Production of Methane Biogas as Fuel Through Anaerobic Digestion - Sustainable Biotechnology: Sources of Renewable Energy - page 123

resulted from the co-digestion of a primary sludge and OFMSW in a TPAD sys-
tem [76]. Additionally, TPAD enhances sanitation of waste streams [73], reducing
potential risks associated with certain types of feedstocks (e.g., municipal sludge
and animal manures). Furthermore, TPAD processes eliminate the AD inhibition
caused by the self-heating of mesophilic AD of high-energy feedstocks (e.g., energy
crop and OFMSW) [49, 52]. The higher energy input required to operate TPAD is
more than offset by the increased biogas and heat produced therefrom [22]. The
TAPD technology will probably be applied more commonly in the near future when
more lignocellulosic feedstocks (e.g., energy crops, animal manure, crop residues,
and OFMSW) are subjected to AD.
Size reduction can dramatically enhance the AD of certain feedstocks, such as
crop residues, OFMSW, and energy crops. Physical and chemical pretreatments can
further enhance AD of these feedstocks [45, 53], but currently they may not be cost-
effective, especially for those feedstocks that contain high water contents and for
wastewaters. Low cost and efficient pretreatments need to be developed.
The entire AD process is often limited by three of the four steps of the AD
process: hydrolysis, syntrophic acetogenesis, and methanogenesis. Hydrolysis of
biomass polymers is typically the rate-limiting step of the entire AD process of
lignocellulolytic feedstocks. Single or mixed cultures of lignocellulolytic microbes
may be used to augment the capability of hydrolysis in digesters as exemplified by
enhanced AD of cattle manure [62] and municipal sludge [30]. Methanogenesis can
become the rate-limiting step when feedstocks containing large amounts of read-
ily fermentable substrates (e.g., starch) are digested. In this scenario, acid-tolerant
methanogen (e.g., Methanobrevibacter acididurans ) cultures may be prepared
and used to enhance the entire AD process or remediate upset AD operation.
Bioaugmentation can also enhance the AD of feedstocks containing high concen-
trations of particular substances, such as lipids [19]. As more and more digesters
are put into operation, there will be increasing needs for such specialty cultures to
enhance existing digesters, start up new digesters, and prevent AD failures.
5.2 Optimizing AD Process Stability
AD process control on current digesters is still relying on input and output data:
primarily biogas yield and composition, and pH. When the output data suggest any
abnormality in performance, it is often too late to intervene, leading to severe disrup-
tion of normal operation. Thus, there is an urgent need for research and development
of on-line systems that can monitor important parameters of the actual AD pro-
cess. Some of the key parameters of AD and their modeling have been reported
[35, 71], which can guide the research effort to develop online monitoring systems.
Propionate was recently identified to be an important indicator of AD performance
[12, 63], and online monitoring of this important SCFA using gas chromatogra-
phy seems promising [12, 70]. Further understanding of the microbial communities
involved in AD processes may also allow for the development of biosensors that