# VOC Removal from Manure Gaseous Emissions with UV Photolysis and UV-TiO\({}_{2}\) Photocatalysis
## 2 Results and Discussion
### UV Treatment of Odorous VOCs in Synthetic Mixture
#### 2.1.1 Effects of UV Light dose, Wavelength, and Catalyst on VOC Conversion
The use of deep UV (UV\({}_{185+254}\)) was found to improve \(R\), particularly when the photolysis was the primary treatment (Figures 3B and 4B). This is to be expected, in that additional photochemical mechanisms are available with 185 nm photolysis, and TiO\({}_{2}\) is not the primary light absorber. Still, acetic and propanoic acids were generated when UV\({}_{185+254}\) was used. Apparent generation of low MW VFAs was likely a result of the partial breakdown of heavier MW VFAs into intermediates such as acetic and propanoic acids. Figure 3: Treatment of target odorous VOCs in a synthetic mixture with UV lights of 254 nm and 185 nm outputs. Effect of UV light intensity (obtained by the varying distance between the UV lamps and the photoreactor) on % conversion \(R\) of odorous VOCs in a synthetic mixture. (**A**) UV\({}_{185+254}\) + TiO\({}_{2}\) (treatment mainly via photocatalysis); (**B**) UV\({}_{185+254}\); no catalyst (treatment mainly via photolysis). Experimental conditions: 25 mg TiO\({}_{2}\) for UV\({}_{185+254}\) + TiO\({}_{2}\) treatment; T = 25 \({}^{\circ}\)C; dry air; sampling conditions: 5 min gas sampling time with CAR/PDMS 85 \(\upmu\)m SPME. The use of a catalyst (TiO\({}_{2}\)) resulted in increased \(R\) for all VOCs and VFAs. The contribution of TiO\({}_{2}\) to treatment effectiveness was most apparent at UV\({}_{254}\) and evident when deep UV (UV\({}_{185+254}\)) was used. The contribution of TiO\({}_{2}\), in general, can be explained by the electron-hole pairs generated on the catalyst surface, producing more radicals that, in turn, are available to oxidize VOCs, compared to relatively inefficient direct absorption of the UV light. The use of catalysts lowered the resulting acetic acid concentration, which is a desirable outcome. A paired \(t\)-test was conducted for assessing the contribution of deep UV (UV\({}_{185+254}\) vs. UV\({}_{254}\) nm; with and without catalyst) and the contribution of catalyst (with TiO\({}_{2}\) vs. no TiO\({}_{2}\); at UV\({}_{185+254}\) and UV\({}_{254}\) nm). The \(p\)-values obtained for each comparison for all target compounds are presented in Table 1. The use of a catalyst improved R significantly (\(p\geq 0.05\)) in most cases, while the use of deep UV (UV\({}_{185+254}\)) had a significant effect mainly without the catalyst. Several exceptions were observed for (i) VFAs with no significant improvement with the use of deep UV, both in the presence or absence of catalyst; except for acetic acid conversion via photolysis, (ii) dimethyl sulfide with no significant improvement with deep UV in the presence of the catalyst, and (iii) \(p\)-cresol with no significant improvement with the use of photocatalysis at deep UV. In some cases, there was even an increase in the concentration of some of these exceptional compounds, i.e., the generation of these substances. Calculated mass VOCs converted (ng) per energy dose (J) for the four treatment processes are summarized in Table 2. Particularly interesting for future scale-up was the mass removal rate of \(p\)-cresol, a characteristic odor indicator associated with livestock operations [14; 19], which was \(\sim\)79 to \(\sim\)110 ng J\({}^{-1}\). UV lamps associated with specific wattage ratings can be selected to achieve desired mass removal fractions for key odorants based on the treatment time and their approximate concentration range. [PERSON] [42] concluded that \(R\) generally increased with radiant flux (light energy). Mass removal rates have practical importance as the key input data needed to scale-up UV treatment, i.e., sizing a minimum UV energy input and cost to treat typical VOC emissions from livestock barn exhaust. Light flux is reduced with distance from a source according to the inverse-square law. In this study, light intensity was manipulated by changing the distance between the UV lamps and the photoreceptor (see Materials and Methods).