Eutrophication is a water pollution phenomenon caused by excessive content of plant nutrients such as nitrogen and phosphorus. Under natural conditions, with the continuous deposition and deposition of impactors and aquatic debris carried by rivers at the bottom of the lake, the lake will transition from a flat nutrient lake to a eutrophic lake, and then evolve into a swamp and land, which is an extremely slow process. However, due to human activities, after a large number of plant nutrients in industrial wastewater, domestic sewage and farmland runoff are discharged into lakes, reservoirs, estuaries, bays and other slow-moving water bodies, aquatic organisms, especially algae, will multiply in large numbers, changing the species and quantity of biomass and destroying the ecological balance of water bodies. A large number of dead aquatic organisms are deposited at the bottom of the lake, decomposed by microorganisms, and consume a lot of dissolved oxygen, which drastically reduces the dissolved oxygen content in the water body and deteriorates the water quality, thus affecting the survival of fish and greatly accelerating the eutrophication process of the water body. When the water body is eutrophic, due to the massive reproduction of plankton, the water body often appears blue, red, brown and milky white. This phenomenon is called water bloom in rivers and lakes and red tide in the ocean. In the waters where red tides occur, some plankton multiply explosively, which makes the water turn red, so it is called "red tide". These algae stink and are poisonous, so fish can't eat them. Algae block sunlight, causing underwater plants to die because of photosynthesis, and releasing nutrients such as nitrogen and phosphorus after corruption, which can be used by algae. In this way, the years are long, resulting in a vicious circle, algae multiply in large numbers, water quality deteriorates and stinks, leading to fish death.

[Edit this paragraph] Case

(1) East Lake in Wuhan, West Lake in Hangzhou, Xuanwu Lake in Nanjing, Daming Lake in Jinan and Dahuofang Reservoir in Fushun in China are all affected by eutrophication. In recent years, red tides have also occurred from time to time in the coastal areas of China. For example, from August to September of 1989, a large-scale red tide rarely seen in the world appeared in the coastal areas from Huanghua County, Hebei Province to Tanggu, Tianjin, which caused serious losses to the shrimp industry.

(2) In recent years, with the increase of sewage discharge in the surrounding areas of Taihu Lake, water eutrophication is becoming more and more serious, and water blooms occur frequently in summer. In severe cases, green algae cells cover the whole water body, water plants stop water supply, and residents in water villages drink sewage. At the same time, the contents of organic matter and ammonia nitrogen in the water seriously exceed the standard, especially the dissolved DOC organic matter accounts for 88% of the total organic matter COD. Because the conventional drinking water treatment process itself has the weaknesses of organic micro-pollutants, ammonia nitrogen and so on. It can not be completely and effectively removed. Chlorination process can not effectively eliminate pathogenic microorganisms such as oxygen-resistant pathogenic parasites in domestic water, and it also leads to the formation of organic chlorides that are more harmful to human health. It is difficult to guarantee the safety of treated drinking water. Ozone biological activated carbon technology uses ozone oxidation and biological activated carbon filter, combined with ozone chemical oxidation, physical and chemical adsorption of activated carbon, biological oxidation degradation and other technologies to remove trace organic substances, chlorine-containing disinfectant by-products and other organic indicators in raw water, and improve the safety of drinking water.

[Edit this paragraph] Water eutrophication

The eutrophication process of water body is closely related to the content of nitrogen and phosphorus and the proportion of nitrogen and phosphorus. Total nitrogen and total phosphorus, chlorophyll a and transparency reflect the level of nutrients, species and quantity of organisms respectively, and they are used as a set of indicators to control lake eutrophication. It has been reported in literature that when the total phosphorus concentration exceeds 0. 1mg/l (if phosphorus is the limiting factor) or the total nitrogen concentration exceeds 0.3mg/l (if nitrogen is the limiting factor), algae will be overproduced. The Organization for Economic Cooperation and Development (OECD) has put forward several indicators of eutrophic lakes: the average total phosphorus concentration is greater than 0.035 mg/L; The average chlorophyll concentration is greater than 0.008 mg/L; The average transparency is less than 3 meters.

[Edit this paragraph] Prevention

To prevent eutrophication, we must first control the entry of nutrients into water bodies. Measures such as dredging sediment, removing aquatic plants and algae, introducing low-nutrient water for dilution and implementing artificial aeration can be taken to control eutrophic water bodies.

[Edit this paragraph] status

The eutrophication trend of lakes, reservoirs and rivers in China is very rapid. During the period of 1978 ~ 1980, most lakes were in a medium trophic state, accounting for 9 1.8% of the investigated area, 3.2% were in a poor trophic state and 5.0% were in a eutrophic state. In a short period of 10, most of the lakes in poor trophic state have transited to those in middle trophic state, and the proportion of lakes in poor trophic state has rapidly decreased from 3.2% to 0.53%, while lakes in middle trophic state have transited to eutrophic state, and the proportion of lakes in eutrophic state has sharply increased from 5.0% to 55.01%[/kloc-] In recent 10 years, the water quality in the lower reaches of Hanjiang River in Wuhan has deteriorated sharply, showing eutrophication. In the 1990s, there were two "water blooms" in which algae proliferated rapidly.

[Edit this paragraph] Injury

The harm of water eutrophication is mainly manifested in three aspects.

(1) Eutrophication reduces the transparency of water, making it difficult for sunlight to penetrate the water layer, thus affecting the photosynthesis of plants in water and the release of oxygen. At the same time, the reproduction of plankton consumes a lot of oxygen in the water, which makes the dissolved oxygen in the water seriously insufficient, and the photosynthesis of plants on the surface may cause local supersaturation of dissolved oxygen. Supersaturation and low dissolved oxygen in water are harmful to aquatic animals (mainly fish), resulting in a large number of fish deaths. (2) Harmful gases produced by the decomposition of organic matter accumulated at the bottom of eutrophic water under anaerobic conditions, and biotoxins produced by some plankton (such as saxitoxin) will also do harm to aquatic animals.

(3) The eutrophic water contains nitrite and nitrate, and people and animals will be poisoned and sick if they drink water with these substances exceeding a certain standard for a long time.

Eutrophication of water often leads to disorder of aquatic ecosystem, decrease of aquatic species and destruction of diversity. The water quality of Dianchi Lake in Kunming was in a state of poor nutrition in 1950s and in a state of rich nutrition in 1980s. The species of macrophytes decreased from 44 in 1950s to 20, phytoplankton from 87 to 45, and indigenous fish from 15 to 4. When the water bloom occurred in Hanjiang River 1992 in Wuhan, the diversity index of algae population also showed a downward trend. The ubiquitous eutrophication has caused serious damage or even complete loss of various water use functions. Due to the phenomenon of water bloom in the lower reaches of the Hanjiang River in Wuhan, Hanchuan Waterworks was forced to close, the purification process of Zongguan Waterworks was difficult, the backwash increased, and the cost of water production increased. In addition, because algae have obvious fishy smell, it affects the quality of drinking water. Toxins produced by algae will endanger the health of human beings and animals.

[Edit this paragraph] Reason

When it comes to eutrophication, people often think that total nitrogen and total phosphorus exceed the standard. It is true that nutrients such as total nitrogen and total phosphorus are necessary conditions for eutrophication. If the concentration of total nitrogen and total phosphorus in water is very low, eutrophication is impossible. On the contrary, the increase of total nitrogen and total phosphorus concentration in water does not necessarily lead to eutrophication. The occurrence and development of eutrophication is a process in which the whole environmental system of water is out of balance, which leads to the growth and reproduction of some dominant algae. Therefore, in order to study the mechanism and conditions of eutrophication, it is essential to understand many differences of algae. Eutrophication will have different symptoms, that is, different dominant algae populations will appear and various types of aquatic organisms will be unbalanced. However, the necessary conditions for eutrophication are basically the same, and the most important influencing factors can be summarized as follows:

① Nutrient salts such as total nitrogen and total phosphorus are relatively sufficient;

② The contents of iron and silicon are moderate;

③ Suitable temperature, illumination conditions and dissolved oxygen content;

(4) The water flow pattern is slow and the water change period is long.

Only when the above four conditions are suitable, some dominant algae species will "grow wildly" and become eutrophic.

Two Theories on the Causes of Eutrophication

1. food chain theory:

This was put forward by martin shaw Dunn, a Dutch scientist, at the "Phosphate Technology Seminar" in June 1997.

According to this theory, there is an aquatic food chain in natural waters. If the number of plankton decreases or the predation ability decreases, the growth of algae will exceed consumption, the balance will be broken and eutrophication will occur. This theory shows that the increase of nutrient load is not the only reason for eutrophication.

2. Life cycle theory:

This is a widely accepted theory in recent years.

It believes that too many nitrogen-containing and phosphorus-containing compounds are discharged into the water body, which destroys the original ecological balance, causes algae to multiply in large numbers, consumes too much oxygen in the water, causes fish and plankton to die of hypoxia, and causes corpse decay, resulting in water pollution. According to this theory, excessive discharge of nitrogen and phosphorus is the root cause of eutrophication, and algae is the main body of eutrophication, and its growth rate directly affects water quality. Under the suitable conditions of illumination, temperature, pH, silicon and other nutrients, the growth of plants depends on the least one or two nutrients provided to them by the outside world. It can be seen from the algae source c65438+oo6h ~ 0110n16p that it takes 358 g carbon to produce 1 kg algae. Therefore, in order to control the eutrophication of water, it is necessary to control the content and proportion of nutrients such as nitrogen and phosphorus in water.

The focus of debate between food chain theory and life cycle theory is whether nitrogen and phosphorus are the main reasons for eutrophication.

There is no conclusion yet. However, judging from the current situation of water eutrophication in China, the reason of eutrophication is mainly explained by the latter (life cycle theory).

There are many reasons for water eutrophication. Today, water eutrophication is becoming more and more serious, and the research on the causes of water eutrophication has made some achievements, but the research on its causes needs to be further deepened. So far, there is no mature theory to predict the occurrence of eutrophication in actual water bodies; There is no widely accepted strict quantitative definition of eutrophication index. Therefore, a systematic and in-depth study of the causes of water eutrophication is of great theoretical significance and practical value for effectively carrying out comprehensive prevention and control of water eutrophication.

A long process of reproduction. For different waters, due to the geographical characteristics, natural climate conditions, aquatic ecosystems and pollution characteristics of the waters.

The specific reasons are:

1 farmland chemical fertilizer

In order to promote the growth of plants and increase the output of agricultural products, people often apply more nitrogen and phosphorus fertilizers, which are easily lost during rainfall or irrigation. The loss mode of nitrogen and phosphorus nutrients is: (1) enters the surface water body with the surface runoff; (2) The subsurface flow (loam flow) is formed by infiltration, moves laterally through the soil, and then discharges into the surface water body; (3) Infiltrate groundwater through soil layer. The first two types are the main reasons for the eutrophication of surface water. Recent studies show that phosphorus can penetrate into groundwater flow in the form of particles dissolved or adsorbed on soil through the movement of soil micropore structure, and then enter rivers, rivers, lakes or bays, while nitrogen (nitrate nitrogen) has strong permeability and can penetrate into groundwater to pollute groundwater [3]. In the process of nitrogen and phosphorus adsorption and desorption, part of soil is dissolved in water, while the other part remains adsorbed, and even deposits with soil particles during the movement, becoming a part of lakes, rivers or seabed sediments. When the flow rate, water temperature and microbial structure change, the pollutants deposited in the bottom mud can return to the water by resuspension and dissolution, which constitutes the secondary pollution of the water source. According to the investigation, the total nitrogen and total phosphorus released by Taihu Lake sediment account for about 25% ~ 35% of the total load every year [4].

2 Livestock manure

Poultry, livestock, especially pigs in captivity will produce a lot of excreta rich in nutrients and bacteria, which can easily flow into rivers and lakes with surface runoff and groundwater flow, polluting water bodies. In addition, excessive application of livestock manure in farmland will also cause nutrients in manure to be lost with surface runoff and groundwater flow, thus polluting water bodies. Overgrazing on grassland leads to a large amount of livestock manure staying on grassland, resulting in excess nutrients and destroying grassland vegetation coverage; When rainfall produces surface runoff, the destruction of vegetation cover will aggravate the erosion of soil and feces, resulting in more nutrient loss and pollution.

3 sewage irrigation

As a reliable water source and cheap fertilizer, sewage is used to irrigate farmland, which is an advocacy way of sewage agricultural utilization. The purpose is to purify sewage through soil purification and crop absorption of nutrients. However, soil and surface water are often polluted due to the high nutrient content in some sewage or technical reasons. According to the investigation of 37 sewage irrigation areas, it was found that the water quality of 32 irrigation areas did not meet the requirements.

4 Urban surface runoff

Most urban roads are impermeable, and nitrogen and phosphorus nutrients mainly enter the surface water body with surface runoff. Nitrogen and phosphorus nutrients in cities and towns mainly come from human domestic garbage, domestic sewage and some industrial and commercial wastewater (such as slaughter, food, paper making, parking lot, etc.). ). The United States Environmental Protection Agency lists urban surface runoff as the third largest pollution source of rivers and lakes in the United States [5].

5 surface runoff in mining area

In the phosphate rock area, the original soil structure and vegetation are destroyed by human activities, leaving the soil surface exposed. Under the condition of rainfall, pollutants such as slag, sediment and phosphate scattered in mining areas will enter lakes, reservoirs, rivers and bays with surface runoff, polluting water bodies.

6 atmospheric deposition

Atmospheric deposition is not only one of the sources of suspended particles and harmful gases, but also one of the sources of nitrogen. When fuel is burned, nitrogen enters the air in the form of nitrogen oxides, and falls on the surface of soil or water with rain and snow, polluting surface water sources.

7. Artificial aquaculture water body

Many water bodies are both water sources and artificial breeding places. With the development of aquaculture, artificial bait and fish excrement bring a lot of nitrogen and phosphorus to water. At present, the feed coefficient of artificial culture in lakes and reservoirs in China is 3.0 ~ 4.0, which has become another source of water eutrophication.