剩余污泥與餐廚垃圾協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.華東師范大學生態與環境科學學院,上海 200241；2.同濟大學環境科學與工程學院,上海 200092)

為了推進污水廠剩余污泥與餐廚垃圾協同厭氧消化在工程規模中的應用,提高其能源回收率,系統分析了協同厭氧消化機制、產物類型及其主要的影響因子,綜述了協同厭氧消化中直接種間電子傳遞作用的重要研究進展,并展望了協同厭氧消化的未來研究方向,包括開發高效經濟的原料預處理方式,表征基質降解特性,基于多組學聯用技術理解微生物代謝調控,緩解消化體系中潛在抑制劑影響,原位耦聯其他種類廢棄物進一步提升消化性能和穩定性,以期為城鎮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；協同厭氧消化；影響因子；直接種間電子傳遞；多種廢棄物協同消化

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產生量將增至1.8億t/a.如何安全、經濟、環保的處置數量龐大的固體廢棄物是我國面臨的重要挑戰.

厭氧消化(AD)技術能夠實現有機固廢的減量化、穩定化和資源化,并將其轉化為甲烷(CH4)、氫氣(H2)和有機酸等能源物質,同時沼渣可用作肥料或土壤調理劑[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酸化,導致消化不穩定甚至體系崩潰[5].

近年來,ES和FW協同厭氧消化(AcoD)技術能夠更加穩定高效的實現廢棄物減量化和資源化,具備稀釋廢物有毒物質、提升高值消化產物產量、改善營養元素均衡性和增強微生物協同效應等突出優勢,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定性,但仍無法突破產酸的熱力學屏障,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)的突破性發現,即產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)直接轉化為CH4,繼而促進電子流向產CH4過程同時加速底物高效定向轉化[10-11].

當前國內外對AcoD工藝的關注和研究較多,在此基礎上較為全面的總結提煉對學界推進和改良AcoD工藝以實現高效定向轉化至關重要.鑒于此,本文綜述了基于ES和FW的AcoD最新研究進展,總結了AcoD機制和產物特性,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 協同厭氧消化機制及產物類型分析

1.1 協同厭氧消化機制

AcoD過程可分為4個連續的底物降解階段:水解、酸化、產氫產乙酸和產CH4階段(圖1)[12-14].首先,ES和FW增溶產生的溶解性多糖、蛋白質和脂質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.即經歷第一步水解階段,其中復雜難降解有機物是此時底物轉化的限速步驟[15].第二步是酸化階段,水解后的單體發酵成短鏈脂肪酸(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,因此極易導致揮發性脂肪酸(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系統酸化并且抑制產甲烷菌代謝[16-17].第三步是產氫產乙酸階段,產氫產乙酸菌能夠降解乙醇和VFAs等轉化為乙酸、H2和CO2,而耗氫同型產乙酸菌將H2和CO2還原為乙酸.第四步是產CH4階段,CH4主要由乙酸營養型甲烷化和氫營養型甲烷化兩種途徑產生[18].在乙酸營養型途徑中,乙酸營養產甲烷菌將乙酸轉化為CH4和CO2.在氫營養型途徑中,氫營養產甲烷菌將H2和CO2轉化為CH4.由于地區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,乙酸營養型是最主要的產CH4途徑,而富含蛋白質的底物厭氧消化過程中,氫營養型產甲烷菌占主導[19].最終AcoD會產生含CH4、CO2、H2和硫化氫等氣體的沼氣.

圖1 ES與FW協同厭氧消化機制及產物類型示意

1.2 協同厭氧消化產物類型

有機物在AcoD體系中降解轉化主要形成有機酸和沼氣等高值產物,沼渣可作為生物肥料實現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酸化階段和產氫產乙酸階段的重要中間產物,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生物柴油、微生物燃料電池發電和合成復合聚合物的重要原料[13].VFAs通過產酸菌催化的一系列生化反應合成.首先,產酸菌將水解產物單體轉化成乙酸、丙酸、丁酸、醇、H2、CO2和其他產物;其次,丙酸、丁酸、醇類和CO2通過還原質子產乙酸途徑或同型產乙酸途徑進一步轉化為乙酸[20].通常,乙酸、丙酸和丁酸是產酸發酵的主要產物[21].產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)同型產乙酸型[13].有機物產酸發酵代謝途徑見圖2.

加快水解速率、強化產酸發酵和減輕抑制因子影響是提高VFAs總產量的有效策略.由于復雜化合物水解很大程度上限制了AcoD產酸發酵效率,因此可通過優化操作參數和對底物預處理加快水解代謝.Jiang等[22]調控消化pH值為6.0時VFAs濃度和產率最高,而Chen等[23]發現在pH值為9.0時,VFAs產量顯著提高并穩定在(25.9±1.5)gCOD/L.酸性和堿性pH值對VFAs產量的影響可能與水解步驟中可溶性蛋白質和糖類的產生有關.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,發現AcoD協同發酵顯著提高了乳酸產量,引入丙酸桿菌則進一步提高了二次消化的高值酸(丙酸)產量.因此如何在提升VFAs總產量的同時,高效、經濟、簡便地提高丙酸、丁酸等高值酸在總VFAs中的比例以實現高效定向轉化與提取,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生產途徑主要有氫營養和乙酸營養兩種,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產甲烷菌協同產CH4[26].對于AcoD產CH4,酸化和產乙酸階段會產生抑制產甲烷菌的酸性pH值和高FA.酸性pH值和FA對產甲烷菌具有協同抑制作用,并對乙酸營養和氫營養產甲烷菌及互營乙酸氧化產生不同程度脅迫,從而改變產CH4代謝途徑[27].Li等[28]為緩解FW消化過程生物乙醇發酵預處理的酸性抑制,混合ES與FW進行協同厭氧發酵.結果表明,在1:2(FW:ES)混合比下CH4產量達到峰值267mL/gCOD, AcoD體系高效穩定互營代謝是由于富集了具有DIET功能的產甲烷菌種和. Chen等[29]通過調控兩相AD中產酸階段的氧化還原電位、pH值和有機負荷(OLR)改變了產酸發酵類型,發現丁酸發酵具有較高的酸化程度(36%)、CH4產率(535mL/gVS)和生物降解率(95%),因此認為調控產酸的發酵類型能夠有效提高CH4產量.

圖2 有機物產酸發酵代謝途徑[13,30]

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

厭氧消化產氫作用主要發生在無光生物厭氧發酵體系中,多糖在黑暗條件下轉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、光發酵和微生物電解等產氫方式,暗發酵可以在無需光照和電能條件下持續產氫,具備高效率和低能耗等優勢.據報道,AcoD體系沼氣產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%,這是由于通過協同消化改善基質特性提供了更多的有機物[32].此外,消化過程產生的FA可能會抑制消化微生物活性,但最近的研究發現了FA對污泥暗發酵產氫的積極影響.Wang等[33]研究證實FA的存在能夠顯著改善污泥暗發酵產氫性能,H2產率隨FA濃度的增加而增加.FA促進胞裂釋放的胞內組分為后續的產氫提供更多底物,FA主要抑制了耗氫代謝而對水解和產乙酸途徑無顯著影響[33].因此,AcoD體系中FW豐富的蛋白質水解釋放的FA可能會促進產氫發酵,但應合理調配FW和ES的進料比例,避免過高濃度FA對微生物產生毒害抑制作用.

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].優質土壤肥料的典型參數是碳氮比,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,發現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,因此在作肥料用途時必須滿足生物安全和衛生質量要求.

1.2.4 藍鐵礦從OFMSW中回收如氮和磷等物質能夠緩解資源匱乏和獲得可觀的經濟收益.相比傳統的鳥糞石法回收磷,從AcoD系統回收穩定的磷-鐵晶體藍鐵礦(Fe3(PO4)2·8H2O)具備明顯的經濟價值優勢(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+從發酵物中有效分離釋放并結合形成藍鐵礦沉淀,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]發現相比單獨添加FW或FeCl3至AcoD體系以回收藍鐵礦,聯合投加FW和FeCl3可生物自調節pH值和提高有機物濃度,從而提高Fe3+還原率和酶活性.另外,還發現污泥停留時間(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體系穩定性和CH4產量的關鍵因子.不同的原料配比將導致消化體系養分含量、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CH4性能最優.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,發現高FW混合比下AcoD體系產CH4量較高,產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CH4.Prabhu等[43]發現相比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產率提升了4.59倍((50.3± 10.4)mL/(gVS·d)).這種較優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范圍內能夠滿足AcoD體系厭氧微生物對養分含量平衡的需求[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體系運行不穩定和沼氣產量下降.一方面,高碳氮比的聯合底物可能引起AcoD體系VFAs大量積累、pH值和緩沖能力下降,進而抑制產CH4作用.另一方面,雖然低碳氮比的聯合底物使得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時厭氧消化丙酸產量極低[13].Azarmanesh等[6]研究發現在碳氮比為8～19.7范圍內,沼氣產量與其呈線性相關,并認為碳氮比是影響AcoD體系沼氣生產的關鍵因素.Li等[32]研究了FW和ES的AcoD生產H2和VFAs,發現聯合基質通過提供適宜的碳氮比(15～23)和pH值(6.1～6.5)有效富集了產酸菌和產氫菌,從而提高了VFAs產量和H2含量.因此,在分析聯合基質化學組分特性基礎上調整原料配比獲得適宜的進料碳氮比,有利于提升AcoD體系發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)%),據報道,油脂厭氧消化產CH4潛力(1014L/kgVS)遠高于碳水化合物厭氧消化產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,油脂粘附生物質引起起泡和生物質浮選,也會降低AcoD效率.LCFAs能夠抑制產甲烷菌代謝,其中乙酸營養型產甲烷菌對LCFAs最敏感,同時LCFAs也可能會抑制氫營養型和互營型產甲烷菌[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過程產生抑制,因此有必要監測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同消化[51-53]、構建DIET富集互營LCFAs β-氧化菌增強產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)%,其值可能因地區不同有顯著差異[55-56].鹽度在適宜范圍內能夠提升厭氧消化相關酶反應活性,維持和調節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型產甲烷菌的代謝生長[57].然而,鹽度較高則會導致沼氣產量下降,甚至厭氧消化系統崩潰.2～ 5gNaCl/L范圍內鹽度能夠提高FW厭氧消化水解和酸化程度,卻會抑制產CH4作用[56].當鹽度>10gNaCl/L將嚴重抑制產甲烷菌活性,鹽度達到50gNaCl/L時將同時抑制產酸菌活性[58].然而,高鹽度卻能夠影響乳酸向VFAs的轉化,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,提高了乳酸的產量同時抑制丁酸生產.Li等[59]發現AcoD體系中高濃度鹽能夠有效提高乳酸產量,在10gNaCl/L時乳酸產量顯著增加,而在鹽度達到30gNaCl/L時獲得了光學純乳酸.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中復雜化合物經水解獲得單糖、氨基酸和LCFAs等小分子化合物,為產酸和產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,現有研究普遍對基質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中加入酸性或堿性物質,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]發現使用HCl在pH值2.0下處理ES有效縮短了CH4生產周期且CH4產量提高了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產量影響,結果表明在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可利用性,從而增強了產甲烷菌活性并提高了CH4產率.Cao等[67]的研究表明ES經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、高分子蛋白質和多糖.Elalami等[68]證實KOH(5g/TS)預處理提高了AcoD的CH4產量(～40%),并且增加了沼渣中氮和磷含量,因此堿性預處理結合AcoD有利于最大限度地提高資源回收率和生產生物肥料.另外,CaO2預處理ES能夠提高CH4產量,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,降低了腐殖質和木質纖維素的芳香性,改變了腐殖質和木質纖維素的結構和官能團,使其轉化為可生物降解的物質,從而為后續的CH4生產提供更多的基質.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產量,基質通常在160～180℃、壓力600～2500kPa條件下分解30～60min.熱水解能夠破壞基質的細胞壁/膜,釋放胞內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)產酸發酵影響,結果表明pH值為7的中溫環境提高了混合酸產量,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]發現單一FW熱水解與ES進行AcoD有利于乳酸生產,預處理縮短了乳酸最大產量的發酵時間.乳酸產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能夠提高發酵微生物及酶的活性,糖類、脂質和木質纖維素更易降解,芽孢桿菌是參與有機物降解的優勢菌屬.另外,堿預處理常與熱預處理相結合,形成熱-堿聯合預處理.該方式結合了熱和堿預處理的優勢并且發揮協同效應. Toutian等[72]研究了熱-堿聯用預處理后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,發現熱-堿預處理后ES的平均CH4產量相比僅熱預處理ES提高了20%,體現了熱-堿聯用預處理在提高能源回收方面的優勢.

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、提高消化效率和穩定性[73].由于微波加熱能夠通過熱效應和非熱效應協同破壞細胞,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,采用微波加熱對油脂和FW進行預處理.結果表明,微波預處理通過熱效應促進脂質混合、固體溶解和分解,從而促進了脂質降解,降低了脂質和大分子脂肪酸的積累,繼而提高了CH4的產率.Liu等[74]以FW和ES為聯合基質,發現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聯合基質進行預處理,發現酶預處理的混合基質CH4產率是對照組的2.5倍,生物質體積也減少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效率的前景技術.

綜上,在ES和FW協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化.另外,預處理的能耗和經濟成本可能削弱甚至抵消其帶來的高能源回收優勢,因此在選擇預處理方式時應進行綜合的能源回收和經濟效益評估.

2.6 攪拌

連續攪拌反應器(CSTR)通常用作厭氧消化器,通過持續攪拌混合實現流體混合物和厭氧微生物的均勻分布,從而提高基質傳質和消化效率.但CSTR連續攪拌的運行和維護成本較高,據估計,消化器中混合所需的能量占總能量需求的8%～ 58%[78].因此,為改進消化經濟性、提高基質降解率,CSTR采取間歇混合模式能夠有效降低能源需求和運行成本,并提高沼氣產量[78-79].Zhang等[80]研究了不同混合模式對FW消化產能影響,結果表明,間歇混合(2min/h)能夠高效產CH4且更加節能,為消化系統提供凈正熱電輸出.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,發現頂部間歇混合模式能夠加速產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CH4性能影響,認為活性炭可穩定高溫消化過程,間歇攪拌耦合活性炭可以進一步改善產CH4代謝、增強功能酶基因表達.因此,應持續優化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(如傳質率、氣體溶解度和化學平衡)過程[50,83].依據不同發酵溫度,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酸菌和產甲烷菌的代謝活性.Fernández-Domínguez等[84]研究了溫度對產酸發酵VFAs產量影響,發現35℃時VFAs產量最高(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]同樣發現乙酸、丙酸和丁酸是產酸過程的主要VFAs產物,在37℃、水力停留時間(HRT)為4d的條件下VFAs產量最高.Arelli等[86]研究了中、高溫條件對AcoD過程CH4產率影響,發現在底物比為2:1(FW:ES,VS:VS)和55℃條件下CH4產率最高(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型甲烷化是產CH4主要代謝途徑.先前的研究同樣報道了嗜熱產甲烷菌屬需要高溫環境來應對不斷升高的OLR[8].最近建立的工程規模消化裝置分別在中溫和高溫下處理FW和ES,結果表明高溫AcoD具有較高的CH4產率[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、沼氣產量高和病原菌滅活率高等優點,但嗜熱產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勢,同時高溫工藝穩定性較低.因此在選擇中溫或高溫AcoD時應在穩定性、能源回收率和經濟成本等方面進行綜合考量比較.

2.8 pH值

pH值是維持AcoD體系穩定性的重要參數.AcoD體系中發酵菌對pH值表現出不同的敏感性,大多數發酵菌能夠在pH值為4.0～8.5之間生存,產酸菌敏感性較低,可在pH值為3.0～11.0時發揮代謝功能,而中性pH值(6.5～7.2)利于產甲烷菌的代謝和生長,當pH<5.5(或5)時將嚴重抑制產甲烷菌活性[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,而維持穩定的pH值體現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節.Wu等[88]在40℃、HRT為7d下運行AcoD反應器,認為VFAs的高效生產是由于系統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值堿性環境也會影響消化液中可被發酵菌群利用的可溶性底物質量,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,發現pH=10時主要進行乙酸生產,而pH=5時主要代謝產物是丙酸和乙酸,VFAs的生產與厚壁菌的豐度呈正相關.Jiang等[22]認為pH值為6.0時VFAs的濃度和產率最高,而乙酸和丁酸是主要的有機酸組分.綜上,考慮到產甲烷菌最適pH值為6.8～ 7.2,因此,調節AcoD體系維持中性或弱堿性pH值6.5～7.5,能夠維持產酸和產甲烷菌生長較優環境,實現高效產CH4.

2.9 水力停留時間(HRT)

HRT是AcoD過程的另一個關鍵參數,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構、底物代謝途徑以及沼渣類型和產量.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.研究發現,在HRT為16～40d、OLR<4.5gCOD/(L·d)條件下,厭氧消化系統能夠穩定運行.較短的HRT通常能夠抑制和淘汰產甲烷菌并且促進消化產氫過程[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產量達到38mL/gVS.然而,其他研究發現隨著OLR由15.10gCOD/(L·d)增加至37.75gCOD/(L·d),H2產量減少,乳酸濃度增加[90].因此,在暗發酵產氫過程中需要優化OLR和HRT來抑制耗氫菌的活性.隨著HRT的延長,水解效率和VFAs產量也隨之提高,而較長的HRT更有利于產甲烷菌的生長增殖.Wang等[7]研究了HRT對AcoD系統性能的影響并揭示了微生物群落結構的差異和主要的產CH4代謝途徑,發現較長HRT(25d)的AcoD系統能夠富集互營型和CO2/H2(甲酸)營養型產甲烷菌,而在HRT為5d時選擇性富集耐酸菌;同時發現互營乙酸氧化和氫營養型甲烷化是主要的產CH4途徑,AcoD系統內菌群通過生態位分化減少了種間的直接競爭.然而,有研究發現單級AcoD體系在較長HRT(25d)下CH4產率(314mL/gVS)接近兩相AcoD體系在較短HRT(15d)下的CH4產率(325mL/gVS),這表明HRT對發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每天的有機底物量,CSTR內一般通過調節HRT或原料配比來改變OLR.據報道,OLR在一定范圍內增加有利于系統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協同消化生產羧酸時發現,提高OLR不影響羧酸的總產量,但己酸的產率增加了44%.De Groof等[93]的研究發現在HRT為8.5d、低OLR(12gCOD/(L·d))下AcoD工藝主要為正丁酸發酵,而乳酸發酵主導了高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構,Li等[94]發現產酸菌和VFAs氧化菌在高OLR(6gCOD/(L·d))脅迫階段豐度顯著增加,而產CH4主要代謝途徑并沒有從乙酸營養型轉變為氫營養型,優勢氫營養型產甲烷菌的演替降低了體系氫消耗能力同時主導的產甲烷菌對乙酸降解性較差,因此消化體系發生惡化.

表1總結了近些年AcoD系統在不同影響因子作用下的運行效能研究,發現提高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產率.

3 協同厭氧消化體系直接種間電子傳遞研究

AcoD系統中CH4的生產高度依賴于互營細菌和產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,因此需要氫營養型產甲烷菌將H2持續轉化為CH4,然而產甲烷菌生長速率緩慢、對環境條件變化敏感等特性影響了消化系統穩定性[95].因此,加速復雜有機物水解速率及打破產酸熱力學屏障,開辟新型產CH4途徑成為了提高有機固體廢棄物厭氧消化效率和穩定性的關鍵[96].

近年來報道了電子轉移效率高于IHFT的另一種間電子轉移方式,DIET,即產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]在共培養體系中首次證明了和通過DIET將乙醇轉化為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為優勢產甲烷代謝菌群的AcoD系統能夠加速電子轉移,消除IHFT固有的熱力學限制,獲得更加高效穩定的AcoD效能.

表1 不同影響因子作用下AcoD系統運行效能研究

據報道,在AcoD體系中可通過代謝產物刺激(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基質的強化微生物丙酸/丁酸互營代謝策略,發現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.在利用乙醇型發酵產物作為基質的消化反應器中,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菌能夠優先利用電子還原CO2為CH4[97].然而,由于乙醇型發酵產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協同消化減輕酸度抑制影響,發現在適宜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CH4性能的提升效果(表2).Kaur等[101]發現添加麥秸生物炭的AcoD體系CH4產量提高了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量.Chowdhury等[46]對比了添加顆粒活性炭和磁鐵礦對FW厭氧消化的影響,證實了相比對照和磁鐵礦消化反應器,添加顆粒活性炭能夠顯著縮短消化延遲期,減少VFAs的積累,CH4產率提高了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體系性能和穩定性的優選導電材料.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CH4性能的適宜導電材料.

表2 不同導電材料對AcoD體系產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化、提高CH4產率、縮短啟動周期、維持穩定高效運行、提高資源回收率等優勢,但缺乏工程規模應用驗證.這主要是由于(1)目前對DIET現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基質仍需進一步研究;(2)相比乙醇代謝,導電材料具備更優良的DIET潛質,但尚未開發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對消化系統穩定性和消化效率的長期觀察,導電材料的回收率和經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中DIET機制見圖3.

4 展望

AcoD技術是提高厭氧消化率和穩定性的前景技術,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物生產等可持續發展和循環經濟要求.為進一步提升AcoD系統高效CH4生產性能,可通過提高底物水解效率、產物高效定向轉化、減輕抑制性因子影響及原位耦連其他廢棄物協同消化等途徑實現.

4.1 提高底物水解效率

水解是限制AcoD效率的主要環節,為此研究人員已開發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-堿聯用[63]、超聲-微波聯用[54]和CaO2-微波聯用[69]等.雖然預處理顯著提升了底物水解速率和產酸產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,開發綠色、高效和經濟的底物預處理方式是AcoD技術應用的關鍵步驟.

4.2 產物高效定向轉化

實際生產中通常會需要特定發酵產物,如特定的VFAs類型、CH4或H2等,然而厭氧消化很難實現發酵產物的定向轉化.通過調控AcoD運行工況(如HRT、OLR、pH值和溫度)[93,104]、添加功能菌[24]和構建DIET體系[100]等途徑對發酵產物進行定向選擇,仍存在操作復雜、產物回收率和經濟性較低等缺點.實際上,產物的定向選擇主要受兩方面影響:基質在AcoD體系中降解轉化和厭氧微生物的代謝功能.研究人員通過對FW和ES的理化性質進行表征,闡明了FW作為產氫產CH4優選原料的機理[25].因此,表征AcoD體系在不同運行條件和環境因子作用下發酵物組成和結構特征能夠明確原料在發酵體系中降解情況,為厭氧發酵的產物定向選擇提供機理見解.不同的厭氧微生物功能代謝調控差異將直接影響發酵產物類型(即菌群-物質的代謝偶聯作用).而近些年迅速發展的宏基因組學、宏轉錄組學、蛋白組學和代謝組學技術為研究微生物群落結構、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],通過多組學聯用技術進一步闡明不同運行條件和環境因子下產物定向選擇和調控背后的微生物學機制,為厭氧消化代謝產物的定向調控和消化性能的有效提高提供微觀依據.

4.3 減輕抑制性因子影響

調控常規的運行條件(原料碳氮比、HRT和OLR等)將會直接影響AcoD系統效能,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、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NAD+/NADH比率誘導細胞凋亡,對水解、產酸和產CH4,尤其是乙酸營養型產甲烷有明顯的抑制作用.Tao等[108]發現大蒜素及其降解產物顯著抑制AcoD體系產CH4,同時促進胞內有機物、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,對厭氧發酵效率和穩定性提出了新的挑戰[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,并考慮在發酵體系內實現協同降解和回收.

4.4 原位耦連其他廢棄物協同消化

此外,為進一步提升固廢處置效率和AcoD消化性能,在AcoD體系中原位耦聯其他物質,如畜糞[102]、園林廢棄物[5]、農作物秸稈[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]發現以FW、ES和園林廢棄物為聯合基質進行厭氧消化能夠增強消化體系微生物協同作用,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同消化,3種原料協同消化CH4產量最高達(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同處理FW與城市污水,結果表明,相比城市污水消化,協同消化FW與城市污水CH4產量提高了1.9倍.這可能是由于FW中富含的蛋白質、糖類等可生化性物質被降解轉化為乙酸和H2,從而提升了體系OLR和CH4產量[115].因此,在FW和ES的AcoD體系中原位耦聯其他廢棄物具有廣闊的發展應用前景,但仍需在基質預處理、物料配比和反應條件等方面進一步優化消化體系,提升消化水解效率和資源回收率.

5 結論

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,因此有必要開發高效低成本的預處理方式.

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

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

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

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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.發表論文10余篇.

2021-10-19

國家重點研發計劃項目(2019YFC1905000)

* 責任作者, 研究員, lpma@des.ecnu.edu.cn