Role of Coagulants in Wastewater Treatment

Role of Coagulants in Wastewater Treatment

Coagulants are used in many industries for a wide range of applications. In this post, Enva explore what coagulants are, how they work, and examine their role in waste water treatment.

This is the second part of our series exploring wastewater treatment chemicals. To learn more about other water treatment chemicals visit part one, which examines the role flocculants play in wastewater treatment.

What is coagulation?

Coagulation is the term used to describe the process of a liquid changing to a solid state; synonyms include curdling and clotting. It is a chemical process, whereby the chemical properties of a liquid change such that particles begin to clump together and form solid matter. Like flocculants, using coagulation to purify water is a technique that’s been used for over 4,000 years, and it is still one of the primary methods used today.

What are coagulants?

Coagulant chemicals are substances that promote the coagulation of liquids into solids. Chemical coagulants for water treatment fall into two main categories – organic and inorganic.

Organic coagulants:

These are generally used to separate solids from liquids and to generate sludge. There are two types of organic water treatment chemicals involved; the first being polyamines (e.g. polydiallyldimethyl ammonium chloride, or polyDADMAC), which are the most widely used organic coagulants. Most effective at treating waste water and high turbidity raw water, they work by simply neutralising the charges of particles so they can bond together. The second type of organic wastewater chemicals are melamine formaldehydes and tannins, which are used to coagulate colloidal material in water. These are especially well-suited to treating hazardous sludge, as they absorb organic materials like oil and grease very effectively.

Inorganic coagulants:

Inorganic coagulants are often more cost-effective than their organic counterparts, and they can be applied to a wide variety of water treatment operations, but they are acidic by character and hence demand due care when handling and storing.. Inorganic coagulants are particularly effective at treating raw water with low turbidity, and can be used to treat this kind of water when organic coagulants prove insufficient. Inorganic coagulants are largely aluminium or iron based. Aluminium sulphate is the most commonly used chemical to treat waste water globally, though other examples include aluminium chloride, polyaluminium chloride, aluminium chlorohydrate, ferric and ferrous sulphate, and ferric chloride.

What are coagulants used for?

In water treatment, coagulants are used to remove a wide variety of hazardous materials from water, ranging from organic matter and pathogens, to inorganics and toxic materials, like arsenic, chemical phosphorous and fluoride. They often form the first step of a pollution reduction scheme for developing countries, especially those with fast-growing urban centres with no centralised waste water treatment. Aside from being water treatment plant chemicals, coagulants are used in medical applications to induce blood clotting. They are also used as part of the pulp, paper and textile manufacturing process; and in oil, gas and mining operations.

How do coagulants work?

Coagulants are comprised of positively charged molecules; they work by neutralising the electrical charge of particles, which destabilises the magnetic forces keeping colloids separated. Once inorganic coagulants have been added to the waste water, and the cationic metal ion has neutralised the colloid particles; iron or aluminium precipitates are formed, which absorb impurities and clean the water. This process is known as the ‘sweep-floc’ mechanism, which, although effective, adds to the overall volume of sludge for treatment and removal. The process for organic chemical coagulants is similar, except the positive charge most commonly comes from an amine (NH4+) group attached to the coagulant molecule, instead of a metal.

Despite being used to purify water for thousands of years, both coagulants and flocculants continue to form vital elements of modern water treatment operations. Due to the hazardous nature of both the chemicals and the work involved, waste water treatment must always be carried out by trained professionals in licensed facilities. If you have any questions about coagulants, flocculants or any other waste water treatment chemicals, contact our expert team.