A Biomat Control Plan Will Give System Users Peace of Mind

From the tap and toilet to the drainfield, onsite professionals should properly manage wastewater flow to ensure system longevity

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We continually receive questions about the biomat and how to best control the formation to provide both treatment of septic tank effluent and maintain the Long-Term Acceptance Rate in systems with gravity distribution. To manage the biomat and systems most efficiently, the approach needs to encompass use and operation of the entire system.

It is not enough to simply say to maintain a BOD level in effluent exiting the septic tank to 170 mg/l or less and everything will be all right. While that number may be key, a lot can be done in all parts of the system to ensure it lasts for a long time.


Everything starts with the user inside the house and the water-use habits of the residents. As we have discussed before, BOD levels can be reduced by keeping solids out of the wastewater stream (not using a garbage disposal). Besides adding solids, water is added to the system to move those solids through the piping to the septic tank. Another kitchen-related problem is the addition of grease down the drain from cooking. It can be wiped off or decanted for composting or disposal in the garbage.

In addition, keeping the use of anti-bacterial soaps and cleaners to a minimum ensures the biology in the tank remains healthy and able to break down solids and reduce BOD. Bacteria in the septic tank can be negatively affected by anti-bacterial additions reducing treatment efficiency in the tank, which translates directly to an increase in BOD exiting the tank and contributing to faster biomat development and formation of a more resistant biomat.

The next obvious place to reduce solids and BOD from being delivered to the soil treatment part of the system is to have regular maintenance of the septic tank and any other sewage tanks in the treatment train. Pay attention to development of and depth of sludge and floating scum and set maintenance intervals to maintain BOD levels in the effluent to less than 170 mg/l. Installing effluent screens, while not impacting BOD levels, prevent larger solids from moving downstream and plugging the infiltrative surface. Plugged screens act as an early warning to clean the tank.

If usage in the house is such the septic tank cannot adequately treat the raw sewage to reduce BOD levels, additional pretreatment can be added. Primary ways this can be done is through addition of a media filter (sand, peat, fabric) or an aerobic treatment unit. The selection of a pretreatment device depends on usage in terms of total flow, frequency of flows and timing of peak flows. These pretreatment devices can reduce BOD levels in effluent significantly slowing biomat development.


In the soil treatment component, biomat can be managed in a variety of ways depending on system configuration, size, and how effluent moves between parts of the system. Using trenches to provide the soil treatment area instead of a bed provides more soil area in contact with sewage effluent to help control biomat development. Trenches afford an easier opportunity to rest parts of the system out of use for periods of time to manage biomat accumulation. This is more to do in bed systems without digging up parts of the system.

Several studies have shown that resting parts of the soil treatment system can have positive impacts on acceptance of sewage effluent. Rest periods of a few months results in increased infiltration rates by 70 to 280%. While this always needs to be balanced with the treatment aspects of the soil being used, it indicates we can control biomat development through system management and increase system longevity.

Our preferred method of managing trenches is to use drop boxes to distribute effluent between trenches. This is called sequential distribution, where effluent does not move out of one trench until the trench is operating at capacity. This distribution mechanism allows taking a trench offline by simply capping off the outlet pipe from the drop box to the trench. It allows effluent to bypass the trench and move onto the next without disturbing any other parts of the system.


Drop boxes and inspection ports in each trench provide access points to evaluate whether the trench is being used and the level of ponding in the trench, indicating biomat development. It allows the homeowner or system professional to observe how each trench is doing and to be able to switch the use when needed.

There are options to manage or shut of parts of the system. One is to have a dual drainfield setup, where one-half of the system is used and then switched periodically to the other with use of a switching device or valve. This is particularly effective for replacing or adding to existing systems that are struggling due to biomat development.

Controlling biomat development by managing the total system from the user through the soil treatment area can pay dividends to the client by extending system life and providing them with peace of mind that the system is ready for use.


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