Commonly used chemicals for sewage treatment mainly include five categories: acid-base regulators, flocculants/coagulants, redox agents, functional agents (such as phosphorus removal agents, ammonia nitrogen removal agents, etc.) and auxiliary agents. Through physical and chemical reactions, pollutants are removed and water quality is discharged or reused in compliance with standards.
1. Acid-base regulators
Used to adjust the pH value of sewage to the neutral range (pH6-9) to ensure the subsequent treatment effect:
1.1. Acids: sulfuric acid, hydrochloric acid, oxalic acid (treatment of high-alkali wastewater such as printing and dyeing wastewater)
1.2 Alkali: sodium hydroxide (caustic soda), lime (neutralization of acidic wastewater such as electroplating wastewater, simultaneous removal of heavy metals, etc.), commonly used to neutralize acidic wastewater, such as electroplating wastewater, pharmaceutical wastewater, etc.
2. Flocculants and coagulants
2.1 Flocculants in sewage treatment often refer to polyacrylamide (PAM), which is a water-soluble high molecular polymer that is insoluble in most organic solvents. It has good flocculation and can reduce the friction resistance between liquids. It is divided into three types: anionic polyacrylamide, cationic polyacrylamide, and nonionic polyacrylamide.
2.1.1 Anionic type: high turbidity wastewater (sand washing, mining wastewater);
2.1.2 Cationic type: sludge dehydration (municipal sewage);
2.1.3 Nonionic type: acidic or metal ion-containing wastewater.
2.2 Coagulants:
2.2.1 Coagulants in sewage treatment mainly refer to polyaluminum chloride (PAC): alias basic aluminum chloride, polyaluminum chloride, hydroxyaluminum chloride, PAC, etc. It is a high-efficiency inorganic polymer coagulant produced by the bridging effect of hydroxide ions and the polymerization of multivalent anions. Compared with traditional coagulants, PAC has the remarkable characteristics of fast floc formation, fast sedimentation speed, wide pH range, and low corrosion to pipeline equipment.
2.2.2 Aluminum sulfate: a solid powder that is acidic when dissolved in water. It has low solubility, and the solution exhibits an acidic reaction. It is low in cost but poor in low temperature effect. As an important coagulant, aluminum sulfate can help suspended matter and colloidal particles in the water treatment process to condense into larger agglomerates, which facilitates precipitation and filtration. It can effectively remove turbid substances in water and improve water clarity.
2.2.3 Ferric chloride: a highly efficient flocculant that is easily soluble in water to form an acidic solution. It can quickly condense suspended impurity particles in wastewater, thereby simplifying subsequent precipitation and filtration steps, and is suitable for a variety of wastewater treatment.
3 Redox agent: acts to decompose organic matter or transform pollutant forms
3.1 Oxidant: degrades organic matter and removes color and odor through oxidation reactions. Common oxidants include sodium pyrosulfate (sterilization, COD removal rate 40%-60%), hydrogen peroxide (Fenton process for treating refractory organic matter). The main functions are oxidation disinfection, removal of organic matter and killing of microorganisms, with the characteristics of high efficiency, energy saving and environmental protection.
3.2 Reducing agent: Reduction reaction removes heavy metal ions (such as Cr⁶⁺, Hg²⁺) or degrades oxidative pollutants. Common reducing agents include sodium metabisulfite (reducing hexavalent chromium to trivalent chromium, a key step in electroplating wastewater), sodium sulfide, and sodium bisulfite. It has a reducing effect under acidic conditions and is used for wastewater treatment in the electroplating industry. For example, to treat hexavalent chromium wastewater, sodium metabisulfite can be added first, and after they have fully reduced, other products can be added after alkali adjustment, and then sodium sulfide can be added to remove heavy metals that have not been completely precipitated. This is the important role of sodium metabisulfite in treating industrial wastewater.
4 Functional agents and auxiliary agents
4.1 Functional agents remove specific pollutants, while other agents prevent scaling, defoaming and cleaning. Common functional agents in sewage treatment processes include ammonia nitrogen removers, phosphorus removers, heavy metal capture agents, decolorizers, defoamers, etc.
4.2 Auxiliary agents are used to assist other treatment processes to quickly purify water quality. Common auxiliary agents include scale inhibitors, demulsifiers, and citric acid.
