Disinfection means the killing of pathogenic organisms (e.g. bacteria and its spores, viruses, protozoa and their cysts, worms, and larvae) present in water to make it potable for other domestic works. The substances used in the disinfection of water are known as disinfectants. At municipal level, chlorine (Cl₂), chloramines (NH₂Cl, NHCl₂), chlorine dioxide (ClO₂), ozone (O₃) and ultraviolet (UV) radiations, are the most commonly used disinfectants. Chlorination, because of its removal efficiency and cost effectiveness, has been widely used as method of disinfection of water. But, disinfection process may add several kinds of disinfection by-products (DBPs) (∼600–700 in numbers) in the treated water such as Trihalomethanes (THM), Haloacetic acids (HAA) etc. which are detrimental to the human beings in terms of cytotoxicity, mutagenicity, teratogenicity and carcinogenicity. In water, THMs and HAAs were observed in the range from 0.138 to 458 μg/L and 0.16–136 μg/L, respectively. Thus, several regulations have been specified by world authorities like WHO, USEPA and Bureau of Indian Standard to protect human health. Some techniques have also been developed to remove the DBPs as well as their precursors from the water. The popular techniques of DBPs removals are adsorption, advance oxidation process, coagulation, membrane based filtration, combined approaches etc. The efficiency of adsorption technique was found up to 90% for DBP removal from the water.

Urine, which is an important waste biomass resource, is the main source of nitrogen in sewage and contains large quantities of emerging contaminants (ECs). In this study, we propose a new method to efficiently remove urine, simultaneously eliminate ECs, and control the generation of toxic chlorate during urine treatment using a photoelectrocatalytic-chlorine (PEC-Cl) system. A type-II heterojunction of WO₃/BiVO₄ was used as a photoanode to generate chlorine radicals (Cl•) by decreasing the oxidation potential of WO₃ valence band for the highly selective conversion of urine to N₂ and the simultaneous degradation of ECs in an efficient manner. The method presented surprising results. It was observed that the amount of toxic chlorate was significantly inhibited by circumventing the over-oxidation of Cl⁻ by holes or hydroxyl radicals (•OH). Moreover, the removal of urea nitrogen reached 97% within 90 min, while the degradation rate of trimethoprim in urine was above 98.6% within 60 min, which was eight times more than that in the PEC system (12.1%). Compared to the bare WO₃ photoanode, the toxic chlorate and nitrate generated by the WO₃/BiVO₄ heterojunction photoanode decreased by 61% and 44%, respectively. Thus, this study provides a safe, efficient, and environmentally-friendly approach for the disposal of urine.

Microplastics have received growing attention as carriers of organic pollutants in the water environment. To better understand the contribution of hydrophobic interaction, hydrogen-bonding interaction, π-π interaction and electrostatic interaction on the adsorption of hydrophilic compounds on microplastics and their adsorption behavior in natural waters, polyethylene terephthalate (PET, <150 μm) was used as an adsorbent and 4-chlorophenol (MCP), 2,4-dichlorophenol (DCP) and 2,4,6-trichlorophenol (TCP) were used as adsorbates. The results of batch adsorption experiments showed that chlorophenols (CPs) reached adsorption sites of PET through film diffusion and intra-particle diffusion. pH greatly affected the adsorption capacity. Hydrophobic interaction was the main adsorption mechanism of undissociated CPs on PET. Hydrogen-bonding interaction was also an adsorption mechanism between undissociated CPs and PET, and the contribution of hydrogen-bonding interaction to adsorption decreased with the increase of chlorine content. Meanwhile, the increase of chlorine content was favorable to the hydrophobic interaction between undissociated CPs and PET. However, higher chlorine content CPs with lower pKₐ values tended to dissociate at neutral pH condition and resulted in stronger electrostatic repulsion with PET. The increase of solution ionic strength and fulvic acid content negatively affected the adsorption of DCP and TCP on PET, but did not show significant impacts on MCP adsorption. Similarly, the adsorption capacity obtained using Taihu lake water and Bohai seawater as matrices was much lower than that using laboratory water for both DCP and TCP, while the adsorption coefficient (Kd) of MCP remained at approximately 10.6 L/kg to 11.4 L/kg in the three different solution matrices. The Kd values exhibited using natural water matrices consistently followed the order of DCP > MCP > TCP. This study provides insights into the fate of CPs in the presence of microplastics and suggests that the potential risks posed by CPs and microplastics to aqueous ecosystems merit further investigation.

Giardia is a protozoan parasite of primary concern for the drinking water industry. High contact times are required for Giardia inactivation by chlorination, while ozonation may be effective at much lower Ct products. In this study, we have assessed the occurrence of Giardia cysts in raw water, and in chlorinated or ozonated water from a drinking water treatment plant (DWTP) in Brazil, over a 16-month period. Moreover, we analyzed the effects of primary disinfection on cysts, and calculated the infection risk caused by the occurrence of Giardia cysts in raw water, chlorinated or ozonated water. Furthermore, we assessed the correlation of Giardia cysts with indicator bacteria in raw water. Data referring to concentration of Giardia cysts in raw water showed adherence to a gamma distribution at a significance level α = 0.05. The detection frequency and the mean concentration of Giardia cysts were higher in raw water (86.6%, 26 cysts∙L⁻¹), than in chlorinated (46.1%, 15.7 cysts·L⁻¹) or ozonated water (43.5%, 11.1 cysts·L⁻¹). Overall, Giardia non-viable cysts were detected more frequently in ozonated water (80%) than in chlorinated water (68.2%) or raw water (37.7%). Ozonation and chlorination resulted, respectively, in ≈27.5- and ≈13- fold reduction of Giardia infection risk, when compared to the risk calculated for raw water. Total coliform and Escherichia coli proved to be suitable surrogates to predict the occurrence of Giardia cysts in raw surface water, however, the indicator bacteria may not be suitable surrogates to predict the disinfection of Giardia cysts, as no correlation was found between indicator bacteria and Giardia cysts in treated water. To our knowledge, this is the first study reporting the efficacy of chlorine and ozone at Ct products actually applied at a full-scale drinking water treatment plant against Giardia cysts naturally occurring in the source water, i.e. real situation. Ozonation has proven more efficient than chlorination against Giardia cysts in surface water. Escherichia coli proved to be suitable surrogate to predict Giardia cysts in raw surface water.

Organic micro-pollutants such as pesticides and endocrine disruptors cause serious harm to human health and aquatic ecosystem. In this study, the potential degradation of atrazine (ATZ), triclosan (TCS), and 2,4,6-trichloroanisole (TCA) by UV-activated peroxydisulfate (UV/PDS) and UV-activated H₂O₂ (UV/H₂O₂) processes were evaluated under different conditions. Results showed that UV/PDS process was more effective than UV/H₂O₂ under the same conditions. Increasing oxidant dosage or decreasing the initial ATZ, TCS, and TCA concentrations promoted the degradation rates of these three compounds. The presence of natural organic matter (NOM) could effectively scavenge sulfate radical (SO₄•⁻) and hydroxyl radical (HO•) and reduced the removal rates of target compounds. Degradation rates of ATZ and TCA decreased with pH increasing from 5.0 to 9.0 in UV/PDS process, while in UV/H₂O₂ process, the increase of solution pH had little effect on ATZ and TCA degradation. In the UV/PDS and UV/H₂O₂ oxidation process, when the solution pH increased from 5 to 8, the removal rates of TCS decreased by 19% and 1%, while when the solution pH increased to 9, the degradation rates of TCS increased by 23% and 17%. CO₃²⁻/HCO₃⁻ had a small inhibitory effect on ATZ and TCA degradation by UV/H₂O₂ and UV/PDS processes but promoted the degradation of TCS significantly (> 2 mM). Cl⁻ had little effect on the degradation of ATZ, TCA, and TCS in UV/H₂O₂ process. Cl⁻ significant inhibited on the degradation of ATZ and TCS, but the influence of Cl⁻ on the degradation of TCA was weak in UV/PDS process. Based on these experimental results, the various contributions of those secondary radicals (i.e., carbonate radical, chlorine radical) were discussed. This study can contribute to better understand the reactivities when UV/PDS and UV/H₂O₂ are applied for the treatment of micro-pollutant-containing waters.

Wastewater treatment plants (WWTPs), usually designed to remove organic pollutants and nutrients, are often poorly equipped to handle pathogens. The present study investigated the multiple barriers provided by WWTPs to understand their role in spreading pathogenic bacteria into the environment. Three types of WWTPs (hospital, domestic, and mixed) differing in the source of raw influent, operating parameters, and reactor configuration (biological and tertiary treatment processes) were compared for the presence of fecal indicators and pathogenic bacteria discharged in their treated effluents. The plate-count technique was used for bacterial enumeration on selective agar. The microbial quality of the treated effluent was observed to be strongly influenced by characteristics inherent to a WWTP rather than depending on the characteristics of the raw influent. Among the different configurations studied, membrane bioreactor (MBR) treatment followed by chlorine disinfection provided an effluent of the highest quality (100% bacterial removal rates) followed by moving bed bioreactor (MBBR) combined with UV disinfection. MBR treatment greatly increased the efficiency of chlorine disinfection. Higher total suspended solids (TSS) removal corresponded to higher bacterial removal rates. Tertiary treatment proved to be an important determinant of the microbial quality of the final effluent. A great heterogeneity was observed in the removal rates of different bacterial groups with different treatment processes. The highest removal was observed in the case of indicators and least in the case of emerging pathogens like Escherichia coliO157: H7 indicating a lack of correlation between traditional indicators and emerging pathogens and also the inefficiency of the current wastewater treatment technologies in dealing with emerging pathogens.

The reuse of wastewater is one effective approach to solving the problem of water resource scarcity. However, deterioration in the quality of reused water, such as increased odor and bacterial growth, restricts its reuse. The objectives of this study were to characterize graywater (GW) treatment technology and to verify the suitability of the reclaimed water for toilet flushing. A membrane bioreactor (MBR) and biological aerated filter (BAF) were used to treat GW in a 1-year laboratory-scale experiment. The optimal operational conditions of the MBR and BAF were as follows: hydraulic retention time = 2–3 h, dissolved oxygen = 4–7 mg/L, mixed liquor suspended solids = 3500–4500 mg/L, and contact reaction time = 1.96–5.89 h, dissolved oxygen = 3–5 mg/L, backwash cycle time = 24–48 h, respectively. The MBR treatment resulted in reductions in COD, NH₃-N, and turbidity of 60–90%, 80–90%, and 95–99%, respectively, whereas those of BAF treatment were 50–90%, 50–90%, and 80–90%, respectively. The BOD₅ values of MBR and BAF effluent were 1.2–4.5 mg/L and 2.5–7 mg/L, respectively. GW treated by both MBR and BAF met the standard for reusing water for toilet flushing. The effluent from MBR, BAF, and BAF + ultrafiltration treatment and purified mixed wastewater was used to simulate toilet flushing at 28 °C, with the addition of 5 mg/L NaClO to the reused water. The residual chlorine levels were 1.5, 0.6, 0.9, and 0.5 mg/L, respectively, after 15 days. No bacteria were detected in any of the reclaimed water after 15 days. The water quality of the effluent of MBR-treated GW was better than that of the mixed wastewater. The results show that it is viable to use GW purified by MBR for toilet flushing. This study provides a scientific basis for the popularization and application of reclaimed water for toilet flushing.

The Campo and Km 119 rivers are sources of irrigation and water supply for the city of Campo Mourão, State of Paraná, Southern Brazil. However, these rivers are under the influence of agricultural and urban activities, which compromise the quality of their waters. The present study evaluated the waters of these rivers in the vicinity of this municipality in two different hydrological periods of 2018 for physical and chemical parameters and potential cytotoxic, genotoxic, and toxic effects. Among the eight sites, in the dry and rainy periods, P1, P2, P3, P4, and P6—sites surrounded by agricultural activities, with nearby residences and with low and/or reduced riparian forest—presented a low concentration of dissolved oxygen and high concentration of nitrite, phosphate, and chlorine. The waters of P1, P2, P3, P4, and P6, in the two samplings, were cytotoxic to the root meristem cells of Allium cepa at 24 and 48 h of exposure, and toxic to Artemia salina nauplii at 24 h of exposure, with LC₅₀ < 100 ppm. The results characterize water contamination by pesticides and urban waste from stormwater drains and runoff from the urban area. Therefore, waters of the Campo and Km 119 rivers in the vicinity of the city of Campo Mourão demonstrate the potential to cause adverse effects to man and the aquatic ecosystem. These results represent an alert to the public authorities of Campo Mourão and the State of Paraná regarding the need to inspect the anthropic activities in that city in the vicinity of these rivers, and in the implementation of a management plan for the replacement of riparian forests in places close to urban area, in order to preserve the health of the population and the ecosystems that depend on these water resources.

Microbiological, physical, chemical, cytotoxic, and genotoxic analyses of waters from ten tubular wells intended for irrigation of a community garden complex in the state of Piaui, northeastern Brazil, were carried out in two periods of 2018 (rainy and drought). All wells in both periods were contaminated with total coliforms, and wells P1, P3, P5, and P10 also had fecal coliforms. The waters, in the two collections performed, presented low concentration of dissolved oxygen, and nitrate and chlorine concentrations higher than allowed by law. Water from P1, P3, P5, and P10 were cytotoxic to root meristem cells of A. cepa in the two periods studied. However, no water sample was genotoxic to root meristems. The results show that the analyzed waters are contaminated with untreated effluents as well as with pesticides. Such conditions are pointed out because the rivers near these wells are degraded by human activities, and the gardens where the wells are found have ditches, sinks, and black cesspools for the disposal of sewage. In view of this, the intervention of the State Government in these places is necessary, since the evaluated wells irrigate community gardens in overcrowded poor neighborhoods, and from there comes part of the food of their residents. Another activity for the government is to put in place management plans for the restoration of rivers and the implementation of public sanitation in the neighborhoods where the wells are located.

Simulation of groundwater quality is important for managing water resources and mitigating water shortages, especially in arid and semiarid areas. Geostatistical models have been used for spatial prediction and interpolation of groundwater parameters. Recently, hybrid intelligent models have been employed for the simulation of dynamic systems. In this study, hybrid intelligent models, based on a neuro-fuzzy system integrated with fuzzy c-means data clustering (FCM) and grid partition (GP) models as well as artificial neural networks integrated with particle swarm optimization algorithm, were used to predict the spatial distribution of chlorine (Cl), electrical conductivity (EC), and sodium absorption ratio (SAR) parameters of groundwater. Results of the hybrid models were compared with geostatistical methods, including kriging, inverse distance weighting (IDW), and radial basis function (RBF). The latitude and longitude values of observation wells and qualitative parameters in three states of maximum, average, and minimum were introduced as input and output to the models, respectively. To evaluate the models, the root mean squared error (RMSE), the mean absolute error (MAE), and CC statistical criteria were used. Results showed that in the hybrid models, NF-GP with the lowest RMSE and MAE and highest CC was the most suitable model for the prediction of water quality parameters. The RMSE, MAE, and CC values were 107.175 (mg/L), 79.804 (mg/L), and 0.924 in the average state for Cl; were 518.544 (μmho/cm), 444.152 (μmho/cm), and 0.882 for electrical conductivity; and were 1.596, 1.350, and 0.582 for sodium absorption ratio, respectively. Among the geostatistical models, the kriging was found more accurate. Using the coordinates of wells will eventually allow the NF-GP to be used for more sampling and replace the visual techniques that require more time, cost, and facilities.