Environmental Engineering Reference
In-Depth Information
from outflow to inflow of the pretreatment process, it takes only slightly more heat than conventional
anaerobic digestion. Such a process is very appropriate for cell like material such as waste sludge.
It is also possible to use biological processes as pretreatment. They are less expensive but emerging
in the world. During the process of silage, disintegration takes place by the lactic acids, which
decompose complex components of certain substrates (Hendriks and Zeeman 2009). Therefore, for
example, clover should be harvested and processed before blooming whereas corn is best cut at the
end of maturation. Recently also disintegration with enzymes has been quite successful, especially
using cellulase, protease, or carbohydrases at a pH of 4.5 to 6.5 and a retention time of at least
12 days, preferably more (Hendriks and Zeeman 2009).
32.3.3.2 anaerobic digestion Process
For anaerobic digestion, several different types of anaerobic processes and several different types
of digesters are applicable. It is hard to say, which digester type will serve best treating the selected
substrate. For treating farm waste, it is suggested that anaerobic digesters are decentralized to serve
each farm separately, to make it an economic and technological unit combined with the farm. It is
important to study the waste of each such unit carefully to be able to determine optimal conditions
for substrate digestion. Farm waste can differ greatly even in the same geographical areas; therefore,
it is strongly recommended to conduct laboratory- and pilot-scale experiments before constructing
the full-scale digester. Considering the costs of the full-scale digester, conducting pilot-scale
experiments is a minor item, especially if you can get good data and construct an optimal digester.
The biggest economic setback is if a digester is constructed and it does not perform optimally and
therefore requires a rebuilding.
There are several processes available to conduct anaerobic digestion. Roughly, the digestion
process can be divided into solid digestion and wet digestion processes. Solid digestion processes
are in fact anaerobic composters. In this process, substrate and biomass are in solid form (although
having approx. 20% of dry matter and 80% water). Such processes have several advantages. The
main advantage is reducing the reactor volume because of less water in the transport. Four times
more concentrated substrate equals approximately four times less reactor volume. It is also possible
that some inhibitors (such as ammonium) can have less inhibitory effects in solid digestion process.
The biggest disadvantage of the solid digestion process is the substrate transport. Substrate in
solid form requires more energy for transport in and out of the digesters. There is also a stronger
possibility of air intrusion into the digesters, which poses a great risk to process stability and safety.
It has been only recently that such processes have gained ground for a wider use. A fine example is
the commercially available Kompogas® process (Kompogas 2009).
A much larger variety of digestion processes are wet digestion processes. In these processes, the
conventional concentration of the digester suspension rarely exceeds 5% of dry solids by mass. There
are several reactor technologies available to successfully conduct anaerobic digestion. Roughly,
they can be divided into batch-wise (Figures 32.4 and 32.5) and continuous processes. Furthermore,
continuous processes can be divided into single stage (Figure 32.6) or two-stage processes
(Figure 32.7). In most of the wet digestion processes, microorganisms are completely mixed
and suspended with substrate in the digester (suspended solids of substrate and microorganisms
are impossible to separate). If the substrate contains few solids and is mostly dissolved organic
liquid, we can apply flow-through processes. In these processes, microorganisms are in granules
and granules are suspended in liquid that contains dissolved organic material. In such anaerobic
processes, microorganisms are easily separated from the substrate. A typical representative of such
a process is the UASB (upflow anaerobic sludge blanket) process (Figure 32.8).
32.3.3.3 Batch Processes
In the batch process, all of the different stages of substrate treatment happen in one tank. Typically
the reaction cycle of the anaerobic sequencing batch reactor (ASBR) is divided into four phases: fill-
load, react-digestion, settle, and decant-unload (Figure 32.4). The reactor is filled with substrate at
