| Abstract: | 多數民眾每天處於室內環境的時間比率為80~90 %，長時間處於空氣品質不佳之室內環境，室內生物性及化學性空氣污染物可能導致呼吸系統健康危害，而電漿技術已廣泛應用在各種領域，但目前尚未應用於同時處理室內甲醛及生物氣膠複合污染物。本研究之目的為選擇非熱介電質放電(Dielectric Barrier Discharges, DBD)電漿系統來探討同時處理連續流動之室內甲醛及生物氣膠兩種複合污染物之可行性，期望未來能利用本研究發展之化學性及生物性空氣污染物控制系統來改善室內空氣品質。
本研究針利用甲醛產生系統及霧化器產生甲醛氣體及生物氣膠，氣流經過Kr85電性中和及調整濕度後進入電漿反應器，而於反應器出口端使用甲醛氣體偵測器及生物氣膠採樣器分別量測甲醛濃度及培養計數生物氣膠濃度及以評估DBD電漿系統對甲醛及生物氣膠之控制效力。實驗變因包含甲醛濃度(0.5 ppm~5 ppm)、細菌性生物氣膠(106 CFU/m3大腸桿菌或枯草桿菌內孢子)、電漿電壓(0~19 kV)、相對溼度(30 %~50 %)及停留時間(1.5sec~10sec)。
結果顯示DBD電漿對細胞型態之大腸桿菌及孢子型態之枯草桿菌內孢子生物氣膠皆具有高殺菌效力；對低濃度甲醛破壞效力又較高濃度甲醛高(0.5 ppm>1 ppm>5 ppm)。不管電漿產生與否，甲醛氣體破壞效力並不受生物氣膠存在之影響；然而在低電壓15 kV電漿尚未完全產生時，生物氣膠殺菌效力會受到甲醛存在影響而降低，而在電漿產生後(電壓16 kV)，生物氣膠殺菌效力則不受甲醛之影響。增加放電電壓、停留時間及相對濕度皆有助於提升電漿對生物氣膠殺菌效力及甲醛破壞效力。
非熱DBD電漿系統之主要殺菌機制可能為帶電粒子(電子與離子)及反應性物質(臭氧、氫氧自由基或其他活性物質)與生物氣膠作用所致；而電漿對甲醛氣體去除之主要機制則可能為高能電子與氣體分子碰撞產生自由基為起始反應，而後自由基與甲醛再進行一連串的連鎖反應所致。
非熱DBD電漿於電漿頻率60 Hz、停留時間1.5秒時及電壓17 kV時，不論相對溼度高低，對單一(甲醛或生物氣膠)或複合污染物(甲醛+生物氣膠)之生物氣膠殺菌或甲醛破壞效力均可達90 %以上，顯示非熱DBD電漿確實具有相當好之同時連續處理室內生物性及化學性污染物之能力，其為一相當有潛力發展成為室內甲醛及生物氣膠之控制技術。
Most people typically spend around 80~90 % of their time indoors everyday. It has been known that lots of air pollutants exist in the indoor environment. It also has been reported that indoor biological and chemical pollutants can cause a serious illness on human respiratory system. Plasma technologies have been widespread applied in many fields. However, plasma was still not to be used in removing indoor formaldehyde and bioaerosols simultaneously. Therefore, the purpose of this study was to select non-thermal dielectric barrier discharges (DBD) plasma system to test its feasibility of removing formaldehyde and bioaerosols from a continuous flow gas stream in the study. It was expected that the plasma system developed in this study could be applied to improve indoor air quality in the future.
Formaldehyde and bioaerosols were generated by a formaldehyde generation system and a nebulizer, respectively. The gas stream then entered the plasma reactor after neutralizing bioaerosols by Kr85 and adjusting humidity. Formaldehyde and bioaerosol concentrations measured at the exit of plasma reactor by a formaldehyde meter and an one-stage Anderson biosampler were used to evaluate the effects of plasma on formaldehyde and bioaerosols. The sterilization efficacy of bioaerosols and destruction efficacy of formaldehyde by plasma were evaluated based on different pivotal parameters including formaldehyde concentrations (0.5 ppm~5 ppm), types of bacterial bioaerosols (Escherichia coli and Bacillus subtilis endospore), applied voltages (0~19 kV), relative humidities (30 %~70 %), and retention times (1.5sec~10sec).
The results showed that the non-thermal DBD plasma system possessed high sterilization efficacies to E. coli and B. subtilis bioaerosols. The destruction efficacy of formaldehyde by plasma was following the order: 0.5 ppm > 1ppm > 5 ppm. Regardless of plasma generation or no plasma, the destruction efficacies of formaldehyde were not affected by bioaerosols. However, the sterilization efficacies of bioaerosols were decreased owing to the present of formaldehyde gas at the applied voltage of 15 kV with no plasma generation. When plasma was generated at the applied voltage greater than or equal to 16 kV, the sterilization efficacies of bioaerosols were not affected by formaldehyde. Increasing applied voltage, retention time, and relative humidity can enhance the non-thermal DBD plasma system to sterilize bioaerosols and destroy formaldehyde.
The charged particles (electronics and ions) and reactive species (O3, OH free radical, or other active species) reacted with bioaerosols in non-thermal DBD plasma system were the main mechanisms of sterilizing bioaerosols. For the main mechanism of destroying formaldehyde by plasma, it should be the high-energy electrons collided with gas molecules to generate free radicals and then initiated a serious of chain reactions with formaldehyde.
The optimal operating conditions of non-thermal DBD plasma system were controlled at power frequency of 60 Hz, retention time of 1.5 sec, and applied voltages of greater than or equal to 17 kV in this study. For single air pollutant (formaldehyde or bioaerosols) or complex air pollutants (formaldehyde and bioaerosols), the sterilization efficacy of bioaerosols and destruction efficacy of formaldehyde were all greater than 90 % regardless the effects of relative humidity. This research demonstrated the potential of non-thermal DBD plasma system to control bioaerosols and formaldehyde from continuous flow gas stream simultaneously, and this system could be applied to improve indoor air quality. |
