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水体富营养化和藻类水华暴发是全球性的水环境问题[1-3]。世界各地,不论发达国家还是发展中国家,都有藻类水华暴发及藻类污染现象的发生[4-8]。蓝藻水华暴发会导致水体缺氧[9-10]、水体异味[11-13]、甚至供水系统堵塞[9];有的藻类还会产生藻毒素,严重威胁水生生物、家畜以及人类的健康[14-15]。
针对蓝藻水华大规模暴发的问题,机械化打捞是一种应急治理措施。如何有效地实现藻水分离是制约蓝藻水华机械化清除技术工程化应用的关键因素之一。目前,常用的藻水分离方法有预氧化/加压+混凝沉淀法[16-17]、加压溶气气浮法[18-19]和物理过滤法,其中预氧化会引入新的物质,破坏藻细胞结构,引起胞内有机物释放[17];气浮法使藻类与气泡黏附,形成密度比水小的结合体,可借助浮力上升并去除,但存在工艺复杂、电能消耗较大等缺点[20];物理过滤法可以不添加絮凝剂等化学药剂,处理规模可根据需要灵活设计,但仅适用于藻密度较低的藻水,当藻密度较高时,筛网易被堵塞,系统易失效[21]。
蓝藻之所以难处理,是由于在自然水体中蓝藻细胞聚集成群,细胞内有伪空胞(gas vesicle,GV),群体外面由胶被包裹。伪空胞为蓝藻细胞提供浮力,使蓝藻上浮不易沉淀去除[22-23];胶被对蓝藻群体起保护作用,阻碍混凝剂结合,影响絮凝性能[24]。有研究表明,超声波能使蓝藻伪空胞破裂,进而减弱其浮力调节能力,使蓝藻细胞沉降下来[25]。目前,超声波对实验室培养藻细胞影响的机理及效果已被广泛认可,但对超声波破坏伪空胞的研究[25-27]多为定性研究(表1),而不同超声波处理条件对伪空胞影响的研究较少,且超声波处理蓝藻的技术参数没有统一的定论。
本研究以滇池蓝藻为研究对象,利用40 kHz超声波进行辐射,研究不同超声波处理条件对滇池蓝藻伪空胞、蓝藻群体沉降性能和水体水质的影响。综合考虑处理效果(藻细胞沉降率≥80%)、出水安全(超声波处理后水体中溶解性总氮、溶解性总磷浓度的变化量<5%)和经济成本等因素,得出最佳超声波处理条件,以期为超声波预处理+混凝沉淀法进行藻水分离提供参考。
超声波对滇池蓝藻伪空胞和群体沉降性能的影响
Effects of ultrasonic treatment on gas vesicles and settleability of Cyanobacteria from Dianchi Lake
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摘要: 为提高混凝沉淀藻水分离技术的分离效果,以滇池新鲜蓝藻为研究对象,研究不同超声波处理条件下,超声波对伪空胞、蓝藻群体沉降性能及水体水质的影响。结果表明:超声波对伪空胞的破坏效果满足准一级动力学规律,反应速率常数k随着超声波功率密度的增大而增大,并逐步趋于饱和;短时间低功率(5 s,5.0~16.7 W·L−1)的超声波能破坏伪空胞,改善蓝藻群体沉降性能,降低水体pH,且对水体DTN、DTP浓度的影响<5%;在保证出水安全的前提下,综合考虑处理效果和经济成本,超声波功率密度16.7 W·L−1、处理时间5 s为超声波处理滇池藻样的最佳条件,该条件下蓝藻伪空胞破坏率、沉降率分别为84%、80%。利用超声波对滇池蓝藻进行处理,确定了超声波对滇池蓝藻的伪空胞有破坏作用,可以改善蓝藻群体的沉降性能,达到大规模、无污染的藻水分离的目的,为蓝藻水华控制提供参考。Abstract: In order to improve the separate efficiency of algae contained water after coagulation-sedimentation, the fresh Cyanobacteria from Dianchi Lake was taken as a study object, and the effects of ultrasonic treatment on the gas vesicles and population sedimentation performance of these algae, as well as water quality improvement, were investigated. The result showed that the destruction of gas vesicles with ultrasound fitted the pseudo-first order kinetics model, and the first order reaction rate constant k increased with the increase of the density of ultrasonic power radiation, then it gradually reached the saturation value. In addition, a short time and low power ultrasonic treatment(5 s, 5.0~16.7 W·L−1) could break the gas vesicles to enhance the population settling performance of Cyanobacteria colonies, and the pH of waterbody decreased, the effects on DTN and DTP concentrations of waterbody were <5%. In the premise of guaranteeing the effluent safety, through a comprehensive consideration of treatment efficiency and economic cost, the optimized treatment conditions of Dianchi Lake Cyanobacteria were determined as follows: the ultrasonic power density of 16.7 W·L−1 and irradiation time of 5 s, which led to 84% gas vesicles destruction rate and 80% Cyanobacteria sedimentation rate. This study tested the destructive effect on gas vesicles of Cyanobacteria by ultrasonic waves, and the result indicated that it could promote the settlement of Cyanobacteria colonies, and could meet the purpose of large-scale and environmentally friendly algae/water separation. Thus it provided a new theoretical approach supporting the Cyanobacteria bloom control.
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Key words:
- ultrasonic treatment /
- gas vesicles /
- Cyanobacteria /
- bloom control /
- settleability /
- Dianchi Lake
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表 1 超声波对微囊藻伪空胞的影响
Table 1. Effect studies of ultrasonic treatment on gas vesicles of microcystis
表 2 采样点藻细胞种类、密度及占比
Table 2. Species, density and proportion of algal cells at sampling point
藻细胞种类 藻细胞密度/(cells·L−1) 占比/% 蓝藻门 1.23×109 96.5 绿藻门 2.70×107 2.1 硅藻门 1.70×107 1.3 合计 1.27×109 100.0 表 3 不同超声波功率密度对超声波破坏伪空胞的准一级动力学模型拟合参数
Table 3. Pseudo-first order kinetics model fitting parameters of gas vesicles destroyed by different ultrasonic power densities
q声/(W·L−1) k/s−1 R2 θ1/2/s 0.5 0.060 0.928 9 11.57 1.7 0.086 0.896 5 8.07 5.0 0.17 0.936 9 4.18 16.7 0.20 0.903 6 3.39 41.7 0.29 0.881 0 2.39 -
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