Basic methods and treatment process of sewage treatment

The basic methods of sewage treatment can be divided into physical methods, chemical methods, and biological methods. The treatment process generally includes pretreatment, primary treatment, secondary treatment, tertiary treatment (deep treatment), and sludge treatment. The following is a specific introduction:

Basic methods

Physical method: Separation and recovery of insoluble suspended pollutants in wastewater through physical action. For example, precipitation method uses gravity to cause suspended particles to settle to the bottom of the water, achieving solid-liquid separation; The filtration method allows wastewater to pass through porous media such as sand filters, activated carbon, etc., to intercept and remove suspended impurities and some organic matter in the water.

Chemical method: using chemical reactions to separate and remove pollutants from wastewater or change the properties of pollutants. For example, neutralization method is used to adjust the acidity and alkalinity of wastewater, making it neutral or close to neutral; Oxidation reduction method, which uses oxidants or reducing agents to oxidize or reduce harmful substances in wastewater into harmless substances; Coagulation method involves adding coagulants to wastewater to aggregate small suspended particles and colloids into larger particles for precipitation.

Biological method: Utilizing the metabolic process of microorganisms to convert organic pollutants and nutrients in wastewater into stable and harmless substances. Activated sludge process is a common biological treatment method that uses aeration to fully mix and contact sewage with activated sludge, and utilizes microorganisms in the sludge to decompose organic matter; Biofilm method is to attach microorganisms to the surface of solid media to form a biofilm. When wastewater flows through the biofilm, organic matter is decomposed by microorganisms.

Process flow

Preprocessing: mainly includes facilities such as grilles and sedimentation tanks. Grilles are used to intercept and remove larger floating and suspended solids in sewage, preventing subsequent equipment blockages; The sedimentation tank removes inorganic sand particles with high specific gravity from the sewage, reducing wear and tear on subsequent treatment equipment.

Primary treatment: Physical methods are usually used, with sedimentation as the main process, to remove most of the suspended pollutants in wastewater. After primary treatment, suspended solids in wastewater can be effectively reduced, but the removal rate of pollutants such as organic matter is relatively low.

Secondary treatment: generally using biological treatment methods, which is the core link of sewage treatment. Through processes such as activated sludge or biofilm, the action of microorganisms is utilized to remove a large amount of organic pollutants from wastewater, significantly reducing indicators such as biochemical oxygen demand (BOD) and chemical oxygen demand (COD), while also removing some nutrients such as nitrogen and phosphorus. After secondary treatment, the water quality of the sewage can be greatly improved.

Third level treatment (deep treatment): In the case where the effluent from the second level treatment cannot meet higher water quality requirements, further removal of residual pollutants in the sewage, such as recalcitrant organic matter, nitrogen, phosphorus, heavy metal ions, etc. Common methods include filtration, activated carbon adsorption, ion exchange, membrane separation, etc. After deep treatment, the sewage can meet higher water quality standards and even be reused.

Sludge treatment: During the sewage treatment process, a large amount of sludge is generated, which contains a large amount of harmful substances such as organic matter, pathogens, and heavy metals, and requires specialized treatment. The main purpose of sludge treatment is to reduce sludge volume, stabilize sludge properties, achieve harmless treatment, and recycle resources. Common treatment methods include sludge concentration, dehydration, anaerobic digestion, aerobic composting, incineration, etc.

The specific treatment methods and processes for different types of sewage, such as domestic sewage, industrial wastewater, etc., may vary. It is necessary to design and optimize them reasonably based on factors such as the quality and quantity of the sewage, as well as the water quality requirements after treatment.