剩余污泥與餐廚垃圾協(xié)同厭氧消化研究進展

張星星,焦彭博,楊匯瑩,吳睿敏,李詠梅,馬黎萍*

剩余污泥與餐廚垃圾協(xié)同厭氧消化研究進展

張星星1,焦彭博1,楊匯瑩1,吳睿敏1,李詠梅2,馬黎萍1*

(1.華東師范大學生態(tài)與環(huán)境科學學院,上海 200241；2.同濟大學環(huán)境科學與工程學院,上海 200092)

為了推進污水廠剩余污泥與餐廚垃圾協(xié)同厭氧消化在工程規(guī)模中的應用,提高其能源回收率,系統(tǒng)分析了協(xié)同厭氧消化機制、產(chǎn)物類型及其主要的影響因子,綜述了協(xié)同厭氧消化中直接種間電子傳遞作用的重要研究進展,并展望了協(xié)同厭氧消化的未來研究方向,包括開發(fā)高效經(jīng)濟的原料預處理方式,表征基質(zhì)降解特性,基于多組學聯(lián)用技術理解微生物代謝調(diào)控,緩解消化體系中潛在抑制劑影響,原位耦聯(lián)其他種類廢棄物進一步提升消化性能和穩(wěn)定性,以期為城鎮(zhèn)有機固體廢棄物的高效能源回收提供指導.

剩余污泥；餐廚垃圾；協(xié)同厭氧消化；影響因子；直接種間電子傳遞；多種廢棄物協(xié)同消化

剩余污泥(ES)和餐廚垃圾(FW)是城市有機固體廢棄物(OFMSW)的重要組成.截止2019年底,我國ES產(chǎn)生量已超5000萬t/a,2025年前預計將增至6500萬t/a[1].而我國FW每年增長6.16%,到2026年預計FW產(chǎn)生量將增至1.8億t/a.如何安全、經(jīng)濟、環(huán)保的處置數(shù)量龐大的固體廢棄物是我國面臨的重要挑戰(zhàn).

厭氧消化(AD)技術能夠?qū)崿F(xiàn)有機固廢的減量化、穩(wěn)定化和資源化,并將其轉(zhuǎn)化為甲烷(CH4)、氫氣(H2)和有機酸等能源物質(zhì),同時沼渣可用作肥料或土壤調(diào)理劑[2].然而,有機固廢中ES具有的難降解、低碳氮比(5～10)和低營養(yǎng)元素的特性, 導致其消化速率緩慢、有機物降解不徹底以及CH4產(chǎn)率低[3-4].FW厭氧消化過程中雖有機物能夠快速水解,相比污泥消化過程CH4得率明顯提高,但短鏈脂肪酸的大量積累易引起消化系統(tǒng)酸化,導致消化不穩(wěn)定甚至體系崩潰[5].

近年來,ES和FW協(xié)同厭氧消化(AcoD)技術能夠更加穩(wěn)定高效的實現(xiàn)廢棄物減量化和資源化,具備稀釋廢物有毒物質(zhì)、提升高值消化產(chǎn)物產(chǎn)量、改善營養(yǎng)元素均衡性和增強微生物協(xié)同效應等突出優(yōu)勢,因此被認為是改良厭氧消化的前景性技術[6-8].

盡管AcoD體系增強了消化效率和穩(wěn)定性,但仍無法突破產(chǎn)酸的熱力學屏障,導致其消化效率無法進一步提升[9].最近報道了厭氧消化過程存在直接種間電子傳遞(DIET)的突破性發(fā)現(xiàn),即產(chǎn)甲烷古菌通過直接的電子傳遞接受來自有機物氧化菌釋放的電子,將二氧化碳(CO2)直接轉(zhuǎn)化為CH4,繼而促進電子流向產(chǎn)CH4過程同時加速底物高效定向轉(zhuǎn)化[10-11].

當前國內(nèi)外對AcoD工藝的關注和研究較多,在此基礎上較為全面的總結(jié)提煉對學界推進和改良AcoD工藝以實現(xiàn)高效定向轉(zhuǎn)化至關重要.鑒于此,本文綜述了基于ES和FW的AcoD最新研究進展,總結(jié)了AcoD機制和產(chǎn)物特性,重點介紹了影響AcoD性能的關鍵因子,分析了AcoD體系中DIET機理和應用,最后對AcoD技術未來研究方向進行展望.

1 協(xié)同厭氧消化機制及產(chǎn)物類型分析

1.1 協(xié)同厭氧消化機制

AcoD過程可分為4個連續(xù)的底物降解階段:水解、酸化、產(chǎn)氫產(chǎn)乙酸和產(chǎn)CH4階段(圖1)[12-14].首先,ES和FW增溶產(chǎn)生的溶解性多糖、蛋白質(zhì)和脂質(zhì)等在水解菌群分泌的水解酶、蛋白酶和脂肪酶作用下,形成單糖、氨基酸、甘油和長鏈脂肪酸(LCFAs)等.即經(jīng)歷第一步水解階段,其中復雜難降解有機物是此時底物轉(zhuǎn)化的限速步驟[15].第二步是酸化階段,水解后的單體發(fā)酵成短鏈脂肪酸(如乙酸、丙酸、丁酸和戊酸等)和醇類.酸化是AcoD反應速率最快的步驟,因此極易導致?lián)]發(fā)性脂肪酸(VFAs)的累積和pH值下降,如果消化體系緩沖能力不足且有機負荷率高將直接導致AcoD系統(tǒng)酸化并且抑制產(chǎn)甲烷菌代謝[16-17].第三步是產(chǎn)氫產(chǎn)乙酸階段,產(chǎn)氫產(chǎn)乙酸菌能夠降解乙醇和VFAs等轉(zhuǎn)化為乙酸、H2和CO2,而耗氫同型產(chǎn)乙酸菌將H2和CO2還原為乙酸.第四步是產(chǎn)CH4階段,CH4主要由乙酸營養(yǎng)型甲烷化和氫營養(yǎng)型甲烷化兩種途徑產(chǎn)生[18].在乙酸營養(yǎng)型途徑中,乙酸營養(yǎng)產(chǎn)甲烷菌將乙酸轉(zhuǎn)化為CH4和CO2.在氫營養(yǎng)型途徑中,氫營養(yǎng)產(chǎn)甲烷菌將H2和CO2轉(zhuǎn)化為CH4.由于地區(qū)飲食習慣的差異導致FW的成分不盡相同,這也造成消化原料中有機大分子種類和含量的差異,同時也影響了產(chǎn)CH4途徑.在富含碳水化合物和脂肪的底物厭氧消化過程中,乙酸營養(yǎng)型是最主要的產(chǎn)CH4途徑,而富含蛋白質(zhì)的底物厭氧消化過程中,氫營養(yǎng)型產(chǎn)甲烷菌占主導[19].最終AcoD會產(chǎn)生含CH4、CO2、H2和硫化氫等氣體的沼氣.

圖1 ES與FW協(xié)同厭氧消化機制及產(chǎn)物類型示意

1.2 協(xié)同厭氧消化產(chǎn)物類型

有機物在AcoD體系中降解轉(zhuǎn)化主要形成有機酸和沼氣等高值產(chǎn)物,沼渣可作為生物肥料實現(xiàn)進一步的資源化利用.另外,從AcoD體系中獲取藍鐵礦回收磷資源也具備較好的應用價值(圖1).

1.2.1 VFAs VFAs是AcoD酸化階段和產(chǎn)氫產(chǎn)乙酸階段的重要中間產(chǎn)物,是一種清潔高值的可再生能源.VFAs可作為生物脫氮的碳源以及生產(chǎn)生物柴油、微生物燃料電池發(fā)電和合成復合聚合物的重要原料[13].VFAs通過產(chǎn)酸菌催化的一系列生化反應合成.首先,產(chǎn)酸菌將水解產(chǎn)物單體轉(zhuǎn)化成乙酸、丙酸、丁酸、醇、H2、CO2和其他產(chǎn)物;其次,丙酸、丁酸、醇類和CO2通過還原質(zhì)子產(chǎn)乙酸途徑或同型產(chǎn)乙酸途徑進一步轉(zhuǎn)化為乙酸[20].通常,乙酸、丙酸和丁酸是產(chǎn)酸發(fā)酵的主要產(chǎn)物[21].產(chǎn)酸代謝途徑一般可分為:(1)乙酸-乙醇型,(2)丙酸型,(3)丁酸型,(4)混合酸型,(5)乳酸型和(6)同型產(chǎn)乙酸型[13].有機物產(chǎn)酸發(fā)酵代謝途徑見圖2.

加快水解速率、強化產(chǎn)酸發(fā)酵和減輕抑制因子影響是提高VFAs總產(chǎn)量的有效策略.由于復雜化合物水解很大程度上限制了AcoD產(chǎn)酸發(fā)酵效率,因此可通過優(yōu)化操作參數(shù)和對底物預處理加快水解代謝.Jiang等[22]調(diào)控消化pH值為6.0時VFAs濃度和產(chǎn)率最高,而Chen等[23]發(fā)現(xiàn)在pH值為9.0時,VFAs產(chǎn)量顯著提高并穩(wěn)定在(25.9±1.5)gCOD/L.酸性和堿性pH值對VFAs產(chǎn)量的影響可能與水解步驟中可溶性蛋白質(zhì)和糖類的產(chǎn)生有關.Chen等[24]在滅菌后的AcoD消化液中接種丙酸桿菌進行二次消化,發(fā)現(xiàn)AcoD協(xié)同發(fā)酵顯著提高了乳酸產(chǎn)量,引入丙酸桿菌則進一步提高了二次消化的高值酸(丙酸)產(chǎn)量.因此如何在提升VFAs總產(chǎn)量的同時,高效、經(jīng)濟、簡便地提高丙酸、丁酸等高值酸在總VFAs中的比例以實現(xiàn)高效定向轉(zhuǎn)化與提取,是未來值得深入探索的研究方向.

1.2.2 沼氣(CH4和H2) AcoD過程產(chǎn)生的CH4和H2是清潔、可再生和高熱值(CH4:50kJ/g,H2:122kJ/g)能源[25].CH4生產(chǎn)途徑主要有氫營養(yǎng)和乙酸營養(yǎng)兩種,而互營乙酸氧化菌(SAOB)在高溫、高游離氨(FA)和高VFAs等條件下可與氫營養(yǎng)產(chǎn)甲烷菌協(xié)同產(chǎn)CH4[26].對于AcoD產(chǎn)CH4,酸化和產(chǎn)乙酸階段會產(chǎn)生抑制產(chǎn)甲烷菌的酸性pH值和高FA.酸性pH值和FA對產(chǎn)甲烷菌具有協(xié)同抑制作用,并對乙酸營養(yǎng)和氫營養(yǎng)產(chǎn)甲烷菌及互營乙酸氧化產(chǎn)生不同程度脅迫,從而改變產(chǎn)CH4代謝途徑[27].Li等[28]為緩解FW消化過程生物乙醇發(fā)酵預處理的酸性抑制,混合ES與FW進行協(xié)同厭氧發(fā)酵.結(jié)果表明,在1:2(FW:ES)混合比下CH4產(chǎn)量達到峰值267mL/gCOD, AcoD體系高效穩(wěn)定互營代謝是由于富集了具有DIET功能的產(chǎn)甲烷菌種和. Chen等[29]通過調(diào)控兩相AD中產(chǎn)酸階段的氧化還原電位、pH值和有機負荷(OLR)改變了產(chǎn)酸發(fā)酵類型,發(fā)現(xiàn)丁酸發(fā)酵具有較高的酸化程度(36%)、CH4產(chǎn)率(535mL/gVS)和生物降解率(95%),因此認為調(diào)控產(chǎn)酸的發(fā)酵類型能夠有效提高CH4產(chǎn)量.

圖2 有機物產(chǎn)酸發(fā)酵代謝途徑[13,30]

AET:乙酸-乙醇型發(fā)酵;ABE:丙酮丁醇乙醇發(fā)酵;PTF:丙酸型發(fā)酵;BTF:丁酸型發(fā)酵;MAF:混合酸型發(fā)酵;LTF:乳酸型發(fā)酵;HMF:同型產(chǎn)乙酸型發(fā)酵;THF:四氫葉酸;[Co-Protein]:corrinoid酶

厭氧消化產(chǎn)氫作用主要發(fā)生在無光生物厭氧發(fā)酵體系中,多糖在黑暗條件下轉(zhuǎn)化為生物氫和乙酸丁酸酯等,見反應式(1)和(2)[31].相比生物光解、光發(fā)酵和微生物電解等產(chǎn)氫方式,暗發(fā)酵可以在無需光照和電能條件下持續(xù)產(chǎn)氫,具備高效率和低能耗等優(yōu)勢.據(jù)報道,AcoD體系沼氣產(chǎn)量和含氫量均顯著高于單一污泥消化過程,而沼氣含氫量比單一FW消化提高了30%,這是由于通過協(xié)同消化改善基質(zhì)特性提供了更多的有機物[32].此外,消化過程產(chǎn)生的FA可能會抑制消化微生物活性,但最近的研究發(fā)現(xiàn)了FA對污泥暗發(fā)酵產(chǎn)氫的積極影響.Wang等[33]研究證實FA的存在能夠顯著改善污泥暗發(fā)酵產(chǎn)氫性能,H2產(chǎn)率隨FA濃度的增加而增加.FA促進胞裂釋放的胞內(nèi)組分為后續(xù)的產(chǎn)氫提供更多底物,FA主要抑制了耗氫代謝而對水解和產(chǎn)乙酸途徑無顯著影響[33].因此,AcoD體系中FW豐富的蛋白質(zhì)水解釋放的FA可能會促進產(chǎn)氫發(fā)酵,但應合理調(diào)配FW和ES的進料比例,避免過高濃度FA對微生物產(chǎn)生毒害抑制作用.

C6H12O6+2H2O→4H2+2CH3COOH+2CO2(1)

C6H12O6+2H2O→2H2+CH3CH2CH2COOH+2CO2(2)

1.2.3 有機肥料 AcoD產(chǎn)生的固體沼渣可作為生物肥料用于土壤修復和肥力提升[34].優(yōu)質(zhì)土壤肥料的典型參數(shù)是碳氮比,因此可通過預處理進料改善沼渣碳氮比以提升沼渣肥料性能[35].Tampio等[36]對比了高壓蒸汽處理和未處理FW的沼渣肥料特性,發(fā)現(xiàn)FW高壓蒸汽預處理(160℃,6.2bar)促進了美拉德化合物的形成,處理后的沼渣土壤改良效果更佳.Grigatti等[37]認為好氧堆肥對沼渣氮和磷的表觀回收率影響較小,而在干式消化過程中控制底物碳氮比能夠有效提升沼渣中氮的表觀回收率.Ma等[38]將真菌預處理后的FW沼渣作為生物肥料,分離出的高濃度有機物沼液進一步與ES進行消化,最終獲得氮和磷含量豐富的生物肥料.然而,OFMSW沼渣可能含重金屬、病原體和蟲卵等潛在土壤污染物質(zhì),因此在作肥料用途時必須滿足生物安全和衛(wèi)生質(zhì)量要求.

1.2.4 藍鐵礦從OFMSW中回收如氮和磷等物質(zhì)能夠緩解資源匱乏和獲得可觀的經(jīng)濟收益.相比傳統(tǒng)的鳥糞石法回收磷,從AcoD系統(tǒng)回收穩(wěn)定的磷-鐵晶體藍鐵礦(Fe3(PO4)2·8H2O)具備明顯的經(jīng)濟價值優(yōu)勢(藍鐵礦10000歐元/t,鳥糞石500歐元/t)[39-41]. pH值是影響藍鐵礦回收磷的關鍵環(huán)境因子[39].藍鐵礦形成的有效pH值范圍是7.0～9.0,同時PO43-和Fe2+從發(fā)酵物中有效分離釋放并結(jié)合形成藍鐵礦沉淀,見反應式(3).然而,隨著pH值的升高PO43-濃度也顯著增加,這可能是由于Fe(OH)2的形成與藍鐵礦競爭Fe2+并促進PO43-的釋放,見反應式(4)[39].Cao等[39]證明pH值介于6.0～9.0,初始PO43->5mg/L和鐵/磷摩爾比為1.5有利于合成藍鐵礦,而在pH值為3.0時投加FeCl3更利于釋放ES中Fe2+和PO43-,同時回收高純度(～93.7%)的藍鐵礦.Wu等[40]發(fā)現(xiàn)相比單獨添加FW或FeCl3至AcoD體系以回收藍鐵礦,聯(lián)合投加FW和FeCl3可生物自調(diào)節(jié)pH值和提高有機物濃度,從而提高Fe3+還原率和酶活性.另外,還發(fā)現(xiàn)污泥停留時間(SRT)與Fe2+和PO43-釋放率密切相關.FW的添加不僅可以提供充分的有機底物、提高微生物活性和VFAs產(chǎn)率,而且可以降低pH值、促進Fe2+和PO43-的釋放[41].因此FW和ES的AcoD能夠有效幫助提升磷回收率.

2PO43-+3Fe2++8H2O→Fe3(PO4)2·8H2O (3)

Fe3(PO4)2·8H2O+6OH-→3Fe(OH)2+2PO43-+8H2O (4)

2 協(xié)同厭氧消化影響因子

2.1 原料配比

原料配比是影響AcoD體系穩(wěn)定性和CH4產(chǎn)量的關鍵因子.不同的原料配比將導致消化體系養(yǎng)分含量、碳氮比和有機物組成差異.ES中主要含氮和微量元素,但可生物降解有機物含量較低,導致生物產(chǎn)CH4潛能不足.相反,FW中的糖類等可生物降解有機物含量豐富且易通過厭氧途徑轉(zhuǎn)化為CH4.因此,提高FW在原料中的配比能夠有效提升AcoD體系CH4產(chǎn)量.Jang等[2]的研究表明,逐步提高FW在原料中的混合比,CH4得率和CH4含量顯著增加, FW:ES(v:v)為75%:25%時AcoD體系產(chǎn)CH4性能最優(yōu).類似地,Cheng等[42]考察了改變FW與ES(v:v)混合比對AcoD體系影響,發(fā)現(xiàn)高FW混合比下AcoD體系產(chǎn)CH4量較高,產(chǎn)CH4性能在FW:ES(v:v)為75%:25%時達到峰值.然而,也有報道稱提高ES的原料配比利于AcoD體系產(chǎn)CH4.Prabhu等[43]發(fā)現(xiàn)相比1:1、1.5:1、2:1和1:1.5(FW:ES,v:v)的原料配比, FW:ES為1:2時沼氣產(chǎn)量最高(823mL/gVS).Pan等[44]對比了FW:ES(v:v)的7種配比對AcoD性能的影響,認為FW:ES比為0.5:0.5時CH4產(chǎn)率最高,同時與單一ES消化相比,滯后期顯著縮短(0.18d),水解速率顯著增加(0.33/d), CH4產(chǎn)率提升了4.59倍((50.3± 10.4)mL/(gVS·d)).這種較優(yōu)原料配比的差異可能與厭氧消化運行模式、消化器類型和容積、原料化學組成及接種微生物/原料比有關.因此,對于不同的AcoD體系,可以通過調(diào)整原料的總固體含量或碳氮比來確定最佳原料配比.

2.2 碳氮比

AcoD體系適宜的進料碳氮比對于維持厭氧消化微生物代謝活性十分重要,因此碳氮比是提高AcoD工藝性能的主要因素.一般,碳氮比在20～30范圍內(nèi)能夠滿足AcoD體系厭氧微生物對養(yǎng)分含量平衡的需求[45].原料的碳氮比主要受ES和FW的化學成分和配比影響,其值低于或高于最佳范圍時可能導致AcoD體系運行不穩(wěn)定和沼氣產(chǎn)量下降.一方面,高碳氮比的聯(lián)合底物可能引起AcoD體系VFAs大量積累、pH值和緩沖能力下降,進而抑制產(chǎn)CH4作用.另一方面,雖然低碳氮比的聯(lián)合底物使得AcoD體系具有較強的緩沖能力,但原料中豐富的蛋白質(zhì)水解釋放高濃度氨可能毒害厭氧消化微生物,有報道稱在原料碳氮比為7.1時厭氧消化丙酸產(chǎn)量極低[13].Azarmanesh等[6]研究發(fā)現(xiàn)在碳氮比為8～19.7范圍內(nèi),沼氣產(chǎn)量與其呈線性相關,并認為碳氮比是影響AcoD體系沼氣生產(chǎn)的關鍵因素.Li等[32]研究了FW和ES的AcoD生產(chǎn)H2和VFAs,發(fā)現(xiàn)聯(lián)合基質(zhì)通過提供適宜的碳氮比(15～23)和pH值(6.1～6.5)有效富集了產(chǎn)酸菌和產(chǎn)氫菌,從而提高了VFAs產(chǎn)量和H2含量.因此,在分析聯(lián)合基質(zhì)化學組分特性基礎上調(diào)整原料配比獲得適宜的進料碳氮比,有利于提升AcoD體系發(fā)酵性能和底物利用率.

2.3 油脂

FW中通常含一定量的油脂((17.5±6.6)%),據(jù)報道,油脂厭氧消化產(chǎn)CH4潛力(1014L/kgVS)遠高于碳水化合物厭氧消化產(chǎn)CH4潛力(如葡萄糖為370L/ kgVS)[46-47].值得注意的是,原料中較高含量的油脂可能會覆蓋在厭氧消化微生物表面影響傳質(zhì)導致細胞功能喪失,同時油脂水解生成的LCFAs會破壞細胞膜并降低細胞通透性,從而影響細胞緩沖能力.另外,油脂粘附生物質(zhì)引起起泡和生物質(zhì)浮選,也會降低AcoD效率.LCFAs能夠抑制產(chǎn)甲烷菌代謝,其中乙酸營養(yǎng)型產(chǎn)甲烷菌對LCFAs最敏感,同時LCFAs也可能會抑制氫營養(yǎng)型和互營型產(chǎn)甲烷菌[48].高溫消化體系中LCFAs引起的抑制比中溫消化體系更加顯著,這可能是由于高溫提升了LCFAs溶解度[49].AcoD體系中FW配比以及OLR較高時會引起LCFAs的積累,當LCFAs濃度在130～ 1000mg/L時會對AcoD過程產(chǎn)生抑制,因此有必要監(jiān)測AcoD體系LCFAs含量防止油脂抑制[50].鑒于LCFAs通過β氧化分解為乙酸和氫氣的過程緩慢復雜,為減輕LCFAs對AcoD體系的潛在抑制,可向消化器中投加陽離子和天然吸附劑(如膨潤土、沸石等)或與含陽離子的廢棄物進行協(xié)同消化[51-53]、構(gòu)建DIET富集互營LCFAs β-氧化菌增強產(chǎn)CH4作用[46]、對FW進行預處理(如微波)[54]及適當增加ES/FW混合比稀釋FW中油脂濃度.

2.4 鹽分

FW和ES中較高濃度的鹽(如鈣、鎂、鉀和鈉)會增大細胞外液滲透壓造成細胞脫水從而抑制微生物生長并降低AcoD中的CH4產(chǎn)量[55].鹽(如NaCl)主要存在于食物中,NaCl在FW中的含量一般為(2～5)%,其值可能因地區(qū)不同有顯著差異[55-56].鹽度在適宜范圍內(nèi)能夠提升厭氧消化相關酶反應活性,維持和調(diào)節(jié)微生物生長所需的滲透壓平衡.0.1～ 0.23gNaCl/L的鹽度有利于中溫厭氧消化菌和乙酸營養(yǎng)型產(chǎn)甲烷菌的代謝生長[57].然而,鹽度較高則會導致沼氣產(chǎn)量下降,甚至厭氧消化系統(tǒng)崩潰.2～ 5gNaCl/L范圍內(nèi)鹽度能夠提高FW厭氧消化水解和酸化程度,卻會抑制產(chǎn)CH4作用[56].當鹽度>10gNaCl/L將嚴重抑制產(chǎn)甲烷菌活性,鹽度達到50gNaCl/L時將同時抑制產(chǎn)酸菌活性[58].然而,高鹽度卻能夠影響乳酸向VFAs的轉(zhuǎn)化,造成乳酸的積累[59].Kim等[60]證實10.14gNaCl/L鹽度脅迫下改變了微生物代謝途徑,提高了乳酸的產(chǎn)量同時抑制丁酸生產(chǎn).Li等[59]發(fā)現(xiàn)AcoD體系中高濃度鹽能夠有效提高乳酸產(chǎn)量,在10gNaCl/L時乳酸產(chǎn)量顯著增加,而在鹽度達到30gNaCl/L時獲得了光學純?nèi)樗?乳酸含量的增加可能與高鹽度提高了底物增溶作用、水解酶活性和乳酸菌豐度有關.

2.5 預處理方式

AcoD中復雜化合物經(jīng)水解獲得單糖、氨基酸和LCFAs等小分子化合物,為產(chǎn)酸和產(chǎn)CH4提供充足底物.加快原料水解是提高AcoD生物降解性和能量回收的重要前提,現(xiàn)有研究普遍對基質(zhì)進行預處理加快底物降解,常用的預處理方式有酸、堿、熱、微波和水解酶(真菌)等[18,59,61-62].

2.5.1 酸和堿 酸和堿預處理是指向基質(zhì)中加入酸性或堿性物質(zhì),通過皂化作用破壞細胞壁,釋放可溶性有機物,同時水解胞外聚合物,增加底物的生物可降解性[63].Devlin等[64]發(fā)現(xiàn)使用HCl在pH值2.0下處理ES有效縮短了CH4生產(chǎn)周期且CH4產(chǎn)量提高了14.3%.Wu等[65]認為酸預處理ES能夠同時高效回收磷和VFAs,酸預處理后的VFAs產(chǎn)量較對照組增加了15.3倍.Saha等[66]研究了乙酸預處理FW對CH4產(chǎn)量影響,結(jié)果表明在62.5℃下利用0.2mol/L乙酸預處理30min后FW的最大糖類回收率達95%,這是因為乙酸預處理FW提高了微生物附著性和基質(zhì)可利用性,從而增強了產(chǎn)甲烷菌活性并提高了CH4產(chǎn)率.Cao等[67]的研究表明ES經(jīng)NaOH處理(pH=10)后消化液含較高的可溶性有機物,但主要是難生物降解有機物,如高復合組分、高分子蛋白質(zhì)和多糖.Elalami等[68]證實KOH(5g/TS)預處理提高了AcoD的CH4產(chǎn)量(～40%),并且增加了沼渣中氮和磷含量,因此堿性預處理結(jié)合AcoD有利于最大限度地提高資源回收率和生產(chǎn)生物肥料.另外,CaO2預處理ES能夠提高CH4產(chǎn)量,當用0.1g/gVS-CaO2輔助微波(480W,2min)預處理時,累積CH4量比對照增加80.2%[69].CaO2的強氧化性和堿性可以有效破壞胞外聚合物和細胞壁,促進難降解有機物或有毒污染物的降解.Wang等[70]的研究表明隨CaO2劑量的增加CH4產(chǎn)量也線性增加,CaO2預處理促進了不飽和共軛鍵的斷裂,降低了腐殖質(zhì)和木質(zhì)纖維素的芳香性,改變了腐殖質(zhì)和木質(zhì)纖維素的結(jié)構(gòu)和官能團,使其轉(zhuǎn)化為可生物降解的物質(zhì),從而為后續(xù)的CH4生產(chǎn)提供更多的基質(zhì).酸和堿預處理是常見的化學處理方式,添加酸或堿可避免消化系統(tǒng)對溫度的依賴,但由于其對pH值和設備防腐蝕的要求較高,并且處理后的污泥需要重新中和,因此酸堿預處理在工程應用時受到一定限制.

2.5.2 熱水解 熱水解通過強化難降解化合物的增溶作用提高CH4產(chǎn)量,基質(zhì)通常在160～180℃、壓力600～2500kPa條件下分解30～60min.熱水解能夠破壞基質(zhì)的細胞壁/膜,釋放胞內(nèi)可溶組分并將絮體和胞外聚合物分解為可溶的小分子化合物.Gong等[4]研究了溫度和pH值對AcoD體系(以FW和熱水解ES為原料)產(chǎn)酸發(fā)酵影響,結(jié)果表明pH值為7的中溫環(huán)境提高了混合酸產(chǎn)量,而堿性(pH=10)中溫以及中性(pH=7)高溫條件促進了丁酸積累.Li等[62]發(fā)現(xiàn)單一FW熱水解與ES進行AcoD有利于乳酸生產(chǎn),預處理縮短了乳酸最大產(chǎn)量的發(fā)酵時間.乳酸產(chǎn)率的提高與熱水解加速底物增溶和水解有關,同時改變了芽孢桿菌和乳酸菌等關鍵微生物的比例.Ma等[71]證實了熱水解FW能夠提高發(fā)酵微生物及酶的活性,糖類、脂質(zhì)和木質(zhì)纖維素更易降解,芽孢桿菌是參與有機物降解的優(yōu)勢菌屬.另外,堿預處理常與熱預處理相結(jié)合,形成熱-堿聯(lián)合預處理.該方式結(jié)合了熱和堿預處理的優(yōu)勢并且發(fā)揮協(xié)同效應. Toutian等[72]研究了熱-堿聯(lián)用預處理后ES的消化性能,在65～70℃下添加NaOH溶液(50%w/w,1～ 2.5mL ES)處理ES達2～2.5h,發(fā)現(xiàn)熱-堿預處理后ES的平均CH4產(chǎn)量相比僅熱預處理ES提高了20%,體現(xiàn)了熱-堿聯(lián)用預處理在提高能源回收方面的優(yōu)勢.

2.5.3 微波 微波是一種低能耗、高效率的預處理方式,能夠促進FW和ES的分解、滅活部分非消化微生物、提高消化效率和穩(wěn)定性[73].由于微波加熱能夠通過熱效應和非熱效應協(xié)同破壞細胞,因此微波預處理顯著縮短了消化時間[54,74].Yue等[54]為減輕油脂對FW消化的抑制和促進CH4生產(chǎn),采用微波加熱對油脂和FW進行預處理.結(jié)果表明,微波預處理通過熱效應促進脂質(zhì)混合、固體溶解和分解,從而促進了脂質(zhì)降解,降低了脂質(zhì)和大分子脂肪酸的積累,繼而提高了CH4的產(chǎn)率.Liu等[74]以FW和ES為聯(lián)合基質(zhì),發(fā)現(xiàn)微波預處理顯著促進了有機物分解同時提高了VFAs濃度和CH4產(chǎn)量.

2.5.4 酶 與物理和化學等預處理相比,酶預處理具有較好的特異性:生物反應簡單、能量需求低和環(huán)境友好性高[75],但相對成本較高.因此,為降低酶處理成本,研究人員從FW中原位提取高效水解酶以替代昂貴的商品酶[38,76-77].Yin等[61]從蛋糕廢棄物中提取富含水解酶的真菌對FW和ES聯(lián)合基質(zhì)進行預處理,發(fā)現(xiàn)酶預處理的混合基質(zhì)CH4產(chǎn)率是對照組的2.5倍,生物質(zhì)體積也減少54.3%.Ma等[77]認為真菌酶能夠快速水解原料,從而實現(xiàn)零固體排放和資源回收,并且探究了集合超快速水解與FW消化、微生物燃料電池、AcoD和生物肥料生產(chǎn)的可行性.因此,酶預處理可以成為增強FW水解和提高生物能源生產(chǎn)效率的前景技術.

綜上,在ES和FW協(xié)同厭氧消化前進行預處理是提高消化效率和能源回收率的有效策略.然而,許多預處理方式主要應用在FW或ES的單一底物處理過程中,如酸和堿主要用于ES預處理,熱水解主要用于FW或ES預處理,而針對混合原料(FW+ES)的預處理方式仍需進一步研究和優(yōu)化.另外,預處理的能耗和經(jīng)濟成本可能削弱甚至抵消其帶來的高能源回收優(yōu)勢,因此在選擇預處理方式時應進行綜合的能源回收和經(jīng)濟效益評估.

2.6 攪拌

連續(xù)攪拌反應器(CSTR)通常用作厭氧消化器,通過持續(xù)攪拌混合實現(xiàn)流體混合物和厭氧微生物的均勻分布,從而提高基質(zhì)傳質(zhì)和消化效率.但CSTR連續(xù)攪拌的運行和維護成本較高,據(jù)估計,消化器中混合所需的能量占總能量需求的8%～ 58%[78].因此,為改進消化經(jīng)濟性、提高基質(zhì)降解率,CSTR采取間歇混合模式能夠有效降低能源需求和運行成本,并提高沼氣產(chǎn)量[78-79].Zhang等[80]研究了不同混合模式對FW消化產(chǎn)能影響,結(jié)果表明,間歇混合(2min/h)能夠高效產(chǎn)CH4且更加節(jié)能,為消化系統(tǒng)提供凈正熱電輸出.Wang等[81]研究了3種間歇混合模式(頂部、中部和底部)對高固體消化性能的影響,發(fā)現(xiàn)頂部間歇混合模式能夠加速產(chǎn)CH4過程,所需的消化時間最短,3種間歇混合模式主要影響消化效率而對CH4累積幾乎無影響.Zhang等[82]評估了間歇混合-活性炭對高溫消化產(chǎn)CH4性能影響,認為活性炭可穩(wěn)定高溫消化過程,間歇攪拌耦合活性炭可以進一步改善產(chǎn)CH4代謝、增強功能酶基因表達.因此,應持續(xù)優(yōu)化AcoD間歇攪拌混合模式,以最大化減少能耗,同時提高消化效率和資源回收率.

2.7 溫度

溫度是影響AcoD性能的關鍵因子.溫度主要影響AcoD中的微生物生理生化過程(如代謝率、酶活性、細菌生長和衰亡率)和物理化學(如傳質(zhì)率、氣體溶解度和化學平衡)過程[50,83].依據(jù)不同發(fā)酵溫度,AcoD過程主要分為中溫(30～40℃)和高溫(55～60℃)兩種.雖然大部分發(fā)酵微生物能夠在較大范圍溫度下生存,但不同溫度可能影響產(chǎn)酸菌和產(chǎn)甲烷菌的代謝活性.Fernández-Domínguez等[84]研究了溫度對產(chǎn)酸發(fā)酵VFAs產(chǎn)量影響,發(fā)現(xiàn)35℃時VFAs產(chǎn)量最高(0.59gCOD/gVS),其組分主要有乙酸、丙酸和丁酸(三者之和占比75%～86%)且不受溫度變化影響.Cavinato等[85]同樣發(fā)現(xiàn)乙酸、丙酸和丁酸是產(chǎn)酸過程的主要VFAs產(chǎn)物,在37℃、水力停留時間(HRT)為4d的條件下VFAs產(chǎn)量最高.Arelli等[86]研究了中、高溫條件對AcoD過程CH4產(chǎn)率影響,發(fā)現(xiàn)在底物比為2:1(FW:ES,VS:VS)和55℃條件下CH4產(chǎn)率最高(0.42L/gVS),微生物群落分析表明甲烷絲菌屬主導古菌群落,宏基因組學研究顯示乙酸營養(yǎng)型甲烷化是產(chǎn)CH4主要代謝途徑.先前的研究同樣報道了嗜熱產(chǎn)甲烷菌屬需要高溫環(huán)境來應對不斷升高的OLR[8].最近建立的工程規(guī)模消化裝置分別在中溫和高溫下處理FW和ES,結(jié)果表明高溫AcoD具有較高的CH4產(chǎn)率[2,17].雖然高溫AcoD工藝具有底物水解速率快、有機物降解徹底、沼氣產(chǎn)量高和病原菌滅活率高等優(yōu)點,但嗜熱產(chǎn)甲烷菌對溫度變化較為敏感,且維持高溫需要的高能量輸入可能會抵消能源回收優(yōu)勢,同時高溫工藝穩(wěn)定性較低.因此在選擇中溫或高溫AcoD時應在穩(wěn)定性、能源回收率和經(jīng)濟成本等方面進行綜合考量比較.

2.8 pH值

pH值是維持AcoD體系穩(wěn)定性的重要參數(shù).AcoD體系中發(fā)酵菌對pH值表現(xiàn)出不同的敏感性,大多數(shù)發(fā)酵菌能夠在pH值為4.0～8.5之間生存,產(chǎn)酸菌敏感性較低,可在pH值為3.0～11.0時發(fā)揮代謝功能,而中性pH值(6.5～7.2)利于產(chǎn)甲烷菌的代謝和生長,當pH<5.5(或5)時將嚴重抑制產(chǎn)甲烷菌活性[3,50,83].Lindner等[87]研究表明,在pH值為5.5時由于較低的微生物氫化酶活性,CH4產(chǎn)量僅為理論值的40.9%.高生物降解性底物在AcoD體系快速水解引起pH值降低,而維持穩(wěn)定的pH值體現(xiàn)出消化過程的強緩沖能力.為了提高消化過程pH值的緩沖能力,可添加石灰或含氮物料來進行堿度調(diào)節(jié).Wu等[88]在40℃、HRT為7d下運行AcoD反應器,認為VFAs的高效生產(chǎn)是由于系統(tǒng)強大的緩沖能力維持了適宜的pH值(5.2～6.4).另外,不同的pH值堿性環(huán)境也會影響消化液中可被發(fā)酵菌群利用的可溶性底物質(zhì)量,進而影響有機物消化VFAs類型和濃度. Khatami等[21]研究了pH值對FW消化工藝VFAs的影響,發(fā)現(xiàn)pH=10時主要進行乙酸生產(chǎn),而pH=5時主要代謝產(chǎn)物是丙酸和乙酸,VFAs的生產(chǎn)與厚壁菌的豐度呈正相關.Jiang等[22]認為pH值為6.0時VFAs的濃度和產(chǎn)率最高,而乙酸和丁酸是主要的有機酸組分.綜上,考慮到產(chǎn)甲烷菌最適pH值為6.8～ 7.2,因此,調(diào)節(jié)AcoD體系維持中性或弱堿性pH值6.5～7.5,能夠維持產(chǎn)酸和產(chǎn)甲烷菌生長較優(yōu)環(huán)境,實現(xiàn)高效產(chǎn)CH4.

2.9 水力停留時間(HRT)

HRT是AcoD過程的另一個關鍵參數(shù),是指底物在消化過程與厭氧微生物接觸時間,它直接影響底物水解效率,同時也影響厭氧微生物的種群結(jié)構(gòu)、底物代謝途徑以及沼渣類型和產(chǎn)量.在CSTR中通常認為HRT等同于SRT,而且HRT也與消化過程OLR有關,HRT越低,OLR越高.研究發(fā)現(xiàn),在HRT為16～40d、OLR<4.5gCOD/(L·d)條件下,厭氧消化系統(tǒng)能夠穩(wěn)定運行.較短的HRT通常能夠抑制和淘汰產(chǎn)甲烷菌并且促進消化產(chǎn)氫過程[31].Angeriz- Campoy[89]報道了高固體AcoD高溫消化工藝中,在HRT=1.9d和OLR=66gVS/(L·d)條件下H2產(chǎn)量達到38mL/gVS.然而,其他研究發(fā)現(xiàn)隨著OLR由15.10gCOD/(L·d)增加至37.75gCOD/(L·d),H2產(chǎn)量減少,乳酸濃度增加[90].因此,在暗發(fā)酵產(chǎn)氫過程中需要優(yōu)化OLR和HRT來抑制耗氫菌的活性.隨著HRT的延長,水解效率和VFAs產(chǎn)量也隨之提高,而較長的HRT更有利于產(chǎn)甲烷菌的生長增殖.Wang等[7]研究了HRT對AcoD系統(tǒng)性能的影響并揭示了微生物群落結(jié)構(gòu)的差異和主要的產(chǎn)CH4代謝途徑,發(fā)現(xiàn)較長HRT(25d)的AcoD系統(tǒng)能夠富集互營型和CO2/H2(甲酸)營養(yǎng)型產(chǎn)甲烷菌,而在HRT為5d時選擇性富集耐酸菌;同時發(fā)現(xiàn)互營乙酸氧化和氫營養(yǎng)型甲烷化是主要的產(chǎn)CH4途徑,AcoD系統(tǒng)內(nèi)菌群通過生態(tài)位分化減少了種間的直接競爭.然而,有研究發(fā)現(xiàn)單級AcoD體系在較長HRT(25d)下CH4產(chǎn)率(314mL/gVS)接近兩相AcoD體系在較短HRT(15d)下的CH4產(chǎn)率(325mL/gVS),這表明HRT對發(fā)酵體系的影響也與運行模式有關[91].

2.10 有機負荷(OLR)

OLR是指單位體積反應器內(nèi)每天的有機底物量,CSTR內(nèi)一般通過調(diào)節(jié)HRT或原料配比來改變OLR.據(jù)報道,OLR在一定范圍內(nèi)增加有利于系統(tǒng)VFAs和CH4的積累,OLR改變也會影響VFAs的類型.Fernando-Foncillas等[92]在利用ES和FW協(xié)同消化生產(chǎn)羧酸時發(fā)現(xiàn),提高OLR不影響羧酸的總產(chǎn)量,但己酸的產(chǎn)率增加了44%.De Groof等[93]的研究發(fā)現(xiàn)在HRT為8.5d、低OLR(12gCOD/(L·d))下AcoD工藝主要為正丁酸發(fā)酵,而乳酸發(fā)酵主導了高OLR(20gCOD/(L·d))的AcoD體系.也有研究認為OLR擾動改變了微生物群落結(jié)構(gòu),Li等[94]發(fā)現(xiàn)產(chǎn)酸菌和VFAs氧化菌在高OLR(6gCOD/(L·d))脅迫階段豐度顯著增加,而產(chǎn)CH4主要代謝途徑并沒有從乙酸營養(yǎng)型轉(zhuǎn)變?yōu)闅錉I養(yǎng)型,優(yōu)勢氫營養(yǎng)型產(chǎn)甲烷菌的演替降低了體系氫消耗能力同時主導的產(chǎn)甲烷菌對乙酸降解性較差,因此消化體系發(fā)生惡化.

表1總結(jié)了近些年AcoD系統(tǒng)在不同影響因子作用下的運行效能研究,發(fā)現(xiàn)提高FW在原料中占比、中性(或弱堿性)pH值和較長HRT(15～25d)條件下能夠顯著提升AcoD的CH4產(chǎn)率.

3 協(xié)同厭氧消化體系直接種間電子傳遞研究

AcoD系統(tǒng)中CH4的生產(chǎn)高度依賴于互營細菌和產(chǎn)甲烷古菌之間的種間H2/甲酸傳遞(IHFT)作用:互營細菌降解VFAs等中間代謝產(chǎn)物并釋放電子載體H2(或甲酸),H2(或甲酸)則通過擴散作用傳遞給產(chǎn)甲烷菌[9].但是,只有在極低的氫分壓下,VFAs在熱力學上才適宜降解,因此需要氫營養(yǎng)型產(chǎn)甲烷菌將H2持續(xù)轉(zhuǎn)化為CH4,然而產(chǎn)甲烷菌生長速率緩慢、對環(huán)境條件變化敏感等特性影響了消化系統(tǒng)穩(wěn)定性[95].因此,加速復雜有機物水解速率及打破產(chǎn)酸熱力學屏障,開辟新型產(chǎn)CH4途徑成為了提高有機固體廢棄物厭氧消化效率和穩(wěn)定性的關鍵[96].

近年來報道了電子轉(zhuǎn)移效率高于IHFT的另一種間電子轉(zhuǎn)移方式,DIET,即產(chǎn)甲烷菌群直接接受電子將CO2還原為CH4[97-98].Morita等[98]在處理啤酒廢水的上流式厭氧污泥床首次觀察到顆粒污泥具備導電特性,且顆粒污泥的電導率是能進行DIET的和共培養(yǎng)體系的4.4倍,微生物群落分析表明,的豐度約占菌群的25%,而是主要的產(chǎn)甲烷菌,這意味著和之間可能存在DIET[98].隨后,Rotaru等[99]在共培養(yǎng)體系中首次證明了和通過DIET將乙醇轉(zhuǎn)化為CH4,而高度表達了乙醇代謝及用以進行胞外電子傳遞的導電菌毛的相關基因.另外,通過DIET過程獲得部分能量時,其增殖速度比基于乙酸為能量來源時更快[99].因此,建立以DIET為優(yōu)勢產(chǎn)甲烷代謝菌群的AcoD系統(tǒng)能夠加速電子轉(zhuǎn)移,消除IHFT固有的熱力學限制,獲得更加高效穩(wěn)定的AcoD效能.

表1 不同影響因子作用下AcoD系統(tǒng)運行效能研究

據(jù)報道,在AcoD體系中可通過代謝產(chǎn)物刺激(如乙醇)[28,97,100]和添加導電材料(如生物炭、活性炭、零價鐵和石墨等)途徑建立DIET[9].Zhao等[100]提出以乙醇為DIET基質(zhì)的強化微生物丙酸/丁酸互營代謝策略,發(fā)現(xiàn)乙醇刺激相比無乙醇投加顯著提高了丙酸(5倍)/丁酸(76倍)降解率.在利用乙醇型發(fā)酵產(chǎn)物作為基質(zhì)的消化反應器中,V/A型ATP酶基因和CO2還原酶關鍵基因豐度均高于以丙酸和丁酸為底物的消化反應器,因此DIET菌能夠優(yōu)先利用電子還原CO2為CH4[97].然而,由于乙醇型發(fā)酵產(chǎn)物所含的過量酸度會嚴重抑制DIET互營代謝,Li等[28]提出以ES與FW協(xié)同消化減輕酸度抑制影響,發(fā)現(xiàn)在適宜FW:ES條件下富集了能夠代謝復雜有機物和帶有導電菌毛的菌屬.

添加不同類型的導電材料將會影響AcoD體系產(chǎn)CH4性能的提升效果(表2).Kaur等[101]發(fā)現(xiàn)添加麥秸生物炭的AcoD體系CH4產(chǎn)量提高了24%(相比無生物炭添加),添加生物炭促進丙酸和LCFAs的降解,同時提高了乙酸和丁酸產(chǎn)量.Chowdhury等[46]對比了添加顆?；钚蕴亢痛盆F礦對FW厭氧消化的影響,證實了相比對照和磁鐵礦消化反應器,添加顆?；钚蕴磕軌蝻@著縮短消化延遲期,減少VFAs的積累,CH4產(chǎn)率提高了50%～80%.而Liang等[11]認為相比磁鐵礦和生物炭,添加零價鐵的AcoD體系累積CH4量最高(394mL/gVS),認為零價鐵是提高AcoD體系性能和穩(wěn)定性的優(yōu)選導電材料.另有研究報道石墨可作為AcoD體系富集DIET菌的導電材料,但其對消化體系CH4產(chǎn)率的提升效果不佳(7.5%～ 20%)[102].因此顆粒活性炭和零價鐵可能是目前提高AcoD產(chǎn)CH4性能的適宜導電材料.

表2 不同導電材料對AcoD體系產(chǎn)CH4提升效果對比

圖3 DIET代謝機理及導電材料作用機制[9-11,46,100-103]

雖然DIET在AcoD體系批次和短期試驗中,具備加速有機物降解轉(zhuǎn)化、提高CH4產(chǎn)率、縮短啟動周期、維持穩(wěn)定高效運行、提高資源回收率等優(yōu)勢,但缺乏工程規(guī)模應用驗證.這主要是由于(1)目前對DIET現(xiàn)象的理解主要基于添加導電材料和乙醇作為底物,而其他有機物能否作為DIET基質(zhì)仍需進一步研究;(2)相比乙醇代謝,導電材料具備更優(yōu)良的DIET潛質(zhì),但尚未開發(fā)出適合長期運行且不與導電材料作用的高效厭氧反應器,同時反應器應滿足導電材料的固定和與微生物充分接觸要求;(3)缺乏DIET對消化系統(tǒng)穩(wěn)定性和消化效率的長期觀察,導電材料的回收率和經(jīng)濟性有待進一步提高.

微生物潛在的DIET代謝機理和不同導電材料在厭氧消化系統(tǒng)中DIET機制見圖3.

4 展望

AcoD技術是提高厭氧消化率和穩(wěn)定性的前景技術,該技術能夠為厭氧微生物提供均衡的營養(yǎng)元素和適宜的碳氮比,并且沼渣可滿足城市固體廢棄物減量、生物能源回收和高值產(chǎn)物生產(chǎn)等可持續(xù)發(fā)展和循環(huán)經(jīng)濟要求.為進一步提升AcoD系統(tǒng)高效CH4生產(chǎn)性能,可通過提高底物水解效率、產(chǎn)物高效定向轉(zhuǎn)化、減輕抑制性因子影響及原位耦連其他廢棄物協(xié)同消化等途徑實現(xiàn).

4.1 提高底物水解效率

水解是限制AcoD效率的主要環(huán)節(jié),為此研究人員已開發(fā)許多底物預處理方式,同時,為了克服單一預處理的不足,進行了不同預處理方式的組合,如:熱-堿聯(lián)用[63]、超聲-微波聯(lián)用[54]和CaO2-微波聯(lián)用[69]等.雖然預處理顯著提升了底物水解速率和產(chǎn)酸產(chǎn)CH4效率,但大量堿性化合物、高氨氮濃度和有毒重金屬等也會進入沼渣(液),增加了后續(xù)沼渣(液)中VFAs分離和氮素脫除的成本,也對生物肥料的安全性進一步造成威脅.因此,開發(fā)綠色、高效和經(jīng)濟的底物預處理方式是AcoD技術應用的關鍵步驟.

4.2 產(chǎn)物高效定向轉(zhuǎn)化

實際生產(chǎn)中通常會需要特定發(fā)酵產(chǎn)物,如特定的VFAs類型、CH4或H2等,然而厭氧消化很難實現(xiàn)發(fā)酵產(chǎn)物的定向轉(zhuǎn)化.通過調(diào)控AcoD運行工況(如HRT、OLR、pH值和溫度)[93,104]、添加功能菌[24]和構(gòu)建DIET體系[100]等途徑對發(fā)酵產(chǎn)物進行定向選擇,仍存在操作復雜、產(chǎn)物回收率和經(jīng)濟性較低等缺點.實際上,產(chǎn)物的定向選擇主要受兩方面影響:基質(zhì)在AcoD體系中降解轉(zhuǎn)化和厭氧微生物的代謝功能.研究人員通過對FW和ES的理化性質(zhì)進行表征,闡明了FW作為產(chǎn)氫產(chǎn)CH4優(yōu)選原料的機理[25].因此,表征AcoD體系在不同運行條件和環(huán)境因子作用下發(fā)酵物組成和結(jié)構(gòu)特征能夠明確原料在發(fā)酵體系中降解情況,為厭氧發(fā)酵的產(chǎn)物定向選擇提供機理見解.不同的厭氧微生物功能代謝調(diào)控差異將直接影響發(fā)酵產(chǎn)物類型(即菌群-物質(zhì)的代謝偶聯(lián)作用).而近些年迅速發(fā)展的宏基因組學、宏轉(zhuǎn)錄組學、蛋白組學和代謝組學技術為研究微生物群落結(jié)構(gòu)、挖掘功能菌群代謝潛能、認識功能基因的表達活性和代謝功能調(diào)控、理解微生物種間相互作用提供了新視角[7,91,105],通過多組學聯(lián)用技術進一步闡明不同運行條件和環(huán)境因子下產(chǎn)物定向選擇和調(diào)控背后的微生物學機制,為厭氧消化代謝產(chǎn)物的定向調(diào)控和消化性能的有效提高提供微觀依據(jù).

4.3 減輕抑制性因子影響

調(diào)控常規(guī)的運行條件(原料碳氮比、HRT和OLR等)將會直接影響AcoD系統(tǒng)效能,甚至抑制厭氧消化過程,如受VFAs累積(>2g/L)、高氨氮(1.5～ 3.0g/L)和酸性pH值(<5.5)等抑制因素影響[83,106].而AcoD原料FW中不僅含豐富的蛋白質(zhì)、碳水化合物和微生物菌群,同時可能含有對AcoD潛在的抑制劑,如鹽分、油脂、辣椒和大蒜等[48,59,107-108].最近的研究關注到FW中常見的辣椒和大蒜等物質(zhì)對AcoD體系的抑制危害,Du等[107]研究表明辣椒素通過改變關鍵激酶或降低細胞內(nèi)NAD+/NADH比率誘導細胞凋亡,對水解、產(chǎn)酸和產(chǎn)CH4,尤其是乙酸營養(yǎng)型產(chǎn)甲烷有明顯的抑制作用.Tao等[108]發(fā)現(xiàn)大蒜素及其降解產(chǎn)物顯著抑制AcoD體系產(chǎn)CH4,同時促進胞內(nèi)有機物、氮和磷的釋放.另外,AcoD體系中可能存在的如納米材料、微塑料和抗生素等新興微生物抑制性污染物,對厭氧發(fā)酵效率和穩(wěn)定性提出了新的挑戰(zhàn)[109-110].未來應進一步關注這些污染物對AcoD體系底物分解和微生物代謝影響,并考慮在發(fā)酵體系內(nèi)實現(xiàn)協(xié)同降解和回收.

4.4 原位耦連其他廢棄物協(xié)同消化

此外,為進一步提升固廢處置效率和AcoD消化性能,在AcoD體系中原位耦聯(lián)其他物質(zhì),如畜糞[102]、園林廢棄物[5]、農(nóng)作物秸稈[111-112]、中草藥渣[112]、黑糞水[113]和城市污水[114]等,能夠達到同時處理多種類型廢棄物和提高能源回收的目的.Mu等[5]發(fā)現(xiàn)以FW、ES和園林廢棄物為聯(lián)合基質(zhì)進行厭氧消化能夠增強消化體系微生物協(xié)同作用,3種原料混合后能夠補充微量元素、提高緩沖能力和獲得適宜碳氮比,相比其中兩種原料協(xié)同消化,3種原料協(xié)同消化CH4產(chǎn)量最高達(314.9±17.1)mL/gVS. Mo?ino等[114]為提高城市污水厭氧消化能源回收率,在厭氧膜生物反應器協(xié)同處理FW與城市污水,結(jié)果表明,相比城市污水消化,協(xié)同消化FW與城市污水CH4產(chǎn)量提高了1.9倍.這可能是由于FW中富含的蛋白質(zhì)、糖類等可生化性物質(zhì)被降解轉(zhuǎn)化為乙酸和H2,從而提升了體系OLR和CH4產(chǎn)量[115].因此,在FW和ES的AcoD體系中原位耦聯(lián)其他廢棄物具有廣闊的發(fā)展應用前景,但仍需在基質(zhì)預處理、物料配比和反應條件等方面進一步優(yōu)化消化體系,提升消化水解效率和資源回收率.

5 結(jié)論

5.1 原料配比、碳氮比、油脂、鹽分、預處理方式、攪拌、溫度、pH值、HRT和OLR等因素會直接影響AcoD效能,其中底物預處理是提高AcoD效率的重要途徑,因此有必要開發(fā)高效低成本的預處理方式.

5.2 構(gòu)建微生物DIET作用是AcoD體系高效定向產(chǎn)CH4的前景性技術,但需要深入評估其大規(guī)模工程應用的可行性.

5.3 表征沼渣(液)在發(fā)酵體系中轉(zhuǎn)化特性有助于理解底物生物降解性和產(chǎn)物轉(zhuǎn)化特點,利用宏基因組學、宏轉(zhuǎn)錄組學、蛋白組學和代謝組學等多組學聯(lián)用技術能夠更好地解析發(fā)酵體系中菌群-物質(zhì)的代謝偶聯(lián),為定向調(diào)控微生物代謝提高發(fā)酵性能提供指導.

5.4 在AcoD體系構(gòu)建及優(yōu)化過程中可綜合考慮通過原料預處理、多種類廢棄物協(xié)同厭氧消化、運行條件優(yōu)化(如溫度、pH、HRT和OLR等)與潛在DIET促進體系構(gòu)建等,以實現(xiàn)產(chǎn)物高效定向轉(zhuǎn)化.

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Recent advances in anaerobic co-digestion of excess sludge and food waste.

ZHANG Xing-xing1, JIAO Peng-bo1, YANG Hui-ying1, WU Rui-min1, LI Yong-mei2, MA Li-ping1*

(1.School of Ecological and Environmental Sciences, East China Normal University, Shanghai 200241, China；2.School of Environmental Science and Engineering, Tongji University, Shanghai 200092, China)., 2022,42(5)：2179～2194

In order to promote the industrial-scale application of anaerobic co-digestion (AcoD) of sewage excess sludge (ES) with food waste (FW) and enhance its energy recovery efficiency, this study systematically summarized the mechanisms of AcoD process, the distribution of co-digestive products and the factors that may affect the AcoD performance, the important research advances of direct interspecific electron transfer in AcoD were then reviewed, followed by the novel perspectives of AcoD process were proposed, such as developing efficient and economic methods for feedstock pretreatment, characterizing substrates degradation, understanding metabolic regulation by omics technologies, mitigating the effect of potential inhibitors in the AcoD systems, and in-situ coupling with other wastes, to improve digestion performance and stability. This study may provide a guidance and reference for efficient energy recovery of urban organic solid wastes.

excess sludge；food waste；anaerobic co-digestion；influencing factors；direct interspecies electron transfer；co-digestion of multiple wastes

X705

A

1000-6923(2022)05-2179-16

張星星(1995-),男,江蘇連云港人,華東師范大學博士研究生,主要從事固體廢物污染控制與資源化技術研究.發(fā)表論文10余篇.

2021-10-19

國家重點研發(fā)計劃項目(2019YFC1905000)

* 責任作者, 研究員, lpma@des.ecnu.edu.cn