When discussing recirculating media filters one important aspect is the establishment and maintenance of the rate of recirculation through the filter. The desired ratio is set based on the level of treatment desired and is determined by the manufacturer or the designer, factoring in the estimated daily sewage flow from the residence. Over time it may be necessary to adjust the ratio based on actual sewage flows.

Typical recirculation ratios range from 3:1 to 4:1. The volume that recirculates over the media filter divided by the volume of sewage generated by the residence minus one is the recirculation ratio.

Here is a simple example of a delivery ratio. If the amount of effluent delivered to the filter over a month was 88,000 gallons and the sewage generated from the residence was 22,000 the recirculation ratio is 88,000 divided by 22,000 = 4.0-1.0 = 3.0.

If the desired level of treatment is being obtained there is no reason to change the ratio. This means that the service provider needs to know the amount of effluent delivered in each cycle by the pump delivering effluent to the filter, as well as the pump delivering effluent for final treatment and dispersal. The number of cycles and pump on-time can be determined from the readings in the control panel. Obviously the service provider needs to also know the starting values.

To determine the level of treatment, the levels of dissolved oxygen are measured at the inlet side of the recirculation tank. Since the goal is to aerate the effluent as it passes through the media filter the effluent returning to the tank should be higher in dissolved oxygen. Desired levels are between 0.1 and 0.3 mg/L since one of the main reasons to have a recirculation system is to reduce the total nitrogen from the systems. These oxygen levels are low enough to promote denitrification.

If the level exceeds about 0.5 mg/L, there is too much oxygen and the ratio needs to be changed so less effluent is passed through the filter. If the effluent coming from the filter is not clear, has a septic odor or dissolved oxygen content less than 0.5 mg/L, the ratio can be increased to have more effluent pass through the filter, which will accomplish more treatment.

In most recirculating filters a programmable timer is set to turn the pump on and off at preset intervals. The pump on-time is set by the manufacturer or designer and is based on the level of treatment desired, so it should not be changed unless specified by the designer. So to change the ratio, the off-time can be either increased or decreased to achieve the desired result. Shorter off-times mean that more effluent is recirculating, and longer times of course mean less. 

About the Author
​Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program, is an emeritus professor in the university’s Department of Soil Water and Climate, and education coordinator for the National Association of Wastewater Technicians. Send him questions about septic system maintenance and operation by email to kim.peterson@colepublishing.com.

This article is part of series on media filters:

• An Introduction to Media Filters
• Designing with Single-Pass Media Filters
• The Basics of Recirculating Media Filters
• Recirculation Ratios 101
• Recognizing A Few Problems with Media Filters