海水淡化排放的高盐废水对海洋生态环境的影响

Influences of the high salinity wastewater from desalination plants on the marine ecological environment

盐度是海洋环境中最重要的生态因子之一,每种生物各有其适宜生长盐度要求,当环境盐度超过该范围时,生物体的生长、发育、生殖、行为和分布都会受到影响。近10年来,随着淡水资源紧缺和海水淡化处理成本的下降,海水淡化厂数量急剧增加,大量浓盐水进入海区导致受纳海域盐度升高。高浓度盐水给海洋生态环境带来许多负面影响,有关浓盐水的海洋生态安全问题备受人们关注。在总结和分析浓盐水对海洋浮游生物、底栖动物、甲壳类动物、鱼类和海草等影响的基础上,指出海水淡化厂排放的高盐废水对海洋生态的潜在威胁,浓盐水的排放使一些经济水产种类产量减少,给渔业资源和海洋经济带来损失。不同地域环境对海水淡化厂排放水的敏感程度不同,且潮流、海流和水团等水文因素以及海水淡化厂的日处理量和规模等因子决定了排放的高盐废水对海域生态影响的程度。为减少排放的高盐废水对出水口附近海域的影响,提出了海水淡化工程合理规划和布局的可行措施,要选择水体交换良好的海域作为海水淡化工程所在地,对排水口要进行因地制宜地设计,做好高盐废水的再利用工作,以最大限度地减少排放的高盐废水对海区环境的负面影响。建议对海水淡化厂附近海域进行有针对性的水质调查和监测,并做好环境风险影响的评价工作;制定及完善相关的法律法规,建立统一的高盐废水排放标准,而该标准则要以排放的高盐废水对当地海洋生态不造成较大的影响为依据,使海水淡化产业的发展有法可依。

Desalination is one of the most important ways to meet water shortage and provide basic support for local life, industry and agriculture in coastal areas. Because of the severe shortage of the fresh water resources and decreased cost of seawater desalination, desalination plants increased conspicuously in last 10 years. The salinity of wastewater discharge from reverse osmosis desalination plant is twice that of the natural seawater,and the salinity of water discharge which is mixed with the cooling water from distillation desalination plant is 10%-15% higher than the natural seawater. A large quantity of hyper-salinity water discharge leads to a great salinity fluctuation at the recipient sea area, which brings about negative effects on the marine ecological environment. Therefore, the environmental problem caused by brine discharge has brought to a wide attention. Salinity is one of the most important ecological factors in the marine environment, and every marine species has its own suitable salinity. Their growth, development, reproduction, behavior and distribution of living organisms will be all influenced when the environmental salinity exceeds the suitable range. The effects of high salinity water on marine plankton, zoobenthos, crustacea, fishes and seagrass are analyzed in this paper, and the potential threats of high salinity wastewater discharge from desalination plants are pointed out. Hypersaline water will cause the decrease of plankton biomass, species richness and diversity index, and the succession towards salinity tolerance species occurs in the community. Laboratory researches show that biochemical and physiological process of the phytoplankton are affected by salinity, and field investigation indicate that salinity difference influences the phytoplankton diversity. The high concentration brine may result in high turbidity of the seawater, prevent the penetration of light, and check the photosynthesis process. The high density hypersaline water sunk to the sea bottom makes the benthos die from the cell dehydration and turgor decrease, and alters the original bottom habitat, thus causing great damage to the benthos. The increased salt stress can result in the decline of chlorophyll content, changes of chloroplast ultrastructure, inhibition of electron flow, diminished enzyme activity and so on. In addition, salinity can influence the survival, growth and development of many economic marine animals significantly, such as shellfishes and fishes. And the same species at different development phase has different suitable salinity range, which for larvae is usually smaller than that for adults. Moreover, the adaptive ability of larvae to low salinity is greater than to high salinity. Thus, brine discharge will make the economic marine species decrease, resulting in the loss of fishery resources and marine economy. The susceptibility to salinity rise varies from place to place. And the environmental ingredients including hydrological factors like tidal current, ocean current and water mass as well as the capacity of desalination plant determine the impact extent of the brine discharge on the marine ecology. To reduce the negative effects of hyper-saline water on the vicinity area of the pipeline outlet, some feasible measures are suggested and a ＂win-win＂ policy is proposed to gain a balance between the construction of desalination plants and marine ecological protection, which include reasonable planning of project construction and operation, choosing the sea areas with good water exchange as the project site, designing the outfall according to the local conditions, and making better reuse of the brine etc. Furthermore, enhanced water quality investigation and monitoring for better environment risk assessment, as well as establishment of the standard of brine discharge for the desalination plant and relative laws and regulations for the development of desalination industry are also suggested.