What is Coagulation in Wastewater Treatment?

In this article the industrial water specialists at Water Treatment Services look at the use of coagulation in the treatment of wastewater and effluent streams.

The article explains the role of chemical coagulants and how they are used in the treatment process, it looks at how they work, and where they are typically used. It concludes by reviewing the different types of coagulants that are commercially available and what they do.

The role of coagulation in wastewater treatment

In the wastewater treatment process, coagulants play a critical role in dealing with sludge. Often used in combination with other mechanical filtering processes and treatment chemicals, using coagulants helps to thicken the sludge into a form which allows the solids and other particles which are contaminating the water to be easily removed.

Companies and businesses which use coagulants in the treatment of their wastewater and effluents are trying to ensure a reliable source of clean water is maintained for all uses in whatever processes they are carrying out on their premises or factory.

The history behind coagulation in the treatment of wastewater

The idea of using coagulation as a way to clean up dirty water is nothing new. There is evidence that the Ancient Egyptians were adding almonds to water in rivers as an attempt to clean it up as early as 2,000 BC. The Romans even added a chemical called alum to water as a coagulant as early as the 8^th century.

Although things have moved on a little since ancient times, coagulation is still an effective way of treating water and wastewater in multiple industries in the 21^st century.

How does coagulation work?

In simple terms, coagulation describes a chemical reaction. It involves adding a special chemical product called a coagulant, something like iron or aluminium salts to the wastewater, which then affect the electrostatic charge associated with the small particles suspended in the water. The chemical reaction destabilises the charges on the solid particles causing the particulates, metals or oily materials in the water to “clump” together, which then makes them easier to remove. Water treated in this way also appears clearer than water which is treated using different methods.

The chemicals used in the coagulation process are generally positively charged, and help to neutralise the negatively charged particles in the water. This neutralisation process is what makes the suspended material in the water clump together… it clots or coagulates the solid particles into clumps which are also known as flocs. These larger flocs eventually settle to the bottom of the wastewater holding tank or clarifier, where they can be more easily removed as a sludge. Typically, as a water treatment process, coagulation is completed before flocculation.

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Where can coagulation be used?

Coagulation can be used in a range of different situations, to deal with specific pollutants affecting your water and causing it to become contaminated. Coagulation is particularly effective against:

Organic Compounds
Whether these compounds are dissolved organic carbon, or natural organic material.
Bacteria & Viruses
Bacteria, and some viruses.
Suspended Particles
Some suspended particles such as iron and other metals.

How coagulation aids mechanical filtration

The main idea behind using coagulation as a treatment for wastewater is to create a state in the water that allows effective mechanical filtration of the effluent. This involves the formation of flocs or clumps of solid material. These clumps start to accumulate at the bottom of the water tank or clarifier and can then be removed using a filter which removes the solids. The treated water can then be fed back into the system or discharged.

When used properly, the combination of clarifiers, coagulants and filters can help companies achieve up to 95% water reclamation through the system. This is almost as effective as an entire closed-loop process for your water use.

What are the different types of wastewater coagulants?

Coagulation doesn’t happen by itself, and in order to kick-start the process you have to add special coagulant chemicals into the wastewater treatment system. The exact combination of chemicals you’ll use will typically depend on the type and concentration of contaminants that are affecting your effluent streams, and the chemical composition. The main choice of coagulant is between inorganic and organic compounds.

Organic coagulants

For solid-liquid separation, one of the best options to think about first is the use of organic coagulation. Organic coagulants are also effective when trying to reduce the total volume of sludge which is created as part of the treatment process.

Organic coagulants often require only a small dose to be effective, and won’t affect the pH level of the water either. There are many organic coagulants on the market, but some of the most commonly used compounds work by neutralising the charge of the particles in the water, forming them into clumps known as micro-flocs.

The other main group of organic coagulants also form flocs, and can also absorb grease or oils in the water. This is the type of coagulant which is used to deal with hazardous sludge, such as the wastewater produced in oil refineries and similar industries.

Inorganic coagulants

From a business perspective, one of the main advantages of using inorganic coagulants is that they are often cheaper to use than organic coagulants. Inorganic coagulants are therefore a very cost-effective option across a wide range of industries and applications.

Inorganic coagulants work by forming precipitates of either iron or aluminium when added to water, and work by absorbing impurities in the wastewater. The downside of this method is that this it can increase the levels of sludge produced.

The most common types of inorganic coagulants used in industry include:

Aluminium Sulphate
The use of alum, also known as aluminium sulphate as a coagulant goes back to Roman times. It’s still one of the most commonly used coagulants today. Alum is added to the water as a liquid. The chemical is mildly toxic, and should be handled with care as it can have the same health risks and corrosion risks as a dilute acid.
Aluminium Chloride
Aluminium chloride is a very similar chemical to alum, but has several drawbacks in that it’s more expensive, more corrosive and has to be handled with more care. It’s therefore usually only used where alum is unsuitable.
Ferrous & Ferric Sulphate
These chemicals both work in similar ways to the coagulants based on aluminium. They are usually used in situation where a reducing agent is required.
Ferric Chloride
Ferric Chloride is a by-product of the steel manufacturing process and is the cheapest inorganic coagulant around. However, despite its low price it’s not often used as it is also very corrosive and dangerous to handle.

Using coagulants to treat wastewater

After you have selected the most appropriate coagulant for your needs, then the process is simple. Add the chemical to the wastewater or effluent stream and mix it quickly and thoroughly. This will ensure that the chemical is evenly distributed through the water.

Usually, the residue left over after using these coagulants doesn’t pose an issue in terms of water quality, as long as you dose them according to the manufacturers’ instructions. This is why it’s so important to get the professionals on board if you don’t have the skills to implement and manage a wastewater treatment process in-house.

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