Production of Methane Biogas as Fuel Through Anaerobic Digestion - Sustainable Biotechnology: Sources of Renewable Energy - page 116

nearly maintenance-free, the gas production is rather slow due to poor mass transfer.
Recently, MPFLR has been built at several dairy farms in the USA by GDH, Inc.
Herrema Dairy located in Fair Oaks, Indiana operates a MPFLR, which receives
more than 400 m 3 of manure slurry of 8% solids that is generated by 3,800 heads
of cattle daily. Operated mesophilically with a HRT of 17 days, this reactor pro-
duces enough biogas to steadily fuel two Hess engine-generators of 375 kWh each.
The separated solids from the effluent are dried and reused for bedding in the barns,
while the heat recovered from the engine-generators is used to heat the digester,
barns, and alleyways.
Both CSTR and CMCR have been used in AD of dairy manure slurry. The con-
tinuous mixing significantly enhances biogas production and reduces HRT (from
months to 10-20 days) [11, 15]. Thus, implementation of CSTR and CMCR signif-
icantly reduces the digester volumes required to digest the manure derived from a
given number of cows or hogs. Although these two types of digesters cost more to
build and operate, the increased costs may be offset by the increased biogas produc-
tion and TS reduction. Other types of reactors that have been tried on AD of manure
slurries include hybrid reactors [26] and anaerobic filter reactors [87, 88]. However,
to prevent clogging of the filter media of these two types of reactors, the SS has to
be separated prior to feeding to these biofilm-based digesters, resulting in reduced
biogas production [88]. The superiority of these digesters remains to be determined.
Recent studies have focused on improvement of VS degradation and concomitant
increase in biogas production. Co-digestion with food wastes or crop residues was
found to dramatically increase (by 2-3 folds) biogas production [51, 59]. This is
attributed to the increased input of readily degradable substrate from these wastes.
Temperature-phased AD (TPAD) also substantially improves AD [78], and TPAD of
dairy manure slurry can be completed within a short HRT. The increased conversion
rates at elevated temperature (55 ◦ C) are responsible for the improvement observed
in TPAD systems [91].
3.3 Anaerobic Digestion of Solid Food and Food-Processing
Wastes, Organic Fraction of Municipal Solid Wastes
(OFMSW), and Crop Residues
These wastes are characterized by varying water contents, but high VS contents
(>95%). However, these parameters vary considerably. Most food wastes have bal-
anced nutrients and large amounts of readily fermentable carbohydrates and thus are
among the most suitable feedstocks for AD. According to a recent study, 348 m 3
of CH 4 can be produced per dry ton of food wastes within only 10 days of AD
[93]. Food wastes amount to approximately 43.6 million dry tons each year in
the USA [81]. This represents a potential of 15.2 billion m 3 of CH 4 per year.
During food processing, a significant portion of foodstuffs also ends up in wastes or
wastewaters. For example, 20-40% of potatoes are discarded as wastes during pro-
cessing. National data on the amount of food-processing wastes are not available.