Page 190 - Materials Chemistry, Second Edition

178 B. Ruggeri et al.

consumption depends to a great extend on the salary of the workers operating in
the plant, and this can introduce a false energy charge on the technology under
study. For all these reasons, only the energy spent to sustain labor as food was
considered, via a dietary evaluation by LCA approach (Sanfilippo and Ruggeri
2009). The AD is not a labor-intensive technology, and hence, only one worker
was considered necessary to run the plant, regardless of the plant dimensions.
Finally, all the energy fluxes crossing the boundaries of the system were consid-
ered diameter dependent, and it has been used as scale-up parameter in scaling
procedure.

3 Sustainability of Anaerobic Digestion

3.1 Introduction to the Anaerobic Digestion Process

AD is a naturally occurring decomposition process, by which organic matter is
broken down to its simplest chemical components under anaerobic conditions.
This process can be very useful to treat organic waste such as: sewage sludge,
organic farm wastes, municipal liquid/solid wastes, green/botanical wastes as well
as organic industrial and food commercial wastes.
The overall of anaerobic digestion process can be schematically divided into 4
sections, as shown in Fig. 4: pretreatment, digestion, gas upgrading, and digestate
treatments. The key point is the digestion unit, which can work at different con-
ditions, e.g., pH, temperature and hydraulic retention time and one or more stages.
Before being digested, the feedstock has to be pretreated. Various types of
pretreatment can be adopted depending on the feedstock; the addition of water or
on towing away undesirable materials such as large items and inert materials (e.g.,
plastic, glass, and metals) to allow a better digestate quality are generally applied.
A more efficient digestion and higher energy production are obtained by means of
acid or base as well as thermal pretreatments (Kim et al. 2009; Wang and Wan
2008a; Yang et al. 2007; Mu et al. 2006; Chen et al. 2002). The digestion process
itself takes place in a digester, which can be classified in relation to the temper-
ature, the water content of the feedstock, the number of stages (single or multi-
stages and the type of biogas produced, that is methane or hydrogen (Ueno et al.
2007; Kraemer and Bagley 2005). During the natural anaerobic digestion process,
some bacteria convert the organic material present in the digester into hydrogen,
carbon dioxide, and water-soluble metabolites, such as ethanol, acetic, butyric, and
propionic acids (Tommasi 2011). These bacteria usually live in close proximity to
other bacteria that consume these metabolites, including hydrogen, and produce
final products such as methane and CO 2 . If the differences between hydrogen
forming bacteria HFB which produce H 2 and hydrogen-consuming bacteria HCB
are known, it is possible to design a 2-stage operation condition process
(Lakaniemi et al. 2011). The combination of multistage processes with the