A whole-farm assessment of the efficacy of slurry acidification in



A whole-farm assessment of the efficacy of slurry acidification in
Available online at www.sciencedirect.com
Europ. J. Agronomy 28 (2008) 148–154
A whole-farm assessment of the efficacy of slurry acidification
in reducing ammonia emissions
P. Kai a , P. Pedersen b , J.E. Jensen c , M.N. Hansen a , S.G. Sommer d,∗
Department of Agricultural Engineering, University of Aarhus, Schüttesvej 17, 8700 Horsens, Denmark
Department of Pig Housing and Production Systems, Danish Pig Production, Vinkelvej 11, 8620 Kjellerup, Denmark
c Extension Service Landbonord, Erhvervsparken 1, 9700 Broenderslev, Denmark
d Faculty of Engineering, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark
Received 6 April 2007; received in revised form 21 June 2007; accepted 24 June 2007
Livestock slurry in animal houses, in manure stores and applied on fields is in Denmark the most important source of ammonia (NH3 ) in the
atmosphere. The emitted NH3 is a source of NH3 and ammonium (NH4 + ) deposition, which causes eutrophication of N-deficient ecosystems and
may form NH4 + -based particles in the air, which are a risk to health. This study examines the reductions in NH3 emissions from pig houses, manure
stores and manure applied in the field achieved by acidifying the slurry in-house. Sulphuric acid was used to acidify pig slurry to pH < 6 and the
system was constructed is such a way as to prevent foaming in the animal house as well as during storage. Acidification of the pig slurry reduced
the NH3 emission from pig houses by 70% compared with standard techniques. Acidification reduced NH3 emission from stored slurry to less
than 10% of the emission from untreated slurry, and the NH3 emission from applied slurry was reduced by 67%. The mineral fertilizer equivalent
(MFE) of acidified slurry was 43% higher compared with the MFE of untreated slurry when applied to the soil. The odour emission from the slurry
was not affected significantly by the treatment. The slurry acidification system is approved Best Available Technology (BAT) in Denmark.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Ammonia; Emission; Acidification; Pig slurry
1. Introduction
Livestock production is the most important source of ammonia (NH3 ) in the atmosphere in Europe (ECETOC, 1994;
Hutchings et al., 2001) and the annual emission from Danish
livestock production is 51,000 t NH3 -N. NH3 readily combines
with sulphate (SO4 2 ) and with nitrate (NO3 − ) to form particulates containing ammonium (Asman et al., 1998). Particulate
NH4 + , and to a lesser extent NH3 , may be transported over
long distances. Deposition of NH3 or particulate NH4 + to
land or water may cause eutrophication of natural ecosystems
(Fangmeier et al., 1994). Furthermore, particulate NH4 + contributes to airborne PM2.5 and PM10 particulates that can be
a health hazard (McCubbin et al., 2002; Erisman and Schaap,
2004). Consequently, ceilings on permitted annual NH3 emissions were included in the Gothenburg Protocol of the United
Nations Convention on Long-Range Transboundary Air Pollu-
Corresponding author. Tel.: +45 65507359; fax: +45 65507474.
E-mail address: [email protected] (S.G. Sommer).
1161-0301/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
tion (CLRTP; United Nations, 2004) and in the EU National
Emissions Ceilings Directive (NECD; EEA, 1999).
For farmers, the loss of NH4 + via NH3 emission from animal
houses, manure stores and applied manure will reduce the fertilizer value of animal manure (Sørensen and Amato, 2002). In
addition, the emission of NH3 will cause variability and uncertainty in the mineral fertilizer equivalent (MFE) of the manure,
reducing farmers’ confidence in manures as a source of N for
crops. Thus, technologies that have the ability to reduce NH3
emissions while still maintaining a high predictability of the
N fertilizer value of manure may contribute to reducing the
oversupply of N to crops.
Acidification of manure is an obvious treatment for the purpose of reducing NH3 emissions from livestock production.
Until now, developments of the technology have failed due to the
risk of foaming and because of the potential hazards associated
with the use of acids (Borst, 2001). However, reducing the pH of
slurry by adding easily fermented biomass to slurry stores has
been shown to reduce NH3 emissions from stored slurry and following land application (Vandre and Clemens, 1997; Clemens
et al., 2002).
P. Kai et al. / Europ. J. Agronomy 28 (2008) 148–154
Introducing a new technology to regulate one source of pollutants on a farm may affect the emissions downstream in the
management chain of manure on the farm. Therefore, it has
been proposed that introducing a technology should be evaluated
using a whole-farm perspective of agricultural waste treatment
and management (Petersen et al., 2007). This study measured
the effect of in-house acidification of slurry in pig houses on the
NH3 emissions from the pig house, slurry store and following
land application. Furthermore, the effect on odour emission from
pig houses were measured and the mineral fertilizer equivalent
(MFE) of acidified slurry and untreated slurry was quantified in
field studies.
2. Materials and methods
The slurry acidification technology used here was developed
by Jens Oestergaard Jensen, Broenderslev, Denmark, and the
first commercial version was produced in cooperation with Staring Maskinfabrik A/S (now Infarm A/S).
2.1. Ammonia emission from animal houses
The reduction of NH3 emissions through in-house acidification of the slurry was examined on a farm with growing/finishing
pigs. The measurements were carried out by Danish Pig Production during three periods (November 2002–January 2003;
February–May 2003; June–August 2003) covering the typical climate variation over the year in Denmark. The fatteners
were housed in pens with a fully slatted floor with 18 mm
slats and 60 cm slurry channels underneath. Two-thirds of the
pen floor had 9 cm beams, and 1/3 had 18 cm beams. The pig
houses were mechanically ventilated with low-pressure ventilation with a diffuse air inlet through the ceiling. The pigs
were fed ad libitum on a standard commercial diet containing 20.0% barley, 49.2% wheat, 20.5% soybean meal, 7.0%
rapeseed meal, 2.4% animal fat and 0.9% premix (minerals, vitamins and amino acids), which contained 4.7 g P kg−1
The equipment for acidifying slurry consisted of a 20 m3
treatment tank to which concentrated sulphuric acid (96%
H2 SO4 ) was added at a rate of 0.5 kg acid to 100 L slurry
(Infarm A/S, Aalborg). Slurry from the slurry channels was
transferred through 315-mm pipes to the treatment tank, where
acid was added to the bottom of the treatment tank and the
slurry aerated to reduce foaming. A fraction of the slurry in
the treatment tank was pumped back to the slurry channels
through a 110-mm pipe system. The slurry channels were flushed
6–10 times per day. After flushing, a layer of slurry about
15 cm deep was left in the slurry channels. Surplus slurry from
the treatment tank was transferred to the slurry store at the
An animal house with 1200 pig places was used for the study.
The animal house was divided into four separate and identical
units with separate slurry channels and ventilation outlets. In
two units the slurry channels were flushed with acidified slurry,
and in two units the slurry was left untreated.
The NH3 emission (E) was determined by measuring the air
exchange rate V and the NH3 concentration in the exhaust air:
V =
ECO2 ,a N
CO2,out − CO2,in
where V (m3 h−1 ) is the air exchange rate, ECO2 ,a the CO2 production from the animals (mg head−1 ), N the number of animals,
CO2,out (mg m−3 ) the concentration of CO2 in the air from the
ventilation outlet and CO2,in (mg m−3 ) is the concentration of
CO2 in the inlet air, i.e. the background concentration. When
the air exchange rate is known, the emission can be calculated
using the following equation:
Ex = V (Cx,out − Cx,in )
where Cx,out and Cx,in are the concentrations of a gas (mg m−3 )
in air collected in the ventilation outlet and in incoming air, and
x is the gas measured.
ECO2 ,a was assumed to be 185 L per heat-producing unit
(1 hpu = 1000 W total heat produced at 20 ◦ C) corresponding to
five fattening pigs each with a weight of 68 kg (Pedersen and
Sällvik, 2002). The concentrations of CO2 in the ventilation
outlet and in incoming air were determined with an IR sensor
(Dräger Polytron IR CO2 ), and the NH3 concentration was measured using a chemoluminescence sensor (Dräger Polytron 1).
The two sensors from Dräger, along with preheating of the sampled air and a valve system to switch between five measuring
points, were included in a measuring device from VengSystem
(Roslev, Denmark). One measuring point was located in the ventilation exhaust of each of the four pig units, while one measuring
point (background) was located on the attic above the pig rooms
above the insulation through which the fresh air was drawn into
the pig rooms using low pressure. NH3 , CO2 and the inside and
outside temperatures were measured every half hour.
2.2. Odour emissions
The odour emission from the pig units with and without acidified slurry was measured using the European standard procedure
(CEN, 2003). Air from the exhausts of each unit was sampled
in 30 L Tedlar bags at a rate of 1.0 L min−1 . A vacuum pump
was used to create a vacuum between a rigid container and the
Tedlar bags inside the container. The air samples were collected
in duplicate from each pig room three or four times during each
batch cycle (feeding cycle of a group of pigs). The samples were
analysed within 24 h at a certified odour laboratory. The unit of
the resulting odour concentration was termed European odour
units per cubic meter (OUE m−3 ). The ventilation rate was determined by use of a calibrated measuring wing (Fancom AT(M)
unit 80). The odour emission was calculated by multiplying the
odour concentration and the concomitant ventilation rate and
divided by the number of livestock units (LU equal to 500 kg
live weight).
For each measuring day, the odour emission was calculated
by multiplying the geometric mean odour concentration of the
duplicate odour samples and the concomitant ventilation rate and
divided by the number of livestock units (LU equal to 500 kg live
P. Kai et al. / Europ. J. Agronomy 28 (2008) 148–154
weight). Before statistical analysis the odour emission factors
were transformed logarithmically. To determine a possible effect
of slurry acidification on the pig room odour emission an analysis of variance (ANOVA) following the generalized linear model
procedure were performed on the data with systematic effects
from treatment, pig weight and external temperature, while batch
number, pig room, and measurement day contributed with random effect. For all the statistics, a significance level of α = 0.05
was applied.
2.3. Ammonia emission from stored slurry
The emission of NH3 was examined by determining the mass
balance of total nitrogen (total-N) in slurry stored in PVC containers 0.95 m high, 0.95 cm wide and 1.15 cm long (volume
1.04 m3 ). The open stores were placed outside, under a roof to
prevent rain and snow from entering the slurry but still allowing
air to flow unhindered over the slurry store. At initiation and
at the end of a storage period, the amount of slurry was determined by weighing, and the slurry was sampled. The stored
slurry was stirred with a propeller mixer before sampling. The
slurry samples were stored at −18 ◦ C until analysis for TAN
(total ammoniacal nitrogen = NH3 + NH4 + ), total-N, dry matter
(DM) and pH. In each experiment, slurry from one pig farm but
with and without acid treatment was stored in two containers.
Two slurry storage experiments were carried out, one during the
6 months from 1 September 2002 until 1 May 2003 and the
second during the 13 months from 1 September 2002 until 1
November 2003.
2.4. Ammonia emission from slurry applied in the field
The effect of acidification on NH3 emission from applied pig
slurry was examined in a comparative field study at Research
Centre Bygholm in May 2003 where NH3 emission from bandspread untreated and acidified slurry was compared. The slurry
was spread with a tractor-driven spreader equipped with 40 trail
hoses mounted 30 cm apart on a 12 m spreading bar. The bar was
mounted 1 m above the soil surface but the trail hoses allowed
the slurry to be placed exclusively on the soil between the rows
of the winter wheat plants in rows sown 12 cm apart.
The NH3 emission FZINST (g NH3 -N m−2 ) from slurry
applied to circular plots with a diameter of 36 m was measured
using the micrometeorological mass balance method described
by Wilson et al. (1982). The emission can be inferred from measurements of the increase in atmospheric NH3 concentration in
the air parcels passing the experimental plot χ in g [NH3 -N] m−3
and wind speed u in m s−1 at a single height HZINST in metres
above the ground:
The term KZINST is the unit flux uχ/F ratio, which is given by
Wilson et al. (1982). The value of HZINST in this study was
1 m. Surface roughness of the wheat crop (height 5–10 cm) was
assessed to be z0 = 0.7 cm.
The horizontal flux (uχ) was measured using passive Leuning
samplers (Leuning et al., 1985) mounted at the HZINST height
1 m above the soil surface, on masts placed at the upwind edge
and at the centre of the plot (radius 18 m). According to Leuning
et al. (1985), uχ is derived from
uχ =
where M is the mass (g) of NH3 -N collected by oxalic acid
coating the interior of the Leuning sampler during the sampling
period t (s), and A is the effective cross-sectional area of the opening of the sampler (m2 ) determined in wind-tunnel calibrations.
After exposure, the coating was dissolved in 0.040 L deionized water, and the NH4 -N content in g L−1 was determined by
the indophenol blue colorimetric method using a spectrophotometer (Shimadzu UV-120-01, Japan). Two Leuning samplers
in the background and two samplers in the centre of each of
the two plots amended with acidified slurry and with untreated
slurry were used to estimate the emission from each. The emission was determined for the following time intervals: 0–1.5,
1.5–5.5, 5.5–24.5, 24.5–48.5, 48.5–72.5, 72.5–96.5, 96.5–144.5
and 144.5–168.5 h.
2.5. Fertilizer efficiency of slurry
In Denmark the mineral fertilizer equivalent (MFE) is defined
as the mineral fertilizer-N (Nfertilizer ) required to achieve the
same yield as 100 kg manure-N (Nmanure ). In this study the MFE
was assessed by surface-applying the slurry to a sandy loam soil
with 5% clay, pH 5.5–6, in 0.01 M CaCl2 , 17.6–26.4 g C kg−1
dry soil. The acidified and untreated slurry was on the fields
near Extension Service Landbonord, Broenderslev (Northern
Jutland) added to the plots (2.5 m wide and 14 m long) at a
rate corresponding to the addition of 150 kg N ha−1 . To ensure
an adequate application of nitrogen, 50 kg of mineral fertilizer (ammonium nitrate) was also added to the plots. In total
the slurry-amended plots received 183–230 kg total-N ha−1 . For
comparison mineral fertilizer (ammonium nitrate) was applied
to plots at rates of 50, 100, 150 and 200 kg N ha−1 . The experiments were carried out in triplicate on winter wheat in 2001,
2002 and 2003 and in triplicate on spring barley in 2003. The
fields are in a rotation where manure is applied to the crop each
year, and there is now grassing on the land. The MFE is defined
as follows (Petersen, 1996):
Y = a + bX
(kg ha−1 )
where Y is the grain yield
and X is the addition
of mineral fertilizer (kg N ha−1 ) in the fertilizer plot trial, a
(kg ha−1 ) is the intercept with the abscissa and b (kg kg N−1
is the inclination of the linear regression. The fertilizer-N equivalent (E, kg N ha−1 ) of manure-N+ mineral fertilizer-N added to
the manure plots is calculated as follows by rearranging Eq. (5):
Ymanure − a
where Ymanure is the grain yield (kg ha−1 ) of the crop in the
plots amended with manure (Nmanure , kg N ha−1 ) and mineral
P. Kai et al. / Europ. J. Agronomy 28 (2008) 148–154
fertilizer (MF, kg N ha−1 ):
E − MF
× 100
where MFE is the amount of mineral fertilizer N (kg N ha−1 )
that would give the same yield as 100 kg ha−1 manure nitrogen
added to the plot. In this study MF was 50 kg N ha−1 .
2.6. Chemical analysis
Total nitrogen (TN) was analysed using the Kjeldahl method
and a Kjellfoss 16200 (Copenhagen, DK). The dry matter (DM)
content was determined after a 24-h drying period at 105 ◦ C.
The manure pH was determined with a pH-meter (Radiometer A/S, Copenhagen, Denmark). The ammonium content of
slurry samples was analysed by means of a QuickChem 4200
flow injection analyser (Lachat Instruments, Milwaukee, WI,
3. Results
Fig. 1. NH3 emission from fattening pig houses divided into two parts, one
with slurry left untreated and one with acidified slurry. The average outdoor
temperature in the measuring periods was: November 2002–January 2003, 3 ◦ C;
February–May 2003, 4.7 ◦ C; and June–August 2003, 19.0 ◦ C.
3.2. N emission from stored slurry
3.1. Ammonia and odour emisson pig houses
The pigs were given a standard commercial diet, therefore, the slurry removed from the pig houses was typical for
slurry produced in Danish pig production systems. NH3 emission factors from the pig units with acidified slurry and from
the untreated control units were obtained from a total of 12
batches (including 1793 pigs produced in the two units with
slurry acidification and 1890 pigs produced in the two units
with untreated slurry (Table 1). Average NH3 emission from the
units with acidified slurry was 0.13 kg NH3 -N per pig produced
(95% confidence interval 0.07–0.19 kg NH3 -N), in contrast,
emission from the units with untreated slurry was 0.43 kg pr.
pig produced (95% confidence interval 0.38–0.49 kg NH3 -N).
There was no significant yearly variation in NH3 emission
(Fig. 1).
No significant difference between the odour emissions from
units with and with out acidified slurry was observed. The
average odour emission from rooms with acidified slurry
was 104 OUE s−1 LU−1 (95% confidence interval 45–239),
whereas in the rooms with untreated slurry the average odour
emission was 98 OUE s−1 LU−1 (95% confidence interval
It has been shown that NH3 emission is the dominant pathway
of N loss from stores with no runoff or leakage (Sommer, 1997);
therefore it is assumed that NH3 emission is the main pathway
of N loss in this study. No surface crust developed on the stored
pig slurry, therefore 5% of the N was emitted during storage
of untreated slurry for 6 months in winter 2002/2003 and 45%
during the 13-month storage period winter, spring and summer
2002/2003 (Fig. 2). The emission from acid treated slurry was
<10% of the emission from untreated slurry.
3.3. Ammonia emission from land-applied slurry
The pig slurry applied to the soil in the field study was from
two different pig production facilities; and consequently there
Table 1
Production parameters obtained during the pig housing study
Production parameter
Acidified slurry
Number of batches
Number of pigs produced
Weight at start (kg)
Weight at slaughtering (kg)
Daily gain (kg)
Feed conversion rate (FEsv kg−1 )
Meat pct. of total weight
Mortality rate (%)
Fig. 2. Nitrogen loss from untreated and acidified pig slurry stored in 1.13-mhigh open PVC stores covered by a roof for 6 months during a winter period and
13 months during a winter, spring and summer period.
P. Kai et al. / Europ. J. Agronomy 28 (2008) 148–154
Table 2
Composition and application rate of untreated and acidified slurry used in the experiment studying NH3 emission from slurry applied to a winter wheat
Type of slurry
DM (%)
Total N (kg t−1 )
NH4 -N (kg t−1 )
Application rate (t slurry ha−1 )
Application rate (kg TAN ha−1 )
Table 3
Estimates of the parameters and the P value of the linear model Y = a + bX fitted
to the yield response data, a (100 kg ha−1 ) is the intercept with the abscissa and
b (100 kg kg N−1 ) is the inclination of the linear model (S.E. of the estimates in
parentheses, n = 4)
Winter wheat
Winter wheat
Winter wheat
Spring barley
52 (2)
39 (4)
33 (4)
24.4 (0.6)
0.09 (0.02)
0.13 (0.03)
0.14 (0.03)
0.080 (0.005)
4. Discussion
4.1. Ammonia and odour emission pig houses
Fig. 3. NH3 emission from acidified and untreated pig slurry applied in the field.
were variations not only in slurry pH, N-total and TAN concentration, but also in DM (Table 2). Almost 50% of the applied
ammonium N was found to be volatilized from the untreated
slurry (Fig. 3). NH3 emission from slurry acidified to pH 6.3
accounted for 16% of applied TAN in the acidified slurry.
3.4. Fertilizer efficiency of the slurry
In the field study, the linear equation shown in Table 3 and
Fig. 4A gave a very significant fit of the relationship between
grain yield and the application of nitrogen in mineral fertilizer.
The mineral fertilizer equivalent (MFE) was therefore assessed
using this relationship. In each experimental year the MFE was
significantly higher in plots amended with acidified slurry compared with the yield in plots fertilized with untreated slurry
(P < 0.05; Fig. 4B). On average, for all four crops, the MFE
equivalent of the acidified slurry was 86 kg fertilizer N as compared with 60 kg fertilizer N in the untreated slurry (Fig. 4B),
i.e. 43% higher than that of untreated slurry.
NH3 emission from pig houses with acidified slurry was
0.13 kg NH3 -N per pig produced and from houses with untreated
slurry was 0.43 kg NH3 -N per pig. The emission was therefore
reduced by 70%. The emissions from units with untreated slurry
were similar to the Danish emission standard for this design
of pens which is 0.44 kg NH3 -N per pig produced (Poulsen et
al., 2001). Emission of NH3 from an animal house is related
to the concentration of TAN in the slurry, slurry pH, and air
exchange from the slurry surface to the atmosphere outside the
house (Aarnink et al., 1996; Ni et al., 1999; Sommer et al.,
2006). These factors seem to have been constant during the
study, as there was very little yearly variation in NH3 emission from the untreated pig house (Fig. 1). Although reducing
the pH of the slurry reduces NH3 emission from the animal
house, this treatment cannot completely eliminate the emission
because NH3 is present not only in the channels but also on
the soiled slats (Rom, 1995; Aarnink et al., 1997a,b; Kai et al.,
Although no statistically significant (P < 5%) difference was
observed, the odour panel evaluating the odour concentration
Fig. 4. Winter wheat grain yield response curve (85% DM) at increasing fertilizer nitrogen application (A) and mineral fertilizer equivalent (MFE) of untreated and
acidified pig slurry applied to winter wheat and to spring barley (B).
P. Kai et al. / Europ. J. Agronomy 28 (2008) 148–154
stated that the odour character of air sampled in rooms with
acidified slurry was different from that of the untreated slurry.
4.2. N emission from stored slurry
It has been shown that denitrification and nitrous oxide loss
from stored slurry is insignificant (Sommer, 1997; Sommer et
al., 2000). Further, there was no leakage of slurry from the slurry
containers; therefore, it is assumed that in this study NH3 emission is the main pathway of N loss. The loss rates from untreated
pig slurry are at a level similar to the emissions of NH3 from
stored and uncovered pig slurry measured in Danish wind-tunnel
studies (Sommer et al., 1993). From acidified slurry the N emission was 0.3% and 5% of total N during 6- and 13-month storage
of slurry. During the 13-month storage period, the pH of acidified
slurry increased 1.1 pH units, indicating a reduction in organic
acids and an increase in bicarbonates (Sommer and Husted,
1995; Sommer and Sherlock, 1996). The pH increase and the
high temperatures during the summer may have caused the loss
of 5% total N from acidified slurry.
The acidification significantly reduced N emission from
stored pig slurry, with the reduction being at the same level as the
reduction in NH3 emission achieved by covering stored slurry
with leca pebbles, straw, natural surface crust or a PVC cover,
where NH3 emissions below 2% of total N have been measured
(Sommer et al., 1993; Sommer, 1997; Hörnig et al., 1999; Xue
et al., 1999).
4.3. Ammonia emission from land-applied slurry
Warm, sunny and windy conditions following application of
the slurry may have caused the high emission of NH3 from
untreated slurry applied to the field (Huijsmans et al., 2001;
Huijsmans et al., 2003; Bussink et al., 1994; Sommer et al.,
1991). The acidified slurry had a higher DM content (4.2%),
which is known to give a high NH3 emission potential (Sommer
and Olesen, 1991), but despite a high DM content the NH3 emission from acidified slurry was low compared to the emission
from untreated slurry with a DM content of 3.7% (Fig. 3). NH3
emission from slurry acidified to pH 6.3 was 33% of the NH3
emission from the untreated slurry and accounted for 16% of
applied TAN in the acidified slurry. Similar results have been
obtained by Stevens et al. (1992), who found a 75% decrease in
NH3 emission from cattle slurry acidified to pH 6.5.
4.4. Fertilizer efficiency of the slurry
In this study a linear regression gave the best fit to data, therefore it was decided to use a linear regression when assessing the
MFE. On average, for all four crops, the MFE equivalent of the
acidified slurry was 26 kg fertilizer N higher than the MFE of
N in the untreated slurry applied in the field, which corresponds
to an increase in MFE of 43% following acidification compared
to the MFE of N in untreated slurry (Fig. 4B). Moreover, the
acidified slurry N was less variable than the untreated slurry, as
reflected in a standard error (S.E.) of 4.8 compared with a S.E.
of 11.0 for the untreated slurry.
Fig. 5. Whole-farm assessement of the gaseous nitrogen losses during manure
management of the slurry produced by one growing/finishing pig (30–100 kg
liveweight) with and without slurry acidification. In the bottom is given the fertilizer equivalent of the manure applied assuming that of the 3.15 kg N excreted,
the amount applied in acidified slurry will be 2.97 kg N and in untreated slurry
2.41 kg N.
4.5. System evaluation
A whole-farm system analysis of the efficacy of slurry acidification was performed (Fig. 5). This was based on the amount of
N excreted by a Danish growing/finishing pig during complete
growth from 30 to 100 kg live weight being 3.15 kg N (Poulsen
et al., 2001). Standard values were chosen for assessing NH3
emission from untreated slurry in order to relate the emission
reduction to standard conditions of animal slurry management
in Denmark, i.e. NH3 losses were calculated using the Danish
manure standard emission factors to estimate emissions from
untreated slurry (Poulsen et al., 2001; Sommer and Hansen,
2004), the coefficients are as follows: housing 16%, store 9%
and application 10.5% of total N in the slurry (NH4 + :totalN ratio = 0.7). For acidified slurry the corresponding emission
coefficients are 4.8%, 1% and 3.5% using the reduction factors
resulting from the present study.
If the emission of NH3 is calculated from housing, storage and
following land application, then of the 3.15 kg excreted N there
will be 2.86 kg N available for the crop in acidified slurry as compared to 2.16 kg N in untreated slurry, i.e. acidification increases
the amount of N available for the crop by 32%. Taking into
account the fertilizer efficiency of the slurries, the amount of N
in acidified slurry produced by one pig corresponded to 2.55 kg
fertilizer N, whereas the untreated slurry corresponds to 1.46 kg
fertilizer N, i.e. an increase of 75%. Thus, not only did acidification reduce NH3 losses during housing, storage and application
significantly but also improved the fertilizer efficiency of the
The annual cost of treating the slurry with this technology
is D 60 per livestock unit. Taking the increase in N MFE into
account, the farmer would pay approximately D 40 per livestock unit for reducing the NH3 emission from pig houses. The
slurry acidification system is approved Best Available Technology (BAT) in Denmark.
P. Kai et al. / Europ. J. Agronomy 28 (2008) 148–154
5. Conclusion
Acidification of slurry in pig houses will reduce NH3 emission from the animal house, the store and after having applied
the slurry to land. Acidification reduces NH3 emission from pig
houses by 70% compared with the standard housing treatment.
Little loss was observed from stored slurry, and the NH3 emission from applied slurry was reduced by 67%. In consequence, a
43% (S.E. 27%) increase in mineral fertilizer equivalent (MFE)
was measured in field studies. The slurry acidification system
is approved Best Available Technology (BAT) in Denmark. The
odour emission from the slurry was not affected significantly by
the treatment.
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