How to Evaluate an Onsite System for Remediation

Failure analysis is critical for successful remediation of a faulty onsite system.
How to Evaluate an Onsite System for Remediation
A system failing due to excessive ponding.

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What does the term “remediation” mean to you? It seems to mean something different to many people, including homeowners. Some property owners' thoughts may be based on a product that they could add to their systems that will solve their current septic problem. Some product manufacturers will market that if you add their treatment systems into an existing system it can help rejuvenate a failing system. The part of the septic system market focusing on remediation tends to focus on recovering soil treatment systems where the biomat has gotten too thick and is limiting the movement of wastewater into the soil, versus making a repair.

A major symptom of system malfunction is plugging or clogging of the soil’s infiltrative surface with a biological mat. This plugging often results in the discharge of inadequately treated sewage onto the ground surface or back into the home or establishment. It could also be indicated by an abnormal amount of ponding in the system. This biological mat is often referred to as a biomat and is a layer of biological growth and inorganic residue that develops at the infiltrative surface. Plugging or clogging of the infiltrative surface can result from a variety of physical, biological and chemical processes, including:                         

  • Physical processes: solids in wastewater (i.e. organic matter, inorganic solids, oil and grease) and fines in backfill or drainfield rock are trapped; the surface soil can be compacted during construction.
  • Biological processes: masses of microorganisms collect at the infiltrative surface.
  • Chemical processes: waste products of microbiological metabolism accumulate.

When evaluating a system for remediation, the depth of biomat should be considered. As the biomat gets thicker, the ability to increase the movement of water will be more challenging. The biomat typically has a low hydraulic conductivity. Soil pores can become filled over time with organic and inorganic residues; this clogging restricts the flow and subsequent infiltration of effluent into the underlying soils. Soil treatment systems can accommodate some degree of clogging. However, severe clogging can produce too large a reduction in the infiltration capacity into the soil, which causes the effluent to pond even more in the soil treatment component (trenches, bed, at grade or mound), which can result in sewage backing up into the house or discharging onto the ground surface. A variety of restorative actions may be proposed to improve the hydraulic performance of an organically overloaded system. Some of the causes are harder to remediate, particularly fines in construction materials and inorganic solids.

Remedial actions may be effective only after properly troubleshooting or diagnosing a system for potential factors that may have contributed to the malfunction. This failure analysis is critical to understand why a septic system is not functioning properly and to determine the root cause(s) of the problem (covered in Article 1). It is important to note that some systems should not be remediated, including systems that do not have the required separation to the limiting condition (seasonal water table, bedrock or other confining layer) and cesspools or drywells. These systems are not adequately treating wastewater due to the lack of aerobic soil conditions, and remediation alone will not solve the problem.

Any practice being considered to remediate a system should meet three basic performance standards for remediation technologies. The remediation practice should:

  1. Restore the infiltration rate into and through the soil below the infiltrative surface to remediate or mitigate the existing malfunction to accommodate the applied flows.
  2. Not result in harm to the septic system.
  3. Not contaminate groundwater (i.e. due to preferential flow, chemical additives or excessive loading).

In a subsequent article, specific remediation practices will be discussed including: reducing hydraulic loading, particularly peak flows, reducing organic loading, resting the system and adding air to the soil.

About the Author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association (MOWA) and the National Onsite Wastewater Recycling Association (NOWRA), and serves on the NSF International Committee on Wastewater Treatment Systems. Send her questions about septic system maintenance and operation by email to


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