Wastewater, usually the discharged water from domestic and production sources, is subject to some pollution. The main types of sewage are domestic sewage, industrial wastewater and initial rainwater. The main pollutants of sewage are pathogenic pollutants, oxygen-consuming pollutants, plant nutrients, toxic pollutants, etc.
According to the viewpoint of the source of sewage, sewage can be defined as discharge from residential, institutional, commercial or industrial areas with groundwater. Surface water, storm water, etc. are mixed with liquid or water carrying waste. Sewage consists of many categories and accordingly there are many techniques and technologies to reduce the environmental impact of sewage. Sewage can be divided into five categories according to its source.
Industrial wastewater, from manufacturing mining and industrial production activities, including run-off live leachate from industrial or commercial storage and processing, and other wastewater that is not domestic sewage. It is the most important cause of water pollution caused by industrial pollution. It accounts for the majority of pollutants discharged by industry. The pollutants contained in industrial wastewater vary greatly depending on the type of plant, and even for similar plants, the quality and quantity of pollutants they contain vary depending on the production process. In addition to the pollution caused by the direct injection of wastewater from industry into water bodies, solid waste and exhaust gases can also pollute water bodies.
Agricultural pollution is firstly due to ploughing or clearing the land to make the surface loose, rainfall when the soil and terrain are not yet stable, and large amounts of sediment flowing into the water, increasing the suspended matter in the water.
There is also an important reason for the increasing use of pesticides and fertilisers, and the use of pesticides and fertilisers only a small amount attached or absorbed, the vast majority of the rest remain in the soil and floating in the atmosphere, through rainfall, through the surface runoff of the wash into the surface water and seepage into the surface water to form pollution.
Sewage from dwellings, offices, institutions or similar; sanitary sewage; sewerage sewage, including industrial wastewater mixed with domestic sewage in sewerage systems. Urban sources of pollution are caused by the concentration of urban population and the pollution of water bodies caused by urban domestic sewage, waste and exhaust gases. The main source of urban pollution to water bodies is domestic sewage, which is a mixture of various kinds of sewage produced by people in their daily lives, including sewage discharged from kitchens, washing rooms, bathrooms and toilets.
As much as 500 cubic kilometres of industrial and domestic wastewater is discharged in urban areas alone in the world in a year, and each drop of sewage will pollute several times or even tens of times more water bodies.
Non-toxic, non-hazardous sewage that comes from commercial facilities and has some components that exceed those of domestic sewage. For example, catering effluent. Laundry effluent, animal feeding effluent, effluent from hair salons, etc.
Surface run-off from rain, snow, highway run-off, water from urban and industrial areas, etc., which does not percolate into the soil and enters groundwater along streets and land.
Of all the water pollution caused by human production activities, that caused by industry is the most serious. Such as industrial wastewater, it contains many pollutants, complex composition, not only in the water is not easy to purify, but also more difficult to deal with. It is therefore particularly important that water quality is tested to meet standards and then discharged.