How Polyethylene Tanks Are Used in Municipal Water Treatment

Polyethylene storage tanks play an important role in municipal water treatment. Drinking, or potable water, originates from two sources: surface water, such as lakes, rivers, and reservoirs, or groundwater, wells or underground aquifers.

This article will explore the five typical steps used in municipal water treatment and examine the use of polyethylene chemical storage tanks in treating water for domestic use.

Municipal Water Treatment Steps

1. Coagulation - In this initial water treatment step, chemicals such as ferric chloride, polymers, and alum are used to produce positive charges to neutralize negative charged solid particles within the water. This causes the solid particles to clump together, making removal easier. Ferrics, alums, and polymers require specialized chemical storage tank solutions.
   
   
2. Flocculation - During this step, the neutralized particles and water are slowly mixed using paddles to create flocs or larger particles. No chemicals are added during this treatment step.
   
   
3. Sedimentation - Next, the water flows into a sedimentation basin. As a result of gravity, the larger particles settle to the bottom of the tank. Again, no chemicals are part of this treatment step.
   
   
4. Filtration - Water is pumped from the sedimentation basin into a concrete box for filtration.  Using sand with a gravel base, the particles are backwashed. Since the particles are lighter than sand, they are suspended and rise up in the filter. These particles are then flushed from the system. No chemicals are used in this step.
   
   
5. Disinfection - Now that the solid particles have been removed, any bacteria and pathogens must be destroyed to make the water safe for drinking. Sodium hypochlorite (bleach) is commonly used to disinfect water. Sodium hypochlorite presents a significant chemical storage challenge for three reasons:

1. Exposure to UV (sunlight) can degrade the chemical.
2. Sodium hypochlorite typically contains transition metals such as copper, nickel, and iron, which can settle in the tank and create off gassing.
3. Since ‘hypo” acts as a potential oxidizer, the tank must rise to a strict standard.

It should be noted here, that the entire Poly Processing chemical storage tank system, not only the tank itself, but the entire system- all fittings, gaskets, and connection points on the tank meet NSF/ANSI Standard 61 for chemical storage used in water treatment.

In addition to use of sodium chlorite during disinfection and beyond, additional chemicals are used for additional water treatment purposes. Hydrofluorosilic acid (Fluoride) is commonly added to municipal water treatment. While we drink small amounts of fluoride in our water, hydrofluorosilic acid is extremely dangerous if not stored properly.

In addition, to balance pH levels, a strong acid, such as sulfuric acid, or a base chemical, typically sodium hydroxide may be added to the water. Poly Processing has an innovative, ideal chemical storage system for sulfuric acid, and a strong, safe storage solution to combat the corrosive properties of sodium hydroxide.

If a reverse osmosis process is used to further clean and filter water, chemicals such as sodium hypochlorite, citric acid, and sodium hydroxide are commonly used for CIP (clean in place) to clean and flush the membranes material used in filtration.

Finally, all the chemicals used in water treatment pose risks of contamination or harm to humans in the event of leaks or spills. Secondary containment is a must. If concrete secondary containment is already in place, Poly Processing’s IMFO tank is the perfect water treatment chemical storage solution. IMFO offers superior tank strength in a one-piece molded tank, with the flange molded into the tank.

Or, if adequate secondary containment is lacking, the safe chemical storage solution for water treatment is the Poly Processing SAFE-Tank System.

For specific questions, or a custom water treatment chemical storage tank system solution, please contact a Poly Processing chemical storage tank specialist.