The waste water that comes out of a hospital also must be filtered, for it is also full of not only pathogens but the residue of the drugs used to treat patients. Reverse osmosis is also used to keep this residue out of the general drinking water.
Reverse osmosis uses the concept of diffusion to work. Osmosis is a type of diffusion where water molecules pass through a membrane from a higher concentration to a lower concentration. In reverse osmosis, the membrane is semipermeable, which means it allows the passage of the fairly small water molecule while keeping out larger molecules, ions and materials whose molecular weight is greater than 100. These materials include bacteria, viruses and other contaminants. Ions include calcium, sodium and chlorine. Poisons like arsenic and lead are also removed by reverse osmosis.
The osmosis is considered reversed because instead of molecules of fresh water passing through the membrane to dilute the contaminated water, the water from the contaminated solution is forced through the membrane to join the fresh water, which leaves the contaminants behind.
The membrane in most reverse osmosis filters is made of polyamides, polysulfonate, cellulose acetate or single or double polymers. The skin of the filter is about .25 microns, with a micron being a millionth of a meter. These filters are either arranged in layers and wrapped around a permeable tube or are made out of hollow fibers. Both types are then arranged in a housing.
When a reverse osmosis system works, water goes through a prefilter after its pH, or acidity or alkalinity level, is adjusted. During prefiltering, the water passes through anything from a bed of sand to a charcoal or anthracite filter to a degasifier to a deionizer. Then, pressure is applied to pump the water through the membrane. The clean water that is produced is pumped into a tank, and the contaminants are drained. Cellulose acetate filters work best at temperatures between 55 and 86 degrees Fahrenheit.
One problem with reverse osmosis is that the system uses a great deal of fresh water, though advancing technologies are being developed to lessen the amount of water that is wasted. Also, the solids left behind after the filtering process can build up next to the membrane. This can be alleviated by flushing the system and eventually replacing the filter. Usually, the membrane module works for up to three years.