Removal Of Algae And The Impact On Subsequent Formation Of Disinfection Byproducts Using UV/Persulfate Process

With the rapid development of China's industrialization and urbanization,the discharge of industrial and domestic wastewater continuously increased,subsequently leading to the rise of nitrogen and phosphorus nutrients,and finally resulting in eutrophication in lakes and reservoirs.Then many phytoplankton,especially algae,would overgrow and even form blooms,causing serious pollution of water environment.Algae were usually difficult to be effectively removed by conventional water treatment due to its small volume.Moreover,both algae cells and algal organic matter could form poisonous and harmful disinfection byproducts(DBPs)during chlorine disinfection,which would endanger human health.Therefore,it is necessary to develop novel efficient water treatment technologies for the removal of algae.Microcystis aeruginosa was selected as target here to help investigate the removal performance and mechanism of algae by ultraviolet/persulfate(UV/PS)process.Furthermore,UV/PS was also used as preoxidation process to assist coagulation on the removal of Microcystis aeruginosa,along with evaluating the formation of DBPs during the subsequent chlorine disinfection.On basis of these works,comparative study of UV/PS and other oxidation processes on algae removal and the changes of DBPs formation in the treatment of real algae-containing waters were investigated.The main research contents and results were as follows:(1)UV/PS process was used to directly remove Microcystis aeruginosa.The removal performance and mechanism were investigated through evaluating the effects of PS dose,U-V intensity,initial pH,algae cell concentration and anions,and analyzing the characterization of algae cells and extracellular dissolved matter.The results showed that the UV/PS could effectively inactivate and destruct Microcystis aeruginos,which was significantly enhanced with the increase of PS dose or UV intensity except anions.Higher removal efficiency was achieved for algae cells in decline phase than those in logarithmic phase.UV irradiation and formed reactive radicals(SO4·-and HO·)contributed to the destruction on cell wall and cytoplasm together,along with the release of intracellular organic matter.(2)UV/PS process preoxidation to improve coagulation-sedimentation on the removal of Microcystis aeruginosa was also studied in this project.To make clear the removal performance and mechanism,the effects of PS dose,UV dose(preoxidation time),initial pH,algae cell phase and extracellular organic matter were studied,along with monitoring the characterization of algae cells and extracellular dissolved matter.The results showed that UV/PS preoxidation significantly enhanced the removal of algae in coagulation-sedimentation,with the optimal PS dose and UV dose being determined to be 60mg/L and 375 mJ/cm2,respectively,under trial conditions.Much higher removal efficiency of algae cells was achieved in logarithmic phase than that in decline phase.Both UV irradiation and formed reactive radicals(SO4·-and HO·)contributed to the improvement of coagulation-sedimentation through the decrease of the absolute value of zeta potential and the impairment of algae cells to release intracellular organic matter which would act as coagulation aid.(3)Chlorination disinfection of the algal solution treated by the combined process of UV/PS preoxidation and coagulation-sedimentation was conducted to investigate the formation of disinfection by-products(DBPs).The effects of PS dose,UV dose,initial pH,bromide and ammonium ions were studied.The results showed that compared to coagulation alone,UV/PS preoxidation overally resulted in the increase of carbonaceous DBPs(C-DBPs)and nitrogenous DBPs(N-DBPs)formation despite decreased several DBPs.Most formed C-DBPs and N-DBPs rose with the increase of PS dose or UV dose(preoxidation time).Combing the aforementioned characterizations of algal cells and the derived organic matter.the primary mechanism was proposed that UV/PS preoxidation not only could destruct cell wall and cytoplasm to release intracellular organic matter which are important precursors of DBPs,but also could produce some precursors containing?-keto or amine and some other intermediate products(aldehydes and ketone).Moreover,both bromide and ammonium ions in waters were also found to remarkably affect the formation of C-DBPs and N-DBPs.(4)UV/PS oxidation and preoxidation were used to remove Microcystis aeruginosa in real waters in comparison with some other usually used oxidation processes including UV/H2O2,NaClO and KMnO4.The results showed that higher removal performance was achieved in ultrapure water than that in real water.The removal efficiency of selected technologies was declined in the order of UV/PS>UV/H2O2>NaClO at a same dose of 2.1 mM.For the preoxidation process,all the four selected technologies achieved approximate performance when chemical dose was fixed at 0.25 mM.Compared to coagulation alone,NaClO and UV/PS preoxidation could increase the formation of most DBPs in subsequent chlorination,while UV/H2O2 and KMnO4 preoxidation remarkably decreased DBPs formation.The economic cost of using UV/PS process in treatment of algal-containing water was also analyzed.For UV/PS oxidation and preoxidation processes,the average costs were calculated to be ?3.654/m3 and ? 0.36/m3,respectively.Overall,UV/PS process,a novel advanced oxidation process,can not only oxidize and remove Microcystis aeruginosa,but also can significantly enhance the coagulation efficiency of algae.Although UV/PS preoxidation would lead to the increase of most chlorinated DBPs,the formation of CH and TCP could be controlled.Thus,it speculated that UV/PS process would have application potential in the treatment of algae-containing water.