In some regions, urban populations are denser than a decade ago. So, we could say urban density is variable. By contrast, the density of a “substance” is a physical property that does not vary. Density of a substance is a known quantity expressed as mass per unit volume. For example, the density of water at 400C, is 1 mg/ml. Why is this relevant?
It is relevant because food also has a density of 1 mg/ml. So, if food waste is 70-90% water to begin with, and it has the same density as human waste and water, there should be no issue with wastewater professionals concerned about “solids” settling out in sewers and causing blockages and overflows. Properly designed and constructed sewers will transport this material very efficiently to the treatment plant without causing any issues. Make sense?
The danger here is some people cling to their well-intentioned perceptions versus scientific fact. This dogmatic attitude can prevent our water resource recovery facilities with anaerobic digesters from taking advantage of a great opportunity to boost biogas and move towards energy independence.
Utilities of the Future
According to a database of information on U.S. infrastructure, there are 1,236 publicly owned treatment works utilizing anaerobic digestion to process sewage sludge into biosolids. The wastewater industry is rebranding itself into water resource recovery facilities – makers of clean water, energy and fertilizer. Just last year the National Association of Clean Water Agencies specifically recognized 61 of these facilities as “Utilities of the Future.” Of these 61 plants, 26 are actively seeking high strength waste to feed directly into their digesters. These feedstocks to boost biogas include food waste.
Certainly, delivering food waste directly to digesters is more efficient than letting it go through the entire plant. This is because only about half would be separated and processed into biogas through primary clarification. The other half would end up being processed in the energy intensive aeration process. This is one of the reasons why the Sustainable Food Waste Evaluation by WERF (2012) determined codigestion of hauled waste delivered directly to wastewater treatment plants results in the lowest greenhouse gas emissions. However, if codigesting food waste improves biogas production, even if only half ends up in the digester, these municipalities should consider encouraging the use of residential and commercial disposers to add more feedstocks to their digesters and boost biogas production even more. There are two reasons why municipalities should at least have a conversation on this opportunity.
First, food waste sent to a water resource recovery facility means it avoids being landfilled. Second, even though less energy is generated than if it were piped directly to a digester, there is still a net energy gain according to research by Leverenz and Tchobanoglous. More importantly, the high carbon to nitrogen ratio will help the plants remove nutrients from their effluent, reducing the negative impacts of eutrophication on our streams, rivers and lakes.
Unfortunately, in my discussions with wastewater professionals, the perpetuated myth that food waste clogs sewers almost always supersede all rational thought, so I would like to remind my esteemed colleagues and friends that in the end, shouldn’t pure science trump preconceived notions?
Food for thought/
I once heard that had the food waste disposer not been invented in 1927, now would be the perfect time. Why? Well, because of the convergence of two environmental megatrends – landfill diversion of organics and resource recovery at wastewater treatment plants, where municipalities produce clean water, and sometimes fertilizer and energy.
Think about it, food waste is at least 75% water, and with the trend of keeping organics out of landfills, it makes sense to leverage disposers, already present in 60 million households in the U.S. (52% of all homes), to manage this waste as a liquid rather than a solid. Right?
Unfortunately, it is difficult to even have a civil conversation with staunch environmentalists that favor composting because of four universal myths and misconceptions about disposers. Never mind that composting is only available to 2.74 million households across 198 cities across the U.S., or less than 2% of all U.S. citizens! I sometimes get exasperated in trying to have a lucid conversation on this subject because many times I don’t even get beyond the myths of environmentalists. For example:
Myths 1 & 2 – Water and Electricity Use
“Disposers can’t be good for the environment – they use water and electricity.”
Ugh! To have a rational discussion, we must move past the debate on water and electricity use associated with disposers, moot points and already discussed ad nauseum in this blog. The perception is disposers use a lot of water simply because they use water. The truth is it is not exorbitant. In fact, it is negligible and totals only 1% of the total household usage. Electricity usage is almost laughable at a mere 3-4 kwh or $0.50 per year. And all treatment plants produce clean water, which is why some of them have adopted the name water reclamation plant! Here’s a little factoid: for every ton of food waste diverted from a landfill to a treatment plant, we recover about 165 gallons of water, because food waste is around 75% water. Enough said.
Myths 3 & 4 – Impacts to Plumbing and Sewers and Overloading of Treatment Plants
“Sewers and treatment plants were made for sewage, not food waste, and mixing food waste with sewage contaminates a potential resource; food waste is just not good for these systems. Besides, sending food waste to the treatment plant disallows composting?”
We inevitably come to the concern that not all wastewater infrastructure is the same, and that is very true, but the only difference between food waste and sewage solids, is one has been eaten first! Since food waste and human waste is essentially the same density, sewers designed to transport sewage are perfectly designed to transport finely ground food waste.
And yes, many treatment plants actually do make fertilizer instead of landfilling their biosolids. According to research gathered by the Northeast Biosolids and Residuals Association, about 55% of all biosolids are beneficially reused. We could do better, but responsible environmentalists should help debunk the myths associated with their safety to increase beneficial use of biosolids. Land application of biosolids is sustainable and completes the circular economy.
As far as energy production, of the 16,000 wastewater treatment plants in the U.S., over 1,200 have anaerobic digestion. This means that only about 8% of all plants have the ability to create energy, either heat or power, or both. Although this seems like a relatively small number of all treatment plants, these plants account for around 50% of all wastewater flows in the U.S. In other words, most major metropolitan areas have anaerobic digestion employed to help manage sewage solids.
Not too long ago, we dumped all our raw sewage into streams and rivers – Cincinnati did so until the late 1950’s! Even after building wastewater treatment plants, some cities like NYC still dumped their sludge into the ocean up until the 1980’s. But cities are moving forward with sustainability efforts. Just this week it was reported that citizens in a Milwaukee program can pay $12.75 to participate in a pilot composting program. Instead of trying to invest in municipal composting programs, which requires new collection and processing systems at very high costs, city leaders should consider leveraging wastewater treatment plants and the existing base of disposers already present in homes.
I wonder if a U.S. city will ever consider subsidizing the purchase and installation of disposers?
By the way, the City of Milwaukee issued a press release in 2009 encouraging residents to use their disposer, and the Milwaukee Metropolitan Sewerage District also encourages their use. In essence, why not feed a disposer and starve a landfill?
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.