The public water treatment process is not perfect. When the processes used by the facilities are added to the steps available for use in the home, the result can be nearly perfect. 99% of most contaminants can effectively be removed.
It is possible to measure the effectiveness of filtration steps by adding a known amount of contaminant to a sample and then running it through the filter. The number of contaminants remaining in the filtered sample is again measured.
It is possible to remove 99% or more of contaminants like chlorine, lead, cysts, THMs, benzene and lindane. Independent laboratories like Underwriter's conduct that kind of testing at the request of companies that manufacture home filters. Not all companies request testing.
The first thing to consider when comparing the filters currently on the market is whether or not the claims have been certified. Manufacturers of home filters are not required to have their claims certified in most states. Only the better companies have testing done to certify their claims, because testing is expensive. So, buying a cheap filter is probably not your best choice.
The main reason the public water treatment process is imperfect has to do with size and output. The volume of water they need to treat is very large. The output or flow rate must be very high.
In the home, you are treating a small fraction of what your facility handles every day. You can handle a lower output even in the shower, if the pressure is high enough.
The flow rate must be reduced in order to ensure that each drop is cleaned. Dual filtration compartments are sometimes used by companies to make doubly sure that every drop is cleaned. The final compartment may include a carbon block with tiny holes less than one micron in size. That step is designed to remove the maximum amount of chemical contaminants and strain out cysts.
Your local water treatment process protects you from most bacteria, viruses and other pathogens except in the cause of flooding or a line break. In those emergencies, your facility will notify you. They might advise you to boil before drinking. One of the things that home filters cannot do is treat biological contaminants other than cysts, regardless of how much it costs or the steps included.
Some believe that the public water treatment process could be improved. With new innovations or technology that may be possible. But, right now, we need home filters in order to get the maximum protection.