The Efficiency And Mechanism Study Of Combined Chlorine/UV Process For Ammonia Removal And DBPs Control

Ammonia nitrogen is one of the common contaminants of drinking water source in China,which has been received widespread concern.Ammonia leads to the formation of disinfection by-products?DBPs?in chlorination disinfection process and posing a threat on human health.The guidelines of ammonia in Chinese standards for drinking water quality?GB5749-2006?was 0.5mg·L^-1.Therefore,the development of an alternative method for removal ammonia and controlling DBPs is necessary and meaningful.This study developed the Chlorine/UV process for ammonia removal.The Chlorine/UV process could be set between the filtration process and disinfection process.The Chlorine/UV process could effectively remove ammnoia at a relative low chlorine:nitrogen mass ratio?less than 5:1?,which significantly lower than the breakpoint chlorination?8:1¹0:1 of chlorine:nitrogen mass ratio?,thereby reducing DBPs formation.In addition,chloramine method have a potential of ammonia recovery.Therefore,the Chlorine/UV process have an advantage for ammonia water treatment.This study investigated the impact effects and mechanism of the Chlorine/UV process for ammonia removal.The chlorine:nitrogen mass ratio,UV doses and pH played the important roles for ammonia removal.The natual orgnic matters?NOM?and bicarbonate restricted ammonia removal.The effect of chloride concentration on ammonia removal could be ignored.The mechanism of ammonia removal in the Chlorine/UV process were the photolysis of chloramine and the oxidation of ammonia by intermediate radicals.UV irradiation can not photolysis ammonia directly.Chlorine reacted with ammonia to form chloramine,which presented better photo sensitivity than ammonia.The molar absorption coefficient and quantum yield of chloramine were 354 M^-1cm^-1 and 0.68 mol·E^-1,respectively.Based on steady-state assumption,a pseudo first order kinetic model was established,which could be used for predicting the reaction rate and estimating the contribution of radicals.The reaction rate constant for chlorine radical and ammonia was determined to be 5.6×108 M^-1s^-1.Chloramine photolysis lead to the formation of amino radical and chlorine radical,and chlorine radical could further generate hydroxyl radical.Chlorine radical and hydroxyl radical react with ammonia to form amino radical,which would further react with dissolved oxidant and generate nitrite,nitrate,as well as nitrous oxide.Amino radical transformed into nitrogen at the absent of oxygen.This study further investigaed the efficiency of the Chlorine/UV process for DBPs control.Compared with breakpoint chlorination,the Chlorine/UV process presented a better NOM removal efficiency,shifting big molecular weight NOM to smaller one.In spect of DBPs,the generation of THMs(31.4 ?g·L^-1)and HAAs(33.8 ?g·L^-1)in the Chlorine/UV process was significantly lower than breakpoint chlorination(65.1 ?g·L^-1 and 43.0 ?g·L^-1,respectively).The generation of DCAN(4.70 ?g·L^-1)was higher than breakpoint chlorination(2.60 ?g·L^-1),whereas the value lower than the guidline of WHO(20 ?g·L^-1).The concentration of NDMA generated in the Chlorine/UV process was less than determination limit(1.0 ng·L^-1).Chlorine dosages and UV doses present significant effects on DBPs formation.The increase of p H promoted THMs formation,but inhibited HAAs and HANs formations.Ammonia leads less THMs and HAAs formation but more HANs generation.NOM obviously promote the DBPs formation.There is a similar trend of DBPs and DBPs formation potential?DBPs-FP?in chlorination and the Chlorine/UV process for nomal water treatment.However,in ammonia containning water treatment,the Chlorine/UV process generated less DBPs but more DBPs-FP than chlorination.Acidic condition promoted the formation of DBPs and DBPs-FP.According to the central composite design,this study established a regression model of three parameters?chlorine:ammonia mass ratio,UV doses,and p H?and two responses?ammonia removal rate and DBPs formation rate?.The two proposed models was reliable with R2 of 0.998 and 0.994,respectively.When the target responses was set as 50% of ammonia removal rate and minimum DBPs formation rate,the optimization parameters of the Chlorine/UV process were chlorine and nitrogen mass ratio of 3.2:1,UV doses 121 m J·cm-2,and p H 7.8,then,21.7 ?g·L^-1 of DBPs was generated.The deviations between model predicted value and experimental value were less than 2%.Under this condition,the electrical cost was 0.025 yuan RMB and chemical cost was 0.006 yuan RMB per cubic meter water.In conclusion,the Chlorine/UV process presents a significant merit for ammonia containing water treatment,in terms of the ammonia removal and DBPs control.The Chlorine/UV process has a bright application future in water treatment plants.