An understanding of the different types of water filter cartridges and how they compare would be handy to know. As water passes through the cartridge, unwanted materials are taken out or altered so that the water coming out the other end is less harmful and tastes better.
The cartridge itself is housed in a vessel and are a cylinder containing various materials---like charcoal and resin beads. The better ones have at least two separate cartridges so that different toxins can be dealt with at different stages. The simple countertop filter; the ones that you fill from the top and stand all by themselves, have a ceramic filter called a candle. Some ceramic candles are filled with carbon such as activated carbon as well.
'Activated' means that the carbon material eg charcoal is processed, giving it a greater surface area for the toxins to be 'adsorbed' (stick to its surface) by. At least one manufacturer I know of also impregnates the ceramic wall of the candle with silver to kill off bacteria.
Gravity pushes the water through the candle and it collects in the bottom 1/2 of the vessel. Turn on the tap and there's your clean water. They are good but most will take an hour to filter one liter of water---not a problem if you fill it up the night before. The water filter cartridge in a filter connected to the mains is different mainly because of water pressure.
Whole house filter systems are designed so that good water pressure is supplied to all of the tap outlets, including the shower. The benefit of these is that all of the water used--the shower, tap, washing machine, fridge, dishwasher as well as the water heater, are all supplied with healthy, non corrosive water.
The under-sink filter and the type that sits on top of the sink use the same cartridges. They slow the water down somewhat. Good enough though, to fill that glass of water or for cooking and washing. A filter fitted to the shower head has only one cartridge. These do a good job of removing chlorine and other toxins while allowing a good flow of water through.
Cartridges do need to be replaced about every six months depending on your usage. However, some ceramic cartridges can be cleaned and reused a few times. While the same basic materials are used, not all filters perform equally and not all are NSA certified. The links below will lead to a comparison between brands.
It would make sense to choose the brand with the best service and which has the best performing water filter cartridge.
