Ferrous sulfate; a commonly used water coagulant. Image via Wikimedia.


Water coagulants are critical to an effective water treatment process. But, with so many options now available on the market, what is the best agent for your coagulation water treatment?

There are many advantages and disadvantages to electrocoagulation or coagulation processes. However, by choosing the right water coagulant, you can help to make sure your business is getting the best results from the process. Take a look at some of the best coagulant candidates for use at your facility.

Inorganic coagulants

Inorganic compounds do not have carbon elements in their molecular structure. As such, they are considered to be “artificial” or unnatural. However, just because a coagulant is inorganic does not mean it is environmentally unfriendly, provided it is handled correctly. This also applies to organic coagulants, which are not necessarily eco-friendly.

  • Aluminum sulfate – Al2(SO4)3

Aluminum sulfate is commonly known as alum, and it is one of the most broadly used coagulants currently on the market. This chemical is a popular choice due to its high availability, cost-effectiveness, and its efficacy as a coagulant. Sold in blocks and easily stored, aluminum sulfate has proved a viable option for water coagulation and treatment in third-world countries, as well as in the United States.

The optimal pH range of aluminum sulfate is between 5 and 7.5, which means the coagulant performs at its best in slightly acidic or neutral solutions. Ferric chloride may be a better option for coagulation below this level of pH, while ferrous sulfate may be suitable for the treatment of more alkaline water.

  • Sodium aluminate

Sodium aluminate is easy to produce and is widely available. It is usually deployed alongside aluminum sulfate to act as a coagulation aid, particularly in applications that require the treatment of very cold water that would be difficult to coagulate on its own without assistance.

This process is sometimes referred to as double coagulation. While sodium aluminate is not usually used as a coagulating agent on its own, it is used in a number of other industrial applications, including lime soda softening.

  • Aluminum chloride – AlCl3

Aluminum chloride works in a similar way to aluminum sulfate. However, it is less commonly used than its other aluminum-based counterpart.

This is largely because AlCl3 tends to be more corrosive and difficult to store, as well as more expensive to source.

  • Polyaluminum chloride (PAC) – Al12(OH)24Cl12

(Just one of a number of different formulae  this is the most common form of polyaluminum chloride used in water purification.)

Polyaluminum chlorides – or PAC – make up a grouping of different compounds, many of which are suitable for coagulant applications. Different compounds within this grouping have different attributes, and they can be deployed at various pH levels for optimum impact.

  • Aluminum chlorhydrate (ACH) – Al2(OH)5Cl•2H2O

Aluminum chlorhydrate is in the PAC family of chemicals, but it is the most concentrated form, with the highest level of alumina and basicity provided by a stable solution. It is a highly effective coagulant, and it is not too difficult or expensive to source – two aspects that make it very attractive to cost-conscious business owners and industry leaders.

As long as facility managers have the requisite expertise, they can fine-tune the amount of the ACH chemical that is used in each dose, reducing the required amount and increasing the cost saving without compromising on coagulation results. It is easy to store and transport. It is also highly effective across a wide range of different water conditions – it does not share the strict pH range needs of other coagulants.

  • Ferric sulfate – Fe2(SO4)3

Ferric sulfate works in a similar way to aluminum sulfate, and it is considered to be a highly effective coagulant for industrial usage. Depending on availability, ferric sulfate may be cheaper to source than aluminum sulfate – or other aluminum-based coagulants – although this may not be true in every location.

For some applications, business owners may opt for ferrous sulfate (FeSO4) – a similar compound to ferric sulfate but with a simpler molecular structure. Ferrous sulfate tends to be more effective in situations where reducing agents are required as the compound increases the availability of iron ions. But, it may not be as effective as aluminum sulfate when treating water with an acidic or neutral pH level.

  • Ferric chloride – FeCl2

Ferric chloride is perhaps the easiest and cheapest coagulant to source, which makes it a popular choice in some industries. It is sourced from waste material from steelmaking facilities, so it may be considered a relatively green option as it is largely recycled.

However, this is only the case if the coagulant is handled properly and if safety solutions such as secondary containment berms are deployed during handling. Ferric chloride is the most corrosive of all the commonly used inorganic coagulants. This may reduce the lifespan of equipment, and it can cause serious damage if released into the water table. Ferric chloride is effective even at pH levels as low as 4.5.

Organic coagulants

As well as the inorganic coagulant compounds listed above, there are a number of different organic coagulants available on the market. These include substances such as polymers, amines, and acrylamides, all of which can be used to achieve effective coagulation at your facility.

Most of these organic coagulants have been manufactured according to proprietary methods and formulae. This makes their specific make-up a closely guarded secret and may affect availability and affordability.

Some organic coagulants are produced with environmental concerns in mind, and they are aimed at providing a safer and more ecologically friendly alternative to traditional coagulant chemicals. However, it should not be assumed that all organic coagulants are necessarily eco-friendly products.

Organic options are becoming increasingly popular in the flocculation stage of water treatment, where polyacrylamide, or PAM, is proving to be highly successful.

Which coagulant is best for your facility?

Modern industry has a wide range of coagulants to choose from, so which is the best option for you and your facility? The specific answer to this question largely depends on your own situation and requirements.

For example, ferric chloride may represent a cost-effective option up front, but you will need to invest in corrosion protection for your pipework and your equipment, as well as ensure you have the capability to handle the substance correctly.

On the other hand, aluminum sulfate may prove effective across many applications, but you also need to factor in the cost of pH control as this coagulant is only effective across a relatively narrow pH band. You may also need to consider the additional cost of the coagulating aid sodium aluminate.

PAC and ACH may provide highly effective coagulation across various conditions, but they may also be more expensive to source.

Consider your own requirements and available resources carefully before you make your choice.