Milk Disposal in Anaerobic Digesters: Some Things to Think About

Michael Best

Milk across the country is being disposed as supply chains are disrupted by the economic shutdown in response to COVID-19.

While milk and food production are exempt from various “Safer at Home” orders, many of the institutions and restaurants to which significant food sales occur are not, resulting in mounting supplies of milk produced 24/7. Bossie has no spigot.



National, regional and state trade associations have called for immediate action by the U.S. Department of Agriculture (USDA) and Congress to address the issue. Meanwhile regulators are providing farmers with guidance as to the safest way to dispose of milk if confronted with that circumstance. There can be no greater disappointment for a farmer than disposing of the very product she has worked her entire life to produce.

One of the disposal options include disposing of milk via anaerobic digesters, be they on-site, at food processors or other “community digester” locations. Indeed, milk contains potential energy content far exceeding that of manure – the usual substrate that feeds digesters. As a result, the disposed-of milk has great potential to create value in the form of renewable natural gas, which in turn can generate revenue from power production or various environmental subsidies, including California’s Low Carbon Fuel Standard (LCFS) or the Federal Renewable Fuel Standard (RFS).



But not so fast. Before putting milk in digesters, farmers need to think through a few important considerations. These include:

  1. Putting milk into the digester substrate mix, and taking it out again down the road could cause big technical issues: Digesters substrates must be carefully controlled in order to maintain the microorganism balance in the digester. Introducing too much high-energy milk can cause microorganism die-off, or imbalance in the microorganism mix. If those things are avoided, other problems can result when milk is removed from the substrate stream. Put simply, the ramp up and ramp down of milk introduction into the digester needs to be managed carefully and scientifically.
  2. If your digester is producing gas to generate RFS credits, putting milk in the digester will change the type and value of those credits. Gas produced exclusively from manure generates credits known as “D3 RINS” under the RFS Program. But, when you add food sources in addition to manure, the digester loses its D3 RINS qualification, and may be eligible only for “D5 RINS” under the RFS, which are generally only about half as valuable as D5 RINs at today’s prices. Impact on RINS needs to be considered. For now, the U.S. Environmental Protection Agency (EPA) has taken the position that qualification for D3 RINS is solely based on the cellulosic content of the digester feedstock. However, EPA is expected shortly to release guidance for industry regarding enforcement of this distinction during the ongoing milk disposal crisis.

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