We have had a couple of questions recently asking why recirculating media filters are used and where they are an appropriate choice for additional pretreatment of septic tank effluent. These questions have come from environmentally sensitive areas where additional treatment is being required for onsite systems. We thought it would be a good time to discuss recirculating filters, where they are most useful, and some considerations for installation.

As the name suggests, the system component recirculates septic tank effluent through a media filter several times before discharging to a final soil treatment and dispersal area. In the past, media in the filter was primarily sand and gravel. Now filter media may be sand, peat, textile sheets and a variety of other artificial products. Many of the textile, peat and other materials are proprietary products and must be installed and serviced by company-approved and trained personnel.

A treatment train including an RMF would be water from the house into a septic tank where the septic tank allows solids to settle and some organic matter is decomposed resulting in biochemical oxygen demand of 170 mg/L and total suspended solids 60 mg/L. Effluent from the septic tank moves — usually by gravity — to the recirculation tank. Here the incoming septic tank effluent mixes with water that has been recirculated through the filter.

Recirculating ratio

Effluent from the recirculating tank is pumped repeatedly through a pressure distribution network in the lined, contained filter. Treated effluent is collected at the bottom of the filter and delivered back to the recirculation tank. The desired ratio is usually 5 treated to 1 septic tank effluent. When the liquid level in the recirculating tank reaches the full level, it is delivered by pump or gravity to the final soil treatment and dispersal area.

Additional treatment of the septic tank effluent occurs in the media filter. BOD and TSS are reduced to 20 mg/L for each. This is a significant improvement. In addition, there are other treatment advantages. Fecal coliform bacteria in the effluent is reduced from millions to 5,000 to 100,000. There is a 30 to 70% reduction in nitrogen levels and 10 to 30% reduction in phosphorus.

These bacteria and nutrient reduction numbers are why RMFs are sometimes required in environmentally sensitive areas such as shallow to bedrock or seasonally high-water tables near lakes, rivers, estuaries, groundwater recharge areas and wellhead protection areas. The reduced BOD and TSS levels make these components ideal for small lot areas, restaurants and other establishments. Also, RMFs have been successfully used to help recover drainfields that have failed due to excessive organic loading.

These systems are often used where nitrogen contamination is a problem. As the anaerobic septic tank effluent moves through the media filter, it becomes oxygenated and the ammonia in the effluent is changed to the nitrate form of nitrogen. When sent back to the recirculation tank, which is low in oxygen, the nitrates are broken down and nitrogen is released to the atmosphere, a process called denitrification.

Must drain freely

The filter itself is housed in a watertight container or liner. The proprietary products are contained in a tank or series of tanks, generally plastic. Concrete tanks are also used but these are usually constructed in place. If the filter is constructed in a lined excavation, it is typically made of 30 mil polyvinyl chloride. The filter is composed of 12 inches of drainfield rock where the pressure distribution system is located, 12-24 inches of drainage media (sand, or other media) and 12-18 inches of gravel at the bottom with the outlet piping.

Outflow from the filter constructed onsite is provided by a 4-inch pipe surrounded by drainfield rock. Proprietary products have their individual ways of providing the outflow drainage. It is important that effluent must drain freely out of the filter to maintain oxygen levels in the filter. If effluent backs up into the filter, the filter’s effectiveness is reduced.

Filters constructed on site include a layer of landscape rock over the top, which helps keep the filter aerated. Site flexibility is one major advantage of these systems. They can be located in a relatively small area and the soil it is placed in doesn’t matter as much since the filter is watertight. It can be put in disturbed or compacted areas, preserving better soil areas for final treatment and dispersal. Landscaping should divert surface water away from the filter. Any amount of surface runoff entering the system could hydraulically overload the system.

RMFs in general require more maintenance than conventional septic tank-drainfield systems. Maintenance includes inspecting flowmeters and the pump, recirculation tank, recirculation pump, distribution system in the filter, media and effluent quality. Tanks and piping will need to be periodically inspected — yearly at a minimum — to make sure the distribution system is working properly. The flowmeter should be read systematically to make sure the application rate is within design limits.

Replacement needed?

Over time, the media in the filter may become plugged with solids or a buildup of organic matter. When this happens, the upper layer of media or perhaps the whole layer will need to be replaced with new media. Manufactured products will have specifications on when this should happen. For constructed-in-place filters, if the surface is growing a biomat, loading rates and septic tank effluent quality should be checked to see if it’s within normal limits. If the amount of water delivered exceeds design specifications, water use must be cut back or the system enlarged. If everything is normal the media should be replaced.

For environmentally sensitive areas and problem sites, recirculating filters may be part of the solution for onsite waste treatment.