Removal of contaminants using carbon filtrationFiltration Media
Removal of contaminants using carbon filtration.
When you think of a “water filter”, what springs to mind first? Is it a basic, stone, sand and gravel-based gravity filter? Is it a high-end reverse osmosis system? Or perhaps a UV treatment system to kill bacteria?
Chances are, most people would think about a carbon-based cartridge. Whether it is a carbon block or granular activated carbon filter, you can find carbon in near enough all drinking water filters.
So, why is this? Granular activated carbon is a specific preparation of activated carbon or activated charcoal. It has been around for hundreds if not thousands of years, being used in ancient India and Egypt to remove impurities.
All forms of activated carbon are produced in the same way. Organic material with a high level of naturally occurring carbon, such as wood, coconut, or coal is slowly heated in a low or no-oxygen environment at around 930°C. This removes water and impurities from the base material, without allowing the material to burn. Coconut carbon is preferred for drinking water applications as it gives water a cleaner, crisp taste which consumers seem to prefer over regular wood-based carbon. What you are left with is a black char, which then goes through a number of chemical and physical processing methods to increase (activating) its surface area. Activated carbon has an amazingly high surface area per granule, covered in sub microscopic pores which allows a process known as adsorption to occur.
Just 4 grams of activated carbon has a surface area the equivalent of a football field (6400 sqm).
The reason carbon is favoured as THE filtration media to include in a drinking water filter, is its desire to seek out and adsorb other contaminants, as it is naturally attractive. Adsorption is different to absorption, as adsorption allows contaminants to bind to a surface which creates a thin film around whatever it is binding to. Absorption is the process where chemicals are taken in, evenly distributed throughout and mixed into the absorbing agent.
Carbon will never stop looking for contaminants to adsorb, it will continue to be attracted to finding things forever really. However, once the surface on the carbon has filled up, it will no longer adsorb contaminants, and this is when the filter/media needs to be replaced.
As activated carbon has an extremely high surface area, is porous and is very attracted to finding contaminants, it makes it a 1st choice pick for contaminant removal. With it being porous, the carbon adsorbs contaminants to its surface, and then deep into its structure, allowing it to hold more than a regular, non-porous media. Activated carbon can be impregnated with various chemicals to improve its binding ability against targeted compounds which it may not be able to originally remove. The best example of this is the inorganic compound chloramine, which water board companies add to the municipal water supply as a substitute for chlorine. Chloramine is a mixture of chlorine and ammonia, and its properties mean that it doesn’t gas off like chlorine does, meaning it can stay in the water supply for longer and be able to disinfect for longer. The downside to chloramine is that it is extremely difficult to remove and is highly detrimental to aquatic life. This leads to a large portion of the aquatic market investing in a treated chloramine removal carbon block or some opting for catalytic carbon, which will separate the chloramine into its separate parts: chlorine and ammonia.
There are different types of activated carbon. The two most popular types being:
- Granular activated carbon
- Powdered activated carbon (usually used in carbon blocks).
Activated carbon is particularly good at removing organic compounds, as well as; chlorine, chemicals, bad odour, taste and smell. Unfortunately, carbon cannot remove all organic contaminants and will not deal with inorganic contaminants which have much stronger bonds which the carbon struggles to break down. The most cited studies from EPA and NSF claim carbon can effectively remove 60-80 chemicals, effectively reduce around 30 and moderately reduce 22 contaminants.
Another factor is the size of the particles that the filter will allow through. This is also known as particulate removal or mechanical filtration and is measured on micron ratings of the filters. Granular activated carbon (GAC) has no specific limit as the material is porous but is usually rated between 20-50 micron. Activated carbon in the form of carbon blocks usually have a pore size of between 0.5 to 10 micron. One thing to note is if you have a sub-micron rated filter with no pre filtration, it could clog prematurely before all of the carbon has been exhausted. Cheaper polypropylene sediment filters are available as a pre filter to make sure your carbon block doesn’t pack in before you have got the best use out of it.
One other important thing to mention regarding carbon and its contaminant removal is contact time.
Contact time is the amount of time the water is given to touch the media and allow the adsorption process to occur.
Unfortunately, granular activated carbon is harder to measure in relation to contact time. Even in cartridge form, granular activated carbon can come in different amounts, and could be potentially challenged with another problem – channelling. Channelling is where water enters the filter and takes a path through the activated carbon. Imagine it as small streams travelling through the filter, as water wants to find the easiest and fastest way to travel through it. Once these channels have been created, water usually follows and sticks to these paths, meaning there is a percentage of carbon in the filter that has not touched any water and won’t do once these channels are created.
Channelling does not always occur, but it is a factor to consider with a GAC filter. The main factor which affect channelling is water pressure, and changes in water pressure which also equates to flow rates.
On the other hand, a carbon block filter is a much more sustainable way of getting the most out of your carbon filter, as it is structured where the water surrounds the cartridge and has to pass through the carbon from outside to inside to be able to pass through the system. In a carbon block, all the carbon is used, and we can fortunately advise on optimal flow rates for contact time. These vary depending on cartridge size.
- 10” = 3.8LPM
- 20” = 7.6LPM
- 10”BB = 7.6LPM
- 20”BB = 15.2LPM