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/