Following the Flow of Urban Organics

Upstream Emphasis: Setting the Table for Food Waste Diversion

With society’s strong appetite for addressing the issue of food waste, this article focuses on defining the players in urban organic waste streams, and the drivers that lead to organic waste policies and practices. It assumes that all upstream efforts have been made to follow the EPA’s organic waste hierarchy by reducing organic waste at the source and directing any remaining edible food towards human consumption.

Downstream Technologies: Anaerobic Digestion and Composting

Also relevant is a general understanding of downstream processing technologies. Anaerobic digestion has two primary flavors: “high solids” which accommodates “stackable” feedstocks (think yard trimmings laced with pasta); “low solids” digestion accommodates “pumpable” feedstocks (think slurries of food scraps mixed with fats, oils and grease (FOG), wastewater treatment plants, or manures). Composting, the notable aerob ic cousin of digestion, converts a range of yard trimmings and food scraps into nutrient-rich soil products. Given this context, let’s eat.

Organic Waste Generators

Fork it over: know thy audience. Wherever there are people, there is organic waste. When thinking about the flow of these wastes, it’s helpful to define the waste generator and characterize the waste composition. Waste generators are typically grouped as follows:

  • Residential: Homes and properties generating yard trimmings and kitchen scraps.
  • Commercial: Grocery stores, restaurants, hotels and other business generating larger volumes of pre- and post-consumer food scraps as well as FOG.
  • Institutional, Commercial, and Industrial (ICI): Concentrated populations such as schools, prisons, campuses, and hospitals; or food-related industries such as food processors, breweries, and dairies, all generating large volumes of organic waste.
  • Other: Multi-family dwellings (housing with four or more units) and events (festivals, conferences) have their own unique characteristics.

Key Characteristics of Organic Waste

Now that you know the audience, it’s time to recognize what they eat. In terms of the composition of wastes generated, not all rinds are created – or collected – equally. The following four drivers shape local policies and practices.

  • Energetic densities of food waste: The same foods that make humans fat help anaerobic bacteria fart and burp. Those farts and burps, known as biogas, can make electricity, pipeline grade natural gas, or vehicle fuel. If you want to optimize biogas generation, be sure to get the donuts to digestion and leave the leafy greens for other technologies.
  • Volumes of food waste: Large volumes of waste generated in a few locations are typically easier to manage than lots of smaller generators. But it’s a balance: you don’t necessarily want to get all of your eggs from one basket.
  • Geographic clusters: Dovetailing with volumes, every hauler knows that a collection route in a same region is ideal because it efficiently shares the fixed collection costs associated with starting up a collection truck and driving it around town.
  • Capture rate versus contamination rate: This can be tricky. The generator and the processor typically do a dance. The processor’s ideal “clean” load of feedstock leads to lower processing costs (removing and disposing of contaminants), increased safety (associated with removing said contaminants from equipment), and optimal product quality (improved biogas yields and fertilizer products). However, the generator might value the ability to include contaminants. This balance is typically struck via acceptance specifications and tip fees.

With this background, you will better understand the organic waste conversations in your community.


Editor and Author’s Note: Content from this post originally appeared in the February 2018 issue of BioEnergy Insight as a contributed article from Harvest.