Effects Of The Decomposition Of Algal Mats On The Odorous Black Water Agglomerate Of Eutrophic Lake And Its Mechanism

The odorous black water agglomerate of eutrophic lake induced by the massive accumulation and death of algae has happened frequently in Meiliang, Gonghu, Zhushan and other bays of Taihu. However, systematic analysis and researches on occurrence conditions, particular pollutants and influencing factors of odorous black water agglomerate are still lacking, which restrict the establishment of a more scientific and rational scheme on the emergency prevention of algae-induced odorous black water agglomerate.This study was based on the field investigation of algae-induced odorous black water agglomerate of Taihu and the water quality monitoring of prone regions, where the odorous black water agglomerate frequently occurred (referred to as "OBWA-prone regions" below). The odorous black water agglomerate was simulated and its occurrence conditions were analyzed based on the responsive relationships between the variation of water quality and the formation of water stinking and. blackening. The influence mechanism of odorous black water agglomerate caused by the massive accumulation and death of algae in lake polluted by the endogenous or exogenous pollutions, and in different hydrological and hydrodynamic conditions were studied. Based on the studies above, the occurrence mechanism of algae-induced odorous black water agglomerate was analyzed systematically. The main conclusions are as follows.(1) Investigation of OBWA-prone regions. Algal species and water qualities of OBWA-prone regions were monitored and analysed throughout the year of 2011. The results showed that the algae densities of OBWA-prone regions were extremely high during summer and autumn, the dominant species was Microcystis aeruginosa, and the average degree of dominance was 0.91. The explosive growth of Microcystis aeruginosa during summer and autumn played an important role in the massive accumulation of algae in OBWA-prone regions. The annual average concentrations of CODMn, NH4+-N, TN and TP of OBWA-prone regions were 10.81 mg/L、1.62 mg/L、5.37mg/L、0.32 mg/L, respectively, which were about 2.6,7.4,2.7,4.6 times of the average concentrations of the whole lake throughout the year.(2) Effect of the algae decomposition on the odorous black water agglomerate. A series of water samples with different algae densities were set to simulate odorous black water. The variations of odorous matter, physical and chemical indicators, organic matter and nutrients, the correlation between algae density and maximum of odorous matter, and the occurrence conditions of the odorous black water agglomerate were analyzed. The results showed that, in the static and dark condition, at a constant high temperature, the water with a density of 1.0×10 cells/L without sediment and exogenous pollutions would emit an appreciable odor. When the algae density reached 1.0×108cells/L or exceeded 1.0×108cells/L, the odorous black water agglomerate occurred. Dimethyl trisulfide (DMTS) was the primary contributor to the odor emission, followed by β-Ionone. There was a time difference between the water stinking and blackening with the stinking first. Although stinking was not always accompanied by blackening, blackening was often accompanied by stinking. Transformation of sulfur forms played a key role in the water stinking and blackening. When the ORP value was between -250 and -50mV, along with the death and decomposition of cyanobacteria, DMTS and other odorous organics were produced. When the ORP value was between -400 and -300mV, waters showed an extremely strong reducing environment and organic sulfides further transformed to inorganic sulfides, which led to water blackening. The maximum concentration of DMTS showed significant correlation with the initial density of algae. The higher the algae density was, the higher the maximum concentration of DMTS was. The relationship between DMTS concentration and the density of algae fitted linear and quadratic model.The massive accumulation and death of algae had direct influence on the concentrations of organic matter, nitrogen and phosphorus. When the algae density reached 1.0×108 cells/L, along with the death and decomposition of cyanobacteria, the concentrations of CODMn、 NH4+-N、TN、TP reached 19.8mg/L,8.5mg/L,10.7mg/Land 1.1mg/L, respectely. Water quality was seriously deteriorated.(3) Effect of sediment on the odorous black water agglomerate. Under the conditions with sediment or without sediment, odorous black water agglomerate was simulated. Physical and chemical indicators, blackening and odorous matter, distribution of microorganisms in the sediment and conventional indicators were monitored to reveal the influence of sediment on the formation of algae-induced odorous black water. The results showed that, the massive accumulation of cyanobacteria was the main basis for the formation of odorous black water agglomerate, and sediment was not the decisive factor. Under the condition of the same algae density, sediment would impel the odorous black water agglomerate occur ahead of time. On the one hand, sediment would accelerate the formation of odorous sulfur organic. On the other hand, the anaerobic environment due to the massive accumulation and death of cyanobacteria made the increase of the number of sulfate-reducing bacteria in the sediment, which induced the reduction of sulfur and iron and exacerbated the odorous black water agglomerate. During the period of water blackening, sulfide and Fe2+ concentration increased by 0.63mg/L and O.lOmg/L in the sediment-water-algae treatment, compared to the water-algae treatment.(4) Effect of flow velocity on the odorous black water agglomerate. Odorous black water agglomerate was simulated when the flow velocities were 1～2cm/s and 5～6cm/s. Flow velocity had an important influence on the formation of odorous black water and showed different influences on water stinking and blackening. As the velocity increased, DMTS concentration decreased. In terms of water blackening, compared with 5～6cm/s, when the flow velocity was 1～2cm/s, the anaerobic reducing environment was unchanged, the generation of inorganic sulfides and the releasing of Fe2+ from sediment were promoted, which aggravated water blackening.(5) Effect of temperature on the algae decomposition and odorous black water agglomerate. Different algae densities were set under different temperatures and the effect of temperature on the algae dying was studied. The increase of temperature had a significant promoting effect on the decomposition of cyanobacteria. While temperature increasing, the dying rate of high density algae increased and 60% of the whole cyanobacteria decayed in early stages, which illustrated the odorous matter which had a close relationship with algae dying, generated during the early stages. The external temperature was also an important factor on water stinking and blackening. The increase of temperature would promote the generation of DMTS.(6) Effect of exogenous pollution on the odorous black water agglomerate. The experimental results of the influence of exogenous pollution on the formation of algae-induced odorous black water agglomerate indicated that the domestic wastewater and the industrial wastewater had a significant improving effect on the formation of algae-induced odorous black water agglomerate. The function of the chemical industrial wastewater was more distinct than the domestic wastewater. The domestic wastewater and the chemical industrial wastewater led to the blackening earlier and increased the quantity of odorous matter.This study explored the formation mechanism and the occurring conditions of the algae-induced odorous black water agglomerate and analyzed the influence of endogenous and exogenous pollution and hydrology on the algae-induced odorous black water agglomerate. These analyses provided theoretical support for the early warning of algae-induced odorous black water agglomerate and contributed to the construction of a prevention system.