pH值調(diào)控檸檬酸污泥厭氧發(fā)酵產(chǎn)酸及碳源潛力研究

孫東霞,周子安,馮志合,胡修玉,祁光霞,董黎明*

pH值調(diào)控檸檬酸污泥厭氧發(fā)酵產(chǎn)酸及碳源潛力研究

孫東霞1,周子安1,馮志合2,胡修玉2,祁光霞1,董黎明1*

(1.北京工商大學(xué),中國(guó)輕工業(yè)清潔生產(chǎn)和資源綜合利用重點(diǎn)實(shí)驗(yàn)室,國(guó)家環(huán)境保護(hù)食品鏈污染防治重點(diǎn)實(shí)驗(yàn)室,北京 100048；2.中國(guó)生物發(fā)酵產(chǎn)業(yè)協(xié)會(huì),北京 100083)

以檸檬酸廢水厭氧顆粒污泥為接種物,在不同pH值調(diào)控條件下開(kāi)展檸檬酸生產(chǎn)廢水剩余活性污泥厭氧發(fā)酵產(chǎn)酸研究.通過(guò)對(duì)發(fā)酵液揮發(fā)性脂肪酸(VFAs)、有機(jī)質(zhì)、氮磷和污泥脫水性能的分析,探討了檸檬酸污泥厭氧產(chǎn)酸機(jī)制.結(jié)果表明,pH310的堿性條件更有利于有機(jī)質(zhì)的溶出從而促進(jìn)VFAs的產(chǎn)生.三維熒光光譜分析發(fā)現(xiàn)在恒定pH值下腐殖酸(HA)和富里酸(FA)會(huì)大量溶出降低VFAs的產(chǎn)量.初始pH=10是檸檬酸污泥厭氧產(chǎn)酸的最佳pH值,發(fā)酵4d的VFAs濃度最高達(dá)(6681.47±126.82) mg COD/L,是文獻(xiàn)報(bào)道中市政污泥產(chǎn)酸量的近2倍,其中乙酸占比49.8%,發(fā)酵后產(chǎn)酸功能菌Chloroflexi、Bacteroidota的相對(duì)豐度分別由初始的9.52%、10.87%增至16.84%、14.39%,污泥歸一化毛細(xì)吸水時(shí)間(CST)為(11.34±0.27) s×L/g,脫水性能良好,發(fā)酵液TP濃度為(20.45±0.33) mg/L.研究表明,利用檸檬酸剩余活性污泥堿性厭氧發(fā)酵產(chǎn)酸作為污水處理過(guò)程中的外加碳源具有較大潛力.

pH值調(diào)控；檸檬酸污泥；堿性厭氧發(fā)酵；揮發(fā)性脂肪酸；污泥脫水性能

城市污水處理廠通常采用生物處理技術(shù)去除廢水中的營(yíng)養(yǎng)物質(zhì)以緩解水體富營(yíng)養(yǎng)化,然而目前國(guó)內(nèi)污水進(jìn)水碳源不足極大地限制了氮、磷的去除效率,因此在廢水處理過(guò)程中通常使用甲醇、乙醇和乙酸作為有機(jī)碳源,但化學(xué)藥品的添加不僅增加了運(yùn)營(yíng)成本也會(huì)造成二次污染[1-2].污水好氧處理的剩余活性污泥富含豐富的有機(jī)物,通過(guò)厭氧發(fā)酵可產(chǎn)生揮發(fā)性脂肪酸(VFAs),將其作為污水處理的有機(jī)碳源,可實(shí)現(xiàn)對(duì)剩余活性污泥的資源化利用[3].

影響污泥厭氧發(fā)酵的因素包括溫度、pH值、微生物、水力停留時(shí)間等[4],其中pH值不僅影響污泥水解和產(chǎn)物組成,還影響微生物群落變化,是污泥厭氧發(fā)酵產(chǎn)生VFAs的最重要因素之一[5].然而產(chǎn)酸發(fā)酵細(xì)菌對(duì)pH值的適應(yīng)性較強(qiáng)[6-7],有研究表明酸性啟動(dòng)(pH=6.0)VFAs最高累積質(zhì)量濃度為1683.5mg/L,比堿性啟動(dòng)模式(pH=10.0)提高了37.5%[8].但堿性條件有利于促進(jìn)有機(jī)物水解,提供高濃度的可溶性底物(SCOD),增加VFAs的產(chǎn)生[5].而通過(guò)不斷調(diào)控pH值,堿性發(fā)酵(pH=10.0)VFAs產(chǎn)量為2901.33mg COD/L,是酸性發(fā)酵的2.7倍[9].由于污泥種類以及實(shí)驗(yàn)條件的不同,產(chǎn)酸條件所需的最優(yōu)pH值不同,但高濃度的SCOD更有利于VFAs的產(chǎn)生結(jié)論一致[4].有研究者[10-12]已建立完整的污水污泥堿性厭氧發(fā)酵產(chǎn)VFAs和作為外部碳源提高污水廠的生物脫氮除磷的工藝系統(tǒng),長(zhǎng)期運(yùn)行結(jié)果表明,該系統(tǒng)可實(shí)現(xiàn)污泥減量和碳源回收,減少約54%的污泥量,平均VFAs產(chǎn)量達(dá)到261.32mg COD/g VSS,該系統(tǒng)凈利潤(rùn)為9.12美元/m3,比污泥厭氧消化產(chǎn)沼氣(3.71美元/m3)有更大的經(jīng)濟(jì)優(yōu)勢(shì).但目前基本是針對(duì)市政污泥產(chǎn)酸條件與機(jī)制的研究,鮮見(jiàn)對(duì)工業(yè)污泥的發(fā)酵產(chǎn)酸研究.而我國(guó)是檸檬酸生產(chǎn)大國(guó),占世界檸檬酸產(chǎn)量的70%以上,產(chǎn)量每年增長(zhǎng)7%,其主要利用玉米進(jìn)行發(fā)酵生產(chǎn),產(chǎn)生的廢水可生化性高,經(jīng)厭氧處理產(chǎn)生的顆粒污泥是重要的微生物源,再經(jīng)好氧處理會(huì)產(chǎn)生大量有機(jī)質(zhì)含量較高的剩余活性污泥[13],其處理處置成本約占污水處理廠運(yùn)營(yíng)成本的60%[14].相比市政污泥,其有機(jī)質(zhì)含量較高,可為發(fā)酵提供充足的底物,因此本研究通過(guò)分析不同pH值對(duì)檸檬酸剩余活性污泥厭氧發(fā)酵產(chǎn)VFAs的影響,探討發(fā)酵過(guò)程VFAs積累的機(jī)制與發(fā)酵液作污水處理過(guò)程中外源碳源的潛力,為發(fā)酵工業(yè)剩余活性污泥的資源化利用提供參考.

1 材料與方法

1.1 實(shí)驗(yàn)材料

選取山東省某檸檬酸生產(chǎn)企業(yè)好氧生化處理的剩余污泥和厭氧顆粒污泥,其中厭氧顆粒污泥作為剩余污泥厭氧發(fā)酵的初始菌種,經(jīng)自然沉降棄去上清液,保存在4℃冰箱中備用.實(shí)驗(yàn)時(shí)剩余污泥過(guò)60目網(wǎng)篩去除沙礫,其含固率(TS)為(4.24±0.64)%,有機(jī)物含量(VS)為(50.10±1.21)%,溶解性化學(xué)總需氧量(SCOD)為(687.50±51.03) mg/L,可溶性蛋白質(zhì)(PN)為(253.33±11.11) mg/L,可溶性多糖(PS)為(19.13±2.37) mg/L,總磷(TP)為(23.17±1.31) mg/L,氨氮(NH3-N)為(294.91±12.02) mg/L.

1.2 實(shí)驗(yàn)方法

采用序批式實(shí)驗(yàn),將剩余污泥和厭氧顆粒污泥按照質(zhì)量比TS=4:1的比例混合均勻,測(cè)得pH= (7.12±0.22),以此為空白對(duì)照組(Control).將300g混合污泥加入500mL厭氧發(fā)酵瓶中,通入氮?dú)?保證厭氧密閉環(huán)境,在(36±2)℃,(120±10) r/min的水浴搖床中進(jìn)行厭氧發(fā)酵.

使用6mol/L的HCl或NaOH,將發(fā)酵罐中混合污泥分別調(diào)節(jié)pH值為5、6、8、9、10、11、12,此后不再調(diào)控pH值,記為初始pH值調(diào)控組(pH),同時(shí)對(duì)產(chǎn)生VFAs的實(shí)驗(yàn)組再次進(jìn)行維持整個(gè)發(fā)酵過(guò)程恒定pH值的實(shí)驗(yàn),記為恒定pH值調(diào)控組(C-pH).

所有發(fā)酵罐均設(shè)置平行實(shí)驗(yàn),在VFAs連續(xù)下降3d后停止實(shí)驗(yàn).取調(diào)節(jié)pH值后的混合污泥樣品記為0d,間隔24h取樣,樣品經(jīng)9000r/min離心10min,上清液過(guò)0.45μm濾膜后用于指標(biāo)測(cè)定,沉淀測(cè)定微生物.

1.3 指標(biāo)測(cè)定

參照《城市污水處理廠污泥檢測(cè)方法》(CTJ221-2005)測(cè)定樣品的TS和VS,TP和NH3-N分別采用鉬酸銨分光光度法和納氏試劑分光光度法測(cè)定[15],用Lowry-Folin法和蒽酮-硫酸法分別測(cè)定PN和PS[16],毛細(xì)吸水時(shí)間(CST)使用CST測(cè)定儀(TR04-304M, Triton,英國(guó))測(cè)定,結(jié)果歸一化[9],見(jiàn)式(1).

式中:CST為歸一化結(jié)果,s·L/g; CST為儀器測(cè)定的毛細(xì)吸水時(shí)間,s; TS為污泥含固率,g/L.

Zeta電位使用激光Zeta粒度分析儀(Zetasizer Nano ZS,馬爾文,英國(guó))測(cè)定;溶解性總有機(jī)碳(DOC)使用DOC分析儀(VarioEL III, Elementar,德國(guó))測(cè)定;使用哈克旋轉(zhuǎn)流變儀(HAAKE MARS III, Thermo Scientific,美國(guó)),選擇速率與黏度模型,CC25DIN Ti轉(zhuǎn)子,剪切率10～300s-1,在25℃下對(duì)污泥樣品流變特性進(jìn)行測(cè)定[17];VFAs采用氣相色譜儀(GC-2014,島津,日本)檢測(cè),換算關(guān)系為: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].樣品經(jīng)處理后(UV254<0.3),使用三維熒光光譜儀(Spectrofluorometer FS5,愛(ài)丁堡,英國(guó))在x/m= 220～550nm/240～600nm,間隔5nm,設(shè)置中扣除空白散射,測(cè)其三維熒光(3D-EEM)譜圖,結(jié)果采用MATLAB 2018b進(jìn)行平行因子(PARAFAC)分析[18-19].微生物由上海美吉生物公司測(cè)定,樣品經(jīng)DNA提取后,使用引物(338F和806R)進(jìn)行PCR擴(kuò)增后,對(duì)16S rDNA的V3-V4可變區(qū)基因進(jìn)行測(cè)序分析[1].所有數(shù)據(jù)使用origin 2018作圖.樣品進(jìn)行了3次平行測(cè)定.

2 結(jié)果分析

2.1 初始pH值對(duì)污泥發(fā)酵性能的影響

SCOD是反映污泥水解和酸化程度的重要指標(biāo),如圖1(a)所示,在0d時(shí)酸堿的加入都促進(jìn)污泥水解,但pH310的條件下SCOD濃度更高,污泥水解的效果更佳.根據(jù)厭氧發(fā)酵的主要產(chǎn)物甲烷和VFAs的變化情況(圖1(b)),初始pH=5～9的條件有利于甲烷的產(chǎn)生,其中Control組累計(jì)最大甲烷產(chǎn)量為(40.25±2.86)mL/g VS,其他條件下甲烷產(chǎn)量降低甚至完全消失,是產(chǎn)甲烷菌的活性受到抑制或喪失所致,甲烷的產(chǎn)生消耗有機(jī)物,與發(fā)酵后SCOD下降結(jié)果一致.初始pH=10～12的實(shí)驗(yàn)組厭氧發(fā)酵后(8d)產(chǎn)生了大量VFAs,導(dǎo)致SCOD濃度增加,其中pH=10的實(shí)驗(yàn)組在8d時(shí)VFAs含量最高為(3149.45±202.53) mg COD/L.因此初始pH=10～12有利于檸檬酸剩余污泥厭氧發(fā)酵VFAs的積累,這與Wu等[2]和Ma等[20]對(duì)不同pH值下污泥厭氧發(fā)酵得出堿性條件更利于污泥厭氧產(chǎn)VFAs的結(jié)論相一致.

圖1 不同初始pH值厭氧發(fā)酵前后SCOD濃度與發(fā)酵過(guò)程累積CH4產(chǎn)量和第8d的VFAs濃度變化

2.2 恒定和初始?jí)A性條件對(duì)檸檬酸污泥厭氧產(chǎn)酸的影響

2.2.1 恒定和初始?jí)A性條件對(duì)VFAs產(chǎn)量的影響 對(duì)產(chǎn)生VFAs的實(shí)驗(yàn)組(pH=10、11、12)進(jìn)行維持恒定pH值的厭氧發(fā)酵實(shí)驗(yàn),如圖2所示.不同條件總VFAs的最大濃度不同,其順序?yàn)?pH=10 ((6681.47± 126.82)mg COD/L)>pH=11((5964.85±524.72) mg COD/L)> C-pH=11((4902.85±596.79)mg COD/L) >C-pH=10 ((4427.41±111.48)mg COD/L)>C-pH=12 ((3321.91±461.07)mg COD/L)>pH=12((2746.54± 55.82) mg COD/ L),pH值為12的兩組VFAs濃度低,是因?yàn)榇蠖鄶?shù)產(chǎn)酸菌不易在pH312條件下存活[3].此外到達(dá)總VFAs最大濃度的時(shí)間亦不同,pH=10時(shí)間最短僅為4d,其次是pH=11和C-pH=10為5d,時(shí)間延長(zhǎng)VFAs出現(xiàn)下降趨勢(shì),可能是底物不足或被產(chǎn)酸菌利用的結(jié)果[2].因此pH=10是檸檬酸剩余污泥厭氧產(chǎn)酸較佳的條件,約為相似條件下的市政污泥厭氧發(fā)酵產(chǎn)VFAs濃度的2倍(最大VFAs濃度為2500～ 3000mg COD/L,時(shí)間為5～6d)[9,21].不同條件對(duì)VFAs的組成有不同影響,其中乙酸占VFAs總量的45%～ 66%,決定VFAs變化總趨勢(shì),因?yàn)榇蠖鄶?shù)微生物都能產(chǎn)生乙酸[22],同時(shí)它是污水處理過(guò)程中受歡迎的碳源,含量越高碳源利用潛力越大[22].其次是異戊酸和丙酸占比為8%～25%,異丁酸、正丁酸和正戊酸由于分解較快[23]僅占2%～13%.

2.2.2 堿性厭氧產(chǎn)酸發(fā)酵過(guò)程中有機(jī)質(zhì)的變化 如圖3所示,SCOD與DOC變化趨勢(shì)基本與PN、PS和VFAs濃度變化相一致.在0d時(shí)PN、PS的水解程度與堿性pH值呈正相關(guān),但PN的水解濃度是Control組的2.66～4.90倍,高于PS(1.10～2.29倍),堿性條件更有利于PN的析出[2,20].隨著發(fā)酵時(shí)間的延長(zhǎng)Control組PN濃度基本不變,而PS有明顯的先升后降趨勢(shì),可能是中性條件下更有利于產(chǎn)甲烷菌對(duì)PS的水解和利用.相反在恒定pH值的厭氧發(fā)酵過(guò)程中PN、PS濃度逐漸升高,是堿性環(huán)境促進(jìn)污泥絮體的破壞所致[3,24],在5～8d時(shí)PN快速下降,而此時(shí)VFAs濃度沒(méi)有明顯上升,可能是因?yàn)閺?qiáng)堿與氨基、羧基反應(yīng)生成鹽導(dǎo)致蛋白質(zhì)變性.pH=11、12的實(shí)驗(yàn)組發(fā)酵過(guò)程中PN和PS呈不明顯上升趨勢(shì),在pH=10的實(shí)驗(yàn)組PN和PS變化趨勢(shì)顯著,0～4d時(shí)VFAs濃度迅速上升,此時(shí)PN濃度下降而PS上升,可能是產(chǎn)酸菌對(duì)PN的利用率高于PN的水解率和PS的利用率,在4～5d時(shí)PN和PS可能達(dá)到此條件下最大水解程度,時(shí)間延長(zhǎng)產(chǎn)酸底物不斷減少VFAs濃度下降.

圖2 不同堿性條件對(duì)VFAs產(chǎn)量及組成的影響

圖3 堿性厭氧產(chǎn)酸發(fā)酵過(guò)程中SCOD、DOC、PN和PS的變化

2.2.3 熒光組分的變化 通過(guò)PARAFAC分析對(duì)上清液3D-EEM光譜進(jìn)行拆分發(fā)現(xiàn),3種主要熒光物質(zhì)[25](圖4),分別為色氨酸類蛋白質(zhì)(TPN):x/m= 275nm/360nm,腐殖酸類物質(zhì)(HA):x/m=360(415) nm/470nm和富里酸類物質(zhì)(FA):x/m=320nm/ 400nm,同時(shí)得到最大熒光max圖5,通常TPN、FA和HA都被認(rèn)為是難生物降解的化合物[25].根據(jù)圖5可知,TPN的max值最高是主要的熒光物質(zhì),且在初始pH值實(shí)驗(yàn)組的變化趨勢(shì)與PN濃度變化幾乎一致,因此TPN可以被檸檬酸污泥堿性厭氧發(fā)酵產(chǎn)酸過(guò)程利用.而在恒定pH值的實(shí)驗(yàn)組尤其是C-pH=11和C-pH=12實(shí)驗(yàn)組的TPN變化趨勢(shì)與PN濃度變化不同,可能因?yàn)榘l(fā)酵過(guò)程中HA和FA的大量溶出對(duì)PN測(cè)定產(chǎn)生干擾,同時(shí)FA和HA已被證實(shí)無(wú)法通過(guò)微生物分解產(chǎn)生VFAs[26],因此FA和HA的大量溶出會(huì)降低產(chǎn)酸效率[27-28],與VFAs的濃度降低相符.

圖4 堿性厭氧發(fā)酵液的熒光組分

圖5 堿性厭氧發(fā)酵液熒光組分Fmax的變化

2.2.4 厭氧發(fā)酵前后微生物群落的變化 由pH=10發(fā)酵前后(0和8d)微生物豐度和多樣性的變化結(jié)果(表1)可知,厭氧發(fā)酵后OUT指數(shù)、ACE指數(shù)、Chao指數(shù)和Shannon指數(shù)都明顯降低,表明堿性厭氧發(fā)酵產(chǎn)酸的菌群多樣性明顯低于初始污泥的多樣性.這一現(xiàn)象在屬(圖6b)水平上尤為明顯,如菌屬消失,以及vadinHA17和SBR1031*等菌屬的大量增加.其中是蛋白質(zhì)降解厭氧菌[29],其消失可能與發(fā)酵后其PN含量降低有關(guān);而HA17*菌屬[30]能夠利用葡萄糖產(chǎn)生乙酸鹽、丙酸鹽和氫氣[30]屬于產(chǎn)乙酸菌,有利于增加乙酸含量;菌屬[31]具有長(zhǎng)鏈脂肪酸(C4及以上)降解功能,降低丁酸、戊酸等長(zhǎng)鏈脂肪酸在總VFAs中占比;SBR1031*菌屬可代謝NH3-N[32],與發(fā)酵罐中的NH3-N含量變化有關(guān).

表1 微生物群落豐度和多樣性變化

注:OTU是操作分類單位,Coverage反應(yīng)測(cè)序深度指數(shù),數(shù)值高于0.99表明測(cè)序深度足夠,結(jié)果可靠;ACE和Chao指數(shù)代表微生物豐度,數(shù)值越高豐度越高;Shannon和Simpson指數(shù)為香濃指數(shù)和辛普森指數(shù),代表微生物多樣性,Shannon指數(shù)越高,多樣性越高,Simpson指數(shù)則相反.

圖6 門、屬水平上的物種相對(duì)豐度

*表示沒(méi)有明確的分類信息或分類名稱

基于樣品OTUs的注釋結(jié)果,門水平和屬水平微生物相對(duì)豐度如圖6所示,主要優(yōu)勢(shì)菌門為Firmicutes, Actinobacteriota, Bacteroidota, Proteobacteria, Chloroflexi,屬于污泥堿性發(fā)酵的優(yōu)勢(shì)菌群[33],但檸檬酸污泥堿性厭氧發(fā)酵過(guò)程改變了初始環(huán)境特征菌群的相對(duì)豐度.Firmicutes具有厚厚的細(xì)胞壁能夠在不同的污泥處理(例如加熱、堿化、酸化)中存活,含有多種產(chǎn)乙酸菌,可將多種VFAs代謝成乙酸、H2和CO2[34-35], Actinobacteriota中的細(xì)菌能降解多糖生成單糖和VFAs[36],然而發(fā)酵后Firmicutes和Actinobacteriota相對(duì)豐度分別由28.37%、21.58%降至10.15%、14.56%,可能是兩者菌門中不適于堿性厭氧環(huán)境下的劣勢(shì)菌種被淘汰所致[37-38].而Chloroflexi和Bacteroidota相對(duì)豐度分別由9.52%、10.87%增加至16.84%、14.39%,這是因?yàn)镃hloroflexi菌門的微生物主要代謝碳水化合物,促進(jìn)VFAs底物降解[39], Bacteroidota的微生物可分泌多種細(xì)胞外水解酶,將葡萄糖、纖維二糖、淀粉等物質(zhì)轉(zhuǎn)化為乙酸、丁酸、異戊酸、H2和CO2[37,40],這兩種菌門中多種微生物適應(yīng)堿性厭氧環(huán)境,有助于促進(jìn)有機(jī)物的水解和VFAs的產(chǎn)生.同時(shí)發(fā)現(xiàn)部分非優(yōu)勢(shì)菌種Desulfobacterota、Thermotogota等相對(duì)豐度增加,據(jù)報(bào)道,Thermotogae菌群可以降解復(fù)雜的有機(jī)物,如木糖和纖維素等[41]. Desulfobacterota的部分菌群在厭氧條件下還原硫酸鹽,競(jìng)爭(zhēng)NO2-電子供體,抑制NO2-還原產(chǎn)生N2O的反硝化過(guò)程[42],與氮含量變化有關(guān).

2.3 堿性厭氧產(chǎn)酸發(fā)酵液外源碳源利用潛力分析

2.3.1 污泥脫水性能分析 如圖7(a)所示,堿處理和厭氧發(fā)酵導(dǎo)致CST增大,是因?yàn)镺H-與金屬鹽離子聚集、發(fā)酵過(guò)程釋放的磷形成的化合物[43]以及上清液有機(jī)質(zhì)含量增加等因素使大量水分被聚合,導(dǎo)致污泥過(guò)濾性能變差,這與Chen等[9]得出的酸性厭氧發(fā)酵可提高污泥的脫水能力結(jié)論一致.但OH-與鹽離子聚集以及VFAs產(chǎn)生的H+中和負(fù)電離子會(huì)降低Zeta電位絕對(duì)值(圖7b),甚至在pH=10的實(shí)驗(yàn)組出現(xiàn)正電位,為維持強(qiáng)堿性環(huán)境的C-pH=11和C-pH=12實(shí)驗(yàn)組,不斷引入OH-,與鹽離子和H+全部反應(yīng)后仍有大量OH-游離,導(dǎo)致Zeta電位絕對(duì)值進(jìn)一步增大,干擾污泥絮體聚集進(jìn)一步增加CST.因而pH=10的實(shí)驗(yàn)組脫水性能相對(duì)較好.由圖7(c)可以看出,堿性厭氧發(fā)酵可以降低污泥表觀黏度,因?yàn)樵诎l(fā)酵過(guò)程中大分子有機(jī)質(zhì)被降解為小分子物質(zhì),網(wǎng)絡(luò)結(jié)構(gòu)被破壞內(nèi)部阻力降低[44],這與Zhang等[45]和Zhang等[46]對(duì)市政污泥厭氧發(fā)酵對(duì)污泥脫水性能的影響研究得出的堿性厭氧發(fā)酵可以增強(qiáng)污泥流動(dòng)性,降低污泥表觀黏度結(jié)論一致.因此可以考慮從流變方面對(duì)發(fā)酵后污泥進(jìn)行脫水研究.

2.3.2 發(fā)酵液N、P的變化 使用厭氧產(chǎn)酸發(fā)酵液作為碳源時(shí),還需要考慮發(fā)酵液中氮磷含量的影響.由圖8(a)可知,pH值不影響NH3-N的變化(0d),在發(fā)酵過(guò)程PN水解生成的氨基酸分子被厭氧菌利用時(shí)會(huì)生成游離態(tài)的NH3-N[1],使發(fā)酵后NH3-N濃度升高,因而代謝NH3-N的SBR1031*菌屬相對(duì)豐度升高.由于厭氧發(fā)酵無(wú)法完成硝化反硝化作用[47],且抑制反硝化過(guò)程相關(guān)的Desulfobacterota菌門相對(duì)豐度升高,使得NH3-N含量不斷升高.然而從圖8(b)可知pH值會(huì)影響TP的變化(0d),當(dāng)pH311時(shí)TP濃度明顯升高,因?yàn)闊o(wú)機(jī)磷酸鹽類在pH311時(shí)不能穩(wěn)定存在[43],而這種高pH值導(dǎo)致的磷的釋放是可逆[48],在初始pH值實(shí)驗(yàn)組中由于VFAs的產(chǎn)生降低pH值使無(wú)機(jī)磷酸鹽類重新沉淀,因此初始pH值的實(shí)驗(yàn)組在8d時(shí)的TP濃度小于恒定pH實(shí)驗(yàn)組,尤其是pH=10的處理組TP幾乎無(wú)明顯變化.

總體而言,污泥堿性厭氧發(fā)酵在增大污泥脫水難度的同時(shí)使得大量氨氮和可溶性磷釋放到發(fā)酵上清液中,已有研究表明[9,49],同時(shí)添加KH2PO4和MgCl2,可以在去除N、P的同時(shí)(NH3-N去除率>75%,TP去除率>80%)提高發(fā)酵后污泥的脫水能力,但藥劑添加會(huì)導(dǎo)致成本增加,影響污泥發(fā)酵產(chǎn)酸再利用的經(jīng)濟(jì)性.

圖8 厭氧發(fā)酵前后發(fā)酵液NH3-N、TP的變化

3 結(jié)論

3.1 在初始pH=5～9的條件有利于檸檬酸剩余污泥厭氧發(fā)酵產(chǎn)甲烷,其中Control組累計(jì)甲烷產(chǎn)量最大為(40.25±2.86) mL/g VS,在初始和恒定pH310的堿性條件下,厭氧發(fā)酵易產(chǎn)生VFAs,同時(shí)更易于PN、PS的釋放.在恒定pH值實(shí)驗(yàn)組,難分解的HA和FA會(huì)大量溶出不利于VFAs的產(chǎn)生.

3.2 初始pH=10的條件是檸檬酸剩余污泥厭氧產(chǎn)酸最佳的條件,發(fā)酵后產(chǎn)酸功能菌Chloroflexi、Bacteroidota的相對(duì)豐度分別由初始的9.52%、10.87%增至16.84%、14.39%,發(fā)酵4d的VFAs= (6681.47±126.82) mg COD/L濃度最高,是文獻(xiàn)報(bào)道中市政污泥產(chǎn)酸量的近2倍,此時(shí)乙酸為總VFAs的49.8%,有很大的碳源利用潛力.

3.3 堿性厭氧發(fā)酵過(guò)程中鹽離子的聚集和有機(jī)質(zhì)的增加惡化污泥脫水性能,同時(shí)還增加N、P等物質(zhì)的溶出,不利于發(fā)酵液作為外源碳源,因此針對(duì)發(fā)酵液用于污水處理的外部碳源,需要進(jìn)一步了解污泥堿性發(fā)酵過(guò)程的SCOD、N、P變化規(guī)律和發(fā)酵液的性質(zhì),以便發(fā)現(xiàn)在提高N、P去除率的同時(shí),還能改善發(fā)酵后污泥脫水性能的成本低、操作簡(jiǎn)單的方法.

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Effect of pH on acid production by anaerobic fermentation of citric acid sludge and carbon source potential of fermentation broth.

SUN Dong-xia1, ZHOU Zi-an1, FENG Zhi-he2, HU Xiu-yu2, QI Guang-xia1, DONG Li-ming1*

(1.Key Laboratory of Cleaner Production and Integrated Resource Utilization of China National Light Industry, State Environmental Protection Key Laboratory of Food Chain Pollution Control, Beijing Technology and Business University, Beijing 100048, China；2.China Biotech Fermentation Industry Association, Beijing 100083, China)., 2022,42(11)：5198～5207

The research of acid production by anaerobic fermentation with different pH control conditions was carried out for the treatment of waste activated sludge from citric acid wastewater, using anaerobic granular sludge of citric acid wastewater as inoculum. The mechanism of anaerobic acid production of citric acid sludge was evaluated by the analysis of volatile fatty acids (VFAs), organic matter, nitrogen and phosphorus contents and sludge dewatering performance. The results showed that the alkaline conditions with pH310 were more conducive to the dissolution of organic matter to promote the production of VFAs. It was obvious that humic acid (HA) and fulvic acid (FA) at constant pH conditions would be dissolved in large quantities with Three-dimensional Excitation-Emission-Matrix Spectra analysis, thus reducing the yield of VFAs. The initial pH=10 was the optimum pH value for anaerobic acid production of citric acid sludge, and the VFAs concentration of (6681.47±126.82) mg COD/L for 4 days was the highest, which was nearly 2 times that of municipal sludge acid production reported in the literature, among which acetic acid was 49.8%. After fermentation, the relative abundances of acid-producing functional bacteria Chloroflexi and Bacteroidota increased from initial 9.52% and 10.87% to 16.84% and 14.39%, respectively. The normalized capillary suction time (CST) value of the final sludge was (11.34±0.27) s·L/g with good dewatering performance, and the TP concentration of fermentation broth was (20.45±0.33) mg/L. Studies have shown that the alkaline anaerobic fermentation of citric acid waste activated sludge to produce acid fermentation broth has a good development potential as an exogenous carbon source in the sewage treatment process.

pH value；citric acid waste activated sludge；alkaline anaerobic fermentation；volatile fatty acids；sludge dewatering performance

X705

A

1000-6923(2022)11-5198-10

孫東霞(1996-),女,山東德州人,北京工商大學(xué)碩士研究生,主要從事清潔生產(chǎn)與資源綜合利用研究.發(fā)表論文1篇.

2022-04-06

國(guó)家自然科學(xué)基金資助項(xiàng)目(41861124004)

* 責(zé)任作者, 教授, donglm@btbu.edu.cn