I just did some basic research since I was planning to buy one water filter.
There are 3 basic types of water purifiers
1. RO - Reverse Osmosis
2. UV - Ultraviolet
3. UF - Ultrafilteration
Each can be either storage type or non storage type, and may have automatic water filing (connected by a pipe to water source) or manual water filing
Here are the benefits / problems with RO filters
A reverse osmosis system physically removes contaminants and dissolved minerals in your water by forcing them through a filter. Some of the benefits of installing a whole-home or point-of-use reverse osmosis system are:
Tasteless water – If you find your water has a funny taste, a reverse osmosis system can help by removing the source of the taste.
No chemicals – Reverse osmosis systems are more environmentally friendly than other water filtration systems because they don’t use any chemicals.
Soft water – Reverse osmosis systems remove the minerals that cause hard water. So if you install a whole-house system, you can benefit from fewer corroded pipes.
Removes odors and unusual colors – Some water contaminants and dissolved minerals can cause strange odors and colors in your water. Reverse osmosis systems remove these minerals and pollutants.
It’s not all sun and daisies for reverse osmosis systems, though. They can be inefficient, returning as little as 5 to 15% of the water that is filtered (with the rest going down the drain.)
Recommended uses: If you have a home with a well or get water from bore wells/ ground source that needs filtration, a whole-home reverse osmosis system may be a good idea. Also, if the water in your home tastes or smells funny, you can install a point-of-use system under your sink for cleaner, better-tasting drinking water.
COMING TO UV systems
UV radiation from ultraviolet water systems alone is not suitable for water with high levels of suspended solids, turbidity, color, or soluble organic matter. These materials can react with UV radiation, and reduce disinfection performance. Water turbidity makes it difficult for the ultraviolet radiation to penetrate water. If your water supply has these characteristics, a sediment prefilter (5 micron or less) should be installed before your UV water purification system to remove particulate matter prior to UV water disinfection.
Ultraviolet radiation can be used as a pretreatment or polishing step to sterilize and disinfect water. UV systems are typically used to pre-treat a water supply that is considered biologically unsafe (lake or sea water, well water, etc). The UV disinfection process is a non-chemical method for destroying microorganisms by altering their genetic material, and rendering them unable to reproduce.
There are several advantages of using UV rather than a chemical disinfection solution (such as chlorination):
No known toxic or significant nontoxic byproducts
No danger of overdosing
Does not require storage of hazardous material
Adds no smell to the final water product
Requires very little contact time
FINALLY UF systems : Like Tata Swach & Pure-it, use gravity to filter water through a filter. Ultrafiltration (UF) is a variety of membrane filtration in which hydrostatic pressure forces a liquid against a semipermeable membrane. Suspended solids and solutes of high molecular weight are retained, while water and low molecular weight solutes pass through the membrane.
For Groundwater / borewell supply or where the water tastes funny / has an odour - USE RO
For water from canal, generally clean and good taste - Use UV or UF.
If you are require larger quantities of water faster - prefer UV, if you want non electric simple very cheap and effective filter - use UF
I have used TATA Swach UF filter when I was staying alone, and now am using Aquaguard Classic UV Filter.
There is generally no reason to have RO + UV + UF filters that are being sold nowadays, unless you are consuming hugely contaminated water.
Read more here
getahead/slide-show/slide- show-1-gadgets-and-gaming- whici-water-purifier-is-right- for-you-reverse-osmosis-ultra- violet/20101215.htm#2
Dr Gaurav Gupta