Biotechnology and Waste - Environmental Biotechnology: Theory and Application - page 179

rich liquors provide heterotrophic micro-organisms with a ready and abundant
source of food. In conditions of relatively low organic loading, a dynamic equi-
librium is reached between the bacteria breaking this material down and the
autotrophic organisms, typically algae, which subsequently make use of these
breakdown products. The oxygen balance works, since the requirements of the
aerobic decomposers is offset by the contribution of the photosynthetic algae
present. However under conditions of high organic loading, the oxygen demand
of the bacteria exceeds the carrying capacity of the water and the algae's abil-
ity 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. How-
ever, this can lead to a significant distortion of the true picture since, particularly
for newer landfills (where the biochemical activity tends more to early aceto-
genic 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 Chemical Oxy-
gen Demand (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 bac-
teria 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 anaer-
obic 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