劉芝宏,魏瑤麗,樊雅欣,段燕青,周愛娟,2,岳秀萍*

游離亞硝酸預(yù)處理對剩余污泥電解及微生物群落結(jié)構(gòu)的影響

劉芝宏1,魏瑤麗1,樊雅欣1,段燕青1,周愛娟1,2,岳秀萍1*

(1.太原理工大學(xué)環(huán)境科學(xué)與工程學(xué)院,山西 太原 030024；2.香港大學(xué)土木工程學(xué)院,環(huán)境工程研究中心,香港 999077)

為打破傳統(tǒng)厭氧發(fā)酵周期長,有機(jī)質(zhì)利用率低等瓶頸,增強污泥的資源利用和能源回收,探討了游離亞硝酸(FNA)預(yù)處理對剩余污泥電解效果及微生物群落的影響.對比分析了FNA預(yù)處理前后剩余污泥在微生物電解池(MEC)中的電流和氫氣產(chǎn)生、溶解性有機(jī)物和揮發(fā)酸的釋放和利用及功能菌群的變化情況.結(jié)果表明,FNA預(yù)處理能有效地促進(jìn)剩余污泥在MEC系統(tǒng)中的水解和酸化,其溶解性糖類、蛋白和揮發(fā)酸的含量遠(yuǎn)高于未預(yù)處理組,進(jìn)而促進(jìn)了水解發(fā)酵菌、產(chǎn)電菌及反硝化菌的生長和富集,最終揮發(fā)酸利用率均在97%以上,表現(xiàn)為電流(1.9mA)和氫氣(0.86mL/g VSS)的增強,分別是空白組的3.8倍和5.1倍.

游離亞硝酸預(yù)處理；剩余污泥；微生物電解池；揮發(fā)性脂肪酸；氫氣；群落結(jié)構(gòu)

剩余污泥作為污水廠生物處理段的伴生產(chǎn)物,其產(chǎn)量隨著生產(chǎn)規(guī)模的擴(kuò)大而逐年增加.由于其中含有大量有機(jī)物?重金屬及病原體,若得不到合理的處置會導(dǎo)致二次污染.厭氧消化(AD)作為有效處理剩余污泥的技術(shù),可通過水解酸化過程將污泥中的有機(jī)物轉(zhuǎn)化為揮發(fā)酸等小分子有機(jī)物,可作為水廠外加碳源或用于產(chǎn)甲烷.但由于微生物細(xì)胞的半剛性結(jié)構(gòu)及污泥胞外聚合物(EPS)的包裹,導(dǎo)致厭氧消化水解和酸化過程受限.因此,預(yù)處理污泥非常必要.游離亞硝酸(FNA)是一種很強的滅菌劑[1],其及其衍生產(chǎn)物(NO, N2O3和NO2等)能破壞脂質(zhì)、蛋白質(zhì)、糖類和脫氧核糖核酸(DNA)[2].金寶丹等[3]和委燕等[4]發(fā)現(xiàn),FNA可有效殺死微生物細(xì)胞(41%~80%),增強污泥溶胞.徐雪芹等[5]將FNA預(yù)處理后污泥與煙草廢物共消化,發(fā)現(xiàn)其強化了有機(jī)物水解酸化進(jìn)程并為產(chǎn)甲烷菌提供基質(zhì).這些研究均證實了FNA用于污泥預(yù)處理強化污泥水解產(chǎn)酸的可行性.

然而,傳統(tǒng)的厭氧消化存在發(fā)酵時間長,有機(jī)物利用率低,能源回收率低等問題[6].微生物電解池(MEC)通過陽極富集的產(chǎn)電微生物(如等)氧化有機(jī)物(乙酸、葡萄糖、蛋白質(zhì)和甘油等),在外接電壓(0.2~0.8V)條件下產(chǎn)生電子和質(zhì)子,分別經(jīng)外電路和溶液內(nèi)部進(jìn)行傳輸,在陰極結(jié)合產(chǎn)生氫氣[7].其庫倫效率和能源回收效率(氫氣)可高達(dá)90%和200%以上[8-9].目前MEC普遍使用的緩沖溶液為磷酸鹽,引入的過量磷酸鹽易導(dǎo)致二次污染.研究表明,使用NaCl溶液作為電解液,仍可得到很好的產(chǎn)電產(chǎn)氫效能[10].

本研究將FNA預(yù)處理技術(shù)用于MEC產(chǎn)氫系統(tǒng)中,通過促進(jìn)污泥中有機(jī)物在電化學(xué)系統(tǒng)中的水解酸化進(jìn)程為產(chǎn)電菌提供基質(zhì),并以NaCl溶液為電解質(zhì),研究FNA預(yù)處理對MEC電解剩余污泥的影響,包括氫氣產(chǎn)量,有機(jī)物釋放及利用效果,采用高通量測序分析陽極功能菌群,旨在為剩余污泥資源化提供參考.

1 材料與方法

1.1 剩余污泥來源及性質(zhì)

剩余污泥取自山西省晉中市第一污水廠濃縮池,經(jīng)4℃條件下靜置濃縮24h后棄去上清液,使用40目篩網(wǎng)過濾污泥中的雜質(zhì)顆粒,置于4℃條件下備用.濃縮后污泥經(jīng)10000r/min 離心10min,上清液經(jīng)0.45μm的濾膜過濾,分析測定相關(guān)指標(biāo).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 FNA預(yù)處理實驗

量取900mL剩余污泥進(jìn)行FNA預(yù)處理:加入亞硝酸鈉儲備液,使污泥中NO2--N濃度為300mgN/L,通過投加1mol/L的鹽酸溶液調(diào)節(jié)pH值為(5.5±0.1),并經(jīng)公式FNA=NO2--N/(a×10pH)和a=e-2300/(273+T)計算得出污泥中FNA的濃度為2.13mg N/L.將處理后的污泥均勻分配到3個容積為500mL的厭氧反應(yīng)器中,通入高純氮氣10~15min,以去除反應(yīng)器中的氧氣.將反應(yīng)器置于恒溫震蕩培養(yǎng)箱中,在(22±1)℃下連續(xù)運行12h后,即得預(yù)處理后污泥.

1.3 微生物電解池(MECs)啟動及運行

同步啟動10個MECs反應(yīng)器.反應(yīng)器容積為28mL,陽極和陰極分別為碳刷和涂有Pt-C催化劑的碳布.每個MEC反應(yīng)器外加0.8V電壓,并接10 Ω電阻以計算電流,采用數(shù)據(jù)記錄儀(Keithley 2700)實時監(jiān)控.啟動前3個周期按照與營養(yǎng)液(1500mg/L乙酸鈉, 3.6g/L的NaCl溶液,維生素和礦物質(zhì)溶液)1:9的體積比接種新鮮的剩余污泥,用1.0mol/L的鹽酸和氫氧化鈉溶液調(diào)節(jié)混合液的pH值為(7.0±0.1);之后停止接種污泥,將乙酸鈉濃度降為1000mg/L,每個周期運行1d,運行10個周期.當(dāng)反應(yīng)器電流和庫倫效率分別達(dá)2mA和90%以上時,即認(rèn)為反應(yīng)器啟動成功.

在成功啟動的MECs中挑選6個運行良好且性能相似的反應(yīng)器進(jìn)行電解實驗.分別以原污泥(空白組)和FNA預(yù)處理污泥(FNA預(yù)處理組)與NaCl緩沖溶液以1:1體積比混合后作為MECs反應(yīng)器基質(zhì)進(jìn)行電解,每組設(shè)置3個平行.采用數(shù)據(jù)記錄儀實時監(jiān)控電壓,運行周期為3d,探究MECs中有機(jī)物變化及產(chǎn)氫性能.

1.4 分析測試方法

TSS、VSS、TCOD、SCOD、PO43--P和NH4+-N、TN和含水率濃度均采用國家標(biāo)準(zhǔn)法測定[11],TP采用SMT分級提取法測定.溶解性蛋白和糖類分別采用BCA試劑盒和苯酚-硫酸法.氣體(H2、CH4和CO2)和TVFAs采用氣相色譜儀測定,檢測器分別為熱導(dǎo)檢測器(TCD)和氫火焰離子化檢測器(FID),以高純氬氣作為載氣.電壓和電流采用數(shù)據(jù)記錄儀(Keithley 2700),每10min記錄一次,庫倫效率(E)和氫氣產(chǎn)率[H2,m3-H2/(m3×d)]分別采用公式(1)和(2)計算.

式中:為法拉第常數(shù),96485C/mol;為1mol底物完全氧化的失電子數(shù);為底物消耗的質(zhì)量,g;為底物消耗的物質(zhì)的量,g/mol.

式中:H2為產(chǎn)氫體積,L;為大氣壓,bar;H2為氫氣的分子量,2g/mol;為理想氣體常數(shù),0.08314L bar/K mol;為熱力學(xué)溫度;ΔCOD為一定時間內(nèi)初始和最終點COD的濃度之差,mg/L;H2為底物的分子量,g/mol.

將MEC反應(yīng)器運行3d后的陽極污泥取樣,進(jìn)行微生物群落分析.污泥樣品經(jīng)DNA提取后,進(jìn)行PCR擴(kuò)增.引物選取Miseq測序平臺V3-V4區(qū)域的通用引物341F和805R.之后采用Illumina測序平臺將不同污泥樣品進(jìn)行高通量測序(上海生工).

為便于比較分析,上述測得的有機(jī)物濃度(mg/L)均換算為COD濃度(mg COD/L),其轉(zhuǎn)化系數(shù)分別為:1.50g COD/g蛋白,1.06g COD/g糖類,1.07g COD/g乙酸,1.51g COD/g丙酸,1.82g COD/g (正/異)丁酸和2.04g COD/g (正/異)戊酸.

2 結(jié)果與討論

2.1 MECs電流及產(chǎn)氫

電流和氫氣的產(chǎn)生可反映出MECs的啟動和運行程度[12].MECs在啟動10個周期后電流高達(dá)4.5mA,庫倫效率和氫氣產(chǎn)率分別為99.8%和105.1%,說明反應(yīng)器已啟動成功.

由圖1可以看到,電流均呈現(xiàn)先下降后緩慢上升的趨勢,第3d電流達(dá)峰值后趨于平穩(wěn).FNA預(yù)處理組的峰值電流高達(dá)1.9mA,是空白組的3.2倍.其原因可能是FNA預(yù)處理有效地促進(jìn)了污泥中溶解性有機(jī)物的釋放,進(jìn)而為陽極產(chǎn)電微生物提供了充足的底物.同時, FNA預(yù)處理組的氫氣產(chǎn)量為0.86mL/g VSS,是空白組的5.1倍.

2.2 有機(jī)物的釋放及利用

2.2.1 溶解性有機(jī)物的變化情況 蛋白質(zhì)是微生物細(xì)胞的主要組成部分,占剩余污泥TCOD的35%~61%,碳水化合物占7%~11%,而油脂和其他化合物占比小于1%[13].因此,剩余污泥的水解效果可通過溶解性糖類和蛋白質(zhì)的釋放來證明.圖2展示了2組剩余污泥中溶解性有機(jī)物的變化情況.經(jīng)FNA預(yù)處理12h后,溶解性糖類和蛋白質(zhì)含量高達(dá)120.9,592.2mg COD/L,相比原污泥分別增加了10倍和1.3倍.說明FNA及其衍生物促進(jìn)了微生物細(xì)胞壁和胞外聚合物(EPS)的破裂,進(jìn)而導(dǎo)致胞內(nèi)物質(zhì)的釋放.據(jù)報道,FNA預(yù)處理會損害細(xì)胞和EPS的脂質(zhì)、蛋白質(zhì)、碳水化合物及脫氧核糖酸等[14],且對細(xì)胞膜的破裂極其有效[15].在MEC運行過程中, FNA預(yù)處理組溶解性糖類在48h達(dá)到峰值259.8mg COD/L,是空白的4.8倍;溶解性蛋白濃度在36h達(dá)最高值1385.1mg COD/L,是空白的1.7倍.上述結(jié)果表明FNA預(yù)處理促進(jìn)了后續(xù)溶解性有機(jī)物的水解.而由于未預(yù)處理污泥細(xì)胞壁的半剛性結(jié)構(gòu)阻礙了有機(jī)物的釋放和溶出,因而其在MEC系統(tǒng)中的釋放和水解效果遠(yuǎn)不如FNA預(yù)處理后污泥的處理情況.

最終,溶解性糖類在兩組中均呈現(xiàn)不同程度的利用.溶解性糖類在空白組和FNA預(yù)處理組的利用率分別為16.7%和22.2%.同時,溶解性蛋白在FNA預(yù)處理組中利用率高達(dá)7%,打破了傳統(tǒng)厭氧發(fā)酵不能利用蛋白質(zhì)的瓶頸.而其在空白組中仍呈現(xiàn)緩慢的上升,說明蛋白在該系統(tǒng)中仍進(jìn)行緩慢的水解過程.溶解性有機(jī)物含量下降可能是由于其在產(chǎn)酸發(fā)酵菌的作用下轉(zhuǎn)化為揮發(fā)酸,以便更好地為產(chǎn)電菌提供理想的基質(zhì).

2.2.2 揮發(fā)酸產(chǎn)生及利用情況 MECs可利用多種物質(zhì)作為底物,如乙酸、乙醇等小分子物質(zhì),蛋白質(zhì)、碳水化合物等大分子物質(zhì),餐廚廢水、生活污水等有機(jī)廢水[16].揮發(fā)酸隨時間的變化情況如圖3所示,揮發(fā)酸濃度隨時間呈現(xiàn)先增加后快速減小的趨勢.揮發(fā)酸總產(chǎn)量在兩系統(tǒng)中均在48h達(dá)到峰值, FNA預(yù)處理組中產(chǎn)量高達(dá)697.1mg COD/L (520.9mg/L),是空白組(140.8mg/L即181.3mg COD/L)的3.8倍.揮發(fā)酸由溶解性有機(jī)物轉(zhuǎn)化而來,因此,FNA預(yù)處理可強化剩余污泥在MEC中的酸化過程,進(jìn)而為產(chǎn)電菌提供底物. MECs運行48h時兩組中揮發(fā)酸的組成成分情況為:乙酸占比在空白組和FNA預(yù)處理組中占比分別為55.8%和46.8%,而乙酸和丙酸的占比分別高達(dá)75.3%和69.4%,恰恰為產(chǎn)電菌提供了最理想的基質(zhì)產(chǎn)電和產(chǎn)氫,因此,VFAs最終得到了有效利用,其在FNA預(yù)處理組和空白組中的利用率分別為98.4%和97.5%.揮發(fā)酸的利用進(jìn)一步表現(xiàn)為氫氣和電流的產(chǎn)生,因此, FNA預(yù)處理組產(chǎn)電和產(chǎn)氫效能優(yōu)于空白組,與圖1結(jié)論一致.

2.3 氨氮的釋放效果

圖4 MECs中NH4+-N釋放情況

厭氧發(fā)酵過程中,細(xì)胞的死亡會導(dǎo)致有機(jī)物和有機(jī)氮進(jìn)一步釋放到上清液[17],而氨氮作為蛋白質(zhì)的副產(chǎn)物,會隨著進(jìn)一步的水解酸化而釋放,氨氮的釋放率可由生物量組成式CH1.93O0.53N0.2來得出[18],因此,氨氮的釋放可側(cè)面反映細(xì)胞的死亡情況及有機(jī)物的水解效果.圖4反映了不同處理系統(tǒng)中氨氮隨時間的變化情況.兩實驗組中氨氮濃度整體上均呈現(xiàn)持續(xù)上升的趨勢. FNA預(yù)處理組氨氮濃度在電解8h后高達(dá)330.0mg/L,是空白組的2.9倍,說明經(jīng)FNA預(yù)處理對污泥中微生物的滅活作用明顯;空白組和FNA預(yù)處理組的氨氮濃度均在MEC運行72h達(dá)最大值,且FNA預(yù)處理組氨氮濃度高達(dá)560.5mg/L,是空白組濃度的1.8倍,該結(jié)果與圖2b中蛋白質(zhì)變化結(jié)果一致,說明溶解性蛋白的逐漸釋放.

2.4 微生物群落解析

MECs反應(yīng)器中有機(jī)物的釋放和利用及產(chǎn)電產(chǎn)氫效果與陽極微生物的種類及豐度有密不可分的關(guān)系.圖5反映了不同反應(yīng)器中陽極功能菌群分別在門,綱和屬水平的相對豐度.可以看出,兩樣品的微生物主要分布在3個門:變形菌門(Proteobacteria)?擬桿菌門(Bacteroidetes)和厚壁菌門(Firmicutes),均為目前已報道的最常見的發(fā)酵菌門[19-20]和產(chǎn)電菌門[21-22].從綱水平來看,擬桿菌綱(Bacteroidia)和鞘脂桿菌綱(Sphingobacteriia)同屬于擬桿菌門(Bacteroidetes),其中擬桿菌綱是典型的發(fā)酵菌,主要參與污泥中固體成分的分解和有機(jī)酸的積累[23],其在FNA預(yù)處理組中豐度高達(dá)19.5%,高于空白組(16.0%),鞘脂桿菌綱是降解纖維素的重要菌群[24].α-變形菌綱(Alphaproteobacteria)?β-變形菌綱(Betaproteobacteria)、γ-變形菌綱(Gammaproteobacteria), δ-變形菌綱(Deltaproteobacteria)和ε-變形菌綱(Epsilonproteobacteria)都屬于變形菌門.相比空白組, FNA預(yù)處理組中δ-變形菌綱由空白組的7.1%顯著增加到18.8%,是絕大多數(shù)產(chǎn)電菌(如菌屬)所屬的菌綱[25].說明FNA預(yù)處理極大地促進(jìn)了產(chǎn)電菌在MECs中的生長與富集.此外,同屬于厚壁菌門(Firmicutes)的梭狀芽孢桿菌綱(Clostridia)和桿菌綱(Bacilli),能夠釋放水解酶,并利用有機(jī)物產(chǎn)酸[26].

根據(jù)屬水平上的群落分布,微生物被分成3類:發(fā)酵產(chǎn)酸菌、產(chǎn)電菌和反硝化菌.總體來講,發(fā)酵產(chǎn)酸菌在FNA預(yù)處理組樣品中相對豐度高達(dá)21.3%,而空白組其含量僅為15.6%.其中,為典型的發(fā)酵菌,可利用碳水化合物產(chǎn)生乳酸鹽?乙酸和丁酸,在FNA預(yù)處理組中含量為7.2%,是空白組的7倍;同時,可代謝多種碳水化合物生成有機(jī)酸,并有生成H2和CO2的能力,其在FNA預(yù)處理組中含量高達(dá)2.7%,高于空白組的1.2%.因此,可得出結(jié)論:FNA預(yù)處理通過破壞EPS和細(xì)胞膜導(dǎo)致有機(jī)物外泄,進(jìn)而強化了污泥中發(fā)酵產(chǎn)酸菌的富集,該結(jié)論證實了圖3中揮發(fā)酸的富集過程.,,和已被證實是生物電化學(xué)系統(tǒng)中典型的產(chǎn)電菌,通過優(yōu)先消耗有機(jī)物酸化過程產(chǎn)生的VFAs,將e-和H+轉(zhuǎn)化為H2[27].這4種產(chǎn)電菌在FNA預(yù)處理組和空白組中的累積豐度分別為6.8%和4.7%,這也是FNA預(yù)處理組中的電流和產(chǎn)氫高于空白組的主要原因.此外,3種典型的反硝化菌:,和在FNA預(yù)處理組中的累積豐度高達(dá)9.5%,而其在空白組中僅占4.2%,且在MECs反應(yīng)器運行末期未檢測到NO2--N和NO3--N,說明FNA預(yù)處理通過富集反硝化菌,強化了反硝化過程.因此,整個預(yù)處理環(huán)保高效,無二次污染風(fēng)險.

3 結(jié)論

3.1 FNA預(yù)處理強化了剩余污泥在MECs系統(tǒng)中的產(chǎn)電和產(chǎn)氫,其值分別為1.9mA和0.86mL/g VSS,分別是空白組的3.2倍和5.1倍.

3.2 FNA預(yù)處理強化了剩余污泥中有機(jī)物在微生物電化學(xué)系統(tǒng)中的釋放和利用.溶解性糖和蛋白在FNA預(yù)處理組中高達(dá)259.8,1385.1mg COD/L,分別是空白組的4.8和1.7倍.由糖和蛋白進(jìn)一步酸化產(chǎn)生的揮發(fā)酸含量在FNA預(yù)處理組中高達(dá)697.1mg COD/L,是空白組的3.8倍,且其最終的利用率均在97%以上.

3.3 FNA預(yù)處理強化了陽極水解發(fā)酵菌,產(chǎn)電菌和反硝化菌的生長和富集,其相對豐度分別高達(dá)21.3%,6.8%和9.5%,均高于其在空白組的含量.

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Role of free nitrous acid on waste activated sludge bio-electrolysis and key microflora shift.

LIU Zhi-hong1, WEI Yao-li1, FAN Ya-xin1, DUAN Yan-qing1, ZHOU Ai-juan1,2, YUE Xiu-ping1*

(1.College of Environmental Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China；2.Environmental Engineering Research Centre, Department of Civil Engineering, The University of Hong Kong, Hong Kong 999077, China)., 2019,39(7)：2953~2959

It is a major bottleneck as efficient energy recovery from waste activated sludge (WAS) often require long treatment time during traditional anaerobic fermentation. In order to further enhance the resource utilization efficiency and shorten the treatment time, bio-eletrolysis, i.e., microbial electrolysis cells (MECs), assisted with free nitrous acid (FNA) was employed for WAS treatment in this study. The performance of current and hydrogen generation during bio-eletrolysis from FNA-treated WAS was compared with that obtained from un-pretreated sludge. FNA significantly boosted the hydrolysis and acidification of WAS in MECs, in detail, the concentrations of soluble carbohydrates, proteins and volatile fatty acids (VFAs) were much higher than that of un-pretreated sludge. The utilization efficiency of VFAs was higher than 97% in the MEC-FNA test with the increase of current (1.9mA) and hydrogen yield (0.86mL/g VSS), which were 3.8 and 5.1 folds higher than that in the control. What’s more, pyrosequencing revealed that the abundance of anaerobic fermentation bacteria, electrochemically active bacteria and nitrate-reducing bacteria were notably enhanced.

free nitrous acid；waste activated sludge；microbial electrolysis cells；volatile fatty acids；hydrogen；key microflora

X703.5

A

1000-6923(2019)07-2953-07

劉芝宏(1993-),女,山西忻州人,太原理工大學(xué)環(huán)境科學(xué)與工程學(xué)院博士研究生,主要從事剩余污泥資源化研究.發(fā)表論文2篇.

2018-11-26

國家自然科學(xué)基金青年基金資助項目(51608345);國家博士后科學(xué)基金資助項目(2017T100170)

* 責(zé)任作者, 教授, yuexiuping@tyut.edu.cn