Point-of-Entry (POE) devices are whole-house treatment systems mainly designed to reduce contaminants in water intended for showering, washing dishes and clothes, brushing teeth, and flushing toilets.
Ion exchange operations and the choice of resins to use is highly dependent on the water analysis, what has to be removed and to what level it has to be reduced. The primary driving force is selectivity. Selectivity is determined by the ionic strength of the charge on the specific ion and the resin type plus this is highly influenced by the other ions in solution that might compete for the reactive sites. No resin is so highly selective that it is exclusive for a specific contaminant. Since most ion exchange processes are reversible, the ion exchangers can be regenerated (put back into their original form) and used over and over.
Cation Exchange Water Softening
Ion exchange water softeners are among the most common ways of softening water. The typical ion exchange system consists of a pressure tank filled with sulfonated, polystyrene beads that are capable of removing hardness ions from water and replacing them with softer ions, such as sodium.
These units are connected to a brine tank that’s filled with salt, which periodically regenerates the resin beads. The unit’s tiny beads attract and hold onto calcium and magnesium ions as water passes through them. When the beads become so saturated they can’t hold any more, the unit rinses them with salt, which scrubs off the mineral deposits and gets them ready to absorb hardness ions again.
If you own this type of water softener, you can set it to regenerate at preset times. More sophisticated units can base their regeneration on your actual water use. Systems that measure water use and regenerate accordingly, called demand initiated regeneration (DIR), may be more efficient because they only regenerate as needed. Systems that automatically regenerate on set time intervals, called time clocks, simplify the process. However, these units sometimes regenerate more often than necessary, wasting salt, or they leave users with hard water when water demand is higher than normal.
Find cation exchange water softeners certified to NSF/ANSI 44 and WQA S-100.
Although water softeners get rid of some heavy metals along with hardness, water filtration systems are the best way to remove organic and inorganic materials (such as microbiological contaminants) and particulates (such as sand, rust and silt). Water filters remove these impurities with a fine physical barrier, chemicals, or some other method to help clean water and make it suitable for drinking or other uses.
While specialty media and membranes are available, activated carbon is a widely used filtration substance. Activated carbon targets various volatile organic compounds, such as benzene, trichloroethylene, and various pesticides and petroleum related compounds. Sediment and tank filtration systems removes contaminants as water enters the home. Large inline filtration systems are installed where water enters the home plumbing system.
Electrochemical Water Treatment Systems
Electrochemical water treatment systems utilize electricity to induce the removal of dissolved contaminants in the water. Positively charged contaminants such as calcium, magnesium, sodium, lead and uranium, are called cations. Negatively charged contaminants such as chlorides, nitrates, nitrites, sulfates and fluorides, are called anions. The introduction of a negatively charged electrode, or cathode, into the water will cause positively charged cations to move towards it. Electrochemical water treatment systems take advantage of this property by combining the electrode with ion exchange membranes. Basically anything that is ionized when dissolved in water will be reduced. A typical target for the product water would be <5 grains per gallon of hardness and <150 ppm of total dissolved solids, but they are not practical if your aim is to produce soft water with <1 grain of hardness.
Antiscaling treatment devices aim to reduce or prevent certain scale deposits that are normal to hard water behavior. This includes technologies such as physical water treatment devices, as well as sacrificial media. These technologies may provide a measurable reduction in hardness depending on the measurement method applied by conversion of calcium and magnesium ions to a precipitate form, but do not provide a reduction of total calcium or magnesium content of the treated stream nor are they proven to achieve soft water (less than 1.0 grain per gallon/<17.1 milligrams per liter or parts per million of total hardness).
A sacrificial media treatment device releases a chemical such as citric acid or phosphate into the water to discourage the formation of scale.
A physical water treatment device modifies the properties of water by physical means as opposed to chemical or mechanical means. Physical water treatment devices do not use, nor do they impart significant levels of any chemical substance(s) to the water stream being treated. Physical water treatment devices use a range of processes, including media induced precipitation, magnetic, electronic, electrostatic, and electromagnetic technologies.
While there is research into the performance of many of these devices, there remains some areas of the technologies that are less understood. Varying water quality in homes throughout the country also make it more difficult to predict the performance of antiscaling devices in any given residential setting. Consumers should investigate their system selection prior to making a purchase, and as with all water treatment devices, look for antiscaling water treatment systems that have been demonstrated to meet performance, structural integrity and safety according to a published standard by an accredited third-party product certifier.