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COMPOSTING

VIEWS: 19 PAGES: 36

									COMPOSTING



          David T. Brown
     Dept. of Tourism and Environment
              Brock University
Composting:
     Composting:


The controlled biological
decomposition of organic
       materials
              Composting

   natural biological process, but for rapid
    composting and consistent quality,
    environmental conditions must be
    controlled

   end product (compost) bears little
    resemblance to original wastes from which
    the compost was made

   typically dark brown to black in colour,
    with crumbly texture and earthy odour
         Finished compost
   humus-like, resembling rich topsoil
   resistant to further microbial decomposition
             Composting
   typical volume reductions in excess of
    50% of the original volume of the waste;
    effective & useful waste diversion strategy

   good compost is devoid of organisms that
    may be harmful to human health
              Uses of compost
   high organic matter content => valuable soil
    amendment

   may be used as low-grade fertilizer to supplement
    plant nutritional needs

   may be used to condition heavy clay or mineral soils

   promotes proper balance between air and water in
    soils

   aids water infiltration, absorption, and ion exchange
    in soils
        What can be composted?
   any waste material with a high organic matter
    content is a potential candidate

   used for centuries to stabilize human and animal
    wastes

   used more recently for:

       sewage sludges

       industrial wastes (e.g. food, pulp & paper)

       yard and garden wastes

       municipal solid wastes (up to 70% organic matter by weight)
  Controlling composting
To achieve maximum composting for any
organic material, certain environmental
conditions must be maintained in the
compost pile

=> may be classified into interdependent
         biological conditions

          physical conditions

         chemical conditions
THE BIOLOGICAL ENVIRONMENT
Key organisms:

     bacteria
     fungi

     Actinomycetes



- play active role in decomposing organic
  matter
THE BIOLOGICAL ENVIRONMENT
Secondary organisms:
     earthworms
     insects

     other soil invertebrates


   play a less significant role in
    decomposition process compared
    to microorganisms
   more important in mechanical
    breakdown of wastes (chewing,
    burrowing, movement, aeration)
    Fate of organic matter in compost
   Carbon-containing compounds are consumed by
    microorganisms and converted to:

             microbial tissues
             carbon dioxide

             water

             humic breakdown products



       Heat is released as a result of microbial
        metabolic activity
        => temperature in pile increases
   Humic breakdown products resulting from
    one type of microbial activity may be used
    as a food and energy source by another
    generation or type of microbes

   Chain of succession continues until there
    is little decomposable organic material
    remaining


                COMPOST
            COMPOST
Stable end product composed of:
 living and dead microbial cells and
  cell fragments
 byproducts of microbial
  decomposition
 undecomposed particles (organic and

  inorganic)
           Microbial succession
             in compost piles
   A wide variety of microorganisms are
    naturally present in most nontoxic
    agricultural wastes, yard wastes, or mixed
    municipal wastes
    ==> number and type of available
    organisms generally not a limiting factor
   Depending upon environmental conditions,
    certain microbial groups may predominate
    at certain stages in the decomposition
    process
   If preferred organic substrate is depleted
    or unavailable, certain microbes may be
    reduced in numbers, go dormant, or die off
   Competition occurs between microbe
    groups
   Dominant groups emerge based upon
    current conditions in the compost pile
   Succession continues as long as there is
    adequate decomposable organic matter
    present
    THE CHEMICAL ENVIRONMENT
 determined largely by the composition of the
  waste materials to be composted
Important factors influencing the chemical
  environment for composting:
       adequate food / energy sources for microorganisms
       balanced amount of nutrients
       adequate water content
       adequate oxygen
       acceptable pH range
       lack of toxic substances that could
       inhibit microbial activity
       Food / energy sources for
          compost microbes
   microbes rely on organic carbon
    compounds to meet energy needs
   Carbon in natural or synthetic organic
    substances varies in degradability (e.g.
    sugars easily metabolized by most
   microbes; lignins in wood or paper
    degraded more slowly, by fewer groups;
    plastic very resistant to breakdown)
       Food / energy sources for
          compost microbes
   As the more easily degradable organic
    compounds are decomposed, a small
    portion of the carbon goes into microbial
    cells, while a large portion is converted to
    CO2 and lost to the atmosphere

    => reduction in weight and volume of
    waste
       Food / energy sources for
          compost microbes
   More resistant carbon compounds form
    the matrix for the physical structure of
    finished compost.
   Most municipal, yard, and agricultural
    wastes have adequate biodegradable
    carbon to support microbial activity
Nutrients for compost microbes
 nitrogen, phosphorus, and potassium are
  most important nutrients
 nitrogen is usually the limiting nutrient

=>
  CARBON to NITROGEN (C:N) RATIO IS
   CRITICAL IN DETERMINING THE RATE
            OF DECOMPOSITION.
   - C:N ratio must be established on the
    basis of decomposable rather than total
    carbon
   generally, a ratio lower than 30:1 is
    considered ideal; higher ratios result in
    slower decomposition rates => adjusted by
    co-composting with different materials
   Typical C:N ratios for waste products:
               Manure - 15:1 to 20:1
            Yard wastes - 20:1 to 80:1
        Municipal wastes - 40:1 to 100:1
           Wood chips - 400:1 to 700:1
   As the composting process proceeds and
   carbon dioxide is lost to the atmosphere,
    the
   C:N ratio narrows => finished compost has
    a C:N ratio of 10:1 to 15:1
     Moisture in compost piles
   ideal moisture: 50% to 60% by weight
   most wastes do not contain enough moisture =>
    composting process slowed down unless water
    is added
   excess water causes problems in compost piles:
    leachate generation, anaerobic conditions,
    rotting, and obnoxious odours
   loss of moisture occurs through evaporation =>
    controlled by adjusting the size and shape of the
    compost pile
     Oxygen in compost piles
   aerobic decomposition is required for
    odour-free, rapid composting
   pile should have enough void space to
    allow gas exchange with the atmosphere
   5% to 15% oxygen concentration is
    considered adequate
   piles aerated bymechanical turning, air
    injection
         pH in compost piles
   pH of 6 - 8 considered ideal

Level of acidity / alkalinity affects:

     nutrient availability
     solubility of (potentially toxic) heavy

      metals
     overall metabolic activity of microbes
         pH in compost piles
   pH may be adjusted upwards by the
    addition of lime (calcium carbonate), but
    most organic substances are naturally
    well-buffered with regard to pH change

   slight tendency towards acidification as
    compost matures, due to production of
    carbonic acid
    THE PHYSICAL ENVIRONMENT
Includes factors such as:

   particle size
   temperature
   mixing
   pile size and shape
   small particle size promotes rapid
    decomposition due to increased surface
    area-to-volume ratio
   However: if all particles are small, they
    pack together and create dense,
    anaerobic compost
   => particles should have enough surface
    area to promote microbial activity, but
    have enough air spaces to permit gas
    exchange with the atmosphere
             Co-composting
   used to achieve better balance of particle
    sizes (e.g. small-particle sewage sludge
    mixed with large-particle wood chips)

   Particle size reduction by grinding is
    occasionally done before composting;
    sometimes undertaken after composting to
    improve aesthetic appeal of finished
    product
  Temperatures in the compost
             pile
 Different microbes have different optimal
  temperature ranges:
     psychrophiles (cool - below 20o C)

      mesophiles (warm - 20o to 40oC)

      thermophiles (hot - 40o to 80o C)

 sub-optimal temperatures interfere with
  metabolic activity and reproduction of
  microbes
   as temperatures increase above the
    maximum threshold, cell proteins are
    destroyed and the microbes die

   most effective temperature range for
    efficient composting is 55o to 75o C
    (thermophile range)
              Thermophiles:
          promote rapid decomposition
                   destroy pathogens

   Temperatures in excess of 55o C are
    required for at least 3 days to ensure
    pathogen destruction
   If compost pile is large enough, internal
    heat will allow composting in subzero
    conditions
 COMPOSTING TECHNIQUES
Small-scale home composting:
     simple compost heaps
     box or barrel composters
     commercial composter units
     digester units
Commercial composting:
     windrows
     aerated static piles
     in-vessel composting systems
     PROCESSING OF MUNICIPAL
           COMPOST
1.   Removal of bulky items
2.   Particle size reduction (grinders, shear
     shredders, hammermills)
3.   Screening (size requirements)
4.   Magnetic separation
5.   Moisture addition and mixing
6.   Composting (numerous techniques)
7.   Postprocessing: screening, curing,
     storage, marketing, application

								
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