Colloidal fouling (iron, clay and organic foulants)
Colloidal fouling is an especially common fouling in pre-treatment systems like UF or MF and despite being quite soft, can be very difficult to remove. Colloidal fouling can also be found in RO/NF systems and ion exchange systems and thus is of great importance across the water treatment industry.
What is a colloid?
The term “colloid”—from the Greek words kolla, meaning “glue,” and eidos, meaning “like”—was first used in 1861 by Thomas Graham to classify mixtures such as starch in water and gelatine.
A colloidal solution is a mixture of water and very fine particles ranging between 1 and 1000 nanometres (or 1um) in diameter and is one of the three primary types of mixtures (the other two being a solution and suspension). The particle size is quite specific as smaller particles will dissolve in the water and not be visible (like sugar for example) and larger particles (such as grains of sand) will settle out. The key defining parameter of colloidal solutions is that the solids will not settle out unless they are forced to by a chemical change. These are also known as colloidal dispersions, or suspensions, because the substances remain dispersed and do not settle to the bottom of the container.
The science behind colloidal stability?
Stability occurs when solid particles are small enough to have a very high electrical charge density. This then causes a secondary layer of counter ions to form around the particle. For example, a colloidal particle of iron (III) hydroxide, does not contain enough hydroxide ions to compensate exactly for the positive charges on the iron (III) ions, thus each individual colloidal particle bears a positive charge. The colloidal dispersion then consists of charged colloidal particles and free hydroxide ions, which keep the dispersion electrically neutral. This secondary layer forms an electrostatic repulsion great enough to prevent agglomeration, and the random motion of the water molecules (Brownian motion) is enough to keep the particles suspended. Technically there is a balance between the attractive Van Der Waals forces and the repulsive electrostatic forces but these act over different ranges. If the particles can get close enough, they will agglomerate and settle out.
Doing things like adding coagulants or high salinity waters will compress the secondary ion double-layer and thus lower the barrier to agglomerating. This is the principle of coagulation. On the other hand, adding charged species (like polymeric antiscalants) puts charge back onto the particle and prevents agglomeration (which is the principle behind dispersing antiscalants).
An easy way of determining whether a mixture is colloidal or not is to take a sample of your water, shake it up and then let it settle overnight. The colloidal particles won’t settle and they scatter light to make the solution appear opaque or milky. On the other hand, a true solution will appear clear or coloured but not disperse light.
Based on this:
- Particles that settle out after a few minutes would be considered gross solids
- Particles that settle out over a number of hours are still fine solids and a flocculant may be required to help increase the settling rate
- Any particles that are still suspended and opaque after settling overnight are considered colloidal
- If the solution is clear or coloured this would normally indicate a solution of organic or inorganic materials
Where do they appear?
Colloids are common across many natural water systems. In water treatment we most commonly see:
- Colloidal clays or silts
- Colloidal iron or aluminium
- Colloidal natural organic mater
These colloids can form in a number of ways, and can either be from washing of fine particles into water streams (which is common for clays and silts), or from the growth of natural biological materials (such as EPS from bacteria or algae). Iron and aluminium are a particular problem and can either form from erosion of a deposit or precipitation of iron oxide or aluminium oxide when they are exposed to oxygen.
What properties do they have?
Dispersed colloidal particles are mostly electrically charged. Most metal hydroxide colloids have positive charges, whereas most metals and metal sulphides form negatively charged dispersions. All colloidal particles in any one system have charges of the same sign. This helps keep them dispersed, because particles containing like charges repel each other.
Colloidal solutions can act like both a solid and a liquid and like a cornflour solution we made as a kid, it can be non-Newtonian. For example, when you put stress on it, it can respond by becoming harder, changing to a solid state. When you release the stress, it again changes state to become a liquid. They have changing viscosity because the particles can’t flow away as fast as the liquid, and are bunched together as a pseudo solid.
As the particle size of colloidal suspensions are so low (1 um or less) then they can often pass through a lot of pre-treatment systems. For example, multi-media or sand filters will often only remove up to 20 um and cartridge filters down to 1 or 5 um. These systems may remove a small partion of the colloids but most likely you will see a increased cleaning frequency and increased pressure drop.
Identifying colloidal fouling
Intially colloids maybe identified by looking at the water analysis and the total suspended solids or turbidity. On a membrane, colloidal fouling is best identified through visual and electron microscopy.
Firstly, when inspecting the membrane, colloidal fouling will appear as a slimy coating across the surface of the membrane. A grainy feeling deposit is generally caused by larger particles and could be gross solids or scale. These colloidal films can usually be easily wiped or scraped off without too much difficulty as they are not adhering to the membrane. The film however, can be un-responsive to many cleaners and difficult to remove.
Colloidal fouling on a hollow fibre UF membrane (left) and a RO membrane (right)
Optical and electron microscope images allow us to see more detail of the fouling layer and the characteristic cake layer with cracks that have developed as it compresses. Zooming in we can often see the finer colloidal particles in the 1um or smaller range.
Optical microscope image of colloidal fouling on a RO brine spacer
Another powerful tool is to use the elemental analysis capability of the electron microscope. Analysis using SEM-EDX (energy dispersive x-ray spectroscopy) will tell us what the major elements are in the deposit and help determine if it’s a clay, iron or organic colloidal fouling. The SEM-EDX is important as if the colloid has limited solubility, then it may not be picked up in other solution chemical analysis techniques like ICP-MS.
Colloidal fouling on a hollow fibre UF membrane (left) and a RO membrane (right)
Cleaning or removing colloidal fouling
Cleaning or removing colloids can be difficult. If the fouling is colloidal iron, then luckily you can usually remove it by re-dissolving the iron either with acids (HCl or Nitric acid) or by reducing it to a soluble form using a reductive cleaner (sodium metabisulphite or sodium hydrosulphite). Likewise biological colloids can be often degraded and dissolved by using caustic (NaOH) or oxidizing cleaners like sodium hypochlorite for UF/MF membranes. However, if this doesn’t work, or if you have a colloidal clay issue, then other approaches may need to be taken.
- Since colloids tend to compress and dehydrate when put under pressure, high flow cleaning can be ineffective. Thus, low flow cleaning or very careful backflushing of the membrane can be effective. Great care must be taken not to contaminate the clean side of the membrane or damage it with excess pressure. A great trick is to use osmotic pressure to clean the membranes. If you apply a 10% NaCl solution to the membrane, this will suck clean water from the permeate side of the membrane back to the brine side, effectively reverse flushing the membrane surface and loosening deposits.
- Surfactant based cleaners can be used in some cases to encapsulate colloidal deposits and allow them to be flushed from the system
- Polymeric dispersants can also be used to help release solids but care must be taken not to affect the membrane itself.
Eliminating the problem
Eliminating colloidal fouling can sometimes be easy and sometimes difficult. Most often, the key is to identify the source of the colloids and then to remove them using organic or inorganic coagulants before the water treatment plant. Its important to have a good dosing system with flow controls to avoid overdosing of coagulants. In fact, if you over-dose a coagulant it can have the reverse effect and actually disperse solids into solution and contaminate the plant further!
If you are looking at a coagulant dosing system, then speak to us about how we can test and design the right system for you.