3.3 Composting technologies

Hi! We already learned the basics
of the science of composting. In this module, we will have a look
at the different composting technologies which are commonly used
in low and middle income countries. These are Windrow composting, passively aerated windrows, forced aerated windrows, in-vessel composting, and bin composting. Let’s start with Windrow composting. Windrow is the general term for an elongated pile
of stacked raw materials. This method is suited
to treat large volumes of bio-waste and produce large volumes of compost. Aeration of the material is crucial. Oxygen needs to be replenished
as it is consumed. Otherwise, the piles
go anaerobic in the center, following a different
decomposition process and producing foul odors. Making sure that the material
is porous enough for air to pass through is always advisable. This is achieved by mixing
fine dusty, wet, or soft materials together with structure-giving materials such as small branches,
cardboard, or hard vegetables. In any case, the piles are normally turned which can be done
mechanically or manually. This way, the porosity is reestablished and air, and therefore oxygen,
get into the pile. Besides, the materials which are
at the exterior of the pile are now in the interior so that they can be
decomposed by micro-organisms. This technology requires
low initial investment and maintenance. However, it needs a lot of space
because the windrows have sloped sides and cannot be put too close together. It ranks medium when it comes to the time needed
for the composting process and it is labor intensive. The second technology
is passively aerated windrows. This technology consists
of perforated pipes located within the bio-waste pile which promotes convected airflow
throughout the material. The pipes can be located
in different dispositions as shown in the picture. The air that gets into the pile brings
the oxygen required by the microorganisms. These piles are normally
not turned during the process. Therefore, it is important
to premix the feed stock thoroughly before placing it on the perforated pipes. It is advisable to insulate
these piles with finished compost to ensure thermophilic temperatures reach
the outer layer of the pile as well. This technology requires slightly higher
initial investment and maintenance than non-aerated windrows, as it involves a passive aeration system. It has similar space requirements. The composting process might take
as long or slightly shorter than with non-aerated windrows and it also requires less labor as the piles do not need to be turned. The next technology
is called forced aerated windrows. They might look very similar
to the previous ones. The difference is that blowers
are installed at the end of perforated pipes or air ducts. These blowers inject air into the piles, preferably during the active phase, when the compost
reaches high temperature. Airflow can be adjusted by changing
the frequency and duration of the blower. These piles do not need to be turned,
and as in the previous ones, insulation with finished compost
ensures that thermophilic temperatures are reached in the outer layers. This technology requires
even higher initial investment, given the need for a blower
and aeration channels. Maintenance is also higher
than the previous two. It has similar or slightly less
space requirements since the composting process
is even shorter and it requires less labor,
as the piles do not need to be turned. Our fourth technology
is called in-vessel composting. This describes a group of methods
that confine the composting materials within a container or vessel. Installations vary from very high-tech
options where parameters are monitored, acting more as a bio-reactor,
to very low tech alternatives. In all configurations airflow
and temperature can be more easily controlled
than with other composting techniques. Turning or stirring takes place manually
or mechanically as seen in the video. (motor noise) This technology is more capital-intensive and requires more maintenance
than the ones mentioned before. It requires less surface area
than with the previous technologies since the composting process is shorter. Besides, little labor is required as the material
is mixed within the vessel. Finally, our last technology
is called bin composting. This technique is practiced
mainly at household level which will then treat
little amounts of waste and produce compost
for self-consumption. Bio waste is located in a container which most of the time
has some sort of perforated wall for air to get through. Some containers include
a stirring mechanism as the one shown on the right. Bin composters can also
be used on a big scale, as in this installation where we see passively aerated big scale
bin composting examples. In this case, the installations
are run by operators and most of the times,
the produced compost is commercialized. Bio-waste is most of the time
inserted from above and removed from the bottom. The material degrades and compacts
slowly as it gets down into the bin. This technology requires
medium investment cost and minor maintenance. In order to treat
the same amount of waste, it will require less space
than windrow composters as the waste is piled up
vertically in the bins without any sloped sides. Besides, no turning is required
and therefore labor is low, but the composting process
might take longer than other options described before. This final table shows
a comparative summary of the presented technologies assuming they are all treating
a similar amount of waste. However, keep in mind that in vessel
and specially in bin composting, can also or are normally used
in smaller amounts. In any case, the table
allows to have a quick impression of which technology could suit
your needs and constraints best. This is the end of the module. Now that we learned the basics
on the science of composting and about the most common technologies
used in developing countries, in the next module I will give you
some more insights on practical aspects of composting such as how to get a proper mix
of different organic wastes, or what environmental impacts
composting as an activity can cause. I hope to see you there!

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