Environmental Engineering Reference
In-Depth Information
biodegradability; a mesophilic process can rarely achieve higher loads than 5.0 kg m
-3
/day and
thermophilic 8.0 kg m
−3
/day, respectively. Locally in the digester for a short period of time, higher
loads can be achieved; however, it is not advisable to run continuously on such high loads.
To achieve better efficiency and higher loads stage separated process can be applied (Figure 32.7).
In this case, the whole substrate or just portions of the substrate that are not easily degradable
are treated first in hydrolysis-acidogenic stage reactor and after that in the methanogenic reactor.
By separating the biological processes in two separate tanks, each can be optimized to achieve
higher efficiency as if in just one tank, where all biological processes are occurring simultaneously.
Much research has been published giving considerable attention to this kind of process (Dinsdale
et al. 2000; Song et al. 2004; De Gioannis et al. 2008; Ponsá et al. 2008). Both stages can be
either mesophilic or thermophilic; however, it is preferred that the hydrolysis-acidogenic reactor is
thermophilic; and methanogenic is mesophilic. Typical HRT for hydrolysis-acidogenic reactor is 1-4
days (thermophilic), depending on the substrate biodegradability. Typical HRT for the methanogenic
reactor is 10 to 15 days (mesophilic) and 10-12 days (thermophilic). Advantages of this process
beside shorter HRT are higher overall volume loads (20% or more). Many authors also reported
slightly better biogas yields (Messenger et al. 1993; Han et al. 1997; Roberts et al. 1999; Tapana
and Krishna 2004). The only disadvantage is that this process requires more sophisticated process
control and equipment, but the construction is more cost effective.
32.3.3.5 Flow-through Processes
Flow-through processes, such as the UASB process (Figure 32.8), are used only for substrates where
organic material is in dissolved form with solids content at maximum 1-5 g/L. In this substrate
category are mostly wastewaters of industrial origin (beverage industry). A detailed depiction of the
process is presented therefore in the wastewater treatment section.
32.3.3.6 Posttreatment and substrate use
After the substrate has been digested, it needs additional treatment. There are several possibilities of
digested substrate use. The most common and most used possibility in farm waste treatment is using
the digested substrate as a fertilizer. It can be used wet or dewatered. Wet substrate (total solids
concentration 1-5% by mass) is taken from the post-treatment tank and used. However, it must be
considered that fertilizing is possible only twice (sometimes once) annually. The post-treatment
container, therefore, must be designed accordingly. A possible solution is a lagoon, where digested
substrate is stored and additionally stabilized and mineralized during the storing time. When using
Biogas
Preparation
buffer tank
Storage-post
treatment tank
Biogas
Hydrolysis-
acidogenic stage
digester
Methanogenic stage
digester
Possible recirculation
FIGure 32.7
Two-stage anaerobic digestion.
