Environmental Engineering Reference
In-Depth Information
offensive odors. High temperatures also destroy fly larvae and weed seeds, yielding a safe, high-
quality finished product.
Finally, aeration expedites the composting process through the mechanism of heating insofar as
the elevated heat will drive biochemical processes faster, so that a finished product can be rendered
in 60 to 120 days. Aerated compost is an excellent source of macro- and micronutrients as well as
stable organic matter, all of which support healthy plant growth. In addition, the microorganisms
in compost aid in the suppression of plant pathogens. Finally, compost retains water extremely well
resulting in improved drought resistance, a longer growing season, and reduced soil erosion.
In Thailand, this system has been used by farmer groups for more than 445 sites (May 2008).
The process needs only 30 days to finish without the need for turning and 10 metric tons of compost
is obtained each time. A blower (6 cm squirrel-caged blower with 2.2 kW motor) is needed to force
the air through 10 static piles of compost 15 minutes for twice a day. The raw materials consist of
agricultural wastes and animal manure in the ratio of 3:1 by volume.
32.3.1.2 Passive composting
Passive composting is composting in which the level of physical intervention is kept to a minimum,
and often as a result, the temperatures never reach much above 30°C. It is slower but is the more
common type of composting in most domestic garden compost bins. Such composting systems
may be either enclosed (home container composting, industrial in-vessel composting) or in exposed
piles (industrial windrow composting). Kitchen scraps are put in the garden compost bin and left
untended. This scrap bin can have a very high water content, which reduces aeration and therefore it
becomes odorous. To improve drainage and airflow, a gardener can mix in wood chips, small pieces
of bark, leaves or twigs, or make physical holes through the pile.
32.3.1.3 natural composting
An unusual form of natural composting in nature is seen in the case of the mound-builders (megapodes)
of eastern Indonesia, New Guinea, and Australia as well as in the case of bowerbirds of New Guinea and
Australia. These megapodes are fowl-sized birds famous for building nests in the form of huge compost
heaps containing leaf litter, in which they incubate their eggs. The birds work constantly to maintain the
correct, almost exact, incubation temperatures, by adding and removing leaves from the compost pile.
In effect, this teaches us that thermophilic high-temperature composting is not man-made.
32.3.1.4 moisture and heat
An effective compost pile is about as damp as a well wrung-out sponge. This provides the moisture
that all life requires. Microorganisms vary by their ideal temperature and the heat they generate
as they digest. Mesophilic bacteria survive best at temperatures of 20°C to 44°C. Thermophilic
(heat-surviving) bacteria grow optimally at around 55°C, and can attain the fastest decomposition,
because metabolic processes proceed more rapidly under higher temperatures. Elevated temperature
is also preferred because it causes the most rapid pathogen reduction, and is more destructive of
weed seeds. To minimally achieve it, the heap should be about 1 m wide, 1 m tall, and as long as
is practicable. This provides enough insulating mass to build up heat but also allows aeration. The
centre of the pile heats up the most.
If the pile does not heat up, common reasons include that
• The heap is too wet, limiting the oxygen which bacteria require
• The heap is too dry for the bacteria to survive and reproduce
• There is insuficient protein (nitrogen-rich material)
The necessary material should be added, or the pile should be turned to aerate it and bring the
outer layers inside and vice versa. You should add water at this time to help keep the pile damp.
One guideline is to turn the pile when the high temperature has begun to drop, indicating that the
