Reverse Osmosis and Septic Systems

RO units will continue to rise in popularity and should be part of every homeowner questionnaire when evaluating a new or existing septic system

Reverse Osmosis and Septic Systems

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Reverse osmosis (RO) has become a common method for the treatment of household water supplies. It can potentially impact septic systems so onsite professionals should be evaluating the presence of RO as part of design, installation and management. 

There are two primary reasons homeowners may install reverses osmosis:

  1. There is a specific contaminant/constituent in the water supply they want to remove. These are typically naturally occurring substances that cause water supplies to be unhealthy or unappealing (foul tastes, smells or colors) or from unnatural contamination sources. 
  2. They want to be assured their drinking water is “clean” removing fluoride, lead, chlorine, pesticides, sulfides, etc. 

The membrane used in RO is semipermeable, meaning it allows the passage of liquid but not of solute or particles. The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. In most cases, the membrane is designed to allow only water molecules to pass through this dense layer while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high concentration side of the membrane.

RO is a separation process that uses pressure to force water through a membrane that retains the solute contaminants on one side and allows the pure solvent to pass to the other side. More specifically, it is the process of forcing a liquid from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure more than the osmotic pressure. This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied.

RO systems are typically used to reduce the levels of total dissolved solids and suspended matter. The principle uses of reverse osmosis are for the reduction of high levels of nitrate, sulfate, sodium, arsenic and total dissolved solids but it can be used for other contaminants. Many RO units are combined with carbon filtration, which is needed for the removal of chlorine and other emerging contaminants. 

When evaluating if an RO unit could be impacting a septic system, it is critical to understand what the RO units is treating: just the drinking water or the entire house. 

1. Under the sink RO/point of use (POU) systems are typically used to treat only drinking and cooking water. These RO units so generally are inefficient at creating “clean” water. The amount recovered for use within the home depends on the unit. The remainder is discharged as wastewater. A typical POU system is 10-20% efficient, meaning it will generate 4 to 9 gallons or more of reject water for every gallon of permeate. In recent years, membrane technology has improved and some POU systems have been designed to operate more efficiently, with some manufacturers advertising a 1:1 ratio of permeate to concentrate production. Regardless of the type of technology, unless it malfunctions a POU RO system should not over-burden a septic system. 

2. A whole-house RO/point of entry (POE) system creates RO water for all sinks, showers and appliances throughout a home and is typically installed in the garage or basement. POE RO systems tend to be more efficient than POU systems because they often include electric booster pumps and/or recirculate some of the concentrate water to improve efficiency. Therefore, POE RO systems can achieve efficiencies between 50% and 75%. Although POE RO systems are typically more efficient, they will still likely create a large wastewater load a septic system was not designed to handle. The wastewater is typically connected to the house drains and will add to the load on the household septic system, but this should not occur with POE RO systems. To determine the best location for the reject water from a POE RO, a wastewater professional should understand the volume of water and the concentration of contaminants removed. It may be that this water could be used for irrigation or discharged to a separate trench but before that determination is made the characteristics of the discharge are needed.   

RO units will continue to rise in popularity and should be part of every homeowner questionnaire when evaluating a new or existing septic system. Hopefully, efficacies will improve over time and reduce the amount of wastewater created in the process.

About the author: Sara Heger, Ph.D., is a researcher and educator in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota, where she also earned her degrees in agricultural and biosystems engineering and water resource science. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is the President of the National Onsite Wastewater Recycling Association and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system design, installation, maintenance and operation by sending an email to


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