Monday, January 20th, 2014

Using RO Water Filters in Drought Conditions

This winter is shaping up to be one of the driest on record for much of western USA. The governor of California has ruled the state under a “Drought Emergency” (which will loosen up some regulations and the flow of federal funds). Here in Oregon, where we just relocated, the winter so far is the second driest recorded, and in Washington State there is also considerably less rainfall this winter than normal.

If you live in an area where the water supply is being constrained due to drought, there are a few things you can do when using a BEV300 system (or any reverse osmosis based system) to help minimize the amount of “brine” water sent to the drain.

Keep in mind a BEV system from Pure Water Systems is more than just a “typical” reverse osmosis (RO) system. All of our BEV systems include deionization after the RO process, cleaning up and removing greater than 97% of any contaminants left behind by the RO process. Where a brand new RO system, using a high quality membrane, might remove 95+ percent of contaminants (decreasing over time), our BEV systems consistently remove greater than 99.6% of all contaminants.

All reverse osmosis units use water to make water pure. The same way we use water to clean ourselves, our clothes, our fruits & vegetables, our cars, etc. Many competitors in the water business, whose systems only slightly purify water, are quick to mention this as “waste” water. We prefer to call it “working” water, i.e. water used to perform a particular purpose.

Poorly engineered RO systems will use 7-12 gallons of water to produce 1 gallon of purified water. We do consider this to be wasteful, when there are reverse osmosis membranes and designs that can use far less. For example, one of our PWS® BEV Systems typically sends 2.5 – 3 gallons of water to the drain for every gallon of pure water that is produced. (Restricting flow beyond this range will cause membranes to become “fouled” quickly with the buildup of mineral plaque or scale, reducing effectiveness or reducing output.)

Also, most under-the-sink RO systems use a pressurized storage tank/reservoir of some kind. These tanks have a diaphragm or bladder that holds the pure water; on the opposite side of the diaphragm there is a small amount of pressurized air. As the tank fills with water it further compresses the air, and this compressed air acts as the driving force to push water out of the tank when the faucet is opened.

As the tank pressure builds it exerts back-pressure on the RO membrane. This reduces the effective RO driving force which will slow the production rate, causing the pure-water/brine-water ratio to increase—sending more water to the drain. (And with many cheap RO membranes, also reducing the rejection rate resulting in TDS creep—another way to say the purity of the water goes down.)

So why do I care you ask? Why are you telling me all this? And what does this have to do with drought conditions?

Well, the answer is simple—if you want to send the smallest amount of water to the drain for every gallon of pure water you produce (with our systems or any other RO system) you could close the valve on your storage tank and fill your own containers directly from your pure water faucet. After you drain your tank your system should be in production mode. Just leave the sink faucet open and the slow dribble that comes from the faucet is the rate your system produces water. Since you are bypassing the tank and making water right into your own container there will be no back pressure—your system will produce water at the fastest rate possible for your membrane.

Depending on your RO system and how much pure water you use each day, you can feel much better knowing you’ve reduced the amount of water you are using to “make” pure water. In less-than-well-designed systems, this can be a significant water savings. Even with the highest quality designs, like those from Pure Water Systems, you may save several gallons each day.


If you really want to reduce your water use but still enjoy the benefits of your RO system, you could, if you were so inclined, disconnect the drain line from your system and route the line into a bucket, collecting the “brine” water for other uses like flushing the toilet. DO NOT, however, extend your drain line more than about 8 feet or you will create back pressure on the brine line which will have a negative impact on your system. This is a little bit more of an extreme measure, but worth consideration should water use become rationed. (WARNING: Don’t be foolish and walk away from a system configured like this or you might waste a ton of water and flood your home at the same time. NOT FUN!)

AND, if you really wanted to use the least amount of water and still enjoy the extreme purity offered by one of our PWS® BEV water filters, you might chose one of our BEV200 water purifiers with a booster pump which would allow you to return the brine water to go through the filter a second or third time (depending on the amount of Total Dissolved Solids, or TDS in your tap water).

Finally, keep in mind that a well engineered reverse osmosis system will still be one of the most cost effective methods of water purification. Compared to distillation, for example, the cost of the tap water used to purify water for your daily cooking and drinking will be far less expensive than the electricity needed to operate a distiller. (And high quality distillers are the only types of water purifiers that compete with our BEV systems in terms of purity.)

2 Comments on “Using RO Water Filters in Drought Conditions”

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