COMPOSTING 101

What is composting?
What can be composted?
Composting technologies
What's so good about compost?
EPA Region IV overview

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Except for the backyard and on-farm, where all materials are generated and composted by the owner, most composting operations are required to be permitted by state regulatory agencies and operated to meet standards known in the trade as the EPA 503[1] regulations.  While the EPA 503 was written for sludge management, in some states the 503’s PFRP (Process to Further Reduce Pathogens) and VAR (Vector Attraction Reduction) attainment requirements are now applied to all manufactured composts without regard to feedstock.   

PFRP/VAR requirements for static aeration and in-vessel are the same.  PFRP for the windrow method requires a longer processing time.   Temperatures required for pathogen kill also get rid of most weed seeds.  

Though there are as many variations on the theme as there are composters and many hybrid systems in use, composting methods fall within three broad categories:

A.  Windrow.  By far the most common and most visible technology, windrow composting can be least expensive with respect to capital, equipment, and labor requirements, but it also needs the most space, has the longest processing time (to meet pathogen and vector reduction targets), and works best with simple formulations like wood chips combined with sludge. 

With this method, feedstocks are blended and piled in the shape of a pyramid at least six feet high and about 12 feet at the base (width = height x 2).  A windrow can be as long as space permits.

Windrows are turned periodically, using hand tools or equipment, to relieve compaction and replenish oxygen.  Frequently turned windrows mature[2] faster, and it is possible to produce decomposition rates that rival forced aeration systems.  But that also pushes up the cost.  Windrows meeting PFRP requirements have also met VAR:

PFRP: Temperatures of 55 degrees Celsius (131°F) or higher must be maintained for 15 days or longer, during which time the windrow is turned at least five times.

B.  Static aerated pile.  Instead of turning piles to inject oxygen, some composting operations force air through a static pile.  Typically, the admixture[3]  is placed on perforated piping connected to blowers set to force air up, pull air down, or both.  The piping may be rigid or flexible. The composting mass doesn’t need to be turned, because a properly prepared pile distributes air evenly and insures uniform biodegradation.  The rate and volume of air moving through the compost pile is controlled to keep the environment within the ideal range for microbial populations. 

Forced aeration is a good choice when composting materials with high moisture content or many different types of feedstocks in the same batch.  It’s more expensive than windrow composting, but offers more control over the composting process and turns out an immature product in 14 days.

PFRP: Temperatures of 55 degrees Celsius (131°F) or higher must be maintained for at least three days.   VAR:  14 consecutive days with the temperature over 40 degrees Celsius (104°F), averaging over 45 degrees Celsius (113°C).

C.  In-vessel.  As the name implies, in-vessel (also en-vessel) composting takes place inside some type of container equipped with equipment to stir or agitate the mix to keep it aerated, or the entire unit may tumble like the rotating drum in a clothes dryer.  This makes for rapid decomposition and, therefore, In-vessel is generally regarded as the fastest composting method.

In-vessel systems also work well for high moisture materials.  But high capitalization and operating costs make in-vessel systems expensive, and they are not widely used for large scale operations.  However, small tub units are popular with college cafeterias and other food service institutions since they are self-contained, fit tidily into a corner of the parking lot, and only need an extension cord to make them work.  Many backyard composters also choose in-vessel composting, using barrels or other devices that can be rolled or turned (manually or electronically) to speed up the composting process.

PFRP: Temperatures of 55 degrees Celsius (131°F) or higher must be maintained for at least three days.  VAR:  14 consecutive days with the temperature over 40 degrees Celsius (104°F), averaging over 45 degrees Celsius (113°C).

D.  Other methods worth mentioning:

(1) SHEET COMPOSTING. For small volumes of grass, sawdust, wood chips, newspapers, leaves and like materials, sheet composting is an easy and economical choice.  It’s called composting because a human decides the where of biodegradation, but beyond that, Mom Nature is in charge all the way.  

Spread a layer of the material on a natural garden path, between beds, or on top of a mulch bed … then leave it alone.  For light materials (i.e, straw, shredded paper) dampen and top with a heavier material to keep it from blowing away.  Sheet composting works well when tucking permanent raised beds in for the winter.  Come spring, the compost layer is ready to fork into the soil.

A word of caution:  this is a cold composting method and will not reach the temperatures required for weed or pathogen kill. Do not use hay or other heavy seeders unless you want their seeds to germinate where you spread the stuff, and do not use raw materials that would be inappropriate for backyard composting.

(2) VERMICOMPOSTING. The use of earthworms to turn waste into a soil amendment, known as vermicomposting, is not an aerobic process and, therefore, not true composting.  Instead of microbes in the composting mass, the gut of the worm does all of the work during digestion.

Vermicomposting is a favorite of children and backyard enthusiasts, and it’s great for households without the time, space, and/or energy for a compost pile.  One can vermicompost in the kitchen or on a patio, and some folks use vermi “septic tanks” to biodegrade pet torpedoes.

But the jury is still out on whether vermicomposting will ever make it into the big leagues of waste management.  Those in the know say worm composting meets the goals of PSRP (Process to Substantially Reduce Pathogens) for biosolids (sewage sludge), but not PFRP.  Therefore, vermicomposted sludge may be restricted to Class B land application, whereas PFRP produces a Class A (exceptional quality) biosolids product for unrestricted end use.[4]  Worm farmers who want to distribute product commercially may need to piggyback the vermicycle with another composting method that meets PFRP and get a state permit.

To play it safe, home vermicomposters can follow use guidelines for Class B[5] land application if using the worms for cleaning up after pets or composting other materials not usually recommended for backyard composting:

·  Use samples of the soil and finished compost to determine appropriate agronomic loading (application) rates.

·  Don’t graze animals on land until at least 30 days after application.

·   Wait 14 months between the time of application and harvest if the vermicompost touches the part of the food crop that will be eaten.

·   Wait 20 months after application before harvesting food grown below the land surface if the vermicompost lays on the surface for four months or more;  38 months if the vermicompost was incorporated less than four months after application.

·   Don’t apply to land with high public exposure (parks, ball fields, etc.) unless you can keep everyone off of it for at least a year.   The waiting period for application sites with low potential for public exposure is 30 days.

 (3) COMPOST TEA.  Easy to make and easy to use, compost tea is prepared just like any other tea and used for watering or foliar feeding. The old-fashioned method is simple: 

Using a old pantyhose, cheesecloth, or another finely-meshed material, bundle a quantity of compost and drop it into a container (glass jar, plastic garbage can or 55-gallon drum[6]) of water to steep for several days. Recommended ratio of compost-to-water is 1:5. 

Give it a stir every now and then to keep oxygen supplies up.  After several days, decant the liquid.  Unfortunately, compost tea made with this method must be used almost immediately (within five hours).

Experts recommend the addition of an aquarium aerator to the container to speed up the brewing time (18-24 hours) and extend the shelf life (up to 15 hours).  The motor on the aerator must be sized to match the container and powerful enough to keep the solids circulating while infusing air.   Kits are available, too.

With either method, stop stirring/aerating and allow the solids to settle before decanting the tea.   Return the solids to your compost pile. 

 Composting technology recommendations: