By Rehan Iqbal
While a carbon-based water treatment system seems like it should be a scaled up version of the filter used in water pitchers, they’re far more complex. These filters come in different chemistries, sizes and locations to address different water quality issues. When do you need to use carbon filters, and which filters work best to address your water quality problems?
What is Activated Carbon?
Activated carbon is the base material of all modern carbon filtration systems. Carbon is activated through a process that breaks it into small molecules. The result is similar to graphite, but with an uneven structure. This creates a bumpy, uneven surface covered in pores of different sizes.
Most commercial filters are made from coconut shells. The shells are heated in an oxygen-free environment, removing everything except pure carbon. Super-heated steam is forced through this charcoal, creating cracks through the carbon’s structure. The result is a powder with an enormous amount of surface area. One gram of activated carbon has up to 32,000 square feet of surface area.
How Does Carbon Filter Water?
Have you ever had a piece of thread get stuck on a Velcro strap? That’s adsorption. Unlike absorption, where something gets stuck inside a substance, adsorption is when something gets stuck on the surface. Carbon filters use adsorption to remove particles from water. With so much surface area for water to pass by, it’s easy for contaminates to stick to the filter.
There are carbon filters that also use absorption, thanks to designs with microscopic pores. If these pores are smaller than one micron in size, the filter can remove most heavy metals, cysts and coliform bacteria. However, other screening methods are usually used to remove these contaminates.
Standard Carbon Filters: How They Work and What They Remove
Plain activated carbon is used in two forms: Granular Activated Carbon (GAC) and carbon block filters.
A GAC filter contains loose activated carbon granules. This type of carbon used in water filter pitchers and aquarium filters. As the water passes through the granules, the carbon picks up contaminates. GAC doesn't restrict flow rate as much as other filters. However, the water can form channels in the filter. This reduces the contact between the water and the carbon, reducing the filter’s effectiveness. Most GAC media tanks use an upflow system, forcing water into the bottom of the tank, where it has to flow up to reach the outlet. Plates and screens are also used to keep the carbon from shifting and forming channels.
Carbon block filters glue the carbon together to form a solid filter. While the adhesive takes up some space in the filter, finer granules of carbon can be used. This increases the surface area of the carbon, improving filtration. It also prevents channeling. A well-designed carbon block filter has up to 10 times more surface area than a GAC filter. However, this structure also makes these filters more restrictive.
Standard Carbon Filters are Good at Removing these Contaminates:
Volatile Organic Compounds (VOCs)
Used in a wide range of products, these chemicals are known to cause irritation of the eyes, nose and throat along with nausea and headaches. Long term exposure can lead to liver, kidney and nerve damage. However, studies of these chemicals and their effects concentrate on direct exposure, such as workplace use of aerosol paints. Information on the effects of small amounts of VOCs isn’t well established. Even if these chemicals don’t cause harm, they can make your water taste bad. The EPA regulates 23 VOCs found in drinking water.
This subcategory of VOCs include chloroform, bromoform, bromodichloromethane, and dibromochloromethane. These are known carcinogens. Most THMs are produced naturally, or when chlorine reacts with organic compounds. This makes THMs a common pollutant in water, no matter the source. The EPA limits THMs in municipal tap water to 80 PPM.
Pesticides and Herbicides
These chemicals can be poisonous in large amounts, but they’re mostly removed to improve the taste of water.
Modifying Carbon to Remove Other Contaminates
There are two other types of carbon used in water filtration: catalytic carbon and KDF. These are modified forms of activated carbon that target specific contaminates.
Catalytic carbon is treated to increase its adsorption abilities. Most catalytic carbon used in water filters is made by spraying it with nitrogen gas during activation. This creates thin sheets of carbon that work better than standard activated carbon at trapping heavy metals, chlorine and chloramides.
Chlorine and chloramine are the main reason most people add carbon filters to their water. Chlorine is used to prevent pathogen growth in tap water. Unlike other treatment methods, like UV light, chlorine remains active, keeping water clean as it goes from the treatment facility to your home. Chloramine is formed when chlorine mixes with ammonia. It’s also used to treat water. Since these chemicals are also used to stop algae growth in pools, we associate their smell and taste with pool water.
Kinetic Degradation Fluxion (KDF) filters use ion exchange to turn soluble metal, which is held in suspension in the water, into insoluble metal, which can be filtered out. In this process, lead and zinc trade places, with the lead bonding to the filter. Other metals attach themselves to the filter media and oxidize. These oxides fall out of suspension, and are trapped by the filter. These filters are more effective at removing heavy metals than catalytic carbon. Metals removed by KDF include mercury, copper, nickel and chromium. These metals can cause long term nerve damage.
Other Carbon Filters
While activated carbon, catalytic carbon and KDF make up the bulk of filter options, there are two more chemistries you may see in water treatment systems.
Silver is bacteriostatic, which means it stops cellular respiration. With no way to break down food, the bacteria die. This isn’t effective at killing off bacteria already in the water, but it does stop spikes in bacterial growth when water sits in the system for long periods of time. Silver is used in carbon filter systems that are shut off for long periods, like those used in remote cabins. However, for most users, it’s easier to use a UV filter to treat all water, whether it’s fresh or stagnant.
Carbon ceramic filters use carbon for adsorption and ceramic for absorption. This makes these filters effective at removing chemicals as well as sediment. These filters are handy for installations in limited space, but most designers opt to split these processes between two filters.
If your water tastes bad, a carbon filter is usually the best treatment solution. Even if the amount of chemicals in the water is far below what is considered safe, they can give it an off taste. However, carbon is not effective at removing sulfur, which gives water a rotten egg smell.
Chlorine and chloramine can cause irritation of the skin and nasal passages after long term exposure. This is mostly an issue with indoor pools, because they trap a large volume of treated water in a confined space. However, if you’ve addressed hard water issues, and your skin still feels dry after a shower, this may be your problem.
If you use chlorine to disinfect your well water, you should use catalytic carbon to remove this chemical before the water passes through the rest of your plumbing system. Most chlorine treatment systems have a built-in carbon filter.
Depending on the type of water softener you use, you may need a KDF carbon filter to address heavy metal contamination. Salt-based water softeners use ion exchange to remove calcium and magnesium from water. This process also attracts heavy metals, including lead and arsenic. Water conditioners, also called salt free water softeners, change the chemistry of calcium and magnesium, so they don’t stick to pipes. This does nothing to remove heavy metals.
Do I Need to Treat All of My Water, or Just Drinking Water?
Point of Use (POU) systems filter water just before it exits the tap. This only treats water from one location in your home. Point of Entry (POE) systems treat water as it enters your home, so all water is treated. Your choice of system depends on the scale of the problem. Do you want healthier, better tasting water to drink, or do you need to address problems that affect bathing, cleaning and plumbing?
Reverse osmosis is commonly used in POU systems, because it removes salt and other contaminates, improving taste. Chlorine destroys RO filters, so most systems have one or more carbon pre-filters. Using carbon to remove contaminates also saves wear and tear on the RO filter.
Chlorine and chloramine is hard on rubber seals and water softener resin. A POE carbon filter protects these parts.
Why Do Most Systems Use Multiple Filters?
The more the filter does, the more expensive it is to make. Just as sediment filters are divided into different pore sizes, carbon filters are divided by their chemistry. By using a large GAC to remove most of the contaminates, you can use smaller carbon block, catalytic and KDF filters, cutting down on the overall cost of the system.
Each chemistry is ideal for a specific task. You need a KDF filter to remove metals and catalytic carbon to remove chlorine. A GAC filter has the lowest impact on water pressure, so you can maintain the flow of water in your plumbing system. Carbon block is more restrictive, but it’s also a lot smaller than an equivalent GAC filter. This makes it a good choice for installations where space is limited.
Why Do Carbon Filters Wear Out?
The surface of activated carbon can only hold so much material. Once those spaces are filled, contaminates are free to pass through the filter. Worse still, worn out filters can break down, releasing captured toxins into the water. Since zinc and lead swap places on KDF filters, eventually the supply of zinc will be exhausted. Pores in carbon and carbon ceramic filters also fill up over time, clogging the filtration system.
Increasing the amount of filter media increases that filter’s lifespan. An under-sink POU filter may only last 6 months to a year, while most POE systems are designed to last 5 or 6 years. Since most water is used in the bathroom, these systems are sized based on the number of bathrooms in your home. A cubic foot of GAC is enough for a 1-3 bathroom home, while a 4-6 bathroom home needs a filter with 1.5 cubic feet of GAC.
Any filter will clog up faster if the water has high levels of dissolved contaminates. Generally speaking, higher grade filters have smaller pores, which makes them easier to clog. While a standard activated carbon filter may have 5 micron pores, a catalytic carbon or KDF filter usually has one micron pores. Installing a pre-filter to remove large contaminates keeps these filters from clogging and failing prematurely.