How does a water filter work? Which filter? Untreated water contains a wide range of contaminates, and it can take several strategies to turn it into something we can use and drink. Depending on the acidity and contaminants in a water source, it may take several filters to make it safe and palatable. Here’s a breakdown of how these filters work, and when they are used.
What is in Water, and Why Do We Need to Filter It?
Pure water is just molecules of H2O. However, since it’s a powerful solvent, it also picks up almost anything it comes in contact with. These are the major contaminates that must be dealt with to make water safe to drink.
- Microorganisms - Untreated water can contain pathogens that can cause diarrhea, vomiting, cramps, nausea, headaches, fever and fatigue. These bacteria, viruses and protozoa can grow in any water. However, they’re more likely to reproduce in water with high nutrient content, including water contaminated with fertilizers and animal feces from agricultural runoff.
New wells should be tested for coliform bacteria. This class of organisms includes E.coli, which can be dangerous by itself, but it’s also a good indicator that conditions are right for the growth of other pathogens. When boil orders are issued for municipal water sources, it’s usually because the network of pipes that link consumers to the water plant is damaged. Boiling kills pathogens that may enter this contaminated system. - Heavy metals - Heavy metal is a broad term that includes any metal or metaloid that is more than 5 times denser than water. All heavy metals are toxic at certain doses, but our bodies requires small amounts of some metals, including cobalt, copper and iron. Arsenic, lead, cadmium and mercury are toxic, even in trace amounts. Over time, these metals can build up in our bodies, causing a range of problems, including cardiovascular and nervous system issues, as well as cancer.
While problems caused by acute exposure to high levels of heavy metals are well known, we don’t fully understand the effects of long term heavy metal toxicity. That’s because it can take years for symptoms to develop, and it coincides with other factors that lead to chronic health problems. - Volatile Organic Compounds (VOCs) - These are carbon-containing chemicals that dissolve easily in water and air. This includes a wide range of chemicals, including everything from gasoline to pesticides. While most people know about airborne VOCs in aerosols and paint, ground water contamination can leach VOCs into well water. The EPA’s National Primary Drinking Water Regulations (NPDWR) lists 53 VOCs that must be monitored and removed from public water systems. Effects can range from short term symptoms, like headaches and dizziness, to long term problems, including cancer and nerve damage.
- Hard and Soft Water - These terms are used to describe water that contains chemicals that make water more acidic or alkaline. However, they don’t directly translate to more acidic or basic water. Water can contain chemicals that both increase and decrease acidity, balancing out the pH, while still causing the same issues.
Hard water contains calcium carbonate and magnesium bicarbonate. These minerals make water taste bad, and they leave deposits on pipes, plumbing fixtures and surrounding surfaces. Soap doesn’t work as well with hard water, and it can leave skin feeling dry after long baths and showers.
Soft water contains calcium and magnesium ions. These ions dissolve iron, manganese, lead, copper and sodium. It erodes copper pipes and lead soldier, leaving a greenish blue film on and around water fixtures. It also causes indigestion, and it tastes bad. Soft water is as good as any other water at rinsing away soap. However, the water itself feels soapy, so it doesn’t feel like it does a good job at cleaning. - Sulfur - Hydrogen sulfur is the bane of many well owners. This chemical smells like rotten eggs, making it hard to drink contaminated water. High levels of hydrogen sulfur can cause diarrhea and other digestive problems. This chemical also causes corrosion, leaving black stains on and around plumbing fixtures. The smell also permeates clothes washed in sulfur-containing water.
Water Filters and How They Work
There is no single filter that can address all of these water problems. Currently, there are 13 types of filtration commonly used to treat water. Their use varies depending on the water source and its quality. Municipal water is already treated for most contaminates, so a home water treatment system may only have a water softener. Well water is untreated, requiring several treatment steps before it’s safe to drink. Portable filter systems, like those used for backcountry hiking, have to balance performance with size. Here’s how these filters work, and what they can do to make water safe to drink.
- Spin-Down Sediment Filtering - These filters work like the cyclones in a vacuum cleaner. As the water enters, it swirls around the outer edge of the chamber. Centrifugal force separates large particles from the water. These particles collect at the base, while a screen removes smaller particles before exiting the filter. These filters are usually the first step in treating well water.
- Direct Filtering - This type of filter screens out anything larger than a water molecule. For example, filters used for backpacking have tubes with microscopic pores, typically 0.1 micron in size. Water can pass through these pores, but all but the smallest bacteria and protozoa can’t. This effectively removes the germs that cause common ailments from untreated water consumption, including giardia and E. coli. These filters can’t remove heavy metals, nor do they treat hard or soft water.
The filter media can be cleaned by backflushing. The filter runs in reverse, with fresh water passing through the outlet and past the filter media. This picks up contaminates and forces them out of the filter chamber. Even pocket-sized filters can clean thousands of gallons of water, as long as they’re periodically backflushed. - Activated Carbon Granules (AC) - Adsorption is a process in which atoms and molecules stick to a solid surface. Activated carbon, also called “activated charcoal” is standard charcoal that is heated in the presence of a gas, creating microscopic pores that increase the carbon’s surface area. One gram of activated charcoal has 32,000 square feet of surface area. This creates massive areas for adsorption to take place. Water isn’t attracted to these surfaces, but many contaminants are. An activated carbon filter is effective at removing VOCs and chlorine. It doesn’t remove pathogens or dissolved solids.
Most faucet and pitcher water filters use activated charcoal. These filters are also widely used in well treatment systems, since they remove a wide range of contaminants. - Reverse Osmosis - Osmosis is the movement of solvent molecules through a semi-permeable membrane from an area of high concentration to low concentration. The process stops when solvent levels on both sides of the membrane reach equilibrium. Osmosis is used for all kinds of biological processes. For example, plant cells have membranes that pump water out of the soil.
Reverse osmosis uses pressure to force solvents through a membrane, concentrating the solvent on one side. Reverse osmosis is great at removing salt, calcium carbonate, nitrates, microorganisms and heavy metals. It doesn’t remove VOCs.
Unlike filtration, the membrane doesn’t trap contaminants. Instead, it keeps them from passing through. The process concentrates contaminates on the membrane side, creating waste water. This process is commonly used to treat water on a large scale, including municipal systems and water bottling plants. - Ion Exchange - An ion exchange filter is filled with zeolite beads loaded with sodium ions. A zeolite is a molecule with large spaces between its bonds. These spaces are large enough to hold water molecules, slowing the flow enough to let ion exchange take place. As water passes through, the molecular bonds of calcium and magnesium carbonate break down, leaving behind calcium and magnesium ions. These ions are more attractive to the zeolite molecule than sodium ions. These ions swap places, so the water contains sodium instead of calcium and magnesium.
Over time, this process depletes the sodium ions in the filter media. Ion exchange filters are recharged by adding sodium bicarbonate (soda ash.) These filters also need to be backflushed to push out collected contaminates.
Zeolite beads are made of crosslinked styrene resin. The links in this long chain polymer can be broken by chlorine and the physical action of filtering. Eventually, the filter media will fail, requiring replacement. The more crosslinks there are between styrene rings, the longer the filter will last. A basic filter uses 8% crosslinked resin, while 10% crosslinked resin is common in premium systems. In areas with high levels of chlorine in the water, the system may use up to 16% crosslinked resin. - Nucleation Assisted Crystallization (NAC) - NAC systems don’t remove anything from the water. Instead, they crystallize the molecules that make water hard. While this doesn’t technically soften water, it has a similar effect.
Calcium carbonate and magnesium bicarbonate attach to the filter media. This forms a nucleation site, creating a chemical reaction. These molecules break down into carbonate crystals, carbon dioxide and water. The crystals don’t react with the water or the surfaces the water comes in contact with. The end result is neutral pH water that doesn’t leave behind limescale.
NAC treatment doesn’t just stop hard water issues, it can reverse them. Normally, mixing carbon dioxide and water forms carbonic acid. This acid is what dissolves calcium and magnesium. The carbon dioxide created during crystalization forms colloidal microbubbles that won’t react with water. These bubbles scour pipes, removing limescale.
NAC filters don’t require backflushing or salt. However, the filter media wears out faster than the crosslinked resin used in ion exchange filters. Once the cost of salt and maintenance is factored in, the lifetime cost of an NAC filter is usually in line with an ion exchange filter.
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