How Does Reverse Osmosis Work?

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.

What is a Reverse Osmosis System

The reverse osmosis system is a very simple process used for filtration of water.  It consists of a series of tubes covered with a semi-permeable membrane through which water is passed at high pressure.  For the filtration of water through reverse osmosis the water needs to be pre filtered to remove suspended particles and contaminants such as sediment, chlorine, and other oxides.  At least 90% to 99% of dissolved impurities get removed by this process.

Reverse osmosis is widely used in home and industry

Reverse osmosis was originally used for desalination of water, but today it is widely used for purification and treatment of water in many homes and industries, such as food industry, where it is extensively used for concentrating food liquids and other diary products.  Research has also been done for the concentration of milk to reduce shipping costs.  It is also used in maple syrup production to remove sap from the boiling syrup, however, microbial decontamination and degradation of RO membranes has to be monitored closely.

The reverse osmosis process is also widely used in hydrogen production, waste water management, chemical industries, and for domestic applications. It is also used by reef aquarium keepers, because ordinary tap water may sometimes contain excessive chlorine, chloramines, copper, nitrogen, phosphates, silicates, and many other dissolved chemicals, which are harmful to the sensitive organisms in a reef environment.

The reverse osmosis process

• A sediment filter traps solid particles such as iron rust and calcium carbonate.
• A second sediment filter with smaller pores, which is optional, is used for removing finer particles.
• An activated carbon filter then traps organic chemicals and chlorine.
• A reverse osmosis membrane, i.e., a thin film composite membrane is placed after the pre filters.
• A second carbon filter is installed to capture the chemicals not removed by reverse osmosis, this is optional.
• An optional ultra violet lamp is used to disinfect the water from any microbes that may have managed to get through the other phases of the ro process.

If properly set up, a system requires a 1-hour cleaning cycle once per month or more frequently depending on use and contaminants.  Usually a filter is alarmed through flow rates so exact monitoring is not always necessary.  Sometimes, it may be necessary to install a pump to maintain the osmotic pressure which insures that the system continues to function efficiently but in most cases this is not required.  Reverse osmosis systems last for many years and rarely fail.   Usually they slowly begin leaking more ions until an unacceptable level is reached and then a new membrane is installed.

Types of reverse osmosis systems

There are several types of reverse osmosis systems available for residential use

• 3-stage system
• 4-stage system
• 5-stage system

The 3 stage reverse osmosis systems operate quite efficiently in houses and commercial setups like restaurants, bars, cafes, offices, etc. They are quick and easy to connect and typically have a 5-micron sediment filter and carbon block filter.

A 4-Stage reverse osmosis system may have an additional pre filter rated at 20-microns installed before the 5-micron filter.  This is to remove larger particles  and help to extend the life of the 5-micron filter, which could become plugged quickly if the water has a lot of sediment.  Depending on conditions there could be a second carbon filter installed instead of a second sediment filter.

A 5-stage system has an additional pre-filter, and is effective in removing lead and other heavy metals.  It would also have a second carbon filter also.

Most reverse osmosis systems also have an optional uv lamp.  This lamp is for eliminating bacteria and other organic compounds from the water.

Components of reverse osmosis filters

Many of these machines also have additional components and features that make them more user friendly.

• Auto have auto shut off valves - A great feature for service.
• Compact - For under sink or counter top installation.
• Installation kits - For easy installation.
• User friendly - easy replacement of filter cartridges.

Manufacturers like BEV and GE design systems to suit every need.  Typically a residential system comes with all filters, storage tank, faucet, tubing, installation hardware, installation manual and are suitable for drinking and cooking use.

Most systems are not designed for whole house use.  Designing a system for whole house applications is not worth the cost.  So these systems are not really suitable for shower and laundry use.  A large system can produce about 50 gallons of pure drinking water per day and it is supplied by a separate faucet mounted on your sink near or beside your main faucets.