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.
Water Filtration Systems
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.