What is anaerobic digestion? How has General Mills’ Murfreesboro Plant leveraged it to save 25,000 gallons of diesel fuel, produce 1.6 megawatts of electricity and return 2,000 acres of farmland back to agricultural use? Sustainable Woman and Energy Engineer Leslie Marshall discusses.

Leslie, you’re the Energy Engineer at General Mills. What does a typical day entail for you?


As the energy engineer for the site, I monitor all of the projects or initiatives that deliver electricity, gas, water, sewer, or solid waste savings. I act as the manager on some of those projects, but others are executed by members of facility team. On a daily basis, I’m completing tasks specific to my projects as well as monitoring the progress of other projects managed by others. After they’re executed, I verify that they’re delivering the calculated savings.

I also make sure I’m spending roughly 10% of my time on technical development … investigating new technologies to see if they’re applicable to our facility, learning about best practices for secondary utility management such as compressed air and steam, etc.

Tell me about your Murfreesboro plant. How big is it? How much electricity, water, and gas does it take to run?

The General Mills site in Murfreesboro, TN has two facilities that make Pillsbury and Yoplait products. Combined, they two plants are about one million square feet.

We use about 95 million kWh of electricity, 240,000 MMBTUs of gas, and 366 million gallons of water (and sewer) annually in order to operate our site. According to the EIA (U.S. Energy Information Administration), the average U.S. household uses about 11,000 kWh a year. Our electricity consumption is the equivalent of about 8,700 residential homes.

A few years ago when you started producing Greek yogurt (a popular consumer product), it caused you to make major changes to the way you processed your waste. Why?

635878682065493690-yoplait-greek-100-black-cherryMost large food processing plants have a waste water pretreatment (WWTP) plant to break down food solids in their process water before sending it to the municipal WWTP for final treatment. Before we started producing Greek yogurt, we used an aerobic bacterial digestion process to treat our process waste water. Once we started making Greek yogurt, our aerobic digester did not have the capacity to treat the increased amount of concentrated dairy whey waste due to its high solids and BOD (Biochemical Oxygen Demand) content. We had to collect it in tanks, load it to trucks, and haul it away to agricultural land to be plowed into the ground. That disposal was costing us well over $1,000,000 annually and occupying over 2,000 acres of farmland.

You’ve found a way to produce electricity from all that Greek Yogurt waste, in fact 10 percent (1.6 megawatts) of the plant’s electrical needs comes from the leftover whey… How?

We use anaerobic digestion. It’s beneficial because the specific bacteria used in the process produce methane (along with other gases.) That biogas can be captured and used to power a biogas-engine-powered generator (a 2,000HP CAT engine in the case of our generator) to produce electricity.

So, what is an anaerobic digester and how does it work? How much power does it generate gas does it produce and in what form?

Anaerobic digestion uses self-renewing bacteria cultures that consume the organic solids and neutralize the BOD content while producing a biogas byproduct.  The amount of biogas produced in an anaerobic digester depends on the amount and type of food solids processed through it. It just so happens that acid whey from dairy solids has a high biogas yield, with a high methane content, which makes it a good candidate for anaerobic digestion.


There are other sustainability benefits of anaerobic digestion (involving C02 emission reduction, a drop in diesel fuel use, water recycling, etc.) — can you highlight those benefits for us?

Before, it took over 15 tanker truck hauls a day to remove the acid whey from our site. Once that process was eliminated and we began using anaerobic digestion, 25,000 gallons of diesel fuel was saved annually.

94% of the whey waste is water. When we process it though our own WWTP, the bacteria turn the dairy solids into sludge that is separated from the treated water. Our pretreated discharge water goes to the municipal WWTP who treats it further before returning it directly to the watershed. In addition, the farmland is not suitable for agricultural use when whey waste is being continuously plowed into it. We were able to return 2,000 acres of farmland back to agricultural use when we switched to anaerobic digestion.

Finally, the 1600 kW that we are able to generate is equivalent to 10,000 tons of CO2 emissions assuming that the offsetting electricity which we no longer need to purchase had been made at a coal power plant. This helps General Mills in its goal to reduce greenhouse gas emissions by 2025.

General Mills has made a public commitment to tackle climate change. Why now?

As Jerry Lynch, General Mills’ VP and chief sustainability officer explained, changes in climate impacts the ingredients we use to make our products. Providing products to our consumers when they want them and in the amount that they want them is critical to our business. We have a vested interest in doing what we can to be a part of solving the climate change crisis.


How has this forced challenged the General Mills team to be more creative and less wasteful in their approach to business and with their energy usage to process product processing?

Energy efficiency technologies and the skill set required to understand, utilize, and optimize them is very different from the technologies and skills required to develop and produce food. One of the things that General Mills did to show their commitment to energy reduction was to dedicate a team of energy professionals assigned to developing a reduction strategy, providing ongoing consumption analysis, executing projects, and influencing behavioral changes.

General Mills has always valued innovation as part of doing business. To be creative, we just needed to apply already existing core values to our focus on energy management.

What are 3 pieces of advice that you would give someone looking to become an energy engineer?

  1. Have passion for energy reduction. What we’re doing is important to our future, and how well we do it directly impacts our community. Plus, some of these technologies we get to work with are very neat. It’s fun to be on the cutting edge of technology.
  2. Enjoy learning, and make sure to put aside time for personal and technical development. Energy efficiency technologies are constantly changing, and energy professionals around the world are discovering better ways to manage their facilities to save money. Read energy magazines, network with other professionals in your area, attend energy conferences, and seek help from experts to conduct audits. If a solution doesn’t work for your plant, be sure to understand why. Circumstances can change (i.e. tariffs, production volumes, etc.) that can make technologies viable for a given application.
  3. Teach others. It’s impossible to implement an energy reduction program by yourself. Remind your team that the work they’re doing is important for the company and for the community. Be sure that they dedicate time to their technical development as well. Share the positive results with them and with others. Everyone who uses energy (which is everyone) needs to know the great work the energy and/or facility team is doing and the positive impact they’re making.

aaeaaqaaaaaaaabcaaaajgfknzvhzjc0ltmzodktndrmns05yjvllwi4odljzwrmmjc5yqLeslie Marshall is an Energy Engineer at the General Mills Murfreesboro facility and a member of the General Mills corporate-wide team that is committed to reducing energy usage at its North American facilities. She manages site utility usage data in order to identify facility equipment and production lines that consume the most energy, determine energy savings opportunities, troubleshoot equipment that perform outside of baseline limits, and target capital projects that will reduce energy consumption. Other project work includes using infrared studies to reduce heat losses through insulation improvements, reducing compressed air losses, recovering waste heat, improving lighting energy usage, and recycling millions of gallons of hot water. Her facility recently installed and is centerlining an anaerobic digester to convert waste food solids into a combustible biogas to be burned in a new generator which, when fully functional, is projected to generate an electricity equivalent of more than 10% of the power consumed by the entire plant. In addition, waste heat recovery from the system will reduce site gas use by 10%. Leslie holds a Bachelor of Science degree in Mechanical Engineering from Cornell University.


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