Home Water Filter Systems
In the last post, we discussed the key characteristics that you should be seeking in a water filter system: it should be able to remove different contaminants while maintaining good mineral content and provide optimum pH levels as well as anti-oxidants and should make the water taste good. There are a number of different home water filter systems and each of them by themselves are not able to satisfy every need. Rather, you will likely be finding a filter solution that is a composite of different technologies.
Carbon Water Filters
A relatively inexpensive and reliable technology, carbon water filters are made from activated charcoal, which has a very high surface area to volume ratio. This treated charcoal presents ample area to which a number of contaminants will readily bond.
Charcoal is created from burning carbonaceous (carbon-based) substances, such as coal, bamboo, coconut shell, or bone char, at extremely high temperatures (600–900 °C) in an inert atmosphere.
During this process, 70% of the carbon-based mass vaporizes, as the larger organic molecules are removed. Activation of the charcoal occurs subsequently either with the chemical treatment or through exposure to oxygen and steam at temperatures between 600 – 1200 °C. The activation process creates a huge array of micro-pores in the remaining carbon substrate, giving the carbon its tremendous surface area.
Not all carbon filters are equally effective with contaminant removal. This is due to the variation in number, distribution, and size of the pores formed, which is ultimately based on the type of carbon substrate and activation method used. Ideally, the pore size will be very close to the size of the contaminants they are trying to attract and trap.
Certain substances are attracted to the granular substrate while others are not. Rather, molecules with properties similar to carbon will have a higher tendency to interact chemically with the carbon, to which they physically attach or stick and become trapped within the porous structure.
Other factors impact the effectiveness of carbon filters at removing these contaminants. Lower temperatures and lower pH levels (i.e., higher acidity) enhance the filter’s performance. Flow speed also provides a large impact on how well-activated charcoal will pick up contaminants. The lower the flow speed, the longer the exposure of contaminants to the carbon surface, the higher the probability of adsorption.
Carbon filters are also extremely useful in removing the disinfectant Chlorine (Cl2) from drinking water, through adsorption or through a reduction process, where molecular chlorine is broken down into its elemental form, Chloride (Cl–), which then oxidizes to CO2.
Carbon filters in conjunction with the process of reverse osmosis will remove chloramines, another disinfectant, from drinking water. Chloramines result from a combination of chlorine and ammonia.
Filters can also improve the taste of water as well as remove bad odors from water.
What carbon filters do not remove well are inorganic contaminants like salts, fluoride, asbestos and heavy metals like mercury and lead, micro-organisms, and minerals. Furthermore, a downside to carbon filters is the combination of large surface area and moist environment, as these create an ideal breeding ground for bacteria, such as Pseudomonas.
Reverse Osmosis Water Filter System
Osmosis is a natural process whereby a solution(solvent) with a lower salt (solute) concentration migrates through a semi-permeable membrane to a solution with a higher salt concentration, in an attempt to equalize solute concentrations on both sides of the membrane. This is the same process used by cells in our bodies for fluid transport and is intimately involved with fluid dynamics in the kidney and plays a pivotal role in filtration.
As you can imagine, reverse osmosis drives the process in reverse, with pressure applied to the channel which has a higher concentration of solute. The pressure must be strong enough to overcome the natural osmotic flow while forcing the solvent through a tight membrane which allows fluids to pass but traps salts and impurities such as heavy metals.
The rejected contaminants are not allowed to block the membrane but are instead forced/washed away from the membrane surface through a reject stream which flows out through the kitchen drain pipes. For every liter of purified water produced, the RO filter throws 3 to 20 liters away as waste.
Small household reverse osmosis units will produce only one to five gallons of drinkable water in a day, while most RO systems have the requirement to 80 psi in the water lines to achieve this capacity. Water pressure to private homes, however, is on the order of 30psi to 50psi, and so the yield will be a fraction of this capacity. As the pressure drops, you get less purified water and will get a larger reject flow from the filter. Your filter simply won’t produce, and what is does produce may be of lower quality. In this case, you will require an electric pump (pressure booster) to drive water across the membrane.
While the carbon filter is very proficient at rejecting organic contaminants, the reverse osmosis filters are very good at rejecting inorganic (as well as organic) contaminants.
Contaminants with a molecular weight of 200 (compared to that of water with a molecular weight of 18) will be forced into the reject stream. That means, roughly 95% to 99% of impurities will be flushed into the reject stream.
Reverse osmosis is able to reject most salts, colloids, ions, bacteria, and organics from the flow of water. Included among the rejected are fluoride, arsenic, radium, sulfates as well as heavy metals such as lead and mercury. Reverse osmosis goes a long way toward improving the appearance, odor, and taste of your water.
Reverse Osmosis systems are very cost-effective, costing pennies per gallon for clean drinking water. Like any system, it requires some degree of maintenance and switching out filters and membranes when they go bad. Systems should be cleaned and sterilized once a year.
Note also that membranes in reverse osmosis systems can be easily damaged by chlorine in the feed water, so the water should be pre-filtered with a carbon bed. The application of sodium sulfite can also be used to neutralize the chlorine.
Another downside to reverse osmosis is that it does take time, and expends a great deal of water to produce only a little usable water. So the flow rate is small, and if you consume a large amount of water, you might quickly deplete your supply of purified water. In such case, use of an electric pump (pressure assist) will aid in improving the rate of purification.
Most reverse osmosis filters are not effective in removing nitrates from the water source. Also hydrogen sulfide – gas that produces the rotten egg smell – as well as other gases are not blocked by RO membranes.
Finally, reverse osmosis is one of those systems which removes essential minerals along with the bad impurities. Consequently, one may want to consider remineralization in the purification process.
One final note: while 99% of impurities are rejected by these systems, not all bacteria and viruses, in particular, are rejected in the process. One may consider the use of a UV source after this stage to further reduce these pathogens.
Water distillers operate off of the principles of evaporation and condensation. Water is brought to it boiling point where it undergoes an evaporative process. Most dissolved impurities and solids will have a boiling point higher than water so these are not vaporized along with the water. Some organic contaminants have boiling points lower than water, so they will be vaporized along with the water and therefore not removed from this distilling process.
The steam will rise into coils which are fan or water cooled. Now free of most contaminants, the steam condenses to form distilled water in a separate chamber. Water distillers are able to remove over 95% of minerals and heavy metals. Because of the heating process, water distillation is able to kill and remove bacteria, viruses, and cysts.
The drawback of water distillers is that it is an energy-intensive process using a heating element of between 1000 – 1500 Watts. Continual use will result in costs accrued to your electric bill.
Also, because it does require energy to operate, it will cease to function in a grid-down emergency situation. Water distillation is a very slow process, taking up to 5 hours to distill only 4 liters of water.
Finally, while distillation removes almost all contaminants from water, there are some volatile organic compounds that it won’t remove.
The rejection of bad impurities (chlorine, arsenic, fluoride, mercury, bacteria, viruses, etc). goes hand-in-hand with the rejection of good minerals (calcium, magnesium, phosphorous, potassium, etc.). One may want to consider re-mineralization of the purified water.
UV Water Purifiers
UV water filters are typically used in conjunction with other filters, as the UV filter does not itself have a means to remove contaminants from water. Water UV filters are very good at disinfecting, and kill 99.99% of bacteria and viruses. Bacteria are killed within 10 seconds of exposure. Typically 60 Watts is all that is needed to run the filter.
Some of the disadvantages of UV purifiers is that it does require power, so in a grid-down situation, it will not function.
It typically can’t operate on its own as a purifier since it has no way to remove impurities from water.
It is typically ineffective at disinfecting muddy (high silt) water. Consequently, pre-filters are required to ensure proper disinfection.
Finally, since UV light is invisible to the eye, it is difficult to tell whether the system is working or not. Some current flow indicator is required. Bulbs are typically changed once per year.
Ion Exchange Filters
Ions are charged atoms or molecule that may be present in water, and for some situations, these may be considered impurities that need to be eliminated. Some ions can have a positive charge, known as cations, while other ions have a negative charge, known as anions. The charge number will differ from one species to the next. Monovalent indicates that the charge number is one, either + or -. Divalent indicates that the charge number is two, either 2+ or 2-. Trivalent indicates that the charge number is three, either 3+ or 3-.
Below are some examples:
These ions are not bound in water (H2O) and are free to move about, though the cations will loosely associate with water’s oxygen atom, while the anions will loosely associate with water’s hydrogen atoms. Some molecules and/or atoms do bond with one another (e.g., NaCl, MgSO4) in the formation of salts.
Ion exchange filters take advantage of the charged nature of these atoms and molecules to purify water. An ion exchange filter employs small polymeric beads ( ~ 0.6mm diameter) which forms a resin-filled cylinder. The beads are porous, creating a lattice to which charged ions ( either negative or positive, never both) have been affixed. These charged ions cannot be displaced by other ions, as they are permanently embedded into the lattice. A negative resin might be made up of sulfonates ( SO3–) while a positive resin might use ammonium complex(CH2-N+-(CH3)3).
In order to maintain an electrical neutral charge for the substrate, these resins are then impregnated with atoms of opposite charge. For example, sulfonate-based resins ( SO3–) may be impregnated with the soluble sodium ion, Na+, while the ammonium-based resins (CH2-N+-(CH3)3) may be impregnated with the soluble chloride ion, Cl–. Unlike the fixed charges, these soluble elements are easily displaced by molecules which have a stronger affinity to the substrate than the sodium or chlorine atoms themselves. Hence, the resin has the ability to facilitate ion exchange.
These ion exchange resins can be used for two different purposes: water softening and water demineralization.
Water which has an excess of Ca2+ and Mg2+ is known as hard water. These elements can precipitate out, causing the formation of scales. Use of a sulfonated resin (as pictured above) will facilitate the removal of the high mineral content of this hard water, thereby producing soft water. The calcium and magnesium ions have a higher affinity to the Sulfonate molecule than does the sodium ion. The sodium ion with therefore be easily replaced by the calcium or magnesium ions, but because these ions have a charge of +2, two of the sodium ions must be removed for one calcium or magnesium ion to maintain electrical neutrality. Eventually, the resin will be consumed by these minerals and will not be able to accept any more, at which point the ionic exchange filter must be replaced.
Both a cation and anion exchange resin are employed in this scenario. The cation exchange resin is impregnated with hydrogen atoms (H+), while the anion exchange resin is impregnated with hydroxyl molecule (OH–). Displacement of H+ and OH– by other cations and anions will result in the formation of pure water.
Ion exchange filters do not typically remove chlorine or organic impurities from water. Magnetic Ion-Exchange Resins (MIEX) are capable of removing organic ions as part of the water purification process.
As the name implies, microporous basic filtration employs membranes which are perforated, such that algae, sediment, protozoa and larger bacteria are blocked while allowing the passage of water as well as ionic contaminants, dissolved organic media and viruses. Pore sizes are typically 0.1 to 10um. Microporous filtration is typically assisted with external pressure applied, typically with the use of a pump. Microporous filters are broken down into three different filter types, surface, depth, and screen, based on their construction and their effectiveness at removing media.
The difference between microfilters and ultrafilters is one of pore size. In microfilters, typical pore sizes are between 0.1 to 10um. On the other hand, ultrafilters have pore sizes in the range of 0.005 to 0.1um. While microfilters may block some viruses, the pore size it such that it allows some through. Ultrafilters, on the other hand, are much more effective at blocking these small particles. The benefits of microfiltration and ultrafiltration are their effectiveness at removing suspended colloids and fine particles when compared to traditional techniques. Furthermore, it is able to remove these contaminants in a relatively small area. The downside is that these methods require high pressures to pass fluids through the membrane, so they require the assistance of pumps and thus use power.
Until next time …