., R2 were 0.86 and 0.91 for the specific endogenous respiration rate (SOUR e) and the specific quasi-endogenous respiration rate (SOUR q), respectively, providing a biologically based approach for monitoring the state of granules. The classification accuracy of models using features of dynamic texture entropy as an input was greater than 0.90, significantly higher than the input of conventional features, demonstrating the significant advantage of this approach. These findings contributed to developing robust monitoring tools that facilitate the maintenance of stable granular sludge operations. [Display omitted] [ABSTRACT FROM AUTHOR]

• Chlor(am)ination-derived odors have extremely low odor threshold concentrations. • The formation pathways of typical chlor(am)ination-derived odors are summarized. • Detection methods of typical chlor(am)ination-derived odors are reviewed. • Combination methods can effectively remove chlor(am)ination-derived odors. • Future development of chlor(am)ination-derived odors is proposed. Odors and tastes have become universal problems related to drinking water quality. In addition to the typical odor problems caused by algae or microorganisms, the occurrence of odors derived from drinking water disinfection have attracted attention. The chlor(am)ination-derived odor substances have certain toxicity and odor-causing characteristics, and would enter the tap water through water distribution systems, directly affecting drinking water safety and customer experience. This study provided a comprehensive overview of the occurrence, detection, and control of odor substances derived from drinking water chlor(am)ination disinfection. The occurrence and formation mechanisms of several typical types of disinfection derived odor substances were summarized, including haloanisoles, N- chloroaldimines, iodotrihalomethanes, and halophenoles. They are mainly derived from specific precursors such as halophenols, anisoles, and amino acids species during the disinfection or distribution networks. In addition, the change of disinfectant during chlor(am)ination was also one of the causes of disinfection odors. Due to the extremely low odor threshold concentrations (OTCs) of these odor substances, the effective sample pre-enrichment for instrument identification and quantification are essential. The control strategies of odor problems mainly include adsorption, chemical oxidation, and combined processes such as ozonation and biological activated carbon processes (O 3 /BAC) and ultraviolet-based advanced oxidation processes (UV-AOPs). Finally, the challenges and possible future research directions in this research field were discussed and proposed. [Display omitted] [ABSTRACT FROM AUTHOR]

Ultraviolet light-emitting diodes (UV-LEDs) are promising alternatives to conventional low-pressure UV (LPUV) lamps, mainly because they contain no toxic mercury and have a potential for less energy consumption and longer lifetime. In this study, UV sources including UV-LEDs (265, 275 and 285 nm) and LPUV (254 nm) were compared in UV/chlorine degradation of an organic contaminant, ronidazole (RNZ). UV-LED/chlorine performed better than LPUV/chlorine at neutral and alkaline pH values for RNZ degradation considering the fluence-based rate constant. However, the wall plug efficiencies of UV-LEDs are relatively low at present and must reach about 20–25% to achieve the same electrical energy per order as the LPUV in UV/chlorine degradation of RNZ at pH 7.5 and 9. Neither the contribution of radical (HO· or Cl·) nor the quantum yield of chlorine could explain the different RNZ degradation rate by UV/chlorine at different wavelengths and pH values, while the chlorine photolysis rate should be the key factor for these phenomena. The effects of common co-existing substances in real water (chloride, bicarbonate and natural organic matter) on UV/chlorine degradation of RNZ were similar at different UV wavelengths. Opposite to other oxidants or reductants, the molar absorption coefficient of chlorine increases when the UV wavelength increases from 254 to 285 nm at neutral and alkaline pH, which makes UV-LED/chlorine one of the best choices for UV-LED-based advanced oxidation/reduction processes. Image 1 • UV-LED/chlorine performed better than LPUV/chlorine at neutral and alkaline pH. • Ronidazole degradation rate was highly dependent on UV wavelength by UV/chlorine. • Chlorine photolysis rate was the key factor for enhancement by UV-LED/chlorine. • Radicals or quantum yield of chlorine was not the key factor for the enhancement. • UV-LED/chlorine has advantage over other UV-LED-based AOPs. [ABSTRACT FROM AUTHOR]

The efficiency of the ultraviolet (UV)/chlorine process strongly depends on UV wavelength because chlorine photolysis and its subsequent radical formation are highly wavelength-dependent. This study compared the degradation of humic acid (HA) during the UV/chlorine process by low pressure mercury lamp (LPUV, 254 nm) and ultraviolet light-emitting diode (UV-LED, 275 and 310 nm). The results indicated that HA degradation followed the pseudo-first-order kinetics, and the fluence-based degradation rate constants (k obs) were significantly affected by UV wavelength and solution pH. HA degradation decreased greatly with increasing solution pH during the UV/chlorine process at 254 nm, while the opposite trend was observed at 275 and 310 nm. In the meantime, k obs decreased in the order of 275 nm > 254 nm > 310 nm at pH > 7.0. The changes of chlorine molar absorption coefficients at different UV wavelengths resulted in the variation of chlorine photodecay rates (k obs, chlorine), and the synergistic effects of k obs, chlorine and chlorine quantum yields (Φ chlorine) affected HA reduction. The formation of disinfection by-products (DBPs) during the UV/chlorine process was also evaluated. A significant suppression on DBP formation and DBP-associated calculated theoretical cytotoxicity were observed at 275 nm high UV fluence and alkaline pHs. These findings in this study demonstrate that UV wavelength at 275 nm is more suitable for HA degradation by the UV/chlorine advanced oxidation process in practical applications. Image 1 • HA degradation rate constants were significantly affected by UV wavelength in the UV/chlorine process. • The UV/chlorine process was more efficient at 275 nm than 254 and 310 nm at alkaline conditions. • Effective mitigation of DBPs and toxicity was achieved at 275 nm at high UV fluence. • DBP-associated cytotoxicity decreased in the order of 254 > 310 > 275 nm at high UV fluence in the presence of Br−. [ABSTRACT FROM AUTHOR]

Iodinated trihalomethanes (I-THMs) formation during chlorination and chloramination of five iodinated X-ray contrast media (ICM) compounds (iopamidol, iopromide, iodixanol, histodenz, and diatrizoate) in the presence of natural organic matter (NOM) was evaluated and compared. Chlorination and chloramination of ICM in the absence of NOM yielded only a trace amount of I-THMs, while levels of I-THMs were enhanced substantially in raw water samples. With the presence of NOM, the order with respect to the maximum yield of I-THMs observed during chlorination was iopamidol >> histodenz > iodixanol > diatrizoate > iopromide. During chloramination, I-THM formation was enhanced for hisodenz, iodixanol, diatrizoate, and iopromide. The order with respect to the maximum yield of I-THMs observed during chloramination was iopamidol > diatrizoate > iodixanol > histodenz > iopromide. With the exception of iopamidol, I-THM formation was favored at relatively low chlorine doses (≤100 μM) during ICM chlorination, and significant suppression was observed with high chlorine doses applied (＞100 μM). However, during chloramination, increasing monochloramine dose monotonously increased the yield of I-THMs for the five ICM. During chlorination of iodixanol, histodenz, and diatrizoate, the yields of I-THMs exhibited three distinct trends as the pH increased from 5 to 9, while peak I-THM formation was found at circumneutral pH for chloramination. Increasing bromide concentration not only considerably enhanced the yield of I-THMs but also shifted the I-THMs towards bromine-containing ones and increased the formation of higher bromine-incorporated species (e.g., CHBrClI and CHBr 2 I), especially in chloramination. These results are of particular interest to understand I-THM formation mechanisms during chlorination and chloramination of waters containing ICM. [ABSTRACT FROM AUTHOR]

Degradation kinetics and pathways of ronidazole (RNZ) by chlorination (Cl 2 ), UV irradiation and combined UV/chlorine processes were investigated in this paper. The degradation kinetics of RNZ chlorination followed a second-order behavior with the rate constants calculated as (2.13 ± 0.15) × 10 2 M −2 s −1 , (0.82 ± 0.52) × 10 −2 M −1 s −1 and (2.06 ± 0.09) × 10 −1 M −1 s −1 for the acid-catalyzed reaction, as well as the reactions of RNZ with HOCl and OCl − , respectively. Although UV irradiation degraded RNZ more effectively than chlorination did, very low quantum yield of RNZ at 254 nm was obtained as 1.02 × 10 −3 mol E −1 . RNZ could be efficiently degraded and mineralized in the UV/chlorine process due to the generation of hydroxyl radicals. The second-order rate constant between RNZ and hydroxyl radical was determined as (2.92 ± 0.05) × 10 9 M −1 s −1 . The degradation intermediates of RNZ during the three processes were identified with Ultra Performance Liquid Chromatography – Electrospray Ionization - mass spectrometry and the degradation pathways were then proposed. Moreover, the variation of chloropicrin (TCNM) and chloroform (CF) formation after the three processes were further evaluated. Enhanced formation of CF and TCNM precursors during UV/chlorine process deserves extensive attention in drinking water treatment. [ABSTRACT FROM AUTHOR]

Abstract: The photochemical degradation of iopamidol with low-pressure UV lamps and the formation of iodinated disinfection by-products (I-DBPs) during sequential oxidation processes including chlorine, monochloramine and chlorine dioxide were investigated in this study. Iopamidol can be effectively decomposed by UV irradiation with pseudo-first order reaction kinetics. The evaluated quantum yield was found to be 0.03318 mol einstein−1. Results showed that iopamidol degradation rate was significantly increased by higher UV intensity and lower initial iopamidol concentration. However, the effect of solution pH was negligible. Degradation of iopamidol by UV photolysis was subjected to deiodination and hydroxylation mechanisms. The main degradation products including –OH substitutes and iodide were identified by UPLC-ESI-MS and UPLC-UV, respectively. Increasing the intensity of UV irradiation promoted the release of iodide. Destruction pathways of iopamidol photolysis were proposed. Enhanced formation of I-DBPs were observed after iopamidol photolysis followed by disinfection processes including chlorine, monochloramine and chlorine dioxide. With the increase of UV fluence, I-DBPs formation were significantly promoted. [Copyright &y& Elsevier]

• IO 3 − can be reduced by e aq − in mixed IO 3 −/I− system under UV irradiation. • Acid pHs and high I− concentration promoted the IO 3 − transformation to RIS. • Mixed IO 3 −/I− system enhanced I-THM formation in UV/post-chloramination. • Humic substances promoted I-THM formation in a mixed IO 3 −/I− system. This study investigated the mechanisms of mixed IO 3 −/I− system under UV irradiation in drinking water and compared the iodinated trihalomethanes (I-THMs) formation of a mixed IO 3 −/I− system to that of single I− and IO 3 − systems during subsequent chloramination. The effects of initial I−/IO 3 − molar ratio, pH, and UV intensity on a mixed IO 3 −/I− system were studied. The introduction of I− enhanced the conversion rate of IO 3 − to reactive iodine species (RIS). Besides, IO 3 − degradation rate increased with the increase of initial I− concentration and UV intensity and the decrease of pH value. In a mixed IO 3 −/I− system, IO 3 − could undergo direct photolysis and photoreduction by hydrated electron (e aq −). Moreover, the enhancement of I-THM formation in a mixed IO 3 −/I− system during subsequent chloramination was observed. The I-THM yields in a mixed IO 3 −/I− system were higher than the sum of I-THMs produced in a single IO 3 − and I− systems at all the evaluated initial I− concentrations and pH values. The difference between I-THM formation in a mixed IO 3 −/I− system and the sum of I-THMs in a single IO 3 − and I− systems increased with the increase of initial I− concentration. As the initial pH decreased from 9 to 5, the difference of I-THM yields enhanced, while the total I-THM yield of a mixed IO 3 −/I− system and single I− and IO 3 − systems decreased slightly. Besides, IO 3 −-I−-containing water with DOC concentration of 2.5-4.5 mg-C/L, which mainly contained humic-acid substances, had a higher risk in I-THMs formation than individual I−-containing and IO 3 −-containing water. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The conversion mechanisms of chlorine species (including free chlorine, monochloramine (NH 2 Cl), dichloramine, and total chlorine), nitrogen species (including ammonium (NH 4 +), nitrate (NO 3 −), and nitrite (NO 2 −)) as well as the formation of disinfection by-products (DBPs) in a UV-activated mixed chlorine/chloramines system in water were investigated in this work. The consumption rates of free chlorine and NH 2 Cl were significantly promoted in a HOCl/NH 2 Cl coexisting system, especially in the presence of UV irradiation. Moreover, the transformation forms of nitrogen in both ultrapure and HA-containing waters were considerably affected by UV irradiation and the mass ratio of free chlorine to NH 2 Cl. NO 3 − and NO 2 − can be easily produced under UV irradiation, and the removal efficiency of total nitrogen with UV was obvious higher than that without UV when the initial ratio of HOCl/NH 2 Cl was less than 1. The roles of different radicals in the degradation of free chlorine, NH 2 Cl and NH 4 + were also considered in such a UV-activated mixed chlorine/chloramines system. The results indicated that OH• was important to the consumption of free chlorine and NH 2 Cl, and showed negligible influence on the consumption of NH 4 +. Besides, the changes of DOC and UV 254 in HA-containing water in UV-activated mixed chlorine/chloramines system indicated that the removal efficiency of DOC (24%) was much lower than that of UV 254 (94%). The formation of DBPs in a mixed chlorine/chloramines system was also evaluated. The yields of DBPs decreased significantly as the mass ratio of HOCl/NH 2 Cl varied from 1 : 0 to 0 : 1. Moreover, compared to the conditions without UV irradiation, higher DBPs yields and DBP-associated calculated toxicity were observed during the UV-activated mixed chlorine/chloramine process. Image 1 • UV-activated mixed HOCl/NH 2 Cl promoted the consumption of free chlorine and NH 2 Cl. • UV-activated mixed HOCl/NH 2 Cl enhanced the formation of NO 2 − and NO 3 −. • OH• was important to the consumption of free chlorine and NH 2 Cl, but not NH 4 +. • Higher DBPs yields were observed in the UV-activated mixed HOCl/NH 2 Cl system. [ABSTRACT FROM AUTHOR]

The effect of bromide and iodide on the transformation of humic acid (HA) and algal organic matter (AOM), and the formation of disinfection by-products (DBPs) during UV/chlorination were investigated. Experimental results indicated that the halides effectively inhibited mineralization, with multiple changes in organic molecule transformation due to differences in formation and speciation of reactive halogen species and free halogen. As a consequence, bromide and iodide also played important roles in DBP formation. The DBP yields in HA-containing water during UV/chlorination decreased in the order of iodide loaded > freshwater ≫ bromide loaded, whereas DBP formation in AOM-containing water decreased remarkably with halides added (freshwater > bromide loaded ≫ iodide loaded) at high UV fluence. Moreover, Pearson correlation analysis exhibited weaker correlation between DBPs and water parameters in AOM-containing water, while DBPs in HA-containing water exhibited better correlation with water parameters. For both simulated waters, the theoretical toxicity was calculated and peaked in bromide-containing water, whereas the calculated toxicity in iodide-containing water was comparable or slightly higher than that in freshwater. Therefore, UV/chlorine treatment may achieve good quality water with reduced DBP-associated toxicity in freshwater or iodide-containing water (iodide only), but careful consideration is needed when purifying source waters containing bromide (bromide only), especially for AOM/bromide-containing water. Image 1 • HA and AOM transformation was affected by halides during UV/chlorination. • DBP formation varied with the addition of Br− and I−. • DBP formed from HA decreased in the order of I− loaded > freshwater > Br− loaded. • Halides mitigated DBP formation from AOM (freshwater > Br− loaded ≫ I− loaded). • The highest DBP-associated cytotoxicity was observed in the presence of Br−. [ABSTRACT FROM AUTHOR]

Bioactivity and settleability of activated sludge are essential for the operation of activated sludge systems in wastewater treatment. In this work, the fractal dimension of sludge image is proposed as a tool to evaluate these two factors. The specific endogenous respiration rate (SOUR e) and the specific quasi-endogenous respiration rate (SOUR q) are found to be more dependent on the 3D structure of sludge than the specific total respiration rate (SOUR t). The relationship between the fractal structure and bioactivity suggests that the bioactivity governs the acceptable upper bound of the fractal dimension (D f), as at its theoretical maximum of 2.0, the non-porous compact flocs are predominant. The settleability or the biomass concentration determines the acceptable lower bound of D f , as at its theoretical minimum of 1.0, the free-swimming microbes are predominant. Our data reveal that the activated sludge has an acceptable fractal dimension D f in a range of 1.07–1.68. In practice, the fractal dimension should be controlled at a reasonable value as there is a trade-off between the bioactivity and physical structure to achieve better performance. A decrease or increase in the fractal dimension can serve as a signal for the change of the operational status, and this is further elucidated from the perspective of settling tanks using state point analysis. Compared with respirogram measurement, measuring fractal dimension is a complex process and its online implementation is challenging. Also, the measured value varies with the methods used. In addition, the difference in their theoretical values depends on the homogeneity of the sludge structure. Since the fractal dimension D f reflects both bioactivity and settleability of the sludge but is difficult to measure, in this work a relationship between D f and the easily measurable respirogram is established, and a method using the respirogram as a proxy of D f is proposed to control the bioactivity and settleability simultaneously. This respiration-based method is able to simultaneously control aeration and settling tanks, and could serve as an efficient tool for the management of wastewater treatment plants. Image 1 • Bioactivity governs the acceptable upper bound of fractal dimension. • Settleability determines the acceptable lower bound of fractal dimension. • A desired fractal dimension has a trade-off between bioactivity and settleability. • Fractal dimension can serve as a signal for the change of plant operational status. [ABSTRACT FROM AUTHOR]