How To Compare Reverse Osmosis Systems
There are hundreds if not thousands of different reverse osmosis systems available for purchase today. It can be very confusing trying to figure out which system has the features you need and which system offers the greatest value. This article looks at the various features offered on today's reverse osmosis systems to help you determine which feature set makes the most sense for you.
3-Stage, 4-Stage, 5-Stage, or more?
Every residential reverse osmosis system is a combination of the reverse osmosis membrane and a series of other filters. Because the reverse osmosis membrane can be damaged by sediment and chlorine, virtually all residential reverse osmosis systems include a sediment filter and a chlorine-removal filter before the reverse osmosis membrane. A system that is comprised of a sediment filter, chlorine filter, and the reverse osmosis membrane is a 3-stage system.
Much more common than a 3-Stage is a 4-Stage reverse osmosis system. The 4-Stage system works in exactly the same way as the 3-stage but offers a post-carbon polishing filter. In all reverse osmosis systems, the purified water is stored in a tank ready for use. In the tank, the water can pick up some slight tastes from the tank liner. In a 4-stage reverse osmosis system, when the water dispenser is depressed, the water travels from the storage tank through the post-carbon filter. This filter absorbs any contaminants that may have been picked up in the tank.
A 5-Stage reverse osmosis system is very similar to a 4-Stage except is adds one extra pre-filtration stage. Where a 4-Stage system would have a sediment filter and single carbon pre-filtration stage, the 5-Stage system has a sediment and two carbon stages. This results in better overall water quality, and does a better job at protecting the membrane.
4-Stage and 5-Stage systems are the most common for residential applications. Because of the presence of chlorine in city water, we recommend a 5-stage system for city use. On rural non-chlorinated supplies, a 4-stage or 5-stage system is appropriate.
More Than 5-Stages?
A 4 or 5-stage reverse osmosis system will produce very high quality water for the vast majority of residential homeowners. But you don't have to look very far to find 6, 7, and even 10-stage reverse osmosis systems for sale. Why? For the vast majority of these products it is simply a marketing gimmick: If 5-stage is good, 10-stage must be twice as good! This simply is not true. A properly functioning and well maintained 4 or 5-stage system will produce outstandingly high water quality. Most of the "extra" stages offered on systems with more than 5-stages are performing secondary functions that have nothing to do with the purity of the water. There are, however, some exceptions:
Ultraviolet (U.V.) Systems: UV systems are used when there is a risk of dangerous microbes in the water such as coliform or e.coli in well water
Re-Mineralizing Stages: Reverse Osmosis systems remove virtually all of the dissolved contaminants and minerals from water. Adding a remineralizing stage will add back calcium and magnesium. It is important to realize that this is done for taste only. Some people prefer the taste of water with some mineral content. This stage should never be chosen for health reasons - the amount of calcium and magnesium received in the water from this stage has no health impact whatsoever.
Gallons Per Day
The output of a reverse osmosis system is measured in gallons per day or GPD. You will see systems advertised as 24, 50, 75, or sometimes 100 GPD systems. To be accurate, the GPD rating is not for the system but is the rating placed on the membrane used in the system. It is important to realize though, that a membrane rated for 50 GPD will only produce this amount of water under ideal laboratory conditions of water temperature, pressure, contaminant concentration, etc. A safe rule of thumb is to assume that each of these membranes will product about one fifth of their maximum rated GPD in a real world application.
The GPD rating of the membrane has two very important implications for how well the system will work in your home. First, will the membrane produce enough water. For all but the rarest of households, a 50 GPD membrane will produce more than enough water for drinking and cooking. The second very important note about GDP, and something that is often overlooked, is that the water quality produced by a 75 GPD membrane will be lower than that produced by a 50 GPD membrane. Yes you'll get more water but that water will have more undesirables in it than you would probably expect.
For most residential applications a system that is rated at 50 GPD will produce ample water and water of a very high quality.
The Faucet
A reverse osmosis system will always come with a dedicated faucet. There are two basic designs in reverse osmosis faucets: airgap and non-airgap. Before delving into the details of these faucets we first need to understand water flow in a reverse osmosis system. There are three feed streams we need to identify: the feed stream, product stream, and waste stream. The feed stream is where raw untreated water is diverted from a cold water line to the reverse osmosis system. Water flows through the system and some is purified and sent to the systems storage tank - this is the product stream. But, some of the water is not purified. This rejected water is called the waste stream and is sent to drain. It is usually adapted to the drain line for the kitchen sink. This process keeps the reverse osmosis membrane clean of debris.
In a non-airgap faucet the product water flows to the faucet and is dispensed. In an airgap faucet the product water is sent to the faucet to be dispensed, but the waste stream is also directed through the faucet. The waste stream does not mix with the product stream. The waste stream is sent through a dedicated line and then to the drain. Why is this done? It's done to help ensure that there is an airgap between the waste stream and the drain to which it is attached. Because the reverse osmosis faucet is mounted on the countertop it sits higher than the reverse osmosis system, and the drain to which it is attached. With an airgap system, if the kitchen drain backs up it is not possible for this dirty water to come into contact with any part of the reverse osmosis system. If this happens with a non-airgap system there is a possibility that water from a backed up drain could be drawn into the reverse osmosis system contaminating it.
In order for a reverse osmosis system to be certified by WQA or NSF it must use an airgap faucet.
Specialized Reverse Osmosis Systems
While a reverse osmosis system will remove the vast majority of contaminants to a very safe level, there are some contaminants that are so dangerous that it is often a smart idea to have a two-stage approach to their removal. Examples of these contaminants are arsenic, lead, and mercury. When these sorts of dangerous contaminants are present, a reverse osmosis system that offers specific contaminant removal is strongly suggested. Reverse osmosis systems designed for specific contaminants are usually based on a 5-stage design. In a typical 5-stage system the first three pre-filtration stages consist of a sediment stage and two carbon stages. In a system designed for a specific contaminant, the sediment stage is replaced with a two-in-one sediment/carbon stage. This makes room for a cartridge dedicated to the removal of the specific contaminant.
