3.4 Operating the Composting Process


Welcome to this module on how to
operate the composting process. In this module, you will learn
some more practical or hands-on aspects necessary to run
a composting plant. These are for example: how to get
a good mix of materials, why the carbon nitrogen ratio is
important or how to avoid foul odors. Let’s start. Composting not always turns
out to be satisfactory. The process can smell bad, take
too long, attract pests and even pollute surface waters when
it is not properly managed. Several operational parameters
need to be considered in order to avoid these problems. The
most important ones are carbon nitrogen ratio, particle size
and porosity, moisture content, temperature and pH. Let’s
briefly describe and mention why these parameters are important. Carbon nitrogen ratio: all organic
materials contain carbon and nitrogen. However the relative proportion
of these nutrients is different for each material. The ratio of
the mass of carbon to the mass of nitrogen in a feedstock is called
carbon-to-nitrogen ratio or CN ratio. The ideal CN ratio ranges
between 25 to 1 and 35 to 1. What happens if the ratio is
greater than 20 to 1? In carbon rich materials such as hay,
straw, wood chips, cornstalks, sawdust, paper, cardboard, etc. microbes will deplete the nitrogen
using it for their own metabolic needs. But what happens if the ratio is
lower than 20 to 1 instead? In nitrogen rich materials such
as manures, vegetable wastes, coffee grounds, grass
clippings, food waste, etc. there is a surplus of nitrogen and
it can be lost to the atmosphere as ammonia gas and
cause other problems. In order to determine the amount of
these 2 nutrients, when available, an elementary analyzer can be used. This gives directly the percentage
by weight of both nutrients. The next parameter is the
particle size of the feedstock. This is a very important parameter
because it will affect the porosity and consequently airflow
within the pile, influencing the oxygen level which is key
for the microbial activity. Porosity is defined as a volume
of pores divided by the total volume of compost. Some of those
pores will be filled with air and the rest with water. During the composting process,
the volume of a pile is normally reduced by compaction and size
reduction, reducing the pore size and consequently
the airfield porosity. These will restrict the airflow
within the pile and consequently the oxygen level. Therefore, it is
recommended to have an initial airfield porosity of 45 to 60%
and keep it higher than 35%, especially during the active
phase when oxygen is required. Porosity can be increased by adding
what is called “structuring materials” such as branches,
hard fruit shells, etc. In the supplementary material,
you will find a quick method to determine porosity. Moisture is the next important
parameter of the list. The ideal moisture content of
the compost pile is between 46 and 60% by weight. The material
should hold together but not exclude excess water when you squeeze
a handful of blended feedstocks. It should feel moist to the touch. Low moisture content will slow the
composting process as microbes need a moist environment. However
moisture content in excess of 60% means that pores spaces in
the compost pile will be filled with water rather than air
leading to anaerobic conditions. Under anaerobic conditions, oxygen
is depleted and consequently anaerobic bacteria start decomposing
the feedstock which creates bad odors and generates methane
which is a greenhouse gas. A pile should always have at
least 5 to 10% of oxygen in the airfield cavities. This can be
monitored with oxygen sensors which tend to be expensive. Therefore
we recommend you to trust your nose. Odors are a clear indication
of something going bad. How can this be avoided? Easy. Blend materials with different
moisture contents to achieve the ideal moisture content. Bulky and dry carbon rich
materials can be used to increase porosity and air cavities. Remember
the active phase when temperatures are high. During this phase,
water will evaporate. What should you do then? The simplest rule is to touch and
feel the moisture of the pile. That will always give you an
indication of its humidity. If it’s dry and if you’re turning
piles, you can add water as you turn. If you don’t turn the materials,
such as with passively and forced aerated windrows,
it is more complicated. The best is to make initial
material blends on the wet site. That means 60, 70% moisture
content by weight. So that when water vapor is lost,
there will still be enough moisture to keep micros active. In the supplementary material,
you will find a quick method on how to determine moisture content. The next parameter
in our list is pH. The acidity, pH lower than 7,
or alkalinity, pH higher than 7, of the organic materials affects
the growth of microorganisms and the ammonia loss
via volatilization. Bacterial decomposers prefer
a pH range between 6 and 7. 5 while fungal decomposers
prefer a pH range of 5. 5 to 8. Gases losses of ammonia
are more likely to occur if the compost pH exceeds 7. 5-8. Certain materials such as
lime, potassium carbonate, dairy manure can raise pH while
other material such as food process in waste, decomposed tree leaves or
pine needles can lower the pH. Our last parameter is temperature. Ambient temperatures affect
microbial growth and activity in the compost pile and hence the rate
at which the raw materials decompose. In tropical climates, as in
Indonesia shown here, composting can take place throughout
the whole year. In temperate climates, however,
composting is fastest in spring, summer and fall months and it is most
likely to stop in winter months. There is however a trick on how
to keep micros ongoing in cold seasons and that is to increase
the pile size or covering it with matured compost. Then, they
can retain heat and remain active. Now you know what parameters
you need to pay attention to in order to produce good compost. However, what kind of environmental
impacts that composting presents? That is what we’re going to see now. There are 3 main issues with
composting: foul odors, ammonia emissions and leachate. Although bad odors can origin
naturally from the feedstocks such as rotted food, manure or sewage
sludge, we saw that anaerobic conditions can play
a big deal as well. Remember that these occurred due to
too much moisture, too compact piles or depletion of oxygen. Replenishing oxygen is often enough
to eliminate these bad odors. Ammonia emissions which have
a characteristic smell, more or less like when people pee in a
corner, can be due to 2 things: either there’s too much nitrogen
in the material and consequently we’re dealing with a material with
a low CN ratio, or the balance between ammonium, which is
dissolved in water, and ammonia, which is volatile, is shifted
towards the ammonia side. This happens when temperatures
are high or, as shown in the figure on the right,
when pH is above 8. See how the relative concentration of
ammonia increases for higher pH values. Leachate is a liquid containing high
amounts of soluble nutrients. This liquid can penetrate soils and
pollute surface and groundwater bodies, often causing
eutrophication conditions. Leachate is generated when piles
have an excess of moisture or when they are not covered
and are exposed to the rain. In order to avoid leachate, the
location of compost installations is an important aspect to consider. RoWSS is the acronym that will
help you remember the four things you need to keep in mind:
first, considered providing roofs to divert precipitation
and keeps stored compost dry. Second, the site should be several
feet above the sustainably high water table. Third, consider
the slope of the site in order to avoid water running into the compost
piles during storms or snow melt. And last, but not least, choose
a soil with low permeability. If still leachate is generated,
the best is to collect it and to moist the piles with it. We arrive to the
end of the module. In this module you learned more
hands-on aspects about composting. We covered the most important
parameters that need to be monitored during composting, as well as the
potential environmental impacts and how to avoid them. You should now be ready to give it
a try at home, in your school, in your university or
your working place. We would like to ask you to upload
pictures of composting plants that you have around you. Try to identify what type of
composting technology it is and make a rough diagnosis of the
quality of the process. Does it smell bad? Is it the
right mix of materials? Is the location adequate? Post your comments in the forum. In the next modules, you will see several
real case composting installations. I’m sure you will find
it very interesting. For those interested in more
details on composting, we strongly recommend you to consult the user
manual Decentralized Composting For Cities of Low And Middle
Income Countries, downloadable from the Sandec website for free. This module also got a lot of
input from the second literature mentioned here, The Art and
Science of Composting.

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