Environmental Engineering Reference
In-Depth Information
household waste, energy crops or sewage sludge. The high water content in
slurry acts as a solvent for the co-substrates, ensuring proper stirring in the
digester and homogeneity of the feedstock mixture. Compared with mono-
digestion, co-digestion of manure and organic wastes results in a higher
stability of the AD process. This is partly due to higher active biomass
concentration inside the digester, which is considered to be more resistant to
inhibitory compounds, and partly because of the presence in the wastes of
inorganic matter such as clays and iron, considered to counteract the
inhibitory effects of ammonia and its respective sulfide (Angelidaki, 2002).
Anaerobic digestion of animal manures and slurries is widely applied and
is increasingly developing in Europe, Asia and North America, in most
places not only for renewable energy purposes but also as means to protect
the environment and recycle materials efficiently into farming systems
(Zafar, 2008). There is a huge potential to increase the use of manures and
slurries as a feedstock for biogas, considering that only a small fraction of
the global production is currently digested in biogas installations. Even in
countries with very developed biogas sectors like Denmark, where manure-
based biogas plants are dominant, only 3-6% of the produced animal
manure and slurries is supplied to biogas plants each year. The co-digestion
of animal manure and suitable organic wastes is likely to increase in the
years to come, concentrated in places where these feedstock substrates are
available and qualitatively suitable.
Plant (crop) residues
The category of plant residues includes various vegetable agricultural by-
products and harvest residues, plants and plant parts, low-quality or spoiled
crops, fruits and vegetables, and spoiled feed silage. Plant residues are
usually digested as co-substrates with animal manures and other feedstock
types. Most of them need to be pre-treated before feeding in to a digester.
Pre-treatments range from simple mechanical particle size reduction to more
complicated treatments aiming at breaking the ligno-cellulosic molecules in
order to facilitate the access of anaerobic microorganisms to these
structures. A particle size of 1 cm (Amon and Boxberger, 1999) allows
proper handling and mixing with other feedstock types and ensures good
digestion.
Energy crops
The cultivation of crops specially dedicated to energy production was
developed in the 1990s in countries such as Germany and Austria, although
Buswell investigated the methane potential of various crops as far back as
the 1930s (Murphy et al., 2011). Many varieties of crops, both whole plants
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