108 Organic waste recycling: technology and management
Parts of NH 3 generated from the above reactions will be lost through
volatilization if pH of the compost piles is above 7. The remaining NH 3 will be
nitrified to become NO 3 - during the maturation or curing of the composted
products. Example 3.3 shows a method to determine aeration requirements
based on the stoichiometric equations.
Example 3.3
Suppose Equation 3.14 represents the reaction occurring during aerobic
composting of septic tank sludge. Calculate the amount of air required to
completely oxidize this sludge.
C 10 H 19 O 3 N + 12.5O 2 ĺ 10CO 2 + 8H 2 O + NH 3 (3.14)
(201) (400)1g of C 10 H 19 O 3 N requires O 2 = 2 gAssume septic tank sludge contains 75% volatile solids of which 50% is
biodegradable (as represented by C 10 H 19 O 3 N):
Therefore, the amount of O 2 required per 1 g of septic tank sludge is
2(0.75)(0.5) = 0.75 g.
Since air contains 23% O 2 (by weight), the amount of air required is
0.75/0.23 = 3.26 g.
The specific weight of air is 1.20 g/L at 25 °C and 1 atmospheric pressure.
The volume of air required to oxidize 1 g (dry weight) of septic tank sludge is
3.26/1.20 = 2.72 L.
Note: This is a theoretical method of calculating aeration requirement. In
practice, the amount of air supply should be adjusted according to the biological
reactions taking place, e.g. thermophilic reactions will require oxygen several
times higher than those of mesophilic and maturation reactions (see Figure 3.2).
Additional amount of air needs to be supplied to compensate for loss to
atmosphere, which can be up to 95-99 percent.
3.4.5 Temperature and pH
The biologically produced heat generated within a composting mass is
important for two main reasons:
- To maximize decomposition rate; and
- To produce a material which is microbiologically ‘safe’ for use