底物對(duì)厭氧消化系統(tǒng)EPS產(chǎn)生及起泡的影響

周瀅月,張 智,李 蕾,楊屏錦,王小銘,彭緒亞

底物對(duì)厭氧消化系統(tǒng)EPS產(chǎn)生及起泡的影響

周瀅月,張 智,李 蕾*,楊屏錦,王小銘,彭緒亞

(重慶大學(xué),三峽庫(kù)區(qū)生態(tài)環(huán)境教育部重點(diǎn)實(shí)驗(yàn)室,重慶 400045)

為明確底物成分對(duì)厭氧消化系統(tǒng)胞外聚合物(EPS)生成及起泡性能的影響,在批次實(shí)驗(yàn)中引入高碳水、高蛋白和高油脂3種廚余垃圾,檢測(cè)了不同試驗(yàn)組EPS的產(chǎn)生特性(濃度、組成、結(jié)構(gòu)、官能團(tuán)等),及各類(lèi)EPS和污泥的起泡特性參數(shù)(粘度、表面張力和相對(duì)疏水性)和起泡潛能參數(shù)(起泡趨勢(shì)、泡沫穩(wěn)定性、泡沫直徑和液膜厚度),并分析了EPS特性與污泥特性的相關(guān)性.結(jié)果表明,底物不同的試驗(yàn)組產(chǎn)生了性質(zhì)各異的EPS.底物組分越均衡越傾向于產(chǎn)氣利用,而高碳水及對(duì)照組因碳水化合物含量過(guò)多,EPS總濃度顯著高于其余實(shí)驗(yàn)組.碳水化合物、蛋白質(zhì)和油脂分別會(huì)通過(guò)改變中間代謝產(chǎn)物、影響微生物產(chǎn)酶情況和增殖情況而改變EPS形態(tài)及組分.EPS結(jié)構(gòu)方面,碳水組和對(duì)照組在高碳水化合物條件下,產(chǎn)生的EPS類(lèi)腐殖質(zhì)更多.官能團(tuán)方面,高碳水底物下烷基含量減少,高蛋白底物下會(huì)產(chǎn)生更多親水性含氮官能團(tuán),高油脂下羧酸含量增加,且3個(gè)試驗(yàn)組的EPS相對(duì)疏水性較對(duì)照組均有不同程度降低.相關(guān)性分析發(fā)現(xiàn),污泥起泡能力與EPS類(lèi)指標(biāo)的相關(guān)性明顯強(qiáng)于底物類(lèi)別.污泥起泡趨勢(shì)(FP)與松散結(jié)合型EPS(<0.01,=0.979)和溶解型EPS(<0.05,=0.678)相關(guān)性最強(qiáng);EPS粘度對(duì)FP與泡沫穩(wěn)定性(FS)也均具有積極作用(<0.05),EPS表面張力增加有利于FS (<0.05).底物對(duì)起泡的作用方面,碳水化合物可能是通過(guò)提高EPS粘度和親水性,使污泥產(chǎn)生直徑更大、穩(wěn)定性更強(qiáng)的泡沫.蛋白質(zhì)利用酶作用EPS表面張力下降,由此產(chǎn)生直徑更小穩(wěn)定性更差的泡沫,起泡現(xiàn)象得以緩解.油脂使松散結(jié)合型EPS減少(<0.05,=-0.649),同時(shí)分解產(chǎn)生的LCFA改變了緊密結(jié)合型EPS粘度(<0.01,=0.788)共同提高了FP.

厭氧消化；廚余垃圾；起泡；胞外聚合物

廚余垃圾(FW)是生活垃圾的重要組成部分,具有含水率高、焚燒效率低、有機(jī)物含量豐富等特點(diǎn)[1-2],厭氧消化(AD)技術(shù)可通過(guò)厭氧微生物將其降解并生成生物質(zhì)沼氣能源,實(shí)現(xiàn)廚余垃圾減量化、資源化、無(wú)害化[3].目前AD已廣泛應(yīng)用于我國(guó)廚余垃圾的處理[4].然而,AD系統(tǒng)中常發(fā)生起泡現(xiàn)象,在廚余垃圾AD系統(tǒng)中該現(xiàn)象尤為嚴(yán)重[5].早前研究發(fā)現(xiàn)工程中很多起泡案例都是底物結(jié)構(gòu)變化造成的,其中大多情況源于高蛋白/高油脂底物的引入[6],也有少量因碳水底物引起起泡的案例[7].但底物成分與起泡的相關(guān)性難以界定,因?yàn)樘囟ǔ煞值囊氩⒉灰欢〞?huì)引起起泡.本團(tuán)隊(duì)近期在廚余垃圾AD系統(tǒng)中發(fā)現(xiàn)不同擾動(dòng)引起的起泡現(xiàn)象都與胞外聚合物(EPS)相關(guān),即EPS總量與系統(tǒng)泡沫高度具有極顯著的相關(guān)性[5],可見(jiàn)EPS可能是決定起泡現(xiàn)象的關(guān)鍵因素.

EPS是復(fù)雜高分子量聚合物的混合物質(zhì),主要由胞外多糖(PS)、胞外蛋白(PN)、腐殖質(zhì)(HU)和核酸(DNA)構(gòu)成.其中,DNA是細(xì)胞裂解后胞內(nèi)核酸釋放到胞外被EPS絮體捕捉吸附而來(lái),其余三類(lèi)組分則由微生物降解底物產(chǎn)生.如PS和PN分別是底物中碳水化合物和蛋白質(zhì)被微生物轉(zhuǎn)化為高聚糖和結(jié)構(gòu)功能性蛋白后分泌至胞外生成;HU是有機(jī)物水解產(chǎn)物.由此可見(jiàn),EPS的產(chǎn)生與底物息息相關(guān)[8],底物組分的改變很可能會(huì)影響EPS的產(chǎn)生特性,進(jìn)而引發(fā)消化液起泡性能的變化.目前已有研究探索過(guò)底物對(duì)EPS濃度與組成的影響,一方面,底物類(lèi)型變化將影響微生物代謝活動(dòng),在不同碳源或氮源下產(chǎn)生的EPS濃度和組成均有所差異[9].另一方面,底物中某一組分波動(dòng)也將影響EPS產(chǎn)生過(guò)程.例如,當(dāng)?shù)孜镏刑妓衔餃p少時(shí),PS傾向于為形成新細(xì)胞供能而不是儲(chǔ)存在EPS中,導(dǎo)致EPS PS的減少[10];底物蛋白質(zhì)過(guò)高時(shí),微生物遭到迫脅,生產(chǎn)更多EPS以抵抗不利環(huán)境[11].然而,在改變起泡性能方面,EPS可能更多的是作為生物表面活性劑影響消化液起泡特征,因此其官能團(tuán)、結(jié)構(gòu)、流體特性以及自身起泡潛能也是消化液起泡性能評(píng)價(jià)中需關(guān)注的參數(shù),但前期尚未有研究探索過(guò)底物對(duì)EPS上述特性及起泡潛能的影響,更未有研究基于此探索底物對(duì)消化液起泡性能的影響.

基于此,本文將碳水化合物、蛋白質(zhì)和油脂引入?；?模擬不同成分的廚余垃圾進(jìn)行厭氧消化,分析底物差異對(duì)EPS濃度、組成、結(jié)構(gòu)等特性的影響,并開(kāi)展EPS特性參數(shù)與消化液起泡特性參數(shù)的相關(guān)性分析,以探究不同底物組成對(duì)EPS衍生規(guī)律及其起泡性能的影響.

1 材料與方法

1.1 接種物與底物

接種污泥取自重慶市某農(nóng)村戶用沼氣池,使用前首先過(guò)10目篩以去除其中的無(wú)機(jī)大顆粒物、秸稈等,并在37℃下預(yù)孵化2周,消耗殘留于其中的底物,待觀察到產(chǎn)氣停止后完成預(yù)孵化.隨后投入廚余垃圾,在2.5gVS/(L·d)的有機(jī)負(fù)荷(OLR)及30d的水力停留時(shí)間(HRT)下馴化60d得到接種污泥.此時(shí)接種物pH值為(7.49±0.2),含固率(TS)為(9.68±0.03)%,揮發(fā)性固體含量(VS)為(5.44±0.05)%,揮發(fā)性脂肪酸(VFAs)為(125±4.15)mg/L,總氨氮(TAN)為(2690.00±7.93)mg/L,總堿度(TA)為(13622.95±1662.45)mg/L.

結(jié)合文獻(xiàn)中廚余垃圾的組分報(bào)道[14],按照蔬菜:果皮:肉:米飯的濕重比例40%:20%:5%:25%來(lái)配制?；?用粉碎機(jī)粉碎并混合均勻后分裝于密封袋中于–20℃冰箱中冷凍保存,用前1d于4℃冰箱中解凍.為避免底物劇烈波動(dòng)對(duì)反應(yīng)器造成嚴(yán)重脅迫而導(dǎo)致其運(yùn)行徹底失敗,結(jié)合前期研究中各組分對(duì)反應(yīng)器造成抑制的閾值[15],按質(zhì)量比將95%模化垃圾和5%淀粉、蛋白胨或油脂混合均勻后依次作為高碳水、高蛋白和高油脂的底物,各組底物理化指標(biāo)見(jiàn)表1.

表1 消化底物理化指標(biāo)

1.2 實(shí)驗(yàn)設(shè)計(jì)

采用模化垃圾、高碳水、高蛋白質(zhì)和高油脂廚余垃圾為底物進(jìn)行批次厭氧消化,4個(gè)實(shí)驗(yàn)組分別命名為對(duì)照組(MG)、碳水組(CG)、蛋白組(PG)和油脂組(LG),以探究不同組分對(duì)廚余垃圾厭氧消化系統(tǒng)中EPS衍生規(guī)律以及污泥起泡性能的影響.實(shí)驗(yàn)選用1L血清瓶作為反應(yīng)容器,按照0.5的食微比(S/I)向血清瓶中添加底物和接種污泥,再加入純水將TS調(diào)至15%,最后充入氮?dú)庖员ＷC血清瓶?jī)?nèi)的厭氧環(huán)境,并放置于(37±1)°C的恒溫水浴鍋中,反應(yīng)器每日手動(dòng)搖晃2次.在累積產(chǎn)甲烷量達(dá)到理論產(chǎn)甲烷量20%、40%、60%和80%時(shí)采樣測(cè)定VFAs和EPS組成成分.考慮到產(chǎn)氣量達(dá)到理論產(chǎn)氣的80%時(shí)反應(yīng)器內(nèi)的底物已被大量消化降解,產(chǎn)生并累積了豐富的EPS,而后若厭氧消化繼續(xù)進(jìn)行,微生物將逐漸分解EPS作為營(yíng)養(yǎng)來(lái)源,導(dǎo)致EPS損耗,不利于后續(xù)分析測(cè)試,因此此時(shí)停止實(shí)驗(yàn).實(shí)驗(yàn)停止后立即進(jìn)行污泥起泡潛能實(shí)驗(yàn),測(cè)定污泥粘度和表面張力,并將所剩污泥至于-4℃冷凍以備其余污泥特性指標(biāo)的測(cè)定以及EPS的分類(lèi)提取和其結(jié)構(gòu)、官能團(tuán)、起泡潛能等的測(cè)定.

1.3 理化指標(biāo)分析方法

日甲烷產(chǎn)量采用排氫氧化鈉法測(cè)定.pH值、TS、VS、TA、TAN采用標(biāo)準(zhǔn)方法測(cè)定,VFAs采用氣象色譜儀(7890A,Agilent,美國(guó))測(cè)定,底物的脂肪和蛋白質(zhì)含量分別采用索氏提取法和凱氏定氮法測(cè)定,碳水化合物用VS與前兩者差減得出.表面張力采用全自動(dòng)表面張力儀(美國(guó)Kino,A101Plus)測(cè)定,粘度采用旋轉(zhuǎn)數(shù)字式粘度計(jì)(上海尼潤(rùn)LDV-2+Pro)測(cè)定,測(cè)定時(shí)剪切速率為100s-1.EPS的提取采用熱提法[12],采取修正Lowry法測(cè)定EPS中的蛋白質(zhì)和腐殖質(zhì)[13],多糖和核酸分別采取苯酚-硫酸法和二苯胺法測(cè)定[14],相對(duì)疏水性采用DAX-8樹(shù)脂吸附法[15].EPS結(jié)構(gòu)采用熒光分光光度計(jì)(日立F-7000)測(cè)定,官能團(tuán)檢測(cè)利用傅里葉紅外變換光譜法(Nicolet iS50).起泡趨勢(shì)與泡沫穩(wěn)定性的測(cè)定采用泡騰片法[16],泡沫直徑與液膜厚度采用體視顯微鏡(SZX16)進(jìn)行觀測(cè).

1.4 統(tǒng)計(jì)與分析

單個(gè)泡沫尺寸采用等容粒徑(ep)表征,泡沫群直徑采用索特爾平均直徑(32)表示,計(jì)算公式如下[17]:

式中:1和2分別代表氣泡的最小經(jīng)線直徑和最大經(jīng)線直徑,mm.

基礎(chǔ)數(shù)據(jù)采用Excel、Orign進(jìn)行整理和繪圖,利用SPSS進(jìn)行Duncan顯著性分析.采用OMNIC進(jìn)行紅外光譜基線校正與官能團(tuán)峰面積測(cè)量.各參數(shù)間交互作用采用R Studio進(jìn)行Spearman相關(guān)性分析計(jì)算,值小于0.05時(shí)為顯著相關(guān),小于0.01時(shí)為極顯著相關(guān),利用Gephi繪制污泥與EPS參數(shù)間的網(wǎng)絡(luò)關(guān)系圖.

2 結(jié)果與討論

2.1 產(chǎn)氣情況及污泥特性

2.1.1 產(chǎn)氣情況 如圖1所示,MG組累積甲烷產(chǎn)率最低,達(dá)到理論產(chǎn)甲烷量80%所用的時(shí)間最長(zhǎng),其VFAs含量隨著AD進(jìn)程逐漸減少.CG組中淀粉被水解酸化產(chǎn)生了更多VFAs,使碳水組在消化末期VFAs仍保持在(6243.75±375.69)mg/L,酸積累現(xiàn)象較明顯.PG組在AD前期VFAs濃度高達(dá)21078mg/L,這可能是由于模化垃圾C/N僅為12.50,低于AD適宜C/N范圍(20～30)[18],底物蛋白質(zhì)的增加導(dǎo)致C/N進(jìn)一步下降,PG組氨氮濃度達(dá)到(2848.34± 176.59)mg/L,造成系統(tǒng)氨抑制,加劇了VFAs積累.然而,后期PG組VFAs濃度急劇下降,產(chǎn)甲烷速率提高,僅19d便到達(dá)反應(yīng)終點(diǎn).LG組累積甲烷產(chǎn)率遠(yuǎn)超其余組,這是由于油脂本身的甲烷產(chǎn)率遠(yuǎn)高于碳水化合物和蛋白質(zhì)[19].

圖1 厭氧消化過(guò)程累積甲烷產(chǎn)率、VFAs和TAN的變化情況

2.1.2 污泥特性 由表2可見(jiàn),起泡趨勢(shì)和泡沫穩(wěn)定性分別代表產(chǎn)生泡沫的難易程度和穩(wěn)定程度,可表征污泥起泡潛能.起泡趨勢(shì)越高污泥能夠產(chǎn)生的泡沫體積越大;泡沫穩(wěn)定性越強(qiáng)意味著泡沫越不易破裂消失,易積聚引起起泡事件.LG組起泡趨勢(shì)最高,表明油脂可以促使污泥產(chǎn)生更多泡沫,部分學(xué)者也觀察到了同樣的現(xiàn)象[5],但這也與部分研究指出的油脂可用于消泡截然相反[20].CG和MG組起泡趨勢(shì)次于LG組,但泡沫穩(wěn)定性顯著高于其余組,指示高碳水化合物也可能在維持泡沫穩(wěn)定中發(fā)揮重要作用.前期Stoyanova等[21]也發(fā)現(xiàn)甜菜厭氧消化系統(tǒng)中所產(chǎn)生的高粘性果膠是起泡的主要原因.意外的是,PG組的起泡趨勢(shì)和泡沫穩(wěn)定都處于較低狀態(tài),顯著小于其余組(<0.05).這與前期研究指出的蛋白質(zhì)加劇了AD系統(tǒng)起泡現(xiàn)象存在矛盾[22].本研究與前期部分研究的不一致性可能恰恰說(shuō)明底物本身并不是影響污泥起泡性能的關(guān)鍵.

泡沫直徑可代表泡沫的大小,液膜厚度表示相鄰氣泡間存在的流動(dòng)液相的厚度,均可作為起泡潛能的參數(shù)指標(biāo).理論上泡沫直徑越大、液膜越厚,泡沫受到的擠壓力越大,穩(wěn)定性越弱[23].由表2可以看出,CG組泡沫直徑顯著大于其余組(<0.05),液膜厚度最小,表明碳水化合物能夠改變泡沫結(jié)構(gòu),產(chǎn)生更穩(wěn)定的泡沫.PG組泡沫液膜最厚,所產(chǎn)生的擠壓力使泡沫更容易破裂.這些現(xiàn)象與起泡趨勢(shì)和泡沫穩(wěn)定性完全一致.表面張力和污泥粘度常用于表征污泥的表面特性和流動(dòng)特性,兩者均是起泡特征最常見(jiàn)的表征指標(biāo).從表2可知,各組表面張力無(wú)顯著性差異,說(shuō)明底物的小幅差異可能不足以改變污泥的表面特性.但PG組和MG組的污泥粘度顯著高于另外兩組,這與PG組最小的起泡趨勢(shì)和泡沫穩(wěn)定性存在矛盾,指示底物對(duì)起泡性能的影響可能不是通過(guò)簡(jiǎn)單的改變表面或流動(dòng)特性造成的.鑒于此,本研究進(jìn)一步解析底物成分對(duì)EPS產(chǎn)生特性的影響,以期明確底物變化引發(fā)起泡現(xiàn)象改變的作用機(jī)制.

表2 污泥起泡特征與起泡潛能參數(shù)

注:采用Duncan多重比較分析,同列標(biāo)有小寫(xiě)字母表示組間存在顯著性差異(<0.05,=3)

2.2 底物對(duì)EPS衍生規(guī)律的影響

2.2.1 EPS濃度及組分 由圖2可見(jiàn), EPS整體含量呈上升趨勢(shì),這與前期研究指出的微生物在營(yíng)養(yǎng)物質(zhì)充足的環(huán)境下會(huì)快速生長(zhǎng)繁殖代謝,不斷分泌代謝產(chǎn)物至胞外,使EPS濃度不斷增加是一致的[8].盡管整體變化趨勢(shì)相同,在不同底物下EPS總濃度顯現(xiàn)出明顯的差距.具體而言,MG組EPS總含量一直保持在最高水平,CG和PG組次之,LG組EPS總含量最低.前期不少研究指出蛋白質(zhì)和油脂引入會(huì)引起氨氮和長(zhǎng)鏈脂肪酸(LCFA)脅迫,惡化微生物生境,使EPS含量增加[5].但本研究結(jié)果并不支持這一論斷,可能是因?yàn)楸M管添加了蛋白質(zhì)或油脂,但各組S/I僅維持在0.5,由此隨底物引入的抑制物有限,使得各類(lèi)抑制的程度都不高,甚至是可逆的.以PG組為例,前期張虹等[24]就在類(lèi)似的氨氮濃度(3000mg/L)下發(fā)現(xiàn)氨脅迫可通過(guò)長(zhǎng)期運(yùn)行而緩解.相比之下,碳水化合物含量更高、底物成分更單一的MG和CG組EPS濃度更高,這也許意味著更均衡的底物成分傾向于產(chǎn)氣利用,而底物的不均衡會(huì)造成微生物代謝物即EPS的增長(zhǎng).各組產(chǎn)氣規(guī)律也支持這一推論.

圖2 厭氧消化過(guò)程中不同試驗(yàn)組的EPS的變化情況

從不同EPS形態(tài)上分析,溶解型EPS(S-EPS)濃度在消化過(guò)程中一直維持在最高水平且呈不斷上升趨勢(shì),占比最高可達(dá)(66.81±4.14)%,松散結(jié)合型EPS(LB-EPS)含量最低,緊密結(jié)合型EPS(TB-EPS)濃度處于兩者之間,且LB-EPS和TB-EPS的濃度呈上下波動(dòng)狀態(tài).各形態(tài)EPS的差異可能與其生成先后順序相關(guān).微生物分泌聚合物至胞外首先生成與細(xì)胞緊密黏附的TB-EPS,TB-EPS被新產(chǎn)生的聚合物擠至外層并逐漸演化為結(jié)構(gòu)松散的LB-EPS[25], LB-EPS在溶解、水解或剪切力作用下生成S- EPS[26].S-EPS的逐漸累積致使其濃度遠(yuǎn)高于結(jié)合型EPS(B-EPS),而B(niǎo)-EPS不斷地被分解轉(zhuǎn)化則導(dǎo)致了其濃度的上下起伏狀態(tài).以反應(yīng)末期的樣品作為代表,比較不同底物中的EPS形態(tài)差異可知,MG組S-EPS與TB-EPS濃度高于其余組,LB-EPS濃度低于PG和CG組,這表明相較于CG和PG組,MG組LB-EPS能更快向S-EPS轉(zhuǎn)化.PG組TB-EPS濃度最低,LB-EPS濃度卻最高,意味著蛋白質(zhì)可以促進(jìn)TB-EPS向LB-EPS的演化.LG組各形態(tài)EPS濃度均維持在低水平,這與其總EPS的規(guī)律相一致.

從組成成分上看,PS總含量在各組的產(chǎn)氣過(guò)程中總體呈下降趨勢(shì),這是由于碳水化合物能夠被微生物優(yōu)先降解并生成胞外PS,當(dāng)微生物快速增長(zhǎng)時(shí),PS又將被分解利用為微生物提供能量.PN、HU和DNA組分含量則在EPS中不斷增加,在營(yíng)養(yǎng)物質(zhì)豐富的條件下微生物不會(huì)將功能性與結(jié)構(gòu)性蛋白作為營(yíng)養(yǎng)物質(zhì)消化分解,因此PN在EPS中逐漸累積.HU因具有難降解性進(jìn)而在EPS中不斷的堆積.AD系統(tǒng)中細(xì)胞不斷死亡裂解并將胞內(nèi)物質(zhì)釋放至胞外,因此EPS中DNA含量也呈逐漸上升趨勢(shì).同樣以末期樣品為代表比較不同底物對(duì)EPS組分的影響可知,MG和CG組PN含量較PG組有小幅度提高,這可能與PG組含氮量高導(dǎo)致其C/N更低有關(guān).前期諸多研究也證實(shí)了在高氮條件下EPS PN含量更低[27].還有學(xué)者指出這是因?yàn)楦逤/N下微生物對(duì)碳源的利用和同化大于氮源,而低C/N下微生物傾向于利用多余的氮合成蛋白質(zhì)和核酸等物質(zhì),而非儲(chǔ)存于EPS中[10]. PG組PS總量明顯低于其余組,這是由于氮源豐富的環(huán)境下微生物的產(chǎn)酶能力提高[17,28],其中的多糖裂解酶在細(xì)胞裂解時(shí)被釋放到胞外促進(jìn)PS分解[29].LG組PN含量為4組內(nèi)最低,PS和DNA含量卻為4組中最高.DNA含量的上升可能是由于微生物總量增加,有理論認(rèn)為當(dāng)微生物快速增殖時(shí)EPS的產(chǎn)量會(huì)減少[26],因此高油脂底物可能促進(jìn)了微生物增長(zhǎng)繁殖,從而使EPS產(chǎn)量下降.同時(shí),PS的代謝過(guò)程受到抑制導(dǎo)致系統(tǒng)中PS的累積;微生物增長(zhǎng)繁殖加快氮源被用于合成新細(xì)胞導(dǎo)致EPS中PN減少.

圖3 厭氧消化末期EPS三維熒光光譜圖

進(jìn)一步分析各形態(tài)EPS組分變化可知,厭氧消化前期PS在S-EPS中占比最高可達(dá)(50.02± 4.61)%,LB-EPS和TB-EPS中PS占比遠(yuǎn)低于S-EPS,這是碳水化合物優(yōu)先降解,生成更多溶解性產(chǎn)物的結(jié)果,但當(dāng)微生物快速繁殖時(shí),PS又將作為能源物質(zhì)被分解利用,導(dǎo)致PS在不同形態(tài)EPS中含量均出現(xiàn)波動(dòng)或下降. DNA也在S-EPS中有最高含量,可達(dá)(188.44±13.22)mg/L,而TB-EPS中低至(38.89± 6.03)mg/L,這可能是因?yàn)榧?xì)胞裂解釋放的胞內(nèi)DNA等物質(zhì)將溶于S-EPS,而TB-EPS多為完整或有活性的細(xì)胞.盡管隨著消化的進(jìn)行,S-EPS PS濃度出現(xiàn)下降,但PN、HU和DNA的累積使S-EPS整體仍呈上升趨勢(shì),B-EPS的濃度波動(dòng)主要?dú)w因于其中PN和PS含量的變化.具體看不同組間的差異可知,CG組TB-EPS和S-EPS中PS含量均低于MG組,而LB-EPS PS卻高于CG組,這表明MG組LB-EPS PS能更快的轉(zhuǎn)化為S-EPS.Ye等[30]分別以乙酸、葡萄糖和淀粉作為底物探究不同碳水化合物對(duì)EPS的影響,發(fā)現(xiàn)乙酸可直接進(jìn)入三羧酸循環(huán),而淀粉和葡萄糖則需要先降解為丙酮酸,氧化形成乙酰輔酶A才能進(jìn)入三羧酸循環(huán).因此底物中淀粉的增加產(chǎn)生了更多中間代謝產(chǎn)物,直接導(dǎo)致LB-EPS中PS的累積.PG組S-EPS和LB-EPS的PN/PS分別為(2.03± 0.05)和(2.45±0.11),MG組分別為(1.52±0.03)和(1.14±0.06),與之相反的是PG組TB-EPS的PN/PS僅為(0.90±0.03),而MG組則高達(dá)(1.63±0.17). TB-EPS PN快速轉(zhuǎn)化為L(zhǎng)B-EPS和S-EPS的PN ,導(dǎo)致PG組S-EPS和LB-EPS的PN含量增加, TB-EPS PN含量驟減,這同樣是由于PG組酶含量更豐富,加速了聚合物的裂解.LG組各形態(tài)EPS的PN/PS均為最低,這也再次證明油脂導(dǎo)致PN分泌減少,與總EPS規(guī)律一致.

2.2.2 EPS的結(jié)構(gòu)特征 EPS中含有大量的熒光特性物質(zhì),圖3展示了在厭氧消化反應(yīng)產(chǎn)氣達(dá)到理論產(chǎn)甲烷量80%時(shí)采樣測(cè)得的各形態(tài)EPS三維熒光光譜圖.Peak A為產(chǎn)甲烷菌特有的輔酶F420的熒光峰x/m=(420nm/475nm),可用于衡量污泥產(chǎn)甲烷活性[31],各組都有著明顯的輔酶F420熒光峰,說(shuō)明在甲烷實(shí)際產(chǎn)量達(dá)到理論產(chǎn)量80%時(shí)污泥中的產(chǎn)甲烷菌仍有著較強(qiáng)的活性.TB-EPS輔酶F420熒光峰最弱,表明在TB-EPS中產(chǎn)甲烷菌的活性弱于S-EPS和LB-EPS,這可能是由于TB-EPS的主要成分是微生物直接分泌的高聚合物,需進(jìn)一步分解和乙酸化才能被產(chǎn)甲烷菌利用[32],因此其產(chǎn)甲烷活性低.Peak C表示類(lèi)色氨酸(x/m=275～280nm/335～345nm)[33],該峰熒光強(qiáng)度在S-EPS中最弱,目前已有研究也證實(shí)了類(lèi)色氨酸在B-EPS中的累積具有明顯優(yōu)勢(shì)[34]. Peak B為類(lèi)腐殖質(zhì)熒光(x/m=220～400nm/380～ 500nm)[35],該峰在MG組與CG組中的熒光強(qiáng)度明顯高于PG組與LG組.關(guān)于腐殖質(zhì)的形成,目前主流觀點(diǎn)認(rèn)為木質(zhì)素和氨基酸是構(gòu)成腐殖質(zhì)的核心[36],在PG和LG組中,微生物利用蛋白質(zhì)合成更多結(jié)構(gòu)與功能性蛋白,因此所生成的類(lèi)腐殖質(zhì)更少.

2.2.3 EPS的官能團(tuán)和疏水性 EPS官能團(tuán)的變化情況可進(jìn)一步明確不同底物和不同形態(tài)下EPS組成結(jié)構(gòu)的差異.EPS官能團(tuán)分布情況見(jiàn)圖4,對(duì)各官能團(tuán)峰面積進(jìn)行測(cè)量,結(jié)果如表3所示.

EPS中-OH與N-H峰面積最大,其中-OH主要來(lái)自PS,N-H主要為PN的官能團(tuán).C=C的峰面積僅次于-OH與N-H,有研究認(rèn)為C=C主要來(lái)源于PN[37].C-O-C主要對(duì)應(yīng)PS與DNA,C-H主要對(duì)應(yīng)PS.進(jìn)一步分析各形態(tài)EPS官能團(tuán)分布差異可知,S-EPS官能團(tuán)種類(lèi)更加多樣,這可能是由于VFAs等物質(zhì)多以溶解態(tài)形式存在,使得S-EPS含有大量烷基.TB-EPS疏水性官能團(tuán)的峰面積大于S-EPS和LB-EPS,表明TB-EPS擁有更多疏水性官能團(tuán),親/疏水官能團(tuán)在各形態(tài)EPS中分布存在差異性.

圖4 厭氧消化末期EPS紅外光譜圖

特征峰A和B對(duì)應(yīng)的氮?dú)滏I(N-H),特征峰A和D對(duì)應(yīng)的羥基(-OH),特征峰C和L對(duì)應(yīng)碳X氫鍵(C-H),特征峰E和F對(duì)應(yīng)的碳碳雙鍵(C=C),特征峰G和K對(duì)應(yīng)的碳氮鍵(C-N),H特征峰對(duì)應(yīng)的烷基(-CH2、-CH3),I特征峰對(duì)應(yīng)羧基(COO-),J峰對(duì)應(yīng)醚鍵(C-O-C),M特征峰對(duì)應(yīng)(-NH2)

從不同底物角度分析,圖4可觀察到MG與CG組的特征峰數(shù)量低于PG和LG組,CG組烷基數(shù)量最低,且不含有-NH2,表明在高碳水底物中EPS的組成結(jié)構(gòu)更加單一.MG組-OH與N-H峰面積在LB-EPS中高于其余組,在TB-EPS中最低,意味著底物能夠影響官能團(tuán)在各形態(tài)中的分布情況.LG組中COO-和C=C特征峰總面積遠(yuǎn)高于其余組,并主要集中在B-EPS,大量COO-和C=C可能源于底物中油脂分解產(chǎn)生的LCFA.PG組S-EPS和LB-EPS在880cm-1和705cm-1出現(xiàn)了C-N和-NH2的振動(dòng)特征峰,在高蛋白底物下EPS具有更多含氮官能團(tuán)且集中分布在S-EPS和LB-EPS,這與PG組不同形態(tài)EPS PN分布情況相符.

表3 EPS官能團(tuán)峰面積

表4 胞外聚合物起泡特征與起泡潛能參數(shù)

注:采用Duncan多重比較分析,同列標(biāo)有小寫(xiě)字母表示組間存在顯著性差異(<0.05,=3).

相對(duì)疏水性為EPS的物理特性,也可表征EPS的起泡特征,受到親/疏水性官能團(tuán)調(diào)控.不難觀察到在EPS中親水性官能團(tuán)占據(jù)主導(dǎo)地位,這與目前主流觀點(diǎn)認(rèn)為的EPS具有高度親水性一致[38].同時(shí)諸多研究也證實(shí)TB-EPS疏水性最強(qiáng)[39],這與其疏水性官能團(tuán)含量最高相對(duì)應(yīng).PS中-OH與PN的-NH對(duì)EPS的親水性貢獻(xiàn)最大,部分研究認(rèn)為PN是EPS中的疏水性物質(zhì),PS為親水性物質(zhì)[40],然而本研究認(rèn)為PS和PN整體親/疏水性受到官能團(tuán)影響.結(jié)合表4各形態(tài)EPS相對(duì)疏水性可知,EPS不同形態(tài)間相對(duì)疏水差異并不顯著.進(jìn)一步分析不同組變化情況可以發(fā)現(xiàn),CG、PG和LG相對(duì)疏水性較MG均有不同程度的降低,CG組各形態(tài)EPS相對(duì)疏水性均為最低,高碳水底物下強(qiáng)疏水性質(zhì)的烷基大大減少,降低了EPS相對(duì)疏水性.PG和LG組含有更多親水性的N-H、C-N和-NH2,LG組中親水性COO-含量最高,可能是相對(duì)疏水性降低的重要原因.底物結(jié)構(gòu)可以改變微生物代謝產(chǎn)物的化學(xué)結(jié)構(gòu),EPS官能團(tuán)的含量和類(lèi)型發(fā)生變化,進(jìn)而對(duì)EPS相對(duì)疏水性產(chǎn)生重要影響.

2.2.4 EPS的起泡特征及起泡潛能參數(shù) 提取消化末期不同反應(yīng)器中各類(lèi)EPS,測(cè)定其起泡特征及起泡潛能相關(guān)表征參數(shù)(表4),發(fā)現(xiàn)不同試驗(yàn)組,EPS粘度和表面張力均表現(xiàn)出顯著的差異性(<0.05),表明EPS粘度和表面張力對(duì)于EPS的變化非常敏感.各組中S-EPS粘度均為最高,TB-EPS粘度處于最低水平.S-EPS是溶解性代謝產(chǎn)物(SMP)中基質(zhì)代謝產(chǎn)物(UAP)部分[41],目前SMP對(duì)粘度的積極作用已受到普遍認(rèn)可[42].進(jìn)一步分析不同組差異可知,各形態(tài)EPS粘度中CG組均為4組中的最高值,PG組均為最低值,同時(shí)PG組各形態(tài)EPS表面張力也為最低.微生物分泌的糖水解酶可以降低EPS粘度[43],高蛋白底物下更高的酶含量促使EPS粘度下降.此外有學(xué)者認(rèn)為結(jié)合酶與解聚酶等同樣可以改變EPS組成結(jié)構(gòu)[26,29],影響EPS粘度和表面張力.因此蛋白質(zhì)可通過(guò)影響微生物產(chǎn)酶以改變EPS的界面和流體特性.

從表4可知,不同形態(tài)EPS起泡趨勢(shì)的差異沒(méi)有明顯規(guī)律,表明EPS形態(tài)并非影響起泡趨勢(shì)的原因.泡沫穩(wěn)定性表現(xiàn)出顯著的差異性(<0.05),S-EPS和LB-EPS泡沫穩(wěn)定性較差,而TB-EPS的泡沫具有較強(qiáng)的泡沫穩(wěn)定性,這可能是因?yàn)門(mén)B-EPS是由微生物直接分泌的高聚合物,較其他形態(tài)EPS具有更高的分子量,諸多研究均證實(shí)更高的分子量有助于泡沫的穩(wěn)定性[44].對(duì)比不同組起泡趨勢(shì)可觀察到,CG和LG組的起泡趨勢(shì)較MG均有不同程度的提高,碳水化合物增加有助于泡沫的產(chǎn)生,油脂產(chǎn)生的LCFA主要分布在B-EPS中,對(duì)B-EPS起泡趨勢(shì)的提高作用更明顯.PG組S-EPS和LB-EPS起泡趨勢(shì)為4組最低,TB-EPS卻為最高,表明蛋白質(zhì)對(duì)各形態(tài)EPS起泡趨勢(shì)的作用存在差異性.對(duì)比泡沫穩(wěn)定性可知,CG組TB-EPS泡沫穩(wěn)定性高于其余4組(<0.05),碳水化合物對(duì)提高TB-EPS泡沫穩(wěn)定性效果明顯.蛋白質(zhì)和油脂對(duì)泡沫穩(wěn)定性的影響與起泡趨勢(shì)相似,蛋白質(zhì)不利于S-EPS和LB-EPS泡沫的穩(wěn)定,但在TB-EPS中具有積極作用;油脂能夠提高B-EPS的泡沫穩(wěn)定性.EPS的泡沫直徑與液膜厚度均無(wú)明顯差異性,EPS泡沫結(jié)構(gòu)不受底物和形態(tài)影 響.

2.3 胞外聚合物與污泥起泡性能的相關(guān)性

2.3.1 EPS特性與污泥起泡性能的相關(guān)性 底物與EPS組成成分、EPS及污泥的起泡特征參數(shù)、起泡潛能參數(shù)的相關(guān)性如圖5所示.從整體上看,污泥起泡趨勢(shì)與LB-EPS起泡趨勢(shì)正相關(guān)性最強(qiáng)(<0.01,=0.979),其次為S-EPS起泡趨勢(shì)(<0.05,=0.678),污泥泡沫穩(wěn)定性與S-EPS泡沫穩(wěn)定性具有極顯著正相關(guān)關(guān)系(<0.01,=0.942),表明S-EPS和LB-EPS對(duì)污泥起泡現(xiàn)象起主導(dǎo)作用.進(jìn)一步分析可知,B-EPS粘度對(duì)污泥起泡趨勢(shì)具有積極作用, (LB-EPS<0.05,=0.600)、(TB-EPS<0.01,= 0.891),S-EPS粘度(<0.05,=0.696)和LB-EPS粘度(<0.01,=0.732)能提高污泥泡沫穩(wěn)定性.EPS表面張力能提高污泥泡沫穩(wěn)定性(S-EPS<0.05,= 0.651)、(LB-EPS<0.01,=0.711)、(TB-EPS<0.01,=0.915),而S-EPS相對(duì)疏水性升高不利于泡沫穩(wěn)定性(<0.01,=-0.714).此外,LB-EPS濃度越高對(duì)污泥起泡趨勢(shì)的抑制作用越明顯(<0.01,=-0.734).EPS對(duì)污泥起泡潛能參數(shù)影響更加明確,其中EPS粘度、表面張力及LB-EPS濃度作用明顯.

S,LB,TB分別代表EPS的形態(tài).Carbonhydrates代表碳水化合物,Protein代表蛋白質(zhì),Lipid代表油脂.FP為起泡趨勢(shì),FS為泡沫穩(wěn)定性,FD代表泡沫直徑,FT代表液膜厚度.粘度表示為Vi,表面張力表示為Su,相對(duì)疏水性表示為Hy.淺灰色曲線代表參數(shù)之間為正相關(guān)關(guān)系,黑色曲線代表參數(shù)間為負(fù)相關(guān)關(guān)系.線條粗細(xì)表征兩參數(shù)間的相關(guān)性強(qiáng)弱

從泡沫結(jié)構(gòu)分析,EPS粘度與污泥泡沫直徑呈極顯著正相關(guān)關(guān)系(S-EPS<0.01,=0.963)、(LB- EPS<0.01,=0.942)、(TB-EPS<0.01,=0.797),與液膜厚度存在負(fù)相關(guān)性(S-EPS<0.05,=-0.657)、(LB-EPS<0.01,=-0.836).EPS表面張力與污泥泡沫液膜厚度呈極顯著負(fù)相關(guān)關(guān)系(S-EPS<0.01,=-0.889)、(LB-EPS<0.01,=-0.940)、(TB-EPS<0.01,=-0.998).S-EPS相對(duì)疏水性與污泥泡沫直徑具有極顯著正相關(guān)關(guān)系(<0.01,=-0.781).由此可見(jiàn),EPS表面張力越高,S-EPS相對(duì)疏水性越低,越能夠產(chǎn)生直徑更大,液膜更薄的泡沫,進(jìn)而提高泡沫穩(wěn)定性,加劇污泥起泡現(xiàn)象.

2.3.2 底物對(duì)污泥起泡性能的影響機(jī)制解析 碳水化合物對(duì)EPS粘度具有明顯提高作用(S-EPS<0.01,=0.718)、(TB-EPS<0.05,=0.621),油脂能提高TB-EPS粘度(<0.01,=0.788).EPS表面張力僅受蛋白質(zhì)影響,兩者呈極顯著負(fù)相關(guān)關(guān)系(S-EPS<0.01,=-0.999)、(LB-EPS<0.05,=-0.993)、(TB-EPS<0.01,=-0.848).S-EPS相對(duì)疏水性在碳水化合物增加時(shí)明顯降低(<0.05,=-0.631). LB- EPS濃度僅與油脂具有顯著負(fù)相關(guān)關(guān)系(<0.05,=-0.649).當(dāng)碳水化合物增加時(shí),S-EPS粘度提高,促使污泥泡沫直徑增大,液膜厚度降低,提高了污泥泡沫穩(wěn)定性,同時(shí)TB-EPS粘度增加使污泥起泡趨勢(shì)升高.S-EPS疏水官能團(tuán)-CH2和-CH3減少,相對(duì)疏水性降低,同樣具有增加泡沫直徑、提高泡沫穩(wěn)定性的作用.因此高碳水底物通過(guò)改變EPS粘度和S-EPS相對(duì)疏水性,影響了污泥泡沫結(jié)構(gòu),對(duì)提高污泥泡沫穩(wěn)定性具有明顯作用.當(dāng)?shù)鞍踪|(zhì)含量增加時(shí),微生物產(chǎn)酶發(fā)生變化從而對(duì)EPS特性造成影響,LB-EPS粘度下降對(duì)起泡趨勢(shì)產(chǎn)生負(fù)面作用,EPS表面張力下降使液膜厚度增加,泡沫所受擠壓力增強(qiáng),導(dǎo)致其穩(wěn)定性更差.當(dāng)油脂增加時(shí),LB-EPS濃度下降,大量分布的C=C提高了TB-PS粘度,對(duì)污泥起泡趨勢(shì)產(chǎn)生了正面作用,因此油脂主要通過(guò)調(diào)節(jié)B- EPS對(duì)污泥起泡現(xiàn)象產(chǎn)生影響.

3 結(jié)論

3.1 底物對(duì)厭氧消化系統(tǒng)EPS濃度、組分、形態(tài)、結(jié)構(gòu)、官能團(tuán)等均具有直接影響.底物結(jié)構(gòu)均衡時(shí)EPS產(chǎn)量更少.碳水化合物影響中間代謝產(chǎn)物從而導(dǎo)致各形態(tài)EPS濃度變化;蛋白質(zhì)主要通過(guò)影響微生物的產(chǎn)酶情況,從而改變EPS的組成結(jié)構(gòu),油脂能夠加快微生物增殖,降低EPS PN/PS.

3.2 EPS參數(shù)不僅對(duì)底物變化敏感,還與污泥起泡潛能參數(shù)間有明確的相關(guān)性.S-EPS和LB-EPS對(duì)污泥起泡趨勢(shì)和泡沫穩(wěn)定性起到主導(dǎo)作用.EPS粘度可以提高污泥起泡趨勢(shì)和泡沫穩(wěn)定性(<0.01),EPS表面張力有助于污泥泡沫的穩(wěn)定(<0.01),S-EPS相對(duì)疏水性不利于泡沫穩(wěn)定性(<0.01,=-0.714). LB- EPS濃度越高對(duì)污泥起泡趨勢(shì)抑制作用越明顯(<0.01,=-0.734).

3.3 底物、EPS特性及污泥起泡性能的交互分析表明,碳水組降低S-EPS粘度(<0.01,=0.718)和相對(duì)疏水性(<0.05,=-0.631),使EPS結(jié)構(gòu)松散,有助于產(chǎn)生直徑更大的穩(wěn)定泡沫.蛋白質(zhì)通過(guò)影響微生物產(chǎn)酶降低了LB-EPS粘度、抑制污泥起泡現(xiàn)象,還通過(guò)降低EPS表面張力使污泥泡沫液膜增厚,泡沫穩(wěn)定性變差.油脂主要通過(guò)影響B(tài)-EPS以提高污泥的起泡趨勢(shì).

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Influence of substrate composition on the production characteristics of extracellular polymers and foaming performance in anaerobic digestion systems.

ZHOU Ying-yue, ZHANG Zhi, LI Lei*, YANG Ping-jin, WANG Xiao-ming, PENG Xu-ya

(Key Laboratory of Three Gorges Reservoir Region’s Eco-Environment, Ministry of Education, Chongqing University, Chongqing 400045, China)., 2023,43(8)：4046～4056

To clarify the influence of substrates composition on the production characteristics and foaming performance of extracellular polymer (EPS) in anaerobic digestion system, three types of food waste, namely high-carbohydrate, high-protein and high-fat, were tested in batch experiments. The EPS production characteristics (concentration, composition, structure, functional groups, etc.), and parameters characterizing the foaming properties (viscosity, surface tension and relative hydrophobicity) and potential (foaming tendency, foam stability, foam diameter and liquid film thickness) of various types of EPS and sludge, were examined in different test groups. The correlation between EPS characteristics and sludge characteristics was also analyzed. The test groups with different substrates produced EPS with different properties. Specifically, the more balanced the substrate composition, the more it tended to be used for gas production, while the high carbohydrate and control groups had significantly higher total EPS concentrations than the rest experimental groups due to the excess carbohydrate content. Carbohydrates, proteins and oils influenced EPS morphology and composition by altering intermediate metabolites, affecting enzyme production and proliferation of microorganisms, respectively. In terms of EPS structure, more humic-like substances were produced under high carbohydrate conditions in the high-carbohydrate and control groups. In terms of functional groups, the alkyl group was reduced in the high-carbohydrate group, the high-protein group produced more hydrophilic nitrogenous functional groups, and the carboxylic acid content increased in the high-fat group. In addition, the relative hydrophobicity of EPS was reduced to varying degrees in all three experimental groups compared to the control group. The correlation analysis revealed that the sludge foaming capacity was significantly more correlated with EPS-related parameters than with substrate categories. Sludge foaming tendency (FP) was strongly correlated with loosely bound EPS (< 0.01,= 0.979) and soluble EPS (< 0.05,= 0.678). EPS viscosity also had a positive effect on both FP and foam stability (FS) (< 0.05), and increased EPS surface tension favored the FS (< 0.05). In terms of the role of substrates on foaming, carbohydrates may be responsible for the production of larger diameter, more stable foams from the sludge by increasing EPS viscosity and hydrophilicity. Proteins use enzymes to reduce the surface tension of EPS, resulting in smaller diameter and less stable foams, and thus alleviated the foaming phenomenon. Fat reduced the loosely bound EPS (< 0.05,= -0.649) and its intermediate metabolite LCFA altered the viscosity of tightly bound EPS (< 0.01,= 0.788), both contributing to increase the FP.

anaerobic digestion；food waste；foaming；EPS

X705

A

1000-6923(2023)08-4046-11

周瀅月(1997-),女,重慶南岸區(qū)人,重慶大學(xué)碩士研究生,研究方向?yàn)楣腆w廢物污染控制與資源化.發(fā)表論文1篇.526511631@qq.com.

周瀅月,張 智,李 蕾,等.底物對(duì)厭氧消化系統(tǒng)EPS產(chǎn)生及起泡的影響 [J]. 2023,43(8):4046-4056.

Zhou Y Y, Zhang Z, Li L, et al. Influence of substrate composition on the production characteristics of extracellular polymers and foaming performance in anaerobic digestion systems [J]. China Environmental Science, 2023,43(8):4046-4056.

2022-12-21

國(guó)家自然科學(xué)基金資助項(xiàng)目(52170124)

* 責(zé)任作者, 副教授, lileich17@cqu.edu.cn