Waste water can be a big deal with reverse osmosis systems, as such it turns off many people from buying them. A reverse osmosis system can reject 3-10 gallons of dirty water (brine) for every gallon of purified drinking water. The amount of waste water depends on several factors such as incoming water pressure, water temperature, efficiency of the membrane, and total dissolved solids (contamination level) in your water. Waste water is obviously a concern for many people who are conservation conscious, it can be costly for those with septic systems that back up quickly due to too much waste water being pumped into them from the reverse osmosis system.
To combat this issue there are so called “zero-waste” reverse osmosis systems that create a loop to keep recycling the rejected water back through the membrane instead of down the drain. The only problem here is that this greatly reduces the useful life of the reverse osmosis membrane from around 2 years for a decent 100GPD membrane to about 6 months or less depending on incoming water quality.
Another popular idea is that users can add a system to capture the waste water in a reservoir and use it for flushing toilets. Of course the big drawback is the added cost to plumb the system and the extra pump required to supply the toilet. In addition, this might not solve the problem of sending excess water the septic system.
The best method, in my opinion, to reduce waste water is by using a permeate pump. This is an inline pump that lies just before the reverse osmosis membrane. It relies only on the hydraulic energy in the brine that usually goes down the drain; there is no electricity required. The permeate pump forces pure water into the storage tank which reduces membrane back pressure and increases the feed pressure going to the membrane. This results in increased membrane efficiency and lifespan and reduces waste water by up to 80%. As a side benefit, storage tank fill time is cut by more than half and reduces the required incoming pressure of 40psi to 30psi – great for those on well water.
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{ 2 comments… read them below or add one }
I’m interested in understanding more about your post.
Can you please explain what you mean when you say “the permeate pump forces pure water into the storage tank which reduces membrane back pressure and increases the feed pressure going into the membrane… Reducing waste water by up to 80%. Storage fill time is cut by more than half.”
I would really like to understand how this works!
Many thanks,
Linda
Linda –
What happens with the permeate pump is just the build up of hydraulic pressure. On the input side, pressure builds up to a certain point, the higher pressure water is now sent to the membrane. The membrane is more efficient at higher pressures – less waste water is produced. With the higher input pressure, the brine water (reject water) has less of an opportunity to seep backwards through the membrane causing TDS creep and reducing membrane life.
Thanks!
Mathew