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.