How is urban sewage treated and where does it end up?

Source of sewage What is sewage? Sewage, usually refers to a certain amount of pollution, from domestic and production waste water. The big white word is that the water that is not needed and wants to be discarded in a particular occasion is sewage. For example, the tap water at home, which is used for drinking by humans, is certainly not sewage, but for some industrial needs, it is sewage. Your honey, my poison. So what is it that makes the originally pure and lovely water king become muddy and disgusting? The pollutants in water can be generally divided into three categories, namely biological, physical and chemical pollutants. Biological pollutants include bacteria, viruses and parasites. Up to now, there are more studies on pathogenic bacteria and parasites, and there are better inactivation methods. However, the research on pathogenic viruses is not sufficient, and there is no recognized standard for virus inactivation. Physical pollutants include suspended solids, thermal pollution and radioactive pollution. Among them, radioactive pollution is the most harmful, but it generally exists in local areas. Chemical pollutants include organic and inorganic compounds. With the development of trace analysis technology, more than 2500 kinds of chemical pollutants have been detected from source water up to now. How do you describe exactly how polluted the water is? Water quality index is what we use to describe water quality quantitatively. Common water quality indicators are COD (chemical oxygen demand), BOD5 (5-day biochemical oxygen demand), ammonia nitrogen, TN (total nitrogen), TP (total phosphorus), pH, coliform bacteria, etc., of which COD should be the most widely known indicators, general introduction of water quality, are using this, more clear.

In general, urban sewage includes domestic sewage, industrial wastewater, stormwater runoff. Domestic sewage accounts for the vast majority, from our daily life (bathing, washing clothes, kitchen, some rainwater, shopping malls, units, car washes, etc., will produce sewage), through the drainage network to the centralized sewage treatment facilities (that is, XX sewage treatment plants, most areas have it). Industrial wastewater comes from centralized production departments, such as factories, laboratories, industrial parks, etc., and is generally treated to the appropriate water quality before it is transferred to the sewage network and treated with domestic sewage. Rainwater is relatively special: in addition to the rainwater runoff in special areas as industrial wastewater treatment, most of them are divided into two situations: economically developed, the construction of a separate rainwater pipe network, that is, the rain pollution diversion mode, so that domestic sewage is sent to treatment, rainwater can be treated and discharged (in China, the initial rain certainly has pollution, but domestic sewage is not too late to treat it, how can you still care about the rain?) ; Or do not build a separate rainwater pipe network, the two share the pipe network, that is, the rain and pollution combined mode, in this mode, there will be no problem in the dry season, the sewage is all sent to treatment, but in the rainy season, due to the surge in water, may exceed the capacity of the pipe network, the excess water will overflow out of the treatment system, because of the mixed part of the sewage, it formed a certain degree of pollution. (From this we should also see that water treatment is obviously subject to economic constraints) Where does the water go after treatment?

There are generally three directions:

(1) Discharge to surface water, which is the most common. This generally includes discharge into oceans, lakes, rivers and even deserts. There is no need to worry about pollution, and the environmental carrying capacity of the receiving water body is already taken into account when the emission standards are set. But if we steal it, it's gonna be contaminated. The "Comprehensive Sewage Discharge Standard" stipulates the discharge standard of sewage in different occasions.

(2) Industrial and agricultural utilization, water quality reaches a certain standard, it can be used, such as green irrigation, flushing toilets, car washing, process water, cooling water, boiler supplementary water, etc.

(3) groundwater recharge. In some areas, due to the excessive use of water resources, it will lead to groundwater depletion, so it is necessary to recharge and maintain a certain amount of water. Be careful when it comes to groundwater, which is much, much harder to repair than surface water. Third, sewage treatment methods This is the core of the industry. There are many ways to treat sewage, including physical methods, chemical methods, biological methods and so on. According to the classification of sewage plants, it generally includes primary treatment, secondary treatment, and advanced treatment. The choice of different methods depends on the intake water quality (that is, raw water quality), effluent water quality, treatment facility footprint, investment, cost and other requirements. The physical method is filtration, precipitation, etc., such as the necessary grates for sewage plants, sand settling ponds, air floating ponds, etc. Chemical methods are generally coagulation precipitation, such as chemical phosphorus removal. Biological methods include aerobic treatment, anaerobic treatment and so on. Activated sludge process is the most classic process of aerobic treatment, on this basis derived delay aeration, deep well aeration, AB method, oxidation ditch, AAO and other processes. The treatment of sewage is also from simple chroma removal, to the removal of organic pollutants, to nitrogen and phosphorus removal, and correspondingly, different process combinations continue to appear. In addition, in order to achieve higher water quality requirements, people also widely use ultrafiltration, nanofiltration, reverse osmosis and other treatment processes, such as the Beijing Olympic Games, and most of the direct drinking water technology on the market.