Let’s start by understanding what coagulation and flocculation of water are. Simply put, these are processes that aggregate small colloidal particles responsible for water turbidity, which do not settle under gravity. A substance is added to the water, which causes particles to clump together, leading to their subsequent settling. The key difference lies in how reagents interact with and affect different particles.

Below, we’ll try to briefly explain the processes of coagulation and flocculation in water treatment and then dive deeper into the specifics of these substances.

Coagulants in water treatment

Coagulants are inorganic salts that, when added to water, create particles with an opposite charge to that of the contaminants. These particles then attract the contaminants’ surfaces, forming conglomerates or flakes.

Coagulants are salts of weak bases that, when added to water, hydrolyze (i.e., turn into an insoluble form). Commonly used reagents are based on two cations:

• Aluminum (e.g., sulfate, hydroxochloride, hydroxy sulfate)
• Iron (e.g., sulfate and chloride)

The reason aluminum and iron are used is due to their high efficiency at a low cost. Besides dirt particles, these coagulants can also bind hardness ions, heavy metals, iron, and organic pollutants, helping not only reduce turbidity but also lower the concentration of other impurities.

Notably, aluminum hydroxochloride is a coagulant used in drinking water at water treatment plants, sometimes exceeding its maximum allowable concentration in drinking water samples.

Currently, the best coagulant for pool water is also a mixture of organic and inorganic aluminum salts.

Principle of coagulants action 

When salts of iron or aluminum are added to water, they dissociate or dissolve in water:

• Al₂(SO₄)₃ → 2Al³⁺ + 3SO₄^2-
• FeSO₄ → Fe³⁺ + SO₄^2-

The aluminum or iron cations then undergo hydrolysis, reacting with water to form insoluble particles:

• Al³⁺ + 3H₂O → Al(OH)₃ + 3H⁺
• Fe²⁺ + 2H₂O → Fe(OH)₂ + 2H⁺

Simultaneously, free hydrogen ions bind with bicarbonate ions:

• H⁺ + HCO₃^- → H₂CO₃ → CO₂ + H₂O

Iron hydroxide, in an alkaline medium (around pH 9, created by lime treatment), oxidizes to form a less soluble and more stable Fe(OH)₃.

• 4Fe(OH)₂ + O₂ + H₂O → 4Fe(OH)₃

The coagulation process lasts 4-5 minutes. First, the liquid becomes turbid due to the appearance of hydrolyzed base particles, which then gradually "stick" to the dirt, forming larger flakes that eventually settle under their own weight.

Factors that affect coagulation include:

• Temperature (0-40°C is optimal for coagulation)
• Coagulant concentration (determined for each water type and may vary by season)
• pH (for aluminum sulfate, a weakly acidic to neutral pH of 5.5-7 is optimal, while for iron sulfate, a neutral or alkaline pH is preferred)

Flocculants for water

Flocculation is often referred to as a form of coagulation, though it is a supplementary process.

A flocculant is an organic polymeric high-molecular substance with long chains. The charges of the flocculant and coagulant are usually opposite, which allows them to interact effectively with similar particles.

Flocculation aims to strengthen the flakes and ensure their stability, enhancing the coagulant's effect. Flocculants work by creating molecular links between the flakes.

In industrial water treatment, polyacrylamide and its modifications are widely used, as they are highly effective and relatively inexpensive.

The simplest organic flocculants are natural substances such as starch, yeast, and alginates. The only commonly used inorganic flocculant is sodium silicate (activated silicic acid).

Flocculants can be divided into four types:

• Anionic – dissociate to form anions, such as polyacrylamide and most of its derivatives.
• Cationic – dissociate to form positively charged complexes. These reagents are becoming more popular due to their ability to work effectively even without prior treatment with a coagulant.
• Non-ionic – do not dissociate. Most natural flocculants (starch, dextrin) and synthetic flocculants like polyethylene oxide fall into this category.
• Amphoteric – dissociate into both cationic and anionic groups.

In Ukrainian water treatment plants, alongside the coagulant aluminum hydroxochloride, flocculants like sodium silicate and polyacrylamide are used.

Flocculants and coagulants: Key differences

Flocculants and coagulants are both substances used in water treatment to remove impurities. Despite having similar principles, there are differences between them. Coagulants remove particles by applying an electrophoretic effect. As a result, the particles lose their charge and aggregate into heavier, larger compounds that can easily be removed from the water. Flocculants, on the other hand, help bind contaminants by forming polymer bridges, maintaining the electrokinetic properties of the particles.

Coagulants form stable sludge that settles to the bottom and can be easily removed, although the sludge may not always be fully captured by filters. Flocculants ensure higher-quality water treatment, forming larger flakes that are easily removed by mechanical filters of any type. Moreover, the coagulation process takes just a few seconds, while flocculation may last several minutes.

Both groups play a vital role in water treatment, ensuring efficient pollutant removal and improving water quality.

Resources:

1. Bratby, John. Coagulation and Flocculation in Water and Wastewater Treatment. IWA Publishing, 2016.
2. Tripathy, Tridib, and Bhudeb Ranjan De. "Flocculation: A New Way to Treat Wastewater." (2006).
3. Tzoupanos, N. D., and I. Zouboulis. "Coagulation-Flocculation Processes in Water/Wastewater Treatment: The Application of New Generation Chemical Reagents." 6th IASME/WSEAS International Conference on Heat Transfer, Thermal Engineering and Environment (HTE’08), August 20–22, Rhodes, Greece. 2008.
4. Yang, Zhen, et al. "Flocculation Performance and Mechanism of Graphene Oxide for Removal of Various Contaminants from Water." Water Research, 47.9 (2013): 3037-3046.