There are several types and installations for water filters.
Point of Entry filters are placed on the main line right after your water meter.
Point of Use filters are typically placed under your kitchen sink with a spigot near your regular kitchen faucet.
Point of Entry (POE) filters are meant to clean the water from fine and coarse sediment and/or "soften" the water.
Those large garbage can size filters that actually remove all the calcium and magnesium are illegal here in Israel for residential use. They are also mighty expensive. That said, there are people who install them none the less. We do not.
The typical POE filter consists of one or two filter housings. One filter catches the sediment as stated above. There are disposable filters that are changed every 6 months or once a year. There are also re-usable filters that can be cleaned every 6 months or once a year. The frequency that a filter is changed or cleaned depends on the water quality, usage, and the size particles the filter removes.
The second filter housing (or sometimes the same housing) contains poly-phosphate crystals, or "cadurey siliphuse in hebrew. They are most effective on cold water lines and closer to the point to be affected. They are far less effective on hot water lines and once oxygen enters the system. Briefly, water passes through the filter. The poly-phosphate then latches onto the magnesium and calcium thereby preventing the minerals from separating from the water (note that the minerals are not removed). This filter housing can be placed on the main water line, as mentioned, and/or can be placed on the water line that supplies the water boiler or washing machine.
The purpose of the poly-phosphate filter is to protect the supply piping, plumbing fixtures in the house (such as faucets and shower heads), water boiler and solar panels.
The crystals are replenished every year to year and a half, depending on the water quality and water usage.
(Disclaimer: We are not experts in water filtration. Therefore, before making a practical decision based on the following information the facts should be verified by the reader.)
Point of Use filters are installed to make the water drinkable. Typical filters are reverse-osmosis that remove almost everything from the water or simpler filters for "aesthetic" purposes and less for health: no harmful metals or minerals are removed (or only a minimal amount); chlorine, sediment and cysts are removed to remove the bad taste and smell.
Reverse Osmosis filters: These filters remove many particles from the water including salt, organic chemicals, heavy metals and minerals (even those that are good for us) and other harmful substances. Although the water supplied throughout the country is potable some people want to purify the water even more either for health concerns or medicals reasons. Some disadvantages: The unit is rather large and takes up much of the cabinet under the sink. Also, for every gallon used, around 3-5 gallons are released into the home waste pipes. Even the healthy minerals are removed from the water. Lastly, it takes some time to fill a cup or bottle due to the design of the system.
"Aesthetic" filters: Typically Granular Activated Carbon filtering. These filters are installed anywhere from one to three filter housings and sometimes with even more filtration before the tap. As stated above, the purpose is to remove sediment, rid of any offensive odor and improve the taste. Aside from chlorine that is removed, some systems remove lead and cysts and perhaps some other particles as well.
It is always important to verify that the filters have been verified by the Ministry of Health in Israel. Filters are available from china or America. The latter are more expensive and typically from Pentek.