Urban sewage is all the drainage water collected through the sewerage pipe, which is a mixture of all kinds of domestic sewage, industrial wastewater and urban rainfall runoff discharged into the sewerage system. Domestic sewage is water discharged from people's daily lives. It is water discharged from households, public **** facilities (restaurants, hotels, theaters, stadiums, institutions, schools and stores, etc.) and factories in the kitchen, toilet, bathroom and laundry and other living facilities. The water quality of this type of wastewater is characterized by a high content of organic matter, such as starch, protein, oil and grease, and inorganic matter such as nitrogen, phosphorus, in addition to pathogenic microorganisms and a high level of suspended matter. Compared to industrial wastewater, the water quality of domestic wastewater is generally more stable and less concentrated. Industrial wastewater is the wastewater discharged during the production process, including production process wastewater, circulating cooling water flushing wastewater and comprehensive wastewater. The nature of industrial wastewater varies greatly due to the different processes, raw materials, and water conditions of the equipment used in various industrial production. Compared to domestic wastewater, industrial wastewater water quality and quantity differences, with high concentration, toxicity and other characteristics, is not easy to be managed through a common technology or process, often requiring it to be discharged before the plant to a certain extent. Rainfall runoff is formed by precipitation or snow and ice melt. For separate laying of sewers and storm drains in the city, rainfall runoff into the storm drains, for the use of rain sewage combined drainage pipe city, can make rainfall runoff and urban sewage to be treated together, but rainfall is larger due to more than the intercepting trunk pipe conveying capacity or sewage treatment plant capacity, a large number of rain sewage mixture overflow, will cause more serious pollution of the water body.
Properties:
Physical properties:
The physical properties of municipal wastewater include color, odor, water temperature, redox potential and other indicators.
(1) color
The color of the incoming water to the sewage plant, which is mainly domestic wastewater, is usually grayish-brown, which is relatively fresh, but in practice the color of the incoming water usually varies, depending on the drainage conditions of the city sewer and the influence of the industrial wastewater discharged into it. If the influent is black in color and has a particularly bad odor, the effluent is stale and may have been in the pipes for too long. If the incoming water is mixed with clearly identifiable other colors such as red, green, yellow, etc., industrial wastewater is entering. For a completed sewage plant, as long as its service area and service object does not change greatly, the incoming sewage color is generally not much change. It is important to observe the effluent on each tank one by one according to the process. The color of activated sludge also helps to determine the operating status of the structure, the normal color of activated sludge is yellowish-brown, the normal smell should be earthy, operating personnel in the site inspection should be conscious of observation and smell. If the color becomes black or smell corruption, it means that the oxygen supply is insufficient, or the sludge has been corrupted.
(2) odor
Sewage plant Baptist Church in addition to the normal fecal odor, sometimes in the vicinity of the catchment well has a rotten egg smell, which is a small amount of hydrogen sulfide gas produced in the pipeline due to sewage decay. Activated sludge mixture also has a certain odor, when the operator workers in the aeration pool next to smell a smell of earth, it can be concluded that the aeration pool is running well. If the city sewage has gasoline, solvents, fragrance, may be an industrial wastewater discharge.
(3) water temperature
Water temperature has a great influence on the aeration and biochemical reaction. A sewage plant water temperature changes slowly and gradually with the seasons, almost no change in a day. If there is a big change in one day, it is necessary to check whether there is industrial cooling water into.
(4) redox potential
Normal municipal wastewater has a redox potential of about +100 mV, less than +40 mV redox potential or negative redox potential indicates that the wastewater has been anaerobically fermented or that there is a large discharge of industrial reducing agents. A redox potential of more than +300 mV indicates that there is a large discharge of industrial oxidizer wastewater.
Chemical Properties
Chemical Indicators There are many chemical indicators of municipal wastewater, which include acidity and alkalinity (PH), alkalinity, biochemical oxygen demand (BOD), chemical oxygen demand (COD), solids, ammonia and nitrogen (NH3-N), total phosphorus (TP), and heavy metal content.
(1) acidity and alkalinity (PH)
The PH of municipal wastewater is generally 6.5-7.5.A small decrease in PH may be due to anaerobic fermentation in the municipal wastewater conveyance pipeline. Lower pH values of influent during the rainy season are often the result of urban acid rain, which is especially prominent in combined flow systems.Sudden and large changes in PH, whether increases or decreases, are usually due to large discharges of industrial wastewater.
(2) biochemical oxygen demand (BOD)
Urban wastewater treatment, commonly used biochemical oxygen demand BOD indicator to reflect the concentration of organic pollutants in sewage. Biochemical oxygen demand is in the development of the temperature and the development of the time period, microorganisms in the decomposition, oxidation of organic matter in the water in the process of the number of samples needed, the unit is mg / L. As the microbial aerobic decomposition rate began to be very fast, about 5 days after the oxygen demand that is to reach the complete decomposition of the oxygen demand of 70% or so, therefore, in practice, commonly used in the 5d Biochemical Oxygen Demand (BOD5) to measure the concentration of sewage in the organic matter. The concentration of organic matter in sewage.
(3) Chemical Oxygen Demand (COD)
Chemical Oxygen Demand (COD) refers to the strong oxidizing agent to make the measured wastewater in the chemical oxidation of organic matter consumed by the amount of oxygen. COD measurement speed, not subject to water quality restrictions, with which to guide the production is more convenient. Commonly used oxidants for KMnO4 and K2Cr4O7. KMnO4 oxidizing capacity is weak, often only part of the oxidized, so the results need to be determined with the actual situation has a big difference, and K2Cr4O7 oxidizing capacity is very strong, can make the vast majority of organic matter in the wastewater oxidation, it is commonly used to determine K2Cr4O7. In the analysis of urban wastewater treatment, the BOD5/COD ratio as a biochemical indicator. When BOD5/COD ≥ 0.3, the biochemistry is good, suitable for biochemical treatment process. Urban wastewater BOD5 and COD mean value maintains a certain correlation between, through a large number of data analysis and comparison, can be approximated from the COD deduction of BOD5.
(4) Dissolved Solids (DS) and Suspended Solids (SS)
Urban wastewater contains a large number of solids, can be divided into suspended solids SS and dissolved solids DS according to its physical properties. Suspended solids (SS) referred to as suspended solids, is an important indicator of the detection of sewage.SS indicators of the significance of: ① indicates that the pollution of sewage, the amount of SS content directly affects the appearance of the water environment, but also not conducive to the process of reoxygenation of the water; ② can reflect the effect of the pollutants removed by a simple precipitation method and the difficulty of the degree.
(5) total nitrogen (TN), ammonia nitrogen (NH3-N) and total phosphorus (TP) Nitrogen and phosphorus content is one of the important sewage water quality indicators, in the process of sewage biochemical treatment of microorganisms metabolism needs to consume a certain amount of nitrogen and phosphorus. If nitrogen and phosphorus are discharged into the water body, it will lead to excessive growth of algae in the water body, resulting in eutrophication as a problem. Total nitrogen is the sum of organic and inorganic nitrogen in sewage. Ammonia nitrogen is a kind of inorganic nitrogen, total phosphorus is the sum of all kinds of organic and inorganic phosphorus in sewage.