94 Organic waste recycling: technology and management
The overall nitrification reaction, combining Equations 3.3 and 3.4 is:
22 NH 4 - +37O 2 + 4CO 2 +HCO 3 - ĺ 21NO 3 - +C 5 H 7 O 5 N + 20H 2 O + 42 H+ (3.5)
The nitrifying bacteria responsible for nitrification consist of two main
groups i.e. the Nitrosomonas which converts NH 4 + to NO 2 - and the Nitrobactor
which converts NO 2 - to NO 3 -. The nitrifying bacteria have a relatively slow
growth rate and are inactive at temperatures greater than 40°C (Alexander
1961), hence they will become active normally after the reactions of organic
waste decomposition (the growth phase and thermophilic phase) are complete.
Since NO 3 - is the form of N which is readily available for crop uptake, the
maturation phase thus becomes an essential step in composting to produce good-
quality compost for use as fertilizer/soil conditioner.
At this stage, the organisms classified as second- and third-level consumers
(Figure 3.3), such as protozoa and beetles, will grow and will feed on the first-
level consumers (e.g. bacteria, fungi, actinomycetes).
The composted products after maturation can be used as fertilizers or soil
conditioners to crops. In this way, the nutrients returned as compost are in the form
of microbial protoplasm and/or organic compounds that break down slowly. Other
nutrients present in the compost such as nitrates are readily available to crops.
In the aerobic composting systems, the degradation of organic matter depends
on the presence of oxygen. Oxygen serves two functions in the metabolic reactions:
as the terminal electron acceptor in aerobic respiration; and, as a substrate required
for the operation of the class of enzymes called oxygenase (Finstein et al. 1980).
Organic matter generally degrades more rapidly and more completely if oxygen
is plentiful. This can be explained by the presence of the large amount of free
energy produced for microbial growth where the prominent electron acceptor is
oxygen. Oxygen can be incorporated into molecules devoid of this element with the
function of the widely distributed, non-substrate specific and inducible enzymes '
oxygenases'. This is often the first necessary step in the metabolic sequences
leading to the degradations of molecules resistant to biological attack. Classes of
organic microcontaminants acted upon by oxygenase include saturated alkanes,
aromatic hydrocarbons, and halogenated hydrocarbons; while anaerobic
environment lack this mechanism (Finstein et al. 1980). In anaerobic composting,
the free energy (heat) produced is much less than that of aerobic composting and
thus the longer time required for organic decomposition and pathogens
inactivation.
The kinetics of composting systems is a subject of vital interest to the design
engineer who must determine the type and size of composting plants and the
detention time required to achieve a certain degree of organic stabilization and