Biotechnology and Waste 179present. However, under conditions of high organic loading, the oxygen demand
of the bacteria exceeds the carrying capacity of the water and the algae’s ability
to replenish it. Hence a downward spiral develops, which ultimate leads to locally
anaerobic conditions.
Although ‘waste’ is itself one of the three key potential intervention points for
environmental biotechnology, it should be clear from the preceding discussion
that there is considerable capacity for biological waste treatment technologies
to contribute heavily to another, namely the reduction of pollution. To try to
set this in context, it is quite common for landfill leachate analysis ranges to
be quoted based on the average values obtained from a number of established
sites. However, this can lead to a significant distortion of the true picture since,
particularly for newer landfills (where the biochemical activity tends more to
early acetogenic fermentation than ‘old’ post-methanogenic or even semi-aerobic
processes) a degree of under-representation often occurs for some substances.
For example, ‘young’, acetogenic leachate is typically below pH 7 and of high
COD, though much of the latter is biodegradable. The bacteria responsible for
the biological breakdown at this point in the site’s life may be anaerobic, aerobic
or facultative anaerobes. In older landfills, methanogenic bacteria predominate,
which are strict anaerobes and can only assume and maintain their dominant
position in the absence of oxygen. Such conditions develop in time as the normal
sequence of events involves the early acetogenic bacteria gradually using up the
available oxygen and producing both the necessary anaerobic environment and
acetate as a ready food source for the methanogens which follow in succession,
as the site ages.
The full picture of the pollution potential of landfill leachate is more complex
than might at first be supposed, if for no other reason than, though it is spoken
of as if it were a single commodity, leachate is a highly variable and distinctly
heterogeneous substance. It is influenced by the age, contents and management
of the landfill of its origin, as well as by the temperature and rainfall of the
site. Moreover, all of these factors interact and may vary considerably, even in
the relatively short term, not to mention over the decades of a typical landfill’s
lifetime. The general range of values for landfill leachate established by the
Centre for Environmental Research and Consultancy (CERC) study (Cope 1995)
makes this point very clearly, as shown in Table 8.2.
Some measures have been written into the legislation in an attempt to minimise
the possibility of pollution, such as the requirement that all sites, except those
taking inert waste, employ a leachate collection system and meet universal min-
imum liner specifications. However, it is obvious that a method of dealing with
waste which removes the bulk of the problem at the outset must be a preferable
solution. The use of biological treatment technologies to process wastes has, then,
considerable future potential both in direct application to waste management itself
and in a number of allied pollution control issues which currently beset this par-
ticular industry. Coupled with the twin external driving forces of legislation and