Generating energy by burning foodMay 16, 2013
In an era when a large number of Americans suffer from “food insecurity,” the title of this post sounds perverse. However, there is a Catch 22 in the US food production and consumption pipeline: According to the Natural Resources Defense Council (link opens a pdf), as much as 40% of US food production is wasted before it can be eaten or donated. The fact of the matter is that produce, milk and meat have short shelf lives that, once passed, make them inedible. Such wastage is a problem everywhere, but is especially pronounced in the US because our agricultural sector is so productive.
One large supermarket chain has come up with something of a solution: Kroger has built a large plant in Southern California that uses a closed-loop anaerobic digester system which reclaims its waste food to produce water, fertilizer and enough energy to provide 20% of its distribution center’s needs. A long article in the LA Times describes the process:
Several chest-high trash bins containing a feast of limp waffles, wilting flowers, bruised mangoes and plastic-wrapped steak sat in an airy space laced with piping. Stores send food unable to be donated or sold to the facility, where it is dumped into a massive grinder — cardboard and plastic packaging included.
After being pulverized, the mass is sent to a pulping machine, which filters out inorganic materials such as glass and metal and mixes in hot wastewater from a nearby dairy creamery to create a sludgy substance.
Mike Vriens, Ralphs vice president of industrial engineering, describes the goop as a “juicy milkshake” of trash.
From there, the mulch is piped into a 250,000-gallon staging tank before being steadily fed into a 2-million-gallon silo. The contraption essentially functions as a multi-story stomach.
Inside, devoid of oxygen, bacteria munch away on the liquid refuse, naturally converting it into methane gas. The gas, which floats to the top of the tank, is siphoned out to power three on-site turbine engines.
The 13 million kilowatt-hours of electricity they produce per year could power more than 2,000 California homes over the period, according to Kroger.
Excess water from the digester is pumped out, purified and sent into the industrial sewer. Leftover sludge becomes nutrient-rich organic fertilizer, enough to nourish 8,000 acres of soil.
The so-called closed-loop system was developed by Boston start-up Feed Resource Recovery and offsets more than 20% of the distribution center’s energy demands — all without producing any pungent odors.
The program helps Kroger reduce its waste by 150 tons a day. The trash otherwise would have been sent to Bakersfield to be composted, hauled away six times a day by diesel trucks traveling 500,000 miles a year.
Trash cogenerationshould be a much greater part of our national energy mix – it is a renewable resource. The Germans have shown that it can be done cleanly and efficiently and even increase the amount and value of recycling. Unfortunately, trash burning plants from the 70s and earlier were dirty and inefficient, and environmental policies and attitudes that were set in place 40 and 50 years ago remain entrenched today, even though they should have been superseded by technological progress. The good news is that private companies are leading the way where governments are unable; let’s hope that some day our political “leaders” can follow them.