Variation And Removal Of Haloacetic Acids In Trace Level At The Endpoint Of Drinking Water Distribution System

Disinfection of drinking water is a vital process to control microbial risks in water.However,the addition of disinfectant can trigger formation of a series of carcinogenic disinfection byproducts(DBPs),which are either regulated already or of emerging concerns.Being a special type of pollutant produced at the endpoint of water treatment plant(WTP),DBP once formed cannot be readily removed by any engineering processes in WTP without harming subsequent microbial safety before delivery to consumers'tap.Previous laboratory works have shown that many DBPs in chlorinated water are cytotoxic and genotoxic to cells and some of them are proven to be carcinogenic to animals or possibly carcinogenic to humans.Meanwhile,epidemiological studies have demonstrated positive associations between DBPs and risk of disruption of in utero development and adverse birth outcomes,bladder cancer in men,and decreased semen quality.Therefore,efforts are needed to decrease the health risks of microbe and DBPs simultaneously in tap.To better understand the occurrence of DBPs in tap water and their real impacts on consumers,this study made a one-year long survey of the temporal variations of a series of DBPs before and after a point-of-use(POU)treatment facility installed in a building serving for?300 people.Water samples were collected every week at a fixed location and time for 1year,and frequent samplings were carried out every 6 h a day for one month at selected seasons,which ultimately amounted to 322 samples.The results show that the concentrations of DBPs were higher in the summer than other seasons,with the lowest DBP levels being observed in spring.Within one week,higher levels of haloacetic acids(HAAs)were identified on weekdays than those on weekends.Diurnally,trihalomethanes,HAAs,and haloacetaldehydes were found to be higher at noon but lower in the evening.Consistent with other studies,the variations of most DBPs were somewhat positively related to the changes of temperature and organic matter,but negatively related to the quantity of free chlorine.With the use of a POU facility,which equips with two activated carbon cartridges and a boiler in sequence,most of DBPs were dramatically reduced,leading to 62-100%lower cytotoxicity for the measured DBPs.The study hence provides real-water evidence about the DBP occurrences in a typical distribution system endpoint and the efficiency of a typical POU on mitigating DBP risks.During the survey,due to their hydrophilicity and ionization properties,haloacetic acids(HAAs)as a prevalent class of DBPs with widespread occurrence and high toxicity are found to be hard to be removed completely by the typical POU facility.Therefore,this research hereafter applied two technologies to treat HAAs.To better understand the HAAs photodegradation process,this study investigated the kinetics,mechanisms,and products of seven Cl-,Br-,and I-HAAs irradiated with 254 nm ultraviolet light(UV₂₅₄).The photodegradation rates of HAAs by UV₂₅₄ alone were found to increase with increasing halogenation degrees(i.e.,tri>di>mono)and molecular weights(i.e.,I>Br>Cl),and the kinetic trends from both literature and this study were well captured by a quantitative structure activity relationship model,which reveals that molar absorptivity of compound is the most important property of HAAs in defining its photo-susceptibility.Interestingly,although direct photolysis was dominant for all HAAs removal,indirect photolysis via generation of hydroxyl radicals due to photodecomposition of degradation intermediates such as formic acid also contributes partially(26?47%)to the losses of Cl-HAAs.Degradation products and pathways varied significantly among HAAs too.While Cl-HAAs mainly underwent mineralization with carbon dioxide being the major product,Br-and I-HAAs experienced stepwise dehalogenation and decarboxylation processes,which resulted in substantial halogen-free organic compounds.The common products of HAAs photolysis are halide(s),formic acid,formaldehyde,and halomethanes,symbolized three concurrent pathways.Overall,the findings clearly demonstrated the dramatic differences between Cl-and Br-/I-HAAs,and indirectly evidenced the presence of hydroxyl radical during HAAs irradiation even without addition of catalyst or photosensitizer.Among available technologies to ensure drinking water security,reverse osmosis(RO)has become the gold-standard for purification due to its maturity and reliability.However,high energy consumption and low water recovery are the major impediments for extensive acceptance of RO.Close-circled RO process are innovative system designs that can offer a more effective way to improve energy efficiency and water recovery,but they were rarely employed for DBP treatment in drinking water.Thus,this study applied close-circled RO to treat water containing HAAs under a variety of environmental and operational conditions.Overall,we found that>85%HAAs were rejected and 80%of water was recovered with close-circled RO sustained 900 s.For compounds with identical number of halogen substitutions,iodinated(I-),brominated(Br-),and chlorinated(Cl-)HAAs were almost equally removed;however,highly halogenated species were easier to be rejected than lowly halogenated HAAs.By developing quantitative structure-activity relationship models,the importances of size exclusion,charge repulsion,and hydrophobic interaction effects on the innovative RO systems removal were revealed.Environmental and operational variables like p H,operating pressure,water matrix,and membrane age also played important roles in the treatment process.Increasing p H from 6.5 to 8.5 and membrane age apparently enhanced HAAs rejections.In contrast,the HAAs rejection increased only slightly from an operating pressure of 0.4 to 0.6 MPa but decreased markedly from 0.6 to 0.8 MPa.Compared to ultrapure water,equal or higher removal efficiency was observed for HAAs spiked to tap water.Considering the need to balance water quality,quantity,and cost,close-circled RO was preferred under this study's condition.