What is Septic Biomat? How Does it Form?


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Septic biomat formation patterns in soil trenches: the Biomat formation pattern in leach field trenches

  • What is a septic biomat? – what is the septic drainfield “biomat” layer?
  • How the biomat forms: how does the Biomat Form in a septic system effluent absorption system?Septic biomat constituents: what is the biomat made from? Septic biomat formation patterns in soil trenches How septic biomat formation, though necessary, clogs & ultimately can cause septic drain field clogging and expensive drainfield repairs Septic drainfield clogging process: how the soakaway bed or drainage trench fails: the biomat clogging process How to Extend the Life of A Septic Drainfield by Protecting the Biomat

Septic drainfield clogging process: how the soakaway bed or drainage trench fails: the biomat clogging process

Biomats tend to restrict the flow of effluent through the drainfield, but are crucial because they filter out viruses and pathogens. As the biomat develops, the soil infiltration rate decreases. Once the hydraulic loading rate exceeds the soil infiltration rate, ponding starts. At some point wastewater will either back up into the home or break out onto the soil surface.

Septic loading and dye tests look for this “breakout” of effluent on the soil surface – a condition which will occur when the biomat has become so thick that septic effluent no longer percolates through it to the soils below. If you dig a neat cross section of a traditional leach field trench, and if it was properly constructed, you’ll see the perforated effluent pipe surrounded first by gravel, and then the sides of the trench as it was originally cut through the soil. You will also see a 1cm (about 1/2″) to 5cm (about 2″) thick gray band around the perimeter of the trench – this is where the soil clogging has occurred.

During the septic loading and dye test, an aggressive volume of water, but not exceeding reasonable septic system design parameters, is run into the system, forcing effluent in a failed system to the surface (maybe). The septic dye itself is a harmless but intense dye – it does not “make anything happen, but its sole purpose is to permit the inspector to distinguish between breakout of (dyed) septic effluent during a septic test and other site water which might be present, say from a spring or surface runoff.

Extend the biomat life – How to Extend the Life of A Septic Drainfield by Protecting the Biomat

Pump the septic tank: Biomat formation cannot and should not be prevented, but septic tank filters, proper organic loading, and proper maintenance of the septic tank can slow the rate at which it forms [thus extending the life of the drainfield].

Septic tank filters prevent excess suspended solids from flowing into the drainfield and can be retrofitted to existing systems.

Washing machine lint filters can reduce the movement of lint into the drainfield. (We’ve seen promotions for this product but not any studies supporting the effectiveness of this measure.)

Septic tank outlet baffle filters are sold to reduce movement of fine particulates into the leach field. Use of these filters will require a suitable access port and regular maintenance at the septic tank. (We’ve seen promotions for this product but not any studies supporting the effectiveness of this measure.)

Drywells to handle graywater can reduce the total liquid load on the leach field – a step often taken at sites with limited drainage capacity.

Reduce unnecessary water usage, and in particular, be alert for plumbing fixtures that run continuously such as toilets and leaky faucets.

WATER SOFTENERS & CONDITIONERS should be checked for proper settings for backwash frequency, volume, and salt dose.

Other maintenance that should be performed on the septic system includes having the system inspected and the tank pumped at regular intervals. Pumping the tank allows it to better settle out solids [by maintaining a larger liquid volume or “net free area” in the septic tank], also reducing the organic load to the drainfield.

Magic bullets which promise to restore drainfields are either unlikely to produce any lasting effect or are at risk of contaminating the environment with toxic and perhaps caustic chemicals. These processes are prohibited in some jurisdictions.


We have seen drain fields still working fine after 25 years and others which failed in the first week of occupancy of a new home. In a properly designed septic absorption field the level of usage of the system, site characteristics such as slope, rock, groundwater level, and soil percolation rate have all been considered.

Barring foul ups such as we discuss in this document, such a field may last from 10 to 20 years. USDA sources assert that a properly operated and maintained ST/SAS (septic tank / soil absorption system) should last at least 20 years.

But it’s easy to ruin or shorten the life of a drainfield/leaching bed. In fact the same USDA source states that

Studies reported at an Environmental Protection Agency seminar, Orlando FL, November 1979, show that over half [ST/SAS] fail prematurely due to improper operation or lack of adequate maintenance. Generally, these failures occur when the soil-absorption system [drainfield] becomes clogged. Preventable clogging, due to a buildup of solids in the [septic] system, is usually extensive enough to require expensive reconstruction of the system. Failures can also cause nearby ground areas, streams, lakes, and water supply systems to become contaminated. This exposed the public [and USDA, EPA, NPS, FPS, and other government employees] to health threats such as hepatitis, typhoid, diarrhea, and dysentery.

… [ in contrast] The unpreventable failure of the soil-absorption system eventually occurs when growth of the organic material in the wastewater [the biomat (SEPTIC BIOMATS) that forms under and along the sides of a drainfield trench] becomes so large [thick] that they plug up the soil.

Similar studies of advanced wastewater treatment systems such as aerobic systems, sand beds, mound systems similarly found that improper or inadequate operation and maintenance were the primary causes of premature failure of those systems as well.

Luis Goncalves, ROWP, IN, PL

I fell into this amazing trade quite by accident, I have always loved being around heavy machinery and moving earth and in contrast love working with designing things.  This trade in the septic field has led me down a wonderful path of exploring all of my skills and passions while working hard at achieving success.

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