What Are Single-Pass Media Filters and Where Are They Used?

Proper maintenance is critical to ensure the best performance from either single-pass or recirculating filters

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In a previous column, I mentioned that sewage treatment mounds are like a single-pass sand filter. That prompted follow-up questions about the use of media filters and how they relate to mounds. In general, media filters — either single-pass or recirculating — are used for advanced pretreatment of septic tank effluent before final treatment and dispersal. Usually, additional pretreatment is used in areas where there are environmental or health concerns and conventional septic tank to trenches, mounds or at-grades do not provide the desired level of treatment.

Media filters consist of a lined or watertight structure containing media of predetermined specifications. This is where working with the local regulatory authority is important to determine what types of media are accepted, and their required specifications are spelled out. a variety of media are approved and recognized in different locations.

Media can be sand, gravel, foam, plastic, textile or peat. In very localized areas, other media such as ash from coal plants and glass are usable. 

Many are proprietary products, so it is important to be certified to service and install them.

These filters follow a septic tank, and the effluent is evenly distributed over the media surface. This means most media filters involve the use of a pump and pressure distribution system in the distribution of the effluent. Treatment occurs like it would in mound sand or soil. There is filtration by the media and bacteria and other microorganisms living on the media assist with the treatment.


It is important that the media is unsaturated, so aerobic microorganisms are a part of the treatment process. Wastewater collects in the bottom of the structure and is delivered by gravity or pump either to a recirculating tank or to the soil treatment and dispersal area.

Single-pass media filters are very effective at removing the organic and total suspended solids from wastewater. They are not good at removing nitrogen from the wastewater. Just like the sand in a mound system, the media in the presence of oxygen (unsaturated) allows for the conversion of organic and ammonium forms of nitrogen to nitrate-nitrogen.

In a mound, the nitrogen is reduced when the effluent infiltrates the original soil surface where there are anaerobic zones. If a similar mechanism is not in place when the single-pass effluent is delivered to the soil treatment unit and the desire is to remove nitrogen as a part of treatment, a recirculating media filter should be used.

Although not common, some media filters have a treatment train where effluent flows from the septic tank by gravity to the surface of the media filter. The effluent infiltrates through the media and is delivered out the bottom of the filter by gravity to a drainfield. Certain types of peat filters have this configuration.

For the service provider, some maintenance tasks should be regularly performed on a single-pass sand filters. Some are like maintenance tasks for mound treatment systems. Others are unique to the sand filter itself.

Since the filter is contained in a watertight structure, it is important to avoid adding additional water to the system due to surface water runoff. If it appears surface water is collecting and infiltrating the filter, it should be diverted away from the filter. This is similar to diverting runoff away from the mound or locating the mound where runoff is not a concern. This should be evaluated for components in any system, but is even more critical where the filter is contained in a box or tank.


Check the effluent quality leaving the sand filter. It should be clear with organics and solids removed. If not, something is wrong with the filter. It may be plugging due to excessive organics or solids in the septic tank effluent, or may simply be hydraulically overloaded. The root cause needs to be determined and fixed for the filter to work as intended.

The surface of the sand should be inspected and evaluated for development of any biomat or plugging. If effluent is ponded at the surface due to biomat development, the system is broken. Sand at the surface may need to be periodically removed and replaced with new sand. This is true for any other media as well.

Since mounds are covered, it is not possible to evaluate the surface of the sand bed. However, a part of the design is to load the mound to prevent biomat development. Similarly, if effluent ponds in the bed of a mound — determined using inspection ports — the mound is broken and needs to be fixed.

In both mounds and single-pass sand filters, the pressure distribution system should be checked to determine if the head pressure at the end of the lines is consistent with system design. Elevated squirt heights would indicate some orifice plugging in the lines. The distribution laterals should be flushed, snaked with a bottle brush or pressure washed to remove solids. This is usually a regular maintenance task for both single-pass filters and mounds.

In mounds, access to the distribution laterals was not always a part of the system design and installation. Now most states require the addition of access ports to allow laterals to be cleaned.

When pumps are present for the pressure distribution system, the pump tanks and pumps should be evaluated to make sure they are delivering the desired amount of effluent at the proper pressure.

Hopefully, this gives some insight into where mound systems and single-pass filters have similar operating conditions and similar maintenance concerns.


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