## 3 Results and Discussion
### Odor Reduction Using Surface Foam-Covering  
#### 3.4.1 Open Soil Reactor System  
The open soil reactor was designed to simulate manure storage in well-ventilated and open areas. As shown in Figure 2, the initial NH\({}_{3}\) concentration generated from the soil was 1000 ppm, which gradually reduced over time in the open soil reactor. The final residual NH\({}_{3}\) concentration after emission from the soil was 350 ppm (\"control\" in Figure 2). Spraying the surfactant foam into the reactor substantially reduced NH\({}_{3}\) emissions. The reduction in NH\({}_{3}\) concentration using surfactant foam can be classified into three stages. In stage 1, which represents the NH\({}_{3}\) soil covered with foam, the NH\({}_{3}\) concentration was reduced from 1000 ppm (initial) to approximately the background concentration in 1 h (25 ppm), which remained until 5 h. The surfactant foam lasted for approximately 5 h. The sprayed foam successfully blocked the emission of NH\({}_{3}\), resulting in an NH\({}_{3}\) reduction of 98%. In stage 2, which represents the phase after foam breaking, the emission of NH\({}_{3}\) restarted. The sprayed foams in the reactor gradually broke and disappeared after 5 h. After foam breaking, the NH\({}_{3}\) concentration at 450 ppm (\"foam alone\" in Figure 2) was further reduced to 325 ppm in 10 h, and was gradually reduced to 150 ppm after 72 h. This implies that the odor was emitted continuously from the soil with time; however, the addition of foam reduced the extent of emission from the soil, which lasted until the foam covered the soil. A similar result suppressing VOC emission from the source was obtained using a 40 cm-thick foam with an efficiency of 93-96% [21; 22]. Here, surfactant foam acted as a good physical barrier for odor emission control. As shown in Figure 2, bio-foam (surfactant foam with bacteria) considerably reduced the NH\({}_{3}\) concentration even after the foam broke. Foam covering and NH\({}_{3}\) emission were also completely blocked in stage 1, and the NH\({}_{3}\) concentration was similar to the background concentration. In stage \begin{table}
\begin{tabular}{c c c} \hline \hline
**Foam Characteristics** & **Unit** & **Value** \\ \hline Foamability (in 1 L cylinder) & s & 2.4 \\ Foam quality (FQ) & \% & 98.5–99.0 \\ Foam stability (half-life) & h & 2.0–2.1 \\ Foam density & g/cm\({}^{3}\) & 0.025–0.026 \\ Foam bubble density & No./cm\({}^{2}\) & 1 \\ Foam expansion ratio & Foam/liquid vol. & 110–112 \\ Foam lamella thickness & mm & 0.12 \\ Bubble shape & & Nonagon \\ Plateau angle & Degree (\({}^{\circ}\)) & 131.5\({}^{\circ}\) (Avg.) \\ Liquid content in the bubble & & Very less \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Characteristics of surfactant foam generated via a foam sprinkling system. Figure 1: Characterization of a single foam bubble. 2, after the foam broke, the NH\({}_{3}\) concentration increased to 375 ppm (at 5 h, \"bio-foam\" in Figure 2); however, this concentration was gradually reduced to 225 ppm in 10 h. The NH\({}_{3}\) concentration in \"control\" and \"foam alone\" at 10 h was 650 ppm and 325 ppm, respectively. The NH\({}_{3}\) concentration in the \"bio-foam\" was further reduced to 10 ppm in 72 h. These results indicate that the microorganisms in the foam infiltrated the NH\({}_{3}\) soil and degraded NH\({}_{3}\) sources. The approximate time for the complete reduction in NH\({}_{3}\) from the source was about 72 h, which is low in comparison with other biological methods. [PERSON] et al. [36] required 45 days for complete NH\({}_{3}\) reduction. [PERSON] et al. [37] achieved 100% NH\({}_{3}\) removal efficiency over 21 d. [PERSON] et al. [38] achieved a 88.55% NH\({}_{3}\) removal efficiency after 61 days. The shorter time required during our study may be due to the addition of foam in the reactor as a bacterial transport medium.