殷霄云,劉芝宏,周愛(ài)娟,李亞男,岳秀萍,崔芷瑄

紫外耦合游離亞硝酸強(qiáng)化剩余污泥厭氧發(fā)酵產(chǎn)酸研究

殷霄云,劉芝宏*,周愛(ài)娟,李亞男,岳秀萍,崔芷瑄

(太原理工大學(xué)環(huán)境科學(xué)與工程學(xué)院,山西 太原 030024)

為打破傳統(tǒng)厭氧發(fā)酵過(guò)程中污泥破壁溶胞困難和產(chǎn)酸效能低的瓶頸,探究了紫外光(UV)耦合游離亞硝酸(FNA)預(yù)處理對(duì)污泥發(fā)酵產(chǎn)酸的影響,并與熱(H)和超聲法(US)耦合FNA預(yù)處理進(jìn)行了對(duì)比分析.結(jié)果表明,UV輔助FNA聯(lián)合預(yù)處理(FNA-UV)對(duì)細(xì)胞破碎和胞外聚合物的剝離具有協(xié)同效應(yīng),?OH和?O2-作為反應(yīng)中間體,其強(qiáng)度遠(yuǎn)高于其他預(yù)處理組(FNA-US和FNA-H),與?NO,?NO2及ONOO-等中間產(chǎn)物共同促進(jìn)了溶解性有機(jī)物的釋放,溶解性碳水化合物和蛋白質(zhì)含量相比FNA組分別提升60%和90%,進(jìn)而為后續(xù)水解產(chǎn)酸過(guò)程提供了充足的底物.FNA-UV組短鏈脂肪酸(SCFAs)濃度于第4d達(dá)到峰值,為(201.8±4.8)mg COD/g VSS,相比FNA組提升67%,乙酸占比高達(dá)56.8%.通過(guò)發(fā)酵末期對(duì)各體系進(jìn)行碳平衡分析表明,紫外耦合FNA預(yù)處理在污泥減量、溶解性有機(jī)物的釋放與轉(zhuǎn)化、SCFAs的產(chǎn)生方面具有重要作用.微生物群落分析表明,FNA-UV對(duì)功能菌群的富集發(fā)揮重要作用,表現(xiàn)為厭氧發(fā)酵菌和反硝化菌的有效增強(qiáng),相比其他各組提升了23.7%～270.6%.

游離亞硝酸；剩余污泥；短鏈脂肪酸；厭氧發(fā)酵；紫外；高通量測(cè)序

近年來(lái),隨著我國(guó)城鎮(zhèn)化進(jìn)程的日益加速,剩余污泥作為市政污水處理廠生物處理段的伴生產(chǎn)物,其產(chǎn)量隨污水處理能力的提升而飛速增長(zhǎng)[1].據(jù)報(bào)道,剩余污泥可作為能源和資源的載體,回收污泥中的能源和資源成為目前研究的重點(diǎn)和難點(diǎn)[2-3].厭氧發(fā)酵技術(shù)(AF)通過(guò)微生物代謝進(jìn)行水解酸化,進(jìn)而實(shí)現(xiàn)污泥減量和資源回收(如短鏈揮發(fā)性脂肪酸SCFAs、甲烷CH4),成為污泥處理的重要途徑[4]. SCFAs具有高附加值,不僅可以作為各種化工產(chǎn)品生產(chǎn)的原材料,還可以作為污水處理廠的有機(jī)碳源[5].然而,由于污泥中有機(jī)物受細(xì)胞壁和胞外聚合物(EPS)包裹,導(dǎo)致其釋放困難,水解過(guò)程成為剩余污泥AF的限速步驟.因此,尋找高效的預(yù)處理技術(shù)成為污泥資源化的先決條件[6].

游離亞硝酸(FNA)作為亞硝酸鹽的質(zhì)子化形式,是一種綠色可再生的抗菌劑[7-8].在百萬(wàn)分之一水平下,FNA及其衍生物可與細(xì)胞/EPS中的脂質(zhì)、蛋白質(zhì)、碳水化合物和脫氧核糖核酸(DNA)反應(yīng),有效破壞微生物細(xì)胞膜,分解胞內(nèi)大分子有機(jī)物,為產(chǎn)酸菌提供底物[9].單獨(dú)采用FNA預(yù)處理可實(shí)現(xiàn)污泥有效溶胞(1～2mg N/L預(yù)處理污泥24～48h,可滅活50%～ 80%的細(xì)胞[10]).但相關(guān)研究表明,經(jīng)FNA預(yù)處理后仍有大量耐受細(xì)胞處于穩(wěn)態(tài),且其對(duì)EPS的破壞作用極為有限,進(jìn)而限制水解產(chǎn)酸效能(3.04mg N/L FNA預(yù)處理污泥,EPS中腐殖質(zhì)等大分子物質(zhì)不能被水解[11])[12-13].因此,越來(lái)越多學(xué)者采用物理/機(jī)械法(冷凍[14]、熱[15]、超聲[16])和投加化學(xué)試劑(Fenton法[13]、過(guò)氧化氫[17]、鼠李糖脂[18])與FNA預(yù)處理耦合,有效強(qiáng)化了水解、產(chǎn)酸和產(chǎn)甲烷過(guò)程.然而,化學(xué)法存在反應(yīng)條件苛刻,易造成二次污染等缺陷,限制了其大規(guī)模應(yīng)用[19].相比其他物理預(yù)處理方法,紫外預(yù)處理具有處理效率高、成本低廉和無(wú)二次污染等優(yōu)點(diǎn),通過(guò)損傷和破壞生物活性,分解胞內(nèi)物質(zhì),導(dǎo)致細(xì)胞破裂,成為重要的污泥預(yù)處理手段[20].研究發(fā)現(xiàn)紫外法(UV)與其它預(yù)處理方式聯(lián)合能有效破壞污泥EPS,使污泥絮體分解(UV輔助零價(jià)鐵活化過(guò)硫酸鈉(PDS)氧化法、UV耦合CaO2法)[21-22].已有研究表明亞硝酸鹽在UV照射下可能產(chǎn)生活性氮物種(×NO、×NO2、ONOO-、ONOOH)和活性氧物種(×OH、×O2-),通過(guò)電子轉(zhuǎn)移、雙鍵加成或氫抽提等方式破壞細(xì)胞內(nèi)各種蛋白質(zhì)、脂質(zhì)和DNA結(jié)構(gòu)[11,20],但UV聯(lián)合FNA預(yù)處理對(duì)污泥細(xì)胞破裂、EPS分解、厭氧發(fā)酵產(chǎn)酸效能及作用機(jī)理尚不明晰.

基于此,本研究通過(guò)超聲、熱和紫外照射3種方式強(qiáng)化FNA預(yù)處理污泥,考察不同預(yù)處理方式對(duì)污泥發(fā)酵產(chǎn)酸性能的影響,以及對(duì)污泥微生物多樣性的影響,旨在為剩余污泥資源化利用提供理論參考.

1 材料和方法

1.1 實(shí)驗(yàn)材料

本實(shí)驗(yàn)所用污泥取自山西省太原市楊家堡污水處理廠的污泥濃縮池,所取污泥首先用200目篩子過(guò)濾,靜置24h后除去上清液,置于4℃冰箱備用.污泥濃縮后10000r/min離心,經(jīng)0.45μm濾膜過(guò)濾后測(cè)定其相關(guān)性質(zhì),剩余污泥初始性質(zhì)如表1所示.

表1 剩余污泥初始性質(zhì)

1.2 實(shí)驗(yàn)設(shè)計(jì)

為了考察3種物理法輔助FNA預(yù)處理對(duì)污泥破壁及厭氧發(fā)酵效果的影響,共設(shè)置5組實(shí)驗(yàn):未預(yù)處理污泥作為空白對(duì)照組,FNA預(yù)處理組,超聲、熱和紫外照射強(qiáng)化FNA預(yù)處理實(shí)驗(yàn)組(FNA-H,FNA-US,FNA-UV).FNA投加量設(shè)置2.13mg N/L[2],FNA預(yù)處理12h后進(jìn)行1h的物理強(qiáng)化處理,具體參數(shù)如表2所示[23-26].預(yù)處理后,測(cè)定EPS中溶解性碳水化合物及蛋白質(zhì)含量.發(fā)酵實(shí)驗(yàn)采用厭氧發(fā)酵瓶,工作容積為400mL,取350mL預(yù)處理污泥和50mL新鮮污泥加入發(fā)酵瓶中,利用1.0mol/L NaOH或10% HCl調(diào)節(jié)pH值至(7.0±0.1),并連續(xù)充氮15min以保證厭氧環(huán)境.每組設(shè)置3個(gè)平行,反應(yīng)器置于35 ℃,120r/min的恒溫?fù)u床中進(jìn)行為期10d的發(fā)酵實(shí)驗(yàn).每隔24h取定量發(fā)酵污泥混合液,10000r/min離心取上清液,經(jīng)0.45μm濾膜過(guò)濾后測(cè)定上清液中的NH4+-N,SCFAs,溶解性蛋白質(zhì),溶解性碳水化合物的含量.

表2 實(shí)驗(yàn)組參數(shù)設(shè)置

1.3 分析方法

采用熱提取法對(duì)預(yù)處理后EPS進(jìn)行提取,pH值采用pH計(jì)測(cè)定,氨氮,SCOD,VSS,TSS采用國(guó)標(biāo)法測(cè)定,溶解性碳水化合物采用苯酚-硫酸法,溶解性蛋白采用改良型BCA法蛋白質(zhì)濃度測(cè)定試劑盒測(cè)定,SCFAs采用配有氫火焰離子化檢測(cè)器(FID)的安捷倫6890氣相色譜儀測(cè)定.

預(yù)處理后用電子順磁共振(EPR)法鑒定自由基種類,以5,5-二甲基-1-吡咯啉-N-氧化物(DMPO)作為自旋捕獲劑對(duì)體系內(nèi)自由基進(jìn)行捕獲,通過(guò)JES- FA300光譜儀進(jìn)行分析.對(duì)發(fā)酵末期污泥樣品進(jìn)行Illumina MiSeq高通量測(cè)序.污泥樣品經(jīng)DNA提取后,進(jìn)行PCR擴(kuò)增.選取Miseq測(cè)序平臺(tái)V3-V4區(qū)域的通用引物338F和806R進(jìn)行Illumina MiSeq測(cè)序.

為便于比較分析,上述所測(cè)得的物質(zhì)濃度(mg/L)均換算為COD濃度(mg COD/L),其轉(zhuǎn)化因子分別為:1.06g COD/g碳水化合物,1.50g COD/g 蛋白,1.07g COD/g乙酸,1.51g COD/g 丙酸,1.82g COD/g丁酸和2.04g COD/g戊酸[2].

2 結(jié)果與討論

2.1 EPS的剝離與釋放

EPS有機(jī)物的剝離和釋放可直觀反映不同預(yù)處理方式的處理效能.一般地,EPS可分為溶解性有機(jī)物(DOM)、緊密附著型EPS(TB-EPS)和松散附著型EPS(LB-EPS)3種[27].如圖1所示,相比空白組,FNA組強(qiáng)化了DOM中有機(jī)物的釋放,溶解性碳水化合物和蛋白質(zhì)的含量分別提升了54%和36%.3組聯(lián)合預(yù)處理均有效強(qiáng)化了EPS的剝離,DOM在FNA-UV組的含量相比FNA-H和FNA-US分別提升了29.9%和62.0%,溶解性碳水化合物和蛋白質(zhì)含量分別達(dá)到最大值(121.1±4.3)和(202.9±6.8) mg COD/L,比FNA單獨(dú)預(yù)處理提升了2.2和2.3倍.同時(shí),經(jīng)FNA預(yù)處理,TP-EPS中溶解性碳水化合物和蛋白質(zhì)含量相比空白組分別下降了20.0%和11.9%,而FNA-UV組的溶解性碳水化合物和蛋白質(zhì)相比FNA組下降了29.8%和36.7%.說(shuō)明UV耦合FNA能明顯促進(jìn)污泥增溶,利于溶解性有機(jī)物從TB-EPS釋放到液相.這是由于FNA-UV預(yù)處理過(guò)程生成的活性氮自由基(×NO、×NO2)和過(guò)氧亞硝酸鹽(ONOO-、ONOOH)可以破壞污泥絮體結(jié)構(gòu),促進(jìn)EPS剝離及污泥細(xì)胞壁破碎,表現(xiàn)為溶解性有機(jī)物的有效釋放.

相比空白組,FNA預(yù)處理及聯(lián)合預(yù)處理后,LB- EPS中有機(jī)物含量?jī)H有少量積累,其原因是LB-EPS處于EPS外層,結(jié)構(gòu)松散且流動(dòng)性強(qiáng),可以吸附細(xì)胞內(nèi)及TP-EPS中有機(jī)物,因此不會(huì)有明顯積累[11].

2.2 不同預(yù)處理方式對(duì)污泥厭氧發(fā)酵的影響

2.2.1 溶解性有機(jī)物的變化 溶解性碳水化合物和蛋白質(zhì)是污泥厭氧發(fā)酵過(guò)程中主要可利用有機(jī)物,因此剩余污泥水解效果可以通過(guò)液相中溶解性碳水化合物和蛋白質(zhì)的濃度來(lái)衡量.如圖2所示,各組中溶解性碳水化合物和蛋白質(zhì)總體呈現(xiàn)先增加后減小的趨勢(shì),其峰值分別在發(fā)酵第2d和第4d達(dá)到.FNA組中溶解性碳水化合物和蛋白質(zhì)含量可以達(dá)到(131.1±5.1)和(684.8±7.6) mg COD/L,相比空白組分別提升了2.8和2.1倍.物理法輔助的FNA預(yù)處理進(jìn)一步提升了溶解性有機(jī)物的溶出.其中FNA- UV組效果最為明顯,溶解性碳水化合物和蛋白質(zhì)分別高達(dá)(251.1±6.3)和(1117.9±12.7) mg COD/L,分別是FNA組的1.9和1.6倍.這主要是由于UV輔助FNA明顯促進(jìn)污泥中EPS水解及污泥細(xì)胞破裂,釋放大量胞內(nèi)外有機(jī)質(zhì)到液相中.各實(shí)驗(yàn)組溶解性碳水化合物和蛋白質(zhì)含量達(dá)到最大值后出現(xiàn)下降趨勢(shì),主要是由于溶解性有機(jī)物作為發(fā)酵產(chǎn)酸菌的底物,逐步轉(zhuǎn)化為SCFAs[28].

2.2.2 揮發(fā)酸的產(chǎn)量及成分 污泥厭氧發(fā)酵過(guò)程中,發(fā)酵菌群利用污泥中多種有機(jī)成分代謝產(chǎn)生SCFAs. SCFAs含量隨時(shí)間變化情況如圖3a所示,不同發(fā)酵條件下,揮發(fā)酸濃度隨時(shí)間呈現(xiàn)先增加后快速下降的趨勢(shì),其峰值均在發(fā)酵第4d達(dá)到.FNA預(yù)處理組中的SCFAs產(chǎn)量((115.3±2.9) mg COD/g VSS)比未預(yù)處理污泥((69.1±4.1) mg COD/g VSS)提升了67%.聯(lián)合預(yù)處理進(jìn)一步強(qiáng)化了SCFAs的產(chǎn)生,其中FNA-UV組SCFAs濃度高達(dá)(201.8±4.8)mg COD/g VSS,是FNA組的1.7倍,相比其他聯(lián)合預(yù)處理提升了16%～35%.與類似預(yù)處理方法相比,產(chǎn)酸效果明顯升高(Wu等[29]采用冷凍-FNA預(yù)處理(1.07mg N/L FNA,-5℃ 48h,SCFAs產(chǎn)量124.0mg COD/g VSS)),且明顯高于Gao等[30]中試發(fā)酵產(chǎn)酸效果(pH值10.0,連續(xù)攪拌反應(yīng)器,SCFAs產(chǎn)量1248.6mg COD/L,約113.5mg COD/ g VSS).其原因是紫外照射與FNA協(xié)同作用產(chǎn)生更多自由基,促進(jìn)污泥解體,破壞污泥細(xì)胞結(jié)構(gòu),釋放蛋白質(zhì)、糖、脂類等大分子有機(jī)物,為水解菌和產(chǎn)酸菌提供充足基質(zhì).各組中SCFAs第4d的組分分布如圖3b所示,各實(shí)驗(yàn)組中乙酸和丙酸占比達(dá)64%～73%,可作為污水廠外加碳源,為反硝化菌和聚磷菌提供最理想的基質(zhì),強(qiáng)化脫氮除磷[31].其中,單獨(dú)FNA預(yù)處理組乙酸占比43.8%,比未預(yù)處理實(shí)驗(yàn)組提高20%,而US、H、UV耦合FNA預(yù)處理污泥體系中乙酸占比進(jìn)一步提高,相比FNA組提高了9%～29%,FNA-UV組乙酸占比達(dá)到最高值(56.8± 0.2)%.同時(shí)UV耦合FNA預(yù)處理強(qiáng)化了丁酸型發(fā)酵,正丁酸含量較FNA組提高24.3%,該結(jié)果可由溶解性碳水化合物的降解來(lái)佐證(圖2a).此外,各聯(lián)合預(yù)處理組中的戊酸含量相比FNA預(yù)處理降低12.6%～29.8%,尤其是紫外照射聯(lián)合組(29.8%)達(dá)到最高值,表明該預(yù)處理強(qiáng)化了戊酸向小分子揮發(fā)酸的轉(zhuǎn)化.

2.2.3 發(fā)酵過(guò)程中氨氮的釋放效果 污泥發(fā)酵過(guò)程中,蛋白質(zhì)分解為氨基酸后,可進(jìn)一步進(jìn)行脫氨基作用生成氨氮,因此,氨氮含量的變化可間接反映細(xì)胞的死亡情況及有機(jī)物的水解效果[32-33].如圖4所示,各實(shí)驗(yàn)組中的NH4+-N濃度都隨著發(fā)酵時(shí)間呈現(xiàn)逐漸升高的趨勢(shì),表明在發(fā)酵過(guò)程中溶解性蛋白不斷水解轉(zhuǎn)化生成小分子有機(jī)物,為產(chǎn)酸菌提供所需基質(zhì)[34].以第4d為例,FNA處理組的NH4+-N濃度是空白組的1.2倍,而聯(lián)合預(yù)處理組的NH4+-N濃度又有了進(jìn)一步上升,FNA-UV組NH4+-N濃度高達(dá)(267±21)mg/L,是FNA組的1.6倍.與溶解性蛋白含量變化一致,各實(shí)驗(yàn)組NH4+-N含量高低順序?yàn)? FNA-UV組(267±21) mg/L>FNA-H組(227±11) mg/L>FNA-US組(204±19)mg/L>FNA組(165±14) mg/L>空白組(137±12) mg/L.

圖4 不同預(yù)處理方式下氨氮濃度變化

2.3 微生物群落結(jié)構(gòu)分析

剩余污泥厭氧發(fā)酵過(guò)程涉及多種微生物的共同參與,圖5反映了不同反應(yīng)器內(nèi)微生物菌落在屬水平下的相對(duì)豐度.對(duì)于門(mén)水平,主導(dǎo)菌群為變形菌門(mén)(Proteobacteria)、擬桿菌門(mén)(Bacteroidetes)、厚壁菌門(mén)(Firmicutes),均為最常見(jiàn)的發(fā)酵菌門(mén),各處理組常見(jiàn)的發(fā)酵菌門(mén)占比高達(dá)80%[35-37].從綱水平分析,-變形菌綱(Alphaproteobacteria)、-變形菌綱(Betaproteobacteria)及-變形菌綱(Gammaproteobacteria)都屬于變形菌門(mén),在水解酸化階段發(fā)揮重要作用.其中-變形菌綱主要參與污泥中溶解性碳水化合物的降解和有機(jī)酸的積累,FNA預(yù)處理組占比相對(duì)空白組提高2.0倍,其在FNA-UV組占比高達(dá)36.6%,是FNA組的1.2倍.擬桿菌綱(Bacteroidia)主要參與污泥中大分子有機(jī)物的降解和有機(jī)酸的積累.

根據(jù)屬水平上的群落分布,可以發(fā)現(xiàn)微生物菌落結(jié)構(gòu)發(fā)生了較大變化.以等為主的碳水化合物降解菌在FNA預(yù)處理?xiàng)l件下得到了明顯富集,相對(duì)豐度為13.7%,是空白組的2.3倍,并在物理輔助下進(jìn)一步積累,相對(duì)豐度達(dá)15.3%～21.6%,是FNA組的1.1～1.6倍,其中FNA-UV組占比高達(dá)21.6%[38-39].可以利用多種碳水化合物產(chǎn)生乙酸,丁酸等,在FNA組含量為11.2%,是空白組的21.0倍,在FNA-UV組含量進(jìn)一步增至14.1%[40].以、為首的蛋白質(zhì)降解菌,在 FNA預(yù)處理組和空白組的累積豐度分別為1.2%和2.1%,而超聲、熱、紫外輔助FNA組相對(duì)豐度達(dá)4.8%～9.8%.其中可有效降解蛋白質(zhì)產(chǎn)生乙酸和丙酸,在FNA組占比為1.1%,是空白組的1.1倍,而在FNA-UV組豐度增至6.9%[41].此外,、和等主要的反硝化菌在FNA組累積豐度為6.9%,是空白組的1.9倍,而FNA-UV組豐度高達(dá)8.2%,是FNA組的1.2倍[42-44].其中在FNA-UV組含量高達(dá)5.9%,是FNA組的18.8倍.說(shuō)明FNA-UV預(yù)處理可以有效富集反硝化菌,強(qiáng)化了反硝化過(guò)程.

圖5 功能微生物在屬水平的相對(duì)豐度

2.4 紫外光耦合FNA預(yù)處理強(qiáng)化污泥厭氧發(fā)酵產(chǎn)酸潛在機(jī)理分析

如圖6所示,對(duì)預(yù)處理后污泥進(jìn)行EPR分析,各組均檢測(cè)出DMPO-OH和DMPO-O2-信號(hào),聯(lián)合預(yù)處理組中DMPO-OH和DMPO-O2-信號(hào)強(qiáng)度均強(qiáng)于FNA預(yù)處理組,其中FNA-UV組信號(hào)強(qiáng)度明顯強(qiáng)于FNA-US組和FNA-H組,表明3種物理預(yù)處理,尤其是紫外預(yù)處理與FNA之間存在協(xié)同作用,強(qiáng)化了兩種自由基的產(chǎn)生與釋放,與預(yù)處理后EPS中DOM釋放效果相吻合,進(jìn)一步為上述分析提供了依據(jù)[45].其潛在的強(qiáng)化產(chǎn)酸機(jī)理如圖7所示,FNA預(yù)處理過(guò)程中產(chǎn)生的活性氮自由基(×NO、×NO2)及活性氮中間體(N2O3和N2O4)通過(guò)與胞內(nèi)或EPS發(fā)生反應(yīng),改變蛋白質(zhì)、脂質(zhì)、碳水化合物和DNA的結(jié)構(gòu),從而促進(jìn)EPS水解及污泥破碎.但FNA對(duì)分子結(jié)構(gòu)的破壞作用有限,細(xì)胞內(nèi)容物溶出較少[46-48]. FNA在UV照射下強(qiáng)化了×NO和×NO2釋放,同時(shí)產(chǎn)生過(guò)氧亞硝酸鹽(ONOO-和ONOOH),ONOO-和ONOOH作為內(nèi)源氧化劑和親核試劑,破壞細(xì)胞結(jié)構(gòu),導(dǎo)致細(xì)胞解體[49]. ONOO-在FNA預(yù)處理的酸性條件下產(chǎn)生×NO和×O2-自由基,ONOOH可分解為×NO2和×OH自由基(式(1 ～ 10)).因此,FNA和UV協(xié)同作用產(chǎn)生的各種活性自由基及中間體(×NO、×NO2、ONOO-、ONOOH、×O2-、×OH),促進(jìn)細(xì)胞裂解,釋放EPS及胞內(nèi)外有機(jī)物釋放到液相中為后續(xù)發(fā)酵產(chǎn)酸菌提供更多底物[50-52].

圖7 UV輔助FNA促進(jìn)污泥厭氧發(fā)酵效能機(jī)理

3HNO2?HNO3+ 2×NO+ H2O (1)

2HNO2?×NO +×NO2+ H2O (2)

2NO2→ N2O4(3)

N2O4+ H2O → HNO3+ HNO2(4)

NO3-×NO +×O2-(< 280nm) (5)

NO3-×NO2+×OH (< 280nm) (6)

ONOOH →×NO2+×OH → ONOO-(8)

ONOO-+×OH →H++×NO+×O2-(9)

×O2-+ H2O ?×OH + OH-(10)

表3 發(fā)酵10d不同發(fā)酵體系碳平衡分析

注: COD轉(zhuǎn)換因子分別為:1.42g COD/g VSS,8g COD/g H2,4g COD/g CH4,1.5g COD/g 溶解性蛋白,1.06g COD/g 溶解性碳水化合物,1.07g COD/g 乙酸,1.51g COD/g 丙酸,1.82g COD/g丁酸,2.04g COD/g戊酸[53].

眾所周知,VSS、H2、CH4、溶解性碳水化合物、溶解性蛋白和SCFAs是發(fā)酵系統(tǒng)中典型的中間產(chǎn)物或最終產(chǎn)物.為進(jìn)一步剖析聯(lián)合預(yù)處理對(duì)剩余污泥厭氧發(fā)酵產(chǎn)酸的影響機(jī)制,對(duì)不同體系進(jìn)行了碳平衡分析.VSS的變化可直觀反映污泥中有機(jī)質(zhì)的降解及其減量化的程度.由表3可知,各預(yù)處理均不同程度實(shí)現(xiàn)了污泥的減量,相比空白組(21.94%),FNA組VSS下降至20.92%,進(jìn)一步在FNA-UV、FNA-H和FNA-US組中降至19.48%,19.25%和20.23%.與VSS變化趨勢(shì)相反,溶解性碳水化合物和蛋白質(zhì)含量相比空白組均有不同程度的提升,并在FNA-UV組中占比達(dá)到最大,分別為3.07%和24.03%,表明紫外耦合FNA預(yù)處理最大程度地強(qiáng)化了溶解性有機(jī)物的釋放.同時(shí),各組中溶解性蛋白質(zhì)占比均高于溶解性碳水化合物,表明蛋白質(zhì)類物質(zhì)相較于碳水化合物更難被微生物所降解,該結(jié)果與Arshad等[54]的研究結(jié)果相一致[54].相應(yīng)地,SCFAs在各預(yù)處理組中的占比均高于空白組,且在整個(gè)體系碳分布中占比高達(dá)28.52%～38.27%,且在FNA-UV組中達(dá)到最高,證實(shí)了紫外光的引入強(qiáng)化了溶解性有機(jī)物的溶出,并在后續(xù)產(chǎn)酸過(guò)程中強(qiáng)化了其向SCFAs的大量轉(zhuǎn)化.

2.5 討論及展望

剩余污泥(WAS)含有豐富的有機(jī)化合物,為能源回收和SCFAs生產(chǎn)帶來(lái)巨大潛力.紫外光和FNA預(yù)處理均可破壞污泥絮狀物和細(xì)胞結(jié)構(gòu),促進(jìn)胞內(nèi)外有機(jī)物釋放[55].研究表明紫外光通過(guò)對(duì)微生物的輻射損傷和破壞DNA中各種結(jié)構(gòu)鍵致使微生物破裂,同時(shí)可改變腐殖酸的結(jié)構(gòu)特性,使得大分子腐殖酸脫穩(wěn)并分解為小分子.相比紫外照射驅(qū)動(dòng)的光催化氧化技術(shù),FNA預(yù)處理污泥通過(guò)FNA及其衍生物(如×NO、×NO2和N2O3)等毒性作用,導(dǎo)致細(xì)胞破裂、EPS剝離.FNA與紫外照射聯(lián)合技術(shù)強(qiáng)化并產(chǎn)生活性自由基及中間體(×NO、×NO2、ONOO-、ONOOH),同步強(qiáng)化EPS剝離及污泥細(xì)胞破裂.與類似的化學(xué)預(yù)處理、物理預(yù)處理及聯(lián)合預(yù)處理相比,紫外光耦合FNA預(yù)處理法對(duì)WAS瓦解和發(fā)酵產(chǎn)酸效果具有優(yōu)越性.如Luo等[57]發(fā)現(xiàn),Ca(OCl)2用量為0.025g/g TSS,SCFAs最大產(chǎn)量為192.8mg COD/g VSS,Zheng等[52]采用紫外照射驅(qū)動(dòng)的光催化氧化技術(shù)進(jìn)行厭氧消化(254nm 1h),獲得SCFAs產(chǎn)量約150mgCOD/ gVSS,Wu等[29]采用冷凍-FNA預(yù)處理(1.07mg N/L FNA,-5℃ 48h),SCFAs產(chǎn)量124.0mg COD/g VSS,低于本研究中使用紫外耦合FNA預(yù)處理獲得的SCFAs最大濃度(201.8±4.8) mg COD/g VSS[29,55,57].據(jù)報(bào)道,FNA可從污泥厭氧發(fā)酵液中原位合成,且采用FNA預(yù)處理在發(fā)酵過(guò)程中FNA會(huì)反硝化為氮?dú)?無(wú)二次污染的風(fēng)險(xiǎn),處理成本約3.60元/m[2].本研究證實(shí)了紫外光耦合FNA預(yù)處理污泥產(chǎn)酸的可行性,然而目前紫外光耦合FNA預(yù)處理技術(shù)的耦合條件及機(jī)理尚不明晰,之后將對(duì)耦合方式、照射時(shí)間、FNA濃度和紫外線照射強(qiáng)度,以及處理時(shí)間等進(jìn)一步優(yōu)化.

3 結(jié)論

3.1 紫外光耦合FNA預(yù)處理可有效促進(jìn)污泥溶胞.其DOM中溶解性蛋白及碳水化合物的含量高達(dá)(202.9±6.8)和(121.1±4.3) mg COD/L,為其他實(shí)驗(yàn)組的1.3～3.1和1.4～3.4倍.

3.2 紫外光耦合FNA預(yù)處理有效提升了污泥厭氧發(fā)酵過(guò)程中SCFAs的產(chǎn)量,在第4d達(dá)(201.8±4.8) mg COD/g VSS,相比其他組提升16%～192%.乙酸和丙酸占比高達(dá)64%,均高于其他各組.

3.3 紫外光耦合FNA預(yù)處理強(qiáng)化了污泥中發(fā)酵菌和反硝化菌的生長(zhǎng)和富集,其豐度分別為31.3%和8.2%,為其他各實(shí)驗(yàn)組的1.2～4.4倍和1.3～2.3倍.

3.4 紫外光耦合FNA預(yù)處理有效強(qiáng)化了×OH和×O2-的產(chǎn)生,與活性自由基及中間體(×NO、×NO2、ONOO-、ONOOH)共同促進(jìn)了污泥的溶胞和胞外聚合物的破解,發(fā)酵末期有效實(shí)現(xiàn)了污泥減量(VSS占比19.48%)和溶解性有機(jī)物的釋放與利用.

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Enhancement of acidification from waste activated sludge via anaerobic fermentation by free nitrous acid (FNA) pretreatment assisted by ultraviolet.

YIN Xiao-yun,LIU Zhi-hong*,ZHOU Ai-juan,LI Ya-nan,YUE Xiu-ping,CUI Zhi-xuan

(College of Environmental Science and Engineering,Taiyuan University of Technology,Taiyuan 030024,China).,2022,42(8)：3770～3779

In order to break the bottleneck of the limited hydrolysis performance and low short chain fatty acids (SCFAs) production efficiency from waste activated sludge (WAS) during the traditional anaerobic fermentation,this work investigated the effect of the ultraviolet (UV) assisted free nitrous acid (FNA) pretreatment on WAS disintegration and acidification,and compared with thermal (H) and ultrasonic (US) coupled with FNA pretreatment. Results revealed that UV assisted FNA co-pretreatment (FNA-UV) had a synergistic effect on disruption of both extracellular polymeric substances and cell envelope.?OH and?O2-,as the main reaction intermediates,their intensities in FNA-UV group were much stronger than that obtained in other pretreatment groups (FNA-US and FNA-H). Moreover,these two free radicals,with the intermediates such as ?NO,?NO2andONOO-,further promoting the release of soluble organics.The contents of soluble carbohydrates and protein were 60% and 90% higher than that obtained in FNA group respectively,which served more soluble substrates for SCFAs generation. Accordingly,SCFAs concentration peaked at 4d in FNA-UV group (201.8±4.8)mg COD/g VSS with 56.8% acetic acid (HAc)),which was 67% higher than that of FNA group. Carbon balance analysis at the final stage of the fermentation showed that UV assisted FNA pretreatment played an important role in sludge reduction,release and transformation of soluble substrates,and finally SCFAs production. The functional microbial consortia analysis indicated the anaerobic fermentation bacteria and nitrate-reducing bacteria were obviously enriched in FNA-UV group,which were 23.7%～270.6% higher than that obtained in other groups.

free nitrous acid (FNA)；waste activated sludge (WAS)；short chain fatty acids (SCFAs)；anaerobic fermentation；physical method；high-throughput sequencing

X703.1

A

1000-6923(2022)08-3770-10

2021-12-28

國(guó)家自然科學(xué)基金資助項(xiàng)目(52100155,52070139);山西省基礎(chǔ)研究計(jì)劃項(xiàng)目(20210302124347);山西省重點(diǎn)研發(fā)項(xiàng)目(社發(fā)領(lǐng)域) (201903D321055)

* 責(zé)任作者,講師,liuzhihong0227@163.com

殷霄云(1998-),女,河南漯河人,太原理工大學(xué)碩士研究生,主要從事廢棄生物質(zhì)資源化.