What Is Chemical Wastewater Treatment?

Wastewater chemical treatment refers to changing the chemical or physical properties of pollutants in wastewater through chemical reactions, causing it to change from a dissolved, colloidal or suspended state to a precipitated or floating state, or from a solid state to a gaseous state, and then removed from water Wastewater treatment methods. Wastewater chemical treatment methods can be divided into: wastewater neutralization treatment method, wastewater coagulation treatment method, wastewater chemical precipitation treatment method, wastewater oxidation treatment method, wastewater extraction treatment method, and the like. In order to effectively treat wastewater containing a variety of pollutants with different properties, the above two or more treatment methods are sometimes combined. For example, when treating small-flow and low-concentration phenol-containing wastewater, a chemical coagulation treatment (removing suspended matter, etc.) and a chemical oxidation treatment (removing phenol) are combined ^[1] .

Chemical treatment of wastewater Chemical treatment is the use of chemical reactions to separate and recover pollutants in wastewater, or convert them into harmless substances. The main processes are neutralization, coagulation, chemical precipitation, redox, adsorption, extraction, etc. ^{[ 2]}

Wastewater chemical precipitation treatment method is to add soluble chemicals to the wastewater to make it chemically react with the inorganic pollutants in the ionic state to produce precipitates that are insoluble or hardly soluble in water, thereby purifying the wastewater . Chemicals that are put into wastewater are called precipitants, and commonly used are lime, sulfide, and barium salts.

The principle of the chemical precipitation method is to convert heavy metals in the wastewater into dissolved heavy metal compounds through chemical reactions, and remove the precipitate from the aqueous solution through filtration and separation, including neutralization precipitation method, sulfide precipitation method, iron Oxygen co-precipitation method. Due to the influence of the precipitating agent and environmental conditions, the effluent concentration of the precipitation method often fails to meet the requirements and further processing is required. The resulting precipitate must be well treated and disposed of, otherwise it will cause secondary pollution.

According to the different precipitants, it can be divided into: the hydroxide precipitation method, that is, the neutralization precipitation method, is an effective and economical method for removing heavy metals from waste water; In particular, the mercury- and cadmium-containing wastewater that still fails to meet the discharge standards after being treated by the hydroxide precipitation method; The barium salt precipitation method is often used for the treatment of electroplating chromium-containing wastewater. Chemical precipitation method is a traditional water treatment method, which is widely used in the softening process of water quality treatment, and is also commonly used in industrial wastewater treatment to remove heavy metals and cyanide.

The wastewater coagulation treatment method is a method of purifying wastewater by adding a coagulant to the wastewater to cause the colloidal particles in the wastewater to be coagulated and flocculated and separated. Coagulation of flocculation and flocculation. The former is caused by the addition of electrolytes, which reduces or eliminates the electromotive potential of the colloidal particles, causing the colloidal particles to lose stability and destabilizing the colloidal particles to coalesce with each other; the latter is caused by the adsorption of polymer materials by bridging and colloidal particles to coalesce produce.

Application of coagulant in sewage treatment: Larger coarse particles suspended in particles can be removed by natural precipitation, but smaller suspended particles, even some harmful chemical ions, especially colloidal particles settle very slowly. It can even remain dispersed and suspended in water for a long time without sinking naturally, and it is difficult to separate and remove it from water by natural precipitation. The principle of coagulant is to destroy the stability of these fine particles, make them contact with each other and aggregate together, form flocs, and sink to separate.

The use of coagulants to treat sewage integrates nine processes including mixing, reaction, coagulation, and flocculation. Since the coagulant is put into water, most of them can provide a large amount of positive ions. Positive ions can neutralize the negative charge carried on the surface of colloidal particles, reducing the repulsive force between the particles, so that they can easily approach and agglomerate into flocculent fine particles, realizing the destabilization and aggregation of fine colloidal particles in water into fine particles. the process of. The tiny fine particles are settled through adsorption, winding and bridging to form larger flocs, which can be separated from the water. Coagulants can be classified into two categories; inorganic salts, such as aluminum salts (aluminum sulfate, aluminum potassium sulfate, potassium aluminate, etc.), iron salts (ferric trichloride, ferrous sulfate, iron sulfate, etc.) and magnesium carbonate Etc .; polymer materials, such as polyaluminum chloride, polyacrylamide, etc. During the treatment, a coagulant is added to the wastewater to eliminate or reduce the mutual repulsive force between the colloidal particles in the water, so that the colloidal particles in the water easily collide with each other and agglomerate to form larger particles or flocs, and then separated from the water. The factors that affect the coagulation effect are: water temperature, pH value, turbidity, hardness, and the amount of coagulant added ^[1]

Wastewater oxidation treatment refers to a chemical treatment method that uses strong oxidants to decompose pollutants to purify wastewater. Strong oxidants can gradually degrade organic substances in wastewater, and finally become simple inorganic substances. They can also oxidize pollutants dissolved in water into substances that are insoluble in water and easy to separate from water. The oxidation treatment method can treat almost all kinds of industrial wastewater, and is particularly suitable for treating organic substances that are difficult to biodegrade in wastewater, such as most pesticides and pesticides, phenols, cyanides, and certain substances that cause color and odor. Such as tannin, lignin, etc. ^[3]

(1) Acid and alkali wastewater (or waste residue) neutralization method: (1) Mutual neutralization of acid and alkali wastewater can be quantitatively calculated according to the law of equivalents: NaVa = NbVb, where: Na and Nb are the equivalent concentrations of acid and alkali respectively; Va And Vb are the volumes of the acid-base solution, respectively. During the neutralization process, when the number of equivalents of both acids and bases are exactly equal, it is called the equivalence point of the neutralization reaction. When the neutralization of strong acid and strong alkali reaches the equivalence point, since the generated strong acid and strong alkali salt does not undergo hydrolysis, the equivalence point is the neutral point, and the pH value of the solution is equal to 7.0. However, if the neutralized side is a weak acid or a weak base, the salt generated during the neutralization is hydrolyzed in water. Therefore, although the equivalent point is reached, the solution is not neutral, and may be acidic according to the hydrolysis of the generated brine. Or alkaline, the size of the pH value is determined by the degree of hydrolysis of the salt produced.

(2) Dosing and neutralization method: Dosing and neutralization method is a widely used method of neutralization. The most commonly used alkaline agent is lime, sometimes caustic soda, sodium carbonate, limestone or dolomite are also used. When choosing an alkaline agent, not only its own solubility, reaction speed, cost, secondary pollution, and ease of use, but also factors such as the properties, quantity, and processing costs of the neutralized product.

(3) Filtration and neutralization method: Generally applicable to the treatment of a small amount of acidic wastewater with low acid concentration (sulfuric acid <20g / L, hydrochloric acid, nitric acid <20g / L, etc.), containing a large amount of suspended matter, oil, heavy metal salts and other toxic Material acid wastewater is not suitable. The filter material can be limestone or dolomite. The reaction speed of limestone filter material is faster than that of dolomite, but the allowable sulfuric acid concentration in the feed water is lower than that of dolomite filter material. Neutralizing hydrochloric acid and nitric acid wastewater, both Both can be used. Dolomite is suitable for neutralizing sulfuric acid-containing wastewater.