Reverse Osmosis Water Filters And Their Effect On Septic Systems

In many rural areas of our country people depend on septic systems to manage their wastewater disposal. But it also happens that in the same areas tap water quality is particularly bad and people are trying to remove some of the nasty contaminants that can be found in their public water supply system by the use of different types of water filters.

An example for such a filter that has gained in popularity over the last decade or so, as the technology is known to be very effective at purifying water, are reverse osmosis filter systems.

Since I know for a fact that many people wonder whether or not a reverse osmosis water filter might cause any harm to a septic system, I decided to address this issues in a new blog post.

Last week Susan P. reached out to me with the following question:

“Hey Chris. My name is Susan and I’m from Pittsboro, NC where a lot of houses have their own septic system (so do we). My husband and I recently found out that the pipes that run through the house we live in add lead to the water that comes out of the kitchen faucet and also one of the bathrooms. My husband assumes that this probably has something to do with the fact that the house is pretty old and years back they used pipes containing lead for the construction.

Anyway here is my question: We would like to install a water filter (probably reverse osmosis) in our house to have all the lead removed. And we already know that those filters waste a lot of water. Do you think that there will be a chance that this could damage the drainfield or tank of our septic system or the system as a whole? Also I have heard that some filters add more filtered material to the waste stream. Is that true?┬áThank you!┬áSusan”


For usability reasons, I’m going to break up my answer into multiple sections.

1) Does a reverse osmosis filter system waste a lot of water?

Yes, it does. Reverse osmosis filter systems produce a lot of wastewater – about five to six gallons for one gallon of purified water give or take. Therefore it makes sense to conclude that such a system might be releasing an unwanted amount of additional wastewater into the tank of your septic system and the drainfield.

2) Can a reverse osmosis filter system have a negative impact on a septic system?

We have to consider two influencing factors at this point to answer this question correctly:

  1. Are you planning to install a point-of-use (POU) filter system in your house, which provides water for a single tap only, or do you want to install a point-of-entry (POE) system that filters the entire water supply before it gets distributed to the kitchen, the bathroom and so on?
  2. How much filtered water do you require on a day to day basis?

Here is why this distinction is important. A POU filter system is used to produce water for drinking purposes and possibly cooking. So it will need to provide just a few gallons of water per day, depending on the size of your family. The impact that the wastewater production will have on your septic system is therefore negligible, I guess.

However, if you require lots of water on a day to day basis, even a POU system might soon exceed the capacity of your septic tank and especially the drainfield. And this particularly applies to POE filter systems. They might be more efficient than POU reverse osmosis filters, but the sheer amount of water they treat and waste is simply too high and cannot be compensated. So if you plan to install a POE system in your house, make sure to talk with both the manufacturer of your septic system and the manufacturer of the filter system first.

3) Can a filter system add more filtered material to the waste stream?

This is just nonsense. A water filter prevents contaminants from passing through by using a membrane or a filter resin, but it doesn’t add more substances to the water. The concentration of contaminants in the waste stream must increase obviously. But these contaminants were going to end up in your septic tank anyway regardless of you using a water filter or not.

There are water treatment systems (water softeners) that do add chemicals to the water they process, but these are not used to filter water, but to condition it and reduce water hardness. In this case a mini-septic system can be installed which receives the discharge from the water softener, which contains high concentrations of sodium or potassium. The salt can cause clay particles to bond forming a water proof barrier. In case this happens, you can use calcium-polysulfide to correct the damaged soils.


In case of a POU reverse osmosis systems, some manufacturers suggest that the best solution would be to route the produced waste water back into the plumbing system and use this so called ‘gray water’ for washing clothes, toilet flushing, bathing, and watering the garden. To me this makes a lot of sense. However, it’s important to make sure that a system that recycles wastewater meets the local plumbing codes, which is not always the case especially for systems that don’t filter the wastewater any further prior to recycling.

E-Mail it to me by clicking here and I will answer it as soon as I can. If don’t get back to you within 5 days I may have missed it so try resending it.