Let’s Probe the Outer Limits of Biomat Benefits

How slow is too slow when effluent starts to enter the clogging zone?

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Recently, a reader in Canada asked about the beneficial characteristics of the biomat. He had read some articles or fact sheets where the author maintained the biomat has no beneficial aspects and if the total system could be oxygenated and biomat eliminated, there would be no problems in septic systems accepting effluent.

As I read an article the reader referred to, I was struck that the article referred to two different distribution methods. The biomat occurred in either of two locations — on the bottom and sidewalls of absorption trenches or beds, and/or the top surface of a pretreatment aerobic filter. In the first case I assume they are talking about a typical sewage treatment trench with gravity distribution since they describe biomat developing at the bottom of the trench where the first few holes in the sewage pipe are located. The second is where effluent is applied by pressure to a media filter (sand, fabric; etc.) These are two entirely different environments and should not be confused with each other.

Considerable research has been conducted since the late 1960s and early 1970s on development of what was then called a crust or clogging zone (now the biomat) and its impact on slowing down the rate of water infiltration into the soil and the effect that has on treatment of bacteria, virus and nutrients.

Here are a couple short quotes from one of the initial research papers on soil as a medium for treatment of septic tank effluent. “In naturally well-drained soils, the soil horizons under the absorption bed will be unsaturated because of the impeded flow through the crust.” And later: “Septic tank effluent passing the crust layer and going slowly through a soil will have time for many reactions to occur at the surfaces of particles and in the soil solution, which is in contact with air-filled voids.” They go on to state that if a crust were not present, flow is through the larger soil pores and voids and there is very little time for purification to occur.


Subsequent research has expanded on the importance of the biomat in treatment of sewage effluent in soil. It has also shown the biomat as being a larger part of the treatment processes than merely slowing down the flow rate into the soil allowing unsaturated (aerated) soil beneath 

the trenches.

There is no question biomat assists in the treatment processes within soil under gravity distribution. Design numbers we use today are based on corresponding research on what happens to flow rates into soil with varying biomat (crust) resistance and different soil moisture and aeration conditions in the soil.

The reader gave me an example of how he was having trouble with systems and gave this account:

“After examining several of these failures it became apparent to me that the problem was high levels of biomat. In our area the bed of choice is often a sand filter bed with 2 1/2 feet of sand filter media with about a foot of stone on top containing the distribution pipe. Alternatively, chambers are placed directly on the filter media.

A typical installation would be a three-bedroom house yielding a daily design load of 422 gallons. The [Ontario Building Code] allows for a daily dosing of (19.8 gallons/10.8 square feet), which yields a bed design of a little over 226 square feet. Given the area of the sides and bottom of the excavation containing the filter media, and even assuming a T-time of 100 minutes/inch, the underlying soil should have been capable of absorbing almost eight times the daily design flow. Further to this, when these failed beds are opened, it is often discovered that the stone layer is totally blocked by biomat, while the filter media appears almost pristine.”

Here is a shortened version of my analysis: His original soil would have a percolation rate of 100 minutes per inch (very slow); a mound system would be the system of choice. If the percolation rate was actually how fast water would move through the soil, he is correct that there should be room under the bed to accept the effluent. However, all of us should realize the percolation rate is not a measure of the actual hydraulic conductivity of the soil but more of an index number.


What is the actual rate in soils he described? A lot less, on the order of 0.40 inches per day, which is equivalent to about 0.24 gallons per square foot per day. This means it will take 1,875 square feet of area to accept the effluent under the sand bed. Anything less than that will cause one of two things (or both) to happen: water backs up into the sand bed reducing oxygen levels and causing a buildup of biomat. This would make the sand and bed look bad as he describes. Or effluent breaks out the sides of the bed to the surface.

I also looked at the numbers he provided for their sand beds compared to what we recommend. We recommend loading our sand beds in mounds at no more than 1.2 gallons per day/square foot. If sand is loaded less than 1.2 gallons per day/square foot, good treatment is provided.

They are loading their sand bed at a rate equivalent to 1.85 gallons per day per square foot or at least 1.5 times our design numbers. Their bed is undersized by about 150 square feet. This combined with not adequately sizing the area of natural soil under the sand fill is a sure recipe for failure and not due to the biomat. The excessive biomat development he observes in the system indicates there is a problem but is not the problem. In a future column I will look more closely at biomat and pressure distribution.   


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