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.
This is the time of year when we gather for a big feast. And afterwards can be a busy time for plumbers, due to the improper disposal of fats, oils and grease (FOG). Remember, fats can be liquefied with heat but solidify in plumbing and sewers after cooling. This can lead to serious issues – plumbing and sewer blockages. Please don’t use your sink, toilet, or disposer to improperly dispose of FOG. It can cause undesirable situations – sewer backups and expensive visits from the plumber.
And what about all those potato peelings and even the leftover turkey carcass from the feast? Here is an interesting new way to peel your potatoes and recycle the skins.
YouTube Video of Ingenious Method for Peeling Potatoes
Over time, potato peels have been named as the culprit in disposer-related sink backups, often by your mother and the plumber. Why? It’s not because potato peels are particularly difficult to grind. As often as not, it’s because people pack the disposer full with the peels of many potatoes and then turn on the disposer. Sometimes without turning on the water or running it through. Or, just as bad – not letting the water run afterwards.
Modern disposers have multiple grind stages, which help alleviate jams and clogs and make grinding even more difficult waste–like the turkey carcass–easy and fast. Yes, I am telling you that with an advanced disposer you not only don’t you have to worry about potato peels but you don’t have to worry about grinding a turkey carcass either.
My hope is that your holiday season is enjoyable and uneventful (at least with regard to kitchen backups) so here are some tips for using your disposer.
- Turn on the water.
- Turn on the disposer.
- Slowly add food scraps to the disposer.
- Leave disposer on until grinding is complete.
- Turn off the disposer.
- Let the water run a few extra seconds.
I’ll have the carcass demo in my next post!
Travel to and around the UK is my excuse for the long time between postings here. (I had never been there before so I had to at least check out St. Paul’s Cathedral and the Tower of London.) One of the reasons for my visit was participation in the European Wastewater Management Conference in Manchester where I shared a paper by Harold Leverenz and George Tchobanoglous on the energy balance and nutrient removal impacts of food waste disposers . To my surprise, audience questions focused on the same concerns I often address in other technical settings back home. One person claimed food waste ends up getting screened out at the headworks of treatment plants, which is untrue because the particles are much too small ‒ less than 3 mm ‒ the typical size of the smallest screens. Some questioned the use of disposers due to the potential impacts on collection systems, i.e. settling in sewers, the contribution to fats, oils and greases, and increased maintenance costs. But sewers designed to transport human waste are perfectly capable of transporting ground food waste. I took the opportunity to educate attendees about FOG and calcium soaps. It seems wastewater professionals still view blockages as grease when in fact they are something entirely different. I was struck how the issues and questions were the same that I’ve heard many times before, thousands of miles away at North American events, until I realized what’s more surprising is – I didn’t expect them.
The highlight of the trip came on the following day when I spoke at the Royal Society at an event hosted by AMDEA to discuss food waste disposers. The Royal Society was founded in 1660 and in centuries past, members included history’s most distinguished scientists including Christopher Wren, Isaac Newton and Charles Darwin. To speak in such a historical setting was quite an experience, to put it mildly! Speakers for the AMDEA event (pictured in order) included Paolo Battistoni from Marche University of Italy, Per Henrik Nielson from Odense, Denmark, yours truly, Douglas Herbison of AMDEA, Jonathon Mattsson from Lulea University in Sweden, and Per Andersson from Surahammar, Sweden.
Speakers of the AMDEA Food Waste Disposer Roundtable at the Royal Society.
During the event we discussed the use and impact of disposers; namely how disposers are helping communities like Surahammar and Odense meet waste diversion goals. But we also discussed more complex issues like the potential concerns of settling in sewers and particulate transformation of organics in sewers, as well as the social implications of whether disposers will help reduce overall “arisings” of waste – otherwise known as generation in the US. This was an interesting point of view that I will discuss in my next post. How about you? Has your work taken you to historic or significant places in history? I’d enjoy hearing from you.