Net Energy Gain from Food Waste
During the course of advocating for disposers as a sustainability tool in managing food waste, and through study of a few decades’ worth of research, I discovered that a scientific gap existed that had not been adequately addressed. Mind you, this was as I neared the end of studying for my Masters degree so it was like a gift from the heavens. It provided a tailor-made subject for my capstone project while providing insight into something useful for my professional work. It was relevant to the drive for organics diversion from landfills and resource recovery at wastewater treatment plants. To be specific, it was, in a nutshell, a model to quantify the impacts of food waste on wastewater treatment in one specific and unstudied area.
Quantifying food waste BOD reaching aeration tanks
The project quantified how much Biochemical Oxygen Demand (BOD) from food waste reached energy-intensive aeration tanks at wastewater treatment plants. Wastewater Engineering lists the value at 20 grams of BOD per capita per day where disposers are used, but that number did not take into account how much decayed in sewers, or how much settled out during primary clarification. Wastewater professionals see these issues as critical for assessing how much food waste actually reaches the treatment plant, and if there is a net energy demand on the entire system – important in rendering judgment whether disposers should be viewed as environmentally responsible.
In 2010, research on the settleability of food waste was completed by Willie Gonwa and Symbiont Engineers. From that study, combined with what we know about particle sizes of ground food waste, I created two mathematical models to describe potential aerobic decay in sewers, and then validated the models by creating a third model using laboratory results from analytical techniques that can be described by this equation (t = time in sewers in days).
BODt = UBOD(1-e-0.39t)
So considering a scenario where the food waste took ten hours to reach the treatment plant, there would be about 15% aerobic decay of the BOD.
Finally, I developed an equation providing quantification of how much food waste BOD entering sewers (from household disposers) ultimately reaches secondary operations at wastewater treatment plants.
kg of BOD5 to Secondary Aeration = BODSA
BODSA = (kg of Food Waste) x (0.12 kg BOD5/kg of food waste) x (e-0.39t) x (1 – Primary Removal Efficiency)
Because primary removal efficiency varies at wastewater treatment plants, and research includes different values for food waste settleability, I created a table to look at three different levels. Even at the most conservative level of only 25% settleability of the food waste BOD, there is a net energy gain.
Net Energy Gain from Food Waste
The conclusion is that at all three primary removal rates, there is a net energy gain for sending food waste through disposers to a conventional activated sludge wastewater treatment plant using anaerobic digestion where the biogas is utilized for heat and power. Given the results of my capstone, using disposers to divert food waste from landfills to advanced wastewater treatment plants is not only environmentally responsible, where the biogas is captured for energy and the biosolids are beneficially reused, disposers are, indirectly, a fiscally responsible tool for municipalities.
4 Replies to “Net Energy Gain from Food Waste”
Very good work, Michael. Actually from the perspective of energy recovery, your thesis examined the worst case scenario, as it is unlikely that conditions in the sewer will remain aerobic for 10 hours. Also, the energy “credit” due to nitrification would further increase the net energy gain.
Thanks George, I appreciate your insight as always. With nutrient removal requirements increasing for treatment plants, I did look at the benefit of the carbon from food waste to offset the needs for external carbon sources, but the benefit for reduced energy demand from nitrification is something I would like to better understand.
These are very encouraging results indeed Michael! Excellent and very useful article and findings! My questions would be, “how much of the methane produced anaerobically in the sewer pipes doesn’t escape but makes it’s way into the wastewater plant digestor tanks, adding possible additional credits to the numbers in these models (making the pipes themselves effectively part of the energy producing system) and what benefit might we be obtaining from the aerobic decay of larger and more complex molecules in the pipes that are then availabe for more thorough decomposition once in the wastewater plant? I ask because Dr. Denecke in Germany (U Essen) as well as Dr. Karve at ARTI India have discussed with me models where an aerobic phase prior to an anaerobic phase can have benefits (where lignocellusic material is concerned particularly). i.e. if the breakdown products of pipe bacterial action are not escaping, where is the stored energy going — could it not also be adding to what is going on in the treatment center tank?
As far as methane production in the sewer that would make it to the treatment plant, it seems plausible that some gas would be solubiized but unless it would somehow get introduced back into the solids, not sure how it could be harnessed in the digesters. At ambient temperatures, methane is much more soluble than at mesophilic temperatures of digesters. It would likely be released to the atmosphere either in the pipe during turbulence, or from the surface of the liquid interface at the primary and secondary tanks.
Your second comment is even more intriguing, because time is indeed necessary for more complex compounds to break down. Lignin would have more time to decompose into a more suitable substrate for digestion, and this material is the most difficult of organics to break down. When organic compounds are at the simpler volatile fatty acid level, however, they will quickly be converted to CO2 and water in an aerobic environment, or methane in an anaerobic environment. This would be pretty difficult to model or analyze in the sewer environment.