How a Reverse Osmosis System Works

by Abbey R Published 9.27.2013

With every passing year we learn more about what is in our water and the effects those contaminants can have on our health. It takes the EPA years of study to figure out what is an acceptable level for contaminants in our water or how best to treat them. Contaminants in residential drinking water can include almost anything, from industrial waste that was dumped in a river, to fertilizers and household cleaning products. Many times, treatment involves adding a chemical to the water to neutralize the contaminant—for instance, chlorine is added to water to control the amount of microbes—but these chemicals can give water undesirable tastes or odors. Technology like reverse osmosis systems exists to remove contaminants from water without adding any chemicals.

Reverse osmosis (RO) systems are becoming an increasingly important, needed appliance in our homes. RO systems utilize your water pressure and a semi-permeable membrane to reduce contaminants for great-tasting water without adding any chemicals. They are typically used to purify drinking water which is where contaminant levels matter the most. Some areas, however, have such terrible water that an RO system is used for the entire home.

Every reverse osmosis system has at least four parts: a prefilter, an RO membrane, a storage tank and a postfilter. Water supplied by the city or a well enters the system through the prefilter, which protects and extends the life of membrane by filtering out the things that can harm it, like chlorine and sediment.

A reverse osmosis membrane uses a semi-permeable membrane to separate water molecules from other molecules. “Semi-permeable” means that some things can pass through and others can’t. A familiar example would be your furnace’s air filter, although, semi-permeable membranes for water treatment allow passage based on the size of the particle as well its molecular charge whereas typical air filters separate the contaminants exclusively by size. Holes or pores in the membrane are sized just big enough for the passage of a water molecule—even small contaminants such as tobacco smoke or paint pigments are too big to go through an RO membrane. At this point, because the membrane only lets certain molecules pass through, there is some waste liquid with a highly concentrated amount of contaminants that goes to the drain. The virtually contaminant-free water that makes it through the membrane, called a permeate stream, is safe to drink and tastes great.

Reverse osmosis technology relies on pressure to push the water molecules through the. Water pressure varies with your water source. City water is usually supplied between 40 and 100 psi (pounds per square inch). Well water is usually less pressure, delivered between 20 and 60 psi depending on your pump. The production rate of the membrane is dependent on factors such as temperature, pressure and Total Dissolved Solids levels. Because flow and production rates vary, most RO systems also have a storage tank, allowing more pure drinking water to be available on demand, so you can fill your glass or pitcher much faster.

Because the water is so pure, bad tastes and odors from the storage tank’s bladder and walls can find their way into the water during prolonged contact, so they must be taken out. That’s why a postfilter is an important part of the reverse osmosis system; any odors or tastes picked up from the storage tank are removed and the water is once again great-tasting.

To raise the pH if it is too low, whole house systems use a “polisher” after the postfilter, which adds minerals to the water which protect the pipes and which come people feel enhances the water’s taste.

Sometimes seeing is believing. Personally, I love seeing ice cubes made with RO water because they are virtually colorless. Ice cubes made from city-supplied tap water are almost white in color, which tells me that there are minerals mixed in with the water molecules. Whenever I see transparent ice cubes, I know the RO system must be removing a lot from my water. If your home has questionable drinking water, maybe it’s time to check out a reverse osmosis drinking water system—you won’t regret it.

Contact Abbey R.


How do water softeners know when to regenerate? Time-initiated vs. Demand-initiated

by Stuart P Published 6.10.2013

Once people learn what water softeners can do to improve water quality, they often ask specific questions about the regeneration process. One of the most common topics is how the softener knows when to regenerate itself. Let’s examine two major categories of water softeners, time-initiated and demand-initiated.

Time-Initiated

Regenerating a water softener based on time was one of the first methods developed for automatic operation. Timed units put an end to the days when the softener operator had to start the softener by hand. Many of these control valve designs incorporate either a motorized or digital clock which has selection options for both the start time and number of days between regenerations. When this type of softener is regenerating, the control valve opens a path for untreated water to go directly to the taps. The clock is often set to start regeneration in the middle of the night to minimize the amount of untreated water going to the faucets or into the water heater. The regeneration interval must be a number of whole days and can be as often as every night, or as infrequent as one week or more. Careful thought must be given to selecting this interval to prevent either salt waste or hard water going to the taps.

Consider this analogy of refilling your car’s gas tank by using a clock. After estimating how many miles you usually drive, and taking into account the size of your gas tank, you may decide to add 12 gallons every four days at noon no matter what. If you drive less than usual, you will overflow your gas tank at the pump. If you drive more than usual, you’ll run out and be stranded in the middle of a trip. 

Time-initiated softeners must be set based on estimates of water usage. If you have extra people visit, do extra loads of laundry, bathe your dog, wash your car or fill your kiddie pool more than what was planned, the softener will deplete and begin delivering hard water to the taps. Likewise, if you go on vacation and use no water at all, the time-controlled softener will regenerate needlessly and give you no value for the salt and water used each cycle. What is often done in practice is to be somewhat generous with the regeneration schedule, and plan for the worst case. This prevents hard water breakthrough for some cases of unplanned over-use, but is more costly to operate than a perfectly adjusted system. 

Some softeners have sensors that can detect when regeneration is needed. These override the clock to some extent, but the unit still has to regenerate itself in the middle of the night in whole-day increments. This still the leaves the possibility for unexpectedly high or low water use in that one-day period to make a softener regenerate too late or too early. Still further sophistication to the time-based system adds an automatic adjustment of the salt to increase efficiency.

Demand-Initiated

Demand-initiated water softeners start their regenerations when the capacity is depleted, which could happen at any time. It’s like filling your car’s gas tank when the gauge says it’s empty. A meter integral to the control valve totalizes water passing through the system. Based on how hard the water is, the meter is adjusted to start regeneration when the softening resin reaches the end of its usefulness. When the meter gives the signal, the control valve begins a sequence of regeneration steps, which typically include drawing in brine, rinsing and backwashing. Afterwards, the tank is ready to put back into the service stream and soften again.

Some systems using demand initiation have small tanks and short regeneration times. That’s how they limit the passage of hard water going to the end user. Other demand systems have a second softening tank which goes into use as the other tank begins its regeneration. This system allows virtually no untreated water to escape to the end user. Both types of demand systems get consistently good salt efficiency since the resin beds are fully depleted and “hungry” for the salt being used every single time they regenerate. More softening capacity is captured in a fully depleted bed than a partially depleted bed just like a hungry person is more likely to finish their dinner than one who just ate.

Comparison / Conclusion

We’ve explored some basic differences between softeners using timers and those using demand-initiated regeneration. Perhaps knowing these differences will be useful to you if you ever seek water treatment for your own home or business.

Contact Stuart P.


Water: Understand it, Value it, Respect it. Learn more about life’s most vital resource.

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