Frequently Asked Questions: Reverse Osmosis

14 Oct.,2022


Reverse Osmosis Membrane Elements

1. What is reverse osmosis (RO)?

Reverse osmosis, often referred to as RO, is an advanced water purification method that was initially developed by the U.S. Navy to produce drinking water from seawater for submarine crews. It is a membrane filtration technology that works by forcing water under pressure through the very tiny pores of a semi-permeable membrane. Modern RO units for the home combine membrane technology with carbon and mechanical filtration to produce highly purified, great-tasting water.

2. How does it work?

To put it simply, the water in modern home units is driven by normal city water pressure and flows through a sediment pre-filter which removes any dirt and small particles that are in the water. Next, a carbon pre-filter removes organic contaminants, including chlorine and its by-products. It then enters the reverse osmosis membrane-- a very tight, sheet-like filter-- which allows water to pass but rejects dissolved solids and impurities such as sodium, lead, and arsenic. Some of the water entering the unit is used to cleanse the membrane surface and flows to the kitchen drainpipes. The purified water is stored in a small storage tank until it is needed. When the faucet mounted on the sink is opened, the purified water is forced through another carbon filter, which gives it a final polish and from there to the faucet. Of course, there is more to it than this simplified description as flow control devices, check valves, and automatic shutoff devices that stop the inflow of water when the storage tank is full all play an essential role in RO, but we'll save you the time for now.

3. What is the best RO system?

All RO membranes produce similar, highly purified water. However, not all systems offer the same features. For example: The FMRO5-MT model is perfect for city water applications. This unit has five stages, meaning the water passes through a 5-micron sediment polypropylene prefilter, then two solid extruded carbon block cartridges for chlorine removal, then the RO membrane to remove the smallest of contaminants, and finally another carbon filter to polish the taste. The FMRO4G-ERP has water conservation in mind with a high-efficiency design. This model produces more water in a shorter period of time, with less water going to the drain than a traditional 50 gpd RO. This is due to the non-electric permeate pump that utilizes the drain flow to reduce the backpressure from the storage tank allowing a more consistent working pressure across the membrane. The WQC4RO13 model is designed for the consumer that would like a low maintenance premium RO. This model is designed with a designer water-dispensing faucet, low profile polypropylene storage tank, and manifold filter heads that are able to pivot 180 degrees with built-in shut off valves to make filter replacements as easy as a 1/4 twist of the filter.

4. Is an RO unit like a distiller?

While both RO units and distillers effectively reduce the "dissolved solids" content of water, the processes are quite different. RO filters water through a very tight semi-permeable membrane, while a distiller is like a big teakettle in that it boils water, catches the steam, condenses it, and captures the resulting water. Most impurities are left behind in the boiling chamber. Both rely heavily on carbon filtration for chemical removal. It's worth mentioning that cheap distillers often have little or no carbon filtration and are not as effective.

5. Is distilled water purer than RO water?

Distillers typically remove a few parts per million more of common mineral constituents like sodium. However, distillers lack efficiency when it comes volatile chemicals with a low boiling point. E.g. chloramines, which many cities now use instead of chlorine as a disinfectant, aren't as effectively removed by distillers. Reverse osmosis paired with carbon filters, however, does a very good job of removing evidence of chloramines. Unless volatile chemicals like chlorine are removed by carbon filtration before they enter the distiller, they will be released into the air or end up in the distilled water. Nonetheless, both methods produce very pure water.

6. Do RO units waste a lot of water?

It depends on what you mean by waste. A home RO unit uses water to clean itself and wash away impurities, much like most other water-using appliances. Considering we also use water to wash clothes, dishes, cars, and to flush toilets, a reverse osmosis unit uses more water in its operation than you actually consume, but not enough to effect your water bill. However, the RO unit only uses water while it's filling its storage tank. Once the tank is full, the unit shuts down and no water runs to drain-- think two or three extra toilet flushes a day.

7. What are the annual maintenance costs?

Only pre filters and post filters need to be replaced on an annual basis to ensure proper performance. Ultimately, the quality of the water and the amount of use will determine the frequency of filter changes. Depending on the point-of-use RO model, the annual cost is less then $0.30 per day. The RO membrane itself will normally last between two and five years.

8. What is the importance of water pressure to an RO system?

It is essential. The water pressure is what forces the water through the membrane for purification and flushes the rejected solids away. Low water pressure will result in reduced production and premature fouling of the membrane. The ideal pressure for operating an RO system is 60 PSI. Pressure below 40 PSI is generally considered insufficient, and should be boosted using a pressure booster pump.

9. Can I hook an RO unit to my refrigerator/icemaker?

Yes, but only if you can reach the appliance with a 1/4" tube from the under sink RO unit. Pressure is a consideration with some refrigerators, so it's a good idea to check with the manufacturer. The pressure you'll get from the RO unit is about 2/3 of the incoming line pressure.

10. How long will an RO unit last?

Virtually forever if you service it regularly and replace parts that wear out, like the storage tank and the faucet. Typical membrane life is about 2 to 5 years, depending on the nature of the water that it's processing.

11. Does RO remove chlorine?

While the RO membrane itself does not remove chlorine, it doesn't have to. A couple of high-quality carbon filters will do that job. In fact, if the first carbon filter didn't remove all the chlorine, the membrane would get eaten alive in no time.

12. Do RO units remove minerals that are essential to health?

RO units remove about 95% of the mineral content, but the mineral issue is probably the most controversial question in drinking water purification. Experts on both sides of the issue speak convincingly. As long as water is palatable, it's within the body's acceptable range. The main issue with water is chemicals, not minerals. Whether water contains 30 or 3 parts per million calcium is not nearly as significant as the difference between 0.5 and 5 parts per million chloroform.

13. Do RO units need electricity?

No, they run on water pressure. You only need electricity if you add an electric pressure-boost pump or an ultraviolet lamp. Standard units have neither.

14. Why are RO units so popular?

Because they produce great-tasting, very pure water at a reasonable price when compared to buying bottled water, and in a trouble-free, fully automatic format to boot. Committed water drinkers know, not all water tastes the same.

15. Will a water softener harm the RO?

No, in fact, a water softener can help extend the life of the RO membrane. Calcium and magnesium (limescale) are two of the hardest minerals for the RO membrane to remove, and sodium (added to the water by the softener) is much easier on the membrane as it rejects 98% of all sodium in the water.