Environmental Engineering Reference
In-Depth Information
Biogas
Preparation
buffer tank
Storage-post-
treatment tank
Digester
Possible recirculation
FIGure 32.6
Single-stage conventional anaerobic digestion.
Better used in farm waste treatment would be batch processes that treat solid substrate (semi-
solid manure). In this case, the cycle is also divided into four phases that are somehow different than
in an ASBR process. This process requires always digesters to be in pairs. The reactor is almost
completely emptied between cycles; therefore, it requires inoculation through leachate exchange
between the two digesters (from the one in the peak biogas production to the one at the start of the
process). In the other phases, leachate is self-circulated (Figure 32.5). Typical cycle time is between
30 and 60 days. Although solid substrate reduces the reactor volume, the volume is still rather
large because of long cycle times compared to conventional digesters that process wet substrates.
The advantage of these digesters is that they are applicable for smaller scale farms and require less
complicated monitoring equipment. For biogas use a buffer gas tank is required.
32.3.3.4 continuous Processes
Most of the commercial biogas plants use a conventional continuous process as the technology for
anaerobic digestion. By conventional it is meant fully mixed, semi-continuous, or continuous load
and unload reactor at the mesophilic temperature range (35-40°C; Figure 32.6). In the majority
of cases, the substrate is loaded to the reactor once to several times a day; rarely is it loaded
continuously. Continuous load can lead to shot circuit, which means that fresh load can directly
flow out of the reactor if mixing is too intense or inflow and outflow are improperly positioned.
The digester is usually single stage. Although they are built in pairs, they do not function as a stage
separated process. Usually digesters are equipped with a preparation tank, where various substrates
are mixed and prepared for loading, which also serves as a buffer tank. In many cases also a post
treatment tank is added (it is also called a postfermentor), where treated substrate is completely
stabilized and prepared for further treatment. The posttreatment tank can also serve as a buffer
to further treatment of the substrate. Generally postfermentors do not add much to overall biogas
yield (up to 5%) if the digester operates optimally. The size of the preparation and posttreatment
tank are determined according to the necessary buffer capacity for continuous operation. The size
of the digester is determined with hydraulic retention time (HRT) and with organic loading rate
(OLR). HRT is defined as digester volume divided by substrate flow and it tells us how many days
it takes on average for a certain portion of the substrate to pass through the reactor. For mesophilic
digesters, the usual values are between 20 and 40 days, depending on the substrate degradability.
In thermophilic digesters, HRT between 10 and 20 days can achieve the same treatment efficiency.
OLR (sometimes also called volume load) is defined as mass of organic material fed to the
digester per volume per day. Typical values for mesophilic digesters is 2.0-3.0 kg m
−3
/day and
for thermophilic digesters, 5.0 kg m
-3
/day. Maximum OLR depends very much on the substrate
