Environmental Engineering Reference
In-Depth Information
productivity the digester should work as close to this as possible. Most
commercial digesters tend to work within empirically established ranges that
do not necessarily represent the point of inflection of the VMP line. There
are, however, other factors that may affect the loading that can be applied:
for example, the ability to separate solids from the liquor in the digestate or
even the ability to mix the digester itself may limit the operation before
optimal biogas production is achieved. Commercial digesters may also work
above the point of inflection if their income stream is mainly dominated by
gate fees rather than by energy production.
Up to now, the OLR has been expressed in terms of kg VSm 3 day 1 even
though some of these VS may not be bio-available because of their
molecular structure or physical position in the substrate. For comparison of
the performance of anaerobic digesters processing different feedstocks the
OLR is not necessarily a good parameter and it would be more logical to
express loading as kg BMPm 3 day 1 . For example, it has often been said
that the so-called 'dry' digestion systems can accept higher organic loadings
than 'wet' systems (Guendouz et al., 2010; Weiland, 2010); yet, in order to
maintain functionality, dry systems often rely on a proportion of the
feedstock applied not degrading but acting as a support matrix, to allow the
use of solids handling equipment rather than pumps. More feedstock is
added, but more undegraded material is removed, and performance in terms
of kg VS destroyedm 3 day 1 may be similar. It is likely that the metabolic
capacity of both types of digester is equal and ultimately this biological
capacity is what determines the process efficiency.
6.4
Overcoming limitation as a result of hydraulic
retention time (HRT)
Where the maximum loading that can be applied to a digester is due to the
safe limit value for the HRT, then the optimisation strategy for the digester
might be to increase the VS content of the substrate, for example by
dewatering if the substrate is a slurry. Where the substrate is a liquid effluent
then the digester design has to be changed to allow retention of the
anaerobic biomass and prevent washout. This can be achieved with designs
such as the upflow anaerobic sludge blanket (UASB) digester (Seghezzo
et al., 1998) or one of the many retained biomass designs aimed at industrial
wastewater treatment applications (e.g. Liao et al., 2006; Chernicharo 2007;
Singh and Prerna, 2009; see also Chapter 5).
￿ ￿ ￿ ￿ ￿ ￿
Search WWH ::




Custom Search