Typical urban sewage water treatment processes mainly include mechanical treatment, biochemical treatment, sludge treatment, and other sections. Systems consisting of mechanical and biochemical treatments are referred to as secondary treatment systems, with BOD5 and SS removal rates reaching 90%-98%. Treatment effects located between the first and second treatments are generally referred to as enhanced first-stage treatment, semi-first-stage treatment, or incomplete second-stage treatment. They mainly consist of two categories: high-load biological treatment and chemical treatment, with BOD5 removal rates reaching 45%-75%. Secondary treatment systems that have biological phosphorus and nitrogen removal functions are usually referred to as deep secondary treatment. In order to remove specific substances, treatment systems set up after the secondary treatment are classified as tertiary treatment, such as chemical phosphorus removal, activated carbon adsorption, and so on.
From the perspective of sewage water treatment, pollutants can be divided into suspended solids, organic pollutants, toxic substances, polluting organisms, and polluted nutrients. Large amounts of organic matter discharged into water from urban wastewater can reduce the dissolved oxygen content in the water, reaching anoxic conditions, seriously polluting the water and making it unsuitable for fish survival. Organic matter concentration in wastewater is generally expressed with biological chemical oxygen demand (BOD5), chemical oxygen demand (COD), total oxygen demand (TOD), and total organic carbon (TOC). Nutrients mainly refer to nitrogen and phosphorus, which can cause algae and floating organisms to reproduce and form "water blooms" and "red tides."
Municipal sewage treatment methods can be divided into physical treatment methods, biological treatment methods, sludge disposal methods from sewage water treatment, and chemical treatment methods according to the water quality types. They can also be classified into first-stage, second-stage, and tertiary treatment processes according to the treatment degree.
The physical treatment method for urban wastewater is to use physical effects to separate and remove pollutants in wastewater. Common methods include screening, interception, gravity separation, centrifugal separation, and corresponding treatment equipment mainly includes screens, sedimentation tanks, settling tanks, and centrifuges, of which the sedimentation tank is the same as the sedimentation tank in urban water supply treatment.
The biological treatment method is to use the metabolic process of microorganisms to remove organic matter in wastewater. Common methods include activated sludge and biological membrane methods, as well as oxidation ponds and wastewater land treatment methods. The chemical treatment method is rarely used in municipal sewage treatment, generally involving other chemical methods in urban water supply treatment, such as neutralization, redox reaction, ion exchange, electrolysis, mainly for industrial sewage water treatment. Sludge needs to be treated to prevent secondary pollution, and its disposal methods often include concentration, anaerobic digestion, dehydration, and thermal treatment. The first-stage treatment mainly targets suspended substances in the wastewater and commonly uses physical methods. After the first-stage treatment, the removal of suspended matter in the wastewater can reach about 40%, and about 30% of organic matter attached to suspended matter can also be removed. The second-stage treatment mainly removes organic pollutants in the wastewater in a colloidal or dissolved state.
The commonly used method is the biological treatment method, including activated sludge and biological membrane methods. Biological sewage water treatment utilizes the function of microorganisms to decompose and oxidize organic matter and takes certain artificial measures to create an environment conducive to the growth and reproduction of microorganisms, enabling the vast reproduction of microorganisms and improving their efficiency in decomposing and oxidizing organic matter. After the first-stage treatment, which has already removed floating substances and some suspended matter, about 25%-30% of BOD5 can be removed. After the second-stage treatment, the removal rate of BOD5 can reach more than 90%, and the effluent from the secondary settling tank can meet the emission standards. The active sludge treatment system is one of the most widely used treatment technologies in the current field of sewage water treatment, with the aeration tank being its reactor. The wastewater and sludge are mixed in the aeration tank, and the microorganisms in the sludge degrade the complex organic matter in the wastewater, using the energy released to achieve the reproduction and movement of microorganisms.