Agriculture Reference
In-Depth Information
One of the more common energy produc-
tion/capture systems for liquid manures is anaer-
obic digestion . Anaerobic digestion is a natural
biological process by which bacteria break down
organic matter in an oxygen-free environment
with moisture content of 85% or higher. The
process produces 'biogas', inorganic salts and
residual organic material. The biogas consists
of CH 4 , CO 2 and trace amounts of other gases
including hydrogen sulphide (H 2 S). Biogas can
be burned to produce heat or to power an elec-
tric generator. Zaks et al . (2011) estimated that
anaerobic digesters have the potential to gener-
ate 5.5% of US electricity and mitigate 151 Mt
CO 2 e, mostly from CH 4 abatement. Rico et al .
(2011) reported that anaerobic digestion of
liquid dairy manure at 37°C produced 1.47 m 3
biogas (m −3 day −1 ) and 1 m 3 CH 4 (m −3 day −1 ) and
could provide 2% of the total electrical power
in the region of Cantabria, Spain. Marañón et al .
(2011) reported that anaerobic digestion of cattle
slurry on dairy farms in northern Spain could
provide enough CH 4 to fulfil the farms' energy
requirements and in some cases provide a sur-
plus that could be used for heating and that
annual GHG emissions savings ranged from
978 to 1776 kg carbon dioxide equivalents
(CO 2 e) per year due to reductions in CH 4 emis-
sions during slurry storage. The amount of
biogas produced and the percentage of residual
organic matter depends on the duration of the
anaerobic digestion process and factors such as
temperature, moisture, nutrient content and
pH. The residual organic material can be used
for animal bedding, a soil amendment, or value
added products such as fibreboards and other
building materials. Additional benefits of
anaerobic digestion are the breakdown of
VOCs responsible for odour, and the destruction
of weed seeds and pathogens. Digestion can
occur in anaerobic lagoons or in engineered
systems. The types of anaerobic digester tech-
nology available include: covered anaerobic
lagoons, plug-flow digesters, completely-stirred
tank reactor, upflow anaerobic sludge blanket
and anaerobic sequencing batch reactor. Due
to the large capital investment, initial set-up
costs and expense of running a digester, they
are not always economically feasible, particu-
larly in areas with low energy prices. Yet, co-
digestion of manure and other biomass is a
potential way to improve the economics of
digesters. The co-digestion of dairy manure and
food processing wastes increased biogas produc-
tion by 110% and tripled gross receipts on a
commercial dairy (Frear et al ., 2011). The addi-
tion of vegetable waste in the anaerobic diges-
tion of swine manure increased methane yield
up to threefold (Molinuevo-Salces et al ., 2012).
Once manure has gone through a solid sep-
aration process or through a digester, the efflu-
ent can be treated to capture valuable nutrients
in the liquid stream and concentrate them to
generate a more valuable fertilizer source.
A common technology to capture P, NH 4 + and K
is the use of struvite (magnesium ammonium
phosphate) precipitation or P capture can be
accomplished with hydroxylapatite (calcium
phosphate) formation as well. Struvite precipita-
tion has been found to remove 70-85% of P,
56-95% of NH 4 + and <10% of K (Zeng and Li,
2006; Song et al ., 2011; Yilmazel and Demirer,
2011). However, a large amount of NH 4 -N can
be lost via volatilization (Song et al ., 2011). The
use of hydrated limes to remove P from wastewa-
ters has been shown to remove >90% of P
(Vanotti et al ., 2003; Szögi and Vanotti, 2009).
The use of algae and other photo-bioreactors
can also remove significant amounts of N and P,
although these require light and the end-product
may need significant processing before the nutri-
ents can be re-used efficiently. The culturing of
microalgae for biofuels production using waste-
water as a nutrient source is also an area under
investigation (Lam and Lee, 2012). Chen et al .
(2012) demonstrated that non-filamentous
green algae were able to tolerate high nutrient
loads and could recover nutrients from waste-
water from anaerobic digestion. It has been
reported that up to 98% of N and 76% of P can
be removed from wastewaters (Kebede-Westhead
et al ., 2006; Chen et al ., 2012). Singh et al .
(2011) reported a maximum biomass productiv-
ity of 76 mg l −1 day −1 for microalgae grown on
poultry litter anaerobic digester effluent with a
60% and 80% removal rate of total N and P,
respectively, from the effluent. The algae con-
tained 39% protein, 22% carbohydrates and
<10% lipids, making it a good animal feed sup-
plement. The processed algae have also been
tested as a slow release fertilizer (Mulbry et al .,
2005, 2007).
In addition to nutrient capture, other prod-
ucts can be obtained or made from solid separated
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