Mechanism And Control Of The Typical Odorous Compounds Formations During Chlorination Of Amino Acids In Drinking Water Treatment

Odor and taste issue in drinking water is one of the main reasons for complaints from consumers,and it is also an important problem that needs to be solved urgently in drinking water treatment.Our research group have found that the odor and taste issue in the outlet water from a drinking water treatment plant around Taihu Lake was very likely to be caused by odorous compounds formed during chlorination of amino acids.However,studies focused on typical odorous compounds formed from chlor（am）ination of amino acids was limited.Therefore,this study has investigated the species,mechanisms and influencing factors of typical odorous compounds formations during chlor（am）ination of free and combined amino acids.In addition,the generation of typical odorous compounds during chlorination of algal organic matters（AOM）from several dominate algal in fresh waters was studied.And the performance of different treatment processes on the removal of amino acids and control of typical odorous compounds formations during subsequent formation of amino acids was explored.This study facilitates the identification of the causes of off-flavour problems in drinking water and the development of technologies to control these problems.The formation evolutions and influencing factors of typical odorous compounds during chlor（am）ination of free and combined amino acids were explored.Results show that free Val,Leu and Ile and short oligopeptides containing these amino acids in combined forms will generate corresponding aldehyde,nitrile and N-chloroaldimine during chlor（am）ination,and using NH₂Cl as a disinfectant can reduce the formation potential of typical odorous compounds to some degrees.The structure and molecular weight of short oligopeptides,different chlorine dosages,pH conditions,the addition orders of chlorine and ammonia and the pre-chlorination contact time will impact the species and concentrations of typical odorous compounds.These typical odorous compounds have low odor thresholds atμg/L levels,and are relatively stable,which will exist in the water supply system at high concentrations for a long time and cause odorous issue in tap waters.The chlor（am）ination experiments conducted in the real water from East Taihu Lake indicated that if the amino acids in the raw water cannot be removed well before disinfection,the concentration of N-chloroaldimines and aldehydes formed during the chlor（am）ination will exceed their odor thresholds,causing odor problems in drinking water.Since algae is the main source of amino acids in natural water,four dominant algae in fresh water were selected to investigated the formation of typical odorous compounds during chlorination of AOM to further explore the source of odorous issue in drinking water.Results showed that AOM contains various and abundant free and combind amino acids,and will generate aldehydes,nitriles and N-chloroaldimine during chlorination.The chlorine dosages and reaction time will impact the formation of typical odorous compounds during chlorination of AOM.And the contribution of free and combined amino acids in AOM to the formation of typical odorous compounds was calculated respectively and compared to help evaluate the occurrence risk of odor problem in real drinking water.The free and combined Leu were selected as model compounds to explore the formation mechanism of typical odorous compounds during chlor（am）ination of amino acid.Kintecus software was used to establish the kinetic model.Results indicated that during chlorination of free amino acids,the reaction pathways are impacted by chlorine dosages.When Cl/N<1.0,free Leu produces N-chloroleucine,and then N-chloroleucine decomposes into isovaleraldehyde.When Cl/N>1.0,N-chloroleucine is replaced by chlorine to generate N,N-dichloroleucine,which then hydrolyze into isovaleronitrile and N-chloroisovaleraldimines,and N-chloroisovaleraldimines can also degrade into isovaleraldehyde and isovaleronitrile.As for combined Leu,after a series of continuous reactions including substitution,dehydrohalogenation,dehydrogenation,β-elimination,hydrolysis,and decarboxylation reactions,combined Leu produces free Leu or other free amino acid,and free Leu will be subsequently chlorinated to generate odorous isovaleraldehyde,isovaleronitrile and N-chloroisovaleraldimines.In addition,some combined Leu can also generate a large amount of isovaleronitrile throughβ-elimination reaction during chlorination.During chlor（am）ination of free and combined Leu,the active chlorine（Cl⁺）in NH₂Cl will substitutes the H⁺on the free amino group in amino acids to produce N-monochloroleucine or N,N-dichloroleucine,respectively.The subsequent reaction steps are almost the same as those during chlorination reaction.However,different from chlorination process,chloramination has its uniqueness by participating in reacting with isovaleraldehyde to produce N-chloroisovaleraldimineUltraviolet/persulfate（UV/PS）and ultraviolet/hydrogen peroxide（UV/H₂O₂）can effectively control the formation of typical odorous compounds during chlorination of free and combined amino acids,and the UV dosages,oxidants dosages,pH and chlorine dosages will impact the treatment effect.In addtion,both UV/H₂O₂ and UV/PS will effectively control the formation of typical odorous compounds during chlorination of amino acids in real waters,and the effect of UV/H₂O₂ is better.