We always spend time at workshops discussing what happens in the soil when septic tank effluent is applied by gravity. We’ve had recent questions about the role suspended solids may play in the formation of the biomat and, ultimately, the plugging of soil pores that reduces the infiltration rate into the soil, causing hydraulic failure. So this is a good time to review how the biomat forms and why it can be a good thing but also can be a problem.  

When septic effluent is introduced to the soil surface, a layer begins to form along the bottom of a drainfield trench consisting of organic material contained in effluent and the living and dead colonies of soil microorganisms. These microorganisms — in the presence of oxygen in the soil — consume and break down the organic material in effluent. The layer of organic matter and microorganisms makes up the biomat.

As the layer forms at the interface with the soil surface, it creates unsaturated flow through the soil with two results: It slows down the flow of effluent through the soil, and it puts the sewage in contact with oxygen in the soil pores and aerobic organisms to break down and consume the organic material, providing treatment.

BIOMAT BASICS

The thickness and resistance of the biomat to flow depends on the initial effluent strength and the original soil conditions. As we have indicated numerous times in this column, most current sizing numbers used for determining drainfield size are based on formation of this biomat. The numbers assume a well-maintained and operating septic tank with BOD values less than 170 mg/L and TSS values less than 60 mg/L. If these values are consistently exceeded, the biomat will be thicker and more resistive. If water use is more than the soil will accept through this thicker biomat, hydraulic failure will result.

So system longevity is directly related to the size of the system and the amount (flow) and organic loading of the wastewater applied. How development of the biomat is managed and controlled will, to a large extent, determine how long the system will last. Properly managed and maintained systems can last 40 years or longer.

We received this comment recently: “My experience with surface ponding or backing up of septic systems often has led to discovery of a biomat as the suspected culprit, a result of accumulated suspended solids over time. This has caused me to consider better ways to reduce TSS.”

We would agree that strategies to reduce TSS delivery to systems will have a positive impact on system longevity. A comment about suspended solids: Most solids are organic in nature and will break down in the soil, but some may consist of materials that have difficulty breaking down in the soil (coffee grounds) or are inorganic in nature and will not break down (plastics in cleaning products). These will permanently block soil pores, reducing infiltration rates and potentially causing hydraulic failure.

REDUCING SOLIDS

So, what are some of the strategies that can be employed to reduce suspended solids? It starts in the house. System performance and longevity will improve if users limit the addition of materials that are difficult to break down, such as coffee grounds and grease, and reduce water usage. Limit the use of cleaning products that contain small plastic beads or are antimicrobial and could affect bacterial action in the septic tank.

Increasing septic tank capacity, which increases effluent retention time in the tank, can lead to additional settling of suspended particles, reducing reliance on effluent screens to catch the particles or break down in soil. While typical effluent screens will not catch the microscopic plastic particles, they will catch other larger solids, preventing them from making their way to the drainfield.

The importance of having a regular maintenance schedule cannot be overstated. Effluent exceeding BOD or TSS limits will contribute to lower system life. Regularly pumping and cleaning tanks and making sure effluent screens are in place and operating are key to preventing problems.

Many localities have instituted mandatory inspection and pumping requirements. In areas that do not have these requirements, it is key to work with the homeowner to impress upon them the importance to have the system periodically evaluated. They need to understand the necessity of checking effluent screens for plugging, and doing so more often than the septic tanks are pumped, to prevent sewage backing up into the house. Hopefully they see the value and are willing to enter into a maintenance agreement.

Finally, additional pretreatment — more than adding septic tank capacity — is a good idea. Better a media filter be plugged than the final dispersal and treatment area. Media can periodically be replaced if it becomes plugged at less expense than replacing the drainfield. We would welcome comments from service providers about their observations and problems encountered due to suspended solids.