Types of coagulation and flocculation in water treatment

Coagulation and flocculation chemicals aid in binding fine suspended particles in water. When added to the treatment system, these chemicals cause the particles to aggregate into larger flocs with sufficient weight to settle at the bottom, thereby effectively supporting the solid-liquid separation and removal processes. Common chemicals used in this stage include aluminum sulfate (alum), ferric sulfate, and various coagulant aids.

Nowadays, thanks to advances in science and engineering, there are many highly effective wastewater treatment methods that meet effluent discharge standards and help protect the environment. However, in many cases, wastewater is polluted by heavy metals and small insoluble solids, making gravity settling ineffective. At that time, wastewater treatment system operators will use coagulation and flocculation methods to make settling easier. In the process, the flocculation process requires flocculation aids. So, what are the types of flocculation chemicals in wastewater treatment? What do they include, and how do they impact the overall treatment process?

1/ Coagulation and flocculation in water treatment

Today, coagulation and flocculation chemicals are widely used because they have many uses. The main function of coagulation and flocculation chemicals is to bind small suspended particles in water that are too small to settle naturally. After gathering together into larger clusters, these flocs are heavy enough to settle to the bottom of the tank due to the effect of gravity. From there, the process of separating solids from water becomes faster, more efficient and easier to control than letting the water settle naturally.

In addition, the use of coagulation and flocculation chemicals helps reduce the load on the next treatment steps. When the amount of suspended solids is significantly reduced, filtration equipment is less prone to clogging, and the required dosage of disinfectants also decreases, thereby optimizing the overall operational cost of the treatment system.

The coagulation process in water treatment

Coagulation is the process of breaking the zeta potential (the difference in electrical potential between the surface of the colloidal particles and the outer coating) and then bonding these colloidal particles together.

To break the bonds of the protective layer, people will add coagulants, which are ions with opposite charges, to the water source. From then on, the colloidal particles will no longer have a protective layer and can easily be separated from the solution.

The flocculation process in water treatment

The flocculation process takes place after colloidal particles have been destabilized and begin to aggregate, forming larger flocs in terms of both size and mass. These flocs can then settle more easily, facilitating subsequent filtration.

At this stage, coagulant aids or sometimes additional coagulants are added. They act as “bridges” that help bind the particles together, promoting the formation of larger, settleable sludge flocs.

2/ Popular coagulants and flocculants in wastewater treatment

Currently, there are many different types of coagulants and flocculants on the market, each with its own advantages, disadvantages and applications.

There are many different types of coagulants and flocculants on the market.

Aluminum sulfate

Alum, also known as Kali Aluminum, is a mixture of sulfate salts of aluminum. Alum includes the following types:

• Aluminum sulfate Al2(SO4)3.18H2O is produced from bauxite ore, nefelin, some clay.
• Aluminum oxychloride Al2(OH)3Cl: produced from Al(OH)3 mixed with HCl.
• Sodium aluminate NaAlO2: produced from Al2O3 or Al(OH)3 + NaOH.

Alum exists in white/slightly yellow powder or granular form.

Ferric Sulphate

Ferric alum is a double salt composed of ferric sulfate (III) and the sulfate salt of an alkali metal or ammonium.

Common types of ferric alum:

• Iron Chlorite: FeCl3.6H2O: manufactured from Fe-Cl2 (at 700 degrees C) or iron ore + Cl2
• Ferric Sulfate II: FeSO4.7H2O
• Ferric Sulfate III: Fe2(SO4)3.2H2O: manufactured from Fe2O3+ H2SO4
• Mixture of Ferric Sulfate (III) and Ferric Chloride (III)
• Mixture of Aluminum Sulfate (III) and Chloride (III)

Ferric alum exists in pure form, and colourless crystals have tiny grains.

Colloidal aids

There are colloidal aids, such as:

• Poly Aluminum Chloride: PAC
• Aln(OH)mCl(3n-m) -> Al12(OH)24AlO4(H2O12)127+
• Poly Aluminum Chloride Sunfat: PACS
• Poly Aluminum Silica Sulfat: PASS
• Poly Ferric Chloride: PFC
• Poly Aluminum Ferric Chloride: PAFC
• Poly Aluminum Sulicate Chloride: PASC
• Poly Ferric Silicate Chloride: PFSC
• Poly Aluminum Ferric Silicate Chloride: PASC

Most colloidal agents are also a type of alum but exist in high molecular form (polymer), in 2 forms: yellowish powder, completely soluble in water and yellowish brown liquid.

3/ Advantages, disadvantages and uses of coagulants and flocculants chemicals

Advantages and disadvantages

Uses of coagulants and flocculants chemicals

4/ Factors affecting the effectiveness of coagulants and flocculant chemicals

To ensure the coagulation and flocculation process runs smoothly and effectively, operators need to check and fix the following factors:

Water temperature

When the water temperature increases, the thermal motion of coagulant particles intensifies, resulting in more frequent collisions and enhanced particle adhesion. Conversely, lower temperatures reduce collision rates and diminish coagulation efficiency. Therefore, maintaining an appropriate temperature range is essential, as it directly influences the effectiveness of wastewater treatment.

Dirty impurities in water

Some types of dirty impurities such as: Organic matter, microorganisms, grease, surfactants can affect the coagulation and flocculation process. Therefore, wastewater needs to be filtered of trash, coarse residue, grease before being put into chemical treatment.

Dosage of coagulant and coagulant aid

Depending on the water source that needs to be treated, we determine the essential amount of coagulant and flocculant in the appropriate ratio. Normally, we need to conduct testing to evaluate the effectiveness before full-scale operation.

pH concentration in the water composition

pH affects the activity and performance of coagulant chemicals. Therefore, it is important to adjust the pH level of wastewater to an optimal range before introducing it into the treatment system.

For example, pH ranges result in high coagulation and flocculation efficiency, such as:

• Aluminum sulfate is most effective at pH:  5.5-7.5
• Ferric sulphate is most effective at pH: 3.5-6.5 and 8-9

Stirring speed

The stirring speed affects the contact between coagulant particles and coagulant chemicals. Normally, the following procedure should be followed:

• Initially, a fast mixing speed is required to help dissolve and evenly distribute the chemicals in the water.
• Then, the stirring speed should be reduced to allow flocs to form and settle properly, avoiding floc breakage.

If you need more detailed consultation regarding types of coagulants and flocculants used in wastewater treatment, or if you have any questions, please contact us at Hotline 0888 167 247. With many years of experience, NTS Engineering Co., Ltd will support you in finding the most optimal and cost-effective solutions.

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