The success of the coagulation-flocculation unit depends heavily on the selection of coagulants and flocculants.

 

Choosing the right coagulant and flocculant is crucial for the success of the coagulation and flocculation units in wastewater treatment. These two units are essential steps in most wastewater treatment systems, as they are well-established, simple in design, and easy to operate, resulting in relatively lower operational costs. However, their apparent simplicity sometimes leads to oversight of the fundamental principles and design philosophy, resulting in subpar wastewater treatment, abnormal effluent quality, sludge flotation, turbid water, and other related issues. Therefore, mastering coagulation and flocculation is fundamental for effective wastewater treatment, providing stable effluent quality or ensuring a consistent influent quality for subsequent treatment stages.

 

Principles of Coagulation

 

Coagulation involves altering the physical state of impurities in water by adding coagulants to form larger particles, facilitating subsequent treatment. Coagulants, usually substances with a positive charge like aluminum or iron salts, react with charged impurities in water, leading to precipitation and the formation of coagulated particles.

 

Principles of Flocculation

 

Flocculation takes the process further by combining small particles into larger flocs, enhancing settling rates. By introducing flocculants, typically high-molecular-weight polymers with a negative charge, small particles are attracted to each other, forming flocs that readily settle, aiding in the removal of impurities from water.

 

Functions and Types of Coagulants

 

Generally, it can be assumed that most of the suspended particles in water carry a negative surface charge. The repulsion between negatively charged particles prevents them from aggregating, resulting in the turbidity of water. The coagulation unit primarily introduces coagulants with a positive charge. Through charge neutralization, the repulsion between stable dispersed particles is disrupted, initiating an unstable state. As the chances of particle collisions increase, the particles aggregate and grow in size.

 

Inorganic Coagulants:

 

The majority of suspended particles in water are assumed to carry a negative charge, resulting in stable dispersion and turbid water. Inorganic coagulants, introduced into the water treatment process, typically fall into two categories: monomeric and polymeric. Commonly used inorganic coagulants include aluminum sulfate (alum), aluminum chloride, iron sulfate, ferric chloride, polyaluminum chloride, and polyferric sulfate. Whether monomeric or polymeric, it is essential to ensure the reliability of the coagulant source. As many of these coagulants are by-products of other industries, verifying the effective component and checking for potential interference in wastewater treatment systems is crucial.

 

Category	Chemical Name	Advantages	Disadvantages
Metal Salt	Ferric Chloride	Typically effective coagulants	Used within the pH range of 5.5 to 8.5, typically requiring alkaline supplements to achieve the optimal pH. Larger doses are generally necessary, and an extended reaction time is required.
	Ferric Sulphate
	Aluminum Sulphate, Alum		Utilized between pH 5.5 and 7.5, typically requiring alkali supplementation to achieve the optimum pH. Performance significantly diminishes at lower temperatures. Prolonged reaction time is necessary.
Polymerization Metal Salt	Polyferric Chloride, PFC	It has low pH sensitivity, functioning optimally within a pH range of 4.5 to 9.5. Short mixing time is required, and the formed sludge is more robust and produced in smaller quantities.	The price is higher compared to 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 organic coagulants. The distinctive feature of organic coagulants is their ability to provide a high concentration of positive charges, requiring only a small amount for effective charge neutralization. Therefore, for wastewater from conditions such as high inorganic coagulant dosage, or containing dyes and paints, the use of organic coagulants can be considered. Under appropriate operating conditions, organic coagulants offer advantages such as decolorization, water clarification, and reduced sludge volume. Currently, there are compounded coagulants available in the market that combine the functionalities of both types. This reduces the complexity of simultaneously using multiple agents and achieves optimal coagulation effects.

 

Flocculants/Coagulants - Functions and Types

 

Polymeric Flocculants/Coagulants

 

When coagulants cause particle aggregation, the particles may still take a considerable time to settle. At this point, the addition of polymeric flocculants/coagulants can increase particle size and precipitation rate. There is a wide variety of flocculants/coagulants, categorized based on their charged charges: cationic flocculants/coagulants, anionic flocculants/coagulants, amphoteric flocculants/coagulants, and non-ionic flocculants/coagulants. Typically, after using a positively charged coagulant, the consideration turns to using negatively charged flocculants/coagulants, aiding in charge neutralization and being the most widely used. In certain situations, the use of cationic or non-ionic flocculants/coagulants may yield better results than anionic flocculants/coagulants. Therefore, when selecting flocculants/coagulants, factors such as the ionic nature, viscosity, molecular weight, and jar test results can serve as criteria for decision-making. Common flocculants/coagulants include: 

 

Category	Chemical Name	Advantages	Disadvantages
Organic Cationic polymer	PolyDiallydimethyl Am monium Chloride, PolyDADMAC	Produces more compact shear-resistant sludge. Typically used in conjunction with inorganic coagulants.	Increases the complexity of the coagulation/flocculation stage, but when used correctly, it can enhance process performance and reduce costs.
	Epichlorohydrin Dimethylamine Polymer, Epi-DMA
	Trimethylammonium Chloride
	Quaternary Ammonium Salt
	Polydimethyl Aminomethyl Polyacrylamide
Nonionic organic polymer	Polyacrylamides
	Polyethylene Oxide
Anionic organic polymer	polyacrylamides	Forms larger-sized sludge through bridging action.
	Polyacrylic Acid
	Polyacrylates

 

Single-Dose Powder Coagulant

 

There are currently single-dose powder coagulants available that combine the functions of coagulation and flocculation/agglomeration into one product. Through a comprehensive formulation design, these products achieve both coagulation and flocculation/agglomeration with a single dose. For field operators, adjusting the dosage of one chemical agent achieves both charge neutralization and particle enlargement, making the operation relatively easy and convenient. In addition, powder coagulants, due to their modified mineral powder formulation, help increase shear force during sludge dewatering, thereby improving sludge dewaterability. However, there are several options for such powder coagulants, and the optimal choice needs to be determined through jar tests. Currently available powder products include the following types:

 

General-purpose single-dose powder coagulant - for treating wastewater containing trace oils and surfactants.

Demulsifying single-dose powder coagulant - for treating wastewater containing moderate levels of oil or surfactants.

Rapid-acting single-dose powder coagulant - accelerates reaction and sludge settling.

Recyclable single-dose powder coagulant - low conductivity without membrane fouling.

 

Conclusion

 

Through the integration of various coagulants and flocculants/coagulants in practical applications, we can clearly see their importance in wastewater treatment. The choice and use of coagulants directly affect the effectiveness of wastewater treatment. In selecting the appropriate chemicals and determining the optimal operating conditions, we rely on the professional judgment and services provided by various chemical suppliers. This helps achieve optimal coagulation effects, allowing particles in wastewater to aggregate effectively. Subsequently, through the synergistic action with flocculants/coagulants, we can further enlarge these particles, enhance their settling properties, and achieve complementary effects.