Water and nature, itself has a certain self-purification capacity, take the river, when a year of sewage into the river, sewage is first mixed by the water, dilution and diffusion, heavier than the water particles on the sedimentation deposited in the riverbed. Then the oxidation process begins, easy to oxidize substances through the oxygen in the water; organic matter through the water microorganisms for biological oxidation and decomposition. At the same time, the surface of the river is constantly obtaining oxygen from the atmosphere, so that the oxygen consumed in the oxidation process to be replenished. In this way, when after a certain period of time, the river water flows to a certain distance, with the mineralization of organic matter, the river water is restored to its original clean state, which is the self-purification effect of the river. This self-purification ability is related to the volume, velocity and flow rate of the water body. The ocean and surface water for general naturally occurring organic substances have great self-purification ability, but for synthetic detergents, organochlorine pesticides, PCBs and other total consortium into organic compounds and such as cyanide, heavy metals, radioactive substances and other toxic substances, self-purification is very limited, it is difficult for these substances to be purified by self-purification.
As for the groundwater, due to the slow flow rate, stay a long time, and no air, sunlight, its self-purification ability to be worse than groundwater from. Therefore, when discharged into the waters of sewage, wastewater, all kinds of waste and other pollutants more than the self-purification capacity of the water body, the water quality will be polluted.
Two. Man-made pollution of waters
Currently, China's annual wastewater emissions of about 36.5 billion tons, and each ton of sewage can pollute more than forty times its natural water. China's industrial wastewater is not only a large amount of emissions and high pollution concentration, the reason for this, mainly because of a considerable number of enterprises with backward production technology, obsolete equipment, low level of management, the majority of small and medium-sized enterprises (accounting for about 90%), a serious waste of resources and energy, water consumption, water consumption per unit of product and sewage discharge compared with the developed countries is a very wide gap. China's average water consumption per ton of ammonia production 500 - 1000 tons, only 12 tons of developed countries; China's water consumption per ton of paper 400 - 500 tons, developed countries for the 5-2002KGB; China's water consumption per ton of steel 30 tons, the United States is only 3-5 tons.
China in 1998 discharged about 25 billion tons of industrial wastewater, the annual amount of wastewater treatment of about 7.2 billion tons of wastewater, industrial wastewater treatment rate of nearly 30%, 92 years of industrial wastewater treatment amounted to 175. 9 billion tons of industrial wastewater treatment rate of 63.6% in 94 years of industrial wastewater treatment amounted to 198.5 tons of industrial wastewater treatment rate of 75%. And treatment of industrial wastewater to meet the standard survey results show that the treatment effect is not satisfactory. Wastewater treatment facilities, equipment operating efficiency is not high. Out of service, scrapped, idle equipment accounted for 12.5% of the total amount of preservation, the cumulative funds accounted for 8.5% of the total investment, the actual operation of the design capacity to reach 80% of only 48%, while 28.7% of the facilities are less than 50% of the design capacity, the treatment facilities and equipment are not fully utilized, resulting in high operating costs. Coupled with China's long-term unpaid use of water resources, water prices are too low, so that enterprises can buy fresh water without investing in wastewater treatment or treatment facilities for decoration, should be examined, arbitrary discharge pollution of the water environment.
The causes of man-made water pollution is multifaceted, according to the source of pollution, can be divided into industrial wastewater pollution, urban sewage pollution, agricultural backwater pollution, solid waste pollution and other pollution.
1. Industrial wastewater pollution
Water in industry is mainly used for washing products, cooling equipment, generating steam, transporting waste and as a raw material for production to have diluted and so on, there is almost no kind of industry can leave water. And industrial water consumption is very large, to account for about 80% of the entire human water consumption. According to statistics, the production of a ton of caustic soda requires more than 100 tons of water; a ton of steel requires more than 30 tons of water; a ton of petrochemicals, a ton of paper or a thousand degrees of electricity requires more than 200 to 500 tons of water, and the manufacture of a ton of man-made fibers requires more than 1,000 tons of water. Such a large amount of industrial water, corresponding to a large amount of wastewater production, industrial wastewater emissions account for about two-thirds of the total amount of wastewater.
Industrial wastewater is characterized by a wide variety of complex composition. There are mainly the following:
1.1 paper industry is China's first household of water pollution. 1992 paper wastewater amounted to 4.2 billion tons, accounting for 17.9% of the country's industrial wastewater, there are nearly 10,000 paper mills in China, with an annual output of 50,000 tons of more than 18, with an annual output of more than 180 tons of more than 10,000 tons of small and medium-sized paper mills have more than 9,000 tons of more than 10,000 tons of paper mill must be required by the state. Engage in alkali recovery, not only beneficial environmental benefits, there are economic benefits, enterprises are willing to on, but the investment is large, difficult to raise funds. Small and medium-sized paper mills are often simply put after treatment, or even not dealt with at all. A paper mill polluting a river is a common occurrence.
2 dyestuff industry wastewater annual discharge of 200-300 million tons, mostly high-concentration organic wastewater, mainly containing difficult to degrade benzene, naphthalene, anthracene, quinone and other polycyclic hydrocarbons and organic compounds, high COD value, acidic, high chromaticity, exceeding the standard of about a few thousand to tens of thousands of times.
3 pharmaceutical industry wastewater annual discharge of 200-300 million tons, chemical oxygen demand of about 150,000 tons, the average treatment rate is less than 30%. Especially the production of antimicrobial wastewater. National production of antimicrobial plant has more than a hundred factories, only Shandong Jining Pharmaceutical Factory, North China Pharmaceutical Factory and Shanghai Second Pharmaceutical Factory three guests of the antimicrobial wastewater treatment, but the streptomycin, vanguardomycin and other production of wastewater is still no good, even if the treatment, the effect is still not good.
4 high concentration of organic wastewater pollution problem is imminent. High concentration of organic wastewater chemical oxygen demand 6,000-20,000 mg per liter, biochemical oxygen demand 3000-10000 mg per liter. The main sources of pollution are food brewing industry, sugar, tannery and slaughtering meat factory.
1.4.1 MSG wastewater: China's manufacturers and workshops have more than 230, the annual discharge of 170,000 tons of wastewater, the vast majority of untreated direct discharge, more than 10 of the country's use of wastewater production of feed yeast, but the chemical oxygen demand is only removed 60%, there are still 40% of the discharged into the environment.
1.4.2 Molasses lees: Molasses alcohol distillation waste liquid; high temperature, acidity, deep color, biochemical oxygen demand, chemical oxygen demand value of up to tens of thousands of milligrams per liter, is the main source of pollution in Guangdong, Guangxi.
1.4.3 At present, China has more than 500 tanneries (excluding township enterprises), the annual discharge of 50-60 million tons of wastewater. Less than 30% of them have certain wastewater treatment measures, and the remaining 70% of the manufacturers discharge wastewater into the environment without any treatment.
1.5 Some other industries such as: mercury electrolysis salt industrial wastewater contains mercury; heavy metal smelting industrial wastewater contains a variety of heavy metals; electroplating industrial wastewater contains cyanide and a variety of heavy metals; coal coke and petroleum refining industrial wastewater contains phenol; pesticide manufacturing industrial wastewater contains a variety of pesticides, etc., which are toxic to the industrial wastewater, has a great deal of harm to human health.
2. Urban sewage pollution
With the development of industry and brought urbanization, a large number of people and industries are highly concentrated in some very small areas, such as the United States, the urban population has accounted for 75% of the country's total population, the German urban population accounted for 70% of the population of Japan, 80% of the population living in the coastal cities accounted for 20% of the national territory. People's daily life produced by the domestic sewage, according to statistics per person per day about hundreds of liters or so (500 liters in the United States, Japan 250 liters), the amount of dirty load of tens of grams of BOD (the United States average of 54 grams, Japan for 36 grams). These sewage, in addition to containing carbohydrates, proteins and amino acids, animal and vegetable fats, urea and ammonia, soap and synthetic detergents and other substances, but also contains bacteria, viruses and other micro-organisms that make people sick. This sewage will consume the dissolved oxygen of the receiving water body, but also produce foam to prevent the oxygen in the air dissolved in water, making the water stink and deteriorate.
3. Agricultural backwater pollution
The largest water use in agriculture is irrigation, of which 60-90% is lost through evaporation, and the remaining 10-40% seeps into the ground or flows away from the surface. As a result of cultivation, spraying pesticides, fertilizers and other work, so that this irrigation return water contains a high concentration of minerals, nutrient-rich fertilizers, toxic pesticides, but also pollute the water body. In particular, organochlorine pesticides like DDT are one of the most dangerous substances for contaminating water. The chemical stability of this substance is extremely high, in nature, it takes more than ten years to completely decompose into harmless substances, become the environment is a long-standing pollutant, but also easily dissolved in fat, can be accumulated in the fatty tissues of animals and humans to cause harm, at the same time, it is difficult to water, with the help of the water flow and migrate to other places, so that many did not use pesticides in the region, and even Antarctica also appeared in the DDT, DDT. It is estimated that two thousand tons of DDT and its residues have accumulated in Antarctic ice water. Due to the excessive use of organochlorine pesticides, some birds have become less and less, or even extinct.
4. solid waste pollution and other pollution
Agricultural waste, industrial waste and municipal garbage are very large in number and variety, if transferred to the middle, will also pollute the water quality, this kind of pollution is quite complex. Organic substances decomposed by microorganisms in the water will consume dissolved oxygen in the water, a variety of toxic substances to make the receiving body of water with toxicity, from the factory exhaust, such as sulfur dioxide, once transferred to the water with the rain, it becomes sulfurous acid, which in turn with the role of the oxygen in the water, oxidation into sulfuric acid, that is, to consume dissolved oxygen in the water, but also to make the water with acidic. In particular, a variety of pollutants flow into the water at the same time, and may sometimes have chemical reactions with each other, thus producing substances with greater danger, for example, various wastes containing inorganic mercury are discharged into the water column and deposited under the water, and most of them can be transformed into methylmercury, which can cause Minamata disease, through the decomposition of microorganisms. Pollution pathways are also varied, such as landfill waste level rain and snow melting may dissolve in water, or chemical action to produce toxic substances, and finally leaked out of the field into the water points of the city of low terrain, or seepage into the ground, contamination of groundwater.
Three. The main pollutants and sources of water
The substances that pollute water quality are extremely complex, from a wide range of sources.
Generally can be divided into ten categories, some of the various types of substances are overlapping each other.
1. Harmful substances
Such substances mainly include benzene, phenol, cyanide, organic matter, arsenic, mercury, cadmium, lead and its compounds, and other carcinogenic substances. Its important source is industrial and mining wastewater, the following table lists some of the industrial and mining wastewater in the main harmful components.
2. Disease-causing microorganisms
Including a variety of viruses, bacteria, protozoa, etc., can infect diseases and have the ability to reproduce, mainly from various sewage, hospital sewage.
3. Oxygen-depleting waste
Including organic and inorganic substances, the former is mainly biodegradable natural organic matter; the latter is mainly reducing substances such as sulfites, sulfides, ferrous salts and ammonia. This kind of material by the biological oxidation and chemical oxidation in water, a large number of water consumption of dissolved oxygen, deterioration of water quality and even make the water malodor, resulting in fish death. It has a very wide range of sources, such as paper mills, fiber mills, instrumentation plants and domestic sewage.
4. Plant nutrients
This is the nutrients needed for plant growth, such as nitrogen, phosphorus, carbon and other compounds, mainly from synthetic detergents, fertilizers, feed and domestic sewage. These substances with the sewers and soil flushing flow to lakes and other waters, will make the poor nutrient lake into a nutrient-rich lake, the result of which reduces the transparency of the water, resulting in water odor, this phenomenon is often difficult to change /,.
5. Oil substances
These substances are lighter than water, and insoluble in water, so covering the water surface to form a thin film layer, preventing the dissolution of oxygen in the air in the water, so that the water to reduce the amount of dissolved oxygen, resulting in a bad smell, affecting the quality of aquatic products, so that the price of the lower, if attached to the crops on the withering; on the other hand, affecting the aesthetics, and the risk of fire. The main sources are the oil industry, machinery processing, automobile and aircraft maintenance, paint, gas, grease processing and other industrial wastewater and ship transportation.
6. Organic chemicals
Such substances include, for example, polychlorinated biphenyls (PCB), synthetic detergents (ABS), etc., are some highly stable synthetic chemicals, biological decomposition is difficult. These substances can be concentrated through the food chain to cause harm and come from a wide range of sources, such as various organic chemical plants.
7. Inorganic compounds and minerals
Including a variety of water-soluble chlorides, salts, and a variety of other acidic and alkaline substances. Concentration is too high will reduce water quality, harm aquatic organisms, or make the receiving water body with acidic or alkaline, corrosion of underwater work equipment. The main source is a variety of chemical plants.
8. Impact and other insoluble solids
Mainly due to the water ridge washed down such as sand, clay and other substances, the deposition of the bottom of the water will be silted reservoirs, irrigation canals and waterways, etc.
9.
9. Thermal effluents
Mainly substances with high temperature such as cooling water from thermal power plants and various industrial processes. It is reported that only one-third of the heat of a typical fuel-fired thermal power plant is converted into electricity, and the remaining two-thirds of the heat is released into the atmosphere or cooling water, while almost all of the waste heat of an atomic power plant is released into the cooling water, accounting for about 75% of its total heat. Such a large amount of heat imported into the waters can cause thermal pollution and dramatic changes in local ecosystems.
10. Radioactive substances
This is a variety of fissionable substances in their fission release of radiation, causing harm. For example, wastewater containing sodium sulfide mixed with wastewater containing sulfuric acid, can react to generate toxic substances hydrogen sulfide; wastewater containing ferricyanide, ferricyanide can be decomposed to generate highly toxic substances such as cyanide.