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Sunday, February 18, 2018

Recirculating Trickle Filter

The recirculating trickle filter (RTF) is the latest alternative onsite wastewater treatment technology to be introduced in the Municipality of Anchorage. It was first introduced into the Anchorage area in July of 1996. There are now several hundred RTF systems in operation within the Anchorage Bowl. Currently there are two systems approved by the MOA that utilize RTF technology; Orenco's Advantex, and the Quanics SCAT AeroCell (formally known as the Zabel SCAT filter).

The RTF systems, like the other alternative onsite technologies, is a biological treatment process in which the microorganisms eat the soluble and suspended organic material in the wastewater. This results in an effluent (the water that is discharged from the treatment system) that is non-biomat forming. In other words, the wastewater is treated to such an extent that it will not cause a progressive failure of the drainfield. This allows for the drainfield to be dramatically reduced in size versus what would be required utilizing conventional septic system technology. This reduction can be as much as 80%! The RTF process is summarized below:


Wastewater from the house is first passed though a septic tank where the settleable solids and scum are trapped/contained. The "clear" liquid from the septic tank (which still contains soluble organic matter and suspended solids that would not settle in the septic tank) then flows into a pump chamber (second compartment of the septic tank) from which it is periodically discharged (for about 1 minute, every 20 minutes) to the trickling filter (see the photograph). The trickling filter provides an oxygenated environment in which microorganisms can grow on the geotextile material that is suspended in the filter housing. The attached microorganisms consume the organic contaminants as it is periodically sprayed over the media. The treated water that flows off of the media is then returned to the pump chamber. When the liquid level in the pump chamber reaches a specific depth, the effluent is diverted to the drainfield for final disposal.


The success of the RTF technology is due to its inherent stability. Most traditional residential wastewater treatment plants (Aerobic Treatment Units or ATUs) utilize what is termed the "extended aeration process". This method involves mixing/aerating the wastewater mixture (including the settleable solids) so as to create an environment in which the microbes present consume the organic constituents. The aerated mixture (referred to as the "mixed liquor") is then passed into a settling basin (referred to as a "clarifier") where ideally the solids can be settled out and the clear liquid (supernatant) discharged to the drainfield. The problem with this method of treatment is that it is inherently unstable, and can easily produce a mixed liquor that does not have good settling characteristics. When these "upsets" occur, significant quantities of organic material can be discharged into the drainfield. The "beauty" of the RTF technology is that only the "clear" liquid in the pump chamber (second compartment of the septic tank) is transferred to the trickling filter and that the ultimate performance of the system is not dependent upon attaining good settling characteristics in a clarifier. Therefore, it is not possible to have an upset that would result in significant quantities of organic material being discharged to the drainfield.

SCAT Spray

The RTF systems, like other alternative technology onsite wastewater treatment systems, should be inspected periodically to ensure that it is operating as the manufacturer intended. Contracts for quarterly inspections will cost the homeowner approximately $500.00 per year, including septic tank pumping. Both RTF systems approved by the MOA are factory equipped with a modem and a programmable controller that monitors and records all system functions, so troubleshooting can often be done remotely by a service technician.

he initial RTF installations have run approximately $25,000 - $30,000, including test holes, engineering fees, MOA permit fees, and contractor installation charges. This is comparable with the other alternative technologies currently being used in the Anchorage & Eagle River area, specifically Intermittent Sand Filters and the BioCycle system.For additional information please contact us or go to the Quanics, or Orenco Systems, Inc. official web sites.

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