Selecting A Reverse Osmosis Drinking Water System
With so many reverse osmosis drinking water systems available for the home, it can be challenging to select the right one. Here we review the reverse osmosis process and describe some of the available product types and features that can help you to select the correct system for your application.
Reverse Osmosis Function
First, a brief non-technical description of reverse osmosis function. In the home, reverse osmosis systems (sometimes referred to as RO), produce purified water used for drinking and cooking. Also called “point-of-use” systems, they are typically designed to be installed beneath the kitchen sink, and include a faucet for dispensing the purified water. In very general terms, this process involves a semipermeable membrane which will only allow relatively pure water to pass through it. On one side of this membrane, there is a high concentration of impurities (raw, unprocessed water). Pressure is applied on this unprocessed water to make it flow through the membrane, and out to the other (low pressure) side. Purified drinking water is collected on the low pressure side of the membrane. Impurities that were not able to pass through the reverse osmosis membrane are washed to drain. Purified water is either stored in a small reservoir tank, or dispensed directly to a faucet.
Types of Reverse Osmosis Membranes
While there are many different types of reverse osmosis membranes, generally only two types are used to prepare drinking water in the home. One system uses a cellulose triacetate membrane, commonly called a “CTA”. The other more common, and generally higher quality, system uses a thin film composite membrane called a “TFC”.
The CTA membrane is organic by nature, and requires disinfection to prevent the growth of bacteria. You most commonly see these systems in municipal (city) water applications, as it is important for the supply water to be chlorinated when using a CTA membrane. Reverse osmosis systems that use CTA membranes can be more affordable, but sometimes can produce a lower volume and quality of purified water. CTA based systems typically contain a sediment pre-filter, followed by the membrane, and a post filter (or filters) that contain carbon. Again, CTA membrane systems should only be used with a chlorinated water supply.
A more common RO system, used in both municipal and well water applications, is one that uses a TFC membrane. TFC membranes are inorganic, and therefore not susceptible to bacterial growth. These membranes also provide a high quality and quantity of purified water, however, TFC membranes are damaged when chlorine is present in the water. For this reason all TFC systems utilize a carbon pre-filter. Carbon easily absorbs chlorine, protecting the downstream membrane from attack, and eventual failure. In chlorinated water applications, it is important to replace the carbon pre-filter on a regular basis, to insure continued protection of the TFC membrane. In well water applications where chlorine is not present, a TFC system is always the preferred choice. TFC systems normally contain a combination sediment/carbon pre-filter, followed by the membrane, and additional carbon post filters.
Tank vs. Tankless
After purified RO water is produced, traditional RO drinking water systems use a small storage tank to hold the water until dispensed. These tanks are typically 4 gallons in size, with an internal plastic bladder. The bladder holds about 2 gallons of water, and the remaining tank volume contains pressurized air, that provides the force required to send the purified water to the dispensing faucet. If RO water remains in a storage tank for an extended period of time, it can develop a “flat” taste. Most better quality systems include an inline carbon filter that is located between the tank and the dispensing faucet. This filter “polishes” the RO water and provides a fresh taste.
The tankless RO system is a new design on the market. This design incorporates multiple TFC reverse osmosis membranes, and provides direct dispensing of purified water to the tap. Tankless systems do not require a water storage tank, are more compact than traditional systems, and occupy less cabinet space. A tankless RO can deliver a large volume of water quickly, and sometimes (depending on the model), to multiple dispensing locations throughout the home. Since the dispensed water is prepared on-demand, it can also have an improved taste. The disadvantage is that cost of these systems is usually higher than traditional RO, and there are also more membranes to replace. Tankless systems typically perform best at pressures above 40 psi, and lower pressure applications may require a pressure booster pump.
Efficiency & Quality
Reverse osmosis systems send water to drain during the water purification process. Measurement of the efficiency of this process system “recovery”. The amount of water consumed during reverse osmosis primarily has to do with water quality, pressure, and temperature. However, the design of the system also plays a large part. Better quality systems send 2 to 4 gallons of water to drain for every gallon of water produced (approximately 25-30% efficient). Lower cost systems usually have a reduced rate of recovery, and can send as much a 6 gallons or more to drain for every gallon of water prepared (less that 15% efficient).
When considering a traditional reverse osmosis system that utilizes a water storage tank, it’s is important to select one that that includes an automatic shut-off valve (also referred to as an ASO valve). This valve stops the water manufacturing process when the storage tank has reached it’s full capacity, and reduces the amount of water sent to drain.
Another important consideration when selecting an RO drinking water system is product certification. The National Sanitation Foundation (www.nsf.com) and the Water Quality Association (www.wqa.com) are two independent third party associations that certify reverse osmosis drinking water systems to a quality standard. A system that meets or exceeds NSF Standard 58 or WQA Gold Seal certification is a tested product, and is assured to provide high quality drinking water.
