Variation Of Biological Stability During Pre-oxidation Of Algae-laden Water With Chlorine Dioxide And Its Mechanism

Lakes（reservoirs）are the main sources of drinking water in our country.Nitrogen,phosphorus and other nutrients discharged into water-body become increasingly pronounced due to human activities,resulting in water eutrophication and algae blooming frequently.Cyanobacteria and chlorella in lakes（reservoirs）grow rapidly in summer and autumn,as a consequence,water treatment processes and qualities can be adversely affected by algae cells and their metabolic products.Those problems are summarized as following: coagulant demand increasing,filter clogging,production of toxins as well as taste and odor substances.In addition,it leads to the increase of carbon source which can facilitate the growth of microorganisms,and then decreases the biostability of drinking water.With the purpose of enhancing algae removal,surface morphology and zeta potential of the algae are changed by pre-oxidation,achieving alga removal by means of enhancing coagulation.However,pre-oxidation can destroy the cellular structure,release undesirable intracellular organic matter which causes the formation of assimilable organic carbon（AOC）,and then result in the blooming of pathogenic microorganisms.Chlorine dioxide is widely used as an oxidant or disinfectant in water treatment,the formation of AOC in the process of oxidizing algae-laden water can be used to quickly predict the biological stability of drinking water.In this paper,microcystis aeruginosa and chlorella that cultivated in laboratory are used as the test object.This thesis is to study the variation and the main mechanism of assimilable organic carbon（AOC）during pre-oxidation of algal-laden water,extracellular organic matter（EOM）and intracellular organic matter（IOM）withchlorine dioxide and the AOC formation characterization of the actual water containing algae during chlorine dioxide oxidation.The main results are as follows:（1）During chlorine dioxide pre-oxidation of water containing cyanobacteria,the changing characteristics of AOC can be divided into two cases: when the amount of chlorine dioxide dosage were 0.5 and 1 mg/L,the change trend of AOC content were all increasing over time due to the release of IOM;when the amount of chlorine dioxide was 2 mg/L,the change trend of AOC content firstly increased and then decreased;the release of IOM was the main process before 180 seconds,and the AOC level increased continuously.As the oxidation time reached up to 180-300 s,the AOC content decreased because the high concentration of chlorine dioxide can attack IOM,which was oxidized into the organic matters that not easy to form the AOC.（2）In the process of chlorine dioxide oxidation,the cell structure and photosynthesis ability of cyanobacteria cells were destroyed.In addition,the algae cell integrity decreased rapidly,releasing the intracellular organic matter（IOM）continuously.When the algae cell integrity was reduced down to 1%,the intracellular organic matter was not released,and the intracellular matter was delayed release,the increase of extracellular DOC value could prove that algae cells were destroyed.The photosynthesis ability of cyanobacteria cells was more sensitive than their cell integrity to chlorine dioxide pre-oxidation.When chlorine dioxide oxidized the EOM and IOM of cyanobacteria,the change trend of AOC content all increased gradually and then decreased,and the high dose（2 mg/L）was faster than the low dose（0.5 mg/L）to raise the AOC to the maximum value.With the time prolonging,the high dose of ClO₂ could oxidize and degrade more AOC so that the AOC content decreased.（3）Comparatively,the AOC change trend of chlorine dioxide oxidation is increasing continuously,the AOC change trend of the chlorine oxidation firstly increased rapidly and then decreased slightly,and the maximum amount of AOC was1701 μg/L and 573 μg/L,respectively.When chlorine dioxide and chlorine oxidized the EOM of cyanobacteria,the former AOC content increased firstly and then decreased,the latter AOC content changed slightly.When chlorine dioxide and chlorine oxidized the IOM of cyanobacteria,AOC changed almost similarly,all increased firstly and then decreased,but the amount of AOC produced by chlorine77 oxidation was less.In summary,in view of maintaining drinking water biological stability,chlorine performed better than chlorine dioxide.（4）Under different chlorine dioxide dosages,the change trend of AOC showed a slight decrease after rapid increase during chlorine dioxide oxidation of chlorella-laden water,and the higher the chlorine dioxide dosage,the higher AOC amount was produced.The increase of AOC in the chlorella is less than that of the cyanobacteria,because the chlorella does not contain fulvic acid-like and humic acid-like compounds.The IOM of chlorella contained soluble bio-metabolites and aromatic structural proteins,but the EOM of chlorella contained soluble bio-metabolites only.During the oxidation of chlorella and cyanobacteria cells using chlorine dioxide,the chlorella cell membrane integrity reduced slower than that of cyanobacteria.During the oxidation of the EOM and IOM of chlorella and cyanobacteria using chlorine dioxide,AOC level showed a trend that firstly increased and then reduced.When the EOM and IOM were oxidized by chlorine dioxide,the produced AOC amount were in order: IOM of cyanobacteria > IOM of chlorella > EOM of cyanobacteria > EOM of chlorella.（5）Organic matter in actual water can consume the chlorine dioxide,which resulted in a longer time to destroy the integrity of algal cell membrane than that in phosphate buffer solution（PBS）,and the membrane of chlorella was more difficult to be destroyed than cyanobacteria.When the same species of algae were placed to the four kinds of water,the decline rate of integrity from fast to slow is as following:PBS,filtered water,sedimentated water,source water.The increased AOC amount was in order: PBS> filtered water> sedimentated water> source water,which was mainly attributed to that the algal organic matters contributing to the AOC more than the NOM in the actual water.The content of organic matter in the source water was higher and chlorine dioxide was consumed heavily,which resulted in the decrease of oxidants for the oxidation of algal cells.Therefore,the AOC increments in the algae-laden pure water was the largest and the AOC increments in the algae-laden source water was the least.