Windrow Composting Process

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 What's happening in the windrow | What affects the composting process

What's Happening in the Windrow?

Active composting stage

  • When the windrow is formed microorganisms consume oxygen (aerobic decomposition.)
  • As oxygen is used up aerobic decomposition slows down.
  • Aeration is provided by mechanical agitation to add oxygen and replenish pore space within the settled medium.
  • Large pore spaces promote air movement through the windrow.
  • Temperature increases due to microbial activity in the windrow.
  • They rapidly increases to 40 - 60°C and are maintained for several weeks.
  • As aerobic decomposition slows the temperature drops off.
  • Temperature will fall if the oxygen decreases too much.
  • Temperatures can get too high thus killing the microorganisms.
  • Aerating the windrows help to control the temperatures.
  • Evaporation is the windrows primary source of heat loss, by maintaining the moisture levels between 50 and 60 % and agitation the windrows temperature will be under control.
    Curing stage
    • When the aerobic decomposition has slowed the curing phase has begun.
    • Oxygen usage is so slow the windrows don't need to be agitated anymore.
    • As the temperature drops to that of ambient air the compost is done.
      What Affects the Composting Process?

      Windrows without sufficient oxygen have a degradation speed that is slowing down. Oxygen must be added to ensure aerobic decomposition.

      Active composting takes place between 40 - 60°C. If the temperature is allowed to go above the range microorganisms begin to die slowing the composting process. If the temperature is lower than this aerobic decomposition slows.

      Composting time depends on many factors within the windrow. If a proper moisture content, carbon to nitrogen ratio, aeration, oxygen content, and temperatures are maintained the shortest possible compost period is ensured. The intended use of the compost will have an effect on the composting time.

      Nitrogen, phosphorus, carbon, and potassium are required by the organisms and plants. A carbon to nitrogen ratio of 25:1 to 30:1 is ideal for composting. An insufficient ratio allows excess nitrogen to be released to the atmosphere. A higher ratio requires a longer composting time.

      Water is required to allow chemical reactions, transport nutrients, and allow microorganisms to move within the windrow. The moisture content should be maintained between 50 - 60%. If levels are too high the water takes the place of the oxygen slowing down decomposition. When levels are too low the microbial activity slows.

      A mixture of materials with a pH level between 6.5 - 8 have a better chance of being effective than materials outside the range. But, if the material has a high nitrogen content then pH levels above 8.5 increase the ammonia loss.

      Particle size
      Particle size and structure affect the natural aeration of the windrow. The smaller the particle size the greater the aerobic decomposition because of the increased surface area. Particles that have good structure prevent the settling of materials which reduces porosity.

      For more information
      Virginia Nelson , E.I.T.
      3000 College Drive South
      Lethbridge, Alberta T1K 1L6

      Phone: 403-329-1212
      Fax: 403-328-5562

      For more information about the content of this document, contact Virginia Nelson.
      This document is maintained by Nicole Huggins-Rawlins.
      This information published to the web on June 5, 2002.
      Last Reviewed/Revised on May 22, 2014.