萬(wàn)順剛，李桂英，安太成*

1. 中國(guó)科學(xué)院廣州地球化學(xué)研究所有機(jī)地球化學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室，廣東 廣州 510640；2. 中國(guó)科學(xué)院研究生院，北京 100049

隨著經(jīng)濟(jì)和科學(xué)技術(shù)的進(jìn)步，在居民的物質(zhì)和精神生活逐步得到提高的同時(shí)，常規(guī)污染物如粉塵、二氧化硫和氮氧化物等大氣污染物的控制以及去除已經(jīng)不能滿足人們對(duì)大氣環(huán)境質(zhì)量日益增長(zhǎng)的要求，惡臭污染物所導(dǎo)致的環(huán)境空氣質(zhì)量惡化的投訴問(wèn)題逐漸增加。因此有關(guān)工農(nóng)業(yè)生產(chǎn)和生活中排放的惡臭氣體污染物的控制標(biāo)準(zhǔn)以及污染治理技術(shù)的研究，正逐步得到重視[1]。通常提到的惡臭污染物(Odor pollutants)，在我國(guó)惡臭污染物排放標(biāo)準(zhǔn) GB14554-93中給出了明確的解釋，即一切刺激嗅覺(jué)器官引起人們不愉快感覺(jué)及損害生活環(huán)境的氣味統(tǒng)稱為惡臭，具有惡臭氣味的物質(zhì)被稱為惡臭污染物。它們主要是通過(guò)刺激嗅覺(jué)細(xì)胞，經(jīng)神經(jīng)的傳遞作用而完成氣味的鑒別。由于惡臭污染物具有揮發(fā)性的特點(diǎn)，因此惡臭污染主要是通過(guò)大氣傳播和擴(kuò)散，作用于人的嗅覺(jué)器官而被感知的一種嗅覺(jué)污染。本文主要對(duì)惡臭有機(jī)污染物的固定化微生物處理技術(shù)的應(yīng)用狀況進(jìn)行了綜述，并對(duì)今后的研究方向和發(fā)展前景進(jìn)行了展望。

1 惡臭有機(jī)物的來(lái)源、分類和危害

首先農(nóng)牧業(yè)是惡臭污染的一個(gè)重要來(lái)源，農(nóng)牧業(yè)的生產(chǎn)過(guò)程中產(chǎn)生的大量含有機(jī)物的各種廢物在微生物的作用下分解，會(huì)排放出具有不同惡臭氣味的空氣污染物[2-5]，特別是隨著農(nóng)業(yè)生產(chǎn)的發(fā)展及農(nóng)產(chǎn)品加工規(guī)模經(jīng)營(yíng)不斷擴(kuò)大，與農(nóng)業(yè)相關(guān)的惡臭污染已成為大氣惡臭污染不可忽視的因素。其次，工業(yè)生產(chǎn)過(guò)程排放的惡臭污染物也是造成惡臭污染的最重要來(lái)源之一，這些工廠產(chǎn)生的惡臭污染物主要包括揮發(fā)性有機(jī)物(VOCs),尤其是揮發(fā)性有機(jī)硫化合物(Volatile Organic Sulfur Compounds,VOSCs)等[6-8]。再次，城市垃圾處理公共設(shè)施也是一個(gè)不容忽視的惡臭污染源，垃圾廢物在通過(guò)城市垃圾廠、污水處理廠、堆肥廠等的處理過(guò)程中，排放出各種惡臭污染物[9-11]。所有這些惡臭污染物會(huì)對(duì)其周邊的空氣質(zhì)量產(chǎn)生不良影響，甚至可以通過(guò)大氣擴(kuò)散作用到幾公里以外的居民區(qū)。除了上述的惡臭污染來(lái)源以外，在工業(yè)生產(chǎn)和居民生活中燃燒煤炭，石油，煤氣等燃料也會(huì)產(chǎn)生含硫和含氮惡臭污染物以及由于個(gè)人生活衛(wèi)生不潔或者患有某種疾病所導(dǎo)致的惡臭污染，如腋臭和口臭等[12,13]。惡臭污染物作為一類重要的空氣污染物，主要包括烴類、含氧有機(jī)物、含硫有機(jī)物、含氮有機(jī)物以及鹵素及其衍生物等[14,15]。目前，種類繁多的惡臭污染物作為世界環(huán)境公害之一，具有自身的獨(dú)特污染特點(diǎn): 具有極低的嗅閾值低(臭味的最低嗅覺(jué)濃度)、排放強(qiáng)度大、污染源多、為點(diǎn)源和區(qū)域性污染以及以心理影響為主等。表1為部分典型的惡臭污染物的嗅閾值及污染特性。

由于惡臭污染物具有上述獨(dú)特的污染特點(diǎn)，不僅會(huì)對(duì)惡臭污染源附近居住及工作人群的心理及感官產(chǎn)生影響，使人產(chǎn)生不愉快，煩躁和厭惡的情緒，降低工作效率和生活的質(zhì)量[18]，而且嚴(yán)重危害呼吸系統(tǒng)、消化系統(tǒng)、血液循環(huán)系統(tǒng)以及神經(jīng)系統(tǒng)，同時(shí)有些惡臭污染物還具有干擾人體的內(nèi)分泌系統(tǒng)，影響機(jī)體的代謝活動(dòng)，甚至具有“三致作用”等[19,20]，因此有必要采取適當(dāng)?shù)募夹g(shù)手段對(duì)大氣中的惡臭污染物進(jìn)行有效去除。

表1 典型惡臭污染物的名稱、分子式、臭味性質(zhì)、嗅閾值Table 1 The designation, molecular formulas, odor character and threshold of typical odorous pollutants

2 大氣惡臭處理反應(yīng)器類型及其應(yīng)用現(xiàn)狀

由于處理惡臭的生物技術(shù)種類較多，本文僅針對(duì)其中的固定化微生物技術(shù)在大氣惡臭處理方面的應(yīng)用進(jìn)行介紹。固定化微生物技術(shù)是指利用將微生物接種到特定的生物反應(yīng)器內(nèi)的載體上，利用微生物自身代謝活動(dòng)降解惡臭物質(zhì)，使之降解礦化為CO2和H2O等最終產(chǎn)物，同時(shí)微生物利用污染物合成自身所需要的營(yíng)養(yǎng)物質(zhì)而進(jìn)行生長(zhǎng)和繁殖，從而達(dá)到無(wú)臭化、無(wú)害化的一種方法[21]。與常規(guī)的方法相比，它擁有物理和化學(xué)方法所不具有的特點(diǎn)：工藝簡(jiǎn)單、操作方便、運(yùn)行穩(wěn)定、處理效果好、無(wú)二次污染等，同時(shí)固定化微生物反應(yīng)器通常在常溫、常壓下運(yùn)行，運(yùn)行時(shí)所需要的能耗低也比較低，尤其對(duì)于工農(nóng)業(yè)生產(chǎn)過(guò)程所排放的大量、低濃度、無(wú)回收價(jià)值的惡臭污染物治理，固定化微生物技術(shù)具有明顯的優(yōu)勢(shì)，具有廣闊的應(yīng)用前景。常用的能夠應(yīng)用于大氣惡臭處理的固定化微生物技術(shù)主要包括生物過(guò)濾池、生物滴濾塔和生物膜反應(yīng)器等。

2.1 生物過(guò)濾池(Biofilter)

生物過(guò)濾池是最早用于去除大氣惡臭污染物的一種固定化微生物方法，目前主要用于去除氣液分配系數(shù)小于1.0的惡臭污染物廢氣[22]。由于生物過(guò)濾池所用的載體通常為含有豐富微生物的土壤、堆肥以及泥炭等天然有機(jī)填料，天然有機(jī)載體本身能夠?yàn)槲⑸锷L(zhǎng)提供所需要的營(yíng)養(yǎng)元素，無(wú)需額外添加營(yíng)養(yǎng)成分。因此，生物過(guò)濾池具有結(jié)構(gòu)簡(jiǎn)單，投資少、運(yùn)行費(fèi)用低等優(yōu)點(diǎn)。Hartikaine等[23]采用接種泥炭的生物過(guò)濾池處理濃度為 14 mg/m3的氨氣，獲得了較好的處理效果，去除能力可達(dá) 1.8 g/m3/d。Shah等[24]采用堆肥添加CaO的形式，研究去除養(yǎng)雞場(chǎng)的氨氣，在濃度為27 ppm時(shí)，去除率可以達(dá)到 97%。生物過(guò)濾池可以對(duì) NH3、H2S和VOCs如甲苯等三種混合污染物進(jìn)行有效降解，最高去除率可以達(dá)到100%[25]。同時(shí)，Chou和 Shiu[26]采用泥炭作為填料，在泥炭濕度為55-60%，pH在7.5-8.5的條件下，對(duì)容積負(fù)荷高達(dá)160 g/m3/h的甲胺可以有效降解。Tang等[27]采用堆肥和稻殼混合物作為填料，可以對(duì)濃度為78～841 ppmv的三乙胺有效降解，最高去除負(fù)荷可達(dá)140 g/m3/h，但高于140 g/m3/h時(shí)，發(fā)現(xiàn)底物具有明顯的抑制作用。同國(guó)外相比，目前國(guó)內(nèi)開(kāi)展氣相含氮惡臭有機(jī)污染物的研究還比較少，殷俊等[28]、丁穎[29]和胡芳等[30]采用接種堆肥或者活性污泥的生物過(guò)濾池對(duì)氣相三甲胺的生物降解方面開(kāi)展了凈化研究工作，去除率可以達(dá)到99%以上。但是，生物過(guò)濾池在降解有機(jī)物時(shí)會(huì)產(chǎn)生酸性物質(zhì)，會(huì)遇到填料的酸化以及設(shè)備腐蝕的問(wèn)題，因此，需要在操作的過(guò)程中對(duì)pH進(jìn)行控制才能更好地發(fā)揮其特性[31]。

2.2 生物滴濾塔(Biotrickling filter)

生物滴濾塔是在生物過(guò)濾池基礎(chǔ)上進(jìn)一步工藝改進(jìn)的固定化微生物技術(shù)，不僅適合處理氣液分配系數(shù)小于1.0的惡臭污染物，也適合小于0.1的惡臭污染物[22]。這是由于生物滴濾塔所采用的填料載體多為機(jī)械強(qiáng)度很高的無(wú)機(jī)或者有機(jī)物質(zhì)，其本身不含有微生物并且不能為吸附在其表面的生物提供營(yíng)養(yǎng)元素。因此，生物滴濾塔運(yùn)行初期一般需要進(jìn)行微生物接種，同時(shí)需要通過(guò)連續(xù)或者間歇噴淋微生物生長(zhǎng)所需要的營(yíng)養(yǎng)液促進(jìn)微生物的生長(zhǎng)與固定化。與生物過(guò)濾池相比，生物滴濾塔具有較高的空隙率和較小的床層壓降。同時(shí)通過(guò)噴淋循環(huán)液進(jìn)而可以有效控制反應(yīng)器內(nèi)微生物的生長(zhǎng)環(huán)境(如pH、營(yíng)養(yǎng)物濃度)，避免反應(yīng)產(chǎn)物在床層內(nèi)的積累。這些改進(jìn)使生物滴濾塔具有可操控性強(qiáng)等優(yōu)點(diǎn)。目前已成為近年來(lái)固定化微生物技術(shù)方面的研究熱點(diǎn)之一。有文獻(xiàn)報(bào)道，采用活性炭作為填料，接種活性污泥等菌種，能夠在停留時(shí)間為4 s時(shí)對(duì)濃度為20 ppmv的H2S有效去除，去除率可達(dá)98%[32]。甚至有文獻(xiàn)報(bào)道在堿性(pH=10)的條件下，接種微生物的滴濾塔在停留時(shí)間為1～6 s的條件下，可以對(duì)濃度范圍為2.5～18 ppmv的H2S進(jìn)行有效地去除，去除率高達(dá)98%以上[33]。同國(guó)外相比，國(guó)內(nèi)采用PVC彈性立體填料的生物滴濾塔研究結(jié)果表明，在H2S質(zhì)量濃度小于1200 mg/m3時(shí)的去除率可以達(dá)到90%以上[34]；黃樹(shù)杰[35]采用滴濾塔處理硫化氫濃度為712.80-948.80 mg/m3的效率同樣可以達(dá)到90%以上。此外，王京剛和張雅旎[36]采用改進(jìn)聚乙烯醇法制成的固定化活性污泥顆粒填充生物滴濾塔在流量低于0.1 m3/h時(shí)，生物滴濾塔對(duì)乙硫醇的凈化效率可達(dá)99.9%以上，當(dāng)高于此流量時(shí)去除率明顯降低。

此外，Kalingan等[37]采用接種微生物的泥炭和無(wú)機(jī)填料的生物滴濾塔，在室溫下處理濃度為200 ppmv的含氨廢氣，去除率高達(dá)100%。有文獻(xiàn)報(bào)道采用活性炭為填料，接種Paracoccussp.CP2可以對(duì)濃度為10～250 ppm的三甲胺(>85%)、二甲胺(>90%)和甲胺(>93%)等三種污染物進(jìn)行有效去除，而三甲胺的生物降解性最差；而在添加菌種Parthrobactersp.CP1的情況下，三甲胺則可以被完全降解[38]。目前，生物滴濾塔技術(shù)也已經(jīng)應(yīng)用于甲苯[39,40]、苯乙烯[41,42]、甲醛和甲醇[43]、酚類[44]、酮類[45]以及揮發(fā)性脂肪酸[46]等的降解。同時(shí)也有文獻(xiàn)報(bào)道了生物滴濾塔降解親水性甲醇和非親水化合物蒎烯的混合物[47]以及甲苯和三氯乙烯的混合物等[48]。

2.3 膜生物反應(yīng)器(Membrane Bioreactor)

膜生物反應(yīng)器是一類新型的用于廢氣處理的固定化微生物技術(shù)，主要是受到新材料的研制開(kāi)發(fā)以及膜生物技術(shù)在廢水處理中的成功應(yīng)用的啟示，人們開(kāi)始關(guān)注膜技術(shù)在廢氣處理中的應(yīng)用。膜生物反應(yīng)器通常采用致密膜、多孔膜或者微孔材料膜作為載體，其中比較常用的膜為微孔膜[49]。用于接種的微生物在膜載體上生長(zhǎng)并形成生物膜或者采取微生物懸浮在營(yíng)養(yǎng)液中，通過(guò)膜的選擇性滲透，污染物通過(guò)濃度的梯度擴(kuò)散作用到達(dá)生物膜，并隨后由微生物降解[50,51]。同生物過(guò)濾池和滴濾塔相比，在膜生物反應(yīng)器內(nèi)由于氣體和生物膜分別位于纖維膜的兩側(cè)，因此氣、液流量可分別控制，同時(shí)膜生物反應(yīng)器可清除過(guò)量的生物量以防堵塞，并且可以提供一個(gè)大的氣液接觸界面提高質(zhì)量傳遞的速率[51]。目前，膜生物生物反應(yīng)器已經(jīng)廣泛用于烷烴等有機(jī)物的生物降解，并取得了較好的效果[49,51]。如采用接種Burkholderia vietnamiensisG4的多孔聚丙烯腈和聚二甲基硅氧烷膜作為載體的膜生物反應(yīng)器可以在停留時(shí)間 2-28 s的條件下對(duì)濃度0.21-4.1 g/m3的甲苯凈化，去除率可達(dá)78-99%[52]。在停留時(shí)間為 8-24 s時(shí)可以對(duì)廢氣中濃度為 38 mg/m3二甲基硫醚進(jìn)行有效降解，去除率可以達(dá)到85-99%[53]。但是總體而言膜生物反應(yīng)器的缺點(diǎn)是投資高，而且隨著時(shí)間的增加，生物膜的活性可能有所下降。因此，同其他的固定微生物技術(shù)，如生物過(guò)濾池和生物滴濾塔相比，膜生物反應(yīng)器降解惡臭有機(jī)物污染物的研究目前還僅限于實(shí)驗(yàn)室階段，未見(jiàn)有工業(yè)應(yīng)用的報(bào)道。

3 固定化微生物技術(shù)的主要影響因素

3.1 底物的影響

固定化微生物反應(yīng)器內(nèi)都裝載有用于固定微生物的載體填料，在載體表面接種具有降解特定污染物的微生物菌種時(shí)，含有惡臭污染物的氣體通過(guò)生物反應(yīng)器內(nèi)的填料層時(shí)，污染物從氣相擴(kuò)散到載體表面的液相或者生物膜，然后被微生物吸附、吸收和降解[10]。因此底物的物理化學(xué)性質(zhì)將會(huì)決定生物反應(yīng)器處理惡臭污染物的效果。有研究表明，污染物的分子組成會(huì)影響底物的可生物降解性，如在苯、甲苯、乙苯和二甲苯這四種物質(zhì)中，最難降解的是鄰二甲苯，降解率只有30%，其次為苯(45%)，最容易降解的甲苯可以得到完全降解[54]。此外也有研究表明，在苯環(huán)上引入取代基團(tuán)或者取代基團(tuán)種類的增加會(huì)導(dǎo)致生物可降解性變差，如苯環(huán)上氯原子的數(shù)目增加到4個(gè)以上或者氯原子和羧基共同存在時(shí)[55]。此外，Ho等[38]的研究也表明，空間結(jié)構(gòu)比較復(fù)雜的三甲胺的可生化降解性明顯差于二甲胺和甲胺。由于大多惡臭氣體污染物在降解過(guò)程主要受制于污染物從氣相到生物膜的擴(kuò)散速率控制，而對(duì)于水溶性好的惡臭污染物則受控于生物膜內(nèi)微生物的降解速率。因此，醇類比醛類易于降解、酮類比酯類易于降解，但是所有這些化合物都要比長(zhǎng)鏈烷烴容易降解，而芳香烴則是最難降解的[56]。所有這些結(jié)果都說(shuō)明降解底物的物理化學(xué)性質(zhì)會(huì)影響到其可生物降解性。因此，針對(duì)不同的物理化學(xué)性質(zhì)的污染物可以有針對(duì)性地選擇合適的固定化微生物反應(yīng)器進(jìn)行有效處理。

3.2 微生物的影響

固定化微生物法降解惡臭污染物，主要是利用固定在填料載體表面的微生物將惡臭污染物作為碳源和能源，同時(shí)通過(guò)新陳代謝活動(dòng)來(lái)降解惡臭污染物。因此，微生物被認(rèn)為是影響惡臭污染物降解效果的最重要因素。目前已經(jīng)分離出的大量可以利用典型惡臭有機(jī)污染物作為單一碳源和能源而生長(zhǎng)的優(yōu)勢(shì)菌種多為細(xì)菌，如含硫惡臭有機(jī)物降解菌、含氮化合物降解菌以及含氯化合物降解菌等，具體總結(jié)如表2所示，這些細(xì)菌大多數(shù)都屬于桿狀菌、絲狀菌以及球菌等。大量研究表明將這些通過(guò)不同手段獲得的微生物作為固定化微生物反應(yīng)器所用的優(yōu)勢(shì)菌種，就可以針對(duì)含有特定目標(biāo)污染物的廢氣進(jìn)行有效降解。例如采用固定化單一菌種RG-1的生物滴濾塔可以對(duì)含硫惡臭有機(jī)氣體污染物進(jìn)行100%凈化[57]，固定化Paracoccussp.CP2的生物反應(yīng)器可以對(duì)含三甲胺(去除率>85%)、二甲胺(去除率>85%)以及甲胺(去除率>85%)氣體污染物進(jìn)行凈化等[38]。

與細(xì)菌相比，真菌具有對(duì)干燥、酸性等環(huán)境具有更強(qiáng)的適應(yīng)性，所以真菌也被廣泛用于氣相中惡臭有機(jī)物的去除[90]，特別是在苯系物的去除方面具有明顯的優(yōu)勢(shì)(表3)。盡管朱國(guó)營(yíng)等[91]對(duì)高效降解乙硫醇的生物過(guò)濾池中的微生物初步鑒定發(fā)現(xiàn)其優(yōu)勢(shì)菌種主要為真菌，但是他們并沒(méi)有具體的優(yōu)勢(shì)真菌進(jìn)行詳細(xì)的鑒定?？傊?，我們也可以看出真菌也是處理惡臭污染物的一個(gè)重要生物資源，如何更有效地發(fā)揮真菌在固定化微生物技術(shù)處理惡臭污染物方面的應(yīng)用是目前微生物資源研究的一個(gè)重要內(nèi)容。

3.3 填料的影響

對(duì)于固定化微生物技術(shù)，固定微生物所用的載體是除了微生物菌種以外最重要的因素。固定化載體不但對(duì)惡臭污染物具有吸附作用，而且還能夠?yàn)槲⑸锏纳L(zhǎng)提供一個(gè)局部生態(tài)微環(huán)境和保留微生物生長(zhǎng)所需要的營(yíng)養(yǎng)等。由于傳統(tǒng)富含微生物的天然填料如堆肥等在使用過(guò)程中容易壓實(shí)引起壓降的增加等缺點(diǎn)，并不太適合用于作為固定化微生物所用的載體。因此，目前大多采用機(jī)械性能好的有機(jī)和無(wú)機(jī)材質(zhì)的合成材料作為固定化微生物的載體。表4總結(jié)了部分典型的固定化微生物所用的載體材料以及在處理各種惡臭污染物的效果比較。這些填料具有共同的特點(diǎn)，即大比表面積、合適的空隙率以及機(jī)械性能好，可以使固定化微生物反應(yīng)器長(zhǎng)期穩(wěn)定運(yùn)行，尤其是采用無(wú)機(jī)填料陶粒作為固定化微生物載體的生物滴濾塔可以長(zhǎng)期穩(wěn)定運(yùn)行處理含硫惡臭氣體污染物[101]。但是，關(guān)于選用什么樣合適的載體用于那種固定化微生物反應(yīng)器是最適合的，目前還沒(méi)有一個(gè)比較一致的結(jié)論。所有這些都需要開(kāi)展大量的工程實(shí)踐的探索，以及設(shè)計(jì)加工更加具有通用性能的新型多功能填料才能充分發(fā)揮固定化微生物技術(shù)在惡臭污染物去除方面的應(yīng)用優(yōu)勢(shì)。

表2 降解不同惡臭污染物的優(yōu)勢(shì)菌種Table 2 List of microorganisms for degrading representative odors pollutants

表3 降解不同惡臭污染物的優(yōu)勢(shì)真菌Table 3 List of fungus for degrading representative odorous pollutants

3.4 微生物固定化方法的影響

總體而言，處理大氣惡臭污染物的微生物固定化方法與用于廢水處理方面的微生物固定化方法基本上是一樣，其固定化方法均主要包括[115-117]：吸附與附著法、包埋法、共價(jià)鍵合法、交聯(lián)法和復(fù)合固定化法等等。而正是由于微生物的固定化方法及其固定化微生物技術(shù)在水處理領(lǐng)域的應(yīng)用方面已經(jīng)具有了很多非常不錯(cuò)的綜述[115-117]，因此本文并不想在該部分進(jìn)行不必要的贅述。

3.5 pH值的影響

固定化微生物最佳降解惡臭污染物的能力一般都在其最佳的pH值范圍(7～8之間)內(nèi)，即大部分細(xì)菌和放線菌的最適宜生長(zhǎng)范圍[118]。而對(duì)于絕大多數(shù)的惡臭污染物在好氧生物降解過(guò)程中將會(huì)產(chǎn)生酸類物質(zhì)，如含硫和含氮污染物中的硫元素和氮元素降解的最終產(chǎn)物為H2SO4和HNO3，繼而會(huì)使固定化微生物反應(yīng)器內(nèi)pH下降，導(dǎo)致微生物自身的活性受到影響，進(jìn)而影響到對(duì)惡臭污染物的去除效果[22]。比如在采用生物滴濾塔降解H2S過(guò)程中，當(dāng)pH從2.0增加到7.0的過(guò)程中，雖然生物反應(yīng)器對(duì)H2S的去除率基本都在95%以上，但是去除能力從13.35 g/m3/h增加到31.12 g/m3/h[119]。因此，如果要持續(xù)有效地發(fā)揮固定化微生物反應(yīng)器的效率，有必要對(duì)固定化微生物反應(yīng)器內(nèi)pH進(jìn)行調(diào)控。通常的方法主要是在固定化微生物反應(yīng)器運(yùn)行初期的固定化載體中添加固體緩沖物質(zhì)來(lái)調(diào)節(jié)pH的變化，如在處理易產(chǎn)生酸性物質(zhì)的惡臭污染物時(shí)，添加石灰和白云石可以有效地減緩固定化載體的酸化過(guò)程并提高污染物的去除能力[58,120]。當(dāng)然也可以采用在固定化生物反應(yīng)器內(nèi)的循環(huán)液中添加適量的堿性或者酸性物質(zhì)來(lái)實(shí)現(xiàn)中和反應(yīng)，使反應(yīng)器內(nèi)的環(huán)境恢復(fù)到微生物生長(zhǎng)所需要的適宜pH值范圍。而對(duì)于處理不同特性的混合污染物時(shí)，如處理NH3和H2S混合惡臭氣體時(shí)，H2S降解生成的酸就可以中和NH3引起的pH上升[121]，因此就不需要采取額外的措施。

表4 典型降解不同惡臭污染物的填料載體Table 4 List of packing material for degrading representative odorous pollutants

3.6 溫度的影響

環(huán)境溫度也是影響固定化微生物反應(yīng)器性能的重要因素之一，這主要是因?yàn)槲⑸锏纳L(zhǎng)有一定的適宜溫度范圍，絕大多數(shù)的微生物的最適宜生長(zhǎng)溫度范圍一般在25～35 ℃之間[22,56]。例如降解苯系物中甲苯、苯、乙苯以及二甲苯的最佳溫度范圍就在30～35 ℃之間[77,122]。但是，也有菌種可以在低溫或者高溫條件下能夠?qū)ξ廴疚镞M(jìn)行有效地降解。如在溫度為2～20 ℃范圍內(nèi)，通過(guò)接種泥炭-土壤或者乙烯氧化菌RD-4的生物反應(yīng)器均對(duì)乙烯具有較好的去除效果[123,124]。以硫化氫和氨氣的混合臭氣為研究對(duì)象，在2～8 ℃時(shí)，氧化硫桿菌和亞硝化細(xì)菌對(duì)混合氣體的去除率也可達(dá)99%[125]。除此之外，Luvsanjamba等[126]利用嗜高溫(52 ℃)二甲基硫氧化菌種的膜生物反應(yīng)器處理含有二甲基硫的高溫廢氣，結(jié)果表明在停留時(shí)間為24 s時(shí)，污染物的去除率達(dá)到84%，容積去除負(fù)荷達(dá)到54 g/m3/h。總之，對(duì)于固定化生物反應(yīng)器內(nèi)的微生物要保持其高效的污染物降解活性，溫度必須控制在其生長(zhǎng)的最佳范圍內(nèi)，而對(duì)于實(shí)際廢氣溫度過(guò)高或者過(guò)低，都需要進(jìn)行預(yù)處理，以保證微生物降解污染物的活性。

5 固定化微生物降解惡臭污染物的機(jī)理研究進(jìn)展

大氣中惡臭污染物在固定而化微生物反應(yīng)器內(nèi)的去除宏觀上主要是通過(guò)底物從氣相中擴(kuò)散到載體表面的生物膜或者液相進(jìn)而被所固定化的微生物所降解，然后生成生物質(zhì)和釋放出CO2等最終代謝產(chǎn)物等[127]。由于氣相污染物的降解首先必須從氣相轉(zhuǎn)入生物膜或者液相中，因此微觀上固定化生物反應(yīng)器降解惡臭氣體污染物的途徑和微生物降解液相中同一種污染物的過(guò)程基本相同。對(duì)于多數(shù)惡臭污染物的降解，首先是在母體化合物上引入羥基或者直接脫去某些基團(tuán)，然后進(jìn)一步發(fā)生系列生化反應(yīng)而將目標(biāo)污染物去除。如甲基叔丁基醚在微生物菌種的作用下先生成叔丁氧基甲醇，進(jìn)一步的生成叔丁基甲酸然后再進(jìn)一步轉(zhuǎn)化成叔丁醇，或者叔丁基甲醚可以直接生成叔丁醇。叔丁醇在微生物的作用下進(jìn)一步降解生成異丙醇以及丙酮等而最終得到完全分解[128]。正己烷在微生物菌種Rhodococcussp. EH831的作用下，首先通過(guò)引入羥基生成2-己醇，經(jīng)2-己酮進(jìn)一步礦化成丁醛，最后經(jīng)丙醛、丙酮和乙醛等途徑生成終產(chǎn)物CO2[79]。對(duì)于含有苯環(huán)結(jié)構(gòu)的物質(zhì)生物降解機(jī)理具有類似的結(jié)果，如Ralstonia pickettiiL2降解氯苯過(guò)程可以通過(guò)兩條途徑完成，首先是在微生物的作用下直接脫去氯原子的同時(shí)引入羥基生成鄰苯二酚或者直接依次引入兩個(gè)羥基生成鄰氯二苯酚，然后這些中間產(chǎn)物在微生物的作用下經(jīng)鄰位或者間位開(kāi)環(huán)而降解[89]。Na等[129]采用Rhodococcus opacus菌種降解液相中苯，首先生成鄰苯二酚然后開(kāi)環(huán)而得到降解。同時(shí)Liang等[130]采用Delftia tsuruhatensisAD9菌種對(duì)苯環(huán)含有氨基的化合物苯胺的研究結(jié)果也表明，苯胺在液相中的降解首先是通過(guò)在苯環(huán)上引入兩個(gè)羥基生成鄰苯二酚作為第一級(jí)降解產(chǎn)物，然后再以鄰位或者間位開(kāi)環(huán)的形式得到進(jìn)一步的降解。Zhang等[131]研究同樣表明液相中苯胺在微生物的作用下首先生成鄰苯二酚，然后進(jìn)一步開(kāi)環(huán)后生成最終產(chǎn)物CO2。對(duì)于含硫化合物的生物轉(zhuǎn)化機(jī)理和上述的引入羥基不同，如Gonzalez等[132]研究二甲基硫在Proteobacteria的作用下，直接生物降解生成中間產(chǎn)物甲硫醇，然后甲硫醇進(jìn)一步礦化生成CO2和硫酸根。同樣二甲基亞砜和二甲基二硫醚在微生物的作用下，首先生成一級(jí)降解產(chǎn)物甲硫醇，然后進(jìn)一步被微生物降解生成硫化氫和甲醛，然后硫元素徹底礦化成硫酸根，而碳元素徹底礦化成CO2或生成生物質(zhì)[133]。而對(duì)于非含苯環(huán)的含氮化合物如三甲胺等在好氧的條件下首先生成三甲胺氮氧化合物，然后依次生成二甲胺和甲胺，最后進(jìn)一步礦化成最終產(chǎn)物，而在厭氧的條件下直接依次生成二甲胺和甲胺等代謝產(chǎn)物后被最終降解[71,72]。

6 研究展望

近年來(lái)固定化微生物技術(shù)在處理大氣惡臭污染物方面顯示了良好的應(yīng)用前景。固定化微生物技術(shù)能夠?qū)⒋髿庵泻械母鞣N有機(jī)惡臭污染物經(jīng)微生物的同化作用和異化作用轉(zhuǎn)化為CO2、水和生物質(zhì)，從而消除惡臭污染物對(duì)環(huán)境的影響。但是在現(xiàn)有的研究基礎(chǔ)上真正實(shí)現(xiàn)固定化微生物技術(shù)在大氣惡臭污染物處理中的工業(yè)化應(yīng)用，今后還需要加強(qiáng)以下幾個(gè)方面的研究工作：

(1)優(yōu)勢(shì)微生物菌劑的培育。盡管目前已經(jīng)有大量能夠降解惡臭污染物的優(yōu)勢(shì)菌種，但是不同的惡臭底物降解所需要的菌種不同，因此針對(duì)不同惡臭污染物的不同特性，需要培養(yǎng)和獲得更多的可用于惡臭污染物降解的優(yōu)勢(shì)菌種，包括能夠適應(yīng)處理實(shí)際環(huán)境條件下的低溫和高溫環(huán)境中使用的優(yōu)勢(shì)菌種以滿足降解不同種類和不同條件下惡臭有機(jī)物的需要，在今后有效實(shí)現(xiàn)固定化微生物技術(shù)在實(shí)際大氣惡臭污染物治理中的應(yīng)用具有非常重要的研究?jī)r(jià)值。

(2)固定化載體的開(kāi)發(fā)與改良。如何針對(duì)現(xiàn)有天然填料容易坍塌壓實(shí)、無(wú)機(jī)填料比表面積小以及有機(jī)合成填料容易堵塞等不足問(wèn)題，設(shè)計(jì)開(kāi)發(fā)出更加廉價(jià)和穩(wěn)定的填料載體，以便有利于固定化微生物的吸附和生長(zhǎng)的新型多功能填料是實(shí)現(xiàn)固定化微生物技術(shù)工業(yè)化應(yīng)用的重要研究方向之一。

(3)固定化微生物反應(yīng)器的構(gòu)建與優(yōu)化。如何針對(duì)實(shí)際不同惡臭污染物的來(lái)源和特性，通過(guò)固定化載體和優(yōu)勢(shì)微生物的組裝，有針對(duì)性地選擇和構(gòu)建高效固定化微生物反應(yīng)器，并且系統(tǒng)開(kāi)展固定化微生物技術(shù)在實(shí)際大氣惡臭污染物工業(yè)應(yīng)用化中操作參數(shù)的優(yōu)化與篩選，有效縮短惡臭污染物在生物反應(yīng)器中的停留時(shí)間，充分和高效地發(fā)揮固定微生物反應(yīng)器的處理效率也是其工業(yè)化應(yīng)用中非常關(guān)鍵的研究?jī)?nèi)容之一。

(4)固定化微生物技術(shù)與其它單元技術(shù)的聯(lián)用。盡管目前有關(guān)固定化微生物技術(shù)凈化惡臭污染物的研究也不少，但是固定化微生物技術(shù)處理實(shí)際惡臭污染物的工業(yè)化應(yīng)用還比較有限，因此如何發(fā)揮現(xiàn)有固定微生物技術(shù)在處理惡臭污染物中的主導(dǎo)作用，并將其它大氣處理的物理化學(xué)單元技術(shù)相結(jié)合，系統(tǒng)研究組合工藝不同組合方式、不同組合順序等對(duì)其聯(lián)用的影響，充分發(fā)揮各單元技術(shù)之間的獨(dú)特優(yōu)勢(shì)，以期實(shí)現(xiàn)固定化微生物技術(shù)在工業(yè)實(shí)際惡臭污染物治理方面的應(yīng)用是目前的一個(gè)重要研究方向。如果今后能夠在以上幾個(gè)方面開(kāi)展更加系統(tǒng)和有成效的研究，可以預(yù)期在不遠(yuǎn)的將來(lái)固定化微生物技術(shù)必然能夠在大氣惡臭污染物的凈化方面發(fā)揮更加出色的作用。

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