3.2.1. Ammonia Emissions
The daily concentrations of NH
3 from treatments along the experiment are shown in
Table 4. Comparatively to biofilter treatments without acid, the daily NH
3 concentrations of the control treatment were significantly higher (
p < 0.05) during the 14 days of experiment, with a peak in the first 6 days (14 to 31 mg NH
3 m
−3) followed by a progressive decrease (31 to 21 mg NH
3 m
−3) until the end of the experiment (
Table 4). The daily NH
3 concentrations did not differ significantly (
p > 0.05) among all biofilter treatments without acid, except in the first 2 days for treatment tomato (
Table 4). The mean NH
3 concentrations (0–14 days) decreased but not always significantly by the following order: treatments tomato and tomato + rice < treatment tomato + pine < treatment control (
Table 4). The addition of oxalic acid had no significant effect (
p > 0.05) on mean NH
3 concentrations (0–14 days) of biofilter treatments without acid, except in treatment tomato + pine + acid that was significant lower (
p < 0.05) relative to the same treatment without acid (4.9 mg NH
3 m
−3 for treatment tomato + pine + acid against 14.9 mg NH
3 m
−3 treatment tomato + pine) (
Table 4).
As can be observed in
Table 5, the NH
3 elimination efficiency did not vary significantly (
p > 0.05) among biofilter treatments without acid (tomato + pine, tomato + rice and tomato) and these treatments had a significant reduction (
p < 0.05) from 51 to 77% relative to treatment control. Comparatively to treatments without acid, the addition of oxalic acid decreased significantly (
p < 0.05) the NH
3 elimination efficiency in treatment tomato + pine + acid (79% reduction with acid against 51% reduction without acid) but not in the other treatments that received acid (
Table 5). The NH
3 elimination capacity of biofilter treatments without acid (tomato + pine, tomato + rice and tomato) decreased significantly (
p < 0.05) when compared with treatment control, but no were found significant differences (
p > 0.05) among these biofilter treatments (reduction of 644 to 908 µg NH
3 m
3 s
−1) (
Table 5). No was found significant differences (
p > 0.05) among treatments with and without oxalic acid on the NH
3 elimination capacity (
Table 5).
The mechanism of NH
3 removal occur by nitrifying bacteria that grow on the moist packing material of biofilter [
7]. The NH
3 dissolves in the water phase and is converted to nitrite and nitrate by nitrification. These compounds could be removed with the percolate water as dissolved NH
4NO
2 and NH
4NO
3 and to some extent also are accumulated in the organic packing material [
7]. In the present study, the three biofilters evaluated presented similar NH
3 elimination efficiency (51 to 77% of reduction) and elimination capacity (644 to 908 µg NH
3 m
−3 s
−1 of reduction) (
Table 5). Although not statistically different, the differences among biofilters evaluated in this study (
Table 3) could be related with the moisture content and characteristics of the media [
11]. Chen et al. [
12], studying three moisture levels (20, 40 and 60%) of biofilters filled with wood chips, reported significant decreases in the NH
3 removal efficiency according to the type of material.
As can be observed in
Table 6, Liu et al. [
18] reported that a biofilter using woodchips as packing material could reduce NH
3 emissions from 64 to 78%, being comparable with the value observed in the present study for the three tomato based biofilters (51 to 77% of reduction). Also, excluding differences among packing materials referred in
Table 6, the reduction of NH
3 emissions by tomato based biofilters was comparable with conventional packing materials like woodchip, pine bark and compost, being a promising solution as packing materials in regions where are available. When the biofilters were coated with oxalic acid crystals to absorb NH
3 from airflow that passed by the packing material [
19,
20], a significant NH
3 elimination efficiency was observed only in treatment tomato + pine + acid, what could be related with the lower pH value and higher moisture content relative to the other two biofilters that were coated with oxalic acid [
20]. In addition, minor advantages were gained from the addition of oxalic acid to the three tomato-based biofilters, with 72 to 79% reduction (average of 75%) with acid against 51 to 77% reduction without acid (average of 66%).
3.2.2. Nitrous Oxide Emissions
The daily concentrations of N
2O from all biofilter treatments and control did not varied along the experiment (
Table 4). The daily N
2O concentrations, including the average values (0–14 days), from biofilter treatments and control did not differed significantly (
p > 0.05) in most measurement days (0.5 to 1.2 mg N
2O m
−3), being numerically higher in treatment tomato (
Table 4). Comparatively to treatment control, the mean N
2O concentrations (0–14 days) were numerically higher in biofilter treatments without acid by the following order: treatment tomato > treatments tomato + rice and tomato + pine > control (
Table 4). The addition of oxalic acid to assigned treatments did not reduce significantly (
p > 0.05) the mean N
2O concentrations (0–14 days) relative to biofilter treatments without acid (
Table 4).
The N
2O elimination efficiency was not significantly decreased (
p > 0.05) in biofilter treatments without acid relative to treatment control, being observed a significant increase (
p < 0.05) in treatment tomato when compared with treatments tomato + pine and tomato + rice (61% increase for treatment tomato against 17% increase for treatments tomato + pine and tomato + rice) (
Table 5). The addition of oxalic acid did not affect significantly (
p > 0.05) the N
2O elimination efficiency of biofilter treatments (tomato + pine, tomato + rice and tomato) relative to these same biofilter treatments without acid (
Table 5). The N
2O elimination capacity was not significantly reduced (
p > 0.05) in treatment tomato (15 µg N
2O m
3 s
−1 of increase) relative to all other biofilter treatments (2 µg N
2O m
3 s
−1 of increase) (
Table 5). In addition, no significant differences (
p > 0.05) were found among treatments with and without oxalic acid on the N
2O elimination capacity (
Table 5).
Ammonia gas from contaminated air streams recovered through absorption is then oxidized into nitrite and nitrate by microorganisms that grow on the surface of packing media [
7]. In anaerobic zones of the biofilter, denitrification can also take place, meaning that part of the nitrite and/or nitrate is converted to N
2 [
7]. In the present study, the three tomato-based biofilters without acid increased significantly the N
2O emissions (
Table 5), particularly for biofilter with 100% tomato, which could be related with the high amount of N in tomato waste (
Table 3). Thus, N
2O is influenced by moisture content and might be produced in the biofilter as a by-product from both nitrification and denitrification [
7,
11].
The results of this study followed the same trend than previous studies [
18,
23,
26] reported in
Table 6, where biofilters using different packing materials increased the N
2O emissions. In the present study, the addition of oxalic acid crystals to the biofilters had no effect on N
2O losses, which could be related with the absence of effect on the reduction of pH of packing materials (
Table 3) and then did not affected the nitrification and denitrification processes.