Re: Why does the brita water filter need to be replaced?

Home water filters need to be replaced periodically for the same reason 
that the filters at the water treatment need to be renewed every 40 hours 
or so:  the filters get clogged, which reduces the flow rate through the 
filter.

The Brita system utilizes two different types of filtration:  a pad of 
activated carbon to absorb taste and odor compounds that may be in the tap 
water and a mixed bed ion exchange resin to reduce the level of cations 
and anions. 

Activated carbon is just charcoal that has been roasted to drive off 
organic compounds remaining from the plant.  There are several different 
kinds made from rice hulls, coconut shells, sugarcane, just about any 
leftover plant material.  The carbon retains the porous structure of the 
plant and this gives the charcoal a large surface area.  When the filter 
is in operation, organic compounds, mostly humic and fulvic acid from the 
breakdown of plants or algae in the source water and the chlorine used as 
a disinfectant, stick to the carbon. 
Eventually the unoccupied absorption sites on the charcoal will be filled 
and/or the filter will get clogged up with particles and bacteria.  
The ion exchanger is probably a cation exchanger for reducing the level of 
calcium and magnesium.  If your water has a lead problem the ion exchanger 
will reduce that as well, but lead in municipal water supplies is pretty 
rare these days.  If your source water is from wells there's a possibility 
that it has nitrate or arsenic.  You can check by requesting a copy of the 
CCR, Consumer Confidence Report, from your water supplier.  Again, the ion 
exchangers have a limited capacity and will eventually fill up.  Brita 
says to change the filter media monthly, but that's only a guesstimate 
based on a typical water supply.  There's no little timer in there that 
counts off 30 days.  Either of the filter media can be exhausted in less 
than a month or can last more than a month.  It depends mostly on how much 
water you run through the filter and what's in the water.