Production of Methane Biogas as Fuel Through Anaerobic Digestion - Sustainable Biotechnology: Sources of Renewable Energy - page 117

The state of California generates more than 4 million dry tons of food-processing
wastes each year [54], potentially producing 1,200 million m 3 of CH 4 . This trans-
lates into an annual potential of several billions m 3 of CH 4 in the USA. Except
for the wastes from animal meat processors, most food-processing streams are rel-
atively poor in nitrogen, but rich in readily fermentable carbohydrates. As such,
food-processing wastes can be co-digested with other nitrogen-rich feedstocks (e.g.,
municipal sludge or animal manures) to enhance AD system stability and CH 4
production [46].
Approximately 250 million dry tons of MSW are produced annually in the USA.
The organic fraction, such as paper, yard trimmings, and food scraps, is biodegrad-
able and can be converted to methane biogas. Although the composition of MSW
varies dramatically depending on society, season, collection, and sorting, OFMSW
accounts for more than 50% of the MSW in most societies. Most OFMSW has little
moisture or readily fermentable carbohydrates and is relatively deficient in N or P,
but has a relatively large BMP (300-550 m 3 CH 4 /ton) if digested adequately [25].
The OFMSW generated annually in the USA has a CH 4 potential of 37.5 billion m 3 .
Crop residues amount to an estimated 428 million dry tons each year in the USA.
Although the majority of crop residues is typically left in the field, approximately
113 million dry tons are recoverable and available for conversion to methane biogas
[69]. Crop residues typically have relatively low water contents, high VS contents,
and variable contents of readily fermentable carbohydrates. Most crop residues are
non-leguminous and are poor in available nitrogen. The BMP of crop residues varies
from crop to crop (from 161 to 241 m 3 CH 4 /ton) (124). If subjected to proper AD, at
least 20 billion m 3 of CH 4 can be produced annually from the crop residues available
for biogas production in the USA. Similarly for other nitrogen-poor biomass, co-
digestion of crop residues with animal manures or municipal sludge substantially
improves CH 4 yield [50]. In the EU, 1,500 million dry tons of biomass are available
each year for biomethanation within the agricultural sector, with half of this being
crops intended for bioenergy production [5]. It should be noted that production of
bioethanol and biodiesel from energy crops only utilizes a fraction of the biomass,
and implementation of AD by the bioethanol industry can generate substantially
more energy (up to 30% of the total energy of the initial biomass) [3, 74]. This also
holds true for many other biomass-based processes producing non-food products.
All these types of feedstocks likely contain bulky materials, such as peeling,
papers, stems and leaves. Pretreatment, especially reduction of particle size by
grinding or milling, is typically required to enhance AD [40]. Other pretreatments
such as alkaline pretreatment [53] have also been evaluated to further enhance the
hydrolysis step in laboratories, but few of them have been implemented in full-
scale AD plants. As mentioned earlier for the AD of livestock manures, co-digestion
with other nitrogen-rich biomass (e.g., municipal sludge or animal manure) can also
substantially stabilize the AD process and increase CH 4 production [50, 94].
The above mentioned wastes have relatively low water contents. They can be
digested using some wet AD processes (e.g., CSTR and CMCR) after dilution. The
Lemvig Biogas plant in Denmark is one example of such wet AD. It is a centralized
biogas plant consisting of three thermophilic CSTR with a total volume of 7,000 m 3