Aerobic septic systems often include a final filter that can clog, requiring frequent maintenance or repair.

As we explain here the problem may not be the filter but rather how the septic system is being used.

This article series discusses problems that occur in aerobic septic systems, also known as aeration septic systems or fine-bubble septic aerators. We identify things that cause problems with or harm aerobic septic systems.

Aerobic Septic Treatment Unit Filter Clogging

Question: gravel filter coating build-up clogs the septic system

2018/09/01 Mark said:

We have an aerobic system with a separate up flow gravel final filter. The aerator pump is checked regularly for function.

Our problem is that the gravel quickly builds up a coating the does not allow the water to flow through at a quick enough rate causing a back up.

We had the system pumped about 6 months ago and they worked on the up flow filter gravel for some time but the unit has already started to back up. any suggestions?

Reply:

Mark,

Thanks for an interesting and important question. A little research shows that clogging of septic effluent filters of all sorts is a recognized problem.

Wastewater Properties that Cause Filter Clogging

The first cause that experts point out, properties of the wastewater itself, assumes that

1. the system's final filter gravel is the properly-selected size and shape

2. upstream filtering and equipment are working properly; for example there is no damaged baffle in the upstream septic tank that might send solids into the filter

3. the septic tank is properly sized for the daily wastewater flow. The concern here is that if daily wastewater flow exceeds the system design the final septic tank chamber wastewater contents will remain agitated, lacking sufficient settlement time.

4. daily wastewater volume exceeding the design level fdor the septic filter selected - for example shown here is the EF-4 Residential Series Effluent Filter (cartridge) that is deisgned for 800 GPD filtration, providing the equivalent filter-area of 86 sq.ft. of 1/16" thick filter surface.

This filter is produced by Tuff-Tite cited at SEPTIC FILTER / GRAYWATER FILTER SOURCES

When there is insufficient settlement time, small solids remain dispersed in the wastewater and these will quickly clog any downstream filter. See details at EFFLUENT RETENTION TIME https://inspectapedia.com/septic/Septic_Tank_Effluent_Retention_Time.php

So you might want to check the conditions in the treatment tanks and chambers upstream from the final gravel filter,

and you want to check the wastewater flow volumes being sent into the system to confirm that you’re not exceeding its design capacity.

You can reduce the capacity problem by spreading out wastewater disposal over a longer time to avoid overloading the system on any single day: don’t run ten loads of laundry in a single day, and don’t let my daughter Mara spend hours in the shower before school.

Fine Solid Discharge Causes Septic Filter Clogging

A second cause of septic filter clogging: food or other fine solid waste discharge:

Garbage grinders add high volumes of finely-ground solid waste to the system that can exceed its design capacity and that push solids out of the septic tank. These solids will clog any septic system filter as well as clogging the drainfield in a conventional (non-aerobic) septic.

I recommend that people do not use garbage grinders when their waste is processed by a private onsite septic system.

SImilarly, lint discharged from washing machines can clog any type of septic filter. Be sure that your washing machine wastewater discharge passes first through a lint filter.

Water Treatment Equipment Can Cause Septic Filter Clogging or Fouling

A third cause of any type of septic filter or effluent filter clogging is overloading from ancillary water treatment equipment.

If your water supply is high in iron and you’re using an iron remover system its discharge adds both water volume and high levels of iron-loving bacteria to the wastewater stream. Discharge this wastewater to a separate facility - a drywell or seepage pit instead of into your septic system.

Household Chemicals that Lead to Septic System Clogging

A fourth cause of septic filter clogging is use of household chemicals that harm the bacterial action in the septic tank.

High levels of bleach, fabric softeners, and possibly de-greasers all can interfere with both the bacterial action in the septic tank, and for non-aerobic systems a grease solvent may prevent the separation of solid waste into settled sludge and floating scum layers - pushing those wastes instead out of the septic tank and clogging downstream filters or fields.

See TOILETS DON’T FLUSH LIST at https://inspectapedia.com/septic/Dont_Flush.php

Also see SEPTIC TREATMENTS & CHEMICALS https://inspectapedia.com/septic/Septic_Tank_Treatments.php since some of these (yeast is a good example) can also cause churning and will push solids and fats out of the septic tank into downstream filters

Research on Gravel Filter Clogging

These articles and standards, while not specifically for aerobic septics, contain helpful guidance about gravel and sand filter clog prevention

• Heger, Sara, Ph.D., "Effluent Screen Troubleshooting", Onsite Installer Magazine, March 6, 2017, retrieved 2018/09/01, original source: https://www.onsiteinstaller.com/online_exclusives/2017/03/effluent_screen_troubleshooting
• Lesikar, Bruce, SAND FILTER SEPTIC SYSTEMS, [PDF] (1999) Texas Agricultural Extension Service, Texas A&M University, retrieved 2018/09/01, original source: https://www.h-gac.com/community/water/ossf/OSSF-Treatment-Systems_Sand-Filter.pdf
• RECIRCULATING GRAVEL FILTER SYSTEMS [PDF] (2015) Washington State Department of Health, Wastewater Management Section, PO Box 47824, Olympia, WA 98504-7824 USA Email: wastewatermgmt@doh.wa.gov Tel: 360-236-3330 retrieved 2018/09/01, original source: https://www.doh.wa.gov/Portals/1/Documents/Pubs/337-011.pdf
• RECIRCULATING GRAVEL FILTER SYSTEMS NETCHD Standards [PDF] (2008) based on Washington State DOH standards from 2007, given in upated form above (2015)
• SEPTIC TANK POLISHING [PDF] (2002) U.S. EPA, Office of Water, Municipal Technology Branch, U.S. EPA, 1200 Pennsylvania Ave. NW, Mail Code 4204M, Washington D.C. 20460 USA, retrieved 2018/09/01, original source: https://www3.epa.gov/npdes/pubs/septic_fs_tank_polishing.pdf

...

Continue reading  at AEROBIC ATU SEPTIC PROBLEMS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see AEROBIC ATU SEPTIC PROBLEM FAQs - questions & answers posted originally at this page

Or see these

Recommended Articles

• AEROBIC SEPTIC SYSTEMS, ATUs - home page for Aerobic Septic Systems
• AEROBIC SEPTIC FILTER CLOGGING
• FILTERS SEPTIC & GREYWATER
• PLUMBING DRAIN NOISE DIAGNOSIS
• MEDIA FILTER SEPTIC SYSTEMS
• SEPTIC FAILURE CRITERIA
• SEPTIC FILTER CLOGGING SIGNS
• SEPTIC FILTER MAINTENANCE
• SEPTIC FILTER / GRAYWATER FILTER SOURCES
• SEWAGE TREATMENT PLANTS, RESIDENTIAL

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Citations & References

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