Compost Bulking AgentIn the food waste composting business, our constant companion, our nemesis, our savior, our persistent crutch and limitation is: bulking agent.  I’d love to not be dependent on it but… bulking agent really is what makes the food waste composting world go round.  If a food waste composting operation is in trouble, bulking agent is usually at the heart of the matter.

What is bulking agent?  Bulking agent is a carbon-based material that adds structure (or bulk) to your compost pile.  Classic examples include: wood chips, wood shavings, saw dust, dry leaves, shredded landscape waste, shredded paper, shredded cardboard and animal bedding.

Almost 20 years ago, I did an experiment.  I took a trash can and began filling it with food waste.  Once that can started to get full, I got a revolting lesson in the importance of bulking agent.  By itself, food waste turns into a kind of an unspeakable porridge.  I live and breathe composting and I can tell you this: rotting food waste by itself is a superb repellent.  It gives visceral meaning to the term, “putrescible.”

Now, add bulking agent to your food waste and, voila, you have a manageable, even somewhat pleasant composting process.  Rotting food does stink but bulking agent is vital to keep those odors under control.  The key is the switch from anaerobic to aerobic decomposition.  Aerobic decomposition smells MUCH better than anaerobic decomposition.  Aerobic decomposition smells like a bunch of foods all cooking at the same time.  Anaerobic decomposition smells sour, acidic, putrid, …perfectly awful.

I’ve never liked the term bulking agent.  It sounds like an evil spy who causes his enemies to gain weight.  The name makes it sound like we are “putting on pounds”.  But what we are really doing is “bulking up”, adding volume to our compost pile.  That is a good thing (and also a liability, as we will see later).

Bulking agents do so much for hot composting, that it’s difficult to fully appreciate its value.

Here are a few of its virtues:

  • We know that bulking agent adds vital air space to the compost pile.  Aerobic microbes have to have air to breath.  They breathe in oxygen and breathe out carbon dioxide, just like us.  Food waste tends to have too high of a moisture content and inadequate structure for air space.  Bulking agent gives the microbes room to breathe.
  • You can tell a good bulking agent has air space in it because it is light in weight.  A good bulking agent has low bulk density (typically less than 600 lbs per cubic yard).  This means it holds a lot of air space.  Wood shavings are a classic example (at about 400 lbs per yard).
  • A good bulking agent is a dry material that effectively balances the high moisture content of the food waste.  Therefore, keeping your bulking agent dry is super important.  If your bulking agent is wet, you are adding wet to wet (wet + wet = WET, aka, “a mess”).
  •  A good bulking agent also provides available carbon to balance out the high nitrogen content of food waste.  If you have a food waste mix that is very wet and goopy, you would be wise to have a supply of saw dust on hand for quickly absorbing the moisture and balancing the high nitrogen in the food waste with lots of available carbon.
  • A good bulking agent provides some air space but not too much.  Imagine trying to compost food waste in a brush pile (tried it:)).  It doesn’t work.  Too much air space.  The compost cannot generate thermal mass to heat itself.  So particle size distribution is an important characteristic of an effective bulking agent.
  •  Bulking agent itself also acts as a biofilter.  Microbes living on the bulking agent literally “eat” odor compounds out of the air.  So adding bulking agent gives your compost pile an ability to control its own odors.  Have a smelly compost pile?  Notice what happens when you put a layer of bulking agent over the top.  Almost immediately smells are extinguished and bugs are driven away.  It’s almost like magic.

So, why is bulking agent a thorn in our sides?  Here are some downsides with bulking agent:

  • Bulking agent takes space, almost always more than we want.  By volume, recipes of 2 parts bulking agent to one part food waste are quite common.  So, in this case, about two-thirds the volume of your compost pile will be bulking agent.  That’s a lot!  So, for in-vessel systems, bulking agent takes up a lot of precious real estate.
  • Bulking agent isn’t always cheap, free, easy to get and readily available (fortunately, sometimes it is).  So one of our main challenges with our clients is helping them source a quality bulking agent that is easy and inexpensive and reliable.
  • Finding a high quality bulking agent, with all of the desirable characteristics isn’t always the easiest thing.  There are plenty of mediocre to bad options for bulking agent and they produce mediocre to bad composting results.
  • People stop putting the bulking agent in.  Probably the #1 reason institutional composting operations get into trouble is this: people stop adding bulking agent.  Kitchen staff add food scraps but not bulking agent.  Do this for two weeks with no bulking agent added, and you’ve got a recipe for a stinky, bug infested compost pile.

So here are my bulking agent tips for a happy composting operation.

  • Keep a ready supply of bulking agent on hand, directly adjacent to your composting operation.
  • Make sure that bulking agent is added to your compost pile on a regular basis.
  • Keep food scraps covered with a layer of bulking agent at all times, to discourage bugs and prevent nuisance odors.
  • Keep a supply of sawdust on hand for compost “emergencies”.  Nothing absorbs odors so quickly as dry sawdust.
  • If you are starting to run low on bulking agent, give yourself plenty of time to get more.  Once you run out, things can turn south in a heart beat.
  • Select your bulking agent carefully.  Your bulking agent should supply a nice combination of air space and available carbon.  A mulchy consistency is good.
  • Keep your bulking agent nice and dry.  Store your bulking agent under cover in rainy/winter conditions.

Much success in hot composting comes from following this time-honored maxim:  honor thy bulking agent.

Happy Composting,