If you’re relying on coagulants to adjust pH, it’s time to rethink your approach. Coagulants are specifically designed to aid in solids coagulation, not to serve as pH adjusters. The distinction is critical, yet many wastewater treatment facilities inadvertently misuse coagulants in this way.
The question is simple: why use iron or aluminum salts for pH adjustment when acids (like sulfuric acid) or bases (such as sodium hydroxide) are specifically designed for this purpose? By understanding the role of coagulants and their proper application, facilities can avoid unnecessary costs, inefficiencies, and complications.
The True Purpose of Coagulants
The primary function of coagulants is to coagulate solids in wastewater, not to adjust pH levels. This fundamental concept is sometimes overlooked by wastewater operators and environmental engineers.
Coagulants, such as aluminum- and ferric-based compounds, generally work by neutralizing the electrical charges to destabilize emulsions enabling them to form together into a floc. These flocs can then be separated from the water more easily.
While many coagulants do have an acidic nature and can reduce pH as a secondary effect, relying on them for pH adjustment is inefficient and problematic.
Misuse of Coagulants
This issue arises when facilities attempt to lower pH levels using coagulants instead of acids. For example:
pH Neutralization Before Treatment: Aluminum- and ferric-based coagulants operate most effectively within a specific pH range (alum: 5.5–7.5; ferric: 5.0–8.5). When wastewater has a higher pH, the correct approach is to use acids like sulfuric or hydrochloric acid to bring the pH into the optimal range. However, some operators incorrectly add coagulants in large quantities to achieve this adjustment, which leads to several unintended consequences.
Negative Impacts of Misusing Coagulants
Under/Overdosing
When pH is used to determine the amount of coagulant being dosed rather than rates that have been determined through jar testing there is the obvious potential to underdose or overdose. The effects of underdosing are self-evident in that adding too little may not achieve destabilization of the emulsion and result in poor performance. Overdosing, however, can also be a problem as it can cause a reversal in charge (overcharge) which can return the colloidal particles back to an emulsified state.
High Costs
Coagulants are significantly more expensive than acids or bases. Using coagulants for pH adjustment results in unnecessary chemical expenses, as large volumes are required to achieve the same effect that a smaller quantity of sulfuric acid could accomplish.
Increased Sludge Volume
Coagulants are metal-based, which means they contribute to the volume of sludge generated during wastewater treatment. Excessive use of coagulants leads to higher sludge production, increasing the costs and labor associated with its storage, dewatering, and disposal.
Elevated COD Levels
When coagulants are added in excess and there are insufficient solids for them to bind with, the unreacted metals dissolve into the water. This can increase chemical oxygen demand (COD), potentially disrupting downstream biological treatment processes.
Best Practices for pH Adjustment
The proper way to adjust pH in wastewater is to use the appropriate chemical reagent:
Use acid (such as sulfuric acid) to lower pH.
Use base (such as sodium hydroxide) to raise pH.
Once pH is within the desired range, apply coagulants to achieve the necessary water clarity. Coagulant dosing should always be determined based on the clarity of the treated water, not to manipulate pH.
Pro Tip:
Perform a jar test to determine the optimal coagulant dosage. When the treated water appears clear and solids are clumped into stable flocs, the coagulant dosage is correctly calibrated.
A Smarter Approach
By minimizing coagulant usage and applying it solely for its intended purpose, facilities can significantly reduce costs, lower sludge production, and maintain a more stable wastewater treatment process.
If your facility struggles with pH adjustment and coagulant misuse, FRC offers systems that automate pH balancing with the correct chemicals, ensuring an efficient and cost-effective treatment process. Contact us to learn more about designing a solution tailored to your needs.
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