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

for some materials, the use of amendments or co-composting with other wastes
can also help optimise conditions for biological treatment. Sewage sludge and
manures are often used in this way, but they can also boost the available nutrient
levels, often in an uncertain way and by variable amounts. Generally, additives
are used where there is a need to improve either the chemical or the physical
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 decompo-
sitional characteristics, which may ultimately influence the nature of the 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 biotreatment
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, anaerobic digestion (AD) is an alternative option
which has been receiving 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