Environmental Engineering Reference
In-Depth Information
the world (Van Horn, 1995; Harkin, 1997). Past practices allowed manure to
be spread without limitation on agricultural land, but now, increasingly
restrictive environmental legislations in many countries require manure
treatment and management technologies such as AD. Zafar (2008)
acknowledged AD in the USA as 'a unique treatment solution for animal
manure, able to deliver multiple benefits, including renewable energy,
avoidance of water pollution, and air emissions'. Anaerobic digestion up-
grades animal manure and slurries from environmentally polluting wastes to
valuable resources used for the simultaneous production of biogas (as
renewable fuel) and digestate (as valuable biofertilizer).
Manures and slurries from a variety of animals can be used as feedstocks
for biogas production (pigs, cattle, poultry, horses, mink and many others).
They are characterized by differing dry matter contents: solid farmyard
manure (10-30% dry matter) or liquid slurry (below 10% dry matter). Their
composition also differs according to the species of origin and the quality of
the animal feed. For simplificity, the general term 'animal manure' is often
used. Manure is an excellent feedstock substrate for AD; with a carbon to
nitrogen (C:N) ratio of around 25:1 and rich in various nutrients necessary
for the growth of anaerobic microorganisms. It has a high buffer capacity,
able to stabilize the AD process in the case of a significant pH decrease
inside the digester, has a natural content of anaerobic microorganisms, is
highly accessible and is cheap.
Solid manure and slurries also have some limitations as a feedstock for
AD. As noted earlier, animal slurries have a low dry matter content (3-5%
for pig slurries and 6-9% for cattle slurries), which gives a low methane
yield per unit volume of digested feedstock, ranging between 10 and 20m
3
methane per cubic meter of digested slurry (Angelidaki, 2002), and biomass
transport costs are high. Both slurries and manures contain various amounts
of straw and fiber particles that are high in ligno-celluloses. The lingo-
cellulosic fractions are known to be recalcitrant to anaerobic decomposition
and usually pass through a biogas reactor undigested, without any
contribution to methane production. As such, although manures have one
of the highest potentials as a feedstock for biogas, their relatively low
methane yield does not provide economic sustainability in the case of mono-
digestion, so they are dependent on co-digestion with co-substrates with a
high methane yield. A number of emerging technologies based on chemical,
mechanic, thermal or ultrasound treatments have been tested in attempts to
disintegrate the recalcitrant matter in animal manure (Angelidaki and
Ahring, 2000) and make it available for the anaerobic microorganisms, with
the aim of enhancing the methane yield and thus the economic efficiency of
manure mono-digestion.
Manure is often co-digested with other co-substrates such as easily
digestible organic wastes from various agro-industries, source-separated
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