Biotechnology and Waste - Environmental Biotechnology: Theory and Application - page 198

Feedstock
As with composting, the particle size and nature of the material to be treated plays
an important role. The ease of breakdown is largely defined by the characteristics
of the biowaste material to be treated, but generally finer particles allow for better
processing and a homogeneous slurry or suspension is the ideal feedstock for AD.
It must be stressed, however, that some biowaste types, particularly the likes of
lignin-rich, woody material, are relatively resistant to this process.
Loading rate
Loading depends on the characteristics of the waste, its degree of wetness,
digester volume, the expected retention period and similar system design param-
eters. It is typically expressed as the COD per cubic metre of digester void-
space (COD/m 3 ) or, for continuous or semi-continuous process, per unit time,
(COD/day, COD/h).
pH and volatile fatty acids concentration
These are interdependent factors which need to be considered together. Adequate
process control and digester optimisation requires suitable pH monitoring, since
many of the bacteria involved are pH sensitive. In particular, acidogens, having
better tolerance to acidity, may produce acids faster than the increasingly inhibited
methanogens can use it, in conditions of low pH, leading to spiralling acidity and
the potential for process collapse. A number of acid-base reactions exist within
the typical AD process, which lead to a measure of natural, in-built resistance to
major pH swings. However, under certain circumstances, the need for external
interference may arise and the amount of such intervention necessary to maintain
proper equilibrium will depend on the nature of the material. For some wastes,
pH control may only be necessary during start up or in overload conditions; for
others where acidity is habitually shown to be a problem, continuous control may
be necessary.
VFA concentration is one of the most important process indicators. Elevated
VFAs are characteristic of AD instability and thus they may be the first indication
of a developing problem, though the actual cause may be less immediately obvi-
ous. Inadequate mixing, excessive loading, poor temperature control or bacterial
inhibition can all lead to an increase in VFAs and a decrease in pH. Considering
the inconvenience and cost of being forced to empty a sick reactor, commercial
AD operations rely greatly on routine monitoring of this kind.
Biogas
The production of methane-rich biogas, which is an essential characteristic of AD,
has been used as an important marketing tool, since the generation of energy from
waste by biological means has much attraction, not least amongst those countries
with a popular dislike of incineration. A cohesive discussion of bioenergy appears