Assessment of NH3 Reduction and N2O Production during Treatment of Exhausted Air from Fattening Pigs Building by a Commercial Scrubber

The use of air scrubbers to reduce ammonia (NH3) emissions from buildings on pig farms is one of the most promising techniques in the Göteborg protocol and other European regulations including the Industrial Emission Directive. In France, some air scrubbers are currently used on pig farms, mainly to reduce odours from livestock buildings. However, recent research revealed the production of N2O resulting from the treatment of air from pig buildings. In this context, a two-month study was conducted on a pig farm with 750 places for fattening pigs to check the abatement of NH3 emissions and to assess the possible production of N2O during treatment of exhausted air from buildings housing fattening pigs by a air scrubber. Concentrations of NH3 and N2O in the inlet and outlet air of the scrubber were continuously monitored using an Innova 1412 infrared analyzer. With the scrubber operating parameters (airflow, design, size), our results confirmed the production of N2O in the order of 5% of NH3-N reduced. N2O was produced by biological nitrification and/or denitrification inside the air scrubber. Statistical analysis (Pearson’s test) showed that the production of N2O was strongly influenced by the rate of airflow and the outside temperature. The abatement of NH3 emissions from the building was only 33%, i.e. much lower than the 70% 90% usually cited in the literature.

environment through the emission of ammonia (NH 3 ) and greenhouse gases (GHG), especially methane (CH 4 ) and nitrous oxide (N 2 O) from livestock housing and manure management [1]- [4]. Among other livestock activities, pig housing is a serious source of NH 3 [5].
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 [6] and the EU National Emissions Ceilings Directive [7]. The Kyoto Protocol under the United Nations Framework Convention on Climate Change-UNFCCC [8] 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 [9], which was recently incorporated in the Industrial Emissions Directive (IED) 2010/75/EU [10].
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" [11] under the Gothenburg Protocol and 2) "Reference Document on Best Available Techniques for The Intensive Rearing of Poultry and Pigs" or BREF [12] 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 [13] [14], 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 [14] [15]. Different types of air scrubber are recommended for the removal of ammonia from exhaust air of piggery buildings. Most are classified in three types [16] [17]: 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 [16] [17]. The use of an air scrubber is expected to reduce NH 3 emissions from buildings by at least 70% [11] [12] [14]. However, some recent studies showed that at the farm level, the actual reduction in NH 3 by a biotrickling filter could in fact be less than 50% and, furthermore, that N 2 O is also produced [17] [18]. 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.) [17] [19] [20]. N 2 O is generally a by-product of nitrification/denitrification processes [21]. The production of N 2 O 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 [20]- [22]. In a comparison of different studies, Van der Heyden et al. [20] reported that an increase in the residence time of the air in the scrubber appeared to increase the production of N 2 O.
Air scrubbers are currently mainly used in French pig farms to reduce odours from livestock buildings to avoid possible conflicts with neighbors [23]. A recent French survey has estimated that air scrubbers are installed in about 5% of pig farm buildings [24]. According to the operating parameters of commercial scrubbers (airflow, design, size, etc.) the ammonia removal rate is lower than that targeted [19] [23].
In this context, a two-month study was conducted on a pig farm with 750 fattening pigs to assess the reduction in NH 3 emissions and the possible production of N 2 O by a commercial air scrubber that had been installed to reduce efficiently odours from the pig building.

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.

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 m 3 •h −1 . This maximum flow rate corresponds to the recommended ventilation rate for the number of pigs in the seven sections (70 -80 m 3 •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 m 3 •h −1 per nozzle). Finally, the air passes through a demister (thickness: 30 cm) before leaving the air scrubber. The washing water (tap water) is stored in a buffer tank (6.2 m 3 ) 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.

NH3 and N2O Measurement
The abatement of NH 3 and the production of N 2 O 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 NH 3 and N 2 O 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

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 NH 3 loading rate, NH 3 removal rate, and NH 3 removal efficiency (%) as described by Melse et al. [16]. N 2 O production was expressed either as the N 2 O production rate (g [N 2 O] h −1 ) or as the percentage of Pearson's correlation coefficient (r) was used to identify significant relationships between the N 2 O or NH 3 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).

Data Analysis
NH 3 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. [21]. Consequently, the photoacoustic analyzer was used to monitor all the NH 3 concentrations. Likewise, no significant differences were found between measurements of the concentrations of N 2 O by GC-ECD and the pho-toacoustic analyzer. The response of the photoacoustic analyzer was sufficiently sensitive for the concentrations of N 2 O present at the inlet or outlet of the air scrubber.
Concerning the efficiency of the removal of NH 3 , the results were sometime negative due to significantly higher concentrations of NH 3 at the outlet than at the inlet. Even though already observed [14] [26], 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 temperature, airflow rate, NH 3 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.

Air Scrubber Operating Parameters
Over the study period, the airflow rate of the air scrubber ranged from 37,538 to 54,800    Based on these data, the NH 3 removal efficiency ranged from 1.4% to 57% with an overall average of 33%. The rate of NH 3 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 [11] [14]. However, the 33% removal found is our study is in agreement with the results of other French expe-  on the concentration of NH 3 at the inlet (r = 0.22, P < 0.05). This does not correspond to the effects observed by Melse et al. [17] who observed a daily pattern between the airflow rate and the concentration of NH 3 at the inlet due to the activity of the pigs, which influenced NH 3 emissions. In the same way, the strong positive link between the efficiency of NH 3 removal and the concentration of NH 3 at the inlet observed in other studies [27] [28] 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 NH 3 at the inlet and hence in fewer fluctuations in the loading rate of NH 3 [29]. 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 NH 3 in the exhausted air from the piggery could be fixed on dust [21] [30]. The NH 3 loading rate is closely correlated with the outside temperature and the airflow rate (r = 0.7, P < 0.05) meaning that more NH 3 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 NH 3 emissions in the pig rooms [5]. However, in our study, there was a very weak correlation between NH 3 removal rate and outside temperature or airflow rate (r < 0.1, P < 0.05). The parameters that most strongly influenced NH 3 removal efficiency (%) were outside temperature (r = −0.5, P < 0.05) and airflow rate, which determined the air contact time between NH 3  NH 3 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 [19] [20] [23]. However, according to the NH 3 removal rate (figure and statistical data), one could assume that the commercial air scrubber parameters only enable the transfer of a certain mass of NH 3 . Beyond this NH 3 mass value, the NH 3 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 NH 3 . All these factors contributed to the reduction of NH 3 removal efficiency (%).

Efficiency of Ammonia Removal
Other parameters that could influence the efficiency of NH 3 removal are the characteristics of the washing water (not measured in this study). The fluctuations in the air scrubber in removing NH 3 could be associated with the accumulation of ammonia and nitrate/nitrite in the solution produced over time [31]. Melsea and Ogink [14] (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 [14] [15] [32], 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 [17]. 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 [17] [20].
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 NH 3 than values normally cited in the literature. It would be possible to enhance the removal of NH 3 by reducing the accumulation of nitrogen in the washing solution [17] [19] 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 [26]. However, Guingand [33] 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 NH 3 would be to add a biological treatment step of the washing water [22] [34]. 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 [14] which is positively linked to the ammonia in solution [19]. From a scientific viewpoint, identifying the parameters responsible for the low NH 3 removal would require a more in-depth analysis than was planned in the present study. In particular, analyzing the washing water would be necessary.

N2O Production
As shown in Figure 6, the concentration of N 2 O at the air scrubber outlet was systematically higher than that measured at the inlet. These results confirm the production of N 2 O reported in other studies [17] [27] with similar air scrubbers to reduce ammonia from exhausted air originating from pig buildings. The production of N 2 O-N observed in this study corresponds to an average of 5% of NH 3 -N eliminated. This mean value is equivalent to that reported by Melse et al. [17] for an average scrubber efficiency of 70% for NH 3 . 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 NH 3 loading rate (r = 0.5, P < 0.05).
Several authors assume that N 2 O 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 [17] [21] [31]. N is biologically degraded by ammonia-oxidizing bacteria such as Nitrosomonas and by nitrite oxidizing bacteria such as Nitrobacter and Nitrospira [35]. The carbon required for nitrification can be obtained from the organic Moreover, N 2 O production is highly dependent on biodegradable carbon, which is expressed as a low COD:N ratio during denitrification [22]. This implies that fluctuations in the NH 3  However, based on our results alone, it is difficult to establish a link between N 2 O production and the operating conditions of the scrubber or the climatic conditions, as these factors are inter-correlated [17]. 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 NH 3 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 2 NO − and 4 NH + as they may inhibit nitrifying bacteria depending on pH value).
However, this was not possible because of the configuration of the commercial air scrubber.

Conclusions
The aim of this study was to assess the reduction in NH 3 emissions and the possible production of N 2 O 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 NH 3 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 NH 3 and the washing solution had the strongest influence on the efficiency of NH 3 removal (%). Another parameter that could influence the efficiency of NH 3 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 N 2 O, which expressed in N-N 2 O, was of the order of 5% of N-NH 3 removed by the air scrubber. This N 2 O 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. N 2 O-N production (% NH 3 -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 N 2 O-N production.
This study shows that the use of air scrubbers to reduce odours for NH 3 regulatory purposes requires some modifications to optimize the efficiency of NH 3 removal and to limit the production of N 2 O. 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 NH 3 removal and N 2 O 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 NH 3 and to control the production of N 2 O, when the target is to reduce odours.