Reverse Osmosis (RO) or, hyper-filtration, is thought by many water filtration experts to be the best system on the market today for home owners. About 90 percent of the world’s premium bottled water companies employ this method of filtration. They rely on reverse osmosis because it has been proven to eliminate or substantially reduce a wide variety of contaminants. So, how does reverse osmosis work and what contaminants can be removed with this equipment?
Some of these contaminants include:
- arsenic
- bacteria and viruses,
- bad tastes and odors
- chlorine
- fluoride
- hydrogen sulfide
- heavy metals
- nitrates
- sediment
- iron
How does reverse osmosis work?
Reverse osmosis uses a semipermeable membrane that allows water to pass through while rejecting contaminants. With pores in a RO system membrane as small as approximately 0.0005 microns, (bacteria are 0.2-1 microns and viruses are 02.-.4 microns) not much more then pure water can pass through the membrane. The semi-permeable membrane allows only pure water to flow through it and filters impurities, providing water of very high quality for drinking.
There are two types of membranes commonly used in reverse osmosis systems today. These are the Thin Film Composite membrane (TFC) and The Cellulose Triacetate membrane (CTA). The TFC membrane filters contaminants at a much higher rate than the CTA membrane. It is more susceptible, however, to chlorine fouling.
The type of membrane used depends on some determining conditions.
- The type of impurities present and their concentration in the feed water is the first parameter.
- The next factor is the amount of water that has to be permitted through the membrane. If large volumes of water are required then you have to use a membrane with a greater permeability composition.
- You can also choose the membrane according to the amount of contaminants it can reject in a given period of time.
Getting the most out of your reverse osmosis system
Quality RO systems use a self cleaning method for the membrane known as cross flow. As some water is passed through the system’s membrane, the rejected water consisting of organic and inorganic insoluble substances will continue on its path downstream; as it does so, it sweeps the rejected contaminant away from the membrane and down the drain.
Cross flow alone is not enough to ensure the efficiency and longevity of a reverse osmosis system. Most work together with an array of granular activated carbon and sediment pre filters. These are used because membranes are subject to degradation from chlorine, iron, manganese, and hydrogen sulfide among other things. Membranes are also vulnerable to bacterial attack. For this reason a sediment pre-filter, an activated carbon pre-filter, and or a post filter should be coupled with your RO system. Most reverse osmosis systems also utilize uv sterilization for bacteria and organic compounds. A pre-softener is sometimes recommended in cases of extremely hard water to help prevent premature membrane fouling from hard water deposits.
Although reverse osmosis is an excellent choice for just about all water purification needs, it’s recommended that a combined RO ultraviolet system be used to purify lake or well water. UV treatment also provides added protection in the rare cases when our municipal water supplies are not managed properly. A standard household water system possesses the necessary water pressure to operate a normal RO system. It requires only 40 psi to function properly.
The filtered water is stored in a storage tank that typically collects about 2-5 gallons of water. Reverse osmosis filters typically produce low quantities of water so a storage tank is used to ensure water is always available.
A post filter consisting of a carbon filter usually is used to remove any element that may contribute to offensive taste and remaining impurities. Usually post filtration is not required when there is a carbon pre-filter. A delivery tap independent of your main tap is used for discharging the pure water.
Some benefits of a reverse osmosis system:
- A high rejection rate for a wide array of contaminants.
- Cost effectiveness is also a positive in an RO system.
- Costs as little as 5 cents per gallon in long time operations of these systems.
Disadvantages of a reverse osmosis system are:
- Requires sediment and carbon pre-filtration to prevent membrane fouling.
- Because RO works against standard osmotic pressure, the process is generally fairly slow.
- Produces roughly 15 gallons of purified water per day and may require 3 to 10 gallons of untreated water to make a single gallon of purified water. This of course varies by manufacturer.
As you can see, reverse osmosis systems are not perfect and are not suitable for whole house applications. Like any water filtration system there is some maintenance required. The big benefit of course is having safe, clean and pure drinking water at your fingertips.
