The Reduction Of Multiple Biotoxicity Posed By Secondary Treatment Effluent In The UV Photocatalysis And Ozone Oxidation Processes

After the secondary treatment of the sewage treatment plant,the effluent still contains many biotoxic substances such as refractory organic matter,and the secondary treatment water discharge is the main contributor to the detection of biologically toxic substances in surface water bodies.In this study,UV photolysis,UV/TiO₂photocatalysis and O₃ oxidation processes were used to further improve the water quality of the secondary effluent from the wastewater treatment plant.The variation of three physicochemical indicators,including UV₂₅₄,total organic carbon(TOC)and three-dimensional fluorescence spectroscopy(EEM)were investigated.In addition,the removal of three biological toxicity from secondary effluent,including the fluorescence inhibition toxicity using the luminescent bacteria,the SOS/umu genotoxicity and the photosynthetic inhibition toxicity using Chlorella,are studied in the UV photolysis,UV/TiO₂ photocatalytic reaction,and O₃ oxidation processes.The relationships between the physicochemical indicators and biotoxicity were analyzed.The aim is to seek technical solutions for further advanced treatment of secondary effluent.The main contents and results of the thesis are as follows:(1)In the irradiation intensity of 3.17 mW/cm²,the UV₂₅₄54 and TOC of secondary effluent was reduced by 31%and 21%,respectively,during the UV photolysis(without the catalyst of nano-TiO₂)of 8 h.In this processthe SUVA was reduced by 43%,which indicated that the biodegradability of secondary effluent was improved.The fluorescence intensity of the secondary effluent was significantly reduced.Protein I,protein II,fulvic acid,microbial metabolites,and humic acids were reduced by 49%,65%,44%,34%,and 44%,respectively,indicating that UV photolysis has a certain efficiency on removal of dissolved organic matter.The total removal of trace organic pollutants in the UV photolysis process was 72%,among which the removal of estrogens,pharmaceuticals and herbicides was removed with high efficiency,and insecticide and the flame retardant was removed with relatively low efficiency.UV photolysis can not effectively reduce the fluorescence inhibition toxicity of secondary effluent,but it can effectively reduce genotoxicity and photosynthetic inhibition toxicity,whose reduction rate is 61%and 81%,respectively.This indicated that fluorescence inhibition toxicity was a relatively stubborn effect to be removed.(2)The adsorption effect of nano-TiO₂ on the reduction of biological toxicity was investigated.In the concentration range of 0²00 mg/L TiO₂,TEQ_phenol,TEQ_4-NQO-NQO and TEQ_diuroniuron did not change significantly,indicating that the adsorption of TiO₂ hardly affected the fluorescence inhibition toxicity,genotoxicity and photosynthetic inhibition toxicity from secondary effluent.The UV/TiO₂ photocatalytic experiment was carried out at a low irradiation intensity of 3.17 mW/cm².The degree of mineralization was less than 10%.The UV₂₅₄54 and SUVA was reduced by 64%and 61%,respectively.Protein I,protein II,fulvic acid,microbial metabolites,and humic acids were reduced by 57%,69%,53%,40%,and 51%,respectively.The total removal of trace organic pollutants was 81%in the UV/TiO₂ photocatalysis process.Of the trace organic pollutants,herbicides and pharmaceuticals was effectively removed,and insecticide and the flame retardant was removed with low efficiency.The reduction rates of fluorescence inhibition toxicity,genotoxicity and photosynthetic inhibition toxicity are 38%,84%and 80%,respectively.When the catalyst TiO₂ was added,the removal of fluorescence suppression toxicity was improved.After the irradiation intensity was increased to 5.16mW/cm²,the reduction rate of fluorescence inhibition toxicity,genotoxicity and photosynthetic inhibition toxicity was further improved,especially the fluorescence inhibition toxicity,whose reduction efficiency increased from 38%to 62%.In addition,the removal of UV₂₅₄,SUVA and trace organic pollutants were improved after increasing the irradiation intensity.The insecticide and the flame retardant plasticizer who were removed with low efficiency under UV photolysis,can be up to 71%and94%removal under high irradiation intensity.(3)Under O₃ oxidation processes,degree of mineralization was low(<10%);UV₂₅₄54 and SUVA can be reduced by 64%and 61%,respectively.when the maximum ozone consumption is 1.31 g O₃/g DOC,the peak in three-dimensional fluorescence spectrum eventually disappeared.Protein class I,protein class II,fulvic acid,microbial metabolites,and humic acids were reduced by 66%,95%,84%,48%,and 83%,respectively,indicating the significant removal of dissolved organic matter.Meanwhile,the removal rate of seven categories of trace organic pollutants was generally up to 95%,of which the removal of estrogens,herbicides,fungicides,medicines and flame retardant was significant and the removal of insecticides and UV absorbers was relatively inferior.The reduction rate of fluorescence inhibition toxicity,genotoxicity and photosynthetic inhibition toxicity was 86%,98%and 92%,respectively.(4)Under UV photolysis process,there was a good correlation between genotoxicity,photosynthetic inhibition toxicity and physicochemical index(fluorescence intensity and UV₂₅₄)of secondary treated effluent.Under UV/TiO₂photocatalysis and O₃ oxidation processes,the fluorescence inhibition toxicity,genotoxicity and photosynthetic inhibition toxicity from the secondary treated effluent were correlated with the physicochemical index(fluorescence intensity and UV₂₅₄)in a certain degree.This provides a basis for the control of biological effects from secondary effluent and the protecting ecological safety.