Livestock production is one of the human activities that has a negative impact on the environment through the emission of ammonia (NH3) and greenhouse gases (GHG), especially methane (CH4) and nitrous oxide (N2O) from livestock housing and manure management  -  . Among other livestock activities, pig housing is a serious source of NH3  .
France has to respect a series of international protocols, European directives and national regulations aimed at reducing the environmental impacts of livestock farming. Limitation of ammonia emissions is part of the Gothenburg Protocol (United Nations Convention on Long-range Transboundary Air Pollution―CLRTP  and the EU National Emissions Ceilings Directive  . The Kyoto Protocol under the United Nations Framework Convention on Climate Change―UNFCCC  targets the emissions of methane and nitrous oxide. An even stricter approach to implementing abatement measures had emerged from the Integrated Pollution Prevention and Control Directive (IPPC) 96/61/EC  , which was recently incorporated in the Industrial Emissions Directive (IED) 2010/75/EU  .
According to this legislative framework, ammonia limitation can be achieved by several available abatement options that are described in official documents: 1) “Guidance document for preventing and abating ammonia emissions from agricultural sources”  under the Gothenburg Protocol and 2) “Reference Document on Best Available Techniques for The Intensive Rearing of Poultry and Pigs” or BREF  emerging from the IED directive.
One of the main techniques used to reduce ammonia emissions from pig housing is to treat the exhaust air with an air scrubber. The principle of this technique, described in more detail elsewhere   , consists of passing the exhaust air from livestock buildings through a trickling bed filter which retains certain pollutants, including ammonia, as well as dust and odours   .
Different types of air scrubber are recommended for the removal of ammonia from exhaust air of piggery buildings. Most are classified in three types   : wet scrubbers (also referred to water-only scrubbers or biotrickling filters), chemical scrubbers (acid for example) and air scrubber filters. Under certain conditions, a wet scrubber could have the same function as a biotrickling filter when a bacterial population develops on the inorganic packing material due to the accumulation of dust contained in the exhaust air from pig buildings   . The use of an air scrubber is expected to reduce NH3 emissions from buildings by at least 70%    . However, some recent studies showed that at the farm level, the actual reduction in NH3 by a biotrickling filter could in fact be less than 50% and, furthermore, that N2O is also produced   . Indeed, the efficiency of an air scrubber depends to a great extent on the characteristics of the equipment (design, maintenance, renewal of the washing water, etc.) and on the operating conditions (ammonia loading rate, air ventilation, etc.)    . N2O is generally a by-product of nitrification/denitrification processes  . The production of N2O also depends on different parameters linked to the air scrubber including the ammonia loading rate, air humidity, temperature, and the composition of the washing water  -  . In a comparison of different studies, Van der Heyden et al.  reported that an increase in the residence time of the air in the scrubber appeared to increase the production of N2O.
Air scrubbers are currently mainly used in French pig farms to reduce odours from livestock buildings to avoid possible conflicts with neighbors  . A recent French survey has estimated that air scrubbers are installed in about 5% of pig farm buildings  . According to the operating parameters of commercial scrubbers (airflow, design, size, etc.) the ammonia removal rate is lower than that targeted   .
In this context, a two-month study was conducted on a pig farm with 750 fattening pigs to assess the reduction in NH3 emissions and the possible production of N2O by a commercial air scrubber that had been installed to reduce efficiently odours from the pig building.
2. Material and Methods
2.1. Pig Housing
The study was carried out from September to November 2012 on a pig farm in Brittany (France). The air scrubber is installed to treat the air of a total of 750 fattening pigs in seven sections of one building. The floor of each section is slatted with a manure storage space underneath for the fattening period. Each room is mechanically ventilated by two fans with variable speed regulation to keep a constant inside temperature of around 26˚C. All the outlet air from all seven sections is combined in a depressurized air corridor and directed towards the inlet of the air scrubber by two large fans.
2.2. Air Scrubber
The commercial air scrubber at the pig farm surveyed had been installed outside the building to reduce obnoxious odours. This air scrubber seems to meet the needs because, according to the farmer, no complaints of local residents have been recorded since the installation process. The air scrubber (3.5 m × 3.6 m × 3.9 m; Figure 1) is a counter-current plastic packed-bed (900 mm thick plastic honeycomb cores with a 1 mm mesh). The outlet air from the seven sections of the fattening building was extracted and directed to the air scrubber unit through a central depressurized duct (at 50 Pa). According to the manufacturer’ instructions the air scrubber is configured to operate at a maximum airflow rate of 2 × 27400 m3∙h−1. This maximum flow rate corresponds to the recommended ventilation rate for the number of pigs in the seven sections (70 - 80 m3∙h−1∙pig−1). The empty bed residence time calculated with the maximum airflow (EBRT = scrubber volume/scrubber airflow rate) is 3.2 seconds. The flow rate of the air entering the filter at a given time, which is automatically applied and recorded by the manufacturer’s data logger, is linked to the flow rate of the outlet air from the pig building which depends on the outside temperature. In these conditions, the flow rate of the scrubber fluctuates resulting in fluctuating loading conditions. The air enters the air scrubber and passes through the plastic packed-bed and is continuously moistened by 16 water spraying nozzles (spray rate = 1 m3∙h−1 per nozzle). Finally, the air passes through a demister (thickness: 30 cm) before leaving the air scrubber. The
Figure 1. Characteristics of the air scrubber monitored on the farm.
washing water (tap water) is stored in a buffer tank (6.2 m3) and is continuously recirculated. A volume-controlled valve allows fresh water to be added automatically to supplement evaporated and discharged water. The discharge water is evacuated every six months to the slurry store and applied to arable land as fertilizer.
2.3. NH3 and N2O Measurement
The abatement of NH3 and the production of N2O by the air scrubber were estimated by measuring the concentration of the gases in the inlet and outlet air of the scrubber. The concentration of NH3 and N2O at the inlet was calculated as the mean of four sampling points located by the two scrubber inlet fans. The concentration at the outlet was calculated as the mean of three sampling points positioned on the diagonal of the scrubber outlet (Figure 2). This design was used to avoid the problem of potential preferential pathways. In addition, chimneys (300 mm in diameter) equipped with a cap were used to protect the outlet sampling points from wind and rain (Figure 2). Each sampling point at the inlet and outlet was fitted with a 0.45 micron dust filter. The filters were replaced twice a week. The inlet and outlet air of the scrubber were continuously sampled by a system of pumps connected to a multiplexer (Secan 2800, EMS). The multiplexer connected a selected inlet or outlet monitoring point sequentially with a photoacoustic infrared gas analyzer (1412 Photoacoustic Field Gas Monitor, Innova
Table 1. Main characteristics of the commercial air scrubber.
aIn dry clean condition; bEmpty bed residence time; calculated as air scrubber volume (m3) divided by the maximum airflow rate (m3∙s−1).
Figure 2. Chimney system for measurement of the concentration of N2O and NH3 at the outlet of the air scrubber.
Luma-Sense Technologies) with filter numbers UA973, UA982, UA985, UA969 and UA0988. The multiplexer was set up to connect one inlet or outlet air point every 8 min 30 s, which corresponds to the time required to measure the concentration of five gases eight times. But, because of possible interference in the detection of NH3 and N2O due to the presence of other gases   only the last three measurements (i.e. those made after six minutes) of the eight recorded measurements were averaged and used for data analysis. According to the measurement specifications provided by the manufacturer, the measurement uncertainty in air was ±0.07 mg [NH3] m−3 and 0.027 mg [N2O] m−3. To check for possibly wrong quantification of gas concentrations by the IR- photoacoustic setup  the concentrations of NH3 were also measured with acid impingers, and the concentrations of N2O by gas chromatography (CG). To check concentrations of NH3, a fraction of the inlet and outlet air was continuously monitored for four days. The acid impingers were replaced at 24 hour intervals. The NH3 in the sampled air was trapped by passing through the impinger containing 50 mL H2SO4 (1 N) at a flow rate of 5 L∙min−1. The ammonia in the total acid solution of the impinger was determined by alkaline distillation and titration. The concentration of NH3 in the sampled air was thus time averaged. To check the concentration of N2O, approximately 15 mL of inlet and outlet air were randomly sampled with a gas syringe and stored in a 4 mL glass tube sealed with a rubber stopper. Over-pressure protected the contents of the vial in the case of imperfect sealing or possible pollution from ambient air. The concentration of N2O was determined with an Agilent 6890 N GC chromatograph (Agilent Technologies, USA) equipped with an electron capture detector (ECD), a 3 m 1/8 stain- less steel pre-column filled with 90/100 Porapack N followed by a 4 m 1/8 stainless steel column filled with 4M Porapack Q. Nitrogen was the carrier gas at 44 ml∙min−1 and temperatures of the column oven and ECD detectors were 70˚C and 300˚C. The injection port temperature was 100˚C.
2.4. Results and Statistical Analysis
Based on the monitored concentration of gas and the recorded airflow rate, the efficiency of the air scrubber was assessed according to the NH3 loading rate, NH3 removal rate, and NH3 removal efficiency (%) as described by Melse et al.  . N2O production was expressed either as the N2O production rate (g [N2O] h−1) or as the percentage of N2O production.
Pearson’s correlation coefficient (r) was used to identify significant relationships between the N2O or NH3 emission rates and environmental and air scrubber working factors with a 95% confidence interval (P < 0.05, r = 0 - 0.25 weak correlation, 0.251 < r < 0.500 moderate correlation, 0.501 < r < 0.750 strong correlation, 0.751 < r < 1.00 strongest correlation).
3.1. Data Analysis
NH3 concentrations obtained with the photoacoustic analyzer were of similar magnitude to those obtained using the acid impinger method taking the difference in the sensitivity of the two techniques into account. This similarity between the two methods was also observed by Dumont et al.  . Consequently, the photoacoustic analyzer was used to monitor all the NH3 concentrations. Likewise, no significant differences were found between measurements of the concentrations of N2O by GC-ECD and the photoacoustic analyzer. The response of the photoacoustic analyzer was sufficiently sensitive for the concentrations of N2O present at the inlet or outlet of the air scrubber.
Concerning the efficiency of the removal of NH3, the results were sometime negative due to significantly higher concentrations of NH3 at the outlet than at the inlet. Even though already observed   , these negative results accounted for 4% of the total results and were not retained for subsequent analyses. These results are questionable because no major differences in the scrubber operating parameters (outside tempera- ture, airflow rate, NH3 inlet concentration) were found that could explain this phenomenon. Other erratic or outlier data due technical problems that are inherent to on-site measurement campaigns, e.g. instrument failure; malfunction of the measuring equipment (pump, analyzer etc.) were also excluded from subsequent analyses. The results discussed hereafter are based on data from the infrared analyzer after the exclusion of the previously described values. Table 2 summarizes these results.
3.2. Air Scrubber Operating Parameters
Over the study period, the airflow rate of the air scrubber ranged from 37,538 to 54,800 m3∙h−1 with an overall average of 45,708 m3∙h−1. Fluctuations in the air scrubber flow rate closely mirrored fluctuations in the outside temperature (Figure 3), which ranged
Table 2. Summary of air scrubber operating parameters, and NH3 and N2O emissions over the 2 months of monitoring of the commercial air scrubber.
(1) Average value of 4 sampling points (2) Average of 3 sampling points.
Figure 3. Fluctuations in outside temperature, inlet air scrubber temperature and air flow rate.
from 5.6˚C to 22.8˚C. The outside temperature impacted the ambient temperature of the pig room, which ranged from 23.5˚C to 28.5˚C. Similarly, the temperature of the inlet air of the scrubber fluctuated over the study period from 9.2˚C to 21.8˚C. This trend was expected as the flow rate to the air scrubber corresponds to the total exhaust air from the seven sections of the piggery, which are regulated individually depending on the outside temperature. Consequently, the EBRT in the scrubber ranged from 3.2 to 4.7 seconds with an overall average of 3.9 seconds and the air speed ranged from 0.7 to 1.1 m∙s−1 with an overall average of 0.9 m∙s−1. The fluctuations of the airflow rate meant that the air scrubber operated under fluctuating NH3 loading conditions, as reported in the study of Melse et al.  .
3.3. Efficiency of Ammonia Removal
Figure 4 partially illustrates fluctuations in the concentration of NH3 at the scrubber inlet, outside temperature, and airflow rate.
The concentration of NH3 at the inlet fluctuated widely from 7.5 to 20.6 mg [NH3] m−3 with an overall average of 12.1 mg [NH3] m−3. The concentration of NH3 at the outlet mirrored this trend and fluctuated between 5.2 and 13.6 mg [NH3] m−3 with an overall average of 8.0 mg [NH3] m−3. The concentrations of NH3 at the inlet and outlet are in the same order of magnitude as those reported in the literature   .
The fluctuations in the concentrations of NH3 at the inlet combined with the fluctuations in the airflow rate resulted in fluctuations in the NH3 loading rate and in the NH3
Figure 4. Fluctuations in the concentrations of ammonia at the inlet and outlet of the scrubber and in the air flow rate during air scrubber monitoring.
removal rate. The NH3 loading rate ranged from 323 to 1035 g [NH3] h−1 with an overall average of 552 g [NH3] h−1. The NH3 removal rate ranged from 5.5 and 535 g [NH3] h−1 with an overall average of 187 g [NH3] h−1.
Based on these data, the NH3 removal efficiency ranged from 1.4% to 57% with an overall average of 33%. The rate of NH3 removal by the scrubber recorded is this study is thus lower than the 70% - 90% range usually cited when a scrubber is recommended for the reduction of ammonia produced in livestock farming   . However, the 33% removal found is our study is in agreement with the results of other French experiments on commercial scrubbers set up to reduce odours   . It is well established that the efficiency of air scrubbers in removing NH3 is strongly dependent on the characteristics of the equipment (ammonia loading rate, airflow rate, etc.) and the operating conditions (design, maintenance, renewal of washing water, etc.)     .
Some general trends emerged from our monitoring of the air scrubber over the whole period. Over time, our results showed an increase in the concentrations of NH3 at the inlet and the concentrations of NH3 ammonia at the outlet with an increase in the airflow rate. This resulted in an increase in the NH3 loading rate, a reduction in the NH3 removal rate (g∙h−1) which in turn reduced the efficiency of the NH3 removal rate (%) with an increase in the air scrubber flow rate (Figure 5).
Statistical analysis (Pearson’s test, P = 0.05) of the overall data highlighted some interesting facts. Our results indicated a significant but weak positive effect of airflow rate
Figure 5. Fluctuations in NH3 removal rate (%) and in the air flow rate during air scrubber monitoring.
on the concentration of NH3 at the inlet (r = 0.22, P < 0.05). This does not correspond to the effects observed by Melse et al.  who observed a daily pattern between the airflow rate and the concentration of NH3 at the inlet due to the activity of the pigs, which influenced NH3 emissions. In the same way, the strong positive link between the efficiency of NH3 removal and the concentration of NH3 at the inlet observed in other studies   was not observed in our study (r = 0.21, P < 0.05). Our result might be due to the central ventilation system, which mixed the exhaust air from several sections of the piggery, thereby reducing fluctuations in the concentration of NH3 at the inlet and hence in fewer fluctuations in the loading rate of NH3  . Another explanation for these different results might be the deposits of dust in the duct that could “smooth” the concentrations at the inlet. Indeed, a significant proportion, (up to 40%) of the NH3 in the exhausted air from the piggery could be fixed on dust   . The NH3 loading rate is closely correlated with the outside temperature and the airflow rate (r = 0.7, P < 0.05) meaning that more NH3 enters the air scrubber. An increase in the outside temperature thus implies an increase in the airflow rate to maintain satisfactory conditions in the piggery. In turn, this affects NH3 emissions in the pig rooms  . However, in our study, there was a very weak correlation between NH3 removal rate and outside temperature or airflow rate (r < 0.1, P < 0.05). The parameters that most strongly influenced NH3 removal efficiency (%) were outside temperature (r = −0.5, P < 0.05) and airflow rate, which determined the air contact time between NH3 and the washing solution (r = −0.4, P < 0.05). An increase in the airflow rate reduced the contact time (EBRT) between the air and the washing solution thereby reducing the mass transfer of NH3 from the air to the water. This is consistent with the results of previous studies, underlining the link between the efficiency of the air scrubber and the air-liquid contact time    . However, according to the NH3 removal rate (figure and statistical data), one could assume that the commercial air scrubber parameters only enable the transfer of a certain mass of NH3. Beyond this NH3 mass value, the NH3 is not transferred to the washing solution. This maximum transfer value is thus reduced by an increase in airflow, which reduces the contact time needed for the transfer of NH3. All these factors contributed to the reduction of NH3 removal efficiency (%).
Other parameters that could influence the efficiency of NH3 removal are the characteristics of the washing water (not measured in this study). The fluctuations in the air scrubber in removing NH3 could be associated with the accumulation of ammonia and nitrate/nitrite in the solution produced over time  . Melsea and Ogink  (2005) reported that up to 90% of the ammonia-N removed was discharged or accumulated in the water as ammonium and nitrate. According to different authors    , the accumulation of nitrogen compounds in washing water could modify the equilibrium between the concentration of ammonia in the outlet air and the concentration of dissolved ammonia in the water  . Such an equilibrium is usually influenced by fluctuations in the composition of the air and of the water, which occurs when the air scrubber is overloaded or when the flow rate of the discharge water is set too low   .
From our study, it appears that the air scrubber installed at a commercial farm to reduce the odours emitted by the exhausted air from the piggery was less effective in reducing the NH3 than values normally cited in the literature. It would be possible to enhance the removal of NH3 by reducing the accumulation of nitrogen in the washing solution   without modifying the operating parameters (airflow rate, water flow rate) of the air scrubber used in this experiment. This could be done by discharging water  . However, Guingand  observed no difference in the ammonia reduction rate between an option in which the washing water was emptied four times and a no emptying option. Another possible way to enhance the removal of NH3 would be to add a biological treatment step of the washing water   . It would also be useful to include a control and monitoring process of the washing water. This could be achieved by installing an electrical conductivity meter  which is positively linked to the ammonia in solution  . From a scientific viewpoint, identifying the parameters responsible for the low NH3 removal would require a more in-depth analysis than was planned in the present study. In particular, analyzing the washing water would be necessary.
3.4. N2O Production
As shown in Figure 6, the concentration of N2O at the air scrubber outlet was systematically higher than that measured at the inlet. The concentration of N2O at the inlet ranged from 0.24 to 1.06 mg [N2O] m−3 with an overall average of 0.55 mg [N2O] m−3. The concentration of N2O at the outlet fluctuated between 0.61 and 1.52 mg [N2O] m−3 with an overall average of 0.93 mg [N2O] m−3. The statistical analysis of all the data revealed a significant difference (P < 0.05) between the concentration of N2O at the inlet
Figure 6. Fluctuations in the concentration of N2O at the inlet and outlet and in the air flow rate during monitoring of the air scrubber.
and at the outlet of the air scrubber. Our results indicate production of N2O by the air scrubber ranging from 14 to 233% with an overall average of 78% compared to the concentration in the air at the inlet. N2O production fluctuated from 5.1 to 31.6 g [N2O] h−1 with an overall average of 17.6 g [N2O] h−1.
These results confirm the production of N2O reported in other studies   with similar air scrubbers to reduce ammonia from exhausted air originating from pig buildings. The production of N2O-N observed in this study corresponds to an average of 5% of NH3-N eliminated. This mean value is equivalent to that reported by Melse et al.  for an average scrubber efficiency of 70% for NH3. Figure 7 clearly reveals fluctuations in the production of N2O-N (% NH3-N removal) with fluctuations in the outside temperature and in the airflow rate. An increase in the outside temperature and in the airflow rate resulted in an increase in N2O-N production. Statistical analysis revealed a strong statistical correlation between N2O production (g [N2O] h−1) and air temperature (outside or inlet air, r = 0.6, P < 0.05), airflow rate and implicitly the EBRT and air speed (r = 0.6, P < 0.05) and NH3 loading rate (r = 0.5, P < 0.05).
Several authors assume that N2O production in the air scrubber is due to biological degradation (nitrification/denitrification) of the nitrogen present in the washing solution by a biomass developing in the packed-bed plastic or in the washing solution due to dust deposition    . N is biologically degraded by ammonia-oxidizing bacteria such as Nitrosomonas and by nitrite oxidizing bacteria such as Nitrobacter and Nitrospira  . The carbon required for nitrification can be obtained from the organic
Figure 7. Fluctuations in N2O-N production (%NH3-N removal) and air flow rate during monitoring of the air scrubber.
materials in the dust particles entering the air scrubber. In general, complete nitrification and denitrification requires the control of pH, temperature, substrate and chemical oxygen demand (COD), dissolved oxygen, etc. as well as preventing the accumulation of inhibitory metabolites such as free ammonia. As mentioned above, up to 90% of the NH3 removed from piggery air was discharged or accumulated in the washing solution and hence available for bacterial oxidation to nitrite () and subsequently from nitrite to nitrate (). It is therefore likely that high concentrations of and in the washing solution affected the activity of Nitrosomonas and Nitrobacter. Moreover, N2O production is highly dependent on biodegradable carbon, which is expressed as a low COD:N ratio during denitrification  . This implies that fluctuations in the NH3 loading rate of the air scrubber might have an influence on the measured N2O production rate, as a change in NH3 loading rate could change the COD:N ratio. A too low COD:N ratio may increase the production of N2O during denitrification. Finally, other parameters can also influence the production of N2O including the temperature of the air to be treated, and the content, temperature, level of oxygen dissol- ved and pH of the washing water   .
However, based on our results alone, it is difficult to establish a link between N2O production and the operating conditions of the scrubber or the climatic conditions, as these factors are inter-correlated  . At commercial scale, the washing dynamics is complex because the physical-chemical reactions and biological reactions occur simultaneously and due to the different media (gas, water, biofilm, and solids) involved. In the same way as for the efficiency of NH3 removal, data on the washing solution during the monitoring period would be required for analysis.
A complete understanding and interpretation of the data could be done with certitude only by making the N balance of the process (including the concentrations of and as they may inhibit nitrifying bacteria depending on pH value). However, this was not possible because of the configuration of the commercial air scrubber.
The aim of this study was to assess the reduction in NH3 emissions and the possible production of N2O by a commercial air scrubber installed to reduce odours from a building housing fattening pigs. The results of a 2 month period of monitoring of a building holding 750 pigs indicated that with the operating parameters of the scrubber concerned (airflow, design), the reduction in NH3 emissions was about 33%, which was much lower than the 70% - 90% reported in the literature. Statistical analysis (Pearson’s test) indicated that the parameters defining the air contact time (airflow, air speed, EBRT) between NH3 and the washing solution had the strongest influence on the efficiency of NH3 removal (%). Another parameter that could influence the efficiency of NH3 removal is the composition of the washing solution (not measured in this study). The instability of the results achieved by the scrubber could be associated with the accumulation of ammonia and nitrate/nitrite in solution produced over time.
This study supported the findings of other studies concerning the production of N2O, which expressed in N-N2O, was of the order of 5% of N-NH3 removed by the air scrubber. This N2O is certainly produced by the biological degradation that takes place inside the air scrubber by nitrification/denitrification of the nitrogen present in the washing solution. The biomass that develops in the packed-bed plastic or in the washing solution due to dust deposition is certainly the cause of this biological activity. N2O-N production (% NH3-N removal) was strongly correlated with fluctuations in the outside temperature and in the airflow rate. An increase in the outside temperature and airflow rate increased N2O-N production.
This study shows that the use of air scrubbers to reduce odours for NH3 regulatory purposes requires some modifications to optimize the efficiency of NH3 removal and to limit the production of N2O. This could be achieved, for example, by setting up a control and monitoring process for the washing water, for example an electrical conductivity meter positively linked to the ammonia in the washing solution. From a scientific viewpoint, exploration of the parameters responsible for the low rate of NH3 removal and N2O production requires more comprehensive analysis than that is planned in the present study, in particular, analysis of the washing water. In conclusion, this study shows that air scrubbers need to be characterized under farm conditions to avoid overestimating the expected efficiency in reducing NH3 and to control the production of N2O, when the target is to reduce odours.
The authors wish to acknowledge the financial support received from the European Regional Development Fund (management under the Atlantic Area Transnational Programme) for the BATFARM Interreg-Atlantic Area Project (2009-1/071) entitled “Evaluation of best available techniques to decrease air and water pollution in animal farms”. The authors would like to thank the livestock farmer for proving access to his farm as well as technical assistance.