An RO is designed to fit neatly under almost any sink. The filter pack typically will slide to the back of the sink next to the garbage disposal. The tank tucks behind the plumbing and garbage disposal to the back of the sink. It uses very little of the sink's practical space. It connects to the cold water supply under the sink. First, water first passes through at a micron sediment filter, which removes large particles that float in the water like silt, sand, rust, dirt, etc. We can see down to 40 microns (a human hair is about 80 microns and most bacteria is around 1 micron).  Second, the water passes through a Bituminous granulated coal filter that removes chlorine, some pesticides, herbicides and toxic chemicals. It consists of three filters, the purpose of the two outside filters is to protect the membrane (in the middle) from chlorine and particles that can prematurely ruin the it.  These two outside filters should be replaced annually.   The membrane is the heart of the system - is should last 4-5 years before it needs replacing.  The membrane filters down to a molecular level (1/10,000/micron). Water is basically forced through the membrane. The water that makes it through is the purified water and the water that doesn't make it is impure water and it is dumped down the drain. The water slows down to a drop at a time and is accumulated in a pressurized storage tank. The water is then pushed out of the tank through a 10 inch post coconut shell carbon filter designed to pick up anything missed by the previous filters insuring great taste on the way to the faucet. The faucet is an air gap faucet and meets all plumbing codes.