Coagulation is one of the preliminary steps in cleaning water and when chemicals are used, a few design practices on the storage tank can bring safety, longevity and operational efficiency. Understanding how coagulants work in wastewater treatment, what to do about the sludge that accumulates, and how to minimize risk will help you configure an effective and efficient coagulation tank system.
There are several different applications for coagulants. Some of the most popular uses include:
- Removal of Natural Organic Matter from Water
- Pathogen Removal
- Removal of Inorganics
- Arsenic Removal
- Fluoride Removal
- Chemical Phosphorus Removal
Understanding Colloids and Turbidity
Small floating particles in river water can make you sick. Floating particles are naturally found in the water source, whether it be river or lake, which your local treatment plant must remove before further processing and sending to our homes. These floating particles, which are too light to settle by gravity, are called Colloids. They are made up of both non-organics (such as clay or silt) and organics (such as algae or bacteria). When ingested by humans, some of the organic bacterias can be fatal.
The more cloudy or muddy resting-water looks, the more floating particles exist. This measure of water clarity is known as turbidity. The more turbid the water, the higher the chance it carries disease. This is because more floating particles create more surfaces for toxic compounds (TC’s) to attach to. These toxic compounds can cause Hepatitis A, E-Coli, Dysentery and others, which kill millions of people per year. A practical way to deal with this problem is to settle these particles out of suspension by bonding them to one another so that they become heavy enough to fall out of suspension by gravity. This process of bringing the particles together is called Flocculation. After the floc forms, it can be separated, which leaves you with clear water. This is known as Coagulation. Therefore, the chemicals that cause these particles to cling together and fall out of suspension can be called coagulants, or flocculants.
The Flocculant / Coagulant Chemicals
The clarification of wastewater dates back to Egyptian times, when people would spread almonds around water channels so that the almond oil would separate solids from the water. Lets examine the two most common coagulation chemical groups (both metal salts) that are used to clump solids together in the water stream today.
1) Those based on aluminum:
- aluminum sulfate (or Alum) Al2(SO4)3
- aluminum chloride
- sodium aluminate
2) Those based on iron:
- ferric sulfate (Iron Sulfate)- Fe2(SO4)3
- ferrous sulfate
- ferric chloride
- ferric chloride sulfate
Other chemicals used as coagulants include hydrated lime and magnesium carbonate. It’s important to note, polymers are man-made organic compounds made up of a long chain of smaller molecules. Polymers can be either cationic (positively charged), anionic (negatively charged), or nonionic (neutrally charged.)
Why Flocculants and Coagulants Work
Almost all colloids have negatively charged surfaces. So, charged particles in the water (having positive ions) will be attracted to the colloids, exactly like opposite poles of a magnet. And just like the same poles of a magnet repel each other, the similarly charged layers of colloids repel each other thus staying small, repellant, and causing turbidity. The added chemicals neutralize the negative charge, allowing the solids to clump together. This makes removal easier since the particulates fall out of suspension with more mass.
Tank Design Considerations
The metal chemicals (alum or ferric) will cause sludge to build in the bottom of a storage tank. This concentrated sludge can even become radioactive and some state departments of environmental quality will even require special hazardous waste removal of old coagulant tanks if present. A full drain is necessary to alleviate this build up and to ensure a safe and clean tank. When tanks simply put an outlet higher off the bottom to allow sludge to settle below the pump feed, this problem is exaggerated. An integrally molded outlet (IMFO) allows fluid to continuously exit at a level flush with the bottom of the tank. Ensuring this outlet is integrally molded into the tank wall eliminates leak concerns where other full drain options containing welds, inserts, attachments, or dissimilar materials fail.
In addition to choosing a tank with a full drain, it’s also important to choose a tank with the ability to handle high temperatures. Alum is frequently delivered at temperatures as high as 125°F (52°C). Cross-linked polyethylene (XLPE) has 30-40% better temperature limits than your standard linear polyethylene (HDPE). For harsh environments, a steel tank lined with polyethylene such as PolyGard will provide extended tank life. With a proper review of the application by an expert, a high-density cross-linked polyethylene tank system can withstand the 100-plus degree chemical that is being supplied and deliver a performance which is far superior to other systems.
Likewise, the unwanted crystallization of many coagulants can occur around 30°F. Your storage tank should be installed indoors or contain heat trac
e and insulation to maintain a temperature between 45 and 60°F (7.2 and 15.6°C).
3) Filling Your Tank
It’s important to note that some coagulants, such as polymers, can be quite viscous. This means that there can be problems with filling a tank via a downpipe. When the alum is pushed into the bottom of the tank with pneumatic pressure from a tanker truck, the contents of the tank can air bubble, causing heavy pressure buildup under the fluid and resulting in a jackhammer effect that can cause stress on the tank, even causing the entire tank to jump. Fill pipes should always end above the highest fluid level the tank will see.
You can also incorporate an anti-foaming connection that leads the chemical to hit the top sidewall and run down the interior tank wall, preventing foaming and splashing while still removing the need for a downpipe.
4) Level Indication
Polyethylene tanks are usually translucent enough to provide a visible indication of the liquid level. However, when these tanks must be insulated, or where other types of storage tanks are utilized, alternative mechanical methods (supplementing possible electronic sensors) are available. A traditional glass site tube will stain from the coagulant and become unreadable over time. A reverse float gauge can be used as an economic option and prevent both visibility issues and potential leak issues caused by tank wall penetrations.