Compost is something that we hear about a lot in the Bay Area, and here, many municipalities even pick it up curbside along with trash and recycling. But what really is compost, and why is it so good for the environment? Here are the answers to Frequently Asked Questions about composting:

What is composting anyway?

Composting is a way of disposing of organic materials (which naturally biodegrade) in a safe, productive, and environmentally-friendly way.

What is produced, and what is it used for?
“Compost” is the actual substance that is produced from the organic materials that are disposed of in a compost bin. It is a soil product that is very productive and that farmers can buy to use on their own land.

How is compost made?

In the spring of 2014, I visited the Newby Island Compost Facility in Milpitas, California, which is where all of Stanford’s compostable materials go to be processed. This facility is the only one of its kind in the South Bay, and it processes over 160,000 tons of compost every year. It was really cool to see the many stages of the process from food waste, compostable containers, and yard trimmings to a pure, homogenous soil product (that actually smells good!) in just over two months.

In general, the composting process has three steps. First, the material is ground up by machines. Then, it is placed in another machine that carefully controls the temperature and oxygen to facilitate the growth of microbes that start decomposing the mixture. After a while, it is moved outside into “windrows”, which are essentially round heaps of soil product where microbes continue to break down the material until it is finished.

Landscapers, farmers, private contractors, and even individuals can go to the Newby Island Compost Facility to buy the soil for use on their own land. People know that this is the best soil around, and even though the facility produces so much soil, it is still not meeting demand.

Newby Island Compost Facility


But wouldn’t compostable materials still biodegrade in a landfill?

Actually, they wouldn’t. Landfills do not provide the right environment, with the right type of soil, exposure to natural elements like light and air, and absence of toxins, for organic materials to naturally biodegrade. That’s why there’s a separate bin and separate facilities for compost versus trash.

What can I put in the compost bin?

  • Food waste
  • Tea bags (without the string and tag)
  • Plain paper (no ink or coloring on them)
  • Paper towels and napkins
  • Paper cupcake/muffin liners
  • Yard trimmings and plants
  • Food containers and utensils that say “compostable” on them
  • Pizza boxes

For municipal composting, be sure to check the specific guidelines in your city.

What is not allowed in the compost bin? 

  • Food containers that do not explicitly say “compostable” (this includes containers that say “biodegradable” – these containers are often not compostable). In general, compostable containers and utensils must be certified by BPI.
  • Plastic bags, wrappers, and other trash
  • Human and pet waste
  • Recyclable materials like glass, plastic, and metal
  • Chemicals
  • Batteries

Compostable versus biodegradable?

Please make sure any plastic you put in the compost bin has “compostable” written on it. Same goes for disposable utensils. Beware of “eco-sounding” companies whose containers don’t actually have “compostable” on the packaging – these are NOT compostable. “Biodegradable” is not compostable.

Other green-sounding names that are not compostable:

  • Biocompostable
  • New generation of plastics
  • Made from corn
  • Taterware, Spudware or Greenware
  • Renewable raw materials
  • “Feels and look like plastics for the most part”
  • ASTM D6400
  • Polylactic Acid (PLA)

What happens if there is contamination in a bag of compost?

If, when a bag gets to the compost facility, there are too many non-compostable items in it, the have to throw the whole bag away because it is not economical for them to sort through it by hand. And if there is even one piece of glass in a bag of compost causes the whole bag to have to go to the landfill for safety and contamination reasons.

What if there is no municipal composting where I live?

If the city doesn’t pick up your compost curbside, consider creating your own compost heap at home. You can learn how to make one here. For this option, you will need to limit the items you put in the compost heap to food waste and paper products. And depending where you live and where you want to out the compost heap, you might want to cut out dairy and meat products from the heap as well.