Introduction to sewage

Sewage source:

From the point of view of the source of sewage, sewage can be defined as the liquid or water carrying waste that is discharged from residential, institutional, commercial or industrial areas and mixed with groundwater, surface water, snowstorms, etc. There are many types of sewage, and there are also many technologies and processes to reduce the environmental impact of sewage. According to the source of sewage, sewage can be divided into four categories.

Category I: Industrial effluents from manufacturing mining and industrial production activities, including runoff leachate from industrial or commercial storage, processing, and other wastewater that is not domestic sewage.

The second category: domestic sewage from residential, office buildings, organs or similar sewage; Sanitary sewage; Sewage, including industrial wastewater mixed with domestic sewage in the sewer system. Garbage, all kinds of atmospheric particles settling, etc., through the surface runoff, soil erosion, farmland drainage and other forms of water environment caused. It has the characteristics of dispersion, concealment, randomness, latent, accumulation and fuzziness, so it is difficult to monitor, difficult to quantify, and difficult to study and control. Pollutant discharge into bodies of water that causes pollution.

The third category: commercial sewage from commercial facilities and some components more than domestic sewage non-toxic, harmless sewage. Such as catering sewage. Laundry room sewage, animal feeding sewage, hair salon sewage, etc.

The fourth category: surface runoff from rain, snow, highway water, water from urban and industrial areas, etc., surface runoff does not penetrate the soil, along the streets and land into groundwater.

Causes of pollution:

Among the water pollution caused by human production activities, the water pollution caused by industry is the most serious. Such as industrial wastewater, it contains many pollutants, complex components, not only in the water is not easy to purify, but also more difficult to treat. Industrial wastewater is the most important cause of water pollution caused by industrial pollution. It accounts for most of the pollutants discharged by industry. The pollutants contained in industrial wastewater vary greatly according to different types of factories, and the quality and amount of pollutants contained in similar factories are not the same, even if the production process is different. In addition to the pollution caused by the direct injection of industrial wastewater into the water body, solid waste and exhaust gas can also pollute the water body.

Agricultural pollution is first due to farming or clearing the land surface loose, in the soil and terrain is not stable when the rainfall, a large number of sediment into the water, increasing the suspended matter in the water.

Another important reason is that the use of pesticides and fertilizers is increasing, and only a small amount of pesticides and fertilizers used are attached or absorbed, and most of the rest remain in the soil and float in the atmosphere, through rainfall, through the surface runoff scouring into the surface water and infiltration into the surface water to form pollution. Urban pollution source is caused by water pollution caused by urban population concentration, urban domestic sewage, garbage and waste gas. The main pollution of urban pollution sources to water bodies is domestic sewage, which is a mixture of various sewage produced in People's Daily life, including the sewage discharged from kitchens, washing rooms, bathrooms and toilets.

The world's urban areas alone discharge as much as 500 cubic kilometers of industrial and domestic wastewater a year, and each drop of sewage will pollute several times or even tens of times the water body.

Main pollutant sources:

Pathogen contaminant

Domestic sewage, livestock and poultry farm sewage, as well as tannery, washing, slaughterhouses and hospitals discharge wastewater, often contain a variety of pathogens, such as viruses, germs, parasites. Contamination of water bodies with pathogens can spread diseases, such as schistosomiasis, cholera, typhoid, dysentery, viral hepatitis, etc. Some of the popular plagues in history are water-borne infectious diseases. For example, in 1848 and 1854, two cholera epidemics in Britain killed more than 10,000 people; In 1892, a cholera epidemic in Hamburg, Germany, killed more than 750 people, all caused by water pollution. In the water polluted by pathogens, microorganisms proliferate, many of which are pathogenic bacteria, eggs and viruses, which often coexist with other bacteria and Escherichia coli, so it is usually prescribed that the total number of bacteria and Escherichia coli index and bacteria value number are direct indicators of pathogen contamination. The characteristics of pathogen pollution are: (1) large quantity; (2) Wide distribution; (3) Longer survival time; (4) Fast reproduction speed; (5) Easy to produce resistance, difficult to eradicate; (6) After traditional secondary biochemical wastewater treatment and chlorination, some pathogenic microorganisms and viruses can still survive in large numbers. Common coagulation, precipitation, filtration and disinfection can remove more than 99% of the virus in the water, such as when the turbidity of the water is greater than 0.5 degrees, it will still be accompanied by the penetration of the virus. Pathogen pollutants can enter the water body through a variety of ways, once the conditions are suitable, it will cause human disease.

Oxygen consuming pollutant

In domestic sewage, food processing and paper and other industrial wastewater, there are carbohydrates, proteins, fats, lignin and other organic substances. These substances exist in a suspended or dissolved state in sewage, and this pollutant can cause a decrease in dissolved oxygen in the water, affecting the growth of fish and other aquatic life. After the depletion of dissolved oxygen in water, organic matter is anaerobic decomposition, producing hydrogen sulfide, ammonia and mercaptan odor, which further worsens the water quality. The composition of organic matter in water is very complex, and the concentration of oxygen-consuming organic matter is commonly expressed by the oxygen consumed in the biochemical decomposition process of oxygen-consuming substances per unit volume of water, unit mg/L.

Generally expressed by Chemical Oxygen Demand, COD (Chemical Oxygen Demand), it is a chemical method to measure the amount of reducing substances in the water sample that need to be oxidized.

A portion of it can be broken down by the biochemical action of microorganisms called biochemical oxygen demand (BOD). Generally, at 20 ° C, the five-day biochemical oxygen demand (BOD5) is expressed.

Plant nutrients

Plant nutrients mainly refer to nitrogen, phosphorus and other substances that can stimulate the growth of algae and aquatic grasses, interfere with water purification, and increase BOD5. The "eutrophication" caused by excessive nutrients in water bodies has become a serious problem in water source protection for lakes and slow-moving waters.

Under the influence of human activities, eutrophication refers to the phenomenon that nitrogen, phosphorus and other nutrients required by organisms enter a large number of slow-flow water bodies such as lakes, estuaries and bays, resulting in the rapid reproduction of algae and other plankton, the decline of dissolved oxygen in water bodies, the deterioration of water quality, and the death of a large number of fish and other organisms. Under natural conditions, the lake will also transition from a nutrient-poor state to a eutrophic state, with increasing sediment, first becoming a swamp and then a land. This natural process is very slow, often taking thousands or even tens of thousands of years. The phenomenon of eutrophication caused by artificial discharge of industrial wastewater and domestic sewage containing nutrients can appear in a short period of time.

The sources of plant nutrients are wide and large, including domestic sewage (organic matter, detergent), agriculture (fertilizer, farm fertilizer), industrial wastewater, garbage, etc. Each person brings about 50g of nitrogen into sewage every day. The phosphorus in domestic sewage mainly comes from washing wastewater, and 50% to 80% of the fertilizer applied to farmland flows into rivers, lakes, seas and underground water bodies. The content of phosphorus and nitrogen (especially phosphorus) in natural water bodies is to some extent a controlling factor for plankton growth. When a large amount of nitrogen and phosphorus plant nutrients are discharged into the water, some organisms (such as algae) are rapidly multiplied and grown, and the growth cycle becomes shorter. After the death of algae and other plankton, they are decomposed by aerobic organisms, constantly consuming dissolved oxygen in the water, or are decomposed by anaerobic microorganisms, constantly producing gases such as hydrogen sulfide, which deteriorates the water quality, causing a large number of fish and other aquatic organisms to die. Algae and other plankton residues in the process of decay, and the biological needs of nitrogen, phosphorus and other nutrients released into the water, for the new generation of algae and other organisms to use. Therefore, after eutrophication, even if the source of external nutrients is cut off, it is difficult to self-purify and return to normal levels. When the water body eutrophication is serious, the lake can be silted up by some numerous plants and their remains, and become a swamp or even dry land. Some areas of the sea can become a "Dead Sea" or a "red tide" phenomenon.

Nitrogen and phosphorus content, productivity (O2) and chlorophyll-α are commonly used as indicators of water eutrophication degree. Table 3-7 shows the index of water eutrophication degree divided by total phosphorus and inorganic nitrogen. To prevent eutrophication, the content of nitrogen and phosphorus entering water must be controlled.

Toxic pollutant

Toxic pollutants refer to substances that accumulate to a certain amount after entering the organism and can cause biochemical and physiological function changes in body fluids and tissues, causing temporary or lasting pathological states, and even endangering life. Such as heavy metals and organic pollutants that are difficult to decompose. The toxicity of pollutants is closely related to the amount ingested into the body. The toxicity of the same pollutant is also closely related to its presence. The toxicity can vary greatly depending on the valence or morphology. For example, the toxicity of Cr (Ⅵ) is greater than that of Cr (Ⅲ); The toxicity of As (Ⅲ) was higher than that of As (Ⅴ). Methylmercury is much more toxic than inorganic mercury. In addition, the toxicity of pollutants is closely related to several comprehensive effects. From the perspective of traditional toxicology, there are three kinds of combined effects of toxic pollutants on organisms:

(1) Additive effect, that is, when two or more poisons coexist, the total effect is roughly the sum of the effects of each component.

(2) Synergistic effect, that is, when more than two poisons coexist, one component can promote a sharp increase in the toxicity of another component. For example, when copper and zinc coexist, their toxicity is 8 times that of when they exist alone.

3, antagonism, when more than two poisons coexist, their toxicity can offset part or most of it. For example, zinc can inhibit the toxicity of cadmium; For example, selenium can have antagonistic effect on mercury under certain conditions. In short, in addition to considering the content of toxic pollutants, it is also necessary to consider its existence and comprehensive effects, so as to fully understand the impact of pollutants on water quality and human health.

Toxic pollutants mainly fall into the following categories:

1. Heavy metals. Such as mercury, cadmium, chromium, lead, vanadium, cobalt, barium, etc., of which mercury, cadmium and lead are more harmful; Arsenic, selenium and beryllium are also more toxic. Heavy metals are generally not easy to disappear in nature, they can be enriched through the food chain; In addition to directly acting on the human body to cause disease, some metals may also promote the development of chronic diseases.

2, inorganic anions, mainly NO2-, F-, CN- ions. NO2- is a carcinogen. Cyanide, a highly toxic substance, mainly comes from industrial wastewater discharge.

3, organic pesticides, polychlorinated biphenyls. There are about 6,000 kinds of organic pesticides in the world, and about 200 kinds are commonly used. Pesticides sprayed in farmland, through leaching and other effects into the water, resulting in pollution. Organic pesticides can be divided into organophosphorus pesticides and organochlorine pesticides. Although the toxicity of organophosphorus pesticides is high, it is easy to degrade and accumulation is not strong, so the impact on the ecosystem is not obvious. The vast majority of organochlorine pesticides have high toxicity, almost no degradation, and high accumulation, which have a significant impact on the ecosystem. Polychlorinated biphenyls (PCB) is a general term for various isomer mixtures formed after some or all of the hydrogen in the biphenyls molecule is replaced by chlorine. Polychlorinated biphenyls are highly toxic, fat soluble, easy to be absorbed by biology, chemical properties are very stable, difficult to and acids, bases, oxidants and other effects, have a high degree of heat resistance, in 1000 ~ 1400℃ high temperature to completely decompose, so it is difficult to degrade in water and biology.

4. Carcinogens. Carcinogens are generally divided into three categories: thick cyclic aromatic hydrocarbons (PAHs), such as 3, 4-benzopyrene; Heterocyclic compounds, such as aflatoxin; Aromatic amines, such as methyl, ethylaniline, benzidine, etc.

5. General organic matter. For example, there are more than 2000 kinds of phenolic compounds, the simplest is phenol, which are highly toxic substances; Nitrile compounds are also toxic, of which acrylonitrile has the most notable environmental impact.

Petroleum pollutant

Oil pollution is one of the important types of water pollution, especially in estuarine and offshore waters. The impact of oil pollution [4] on people is also gradually far-reaching. Oil pollution of the sea has become a serious problem in the world... Petroleum Petroleum, also known as crude oil, is a brown-black flammable viscous liquid extracted from deep underground. The amount of oil released into the ocean is estimated to be between millions and tens of millions of tons per year, or about five-thousandths of the world's total oil production. Oil pollutants mainly come from industrial discharge, which can be caused by cleaning the cabins and machinery of oil transport vessels, accidents and offshore oil production. The accident of oil tanker is a concentrated source of explosive pollution, and the harm is devastating.

Petroleum is a mixture of alkanes, alkenes and aromatic hydrocarbons, and its harm after entering the water body is manifold. If the oil film is formed on the water, it can hinder the reoxygenation of the water body, and the oil adheres to the gills of the fish, which can asphyxiate the fish. It sticks to algae and plankton and kills them. Oil can inhibit the laying and hatching of water birds, and in severe cases, cause a large number of bird deaths. Oil pollution can also reduce the quality of aquatic products.

Radioactive contaminant

Radioactive contamination is caused when radioactive materials enter the water. Radioactive pollutants mainly come from cooling water discharged from nuclear power plants, radioactive waste thrown into the sea, nuclear explosions falling into the water body, nuclear fuel accident leakage; When mining, refining and using radioactive materials, if not handled properly, it can also cause radioactive contamination. Radioactive pollutants in the water can be attached to the surface of organisms, can also enter the accumulation of organisms, and can also produce internal irradiation through the food chain.

The main natural radioactive elements in water are 40K, 238U, 286Ra, 210Po, 14C, tritium and so on. 90Sr and 137Cs can be detected in almost any sea area in the world.

Inorganic pollutant

Various acids, alkalis, salts and other inorganic substances into the water body (acid, alkali and salt, they interact with some minerals in the water body to produce some salts), so that the salinity of fresh water resources is improved, affecting the quality of various water. Salt pollution mainly comes from domestic sewage, industrial and mining wastewater and some industrial wastes. In addition, due to the increasing scale of acid rain, soil acidification and groundwater salinity increase.

The increase of inorganic salt in water can increase the osmotic pressure of water, which has a bad effect on the growth of freshwater organisms and plants. In salinized areas, the salt in surface water and groundwater will have a greater impact on soil quality.

Thermal pollution

Thermal pollution is a kind of energy pollution, which is caused by the discharge of high temperature wastewater from industrial and mining enterprises. If the cooling water in some thermal power plants and various industrial processes is discharged directly into the water body without taking measures, the water temperature can be increased, the speed of chemical reactions and biochemical reactions in the water will be accelerated, the toxicity of some toxic substances (such as cyanide, heavy metal ions, etc.) will be increased, the dissolved oxygen will be reduced, the survival and reproduction of fish will be affected, and the reproduction of some bacteria will be accelerated. Encourages water weeds, anaerobic fermentation, foul odor.

There is an optimal water temperature range for fish to grow. Water temperatures that are too high or too low are not suitable for fish to grow, and can even lead to death. Different fish also have different adaptations to water temperature. For example, tropical fish are suitable for 15 ~ 32 ° C, temperate fish are suitable for 10 ~ 22 ° C, and cold fish are suitable for the range of 2 ~ 10 ° C. For example, although trout live in 24 ° C water, its breeding temperature is lower than 14 ° C. The upper limit of water temperature that aquatic organisms can live in is 33 ~ 35℃.

In addition to the above eight types of pollutants, the main harm of surfactants such as detergents to the water environment is to make water foam, prevent air and water contact and reduce dissolved oxygen, and cause water hypoxia due to the biochemical degradation of organic matter, which consumes dissolved oxygen in water. High concentration surfactants have obvious toxicity to microorganisms. There are many examples of water pollution. For example, there are many factories on both sides of the Beijing-Hangzhou Grand Canal (Hangzhou section), and a large amount of waste water is discharged into the canal every day, making the content of solid suspended matter, organic matter, heavy metal (Zn,Cd,Pb,Cu, etc.), phenol and cyanide in the water body greatly exceed the surface water standard, and some exceed tens of times, so that the water body is in an anaerobic reduction state, black and stinky. Fish and shrimp disappeared, can not be used for living, agriculture and other water; The self-purification ability of water body is poor, if we do not control and control the pollution source, the water body pollution will further expand.

The pollutants in the water environment can be divided into inorganic pollutants and organic pollutants in general. Heavy metals and organics are the most important and studied pollutants in water environmental chemistry. The research on water pollution chemistry in China began in the 1970s, starting from the pollution of heavy metals, oxygen-consuming organic matter, DDT, 66C and other pesticides. The research focus has shifted to organic pollutants, especially refractory organic matter, which has attracted increasing attention due to its long life in the environment, easy transfer and accumulation along the food chain (network), threatening biological growth and human health. This chapter focuses on the environmental chemical behavior of heavy metals and organic pollutants in water.