Polymers in Wastewater Treatment
Polymers separate solids from liquids through coagulation and flocculation. Their ability to do so is central to their critical role in wastewater treatment.
Most industrial process water or wastewater includes sediment and particles with a negative electric charge. The negative charge around each particle keeps them from coming together, creating colloidal dispersion. Negatively charged particles won’t come together and can float in liquid suspension for hours, weeks, and even years. A simple example is muddy water created by aggregate producers who wash sand and gravel.
When a coagulant is added to the muddy water, it creates a coagulation process that neutralizes the particles’ negative charge. Once neutralized, the particles can come together to form larger particles called micro-flocs or pin flocs.
The next chemical reaction is called flocculation, which helps create even larger flocs or macro-flocs. In this step, chemical products called flocculants help bring together the coagulated particles to form longer and larger particle chains. As the flocculant chains become larger, they become heavier and even visible to the naked eye. The flocculants create a larger particle with a higher mass-to-drag ratio, so they speed up the natural process of sedimentation.
As the sedimentation process continues, a sludge forms at the bottom of whatever is holding the wastewater. In nature, we see this in the delta at a river’s mouth; in industry, this sludge accumulates in settling ponds, sludge ponds, clarifiers, or thickeners. Once the sludge has formed, it can be managed and disposed of through different methods.
As you can tell, polymers play an important role in wastewater treatment. Besides separating solids from liquids, they also help thicken sludge and dewater contaminated material for easier handling and disposal. Removing the water content from a waste sludge can change the waste properties from liquid to solid waste. This change has significant impacts on the disposal cost or tipping fee.
The industry standard that determines if the waste is solid or liquid is EPA Method 9095B or “The paint filter test.” In this procedure, a sample of the waste sludge is placed in a standard 60 mesh conical paint filter for 5 minutes. If no liquid passes through the filter, the waste is deemed solid and can be disposed of more cost-effectively than liquid waste.
In industry, we would consider sludge passing the paint filter test to be dewatered. Polymer flocculants expedite the dewatering processing by allowing water to leave the flocculated solids more freely.
This video shows the coagulation, flocculation, and sedimentation process in real-time.
Industries That Use Polymers to Treat Wastewater
Almost any industry that needs to remove solids from their wastewater stream can use polymers in their treatment process.
For instance, aggregate producers use water to wash the sand, gravel, or other aggregates they produce. The wash water picks up dirt, clay, and silt during the washing process. To reuse or safely discharge this wash water, the solids need to be separated from the water.
Many aggregate producers use settling ponds to hold the wastewater and allow sedimentation. As we noted, it can take a long time for sediment to fall out of liquid suspension. Aggregate producers must stop production while cleaning their ponds and waiting for clean water. These shutdowns cause producers to lose production hours and increase labor hours.
However, polymers can increase the rate of sedimentation for aggregate producers. By using a polymer dosing system, producers can inject the polymer solution directly into their settling ponds. The polymer dosing increases the sedimentation rate, provides clean water, and thickens the sludge at the pond’s bottom for easier management and disposal.
Many operations also use polymers to prepare wastewater before it’s mechanically treated in clarifiers, thickeners, filter presses, centrifuges, and other dewatering equipment.
The following video displays a polymer system coupled with a high compaction clarifier. This setup creates a thick and manageable mud discharge plus clean water for reuse.
It’s important to note that not all wastewater treatment polymers are the same. Specific chemistry is selected depending on the wastewater and the operation’s needs. Plus, as we said, dry and emulsion polymers require different dosing equipment. It can be tough to know what’s best for your application.
An excellent place to start is by speaking with an experienced application engineer. They will provide options that best fit your needs and use jar testing to determine the best polymer chemistry. The right type of polymer will help you meet regulations, save money, and increase your operation’s success.