Case Study: The Importance of Proper Mixing in the Coagulation and Flocculation Stages

The coagulation and flocculation stages enable wastewater to be treated. However, to achieve the desired performance, the coagulant and polymer must be added to the water being treated under optimal conditions to ensure uniform contact between the reagents and the water.

In fact, coagulation and flocculation are collision reactions:

• Between the coagulant AND the colloidal particles contained in the water when it comes to the coagulation reaction
• Between the pin flocs formed during coagulation AND the polymer charges when it comes to the flocculation reaction

If the mixing of the coagulant and the water being treated and/or the mixing of the polymer solution and the water being treated is not vigorous, the probability of collision is reduced, thereby decreasing the effectiveness of these two steps.

We invite you to read the following article, which provides detailed explanations and clear illustrations of the two stages of coagulation and flocculation:

https://aquasan.ca/articles/fundamentals-of-coagulation-and-flocculation/

The following example concretely illustrates how inadequate mixing of the polymer solution and coagulated water significantly impacts the flocculation efficiency.

For this customer, the coagulation stage and the flocculation stage take place in mixing chambers with wastewater retention times that are normally sufficient to allow for the desired chemical reactions to occur, even in cold weather.

Before our intervention, the polymer solution was injected at the surface of the flocculation chamber, creating a local overdose of polymer solution. A thick layer of sludge formed on the surface, which made it even more difficult for the polymer solution to come into contact with the water to be flocculated.

Before our intervention
Injection of the polymer solution at the surface of the flocculation chamber (red tube). The polymer solution falls on the sludge accumulated on the surface and does not come into contact with the coagulated water	Visual of the flocculation chamber. The formed flocs are very small.

To improve the flocculation stage by ensuring adequate mixing between the polymer solution and the coagulated water, we simply lengthened the polymer solution feed pipe. This allows the polymer solution to be injected directly into the water being treated near the agitator.

After our intervention
The polymer solution is injected directly into the coagulated wastewater thanks to the lengthened polymer solution feed pipe	Visual of the flocculation chamber – significant improvement in flocculation with the formation of dense, compact flocs – the clarified water can be seen between the high-quality flocs

Very quickly, the modification of the injection point of the polymer solution led to a significant improvement in floc formation in the flocculation chamber.

If coagulation and flocculation take place in mixing chambers, we recommend mechanical mixing rather than mixing with air.

In fact, we have already encountered treatment issues with customers who mixed the coagulant and the water being treated with air rather than using mechanical mixing.

Injecting air into the water being treated if the solid/liquid separation stage takes place in a settling tank is strongly discouraged, as microbubbles could attach themselves to the flocs and make them float, resulting in flocs being carried away into the clarified water.

In the same vein, the polymer solution used for the flocculation stage should not be too diluted.

As this involves a probability of collision between the polymer charges and the pin flocs formed during coagulation, if the polymer is too diluted, its charges will be too dispersed in the dilution water to react properly. The contact time for flocculation will therefore need to be increased to ensure that the polymer charges have time to come into contact with the pin flocs.

We recommend preparing polymer solutions at the following concentrations:

• 0.2% or 2 grams/litre of dilution water for dry polymers
• 0.5% or 5 grams/litre of dilution water for emulsion polymers

https://aquasan.ca/articles/do-you-know-how-to-properly-prepare-your-polymer-solutions/

At AQUASAN, our goal is to offer you an innovative and easy-to-use chemical program that is efficient and corresponds to your performance objectives. We have unparalleled training and technical know-how, and we know how to provide creative solutions that ensure the quality of the treated water is maintained.