In order to achieve standard discharge or reuse of wastewater after treatment, multiple chemical agents need to be used in the treatment process. According to their different uses, these drugs can be classified into the following categories:

⑴ coagulants: sometimes also known as coagulants, can be used as a means to enhance solid-liquid separation, and are used in primary sedimentation tanks, secondary sedimentation tanks, flotation tanks, and tertiary or deep treatment processes.

⑵ Coagulant: The auxiliary flocculant plays a role in enhancing the coagulation effect.

⑶ Conditioning agent: also known as dehydrating agent, used for conditioning the remaining sludge before dewatering. Its varieties include some coagulants and coagulants mentioned above.

⑷ Demsifier: sometimes also known as a destabilization agent, mainly used for pre-treatment of oily wastewater containing emulsified oil before water flotation. Its varieties include some coagulants and coagulants mentioned above..

(5) Defoamer: mainly used to eliminate a large number of foam in the process of aeration or mixing.

PH adjuster: used to adjust the pH value of acidic and alkaline wastewater to neutral.

⑺ Redox agent: used for the treatment of industrial wastewater containing oxidizing or reducing substances.

Disinfectant: Used for disinfection treatment after wastewater treatment before discharge or reuse.

Although there are many types of the above drugs, each drug has different names due to its different effects when used in different situations. For example, CL2 is called a coagulant aid when used to enhance the coagulation treatment effect of wastewater, an oxidant when used to oxidize cyanide or organic matter in wastewater, and a disinfectant when used for disinfection treatment.

What is a flocculant? What is its function?

Coagulants, as a means of enhancing solid-liquid separation in the field of wastewater treatment, can be used to strengthen the initial precipitation, flotation treatment, and secondary precipitation of wastewater after activated sludge process. They can also be used for tertiary or deep treatment of wastewater. When used for conditioning the remaining sludge before dewatering, coagulants and coagulants become sludge conditioning agents or dewatering agents.

When using traditional coagulants, the method of adding coagulants can be used to enhance the flocculation effect. For example, using activated silicic acid as a coagulant aid for inorganic coagulants such as ferrous sulfate and aluminum sulfate, and adding them in sequence before and after, can achieve good flocculation effect. Therefore, in layman's terms, inorganic polymer flocculant IPF is actually prepared by combining coagulants and coagulants together and then adding them together to simplify the user's operation.

Coagulation treatment is usually placed in front of solid-liquid separation facilities and combined with separation facilities to effectively remove suspended solids and colloidal substances with particle sizes ranging from 1nm to 100 μ m from raw water, reducing effluent turbidity and CODcr. It can be used for pre-treatment and deep treatment of sewage treatment processes, as well as for residual sludge treatment. Coagulation treatment can effectively remove microorganisms and pathogens from water, as well as emulsified oil, color, heavy metal ions, and other pollutants in wastewater. The removal rate of phosphorus in wastewater can reach up to 90-95% when using coagulation sedimentation, making it the cheapest and most efficient method for phosphorus removal.

What is the mechanism of action of flocculants?

Colloidal particles in water are small in size, and their surface hydration and charge make them stable. When flocculants are added to water, they hydrolyze into charged colloids that form a double layer structure with the surrounding ions. By using rapid stirring after administration, the collision opportunities and frequency between colloidal impurity particles in water and the gel clusters formed by the hydrolysis of flocculants are promoted. Impurity particles in water first lose stability under the action of flocculants, and then coalesce into larger particles, which precipitate or float up in separation facilities.

The product GT of the velocity gradient G generated by stirring and the stirring time T can indirectly represent the total number of particle collisions during the entire reaction time. By changing the GT value, the coagulation reaction effect can be controlled. Generally, the GT value is controlled between 104 and 105. Considering the influence of impurity particle concentration on collision, the GTC value can be used as a control parameter to characterize the coagulation effect, where C represents the mass concentration of impurity particles in the sewage, and it is recommended that the GTC value be around 100.

The process of promoting the rapid diffusion of flocculants into the water and mixing them evenly with all wastewater is called mixing. The process in which impurity particles in water interact with flocculants and lose or reduce stability through mechanisms such as compressing the double layer and electrical neutralization, resulting in the formation of micro flocs, is called coagulation. The process of aggregating and generating micro flocs, which grow into large flocs through mechanisms such as adsorption bridging and sediment trapping under the agitation of bridging materials and water flow, is called flocculation. The combination of mixing, coagulation, and flocculation is called coagulation, and the mixing process is generally completed in a mixing tank, while coagulation and flocculation are carried out in a reaction tank.

What are the types of coagulants?

Coagulants are a type of substance that can reduce or eliminate the precipitation stability and polymerization stability of dispersed particles in water, causing them to aggregate and be removed. According to their chemical composition, coagulants can be divided into three categories: inorganic coagulants, organic coagulants, and microbial coagulants.

Inorganic coagulants include aluminum salts, iron salts, and their polymers. Organic flocculants can be classified into several types based on the charge properties of the charged groups of the polymerized monomers, including anionic, cationic, nonionic, and amphoteric. They can also be divided into two categories based on their sources: artificially synthesized and natural polymer flocculants. In practical applications, inorganic and organic flocculants are often compounded based on their different properties to produce inorganic organic composite flocculants. Microbial flocculants are a product of the combination of modern biology and water treatment technology, and are an important direction for the current research, development, and application of flocculants.

What are the types of inorganic flocculants?

The traditional inorganic coagulants used are low molecular weight aluminum and iron salts. Aluminum salts mainly include aluminum sulfate (AL2 (SO4) 3 ∙ 18H2O), alum (AL2 (SO4) 3 ∙ K2SO4 ∙ 24H2O), and sodium aluminate (NaALO3). Iron salts mainly include ferric chloride (FeCl3 ∙ 6H2O), ferrous sulfate (FeSO4 ∙ 6H2O), and ferric sulfate (Fe2 (SO4) 3 ∙ 2H2O).

Generally speaking, inorganic flocculants are widely used in water treatment due to their easy availability of raw materials, simple preparation, low cost, and moderate treatment efficiency.

What are the characteristics of inorganic flocculant aluminum sulfate?

Since the late 19th century, when the United States first used aluminum sulfate for water treatment and obtained patents, aluminum sulfate has been widely used for its excellent coagulation and sedimentation performance. Aluminum sulfate is currently the most widely used flocculant in the world, with an annual production of approximately 5 million tons worldwide, of which nearly half is used in the field of water treatment. There are two forms of commercially available aluminum sulfate: solid and liquid. The solid form is divided into refined and crude according to the content of insoluble substances. In China, the commonly used solid product for drinking water purification is alum, which is a double salt of aluminum sulfate and potassium sulfate. However, it is not widely used in industrial water and wastewater treatment.

The pH range suitable for aluminum sulfate is related to the hardness of the raw water. When treating soft water, the appropriate pH value is 5-6.6. When treating medium hard water, the appropriate pH value is 6.6-7.2. When treating high hard water, the appropriate pH value is 7.2-7.8. The suitable water temperature range for aluminum sulfate is 20oC to 40oC, and the coagulation effect is poor below 10oC. Aluminum sulfate has low corrosiveness and is easy to use, but its hydrolysis reaction is slow and requires a certain amount of alkali consumption.

What are the characteristics of inorganic flocculant ferric chloride?

Ferric chloride is another commonly used inorganic low molecular weight coagulant, with solid black brown crystals and high concentration liquids. It has the advantages of easy solubility in water, large and heavy alum flowers, good precipitation performance, and wide adaptability to temperature, water quality, and pH.

The suitable pH range for ferric chloride is 9-11, and the formed flocs have a high density and are easy to precipitate. The effect is still good at low temperatures or high turbidity. Solid ferric chloride has strong water absorption, strong corrosiveness, and is prone to corrosion of equipment. It requires high anti-corrosion requirements for dissolution and dosing equipment, has a pungent odor, and operates under poor conditions.

The mechanism of action of ferric chloride is to use various hydroxyl iron ions generated by the stepwise hydrolysis of trivalent iron ions to achieve flocculation of impurity particles in water. The formation of hydroxyl iron ions requires the use of a large amount of hydroxyl groups in water, so a large amount of alkali will be consumed during use. When the alkalinity of the original water is not sufficient, lime and other alkali sources need to be supplemented.

Ferrous sulfate, commonly known as green alum, forms flocs quickly and stably, with a short precipitation time. It is suitable for situations with high alkalinity and turbidity, but its color is difficult to remove and it is also highly corrosive.

What are the types of inorganic polymer flocculants?

Inorganic polymer flocculant (IPF) is a new type of flocculant developed since the 1960s. Currently, the production and application of IPF have made rapid progress worldwide. The inorganic polymer flocculants of aluminum, iron, and silicon are actually the intermediate products of their hydrolysis, sol, and precipitation processes, namely the hydroxyl and oxygen based polymers of Al (III), Fe (III), and Si (IV). Aluminum and iron are positively charged cations, while silicon is negatively charged anions. In their water-soluble state, their unit molecular weights range from several hundred to several thousand, and they can combine with each other to form aggregates with fractal structures. Their coagulation flocculation process is a comprehensive manifestation of the two effects of electric neutralization and adhesion bridging of particles in water. The particle size of suspended particles in water ranges from nanometers to micrometers, and most of them carry negative charges. Therefore, the charge polarity, electrical strength, molecular weight, and aggregate size of coagulants and their forms are the main factors determining their flocculation effect. At present, there are dozens of types of inorganic polymer flocculants (the main varieties are shown in the table below), and their production accounts for 30% to 60% of the total flocculant production, among which polyaluminum chloride is widely used.

Types and varieties of commonly used inorganic polymer flocculants

What are the characteristics of inorganic polymer flocculants?

The hydroxyl and oxygen based polymers of Al (III), Fe (III), and Si (IV) will further combine to form aggregates, which will be maintained in aqueous solution under certain conditions. Their particle size is roughly in the nanometer range, and the coagulation flocculation effect will result in low dosage and high effect. If we compare their reaction polymerization rates, the trend from Al → Fe → Si tends to be strong, and the trend from hydroxyl bridging to oxygen bridging also follows this order. Therefore, the reaction of aluminum polymer is relatively mild and the morphology is relatively stable. The hydrolytic polymer of iron reacts rapidly and is prone to lose stability and precipitate. The silicon polymer tends to generate sol and gel particles.

The advantages of IPF are reflected in its superior performance compared to traditional flocculants such as aluminum sulfate and ferric chloride, and its lower price compared to organic polymer flocculants (OPF). Now it has been successfully applied in various treatment processes of water supply, industrial wastewater, and urban sewage, including pretreatment, intermediate treatment, and deep treatment, gradually becoming the mainstream flocculant. However, in terms of morphology, polymerization degree, and corresponding coagulation flocculation effect, inorganic polymer flocculants still occupy a position between traditional metal salt flocculants and organic polymer flocculants. Its molecular weight, particle size, and flocculation bridging ability are still much worse than organic flocculants, and there is also instability in further hydrolysis reactions. These weaknesses of IPF have promoted the research and development of various composite inorganic polymer flocculants.

What are the characteristics of polyaluminum chloride?

Polyaluminum chloride (PAC), also known as basic aluminum chloride, has the chemical formula ALn (OH) mCL3n-m. PAC is a multivalent electrolyte that can significantly reduce the colloidal charge of clay impurities (mostly negatively charged) in water. Due to its high relative molecular weight and strong adsorption capacity, it forms larger flocs with better flocculation and sedimentation performance than other coagulants. PAC has a high degree of polymerization, and rapid stirring after addition can greatly shorten the formation time of flocs. PAC is less affected by water temperature and works well when used at low water temperatures. It reduces the pH value of water less and is suitable for a wide pH range (can be used in the pH range of 5-9), so alkaline agents can be omitted. The dosage of PAC is small, the sludge production is also low, and the use, management, and operation are relatively convenient, with little corrosiveness to equipment, pipelines, etc. Therefore, PAC has a trend of gradually replacing aluminum sulfate in the field of water treatment, but its disadvantage is that it is relatively expensive.

Furthermore, from the perspective of solution chemistry, PAC is a kinetic intermediate product in the hydrolysis polymerization precipitation reaction of aluminum salts, which is thermodynamically unstable. Generally, liquid PAC products should be used within six months. Adding certain inorganic salts (such as CaCl2, MnCl2, etc.) or polymers (such as polyvinyl alcohol, polyacrylamide, etc.) can improve the stability of PAC and increase its coagulation ability. In terms of production technology, introducing one or several different anions (such as SO42-, PO43-, etc.) into the manufacturing process of PAC can change the structure and morphology distribution of the polymer to a certain extent through polymerization, thereby improving the stability and efficacy of PAC; If other cationic components, such as Fe3+, are introduced during the manufacturing process of PAC, and Al3+and Fe3+are alternately hydrolyzed and polymerized, composite flocculant polymeric aluminum iron can be prepared.

The content of aluminum oxide is a measure of the effective components of polyaluminum chloride. Generally speaking, the higher the density of flocculant products, the higher the content of aluminum oxide. Generally speaking, the higher the alkalinity of polyaluminum chloride, the better its adsorption bridging ability. However, due to its proximity to [Al (OH) 3] n, it is prone to precipitation and therefore has poor stability.

The above are the chemicals used for sewage treatment.

This article is compiled by chemical wastewater treatment equipment and does not represent the views of this website.