Environmental Engineering Reference
In-Depth Information
consider several points before considering the process and biomass pretreatment. The contents and
concentration of substrate should match the selected digestion process. For anaerobic treatment of
liquid farm waste, the most appropriate concentration is between 2 and 8% of dry solids by mass. In
this case, conventional single-stage digestion or two-stage digestion is used. If the treatment of solid
farm waste using a solid digestion process is considered, the concentration is between 10 and 20%
by mass. The concentration of dry matter in farm waste depends on the feces/urine ratio, whether
straw or other material is used for animal litter, and the quantity of water used to flush the material
away. The empirical value of used water should not exceed 100 L per LSU (livestock unit) to achieve
desired liquid manure concentration suitable for anaerobic digestion. The liquid manure can also
contain other foreign matter; fodder residue can for instance contribute to better biogas yield, others
usually impair the process of digestion. Such materials are
• Sand from mineral materials in animal feed
• Sawdust from scattering
• Large pieces of straw from litter
• Soil from roughage
• Soil which is carried from meadows
• Skin and tail hair, bristles, and feathers
• Cords, wires, plastics, stones, and other similar material.
The presence of foreign matter in the substrate can lead to increased complexity in the operating
expenditure of the process. During the process of digestion of liquid manure from pigs and cattle, the
formation of a scum layer on the top of the digester liquid can be formed, caused by straw and muck.
The addition of rumen content and cut grass (larger particles than silage) can contribute to its formation.
If the substrate consists of undigested parts of corn and grain combined with sand and lime, the solid
aggregates can be formed at the bottom of the digester and can cause severe clogging problems.
In all such cases, the most likely solution is pretreatment to reduce the size of solids. Naturally,
all of the nondigestible solids (soil, plastic, metal, stones, etc.) should be separated from the substrate
flow. On the other hand, grass, straw and fodder residue can contribute to the biogas yield. When
properly pretreated they are accessible to the digestion microorganisms. Such pretreatments are
generally physical, chemical, or combined in nature.
As physical pretreatment, disintegration is the most common (the most known are grinding and
mincing). In grinding and mincing, the energy required for operation is inversely proportional to
the particle size. Because such energy contributes to the parasitic energy, it should be kept in the
limits of positive margin (the biogas yield increased by pretreatment is more than energy required
for it). In the case of farm waste, the empirical value for such particle size is between 1 and 4 mm.
Chemical pretreatment can be used when treating lignocellulosic material, such as spent grains
or even silage. Very often chemical treatment is used combined with heat, pressure, or both. It is
common to use acid (hydrochloric, sulfuric, or others) or an alkaline solution of sodium hydroxide
(in some cases potassium hydroxide). Such a solution is added to the substrate in quantities that
surpass the titration equilibrium point, and then it is heated to the desired temperature and possibly
pressurized. Retention times are generally short (up to several hours) compared to retention times
of the anaerobic digesters. The pretreated substrate is then much more degradable. The downside
of this pretreatment is an excessive demand for parasitic energy and the cost of chemicals required.
It rarely outweighs the costs of building a bigger digester. Therefore, it is used mostly in treating
industrial waste (such as brewery) where there is plenty of waste lye or acid present and waste heat
can be regenerated from the industrial processes as well.
Thermal pretreatment can yield up to 30% more biogas if properly applied. This process occurs at
temperature range of 135-220°C and pressures above 10 bar. Retention times are short (up to several
hours) and hygienization is automatically included. Pathogenic microorganisms are completely
inhibited. The process runs economically only with heat regeneration. When heat is regenerated
