What To Do With It?

The City of Milwaukee’s priorities for food waste reduction, set out in the Climate and Equity Plan, are influenced by the EPA’s hierarchy for managing food waste. In order of priority:

Edible wasted food should go to feeding people, then animals.
Remaining food waste should be sent to biodigestion or compost.
Sending food waste down the drain or to an incinerator is a last resort.

Quantities to handle:

11,550 tons of wasted edible food can be redistributed to humans
75 tons of wasted edible food can be redistributed to animals
17,650 tons can be managed with composting
112,735 tons will need to be managed through biodigestion

Facilities Required:

240,000 sq ft of food banks will be needed to redistribute edible food to people and animals
35 acres of composting will be needed to process a mix of food and yard waste
5 million gallons of digestion facilities will be needed to handle the remaining food waste

Capacity calculations:

Edible food demand from people: Feeding America tracks the number of meals needed by county to eliminate food insecurity on an annual basis. The USDA estimated the weight of a nutritious meal in the US.
Edible food demand from animals: Interviews with Feeding America identified shelter and rescue operations that they have been supplying and their unmet needs.
Composting: Windrow composting cannot handle more than a 1:1 volume ratio of yard waste to food waste. The 9,800 tons of compostable yard waste going to landfill in MKE sets the upper limit on the amount of food waste that can be composted using traditional methods.
MKE MMSD processing calculations: Large scale usage of kitchen disposals to process food waste would overwhelm the city’s current infrastructure and is not desirable. Planned increases to processing facilities could handle up to 3,000 additional tons of food waste per year through the sewer system, beyond population growth. MMSD also has a high-strength digester that can handle up to 2,000 more tons of food waste.
Biodigestion calculations: All other outlets limited by demand or capacity, biodigestion would have to take on the remaining food waste. Milwaukee’s energy demand far exceeds the maximum generation capabilities, so digestion is not limited in demand, though may have issues with profitability. Low-production inputs may need to be sent to vermicomposting facilities instead.
Vermicomposting: Using worms to process food waste is a bridge between composting and digestion - it can handle food waste material that would not be profitable for a biodigester in a smaller footprint than a windrow composter, and produces compost as an output.

Basic digester sizing diagram, care of Biocycle

Facility sizing calculations:

Feeding America’s Milwaukee facility was used as a baseline to determine the number of square feet of new facilities required for feeding people and animals - however, the total is likely larger, as smaller facilities are less efficient, but dispersion of them can improve distribution.
The 2017 SARE Composting Study reviewed composting facilities and processing capabilities, finding an average processing rate of 3,850 cubic yards of material per year per acre (larger facilities are more efficient than smaller ones).
Barring changes to the 2035 and 2050 plan, the previously cited figures set the maximum the MMSD can take.
Facility design guidance from BioCycle suggests that a 1 million gallon digester can process around 26,300 tons of food waste per year, given 80 tons of input per day and a 90% run time.
Vermicomposting can help balance Milwaukee’s equation, but we did not factor it into our sizing calculations due to its overlap with both digestion and composting