Sewage/wastewater treatment methods and processes

The basic method of sewage treatment is to use various technologies and means to separate and remove the polluting substances contained in sewage, recycle them or convert them into harmless substances, so that the water can be purified.

Modern sewage treatment technology can be divided into three categories according to the principle of physical treatment, chemical treatment and biochemical treatment.

Physical treatment method: The use of physical action to separate the solid pollutants in the suspended state of sewage. The methods are: sieve method, precipitation method, upper float method, gas float method, filtration method and reverse osmosis method.

Chemical treatment: The use of chemical reactions to separate and recycle polluted substances in various forms of sewage (including suspended, dissolved, colloidal, etc.). The main methods are neutralization, coagulation, electrolysis, REDOX, stripping, extraction, adsorption, ion exchange and electrodialysis. Chemical treatment method is used to treat production sewage.

Biochemical treatment method: It is the use of microbial metabolism, so that the organic pollutants in the sewage is dissolved, colloidal state into stable harmless substances. The main methods can be divided into two categories, namely, the aerobic method using the action of aerobic microorganisms (aerobic oxidation method) and the anaerobic method using the action of anaerobic microorganisms (anaerobic reduction method). The former is widely used to treat municipal sewage and organic production sewage, including activated sludge method and biofilm method. The latter is mostly used to treat high-concentration organic sewage and sludge generated in the sewage treatment process, and is now also being used to treat urban sewage and low-concentration organic sewage.

The pollutants in urban sewage and production sewage are diverse, and it is often necessary to use a combination of several methods to deal with pollutants and sludge of different properties, so as to achieve the purpose of purification and discharge standards.

Modern sewage treatment technology, according to the degree of treatment, can be divided into primary, secondary and tertiary treatment.

Primary treatment: requires the removal of solid pollutants in the suspended state of sewage, and most of the physical treatment methods can only complete the requirements of primary treatment. After primary treatment of sewage, BOD can generally be removed about 30%, can not meet the discharge standards. Primary treatment belongs to the pre-treatment of secondary treatment.

Secondary treatment: The main removal of colloidal and dissolved organic substances in sewage (i.e. BOD, COD substances), the removal rate can reach more than 90%, so that organic pollutants can meet the emission standards.

Tertiary treatment: After primary and secondary treatment, further treatment of refractory organic matter, phosphorus and nitrogen and other soluble inorganic substances that can lead to eutrophication of water bodies. The main methods include biological dephosphorization, coagulation precipitation, sand filtration, activated carbon adsorption, ion exchange and electrodialysis. Tertiary processing is the same word for deep processing, but the two are not the same, tertiary processing is often used after secondary processing. Advanced treatment is a treatment process added after primary or secondary treatment for the purpose of sewage recovery and reuse.

Sludge is the product of sewage treatment. The sludge produced by urban sewage treatment contains a large amount of organic matter, rich in fertilizer, and can be used as agricultural fertilizer, but it also contains a large number of bacteria, parasite eggs and heavy metal ions brought from the production of sewage, which needs to be stable and harmless treatment. The main methods of sludge treatment are reduction treatment (such as concentration, dehydration, etc.), stable treatment (such as anaerobic digestion, aerobic digestion, etc.), comprehensive utilization (such as digestive gas utilization, sludge agricultural utilization, etc.), and final disposal (such as dry incineration, land reclamation, construction materials, etc.).

The treatment process of production sewage varies with the nature of the industry, raw materials, finished products and production processes, and the specific treatment methods and processes should be determined according to the water quality and amount of water and the object of treatment.