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.
Bad odors often elicit gnarled faces, loud incomprehensible reactions, and even gagging. This is because we are created in such a way to instinctively react to things we should avoid. Garbage, and specifically food waste, provokes this reaction because the decay of organic material results in emissions such as ammonia, sulfides, mercaptans, and butyric acid, as well as bioaerosols. Both types of emissions are to be avoided, because there are potential health implications from exposure to either of them.
Ew! What’s That Smell?
Decay of organic material requires microorganisms like bacteria to convert complex molecules and break them down into simpler ones like carbon dioxide and water. Unfortunately, during the process of decay, some malodorous chemicals from the garbage bin are telling us that not only should we have gotten rid of that rancid stuff sooner, but also that there are potentially harmful bacteria lurking in there!
Recent research from Open University in the UK investigated the release of bioaerosols from trash and revealed that after four weeks, bacteria types increase, resulting in the release of higher levels of endotoxins. The research did not indicate a significant increase earlier than two weeks. While the authors mention more study is needed, they also recommend conducting risk assessments for waste collectors since there is a known dose response relationship for endotoxins.
A few cities in the U.S.A. recently started biweekly trash pick-up. However, putrescible waste must be collected weekly, so where cities invest in green bin programs to collect organics, they can justify less frequent, biweekly trash pick-up. Nevertheless, it is not a good idea to let your organics stay in the trash for a lengthy time. Most everyone has experienced the smell of rotten garbage in much less time than two weeks. That being said, most trash is picked up much more frequently than every four weeks, but if you want to end the odors, why not avoid bioaerosols or endotoxins altogether and use your garbage disposer!
Can WWTPs go from Energy Drain to Energy Gain?
As many of us know, the wastewater treatment process uses a lot of energy, and with increasing economic pressures on wastewater utilities, facility managers are looking for high strength waste to help them become energy neutral — even net energy producers. The creation of energy would not only provide relief from normal operational expenses, it could help demonstrate sustainability to the surrounding communities.
One such utility, Milwaukee Metropolitan Sewerage District, cleans billions of gallons of wastewater every year at two reclamation facilities that serve 1.1 million people in 28 communities. MMSD wants to achieve energy neutrality by 2030 and they see food waste as an integral part of it.
This isn’t another fantasy. In the first quarter of 2016, MMSD received over 200,000 gallons of high strength food waste slurry at one of their two facilities.
The slurry is virtually contaminant-free and readily biodegradable, which enables water resource recovery facilities like MMSD to make more biogas to produce onsite energy. As a result, they are able to help offset operational expenses by reducing their dependence on purchased electricity.
The slurry is high in solids compared to sludge pumped into municipal digesters – about 10-12% TS. But it is pumpable and contains very high volatile solids — over 90% on average. The high COD content and readily biodegradable composition is a result of the food waste being held in a storage tank at the waste generator for 7-10 days.
Plants say, “Keep it coming”
The treatment plants already accepting food waste slurry say, ‘We want more of this!’ It has very high volatile solids content (more than 90 percent) and it’s rapidly biodegradable. The methane potential testing we’ve done in-house shows that the organics are converted to methane in five to 10 days. It doesn’t take the normal 20- to 30-day detention time that a digester requires to process wastewater sludge.
If you’re thinking it’s too good to be true, you should take a look at this video. Feel free to drop me a note if you have questions or need convincing you’re not dreaming.