The selection of coagulants and flocculants may related with the success or not in wastewater treatment system~

 

The two units of coagulation and flocculation/coagulation are indispensable and important steps in the general wastewater treatment system, and almost every wastewater needs to go through these two physical treatment units. The reason is that the coagulation and flocculation/agglutination technology is relatively skilled, the design is simple, the operation is easy, and the operation cost is relatively low in comparison. However, due to such easy characteristics, it is often easy to ignore the essence of coagulation and flocculation/coagulation and the design concept of the system in actual operation, resulting in poor wastewater treatment effects, abnormal discharge water quality, sludge floating, water turbidity, excessive sludge, etc. question. Therefore, to properly treat wastewater, the first basic skill is to do a good job of coagulation and flocculation/agglutination to obtain stable discharge water quality, or to provide stable pre-treatment water quality to enter the second stage of treatment.

 

Function and type of coagulant

Generally, it can be assumed that most of the suspended particles in the water are negatively charged on the surface. As a result of negative and negative repulsion, the particles are dispersed and unable to aggregate. This is the reason why we see turbid water quality. The coagulation unit mainly uses a positively charged coagulant to neutralize the positive and negative charges so that the stable dispersed particles that repel each other begin to be in an unstable state, and the chance of collision between the particles increases gradually, which promotes the aggregation of the particles. and increase.

 

Inorganic coagulants

The coagulants added can generally be divided into two categories, mainly inorganic coagulants and organic coagulants, and inorganic coagulants can be divided into monomers and polymers. The common ones on the market are aluminum sulfate (alum), aluminum chloride, ferric sulfate, ferric chloride, polyaluminum chloride, and polyferric sulfate. Regardless of whether it is a monomer or a polymer, it is necessary to pay attention to whether the quality of the source is reliable when using it. In practice, since these coagulants are mostly by-products produced by other industries, in addition to confirming the effective components of the unit, whether they are mixed with other possible Components that interfere with wastewater treatment systems also need attention.

 

Type	Chemcials	Pro	Con
Metal Salt	Ferric Chloride	Typical and effective coagulants.	Use between pH 5.5 and 8.5, usually requires supplemental caustic soda to achieve the optimum pH. Usually larger doses are required. Requires longer reaction time.
	Ferric Sulphate
	Aluminum Sulphate, Alum		For use between pH 5.5 and 7.5, supplemental caustic soda is usually required to achieve the optimum pH. Performance degrades significantly at lower temperatures.
Polymerization Metal Salt	Polyferric Chloride, PFC	Less sensitive to pH, reacting at pH 4.5 - 9.5. The mixing time is shorter. The sludge formed is tougher and less in volume.	The price is higher than traditional coagulants.
	Polyaluminum Sulfate, PAS
	Polyaluminum Chloride, PAC
	Poly Aluminum Ferric Sulfate, PAFS
	Poly Aluminum Ferric Chloride, PAFC

 

Organic coagulants

Another type of coagulant is an organic coagulant. The characteristic of organic coagulants is that they can provide a high concentration of positive charges, and only need a small amount of addition to achieve electrical neutrality. Therefore, organic coagulants can be considered for treatment of wastewater with high additions of inorganic coagulants, dyes, paints, etc. Under proper operating conditions, it has the advantages of decolorization, water clarification, and reduction of sludge volume. At present, a compound coagulant has been launched on the market, which combines the functions of the two types together to reduce the complexity of operating several medicines at the same time and achieve the best coagulation effect.

 

Function and type of flocculant/coagulant

Polymer flocculant/coagulant

When the coagulant causes particles to aggregate, it still takes a long time for the particle size to settle down. At this time, the particle size and sedimentation speed can be increased by adding a polymer flocculant/coagulant. There are many kinds of flocculants/coagulants, which can be divided into cationic flocculants/coagulants, anionic flocculants/coagulants, zwitterionic flocculants/coagulants and non-ionic flocculants/coagulants according to the charged charge. Usually after the use of positively charged coagulants, the use of anionic and negatively charged flocculants/coagulants is considered, which is helpful for electrical neutralization and is the most widely used. In some cases, the effect of using cationic or non-ionic flocculant/coagulant is better than that of anionic flocculant/coagulant. Therefore, in the selection of flocculant/coagulant, in addition to the factors of anion and cation, the viscosity, molecular weight and bottle-cup test results of the flocculant/coagulant can be used as the basis for selection. The following are common flocculants/coagulants:

 

Type	Chemicals	Pro	Con
Organic Cationic polymer	PolyDiallydimethyl Am monium Chloride, PolyDADMAC	Produces denser, shear-resistant sludge. It is generally used in conjunction with inorganic coagulants.	Add complexity to the coagulation/flocculation stage, but when used correctly can improve process performance and reduce costs.
	Epichlorohydrin Dimethylamine Polymer, Epi-DMA
	Trimethylammonium Chloride
	Quaternary Ammonium Salt
	Polydimethyl Aminomethyl Polyacrylamide
Nonionic
	Polyethylene Oxide
Anionic organic polymer	Polyacrylamides	A larger form of sludge is formed by bridging.
	Polyacrylic Acid
	Polyacrylates

 

One-dose powder flocculants

At present, there is also a one-dose powder flocculant that combines coagulant and flocculant/coagulant. The functions of coagulation and flocculation/aggregation are combined in one dose mainly through the compound product formula design. For the on-site operators, only need to adjust the dose of a drug to achieve the physical effect of electrical neutralization and particle enlargement, which is relatively easy and convenient in operation. In addition, the powder flocculant has a modified mineral powder formula, which helps to increase the shear force during sludge dewatering and improve sludge dewaterability. However, there are several options for this type of powder flocculant, and the best choice can only be made after a bottle-cup test. There are several types of powder products currently on the market:

Neosorb^® GS-A N

Universal one-package powder flocculant - treatment of waste water containing trace amounts of oil and surfactants

Neosorb^® EMU-V Q

Demulsifying one-package powder flocculant - treatment of waste water containing moderate oil or surfactant

Neosorb^® EMU-V DES

Quick-acting one-dose powder flocculant-accelerates reaction and sludge settlement

Neosorb^® MB

Recyclable one-dose powder flocculant- low conductivity and no membrane blocking

 

Conclusion

Based on the current experience of using various coagulants and flocculants/coagulants in practical cases, the selection and use of coagulants play a key role in the quality of wastewater treatment. The selection of chemicals and the optimum operating conditions require the professional judgment and service provided by each chemical supplier to achieve the best coagulation effect and effectively aggregate particles in wastewater. The size of the particles increases, and the sedimentation property is increased to achieve the effect of complementing each other.

The video below uses a one-dose powder flocculant (left: PAC/PAM, right: powder flocculant) for metal pretreatment wastewater. The use of powder flocculant can get better flocculation effect, faster sedimentation speed, and less sludge amount than general coagulant (under the premise of achieving the same effect).