Biotechnology and Waste 183There are a number of processes currently available in varying degrees of
commercial readiness, and others under development, to deal with biowaste.
While the underlying aims and basic requirements of all these biotechnologies are
essentially the same, there is some variance of detail between individual methods.
Two general approaches in particular, composting and anaerobic digestion, are
so well established and between them account for such a large a proportion of
the biowaste treated worldwide, that the discussion of specific technologies must
begin with them.
Composting
For centuries, gardeners and horticulturalists have encouraged biodegradable
waste to break down to produce stable, nutrient-rich compost for use in pots
or directly for improving the soil. This application of the natural, exothermic
process of aerobic decomposition, is familiar and time-honoured. More recently,
however, composting has been the recipient of increased attention as a poten-
tial means of treating biowaste on a municipal basis. Though the scale of such
operations imposes certain restrictions of its own, generally, putrescible matter
decomposes more efficiently and completely when oxygen is readily available.
This leads to proteins being degraded to nitrogen or ammonia and ultimately
mineralised to nitrate, while fats and carbohydrates are broken down to carbon
dioxide and water, via organic acids. This is, of course, purely a mass flow
overview of the process since a proportion of the material becomes incorporated
into microbial cells, as the decomposers themselves multiply and grow. Even
under optimised environmental conditions, there are a number of rate-limiting
factors in the process, which include extracellular hydrolytic enzyme production,
the speed of hydrolysis itself and the efficiency of oxygen transfer. These may,
in turn, be influenced by other aspects such as the particle size and nature of the
biowaste material to be treated.
In a practical application, this can be a major consideration as the kinds
of biowaste to be composted can vary greatly, particularly when derived from
municipal solid waste, since seasonal variation, local conditions and climate may
produce a highly heterogeneous material. On the other hand, biowastes from
food processing or horticulture can be remarkably consistent and homogeneous.
Accordingly, the details of breakdown may be very complex, involving a number
of intermediary compounds and different organisms utilising various biological
pathways. However, in broad terms, the composting process can be split into
the following four distinct general phases, which are chiefly defined by their
temperature characteristics.
The composting process
- Latent phase (Ambient temperature–c. 22◦C). Composting microbes infiltrate,
colonise and acclimatise to the material.