Organic Waste Recycling

Composting 107

When mechanical aeration is to be employed, the quantity or rate of air flow
has to be properly controlled. Too much aeration is wasteful and can cause a
loss of heat from the compost piles, while too little aeration would lead to the
occurrence of anaerobic conditions inside the compost piles.
A simple method to determine aeration requirements is based on the
stoichiometric reaction of waste oxidation. A knowledge of the chemical
compositions of the wastes to be composted or oxidized (Table 3.2) is useful for
the calculation. Since compost feed is a combination of various organic
compounds, for practical purposes, the formulae C 10 H 19 O 3 N and C 5 H 7 O 2 N have
been used to represent its chemical composition. The stoichiometric equations
for complete oxidation of these compounds are as follows:

C 10 H 19 O 3 N + 12.5 O 2 ĺ 10 CO 2 + 8 H 2 O + NH 3 (3.14)

C 5 H 7 O 2 N + 5O 2 ĺ 5 CO 2 + 2 H 2 O + NH 3 (3.15)

Table 3.2 General chemical composition of various organic materials (Haug 1980)

Waste component Typical chemical composition

Carbohydrates (C 6 H 10 O 5 )x

Protein C 16 H 24 O 5 N 4

Fat and oil C 50 H 90 O 6

Sludge

Primary C 22 H 39 O 10 N

Combined C 10 H 19 O 3 N

Refuse (total organic fraction) C 64 H 104 O 37 N

C 99 H 148 O 59 N

Wood C 295 H 420 O 186 N

Grass C 23 H 38 O 17 N

Garbage C 16 H 27 O 8 N

Bacteria C 5 H 7 O 2 N

Fungi C 10 H 17 O 6 N