Biotechnology and Waste - Environmental Biotechnology: Theory and Application - page 191

nature of the composting material. Clearly, for a large commercial operation, it
is essential that whatever is used does not significantly affect the economics of
the plant and for this reason, although artificial fertilisers are an ideal way of
increasing nutrient content, they are seldom used in household waste applications.
Their expense relative to these low-cost biowastes effectively rules them out;
they are, however, often to be seen in ex situ bioremediation operations, since
composting contaminated soil commands a higher price.
Additions to the original material typically accelerate processing, but careful
monitoring is essential since the blend may exhibit very different decompositional
characteristics, which may ultimately influence the nature of final product derived.
Accelerants
Although gardeners have a number of proprietary brands of compost accelerants
available to them, this is not an approach often used at commercial facilities,
mainly due to the scale of these operations and the consequent expense. As with
nutrient addition, this tends to be reserved for use on high value wastes, though
many common substances used in co-composting programmes, like manures, are
themselves widely accepted to act as natural accelerants. Though their effect is
more variable, it seems likely that this is the only form of enhanced processing
applicable for general biowaste use.
Processing time
In many respects, the time required is a function of all the other factors. Process-
ing garden or food waste can be achieved in under three months using aerated,
in-vessel or turned windrow systems, while in a simple static pile, it may take a
year or more to reach the same state. Inevitably much depends also on the man-
agement regime, since process optimisation is the key to accelerated bio-treatment
and good operation practice is, consequently of considerable importance.
Anaerobic Digestion
Although composting certainly accounts for the majority of biowaste treatment
applications around the world, AD is an alternative option which has continued
to receive increasing interest over recent years. In many respects, it is a regulated
version of the natural events of landfill, in that it results in the controlled release of
methane-rich biogas, which offers the potential for a very real form of energy-
from-waste. This technology is viewed in certain circles as rather novel, but
this is not really the case. It has been used in the water industry for around a
100 years to treat sewage and, more recently, been successfully applied to the
processing of agricultural and household wastes, most notably in Germany and
the Netherlands. However, waste management tends to be a naturally cautious
field and the relative lack of a proven track record with MSW derived biowaste
compared to composting has made the uptake of this approach slow.