Coffee and Soda Down the Drain: What are the Impacts on Septic Systems?

An OSTP study looks at how these beverages change the pH and other levels in an onsite system

Coffee and Soda Down the Drain: What are the Impacts on Septic Systems?

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The Onsite Sewage Treatment Program (OSTP) at the University of Minnesota Water Resources Center conducted a brief study to understand more about the effect that coffee and soda beverages have on the functionality of a septic system.

Many septic professionals can agree that putting coffee grounds into a septic system is a bad practice and could negatively affect its function. Coffee grounds will not break down in a septic tank; they will build up over time and might cause the tank to have to be pumped more often. Also, because they are so acidic, they can compromise the pH of a tank. If the contents of a septic tank become too acidic, it can create an unhealthy environment for the bacteria that contribute to the healthy ecosystem of the tank and help break down waste. 

For this study, the OSTP was interested particularity in restaurants, convenience stores or gas stations that are serviced by a septic system and what the possible impacts could be to the system when coffee and soda are dumped down the drain.

Five different types of product were used for this study:

  1. Black coffee
  2. Iced coffee
  3. Coffee with a sweetened creamer
  4. Coffee with regular half and half added
  5. Coke (regular)

These products were selected to represent a wide range of products offered at gas stations, convenience stores and restaurants. Of these beverages, it would seem most likely that more regular soda or black coffee would be put down the drain most often. A common scenario might be if someone pours a soda and decide they want another type, so they pour their first choice down the drain. Similarly, someone pours a coffee, and they decide they want another kind, or they pour off the top to add creamer. There are also convenience stores and restaurants that regularly dump coffee to assure freshness. It can be assumed from these inferences that most of the ingredients going down the drain would come from either the regular soda or the black coffee products.

As can be seen in the table below, the concentrated beverages all had levels significantly higher than septic tank effluent. Generally black coffee had the lowest levels and as there were cream or flavor additions to the coffee the waste strength increased. Iced coffee is a product that is increasingly being offered at convenience stores which was found to have very high levels. The pH of the soda was very low. 

For septic system designers this data indicates it is important to characterize the facility to determine if there are self-serve soda or coffee dispensers and if soda and coffee are regularly refreshed. The CIDWT forms for Analyzing Wastewater Treatment Systems for High Strength Waste and Hydraulic Loading available on the UMN website at are useful in performing these characterizations.

After this assessment or while providing service, it is worthwhile to discuss with the facility owner or manager if changes can be made to the equipment or management practices to reduce the amount of beverage waste entering the system. During design or management for all existing facilities, several samples should be taken and analyzed, as the influent levels are likely to be quite variable. 

A full version of the report from this study can be found at

About the author
Sara Heger, Ph.D., is a researcher and educator in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota, where she also earned her degrees in agricultural and biosystems engineering and water resource science. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is the president-elect of the National Onsite Wastewater Recycling Association and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system design, installation, maintenance and operation by sending an email to


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