Agriculture Reference
In-Depth Information
Chemical flocculants have also been used in
conjunction with solid-liquid separation sys-
tems in order to improve separation efficiencies.
For example, solid separation using screens has
low efficiency with solids removal rates of 5 to
15% (Vanotti and Hunt, 1999), whereas solid
separation using screens with a flocculant agent
can remove >90% of total and volatile solids
and >70% of chemical oxygen demand (COD)
and total N and greater than 50% of TP (García
et al
., 2009; Paz Pérez-Sangrador
et al
., 2012).
Precipitation or flocculation in a treatment cell
where the material can be harvested is beneficial.
Although there are many separation techniques
available for use on livestock farms, there is still
a need to improve the cost effectiveness of these
technologies. Once manure has gone through
a solid separator, the remaining liquid is either
transferred to a storage pond where it is kept
until land application, or it can be further pro-
cessed to produce recycled cleaner water, energy
and other by-products.
Once the liquid fraction is transferred to a
storage pond, basin or lagoon, there can be addi-
tional losses of N due to NH
3
volatilization as
well as the generation of CH
4
and volatile
organic compounds (VOCs; responsible for
odour) due to the development of anaerobic
conditions. One way to minimize these emis-
sions is to utilize a
cover
. Guarino
et al
. (2006)
tested several permeable covering systems
(maize stalks, wood chips, vegetable oil,
expanded clay, wheat straw) to reduce emissions
from livestock slurry tanks and lagoons. They
reported reductions of NH
3
emissions from
swine and dairy slurry in the range of 60-100%
with 140-mm solid covers or 9-mm liquid covers.
Miner
et al
. (2003) reported that a permeable
polyethylene foam lagoon cover reduced NH
3
emissions by approximately 80% on an anaero-
bic swine lagoon. Floating an impermeable cover
over the surface of a lagoon or pond can also
capture up to 80% of methane and reduce
odours. The trapped gas can be flared or used to
produce heat or electricity. Craggs
et al
. (2008)
reported that placing a floating polypropylene
cover on anaerobic swine and dairy ponds
yielded biogas recoveries of 0.84 m
−3
m
−2
day
−1
and 0.032 m
−3
m
−2
day
−1
, respectively. They esti-
mated that this could produce 1650 and
135 kWh day
−1
from fully covered anaerobic
swine and dairy ponds. Permeable surface covers
not only act as a physical barrier to gas trans-
port, but can also support microbial communities
that are capable of utilizing reduced gases emit-
ting from the slurry (Petersen and Miller, 2006)
providing additional mitigation benefits.
When land for application of liquid manures
is limited, treatment systems that remove N via
biological nitrification/denitrification
have been
employed. Uncovered anaerobic lagoons have
commonly been used to treat livestock wastewa-
ters with the main treatment focusing on N vola-
tilization and reduction of solids. While the goal is
to reduce N by conversion to N
2
gas there is still a
large amount of NH
3
volatilized into the atmos-
phere. These lagoons are also large sources of CH
4
as the solids are broken down in anaerobic condi-
tions, and they have been identified as one of the
major GHG emitting sources in livestock produc-
tion systems (Leytem
et al
., 2011). Systems that
capture the CH
4
and use it for energy generation
or flare the CH
4
help mitigate this impact (see
below). The design of batch reactors to convert
NH
4
-N to N
2
has also been an area of research
(Loughrin
et al
., 2009; Wang
et al
., 2010). Related
to this is the anaerobic ammonium oxidation
technology where ammonium (NH
4
+
) is oxidized
to N
2
and has been shown to remove up to 92% of
NH
4
-N from swine manure effluent (Molinuevo
et al
., 2009). Another cost effective and passive
method for treating wastewaters is the use of con-
structed wetlands which are primarily designed
to remove N prior to land application through
plant uptake and denitrification. These con-
structed wetlands also have the added benefit of
reducing the total suspended solids (TSS), COD
and P, which are important from a water quality
standpoint. A marsh-pond-marsh constructed
wetland was shown to remove up to 51% of
TSS, 50% of COD, 51% total N and 26% total P
from swine wastewater (Poach
et al
., 2004).
Constructed wetlands can also release N as NH
3
,
although this has been shown to be a relatively
small portion of total N loss (Poach
et al
., 2002).
The main limiting factor for denitrification in
these systems is the conversion of N to nitrate
(Hunt
et al
., 2006). While these management
strategies can reduce the potential for water
quality impairment from over-application of N,
manure management strategies that are desig-
ned to waste valuable N are difficult to justify as
replacing the lost N via chemical fertilization
requires considerable expenditure of energy.
