OSTP completes analysis of CEC concentrations within on-site septic systems at MNDOT rest areas

by Jack Distel, Sara Heger, Sondra Larson, Dan Wheeler and Jessica Doro

septic field

Example of sampling site. The monitoring well is down gradient from septic mound with poles marking locations for soil

An analysis of contaminants of emerging concern (CEC) within on-site treatment of human waste identified the prepotency of such systems to process CECs or transport them into the environment.  The systems sampled ranged from trench,at-grade, and mound septic tanks among five respective sites and one septage land application site.  All septic systems treated public rest stops managed by the Minnesota Department of Transportation (MNDOT).  Septage was sourced from a variety of public facilities and private businesses.  Public restrooms are heavily used and thus provide a good representation people’s CEC use and environmental impact, compared to a single-family home system. Researcher chose a diverse distribution of individual CECs and treatment technologies. Examples of CECs chosen include antibiotics, chronic disease prescriptions, veterinary medicine, stimulants, hormones, antihistamines, pain and fever relief prescriptions, and a plasticizer.    

On-site treatment of human waste greatly affects how CEC finds its way into the environment.  How on-site treatment systems handle influent CECs is important for future system design and use.  The sampling methods of this analysis allowed a comparison of CEC concentrations from a system’s influent sewage/septage to those found in the treatment area soil and then the groundwater.  All CECs were analyzed following standardized laboratory procedures.  General waste water characteristics (e.g. biological oxygen demand and total phosphorus) were also sampled and compared at all sites.


Concentration of all CECs and sources based on use categories.

There were 58 individual CECs tested, split among two main groups based on criteria of laboratory analytics.  Among the 58 CECs, 13 were not found in any samples.  Sewage samples, across all treatment types, had the highest prevalence of CEC occurrence.  Groundwater had the second most CEC occurrences. Soil had the fewest CECs detected.  The occurrence pattern was similar to the distribution of concentrations.  Sewage and groundwater CEC concentration, in milligrams per liter, were similar.  Soil was at an order of magnitude lower.  This trend represents the potential ability for soil to either bind or breakdown CECs to a point of non-detection.  Furthermore, it illuminates the ability of CECs to be hydraulically transported through a system and into the environment with minimal depletions in concentration.      

Due to the potential risk posed by CECs in the environment, it is imperative to understand the abilities of on-site treatment systems to effectively process CECs.  This analysis, on the occurrence and concentrations of CECs within on-site treatment of human waste, builds a foundation of observations that can be used to prompt further studies.

In conclusion, here are the main take-aways of this study:

  • On-site sewage treatment practices tend show promising removal rates of CECs 
  • No treatment type seemed to be more or less adequate at processing CECs    
  • Within soils, CECs did not seem to occur prevalently or at large concentrations 
  • Concentrations and occurrences of CECs in groundwater were not uncommon and were reflective of the CECs within the respective sewage influent