Septic Care: Explaining the Effects of Detergents

Help out customers who want to take better care of their onsite system with some recommendations for septic-safe detergents

Septic Care: Explaining the Effects of Detergents
Foaming in an aerobic treatment unit.

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Explaining to customers why their ATU looks like the photo at right takes patience. Here’s some background info to help you out. 

Dishwashing detergents come in many forms: cartridges, gel, liquids, pacs, powders and tablets. These products may contain bleach, enzymes and rinsing aids. Some dishwashing detergents still contain phosphorus. Phosphorus is removed from wastewater by being chemically bound to minerals and held on exchange sites on soil particles and it is chemically bonded to iron, calcium and aluminum in a process called adsorption.

If the soil treatment system is functioning correctly, and proper setbacks are maintained from surface waters and vertical separation from periodically saturated soil, problems from phosphorus movement to surface water or groundwater should be minimal. Phosphates that pass through the septic system due to improper design can enter surface water, causing very high growth rates of algae. The algae growth, in turn, suffocates fish and other life in rivers, streams, lakes and ponds.

The main ingredient in liquid soaps is water, while in powders it is alkaline salt such as sodium carbonate. The main active ingredients are detergents. Many products contain thickening and stabilizing agents. Other ingredients include surfactants, hydrotropes (which allow for concentrated formulations), preservatives, fragrances, perfumes and dyes. Surfactants typically cause foaming or suds in water. Anionic surfactants have poor anaerobic degradability in the septic tank and may inhibit hydrolysis at 10 mg/L. Slow hydrolysis may cause the accumulation of solids in septic tanks and therefore may necessitate larger system design for adequate wastewater treatment.

Problems in septic systems
Surfactants do not typically pose a risk to surface or groundwater, unless under saturated conditions, because of a strong tendency for soil sorption. Hydraulic conductivity and soil structure can be negatively affected at concentrations over 30 mg/L. Because sorption leads to an accumulation of anionic surfactants, it is recommended that wastewater applied to soil have surfactant levels not exceeding 1 mg/L. In 2014, seven systems in Minnesota were sampled to determine their level of surfactants and all were above 1 mg/L with a range of 1.3 – 9.4 mg/L. Therefore, the amount of dishwashing soap should be kept to a minimum. Excessive surfactants are often easy to see in aerobic treatment units due to addition of air into the system.                              

Many of the chemicals in dishwashing products that contain fragrance are the same chemicals in cigarette smoke, and yet there are no regulations for the fragrance industry. About 95 percent of the chemicals in fragrances are synthetic compounds derived from petroleum. For these reasons, fragrance (unless natural) should be avoided in dishwashing soap.

Recommendations for your customers
Here are some tips to pass along to homeowners to help them take better care of their onsite system.

Dish soap criteria:

  • Plant-based surfactants (or just plain soap, like Dr. Bronner’s)
  • 1,4-dioxane-free
  • Phlathate (synthetic fragrance)-free
  • Dye-free
  • Petrochemical-free
  • Glycol-free
  • Phosphate-free
  • Caustic-free

The Environmental Working Group (EWG) is a nonprofit with the goal of educating and empowering consumers to make safer and more informed decisions about the products they buy and the companies they support. EWG has a great website that rates products including dishwashing detergent:

The brands making the list are Earth Friendly, Seventh Generation, Biokleen, Sun&Earth, Green Shield, The Honest Co., Nature Clean, and Nice!.

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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