Environmental Engineering Reference
In-Depth Information
ethanol plants, all silage fractions are typically anaerobically degradable
(Rosentrater et al., 2006; Cassidy et al., 2008; Drosg et al., 2011). In sugar
cane bio-ethanol plants, the cane juice silage is also a suitable substrate for
AD (Callander and Barford, 1983; Cail and Barford, 1985; Russo et al.,
1985), whereas the bagasse is mainly incinerated for energy recovery. In
biodiesel production, the glycerol (Siles et al., 2009) as well as the
wastewaters (Siles et al., 2010) are suitable substrates for AD, while the
residual cake after oil extraction is often used as animal feed. As the biofuels
and biorefinery industries become more prominent, there will be a greater
need for integrating industrial AD processes.
These large quantities will demand proper strategies for the use and
treatment of the anaerobic digester effluent (Fuchs and Drosg, 2011). The
rising importance of utilizing by-products from the biofuel industry can be
seen best in the case of the bio-ethanol industry. In the last decade, US bio-
ethanol production increased almost tenfold due to the increasing demand
for ethanol as a fuel additive (6.2 million m
3
in 2000 and 50.1 million m
3
in
2010 according to the Renewable Fuels Association).Worldwide bio-ethanol
production rose from 65 million m
3
in 2008 to 95 million m
3
in 2010, an
increase of almost 50% within only two years. However, one important
drawback is that high volumes of bio-ethanol produce high amounts of
effluents. The dry-grind bio-ethanol process from grains produces up to 5.6 t
of stillage per m
3
of ethanol (Drosg et al., 2008). Since this process is the
prevailing process in the USA, it can be estimated that roughly
280 million t/yr of stillage are accumulated in US domestic ethanol
production. The state-of-the-art stillage treatment process is drying to
animal feed. This consumes a considerable amount of energy, since grain
silage has a water content of about 85-90%. Depending on the price of
animal feed and energy, AD can be a valuable option. Using AD on the
annual stillage produced in the USA, roughly 16,300 million Nm
3
/yr of
methane could be recovered. In practice, the nitrogen and sulfur content of
stillage can be a challenge for the stability of the AD process, although a
stable process can be achieved through process optimization measures.
The main limitation for using industrial organic wastes for biogas
production is related to their potential content of undesirable matter such
as biological, physical or even chemical pollutants. Depending on the process
of their origin, industrial wastes can contain physical impurities, pathogens,
heavy metals or persistent organic compounds in such amounts that they
could become sources of environmental pollution or pose health risks for
humans and animals when the produced digestate is used as crop fertilizer.
Environmental legislation, certification systems for digestate and 'positive
lists' have therefore been introduced in many countries, prescribing limit
values for pathogens and pollutants and requiring product declaration of the
feedstock used and the digestate produced. Specific materials (e.g. food
Search WWH ::
Custom Search