Avoid unfortunate incidents by knowing chemical incompatibility between different coagulants and polymers

Each product is recognized by the atoms it consists of and the charges associated with it. The chemical composition of various products not only defines their functionalities but also their incompatibility with other compounds. Some molecules simply do not mix well together, it is important to be able to identify them. Educating yourself about the incompatibility between your different products will help you avoid dangerous errors.

COAGULANTS

Coagulants are used to neutralize the negative charge of colloidal particles present in wastewater. These coagulants can be found in organic or inorganic forms.

At Aquasan, we offer two families of organic coagulants: polyamines and polyDADMACS. Transitioning between these two types of products poses little risk. We simply recommend thoroughly rinsing your lines with hot water for 15-20 minutes and then ensuring they are properly dried before making the transition.

Inorganic coagulants are divided into two families: aluminum-based salts (e.g., Al₂(SO₄)₃ = aluminum sulfate, also known as alum) and iron-based salts (e.g., Fe₂(SO₄)₃ = ferric sulfate). These two types of coagulants also differ in their sulfate or chloride content. Depending on their chemical composition and relative pH to each other, the contact between two different inorganic coagulants can become very dangerous.

In a wastewater treatment plant, changing coagulants requires several precautions. The compatibility chart provided below will help you understand the necessary actions in the event of a change of an inorganic coagulant in your facility. When it comes to compatible coagulants, transitioning between these two products should not pose a risk. However, as a precaution, we recommend rinsing with hot water for 15 to 20 minutes and drying the lines as a best practice. However, for a change between non-compatible compounds, first inquire about the compatibility between your new product and the materials used in your installations. It is important to know, for example, that ferric chloride should never be used in a system made of stainless steel. If your new coagulant is compatible with your current installations, we then recommend a thorough pipe flushing and powerful air drying. We recommend flushing all your lines and pumps with hot water for at least 30 minutes. Contact between a few drops of two non-compatible products could generate a precipitation reaction and clog your pipes and tanks.

Figure 1. Contact between AQ-8920 and AQ-8810

WARNING: A change of coagulant involving AQ-8920 requires a complete replacement of your piping system. This basic coagulant should never come into contact with other acidic coagulants.

As shown in Figure 1, the contact between AQ-8920 and AQ-8810 instantly triggers a precipitation reaction between the two liquids. This reaction is also exothermic. Therefore, one can easily envision the damage that this interaction can cause on a large scale in your installations.

Table 1. Compatibility chart for Aquasan inorganic coagulants

POLYMERS

Polymers, on the other hand, facilitate the agglomeration of flocs generated by coagulation. These products come in two distinct forms: anionic (negatively charged) and cationic (positively charged). For each type of charge, polymers can further differ based on their molecular weight (chain length) and their charge percentage. Changing between anionic polymers poses no risk because they are compatible. The same goes for changes between cationic polymers. A simple 15–20-minute rinsing and drying of lines are sufficient. However, transitioning between polymer of opposite charges requires thorough cleaning of your installations. We recommend flushing the piping and pumps with a bleach solution for at least 30 minutes. Subsequently, we advise intensive drying. It is essential to remove all residues of the old polymer before introducing the new one.

Figure 2. Clogging of the plant’s polyblend following the addition of an opposing filler polymer.

The attached Figure 2 clearly illustrates the kind of problem that can occur when polymers of opposite charge come in contact. No rinsing was carried out before switching to a polymer with the opposite charge, despite our recommendations. The new polymer then reacted with residues from the previous polymer, completely clogging the mixing chamber. The customer had to dismantle the entire solution delivery system and thoroughly wash it with a bleach solution to eliminate the sticky residue that formed when polymers with opposite charges reacted together.

In summary, knowing the compatibility level of your products before each change will help you avoid such situations. It is better to prevent this risk by conducting a thorough cleaning and drying process rather than having to repair the damage that this type of reaction can cause, not to mention the potential injuries it can avoid.