Compost FAQs

What is Composting?

Composting is the natural process by which organic materials (e.g. food waste, yard waste, plants, etc.) decompose into a nutrient-rich soil amendment known as compost. The difference between composting and natural decomposition is that composting is purposefully managed. A banana peel thrown on the ground will decompose, but will not create compost. However, the same peel placed with other organic material in a pile will break down into compost.

There are a number of different ways composting can be achieved. There are 3 common types:

Aerobic composting: This is the type of composting that occurs in backyard compost piles, other outdoor composting operations and certain enclosed composting systems (often called “in-vessel composters”). Organic matter is broken down by bacteria naturally found in the soil. The term “aerobic” means that this process requires oxygen (just like aerobic exercise!); specifically, the bacteria that decompose the material require oxygen to do their job. Compost piles must be occasionally turned or stirred up to ensure that the bacteria are getting the oxygen that they need to break down the organics without producing bad odors (we’ll get to that next…).

Anaerobic composting: This is the type of composting that occurs in enclosed composting systems called “digesters” or “anaerobic digesters.” Organic matter is broken down by anaerobic bacteria, meaning that the bacteria work without oxygen (the opposite of the term ‘aerobic’). These systems are kept enclosed for 2 reasons: the presence of oxygen can be better managed indoors and one of the byproducts of this process is methane, an odorous gas. While anaerobic composting smells terribly, the methane generated can be used as a fuel source. If an aerobic composting operation isn’t managed properly and the bacteria don’t receive the oxygen they need to thrive, the compost pile will “go anaerobic,” begin creating methane and STINK! An outdoor composting operation that smells badly is a poorly managed composting operation for this very reason.

Vermicomposting: This is an aerobic type of composting that relies heavily on worms to break down the organic matter instead of just bacteria. One of the benefits of this system is that it can be done indoors! “Vermi” is Latin for “worm” and a vented bin with some shredded paper, food waste and worms is all that is required to begin a vermicomposting system. Mary Appelhof is a great resource for more information about vermicomposting.

Does Compostable and Biodegradable Mean the Same Thing?

No. Uh-uh. Not in your life. No-sir-ee-Bob. Here’s a great explanation of the difference.

Well, biodegradable is still good for the earth because these items break down in a landfill, right? That’s what the vendor told me…

WRONG! Landfills are designed to prevent items from breaking down. Learn more.

Does U-M Compost?

The short answer: YES! Read on for the long answer…

Yard waste composting: U-M Grounds has composted yard waste from campus for many years. This is an aerobic composting operation and the finished compost is used throughout campus. Additionally, they have experimented with the use of ‘compost tea’ as a natural fertilizer. For more information on Grounds’ use of compost tea, check out these articles:

Landscape chemical reductions planned for ‘normal’ spring

An In-depth Look at Using Compost Tea in Lieu of Chemical Fertilizers

Food waste composting: U-M Waste Management Services(WMS) has offered a ‘pre-consumer’ food waste composting program since 1997. Pre-consumer food waste, also known as prep waste, is unwanted or unusable foods that have not been served to people and is generated during meal preparation. This includes fruit & vegetable trimmings and peelings, spoiled produce, egg shells, stale bakery items, etc.

In 2012, WMS began offering a ‘post-consumer’ food waste composting program. Post-consumer food waste is unwanted, leftover food that has been served to customers as well as compostable disposables. Plate scrapings, apple cores, half-eaten sandwiches, etc. are all collected and composted. Additionally, certified compostable disposables can be collected for composting. These include compostable plates, cups, flatware, bowls, napkins, etc. Many items claim to be ‘compostable’ or ‘biodegradable,’ but without certification, will not break down in a compost pile. All campus dining halls and unions participate in this program.

Where Does the Food Waste Go and Does It Come Back to Campus?

U-M does not manage the food waste compost site. We have partnered with WeCare Organics, the private operator of the City of Ann Arbor’s compost site, to compost this material for us. Once the compostables are collected from building loading docks by WMS staff, the material is taken to the City’s compost site and dumped. It is then mixed with yard waste and formed into long piles, known as windrows, where air can circulate and aid in the aerobic decomposition process. After the material has fully decomposed, the finished compost is tested to assess its potential as a fertilizer or soil amendment.

Composted food waste does not often come back to campus. The majority of our compost needs are served by the compost generated from U-M yard waste at our Grounds facility. Occasionally, we need to purchase additional compost from WeCare Organics, so there is a chance that it contains food waste from U-M. However, as the Ann Arbor Compost Site accepts organics from so many places, we can’t guarantee it.

Why Isn’t All Food Waste Generated at U-M Composted?

Contamination is the primary reason all food waste is yet to be collected for composting on campus. Contamination is a HUGE concern when it comes to the composting process. While many assume that diverting wastes to compost is just like recycling, it couldn’t be farther from the truth.

Here at U-M, we send our mixed recyclables to a Materials Recovery Facility (MRF) where they are sorted, baled and sold. If non-recyclables are mixed in, it’s not that big of an issue because they are sorted. However, compostables are not sorted. Since compost sites typically deal with yard waste, there hasn’t been a need to sort the incoming material. As a result, any non-compostable items mixed in remain that way through the composting process and result in compost with bits of trash in it. No one wants to buy compost with trash in it to put on their crops, garden, etc. As a result, we need to ensure that there is no contamination in the food waste we send to the compost site.

Collecting contaminant-free, or clean, compostables is relatively simple in places like prep-kitchens, dining halls and catering operations where trained staff are the ones deciding where the waste should go. In areas where the campus community is generating compostables, like coffee shops, fast food restaurants, and events, this gets to be much more difficult. It will take a full culture shift on campus before collecting food waste from these types of locations is feasible.

How Can I Compost at My Event?

Visit the Zero Waste Events program.

How Can I Start Composting at My Building?

We have minimum requirements for participation in the food waste composting program. You must meet all of the following:

  • Have a loading dock or dumpster area where the food waste collection carts will be emptied from.
  • Have a plan to deal with contamination of the carts. This can include limiting the food waste collected to prep-waste, plate waste or event waste.
  • Agree to meet with representatives from the Office of Campus Sustainability prior to participation to address operational concerns.
  • Participants who have ongoing contamination issues will be removed from the program.

The first step is to email us at to express your interest in participating in the program.