What are flocculants, coagulants and conditioners？

What are flocculants, coagulants and conditioners？

Summary

In sludge filter press treatment, these agents can be divided into the following categories according to their different uses:

What are flocculants, coagulants and conditioners？
What are flocculants, coagulants and conditioners？
What are flocculants, coagulants and conditioners？
1. What are flocculants, coagulants and conditioners?

In sludge filter press treatment, these agents can be divided into the following categories according to their different uses:

(1) Flocculants: Sometimes also called coagulants, they 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 treatment or deep treatment process links.

(2) Coagulants: Assist flocculants to play a role and enhance the coagulation effect.

(3)Conditioners: Also known as dehydrating agents, they are used to condition the residual sludge before dehydration. Their varieties include some of the above-mentioned flocculants and coagulants.
2. Flocculants

Flocculants are a type of substance that can reduce or eliminate the sedimentation stability and aggregation stability of dispersed particles in water, causing the dispersed particles to condense and flocculate into aggregates for removal.
According to their chemical composition, flocculants can be divided into inorganic flocculants and organic flocculants.

(1) Inorganic flocculants
The traditionally used inorganic flocculants are low molecular weight aluminum salts 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∙6H20), ferrous sulfate (FeSO4∙6H20), and ferric sulfate (Fe2(SO4)3∙2H20).
Generally speaking, inorganic flocculants have the characteristics of easy availability of raw materials, simple preparation, low price, and moderate treatment effect, so they are widely used in water treatment.

(2) Inorganic polymer flocculants
The hydroxyl and oxygen polymers of Al(III) and Fe(III) will further combine into aggregates and remain in aqueous solution under certain conditions. Their particle size is roughly in the nanometer range. In this way, the coagulation-flocculation effect will be exerted to obtain the result of low dosage and high effect.
If their reaction polymerization speeds are compared, the reaction of aluminum polymer is relatively slow and the morphology is relatively stable, while the hydrolysis polymer of iron reacts quickly and easily loses stability and precipitates.
The advantages of inorganic polymer flocculants are reflected in their superior performance compared with traditional flocculants such as aluminum sulfate and ferric chloride, and their lower price than organic polymer flocculants. PAC is now successfully applied in various treatment processes of water supply, industrial wastewater and urban sewage, including pretreatment, intermediate treatment and deep treatment, and has gradually become a mainstream flocculant. However, in terms of morphology, degree of polymerization and corresponding coagulation-flocculation effect, inorganic polymer flocculants are still between traditional metal salt flocculants and organic polymer flocculants.
(3) Organic polymer flocculants
Man-made organic polymer flocculants are mostly polypropylene and polyethylene substances, such as polyacrylamide and polyethyleneimine. These flocculants are all water-soluble linear polymer substances. Each macromolecule is composed of many repeating units containing charged groups, so they are also called polyelectrolytes. Those containing positively charged groups are cationic polyelectrolytes, those containing negatively charged groups are anionic polyelectrolytes, and those containing neither positively charged groups nor negatively charged groups are called non-ionic polyelectrolytes.
Currently, the most commonly used polymer flocculants are anionic, which can only play a coagulant role on negatively charged colloidal impurities in water. They are often not used alone, but are used in combination with aluminum salts and iron salts. Cationic flocculants can play both coagulation and flocculation roles and are used alone, so they have developed rapidly.
3. Coagulant aids

In the coagulation treatment of wastewater, sometimes a single flocculant cannot achieve a good coagulation effect, and it is often necessary to add some auxiliary agents to improve the coagulation effect. Such auxiliary agents are called coagulants. Commonly used coagulants include chlorine, lime, activated silica, gelatin and sodium alginate, activated carbon and various clays.
Some coagulants do not have a coagulation effect by themselves, but play a role in assisting flocculants to produce coagulation effects by adjusting and improving coagulation conditions. Some coagulants participate in the formation of flocs and improve the structure of flocs, which can turn the fine and loose flocs produced by inorganic flocculants into coarse and dense alum flowers.
Common types of coagulants:
There are many types of coagulants, but they can be roughly divided into the following two categories according to their role in the coagulation process:

1) Agents that adjust or improve coagulation conditions
The coagulation process should be carried out within a certain pH range. If the pH value of the raw water cannot meet this requirement, the pH value of the raw water should be adjusted. This type of coagulant includes acids and alkalis. When the pH value of the raw water is low and the alkalinity is insufficient, making it difficult for the flocculant to hydrolyze, alkaline substances such as CaO, Ca(OH)2, Na2CO3, and NaHCO3 (commonly lime) can be added; when the pH value is high, sulfuric acid or CO2 is often used to reduce the pH value of the raw water.
For wastewater with a large content of dissolved organic matter, oxidants such as Cl2 can be used to destroy organic matter and improve the removal effect of dissolved organic matter. In addition, when ferrous salts are used as flocculants, chlorine can be used to oxidize ferrous iron (Fe2+) into high-valent iron (Fe3+) to improve the coagulation effect.
The above alkaline agents, sulfuric acid, CO2, chlorine, etc. do not play a coagulation role by themselves, but only play a role in assisting coagulation.

2) Coagulants that increase the particle size, density and firmness of alum flocs
The result of coagulation requires the generation of alum flocs with large particle size, high density and firmness, which is conducive to precipitation and not easy to break. In order to obtain this result, combined with the characteristics of water quality, it is sometimes necessary to add certain substances or agents to the water. For example, in low-turbidity wastewater containing light impurities that are not suitable for sedimentation, adding coarser particles such as silica, activated carbon, clay, or returning part of the precipitated sludge can play a role in aggravating and enlarging the alum flocs; when aluminum salts and iron salts are used as flocculants, only small and loose flocs can be produced, high-molecular coagulants such as polyacrylamide, activated silica and gelatin can be added, and their strong adsorption and bridging effects can be used to make the small and loose flocs become coarse and dense.
4. Conditioning agent

Conditioning agent, also known as dehydrating agent, can be divided into two categories: inorganic conditioning agent and organic conditioning agent. Inorganic conditioning agent is generally suitable for vacuum filtration and plate and frame filtration of sludge, while organic conditioning agent is suitable for centrifugal dehydration and belt filter press dehydration of sludge.
Types of commonly used sludge conditioning agents:

1) Inorganic conditioning agent
The most effective, cheapest and most commonly used inorganic conditioning agents are mainly iron salts and aluminum salts. Iron salt conditioning agents mainly include ferric chloride (FeCl3∙6H2O), ferric sulfate (Fe2(SO4)3∙4H2O), ferrous sulfate (FeSO4∙7H2O) and polyferric sulfate (PFS) ([Fe2(OH)n(SO4)3-n/2]m), etc. Aluminum salt conditioning agents mainly include aluminum sulfate (Al2(SO4)3∙18H2O), aluminum chloride (AlCl3), basic aluminum chloride (Al(OH)2Cl), polyaluminum chloride (PAC) ([Al2(OH)n∙Cl6-n]m), etc.
After adding inorganic conditioning agents, the concentration process of sludge can be greatly accelerated and the filtering and dehydration effect can be improved. Moreover, the combination of iron salt and lime can further improve the conditioning effect. The disadvantages of adding inorganic conditioning agents are that the dosage is large. Generally speaking, the dosage should reach 5% to 20% of the dry solid weight of the sludge, which will increase the volume of the filter cake; second, the inorganic conditioning agent itself is corrosive (especially iron salts), and the addition system must have anti-corrosion properties. It should be noted that when ferric chloride is used as a conditioning agent, it will increase the corrosion of the metal components of the dewatered sludge treatment equipment, so the anti-corrosion level of the equipped dewatered sludge treatment equipment should be appropriately increased.

2) Organic conditioning agents
There are many types of organic synthetic polymer conditioning agents, which can be divided into low polymerization degree (molecular weight of about 1,000 to tens of thousands) and high polymerization degree (molecular weight of about hundreds of thousands to millions) according to the degree of polymerization; according to the ionic type, they can be divided into cationic, anionic, non-ionic, anionic and cationic types. Compared with inorganic conditioning agents, the dosage of organic conditioning agents is less, generally 0.1% to 0.5% of the dry solid weight of the sludge, and they are not corrosive.
The organic conditioning agents used for sludge conditioning are mainly high-polymerization polyacrylamide series flocculant products, mainly cationic polyacrylamide, anionic polyacrylamide and non-ionic polyacrylamide. Among them, cationic polyacrylamide can neutralize the negative charge on the surface of sludge particles and produce a bridging effect between particles, showing strong cohesion, with significant conditioning effect, but the cost is high. In order to reduce costs, the cheaper anionic polyacrylamide-lime combination method can be used, using positively charged Ca(OH)2 flocculants to adsorb negatively charged flocculants and sludge particles together to form a composite coagulation system.
5. Relationship between flocculants, coagulants and conditioners

Dehydrating agents are agents added before sludge is dehydrated, that is, conditioners for sludge, so the meanings of dehydrating agents and conditioners are the same. The dosage of dehydrating agents or conditioners is generally calculated as a percentage of the dry solid weight of sludge.
Flocculants are used to remove suspended matter in sewage and are important agents in the field of water treatment. The dosage of flocculants is generally expressed as the amount added per unit volume of water to be treated.
The dosage of dehydrating agents (conditioners) and flocculants and coagulants can all be called dosages. The same agent can be used as a flocculant in sewage treatment and as a conditioner or dehydrating agent in the process of residual sludge treatment.
When a coagulant is used as an auxiliary agent for a flocculant in the field of water treatment, it is called a coagulant. The same coagulant is generally not called a coagulant in the treatment of residual sludge, but is generally called a conditioner or dehydrating agent.