Comparing Media and Membrane Filtration for Stormwater Treatment

EPA stormwater regulations are changing with great frequency and requiring an increased level of testing, monitoring and treatment. In fact, any business that uses, produces, transports or stores chemicals or petroleum products, and any construction projects that have the potential for harmful discharges, must follow strict guidelines for stormwater runoff. The most important reasons for these regulations are to keep humans safe and to preserve our natural environment and the criteria for these fall into two categories:

1. Water quality: keep water safe by reducing pollutant runoff from sites.
2. Water quantity: flood and erosion control and protection.

The burden for compliance falls on localities, businesses, developers and others. Because regulations are requiring continually lower levels of pollutants to be released into creeks, rivers and other water sources, a lot of importance for property owners rests in the capabilities of their filtration systems.

There are many ways to filter stormwater that flows from rooftops, parking lots, streets and driveways and collects into retention ponds or tanks, or flows through drainage and sewer systems. For those businesses that must use filtration systems that are attached to their stormwater systems, there are two different types of systems, media and membrane filtration, that have proven effective at cleaning stormwater before returning it to natural streams and rivers.

When comparing media and membrane filtration systems it is important to know what regulations apply to your stormwater situation.

Media Filtration systems work by physically capturing pollutants and by the adsorption of pollutants with the help of chemical reactions. The common media-based filtration systems are formed by layering sand, stone, organics, carbon and/or other materials in a filtration system or below a stormwater retention area. Media filtration is typically used when there are specific pollutants targeted for removal.

Stormwater runoff must pass through layers of sand, organics, stone and/or other similar materials that are placed in its path. Block carbon is another highly effective media found in these types of filtration systems because it provides a faster filtering process than sand filters. Media filters are created in such a way that the water must pass over and through the media (often many times, as needed). Pollutants in the runoff become trapped in the spaces and voids between the media particles as well as on the surfaces of these media particles.

A second way that media filtration works is when chemical reactions or molecular cation exchanges occur with pollutants allowing them to be more easily isolated and removed. This process of media filtration is especially effective with nutrient and metal removal. Successful media filtration leads to cleaner water being released back into the environment or being designated for reuse.

Media filtration is used by municipal drinking water systems, stormwater filtration, sewage and wastewater applications, and by the oil and gas industry. The media for each use may vary some, but sand, gravel and natural stone fiber are common among all uses.

Membrane filtration varies a bit in that it uses a thin, permeable layer or a sheet of material that helps to remove impurities and targeted pollutants as the stormwater passes through it. Although secondary mechanisms are set in place, the primary removal mechanism in membrane filtration occurs when the membrane material physically blocks contaminants and contaminated particles from passing through. Secondary removal mechanisms include the removal of substances that are attached to the particles captured during the primary removal process.

Membrane filtration uses a permeable layer or sheet of material which the water passes through to remove the pollutants or targeted impurities. Membrane filtration has two removal mechanisms: physical blockage of particles by the membrane material and the removal of substances that are attached to the particles captured during the primary removal process.

Membrane filter manufacturers use a wide variety of materials to build these filtration systems. The two key factors used in selecting a membrane material are pore size and durability. Pore size refers to the size of the hole or pore in the membrane material. The smaller the pore size the fewer particles are allowed to pass through the membrane.

There are four categories of membrane filtration: 

1. Microfiltration – has the largest pore size and is often used in pre-treatment for further filtration in some processes.
2. Ultrafiltration – finer pore size also used in pre-treatment.
3. Nanofiltration – finest pore size 
4. Reverse osmosis filtration – pressure pushes untreated water through a semipermeable membrane removing all pollutants and impurities.

For those asking, “what is membrane filtration used for?”  The answer is, the options are widely varied. One or more of these filtration membranes are applied to stormwater filtration in order to remove biomass, bacteria, heavy metals, chemical compounds, petroleum products, etc.

When seeking to answer stormwater filtration issues for your property, it is important to know the requirements set for you as well as your own goals when comparing media and membrane filtration. Knowing these will allow you to select the appropriate media gradation and composition for your stormwater project.

If you need help with any step of the process for stormwater collection, testing, analysis, remediation and monitoring, it is extremely valuable to have experts to consult with and trust with your samples, tests, reports, monitoring systems, filtration system advice and much more.

Torrent Laboratory is the premier testing laboratory for California and Hawaii. With almost three decades of experience, stellar customer service, the highest national and state certifications, the most advanced laboratory equipment and technologies and the fastest turnaround times, Torrent is the right laboratory to meet all of your needs.

Torrent Laboratory will partner with your business or development project for stormwater testing, stormwater runoff analysis, remediation and stormwater monitoring plans. We have been privileged to work in partnership with regulatory boards, industries, developers and communities to meet and exceed all regulatory requirements. We will work with you to preserve and protect the health and future of us all.