Back to the Basics

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A couple months ago in Reno, Nevada I presented at the Onsite Wastewater Mega-Conference – mega because it was a joint conference for the National Association of Wastewater Technicians and the National Onsite Wastewater Recycling Association.

Much of my work is focused on centralized wastewater treatment, but this event was centered on decentralized treatment (septic systems). Since I recently wrote a whitepaper on how it is okay to have a disposer on a septic system, it was the perfect venue for further discussion on the topic.

After listening to the plenary speakers Robert Siegrist and George Tchobanoglous, as well as Andrew Sawyers of the USEPA, the conference theme was clear – “managing a precious resource, water.”

Emphasis was placed on new terminology and a paradigm shift in how we view decentralized systems. As wastewater professionals, we were encouraged to adopt the lexis of treatment systems instead of septic systems and “water recycle and reuse” versus disposal. We were also encouraged to consider the optimum use of carbon, in which food waste and sewage can be used to make energy and fertilizer.

Most people I spoke with didn’t express concern about disposers on septic systems. However, there were questions at the conclusion of my presentation on the impacts of the additional 20-30% load and FOG, especially where new systems are now requiring effluent screens on the tanks. This might be resolved by simply requiring a two-compartment tank, as is the case in Minnesota.

I came away from the conference with thoughts regarding the fundamental science that is overlooked with disposers. Food waste is 70-90% water, and disposers were designed to quickly and conveniently grind the remaining solids into fine particles that can be sent through plumbing pipes without clogs.* By grinding food waste into very small particles, disposers exponentially increase the surface area available for microorganisms to break down the organics.


By adding water, the slurry starts to undergo hydrolysis, fermentation and acidification. Soon-to-be published research from Cornell shows that this happens very quickly.  Even when the slurry is held in a tank. Mix slurry with the biology of a sewer or a septic tank, and it’s easy to see how the food waste rapidly breaks down.

Perhaps this is why treatment plants in areas where 90% of homes have a disposer are not experiencing organic overload and why there is no evidence that disposers are the cause of septic system failures.

*ASSE standards require disposers to grind so that all particles are less than one-half inch. Most particles are actually between one-eighth and one-quarter of an inch.






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