## 3 Results and Discussion
### Odor Reduction Using Surface Foam-Covering
#### 3.4.3 Open Swine Manure Reactor  
Figure 4 shows the changes in NH\({}_{3}\) concentration from the swine manure in the open reactor. Similar trends were observed for swine manure as those for artificially contaminated soil in all three types of treatments. In the first foam covering stage, NH\({}_{3}\) gas emission was completely prevented by surface foam in both \"foam alone\" and \"bio-foam\" treatments until foam was stable for approximately 6 h. The NH\({}_{3}\) gas concentration in the foam covering reactor was similar to its background concentration (10 ppm). NH\({}_{3}\) gas emission from swine manure in the control reactor (the initial NH\({}_{3}\) concentration; 450 ppm) remained constant until 2 h, which gradually decreased to 375 and 350 ppm at 5 and 6 h, respectively, and reached 275, 250, and 225 ppm (50% reduction) after 24, 48, and 72 h, respectively. NH\({}_{3}\) gas was re-emitted in \"foam alone\" and \"bio-foam\" reactors after all foams disappeared in 6 h. The NH\({}_{3}\) gas concentration was 150 and 100 ppm at 6 h in \"foam alone\" and \"bio-foam\" reactors, respectively. However, the \"bio-foam-treated\" manure significantly reduced NH\({}_{3}\) emission after 12 h, which further reduced to 10 and 0 ppm after 24 and 48 h, respectively. This result indicated that microbial degradation was active even after foam breaking (after 12 h). However, the \"foam-alone-treated\" manure after foam breakage presented an NH\({}_{3}\) gas concentration of 125 ppm in 12 h, which remained at a relatively stable NH\({}_{3}\) concentration of 100 ppm up to 72 h. This apparent difference in NH\({}_{3}\) gas reduction from \"foam alone\" and \"bio-foam\" treatments was due to the effective bacterial degradation in the bacterial-foam spraying method. Figure 3: Change in NH\({}_{3}\) concentration from NH\({}_{3}\)-contaminated soil with time in the closed reactor; control (no treatment), foam alone (application of surfactant foam only), and bio-foam (surfactant foam containing bacteria). As shown in Figures 2-4, bacteria require time to degrade odor sources because bacterial degradation is a slow process. In a case where the bacterial solution was applied individually to the swine manure, the initial NH\({}_{3}\) concentration would be high until bacteria became active. However, the sprayed foam reduced the NH\({}_{3}\) concentration to the background level during the initial period. Thus, there exists a synergetic effect between surfactant foam and odor-degrading bacteria for the complete reduction in malodorous gas emission from swine manure by individual contribution. Further studies should be conducted to completely suppress the malodors from the swine manure by applying the surfactant foam twice or thrice just before its complete breakage, which would prevent the release of odors after foam breaking. The final residual NH\({}_{3}\) concentration in the reactor ranged from 0 to 30 ppm, thereby achieving a 97-100% removal efficiency within 72 h. The malodor suppression efficiency using surfactant foam and bacteria was the highest when compared with that obtained with other physical and biological methods for odor removal from manure. [PERSON] et al. [40] used biochar as an odor suppression agent and achieved a NH\({}_{3}\) removal efficiency of 53% over 30 days with the repeated re-application of biochar for odor suppression. [PERSON] et al. [41] achieved a maximum NH\({}_{3}\) removal efficiency of 80% after using the manure for approximately two months. Although other methods were found in the literature, they were either time-consuming or costly. The cost for preparing stable foam was compared between our experimental conditions and other existing experiments in the literature. The comparison was made by assuming that the same amount of foaming reagent was required (1 L) to fill the foam completely in a 60 cm \(\times\) 60 cm \(\times\) 23 cm reactor.