Composting 99Percent ash in Equation 3.6 refers to materials remaining after being burnt at
550 °C for 1 hour. For some types of wastes that contain a large portion of
plastics (which do disappear at 550°C), the use of Equation 3.6 will give a high
value of % C which is largely non-biodegradable.
Based on the above reasons, the initial C/N ratios of between 20/1 to 40/1
should serve adequately as an optimum range for composting.
If the initial C/N ratio of materials to be composted is greater than the
optimum value (such as sawdust and wheat straw), the microorganisms will
have growth limitations due to the lack of N. They will have to go through many
life cycles, oxidizing the excessive C until a final C/N ratio of about 10/1 is
reached in the composted products. Therefore, an extra composting time is
needed and a smaller quantity of final humus is obtained. The relationship
between aerobic composting time and C/N ratio observed during studies at the
University of california, USA, is as follows (reported in Haug 1980):
Initial C/N ratio = 20/1,
Composting time about 12 days
Initial C/N ratio = 20/1-50/1,
Composting time about 14 days
Initial C/N ratio = 78/1,
Composting time about 21 daysWith lower than optimum initial C/N ratio (such as the cases of nightsoil and
sludge), N will probably be lost as NH 3 gas, especially under conditions of high
temperatures and pH and forced aeration, hence a loss of the valuable nutrient to
atmosphere.
An example showing a method of calculation of appropriate mixing ratio of
some raw materials to be composted is given below.
Example 3.1
Sludge from a septic tank has the following characteristics:
C/N ratio = 15/l (dry weight basis)
Total solids = 10%
Volatile solids = 90% of total solids
Specific gravity = 1.1Determine the quantity of rice straw needed to be mixed with this septic tank
sludge to raise the C/N ratio of the mixture to 30/l, suitable for composting. Rice
straw has a C/N ratio of 80/l, moisture content 50%, bulk density = 100
kg/m^3 and N = 0.2 % of dry weight.