What is coagulation, how does it work and what types of coagulants exist?
Coagulation is one of the common methods used by water treatment plants to provide safe, clean drinking water to public water customers.This method is often used alongside processes including filtration, disinfection and sedimentation to remove select contaminants from water.
This article will look at this method in water treatment: what it is, how it works, and what is removed in this process.
What is a Coagulant for Water Treatment?
Ferric sulfate, aluminum sulfate, or ferric chloride, classed as aluminum or iron salts, are common coagulants for water treatment.
A coagulant is a chemical that is used to remove suspended solids from drinking water. They are made up of positively charged molecules, which help to provide effective neutralization of water.
How Does Coagulation Treatment Work?
Coagulation treatment is usually carried out before sedimentation and filtration. During the process, a coagulant is added to water, and its positive charge neutralizes the negative charge of suspended contaminants.
Neutralization causes suspended particles to bind together (hence the term). In clumps known as “flocs”, these particles sink to the bottom of the treatment tank. They can then be more easily filtered out of water.
During this process, the coagulant is quickly added to the water and mixed, allowing it to be distributed throughout the entire sample of water. When the water is coagulated, it can be filtered through an ultrafiltration or microfiltration membrane, or a medium filter, to remove the settled particles. Water can also be moved into a settling tank, in which the heavy particles will sink to the bottom, where they can then be removed.
What Is Removed During Coagulation?
Coagulation is most effective at removing suspended solids and natural organic matter like gravel, sand, algae, clay, iron, protozoa, and even bacteria. Many of these contaminants can give water an unpleasant taste when present in large quantities, and can also give water a brown or orange color.
However, not all contaminants can be coagulated within the same timeframe, which is why other methods of water cleaning are used alongside this treatment method.
Gravel, sand and fine sand can all coagulate during neutralization within two minutes. It takes algae, clay and protozoa up to 2 hours by comparison.
Anything else that can coagulate takes much longer: bacteria and algae at a diameter of 1 micron take around 8 days to coagulate and settle to the bottom of the water supply, while viruses of 0.1 microns in diameter take 2 years to coagulate and settle. That’s why disinfection is needed to kill microbiological contaminants much more quickly and efficiently.
With that said, many pathogens actually attach themselves to coagulated particulates, and are removed during filtration.
Coagulation does not guarantee safe drinking water, but it is still an essential water treatment process. It removes suspended substances that make water more difficult to properly treat with a disinfectant, and means that less chlorine can be added to disinfect the water.
As well as the money-saving benefits of this chemical treatment process, municipal water suppliers can also guarantee safer drinking water, as certain organic particles can react with chlorine and produce a dangerous byproduct known as trihalomethanes (THMs).
What Are the Most Common Types of Coagulants?
The most commonly used chemical for coagulation is aluminum sulfate. Ferric sulfate, ferric chloride, or sodium aluminate are also popular types of coagulants.
Let’s take a look at these coagulants in more detail:
Aluminum sulfate is available in several forms, including ground, kibbled or block. When added to naturally alkaline water (which typically contains calcium bicarbonate), aluminum sulfate produces an aluminum hydroxide floc.
Ferric sulfate is a type of iron coagulant that is second-best to aluminum sulfate. This chemical combination is often used in conjunction with chlorine, and can provide a denser floc than aluminum sulfate. However, it does produce a significantly heavier hydroxide sludge.
Ferric chloride is an alternative to ferric sulfate, but is the less popular choice, as chloride can increase water’s corrosivity.
Sodium aluminate is a combination of sodium oxide and aluminum oxide. Solid forms of this chemical usually contain 70-80% sodium aluminate, while liquid forms contain around 30% sodium aluminate.
How to Choose a Coagulant for Water Treatment
The type of coagulant used by your local water treatment facility will usually depend on availability and affordability. With aluminium sulfate being available, affordable and highly effective, it is the preferred choice for public water treatment around the world.
Metal coagulants are some of the most popular for water treatment as well. However, synthetic coagulants and biopolymer coagulants (including natural biopolymers sourced from fungus, plant sources and animals) are also available. These have the advantage of producing less sludge, and they pose fewer toxicity or safety issues.
Coagulation is a necessary water treatment process, but it cannot work alone. Filtration, sedimentation and disinfection are also required to ensure that water is free from harmful contaminants and safe for drinking.