Biorefineries using waste – production of energy and chemicals from biomasses by micro-organisms - Biomass as Energy Source: Resources, Systems and Applications - page 240

• municipal household waste requires quite an effort to be digested as disturbing materials have
to be sorted out and sanitation is necessary, but the organic matter left in the household waste
makes it worth taking the effort

• industrial organic wastes from alcohol or food industry often contain high energy but most

of them also require high attention concerning process stability and they perfectly suit as
co-substrates to enhance the methane yield of a mixture.

13.7.1 Enzymatic hydrolysis of macromolecules
One major obstacle in the biocatalysis is formed by the so-called steric hindrances in the molecular
structures (Rosenberg et al. , 2011). This means the space limitation around the target substrate,
and enzyme molecules functional, active sites. These are lowering the reaction speed if not
completely preventing the reaction from happening. For example, in the case of plant mate-
rial, cellulose fibers are not always fully accessible for the hydrolytic enzymes. The hydrolysis
of macromolecules is the basis for the effective utilization of organic wastes. Plant materials
are often degraded by either amylolytic enzymes in case of starch polymers, or by cellulases
and hemicellulases attacking the corresponding cellulose fibers or their hemicellulose bonds.
Regarding the slaughterhouse wastes, also such enzymes as proteases and lipases are needed for
the pretreatment of the biomasses.

13.7.2 Hemicellulose, cellulose and lignin
There are remarkable physiochemical barriers in the lignocellulosic materials hindering the
hydrolysis of cellulose and hemicelluloses to fermentable sugars (Alvira et al. , 2010). The pre-
treatment methods aim at loosening the biomass structures, improving enzyme accessibility and
thus making the macromolecular structures more digestible. Such methods are including hot
water treatment, steam explosion, ammonia filter explosion (AFEX), wet oxidation, microwave
or ultrasound pretreatments and CO 2 explosion. Besides these methods, also traditional acid or
alkali treatments, as well as mechanical communition are available. Chemical treatments with
ozone, organic solvents and ionic liquids have also been in use for achieving an improved recov-
ery of the chemical energy generating potential of the plant biomass wastes, as well as urban or
industrial, paper or board disposals. Increased efficiencies of the pretreatments pave way for the
replacement of combustion as the primary waste processing methods. The more is the sustainable
microbial biotechnology applied for the refinement of the organic wastes, the more beneficial
are the consequences for the air, water and ground. In this fashion, the organic matter is truly
circulated based on the natural cycle of carbon and other substances.
Hemicellulases consists of chains of pentose sugars, whose presence is selecting the organisms
available for their utilization (Papoutsakis and Meyer, 1985). In the wastewater pools of a paper
and pulp mill, it is self-evident to find mixed cultures of hexose and pentose-fermenting bacteria.
Among the Nordic wood materials, especially the birch is containing high levels of hemicelluloses
(35%) (Alén, 2000). The role of hemicelluloses in wood is to cement the cellulose fibers together.
The resultingmonosaccharides from the hydrolysis of hemicelluloses include the hexoses glucose,
mannose and galactose, and the pentoses xylose and arabinose, and the disaccharide cellobiose
(Chandel et al. , 2011). Lignin in turn is the most difficult biomaterial in wood for hydrolysis. If
it's concentration in wood is rising up it increases the HVV s of the biomass fuels ( HHV = higher

heating value) (Dermirbas, 2010).
Therefore, the more lignin-containing the material is (and the more difficult is its hydroly-
sis) the more tempting it is to choose the combustion for the method of choice in treating the
particular biowaste. In this context, it is important to keep in mind that the extraction of hemi-
celluloses and celluloses is increasing the HHV s of the remaining material. Consequently, the
microbial bioprocesses could be extremely feasible as pretreatments for the combustible rem-
nants prior to burning. This approach, besides saving the environment, could increase the overall
economy of the process. It is clear that as the combustion is currently responsible for about 97%