The Key Water Quality Conditions And Preliminary Mechanism Analysis For The Formation Of Black And Odorous Water Body

Black and odorous water body is a general term for water that presents unpleasant colors and/or releases unpleasant odors.With the continuous improvement of China’s urbanization level,black and odorous water bodies are ubiquitous in some urban areas in China,which have become an urgent urban water environment problem.Due to the complex formation factor,there existed many problems in actual treatment for actual black and odorous water body.Therefore,it is necessary to explore the key water quality conditions and mechanisms in the formation of black and odorous water body,so as to provide a scientific basis for its accurate management.In this study,based on the one-year water quality monitoring of Zuogan Canal,an actual black and odorous water body in a certain urban area,the formation process of the black-odor water sample was simulated in our laboratory by this Zuogan Canal black-odor bacteria liquid.The effects of organic carbon,total nitrogen（TN）,total phosphorus（TP）,ferrous iron(Fe²⁺)and sulfate(SO₄^2–)on the formation of the black-odor water sample were investigated in single factor experiments.In addition,the interaction of organic carbon,TN and TP was investigated by response surface methodology.The succession of microbial community during the formation of the black-odor water sample was studied by 16S r DNA high-throughput sequencing technology.The formation mechanism of black and odorous water body was preliminarily analyzed.The main conclusions of this study were as follows:（1）The annual average values of dissolved oxygen（DO）,oxidation and reduction potential（ORP）,ammonia nitrogen（NH₃-N）,TP and chemical oxygen demand(COD_Cr)in the Zuogan Canal,the actual black and odorous water body,were2.93 mg/L,–21 m V,5.35 mg/L,0.63 mg/L and 53.71 mg/L,respectively,which were belonged to the mild black and odorous water body and the inferior V-class surface water.The dominant phyla at the phylum level were:Proteobacteria（25.3–56.7%）,Firmicutes（11.6%–31.4%）,Bacteroidota（6.2%–23.0%）,Epsilonbacteraeota（0.7%–41.9%）and Actinobacter Ia（0.3%–9.7%）.The suitable temperature was 28℃and the suitable ratio of bacteria to liquid was 1:250 in the formation of the simulated black-odorous water sample.Moreover,all the simulated water samples with bacteria liquid appeared black and odorous phenomenon compared to the control group,indicating that the formation of the black-odor water samples was driven by microorganisms.（2）In this study,in the formation of the black-odor water sample,organic carbon sources,TN and TP were key water quality conditions,and Fe²⁺and SO₄^2–were necessary conditions.The thresholds of water quality conditions for the formation of the black-odor water sample were as follows:organic carbon source（TOC）≥100 mg/L,TN≥0 mg/L,TP≥0.1 mg/L,Fe²⁺≥0.5 mg/L,SO₄^2–≥10 mg/L.The interaction between organic carbon source and TP on the changes of ORP during the formation process of the black-odor water sample was significant.Besides,water sediments contain a large number of iron compounds and sulfate,which results in higher concentrations of Fe²⁺and SO₄^2–in natural water bodies.Therefore,organic pollutants and TP should be removed preferentially in treatment of black and odorous water bodies.（3）In this study,with the formation of the black-odor water sample,the relative abundance of Proteobacteria decreased at the phylum level,while the relative abundance of Bacteroidota,Firmicutes and Desulfobacterota increased significantly.On the genus level,anaerobic bacteria such as Macellibacteroides,Lentimicrobium,Paludibacter,Bacteroides,Desulfovibrio had become dominant bacteria genus.According to the results of canonical correlation analysis,the environmental influence factors on the microbial community of the black-odor water sample at aerobic decomposition stage,anoxic reduction stage and nutrient deletion stage were as follows:NH₃-N,TOC,odor threshold,Fe²⁺,chromaticity,p H,ORP,S^2–,PO₄^3–,and DO.（4）According to preliminary analysis of the experimental data,the formation mechanism of black and odorous water body was as follows:DO was consumed by aerobic microorganisms in the process of decomposing organic matter,which resulted in the state of hypoxia reduction of the water sample.In this state,Fe³⁺and SO₄^2–were reduced to Fe²⁺and H₂S respectively.H₂S produced a bad odor,meanwhile formed Fe S to blacken the water sample.