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agronomy
Article
Evolution of NH3Concentrations in Weaner Pig
Buildings Based on Setpoint Temperature
Manuel R. Rodriguez 1, * , Eugenio Losada 2, Roberto Besteiro 1, Tamara Arango 1,
Ramon Velo 1, Juan A. Ortega 3and Maria D. Fernandez 1
1Department of Agroforestry Engineering, University of Santiago de Compostela, 27002 Lugo, Spain;
roberto.besteiro@rai.usc.es (R.B.); tamara.arango@rai.usc.es (T.A.); ramon.velo@usc.es (R.V.);
mdolores.fernandez@usc.es (M.D.F.)
2Xunta de Galicia, Consellería de Educación e Ordenación Universitaria, 27370 Rábade, Spain;
elvelv@edu.xunta.es
3Xunta de Galicia, Consellería do Medio Rural, 36500 Lalín, Spain; juan.antonio.ortega.martinez@xunta.es
*Correspondence: manuelramiro.rodriguez@usc.es
Received: 28 December 2019; Accepted: 9 January 2020; Published: 11 January 2020
Abstract:
Ammonia (NH
3
) concentration has seldom been used for environmental control of weaner
buildings despite its impact on environment, animal welfare, and workers’ health. This paper aims to
determine the effects of setpoint temperature (ST) on the daily evolution of NH
3
concentration in the
animal-occupied zone. An experimental test was conducted on a conventional farm, with ST between
23
◦
C and 26
◦
C. NH
3
concentrations in the animal-occupied zone were dependent on ST insofar
as ST controlled the operation of the ventilation system, which effectively removed NH
3
from the
building. The highest NH
3
concentrations occurred at night and the lowest concentrations occurred
during the daytime. Data were fitted to a sinusoidal model using the least squares setting (LSS)
and fast Fourier transform (FFT), which provided R
2
values between 0.71 and 0.93. FFT provided a
better fit than LSS, with root mean square errors (RMSEs) between 0.09 ppm for an ST of 23
◦
C and
0.55 ppm for an ST of 25
◦
C. A decrease in ST caused a delay in the wave and a decrease in wave
amplitude. The proposed equations can be used for modeling NH
3
concentrations and implemented
in conventional controllers for real-time environmental control of livestock buildings to improve
animal welfare and productivity.
Keywords:
ammonia daily variation; sinusoidal pattern; environmental control; animal-occupied
zone; fast Fourier transform
1. Introduction
Ammonia (NH
3
) release in livestock buildings originates from the nitrogen content in the urine
and feces deposited in pits or on the building floor surfaces with or without bedding material [
1
].
Currently, NH
3
, together with hydrogen sulfide (H
2
S) is one of the most critical pollutants for pig
production [
2
–
4
] because of its direct relationship with animal and workers’ welfare and health [
3
–
6
].
Accordingly, many authors have analyzed the effects of NH
3
concentration on animal behavior, health,
and productivity [
7
–
14
]. Generally, the negative effects of NH
3
concentrations on the physiological state
of pigs in terms of growth and health have been acknowledged, but no consistent experimental results
have been obtained. Actually, whereas some authors have claimed that high NH
3
concentrations have
physiological effects on pigs [
10
,
13
], other authors have not found a clear influence on hepatic gene
expression [7] or pig growth performance [11,13].
In accordance with Directive 91/630/EEC, gas concentrations must be kept within limits that are
not harmful to pigs through building insulation, ventilation, and heating. Yet, the directive does not
Agronomy 2020,10, 107; doi:10.3390/agronomy10010107 www.mdpi.com/journal/agronomy
Agronomy 2020,10, 107 2 of 14
establish any numerical limits [
15
]. Likewise, there is no consensus on the maximum allowable NH
3
levels. Whereas the International Commission of Agricultural and Biosystems Engineering, CIGR,
(2002) [
16
] recommended a maximum concentration of 20 ppm, Bottcher et al. [
17
] were more cautious
and considered concentrations below 15 ppm as adequate. They also recommended caution at levels
between 15 and 25 ppm, and considered levels above 25 ppm as dangerous. More strict and safe
exposure limits were proposed by Cargill et al. [
18
] and Donham [
19
], with 10 and 11 ppm, respectively.
Drummond et al. [
9
] found much higher NH
3
levels than usual in closed swine buildings, which
depressed pig growth by 12% to 30%. However, there was evidence that 20 ppm atmospheric NH
3
may have an adverse influence on the well-being of growing pigs. [
11
]. Similarly, NH
3
concentrations
between 0.6 and 37 ppm showed no direct effects on the growth, food conversion efficiency [
14
], or
respiratory health of weaned pigs [8].
From an environmental perspective, odor and NH
3
show a strong impact on animal
production
[2,20]
. NH
3
emissions and deposition play a critical role in the acidification and
eutrophication of ecosystems and contribute to indirect emissions of nitrous oxide [
21
]. Adverse effects
such as the acidification and eutrophication of ecosystems [
22
] lead to a loss of biodiversity [
23
] and
contribute to a high share to the total mass of particulate matter smaller than 2.5
µ
m and 10
µ
m [
24
–
26
].
Furthermore, significant air concentrations of NH
3
have been found around swine farms [
27
–
29
].
Time-average concentrations of NH
3
outside a pig farm ranged from 38.4
µ
g m
−3
at a distance of 10 m
to 14.0
µ
g m
−3
at a distance of 650 m [
29
]. Actually, the highest concentrations and depositions of
more than 5
µ
g NH
3−
Nm
−3
were seen around point sources (animal houses and manure storages)
in Denmark [
27
]. Similarly, the highest estimated annual average concentrations of NH
3
exceeded
7µg m−3
for 5 km
×
5 km grid squares and were found in the area of emissions originating from pigs
and cattle breeding in Poland [28].
NH
3
contributes significantly to animal well-being and to the extension of the useful life of
equipment and facilities [
30
]. For these reasons, NH
3
emissions are one of the concerns related to
environmental control. Actually, most European countries have stressed the importance of reducing
NH
3
and odor emissions in order to limit their negative impact on local communities and the
environment [3].
NH
3
concentrations in swine buildings show large variations and are related to a number of
factors, including animal age, activity and density, outdoor temperature, ventilation control, time
of day or time of year [
5
,
31
,
32
]. For example, daily mean NH
3
concentrations in swine finishing
buildings on slatted floors where flushing was applied ranged from 14.20 to 16.90 ppm in the control
barn [
33
], and similar daily average values (12.10 to 18.20 ppm) were reported in Northern Europe [
34
].
For experimental rooms with partial pit ventilation systems, NH
3
concentrations decreased down to
6.50 ppm [3], or even 2.10 to 3.40 ppm in summer and 4.20 to 4.30 ppm in winter [35].
The decrease in ventilation rates caused by the decrease in outdoor temperature has led to seasonal
variations in NH
3
concentrations, with generally higher values in winter than in summer [
5
,
28
,
35
,
36
].
However, some authors have reported higher values in the summer period, stressing that the conditions
that lead to an increase in NH
3
generation rates, such as building management, hygiene or volume,
affect NH
3
concentrations more strongly than the factors that reduce concentration rates [
37
]. NH
3
concentrations do not show an evident daily pattern, but the lowest peaks tend to be during the middle
of the night [38]. Likewise, many authors have reported higher NH3concentrations during the night
or early in the morning and between 16:00 and 20:00 [
5
,
32
]. Contrary to the general belief that NH
3
concentrations are closely associated with ventilation rates, NH
3
levels are more closely associated to
evaporation levels that are at the maximum at higher temperatures [
38
]. After weaning, the thermal
requirements of pigs are 26–28
◦
C [
34
] and then they decrease by 10
◦
C throughout the cycle [
39
–
41
],
and are generally controlled by conventional systems composed of heating and ventilation systems
regulated by at least one temperature sensor [
42
] that does not directly control parameters such as
relative humidity or other pollutants [
43
]. This paper aims to determine the patterns of daily NH
3
concentrations in the animal-occupied zone of a weaner building and the variations in the setpoint
Agronomy 2020,10, 107 3 of 14
temperature defined in climate control systems. By finding these patterns, NH
3
concentration can
be incorporated in conventional control systems with temperature as the single input variable by
applying a simple algorithm based on the variables used by the climate control system. As a result,
environmental control systems will contribute to decreasing the environmental impact of livestock
production [44] and improving animal welfare status [45] while maintaining productivity.
Our results add to the results reported in previous research on NH
3
concentrations and emissions
in buildings for rearing various species and their influencing factors, which were aimed at determining
pollution levels and designing NH3reduction strategies [46–52].
2. Materials and Methods
2.1. Experimental Test
The study was carried out on a conventional intensive pig farm with an authorized farm size
of 4895 sows, where piglets were reared to a live weight of 20 kg in 2013. The farm is located in
Abegondo, A Coruña, NW Spain (43
◦
10
0
12” N, 8
◦
19
0
30” W), where temperatures are mild and frost is
unusual. In 2013, mean annual temperature was 13.20
◦
C, mean annual relative humidity was 86.67%,
and there were 17 frost days, according to the public network of weather stations, Meteogalicia [
53
].
The experimental test was conducted in a weaning room, where piglets were reared from 6 to 20 kg
live weight. The inside dimensions of the weaning room, with polypropylene slat floors, were
11.82 m in length by 5.86 m in width and 2.50 to 2.25 m in height (Figure 1). The room contained six
2.53 m ×1.97 m
pens on each side of a central aisle and housed 50 piglets per pen, up to a maximum
of 300 piglets. The manure pit was empty at the beginning of the production cycle and manure was
removed at the end of the cycle.
Agronomy 2020, 10, x FOR PEER REVIEW 4 of 14
measurement range and a thermistor interchangeability error of < ± 0.20 °C over the range 0–50
°C.
• Average temperatures measured with temperature probe model 107 were stored in a CR-10X
datalogger (Campbell Scientific Ltd., Loughborough, United Kingdom).
• Average C, RH, T, and the voltage and intensity supplied to the fan were stored in one
HOBO H-22 datalogger (Onset Computer Corporation, Bourne, MA, USA).
All the variables were sampled at 1-s intervals and stored every 600 s.
The sensors used to measure relative humidity ( RH), NH3 concentration ( C) and
temperature in the animal-occupied zone (T) were installed in a central pen at 0.40 m height inside
a metal structure that protected the equipment against aggressions from animals (Figure 1). The
sensor used to measure air temperature at the corridor outside the room (T) was installed in the air
inlet at 2.40 m height (Figure 1). Air temperature at the corridor outside the room (T) and outdoor
temperature (T) were the variables used to characterize outdoor climate. Outdoor temperature data
were provided by Meteogalicia public weather station network, particularly by Abegondo weather
station (43°24′14″ N, 8°26′22″ W; elevation: 94 m).
Table 1. Setpoint temperatures for environmental control and measurement periods.
Setpoint Temperatures (ST) (°C)
26 25 24 23
Onset date 2 March 8 March 19 March 27 March
End date 6 March 17 March 25 March 7 April
No. of days 5 10 7 12
Fan air inlet section (m2) 0.0707 0.0962 0.1256 0.1963
Figure 1. Location of sensors used to measure the study variables.
2.2. Mathematical Analysis
Figure 1. Location of sensors used to measure the study variables.
In the experimental test, we used the climate control system of the farm, which was composed of
a ventilation system and a hydronic radiant floor heating system. The environmental conditions of the
Agronomy 2020,10, 107 4 of 14
building were controlled by a temperature probe that did not alter the farm management. The ventilation
system was composed of a 500 mm helical extractor fan with the following specifications: 230 V AC,
50 Hz, 1330 rpm, 480 W power, and cos
ϕ
=0.96, 8746 m
3
h
−1
. Fan minimum flow was 20% and
acceleration was 3
◦
C. In addition, the fan incorporated a system to reduce the section of the air inlet
duct, the diameter of which could be adjusted between 0.30 m and 0.50 m. The radiant floor heating
system was composed of two 1.20
×
0.40 m polyester spreader plates for hot water, each with a capacity
of 2.90 L. The temperatures of the heating fluid ranged from 37
◦
C to 41
◦
C. The flow rate of the
heating fluid was adjusted manually on the dates on which setpoint temperature (ST) was changed
for ventilation purposes. The ST defined for the environmental control was in the range of 26–23
◦
C
(Table 1) and decreased with the increase in animal age and weight. Fresh air entered the room through
two 1.50
×
0.70 m windows with air deflectors installed on the wall opposite to the fan, at both sides of
the entry room door.
Table 1. Setpoint temperatures for environmental control and measurement periods.
Setpoint Temperatures (ST) (◦C)
26 25 24 23
Onset date 2 March 8 March 19 March 27 March
End date 6 March 17 March 25 March 7 April
No. of days 5 10 7 12
Fan air inlet section (m2)0.0707 0.0962 0.1256 0.1963
The environmental variables measured inside the building and the measurement sensors and
dataloggers used were as follows:
•
NH
3
concentration in the animal-occupied zone (
CNH3
): ST–IAM IP66 electrochemical detector
(Murco Ltd, Dublin, Ireland) with splash guard, 0–100 ppm detection range, 5% accuracy,
temperature correction and auto-zero factory calibration before installation, implemented with a
particulate filter made of wire cloth with 0.168 mm aperture width and 0.110 mm wire diameter.
•
Relative humidity (
RHaz
) and temperature (
Taz
) in the animal-occupied zone: temperature/relative
humidity sensor, model S-THB-M008 sensor (Onset Computer Corporation, Bourne, MA, USA),
with 40–75
◦
C temperature measurement range,
±
0.21
◦
C accuracy over the range 0–50
◦
C, and
0%–100% relative humidity range, and ±2.50% accuracy from 10% to 90% RH.
•
Fresh air temperature at air inlets (
Tac
): negative temperature coefficient type sensors, model 107
sensor (Campbell Scientific Ltd., Loughborough, United Kingdom), with
−
35–50
◦
C measurement
range and a thermistor interchangeability error of <±0.20 ◦C over the range 0–50 ◦C.
•
Average temperatures measured with temperature probe model 107 were stored in a CR-10X
datalogger (Campbell Scientific Ltd., Loughborough, United Kingdom).
•
Average
CNH3
,
RHaz
,
Taz
, and the voltage and intensity supplied to the fan were stored in one
HOBO H-22 datalogger (Onset Computer Corporation, Bourne, MA, USA).
All the variables were sampled at 1-s intervals and stored every 600 s.
The sensors used to measure relative humidity (
RHaz
), NH
3
concentration (
CNH3
) and temperature
in the animal-occupied zone (
Taz
) were installed in a central pen at 0.40 m height inside a metal
structure that protected the equipment against aggressions from animals (Figure 1). The sensor used to
measure air temperature at the corridor outside the room (
Tac
) was installed in the air inlet at 2.40 m
height (Figure 1). Air temperature at the corridor outside the room (
Tac
) and outdoor temperature (
Tao
)
were the variables used to characterize outdoor climate. Outdoor temperature data were provided by
Meteogalicia public weather station network, particularly by Abegondo weather station (43
◦
24
0
14” N,
8◦26022” W; elevation: 94 m).
Agronomy 2020,10, 107 5 of 14
2.2. Mathematical Analysis
We estimated the average NH
3
concentration at each setpoint temperature every ten minutes,
which provided an average daily evolution pattern that was fitted by least squares setting (LSS) using
the following equation:
CNH3(t)=Asin(ωt+ϕ)+B(1)
where:
CNH3: NH3concentration (ppm)
A: amplitude (ppm)
ω: angular frequency (rad min−1)
ϕ: initial phase angle (rad)
B: independent variable or vertical shift (ppm)
To fit the series of values of NH
3
concentrations to Equation (1), we derived the characteristic
values of A,ω,ϕ,and Bfrom the following equations:
A=CNH3MAX−CNH3MIN
2(2)
ω=2π
T=4.36E−3 (3)
ϕ=ωt0(4)
B=CNH3AVE =Pn
1CNH3i
n(5)
where:
CNH3MAX: maximum NH3concentration in the animal-occupied zone (ppm)
CNH3MIN : minimum NH3concentration in the animal-occupied zone (ppm)
T: period of the wave, 1440 min
t0: time during which the wave takes the average value (min)
B=CNH3AVE: daily average NH3concentration in the animal-occupied zone (ppm)
Time, t
0
, was considered positive if the wave was advanced or negative if the wave was delayed.
Initial time was obtained from experimental data, which was used to maximize the coefficient of
determination R2for fitting data to the sine wave function.
The goodness of fit was defined by the coefficient of determination (R
2
), the root mean square
error (RMSE), and the standard deviation of the error (SDE), in ppm. The equations for RMSE and
SDE can be written as:
RMSE =1
NXN
1CNH3C−CNH3M20.5
(6)
SDE ="1
N XN
1CNH3C−CNH3M2−XN
1CNH3C−CNH3M2!#0.5
(7)
where:
N: number of observations
CNH3C: estimated NH3concentration (ppm)
CNH3M: measured NH3concentration (ppm)
In addition, fast Fourier transform (FFT) was used for harmonic analysis. Before applying the
FFT, the additive time series was decomposed into the trend, seasonal, and random components. FFT
was applied only to the seasonal component of the series, which provided a good representation of the
average of the analyzed days. The trend component was determined by using a moving average and
was then subtracted from the series. The seasonal part was computed by averaging for each time of the
day over all days. Finally, the random component was the remaining part of the original time series.
Agronomy 2020,10, 107 6 of 14
3. Results
We analyzed the daily evolution of NH
3
concentrations in the animal-occupied zone of a weaner
building where pigs were reared for 44 days, from 5.36 kg to 20.34 kg average live weight. Since
thermal requirements during this phase are strict and changing, setpoint temperature was modified
according to the usual production process. The days in which setpoint temperature was changed were
not considered in the analysis insofar as two different temperatures were used during the same day to
control the heating and ventilation systems.
The analyzed days were grouped according to setpoint temperature (Table 2). Overall, average
NH
3
concentrations decreased with the decrease in setpoint temperature and ranged between 3.79
and 0.30 ppm for 26 and 23
◦
C, respectively. However, at a setpoint temperature of 25
◦
C, the average
NH
3
concentration was 5.24 ppm. A sharp decrease in NH
3
concentrations was observed when
setpoint temperature decreased from 25 to 24
◦
C. Such a decrease was caused by a change in setpoint
temperature that was related to the increase in the ventilation rate. NH
3
concentrations were within
the established limits [20–23].
Table 2. Statistical values of environmental variables for different setpoint temperatures.
ST (◦C) CNH3 (ppm) RHaz
(%) AVE
Taz (◦C) Tac (◦C)
AVE
Tao (◦C)
AVE
AVE SD MAX MIN AVE MAX MIN
26 3.79 2.48 6.84 1.38 58 28.07 29.43 26.85 14.51 11.74
25 5.24 2.55 7.82 2.45 57 27.88 28.62 25.72 10.74 8.33
24 1.00 0.78 2.00 0.25 59 26.56 28.02 24.94 10.97 10.69
23 0.30 0.48 0.72 0.05 61 24.56 26.33 22.87 11.05 10.88
where: ST: setpoint temperature C
NH3
: NH
3
concentration RH
az
: relative humidity in the animal-occupied zone
T
az
: temperature in the animal-occupied zone T
ac
: temperature at the corridor outside the room T
ao
: outdoor
air temperature AVE: average SD: standard deviation MAX: maximum MIN: minimum
Relative humidity and average NH
3
concentration showed an inverse behavior at all setpoint
temperatures, which was fitted by the least squares method to a potential function (Figure 2).
Agronomy 2020, 10, x FOR PEER REVIEW 6 of 14
3. Results
We analyzed the daily evolution of NH
3
concentrations in the animal-occupied zone of a weaner
building where pigs were reared for 44 days, from 5.36 kg to 20.34 kg average live weight. Since
thermal requirements during this phase are strict and changing, setpoint temperature was modified
according to the usual production process. The days in which setpoint temperature was changed
were not considered in the analysis insofar as two different temperatures were used during the same
day to control the heating and ventilation systems.
The analyzed days were grouped according to setpoint temperature (Table 2). Overall, average
NH
3
concentrations decreased with the decrease in setpoint temperature and ranged between 3.79
and 0.30 ppm for 26 and 23 °C, respectively. However, at a setpoint temperature of 25 °C, the average
NH
3
concentration was 5.24 ppm. A sharp decrease in NH
3
concentrations was observed when
setpoint temperature decreased from 25 to 24 °C. Such a decrease was caused by a change in setpoint
temperature that was related to the increase in the ventilation rate. NH
3
concentrations were within
the established limits [20–23].
Table 2. Statistical values of environmental variables for different setpoint temperatures.
ST (°C) C
NH3
(ppm) RH
az
(%)
AV E
T
az
(°C) T
ac
(°C)
AV E
T
ao
(°C)
AV E
AVE SD MAX MIN AVE MAX MIN
26 3.79 2.48 6.84 1.38 58 28.07 29.43 26.85 14.51 11.74
25 5.24 2.55 7.82 2.45 57 27.88 28.62 25.72 10.74 8.33
24 1.00 0.78 2.00 0.25 59 26.56 28.02 24.94 10.97 10.69
23 0.30 0.48 0.72 0.05 61 24.56 26.33 22.87 11.05 10.88
where:
ST: setpoint temperature
C
NH3
: NH
3
concentration
RH
az
: relative humidity in the animal-occupied zone
T
az
: temperature in the animal-occupied zone
T
ac
: temperature at the corridor outside the room
T
ao
: outdoor air temperature
AVE: average
SD: standard deviation
MAX: maximum
MIN: minimum
Relative humidity and average NH
3
concentration showed an inverse behavior at all setpoint
temperatures, which was fitted by the least squares method to a potential function (Figure 2).
Figure 2. Exponential fit of NH
3
concentration and relative humidity (RH) in the animal-occupied
zone.
Figure 2.
Exponential fit of NH
3
concentration and relative humidity (RH) in the animal-occupied zone.
The daily evolution of NH
3
concentration (Figure 3) was fitted to a sine wave function by the
least squares method and FFT method. In the fast Fourier transform, a decomposition of the additive
time series (Figure 4) was performed, and only the seasonal component was considered. The FFT
determined the amplitude (A), the initial phase angle (
ϕ
), and the average value (B) of the sine wave.
Table 3summarizes the values obtained at every setpoint temperature (Figure 5).
Agronomy 2020,10, 107 7 of 14
Agronomy 2020, 10, x FOR PEER REVIEW 7 of 14
The daily evolution of NH3 concentration (Figure 3) was fitted to a sine wave function by the
least squares method and FFT method. In the fast Fourier transform, a decomposition of the additive
time series (Figure 4) was performed, and only the seasonal component was considered. The FFT
determined the amplitude (A), the initial phase angle (φ), and the average value (B) of the sine wave.
Table 3 summarizes the values obtained at every setpoint temperature (Figure 5).
Figure 3. Daily evolution of average NH3 concentration in the animal-occupied zone at 26, 25, 24, and
23 °C setpoint temperatures.
0
3
6
9
0 4 8 12162024
NH3 concentration (ppm)
Time (h)
ST = 26 ºC ST = 25 ºC ST = 24 ºC ST = 23 ºC
Figure 3.
Daily evolution of average NH
3
concentration in the animal-occupied zone at 26, 25, 24, and
23 ◦C setpoint temperatures.
Agronomy 2020, 10, x FOR PEER REVIEW 8 of 14
.
Figure 4. Decomposition of additive time series.
Table 3. Characteristic values of the sinusoidal curve at different setpoint temperatures. LSS: least
squares setting; FFT: fast Fourier transform.
ST (°C) Method A (ppm) B (ppm) φ (Rad) Wave Onset Time
26 LSS FFT 2.73 2.08 3.79 0.26 0.33 23:00 22:44
25 LSS FFT 2.69 2.10 5.24 0.44 0.47 22:19 22:13
24 LSS FFT 0.87 0.60 1.00 −0.17 −0.14 00:39 00:31
23 LSS FFT 0.33 0.22 0.30 −0.31 −0.32 01:11 01:12
where:
ST: setpoint temperature
A: amplitude
B: independent variable or vertical variation obtained as the average daily NH3 concentration in
the animal-occupied zone.
φ: initial phase angle
Figure 4. Decomposition of additive time series.
Agronomy 2020,10, 107 8 of 14
Table 3.
Characteristic values of the sinusoidal curve at different setpoint temperatures. LSS: least
squares setting; FFT: fast Fourier transform.
ST (◦C) Method A (ppm) B (ppm) ϕ(Rad) Wave Onset Time
26 LSS FFT 2.73 2.08 3.79 0.26 0.33 23:00 22:44
25 LSS FFT 2.69 2.10 5.24 0.44 0.47 22:19 22:13
24 LSS FFT 0.87 0.60 1.00 −0.17 −0.14 00:39 00:31
23 LSS FFT 0.33 0.22 0.30 −0.31 −0.32 01:11 01:12
where: ST: setpoint temperature A: amplitude B: independent variable or vertical variation obtained as the average
daily NH3concentration in the animal-occupied zone. ϕ: initial phase angle
Agronomy 2020, 10, x FOR PEER REVIEW 9 of 14
Figure 5. Measured (Meas.) and modeled sine fit using least squares setting (LSS) and fast Fourier
transform (FFT) for daily evolution of NH3 concentration at different setpoint temperatures.
The amplitude of the sine wave function decreased with the decrease in setpoint temperature
because of the lower NH3 levels. However, the values obtained at setpoint temperatures of 26 °C and
25 °C were almost identical, whereas amplitude decreased dramatically at lower setpoint
temperatures. The LSS model showed a greater wave amplitude than the FFT model at all setpoint
temperatures. Moreover, air temperature in the animal-occupied zone was higher than the setpoint
temperature in all cases, with values above 1.50 °C (Table 2).
The initial phase angle was positive for setpoint temperatures of 26 °C and 25 °C, and negative
for 24 °C and 23 °C. For 26 °C and 25 °C, the initial NH3 concentration was higher than the average
NH3 concentration by 19% and 22%, respectively. For setpoint temperatures of 24 °C and 23 °C, the
initial concentrations were 15% and 34% lower than the average concentrations, respectively.
The statistics included in Table 4 show the goodness of fit of the sine wave pattern to the daily
evolution of NH3 concentrations in weaner buildings based on setpoint temperature, using LSS and
FFT.
Table 4. Goodness of fit of the daily evolution of NH3 concentrations to a sinusoidal curve at different
setpoint temperatures.
ST (°C) Method R2 SDE (ppm) RMSE (ppm)
26 LSS FFT 0.93 0.92 0.64 0.42 0.64 0.42
25 LSS FFT 0.88 0.88 0.70 0.55 0.70 0.55
24 LSS FFT 0.84 0.84 0.26 0.19 0.26 0.19
-1
0
1
2
3
4
5
6
7
8
0 6 12 18 24
Meas. 26 ºC LSS 26 ºC FFT 26 ºC
Meas. 25 ºC LSS 25 ºC FFT 25 ºC
Meas. 24 ºC LSS 24 ºC FFT 24 ºC
Meas. 23 ºC LSS 23 ºC FFT 23 ºC
Figure 5.
Measured (Meas.) and modeled sine fit using least squares setting (LSS) and fast Fourier
transform (FFT) for daily evolution of NH3concentration at different setpoint temperatures.
The amplitude of the sine wave function decreased with the decrease in setpoint temperature
because of the lower NH
3
levels. However, the values obtained at setpoint temperatures of 26
◦
C and
25
◦
C were almost identical, whereas amplitude decreased dramatically at lower setpoint temperatures.
The LSS model showed a greater wave amplitude than the FFT model at all setpoint temperatures.
Moreover, air temperature in the animal-occupied zone was higher than the setpoint temperature in all
cases, with values above 1.50 ◦C (Table 2).
The initial phase angle was positive for setpoint temperatures of 26
◦
C and 25
◦
C, and negative
for 24
◦
C and 23
◦
C. For 26
◦
C and 25
◦
C, the initial NH
3
concentration was higher than the average
Agronomy 2020,10, 107 9 of 14
NH
3
concentration by 19% and 22%, respectively. For setpoint temperatures of 24
◦
C and 23
◦
C, the
initial concentrations were 15% and 34% lower than the average concentrations, respectively.
The statistics included in Table 4show the goodness of fit of the sine wave pattern to the daily
evolution of NH
3
concentrations in weaner buildings based on setpoint temperature, using LSS
and FFT.
Table 4.
Goodness of fit of the daily evolution of NH
3
concentrations to a sinusoidal curve at different
setpoint temperatures.
ST (◦C) Method R2SDE (ppm) RMSE (ppm)
26 LSS FFT 0.93 0.92 0.64 0.42 0.64 0.42
25 LSS FFT 0.88 0.88 0.70 0.55 0.70 0.55
24 LSS FFT 0.84 0.84 0.26 0.19 0.26 0.19
23 LSS FFT 0.71 0.71 0.13 0.09 0.13 0.09
where: R
2
: coefficient of determination ST: setpoint temperature SDE: standard deviation of the error RMSE: root
mean square error
The goodness of fit of the data to a sinusoidal function was characterized by the coefficient of
determination, R
2
, which showed reasonable values, in the range 0.71–0.93 at setpoint temperatures of
23 and 26
◦
C, respectively. R
2
values increased with ST, which suggest a better fit and greater variations
for high NH
3
concentrations. These findings were supported by other statistics, such as the standard
deviation of the error (SDE), which was in the range 0.70–0.09. As SDE and RMSE were identical, mean
errors were null. The values of R
2
were almost identical in both methods, whereas SDE and RMSE
values suggest a better estimation of NH3evolution by FFT.
4. Discussion
The air temperatures recommended for weaned piglets housed in pens with plastic slatted
floors range between 30–32
◦
C for 5 kg live weight and 19–25
◦
C for 20 kg live weight [
40
,
41
]. Many
authors [
5
,
32
,
52
] have related the effect of setpoint temperature on NH
3
concentration with the influence
of setpoint temperature on ventilation and, consequently, on NH
3
extraction from the building. During
approximately the first two weeks of weaning, which correspond to the critical period [
39
], the setpoint
temperatures were 26
◦
C and 25
◦
C and ventilation was highly restricted by reducing the fan air
inlet section due to the strict thermal requirements for pig growth and the sensitivity of weaners to
air currents. The highest NH
3
concentrations occurred during this period. During the postcritical
period, when regular food intake was already established [
39
], setpoint temperatures decreased to
24 and 23
◦
C, and ventilation restrictions were lower, which led to a substantial decrease in average
NH
3
concentration, from 5.24 ppm (critical period) to 1.00 ppm (postcritical period) for setpoint
temperatures of 25
◦
C and 24
◦
C, respectively, as shown in Table 2. In addition, air temperature in the
animal-occupied zone (T
az
) was always above setpoint temperature, with variations between 2.07 and
1.56
◦
C for setpoint temperatures of 26 and 23
◦
C, respectively, which shows the thermal inertia of the
heating system. These findings suggest a better performance of the environmental control system at
lower setpoint temperatures.
Since NH
3
density is lower than air density, NH
3
settled in the upper areas of the room and was
easier to extract than other gases such as CO
2
, which concentrated in the lowest areas. Accordingly,
NH
3
concentration patterns were strongly influenced by ventilation, which, in turn, was affected by
setpoint temperatures, which were in the range 26–23
◦
C and decreased with the increase in animal
age and weight.
Average NH
3
concentration and relative humidity showed an inverse behavior (Figure 2). This
is in agreement with Banhazi [
38
], who demonstrated that NH
3
levels are more closely related to
evaporation levels than to ventilation rates and that evaporation levels are at the maximum at higher
temperatures. However, the small margins of relative humidity in this study (57%–61%) and the
Agronomy 2020,10, 107 10 of 14
sensor accuracy (
±
2.50%) do not allow a strong relationship to be established between the relative
relationship and the NH3concentration.
Many authors have reported higher measured NH
3
concentrations than the concentrations
reported here due to factors such as animal age and weight [
32
,
38
,
50
], ventilation system [
35
], cleaning
system [
32
], location, climate [
34
,
38
], or season of the year [
35
,
38
]. The values reported by other authors
were above the values obtained during the last phase of our research, during which the conditions for
pigs with a weight of approximately 20 kg came nearer to the conditions that are usual for finishing
pigs, with NH3levels of 0.30 ±0.48 ppm at 23 ◦C setpoint temperature.
The daily evolution of NH
3
concentrations observed in our research differs considerably from
the pattern observed under laboratory conditions for finishing pigs in rooms ventilated with negative
pressure systems [
32
]. Such differences may be due mainly to the use of dissimilar ventilation and
cleaning systems. In the experimental test conducted, the forced ventilation system effectively removed
NH
3
at midday, thus avoiding a trend of NH
3
concentration parallel to that of air temperature.
In addition, daily manure removal abruptly affected the daily evolution of NH
3
concentration [
32
],
which did not happen in our study.
The results of our experimental test suggest a sinusoidal response of the daily evolution of NH
3
concentration, which is in agreement with the results reported for rabbits [
47
]. Likewise, a sinusoidal
response was found for daily NH
3
concentration, which was directly related to odor and pollutant
emission from finishing pigs [
31
]. Saha et al. [
50
] incorporated the daily activity of pigs into the model
as a sine wave equation to predict NH
3
emissions from dairy livestock with natural ventilation and
found that including the sine and cosine of circular variables such as the hours of the day, days of the
year, and wind direction improved the dynamic nature of the models used to predict NH3emissions.
Similarly, clear sine patterns were found for the daily emission of NH3for broilers [48].
The daily evolution of NH
3
concentration in weaner buildings showed a similar pattern to the
pattern obtained by Calvet et al. [
47
], with maximum values at night, when ventilation rates were
minimum and minimum values during the day, when ventilation rates were maximum. Therefore, the
sinusoidal response was strongly conditioned by the ventilation rates inside the building, which was
controlled exclusively by indoor temperature. This pattern affected NH
3
emission, which followed the
opposite trend to NH
3
concentration and increased with the increase in ventilation rates. As a result,
NH3emission was higher during the daytime [48,50].
Overall, a decrease in setpoint temperature caused a decrease in the amplitude of the modeled sine
wave function and a delay in the wave. However, we observed only a small difference in amplitude
when fitting the model to temperatures of 26 ◦C and 25 ◦C, around 2.70 ppm. Yet, amplitude sharply
decreased at lower setpoint temperatures (0.33 ppm at 23
◦
C), when higher ventilation rates are allowed
because of the lower sensitivity of weaners to air currents. This is in agreement with high temperatures
leading to considerable NH
3
emissions, which increase when combined with high pH levels in bedding
material [54], although this not the case.
SDE was the main component of the error because the bias was null insofar as the mean of the
experimental data coincided with the mean of the sinusoidal curve obtained in one period (1440 min).
From among the two methods used, FFT showed a better sine fit than LSS, probably because the FFT
used only the seasonal component of the series and neglected the trend component (Figure 4). Figure 5
shows the better fit of FFT, confirmed by statistics.
5. Conclusions
The following conclusions can be drawn:
1.
NH
3
concentration in the animal-occupied zone varies with the temperature setpoint defined for
the climate control system. At night, when air temperature is lower, the ventilation rate decreases,
which causes an increase in NH
3
concentration. The increase in outdoor temperature during the
daytime causes an increase in the ventilation rate and, consequently, in the rate of gas removal.
Agronomy 2020,10, 107 11 of 14
2.
The daily sine wave for NH
3
concentrations provides a reliable pattern at every setpoint
temperature, with R
2
values between 0.93 and 0.71 for the two methods used, LSS and FFT.
The FFT method showed a better sine fit, with RMSE values below 0.55 ppm as compared to
0.70 ppm in the LSS method. This occurs because the trend component of the series is neglected
and only the seasonal component is considered. With the decrease in setpoint temperature, the
amplitude of the wave diminishes and, generally, the sine wave is delayed. These sine waves
were obtained by using inexpensive electrochemical sensors that could be easily incorporated
in livestock farms. Our results show that these sensors, if maintained properly, can accurately
represent the daily evolution of NH3concentration.
3.
The use of sine wave equations to estimate NH
3
concentrations can be beneficial for farmers, insofar
as sine wave equations provide a reliable pattern for real-time estimation of NH
3
concentration
and can be included as a parameter in control strategies considering daytime. In addition, sine
wave equations can be implemented in many conventional controllers because of their simplicity.
Sine wave equations based on setpoint temperatures could be useful for real-time environmental
control, which would substantially improve animal welfare.
Author Contributions:
Conceptualization, M.R.R. and M.D.F.; methodology, M.R.R., E.L., and R.B.; formal
analysis, M.D.F., J.A.O., and T.A.; investigation, R.V., M.R.R., and M.D.F.; writing—original draft preparation,
review, and editing, M.R.R. and M.D.F. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by Xunta de Galicia, grant number GPC-ED431B 2018/012.
Conflicts of Interest: The authors declare no conflicts of interest.
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