How are coagulants and flocculants used in water and wastewater treatment?

By: James Greenwood on Mar 24, 2020

Coagulation and flocculation are essential components of both drinking water and wastewater treatment. They provide a reliable process for treating water turbidity (the cloudiness or haziness of a fluid typically invisible to the naked eye) which is a key test of water quality. In wastewater treatment, they enable up to 90% reduction in suspended solids and organic loads.

All waters contain suspended particles. The smallest particles (colloids) are stabilised by the action of physical forces (static electricity) on the particles themselves and, because they all have a negative charge when suspended in water, they repel each other. This causes them to remain suspended rather than clumping together and settling out of the water. They might take days or even up to centuries to settle out!

Coagulation and flocculation are two separate processes, used in succession, to overcome the forces stabilising the suspended particles. While coagulation neutralises the charges on the particles, flocculation enables them to bind together, making them bigger, so that they can be more easily separated from the liquid.

The coagulation process

This destroys the process whereby tiny particles repel each other and promotes their consolidation to bigger ones that are able to stick together. The bigger the particle, the easier it is to separate from the liquid. The use of coagulants for treating water goes all the way back to around 2000 BC when the Egyptians used almonds, smeared around vessels, to treat river water.

These larger ‘clumps’ of particles are called micro-flocs and still cannot always be seen by the naked eye. The water surrounding these newly formed particles should be clear - and this will signal that the particles’ charges have been neutralised. If it isn’t, more coagulant may be needed. Too much coagulant and the particles will revert to repelling each other – but predominantly by the reverse charge.

Rapid mixing ensures the coagulant is properly dispersed to promote particle collisions. The metal coagulant hydrolysis products formed within 0.01 to 1.0 seconds tend to be the most effective for destabilisation – this is why adjustment of pH and post-dosing of more coagulant is rarely effective after the initial coagulant addition.

One common type of rapid mixer is called a back-mix reactor, which normally consists of square tanks with vertical impellers. In many instances, they produce poor results and WCS tends to design in-line mixers with velocity gradient control to provide the best conditions for rapid mixing.

Types of coagulants

Today, there are two types of coagulants that are most commonly used in water and wastewater treatment. Organic and inorganic.

Inorganic coagulants include:

  • Aluminium coagulants - e.g. aluminium sulphate, aluminium chloride and sodium aluminate
  • Iron coagulants - e.g. ferric sulphate, ferrous sulphate, ferric chloride and ferric chloride sulphate

Organic coagulants include:

  • Polyamines
  • Poly DADMAC’s
  • Polytannate

Inorganic coagulants

Both aluminium and iron coagulants have been proven to be very effective at removing most suspended solids. They offer a number of advantages:

  • Enable highly charged ions to give a high charge density to neutralise suspended particles, which allows hydrated inorganic hydroxides to form and produce short polymer chains that enhance microfloc formation and heavy floc
  • Capable of removing a portion of the organic precursors which may combine with chlorine to form disinfection by-products
  • Low unit cost and widespread availability
  • Enable relatively low charge density to neutralise lower charged suspended particles, more effectively. Produce longer polymer chains that enhance microfloc formation without metals or hydroxides
  • Capable of removing a portion of the organic precursors which may combine with chlorine to form disinfection by-products
  • Produce small floc volume
  • Liquid forms, non-corrosive, ready for direct use.
  • Do not impact and are rarely or marginally affected by pH

They have some disadvantages:

  • They create large volumes of floc, rich in metal, which must be disposed of in an environmentally appropriate manner, which can add significant cost to disposal
  • They can significantly alter the pH of the water, where pH is critical for effective coagulation, necessitating pH control. They also require corrosion-resistant storage and feed equipment
  • Aluminium sulphate and chloride, ferric sulphate and chloride and ferrous sulphate are highly acidic, destroy alkalinity and lower pH. Sodium aluminate, on the other hand, will add alkalinity and raise pH
  • Higher unit cost
  • High dosages needed if charge demand is high
  • Low density floc – does not always settle well

Organic coagulants

Both polyamine and poly-DADMAC coagulants have been proven to be very effective at removing most suspended solids. Tannates are particularly good at oils and fats.

They offer a number of advantages:

  • Enable relatively low charge density to neutralise lower charged suspended particles, more effectively. Produce longer polymer chains that enhance microfloc formation without metals or hydroxides
  • Capable of removing a portion of the organic precursors which may combine with chlorine to form disinfection by-products
  • Produce small floc volume
  • Liquid forms, non-corrosive, ready for direct use
  • Do not impact and are rarely or marginally affected by pH

They have some disadvantages:

  • Higher unit cost
  • High dosages needed if charge demand is high
  • Low density floc – does not always settle well.

The flocculation process

Following coagulation (“Charge Neutralisation”), a second process called flocculation needs to take place. This is the growth of small, neutral particles into larger particles. Flocculants are the agents which promote this clumping of fine particles into ‘floc’ that can then be readily separated from the water. They are invariably polymers.

The flocculation process is a gentle mixing stage that increases the size of the particles from micro-floc to large, visible suspended particles called pin-flocs. Additional collisions between pin-flocs cause them to produce even larger, ‘macro-flocs’. The flocculants aid in this by being long chain polymers with low charge to involve entanglement, enhanced van der Waal’s forces and hydrogen bonding between the particles. Once these flocs have reached an optimum size and strength, the water is ready for solids-liquid separation. This can be filtration; centrifugation; sedimentation or flotation.

The role of polymers

Polymers are a range of water-soluble macromolecular compounds that have the ability to stabilise or enhance flocculation of the constituents in a body of water. They are added as part of the flocculation process to help strengthen and increase the settling weight of the floc.

Polymers can be either natural or synthetic. Natural polymers also date back to ancient times, with Sanskrit literature from around 2000 BC referencing the use of crushed nuts to clarify water. Natural polymers are virtually free of toxins and are biodegradable. Synthetic polymers are more commonly used because they are more effective, reliable, reproducible and cost-effective.

Summary

Coagulation and flocculation are two, separate, crucial parts of water and wastewater treatment. Coagulation destabilises the minute suspended particles by static charge neutralisation, while flocculation helps them to bind together to form much bigger morphologies, so they can be more easily separated from the liquid phase.

At WCS Group, our range of coagulants and specially selected flocculants can significantly reduce sludge generation and create alternative disposable routes or new wastewater options for our customers. To find out more, get in touch.

Wastewater Treatment

Topics: Wastewater

James Greenwood

Written by James Greenwood

James Greenwood as been working in the Water Treatment and Water Hygiene Industry for over 20 years. He is currently the Sales and Marketing Director for WCS Group the largest water hygiene and treatment Company in the UK. James has been instrumental in bringing significant innovations to the UK market over the years always focusing on enhancing client’s compliance and delivering true return on investment projects offering monetary and environmental savings.