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There are many complex mechanisms at work in the processes of coagulation and flocculation

In the world of water and wastewater treatment, coagulation and flocculation are processes that have been used for many years (reportedly, both the Egyptians and Romans dabbled). At face value, they are simply ways of producing larger particles that can be quickly and easily separated from the water being treated, but there are many complex mechanisms at work. In this article, I explore these mechanisms and look at what is driving development.

When removing a contaminant from water, in general, the bigger it is, the easier your task. Chemical engineers must develop a clear understanding to optimise performance. When the mechanisms underpinning coagulation and flocculation are understood, they can reveal new opportunities to improve target contaminant removal, reduce chemical and energy use, and manage waste production. Developments in this area of process engineering have been driven by these benefits.

Contaminants and coagulation

 Broadly speaking contaminants can take several forms: suspended solids, colloids and dissolved solids.

  • Suspended solids range in diameter from a few millimetres down to tens of microns.
  • Colloids are very small particles ranging from 10 microns down to 10 nanometres.
  • Dissolved solids are individual molecules or ions and in the nanometre range.

There is not a hard cut-off between these as, for example, large organic molecules are colloidal in nature.

Suspended solids will tend to settle under gravity (or float if buoyant) therefore they can be removed relatively easily. Colloids have such a high surface area to volume ratio, that surface forces are more important than gravitational forces. Coagulation is the most common way of destabilising colloids, and allowing them to clump together to form larger particles that can more easily be removed.

So – how do you destabilise these minute particles?

The majority of colloids have a negative surface charge, so they naturally repel each other. Coagulation is the process of overcoming or neutralising this repelling force such that they stick together and form larger particles.

There are four key mechanisms, any or all of which take place when a coagulant chemical is added:

  • Double layer compression – addition of an electrolyte reduces the diameter of the electrical double layer surrounding the colloids. The reduced electrical repulsion allows the particles to get closer and short-range attraction becomes dominant.
  • Charge neutralisation – when ions of the opposite charge are added, some adsorb onto the surface of the colloid, neutralising the electrical repulsion and allowing them to agglomerate. Iron and aluminium salts are commonly used with their high (+2 or +3) ionic surface charge.
  • Entrapment in, and adsorption on the precipitate – soluble metal salts will, at the correct pH, precipitate as metal hydroxides. Colloids form energetically favourable nuclei for this precipitation, thus the colloids are entrained within the suspended solids or adsorbed on to the surface.
  • Bridging – long chain organic molecules (polymers) with multiple surface charge sites ‘bind’ colloids and other suspended particles together, forming larger solids that may be removed.
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Principals of coagulation

A general rule is that coagulation requires high-energy mixing. This is because where metal salts effect coagulation by double layer compression and charge neutralisation, their effectiveness is greatest when the salts are present as ionic complexes. These complexes only exist for a very short time, in the order of seconds. For effective economic coagulation by double layer compression and charge neutralisation, intense mixing is required to ensure that the metal coagulant is distributed rapidly through the water before insoluble salts are formed.

Flocculation

Once we have chemically modified the colloids so they are no longer repelling each other and they are starting to clump together, what next? Well, the next stage is flocculation – bringing these tiny particles close enough to each other so they grow into larger particles (called flocs) that are easily removed in downstream solid-liquid separation.

Flocculation is achieved through three main mechanisms: Brownian motion, stirring and differential settling. For initial flocculation of particles smaller than 0.5 microns, Brownian motion is the main process. However, as particles increase in size it is necessary to encourage collisions by stirring. The amount of energy used for this needs to be appropriate to the size and strength of the flocs. Too little energy will result in low rates of floc formation, but excessive energy input will lead to floc breakage.

Optimisation

The next challenge is optimising the process to achieve the desired contaminant removal while minimising chemical use, energy demand and waste stream production. But what is optimum? That depends on the contaminant(s) targeted.

Coagulation and flocculation are mainstays of drinking water treatment – clarifiers and granular media depth filters would not meet regulatory water quality targets without it. Historically, turbidity removal was the metric used to optimise performance. More recently, removal of organic compounds has been used too. This is because natural organic matter (NOM) such as humic and fulvic acids are precursors to regulated by-products formed during disinfection. The optimum coagulation conditions for turbidity removal will often occur at a higher pH than for NOM.

Jar testing

Selecting the optimum coagulation conditions is normally done with jar testing. This allows a range of coagulant types, dose rates and pH conditions to be compared directly. Setting a target for the desired degree of contaminant removal is the starting point (eg turbidity, NOM or phosphorous), but there will often be a variety of ways achieving this. Selecting the optimum conditions involves a complex balance: considering the quantity and cost of coagulant used, likewise any pH correction up or downstream of coagulation and importantly the cost of treatment and disposal of the additional solids produced in the form of metal hydroxides. On a 100Ml/d water treatment works, an increase in aluminium dose of 1mg/l produces an additional 105 tonnes per year of metal hydroxide solids that must be disposed of – that’s over 130 road tanker movements a year if exported as a four per cent d.s. liquid sludge! However, coagulant dosing for phosphorous removal during primary settlement in wastewater treatment can yield a significant improvement in performance and downstream biogas production – a countering factor in the cost benefit calculation.

With many different mechanisms occurring, jar testing allows different chemicals, at different doses and pH values to be assessed. Destabilising colloids through double layer compression or charge neutralisation requires that the coagulant metal is in ionic form. Metal ions quickly hydrolyse and then form hydroxides when dosed, so achieving rapid and effective mixing is essential. Similarly, removing phosphorous from wastewater is most effective through precipitation with the metal ions before they form hydroxides.

Also, there’s little benefit in achieving optimal coagulation without the crucial next step – flocculation. Flocculation grows these destabilised particles to a size at which the downstream solid-liquid separation is effective. For drinking water, the size of these flocs is tailored to the downstream process – large for sedimentation, smaller (but not too small!) for floatation or filtration.

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Flocculated solids settle quickly or can be floated to the surface using microbubbles

The same is true for wastewater, where tertiary treatment often involves a filtration step. Having successfully precipitated metal phosphate, it’s an own goal to allow these to pass through the solid-liquid separation process. Environmental permits are based on total phosphorous, so metal phosphate in final effluent still affects compliance.

What does best practice look like?

Raw water abstracted from rivers and reservoirs varies in quality significantly with the seasons. Municipal wastewater quality varies significantly over the course of each day. Faced with this change in influent quality, applying a fixed dose of coagulant chemical is only ever going to be optimal some of the time – even a stopped watch tells the right time twice a day! Jar testing provides a snapshot in time, but it’s not practical to do this with the frequency required to capture daily or hourly changes.

Process instrumentation and control is the key to coagulant dosing. This can be based on influent quality with feed forward control or treated water quality informing feedback loops. There are also instruments that directly measure the charge neutralisation effects. For very stable contaminant loads, the dose adjustment may be made by operational teams if they’re going to be watching at the right time to make them. Flocculation control is less variable – once the system has been set up to provide sufficient energy to grow the correct size of floc, adjustments are rare.

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Coagulation requires high-energy mixing

Coagulation and flocculation are not novel processes. However, the increasingly stringent targets for certain contaminants has meant that the mechanisms have become better understood. The drive for more efficient use of chemicals and minimising waste streams has stimulated further research. Most water and wastewater treatment plants would not comply with treated water targets without these processes.

So, coagulation and flocculation are here to stay, but the big advances ahead are around process monitoring and control.

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About the author

Hugh Thomas is Chief Process Engineer at Atkins (part of the SNC-Lavalin group)