Chen Changjie, a researcher at the Institute of Environmental and Health-related Product Safety of China Center for Disease Control and Prevention, further pointed out: "Cyanobacteria need more nutrients.
Generally, there is a lot of nitrogen in water, and other nutrients are enough, but phosphorus is scarce. Once a large amount of phosphorus appears in the water, it is natural for cyanobacteria to erupt. So phosphorus is the decisive factor. "
When it comes to water eutrophication, people usually think of industrial pollution immediately. In fact, the environmental protection department of China has also formulated a series of policies and regulations to ensure that the contents of nitrogen and phosphorus in industrial emissions are within the range permitted by the environment.
The problem is that industrial emissions are not the only channel of nitrogen and phosphorus pollution. "Pollution is a comprehensive problem. People pay more attention to industrial pollution, which is easy to find and therefore easy to control.
Agricultural pollution is not well known to the public, but it is also very serious. There are many farmlands around Taihu Lake. Fertilizers and pesticides used in agricultural production either enter Taihu Lake through soil infiltration into groundwater, or are directly discharged into rivers and flow into Taihu Lake.
Fertilizers and pesticides are rich in nitrogen and phosphorus, which is an important reason for the eutrophication of Taihu Lake. In addition, domestic pollution has also accelerated the eutrophication of toxins. Organic wastes discharged by local residents, such as feces, are washed into sewers with sanitary equipment and discharged into rivers and lakes after certain treatment. It also contains a lot of nitrogen and phosphorus.
Li also believes that cyanobacteria outbreaks in Taihu Lake, Chaohu Lake and Dianchi Lake in recent years are closely related to the population explosion around the lake in recent years.
Cyanobacteria release odor when they rot. This smell includes two gases, "one is the smell of earth that smells like earth, and the other is 2- methyl isoborneol that smells like 666." As long as the concentration is very small, they can produce stench. "Liu Wenjun said.
Odor and foul smell can make people feel uncomfortable, but the study has not found any substantial harm to human body. In the process of purification, activated carbon is generally used to adsorb these two odorous substances, but the effect is limited. At present, there is also ozone plus activated carbon adsorption method, which has a good effect, but the price is higher.
Microcystins are not that simple. Although cyanobacteria can't parasitize humans or animals and cause diseases, they may produce a series of natural toxins that are harmful to human health.
These toxins usually exist in the cell membrane of cyanobacteria, so when cyanobacteria are alive, they are fine. When they die and rot, the cell membrane breaks and microcystins are released into the water.
Microcystins can be produced by Microcystis, Anabaena, Tremella and other species in cyanobacteria. Among more than 80 microcystins identified so far, microcystin -LR is the most common and toxic one.
Shanghai Medical University has done experiments on the damage of microcystins to mice. The results show that about 70% of microcystins will gather in the liver after being transported through the small intestine and liver, resulting in organ damage.
Chen Changjie pointed out: "Animal experiments show that microcystins may cause cancer in mice, but the impact of this situation on human body is still unclear." Some researchers also believe that this damage may not be directly caused by microcystins, but by other organic substances contained in polluted water.
For safety reasons, the World Health Organization regards microcystin -LR as one of the criteria for judging the quality of drinking water. 1 in July, the country promulgated a new drinking water standard, referring to the relevant standards of the World Health Organization: the content of microcystin -LR in each liter of water should not exceed 1 microgram.
What is alarming is that even the drinking fountains we usually use will produce cyanobacteria. Liu Wenjun said that if there is too much organic matter in the filter of the water dispenser, cyanobacteria may accumulate, so it should be replaced once every six months. He also believes that it is hard to say what harm there is before the new drinking water standard is formulated.
However, there is no need for the public to panic about microcystins. Even if the concentration of microcystins in water source is high, follow-up water treatment and other measures can be taken to ensure that the content of microcystins in tap water meets the new national standard. As for some aquatic products with high content of microcystins, it is possible to cause potential health hazards only if they are eaten in large quantities for a long time.
Microcystins can be produced by Microcystis, Anabaena, Tremella and other species in cyanobacteria. Among more than 80 microcystins that have been identified, microcystin -LR is the most common one. The new drinking water standard stipulates that the content in each liter of water should not exceed 1 microgram.
Once the cyanobacteria break out, salvage is the first step of emergency treatment. Therefore, we often see such a scene recently: water as thick as paint is scooped out of rivers and lakes by spoonfuls.
"Salvaging cyanobacteria floating on the water surface is only an emergency measure and can only produce invisible effects. Most of cyanobacteria exist in water and sediment and cannot be salvaged. " Li said to him. This method of mechanical algae fishing is mainly used in local areas where cyanobacteria break out.
Another emergency method is "physical absorption". Adding clay to water can make cyanobacteria adsorb on clay particles and make them sink to the bottom of water. "But this is a temporary solution, because cyanobacteria still live underwater and will not die. It will continue to erupt. "
There are more advanced ways to kill cyanobacteria, such as crushing cyanobacteria cells with ultrasonic waves, killing cyanobacteria cells with radiation, or affecting the activity of cells with electric fields.
"There are also electromagnetic waves and ultrasonic waves sinking cyanobacteria abroad, but even this will cause secondary pollution, because cyanobacteria are still alive and dead cyanobacteria will release toxins."
Chemical methods can also be used to remove cyanobacteria. For example, put algaecide or flocculant into water. However, these chemicals may pollute lakes, so there are high requirements for chemicals: high efficiency, low toxicity or non-toxicity, no pollution, no corrosion, low cost, safe production and transportation, and convenient administration.
Among qualified chemicals, copper sulfate can kill cyanobacteria, and alum and lime can make it sink.
Because the cells of cyanobacteria are broken after being killed, which leads to the release of microcystins, chemicals with toxin adsorption function must be used at the same time. Some smaller lakes can replace the mud bottom containing cyanobacteria by changing the lake bottom to prevent the release of microcystins.
Compared with the elimination of cyanobacteria, it is more difficult to restore the damage caused by cyanobacteria to the surrounding ecology. Therefore, ecological restoration has become the focus of cyanobacteria control research. "In ecological restoration, aquatic plants can be planted to absorb nutrients in water and compete with cyanobacteria."
Li said to him, "But problems have also arisen. Plant aquatic plants, and the aquatic plants will soon die, because the water body propagated by cyanobacteria is very opaque, or even completely opaque. Because photosynthesis cannot be carried out, it is difficult for aquatic plants to grow. "
Some experts also envisage the elimination of cyanobacteria through traditional fish farming. For example, increase filter-feeding fish and zooplankton that eat algae, such as silver carp. But this method also has limitations.
If there are fewer fish, it is difficult to kill cyanobacteria in a large area, but if the density of fish is too high, it will also lead to new pollution. And it is not easy for fish to compete with cyanobacteria.
"As a result of the outbreak of cyanobacteria, the oxygen content in the water will decrease due to the increase of organic matter, and the fish will die of lack of oxygen, so you can't raise too much." Kong Fanxiang said that fish, as a link in the food chain, may also spread algal toxins from the food chain to people.
The more advanced ecological control method is to directly attack cyanobacteria cells with microorganisms, which is very effective, but the disadvantage is that it is expensive.
Some people say that cyanobacteria is an "ecological cancer", while others say it is a "green psoriasis". Once infected, it is difficult to remove it. "Cut off the pollution source first, and then restore the water source of the lake, including ecological restoration and physical restoration."
Liu Wenjun said, "The most fundamental way is to control pollution sources, change the mode of economic growth and change the mode of industrial structure." It is more important to strengthen environmental early warning to prevent problems before they happen.
Some people say that cyanobacteria is "green psoriasis", and once it is obtained, it is difficult to remove it. First, cut off the pollution source, and then restore the water source of the lake, including ecological restoration and physical restoration. The most thorough way is to control pollution sources, change the mode of economic growth and change the industrial structure model.