Homeowners hear about all of the new onsite products and technologies that have come online over the last decade and they naturally have questions about how systems operate. There are more and more references to oxygen in the system, which can be confusing for homeowners. For the most part, they don’t understand it. But they have been told it’s important, so they want to make sure they have it.

When customers ask questions, it’s a perfect time to pass on information about what actually goes on in onsite systems, including explaining the importance of having “good” soil that has not been disturbed and keeping tank maintenance up to date.

MOST ARE GRAVITY SYSTEMS

The vast majority of residential systems still start with a septic tank and end with a gravity-fed set of trenches. In this scenario, oxygen in the system is in the soil itself. The septic tank, as the name indicates, is anaerobic, or without oxygen. So liquid in the tank or in the effluent passing to the trenches is septic, or without oxygen. So here is a story that can be told in varying ways.

As septic tank effluent flows into a soil treatment system (most commonly a trench), it moves vertically through the trench media to the soil, where treatment begins. A biological layer, the biomat, is formed by anaerobic bacteria in the trench that secrete a gluey substance to anchor themselves to the soil or rock particles. Biomat forms first along the trench bottom. As liquid ponds in the trench, biomat forms along the soil surfaces on the sidewalls. When fully developed, the gray-to-black slimy biomat layer will vary in thickness from about 3/16 of an inch (5 mm) to 1 3/8 inches (30 mm). The thickness increases and decreases with changes in wastewater quality and quantity, the growth and decay of microbes, and the amount of oxygen in the soil.

Flow through a biomat is considerably slower than flow through natural soil, allowing unsaturated conditions to exist in the soil beneath the trench. Unsaturated soil has pores containing both air and water so aerobic microorganisms living in the soil can effectively treat the wastewater as it travels through the soil system. In unsaturated flow conditions, wastewater flows around the soil particles. In saturated flow conditions, wastewater flows through the soil pores. Unsaturated flow increases travel time of effluent through the soil, ensuring it has sufficient time to contact the surfaces of soil particles and microorganisms.

A developed biomat reaches equilibrium over time, meaning it remains at about the same thickness and the same permeability if effluent quality is maintained. The biomat and the effluent ponded within the trench are in anaerobic conditions. Organic materials in the wastewater feed the anaerobic microorganisms, which grow and multiply, increasing the thickness and decreasing the permeability of the biomat. On the soil side of the biomat, beneath the trench, oxygen is present. So conditions allow aerobic soil bacteria to feed on and continuously break down the biomat. These two processes occur at about the same rate so the thickness and permeability of the biomat remain in equilibrium.

BIOMAT FACTORS

If the quality of the wastewater leaving the septic tank decreases because of failure to regularly pump out the septic tank, more food will be present for the anaerobic bacteria. This will cause an increase in biomat thickness, decreasing its permeability. If saturated conditions occur seasonally in the soil outside the trench, aerobic conditions will no longer exist. This will prevent aerobic bacteria from breaking down the biomat. Under these conditions, the biomat will thicken, reducing its permeability and the effectiveness of treatment.

A number of factors affect the development of the biomat, including:

Soil temperature: Colder temperatures should cause more biomat development. So there will be more development in areas like Minnesota that have colder soil temperatures. In areas of higher soil temperatures, biomat development will be less as long as the systems are installed shallow. Temperatures drop in deeper trenches.

Soil moisture: Wetter soils should cause more biomat development. Along with permeability concerns, this means systems should be installed as shallow as possible to avoid limiting soil conditions. Other significant limiting conditions include shallow bedrock and dense soil layers restrictive to water and air movement.

Soil aeration: Aerobic soil conditions should decrease the intensity of biomat development. This is one of the reasons why aerobic conditions in the soil are so important. The most permeable parts of a soil for both water and oxygen occur in the upper horizons, another reason to keep systems shallow and why the KISS principle (keep it simple) is a guiding installation factor.

Wastewater quality: Pretreatment of wastewater to remove much of the BOD and TSS beyond typical septic tank effluent values should decrease the intensity of biomat development. This is why maintenance of the septic tank is important. Extra pretreatment, such as a media filter prior to discharge to the soil treatment area, should decrease the intensity of biomat development even more.

Wastewater loading approach: Gravity distribution, such as what occurs in conventional systems, should cause more biomat development, while pressure distribution with dosing and resting cycles should decrease the intensity of biomat development, particularly at low loading rates.

Wastewater loading rate: Higher loading rates should cause more biomat development since more solids and nutrients are added to the soil. In cases where the organic loading rate contains high levels of fats, oils and grease, the ability to supply oxygen through the soil to aerobic organisms often can’t keep up with the waste strength. This demands the use of additional pretreatment and introducing oxygen elsewhere in the system to effectively treat effluent. For homeowners, use of a garbage disposal dramatically increases the organic loading component. So to avoid excessive biomat development, they either need to avoid using the appliance or increase septic tank maintenance.

EDUCATIONAL TOOL

Using this as a primer for gravity systems, you are now ready to answer that homeowner’s question. In the future I will look more closely at pretreatment technologies and where oxygen is involved in those treatment processes.