The Role of Membranes in Decentralized Wastewater Treatment

The Role of Membranes in Decentralized Wastewater Treatment
The reject water from whole-home reverse osmosis units, like this one, will overload a septic system.

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There are two types of membranes an onsite professional may run into: reverse osmosis and membrane bioreactors. Although the process is similar when treating water versus wastewater, the systems are very different. They both use a separation process to remove issues in the source, although the contaminant load is very different in water compared to wastewater treatment.

What is reverse osmosis?
Reverse osmosis is a separation process that uses pressure to force water through a membrane that retains the solute 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 in excess of 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. The membrane here is semi-permeable, 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 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, usually 2 to 17 bar (30 to 250 psi).

Extra water
Reverse osmosis units sold for residential purposes offer water filtration at the cost of large quantities of reject water. For every 5 gallons of output, a typical residential reverse osmosis filter will send around 10 to 20 gallons of water down the drain (although many people capture it and use it for watering plants and lawns). In some states, this water is used for irrigation. For home units used only under the sink for drinking water, this volume should not impact the septic system, but the extra water from whole-home units will overload the system. The most practical solution is to route this water out of the system and discharge to the surface or use it for irrigation.

What are membrane bioreactors?
Membrane bioreactors are a type of aerobic treatment unit with an added membrane filtration process. MBRs combine two basic processes — biological degradation through the activated sludge treatment process and membrane separation — into a single process during which suspended solids and microorganisms responsible for degradation are separated from treated water by membrane filtration units, which pull water through a membrane with very small pores.

While there are many designs of MBRs, the MBR systems commonly available share typical characteristics. For instance, most MBRs use ultrafiltration membranes with a 0.02- to 0.05-micron pore size, which traps solids on the outside. The membrane is typically made of polypropylene, cellulose acetate, aromatic polyamides, or thin-film composite. The activated sludge process combined with membrane separation is able to achieve very high removal efficiencies of organic material, with greater than 95 percent common for biochemical oxygen demand and total suspended solids. In addition to removing biodegradable organics and suspended solids, MBRs remove a very high percentage of pathogenic organisms, providing disinfection of the effluent with 99.9 percent removal of fecal coliform.  

MBR applications in the decentralized field
Particularly in individual systems, MBRs are an innovative technology. Most MBR installations are less than 10 years old and are located in small cluster treatment plants with varying design parameters. MBR use will continue to develop, particularly in areas where the demand for wastewater reuse is increasing.

About the author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association (MOWA) and the National Onsite Wastewater Recycling Association (NOWRA), and serves on the NSF International Committee on Wastewater Treatment Systems. Send her questions about septic system maintenance and operation by email to


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