1、Eutrophication is one of the water pollution .It is mainly caused by human activities.When a large amount of nutrients , such as nitrogen and phosphorus ,are discharged into large lakes, estuaries, bays and other water bodies which flow slowly, it will cause a phenomenon that algae and other plankton in water multiply rapidly, dissolved oxygen decreases , water quality deteriorates and a large number of fish and other organisms die.Excess nutrients(N、P)Autotrophic organisms (especially blue-gree
2、n algae and red algae) grow quicklyOther algae species gradually reduce Plankton and other small organisms die Be decomposed by aerobic microorganismconsume dissolved oxygen in waterFish and other aquatic life die Or be decomposed by anaerobic microorganismProduce hydrogen sulfide and other gasesNitrogen content 0.20.3 ppmBiochemical oxygen demand(BOD) 10 ppmPhosphorus content 0.010.02 ppmThe number of bacteria is more than 100000 in per ml of water(pH=910) The content of chlorophylla in charact
3、erized algae 10mg/LExcess nitrogen and phosphorus in the water body mainly come from industrial waste water and domestic sewage, organic waste, livestock and poultry manure and agricultural chemical fertilizer.Engineering methodsuDigging up the lake sediments It is the main ways of reducing the self-concentration of nitrogen and phosphorus in eutrophic lake.uDeep aeration Taking artificial lake deep aeration and supplementing oxygen from time to time is conducive to inhibit the release of phosph
4、orus from sediment. uInject water Injecting water containing low concentrations of nitrogen and phosphorus into lakes can dilute nutrient concentration.Chemical methodsuCoagulation and sedimentation Many kinds of cations can cause the phosphorus to precipitate from the aqueous solution effectively , such as iron, aluminum and calciumu For example, the long lake in western Washington state is a eutrophic water body. In October 1980,people add aluminum salt into the lake to precipitate the lakes p
5、hosphate. In the summer of fourth years after the addition of aluminum salt, phosphorus concentration in the lake water became from the original 65 g/L to 30 g/L, the water quality of the lake has been significantly improved.uIn the chemical method, another way is to kill algae with algaecide. uThis method is suitable for the water body of the lake water bloom. After algaecide kills the algae, the decomposition of algae will still release phosphorus. Therefore , algae killed by algaecide should be promptly removed, or some appropriate chemicals should be added into the water body to make phosphate precipitate.Biological methodsuUse aquatic life to absorb nitrogen and phosphorus in order to remove nitrogen and phosphorus in water.uFor example, some countries try to purify the eutrophic water body with large aquatic plants , such as water hyacinth, reed , typha angustifolia and so on.
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