K2FeO4-FeCl3聯(lián)合強(qiáng)化剩余污泥厭氧消化產(chǎn)酸

田夢佳,劉 鋒,2,李 祥,2,*,馬 軍,王加恩,趙魏東

K2FeO4-FeCl3聯(lián)合強(qiáng)化剩余污泥厭氧消化產(chǎn)酸

田夢佳1,劉 鋒1,2,李 祥1,2,3*,馬 軍3,王加恩1,趙魏東1

(1.蘇州科技大學(xué)環(huán)境科學(xué)與工程學(xué)院,江蘇 蘇州 215000；2.蘇州科技大學(xué)江蘇水處理技術(shù)與材料協(xié)同創(chuàng)新中心,江蘇 蘇州 215000；3.哈爾濱工業(yè)大學(xué)城市水資源與環(huán)境國家重點(diǎn)實(shí)驗(yàn)室,黑龍江 哈爾濱 150090)

通過堿性高鐵酸鉀(K2FeO4)-FeCl3預(yù)處理聯(lián)合厭氧消化對剩余污泥進(jìn)行減量化處理,探討不同形態(tài)的鐵源及不同鐵濃度對以有機(jī)酸回收為目標(biāo)的污泥減量化過程的影響,并尋找最適鐵投加量.結(jié)果表明在預(yù)處理過程中堿性K2FeO4對污泥聚集體結(jié)構(gòu)破壞起著重要作用.與空白相比,揮發(fā)性懸浮物固體(VSS)下降了26.79%, 中值粒徑(Dx(50))下降了90%,污泥沉降比(SV30)下降了33%,污泥沉降性能變好,減量效果明顯. FeCl3在預(yù)處理過程僅具有絮凝作用.在厭氧消化過程,經(jīng)堿K2FeO4預(yù)處理后的污泥產(chǎn)酸量高于空白組.適量鐵離子的增加可加速有機(jī)酸的產(chǎn)生速率.最適鐵投加量20mg Fe/g VSS的K2FeO4及21mg Fe/g VSS的FeCl3(即TFe投加量為41mg Fe/g VSS)的產(chǎn)酸效果最好, 在消化第3d揮發(fā)性有機(jī)酸達(dá)到峰值(436.1mg COD/g VSS),是空白組峰值的4.45倍.當(dāng)經(jīng)過15d消化后,PO43--P去除率達(dá)69.8%,是僅投加K2FeO4預(yù)處理的2.03倍.此外,適量鐵的投加還有利于產(chǎn)酸菌(Actinobacteria門和Chloroflexi門)富集,促進(jìn)了厭氧消化產(chǎn)酸速率提升,而鐵投加量高于48mg Fe/g VSS反而會抑制厭氧環(huán)境中產(chǎn)酸菌的活性.

K2FeO4；FeCl3；剩余污泥；厭氧消化；揮發(fā)性脂肪酸

據(jù)統(tǒng)計(jì),截至2020年,我國剩余污泥總產(chǎn)量增至6000萬t,處理成本占總運(yùn)營成本的20%～50%[1].剩余污泥中含有大量重金屬、寄生蟲及病原體等有毒有害物質(zhì),直接排放會危害環(huán)境[2].然而剩余污泥含有豐富的蛋白質(zhì)、多糖,其含量占固體總量的40%～ 60%,可作為具有經(jīng)濟(jì)價(jià)值的資源進(jìn)行回收[3].尤其是厭氧消化階段產(chǎn)生的揮發(fā)性脂肪酸(VFAs),不僅能作為生產(chǎn)聚羥基脂肪酸(PHAs)的工業(yè)原料[4],還能作為污水脫氮除磷的優(yōu)質(zhì)碳源[5],具有較高的經(jīng)濟(jì)價(jià)值.因此,剩余污泥資源回收已成為環(huán)保界的熱點(diǎn)問題.

厭氧消化是目前最常使用,效果最穩(wěn)定的污泥處置方式之一[6].它由水解、酸化、甲烷化3個(gè)階段組成,其中水解階段是污泥消化的限制性步驟[7].因此,學(xué)者利用不同方式將污泥進(jìn)行預(yù)處理,破壞污泥聚集體結(jié)構(gòu),促使大分子難降解物質(zhì)快速破解為易于利用的小分子物質(zhì),提高水解階段的速率,從而提高后續(xù)消化速率[8].堿性高鐵酸鉀(K2FeO4)作為綠色強(qiáng)氧化劑具有殺菌絮凝等效果,不僅能氧化污泥中大分子有機(jī)物,加速其水解、促進(jìn)厭氧微生物生長代謝[9],而且其氧化后的堿性環(huán)境對產(chǎn)酸菌具有促進(jìn)作用,有利于VFAs的回收[10].李林等[11]研究發(fā)現(xiàn),當(dāng)K2FeO4投加量為500mg Fe/g VSS時(shí),發(fā)酵第5d,VFAs達(dá)到峰值322.6mg COD/g VSS,是空白的2.39倍.在李林等[11]的基礎(chǔ)上,何張煒等[8]提出用56mg Fe/g TSS的K2FeO4進(jìn)行預(yù)處理,然后接種新鮮污泥進(jìn)行消化, 5d內(nèi)VFAs最大產(chǎn)量達(dá)到343mg COD/g VSS,是空白的6.72倍. K2FeO4價(jià)格昂貴,為降低其用量,李祥[12]采用堿性K2FeO4預(yù)處理污泥,發(fā)現(xiàn)在K2FeO4投加量為20mg Fe/g SS時(shí),可達(dá)到破壞污泥聚集體而不破壞微生物細(xì)胞的氧化極限,此時(shí)消化第3dVFAs積累量達(dá)到408.21mg COD/g VSS,是堿空白的3.08倍.可見,K2FeO4預(yù)處理對污泥消化具有顯著的促進(jìn)作用.

在厭氧消化過程中, 鐵是微生物代謝酶的重要組成部分,不僅能促進(jìn)微生物種間電子直接傳遞[13],還能富集異化鐵還原菌,加速水解階段還原型輔酶Ⅰ(NADH)和H2的消耗,同時(shí)高濃度的鐵還能抑制甲烷菌產(chǎn)CH4[14].在K2FeO4氧化后形成的Fe3+也應(yīng)具有同樣效能.然而為了降低預(yù)處理過程成本,在低K2FeO4使用量的情況下,其形成的鐵是否滿足需求,需不需要額外投加鐵加速其消化尚未獲得關(guān)注.K2FeO4在氧化過程中產(chǎn)生的Fe3+會在產(chǎn)氫產(chǎn)乙酸過程中被還原為Fe2+,與PO43--P反應(yīng)生成Fe-P沉淀,以藍(lán)鐵礦的形式實(shí)現(xiàn)磷回收[15].藍(lán)鐵礦是一種非常穩(wěn)定的鐵磷化合物,不僅能作為磷肥生產(chǎn)原料,還可以作為鋰電池合成原料.因此,額外投加鐵對該過程的作用還需進(jìn)一步研究.也有研究表明,NH4+-N和Fe3+在厭氧環(huán)境下會發(fā)生厭氧鐵銨氧化反應(yīng),即將NH4+-N轉(zhuǎn)化為N2或NO3-,達(dá)到脫氮效果[16].可見,鐵在厭氧消化過程中具有諸多益處.因此,深入探討堿性K2FeO4預(yù)處理污泥后的鐵在厭氧消化階段的作用和影響對利用該技術(shù)進(jìn)行污泥減量和有機(jī)物回收具有重要意義.

由于零價(jià)鐵難溶解,Fe2+不具有異化鐵還原作用,故選擇三價(jià)鐵氧化物作為鐵源.其中FeCl3更易在水中溶解,且價(jià)格相對低廉.因此,本實(shí)驗(yàn)以VFAs回收為目標(biāo),采用堿性K2FeO4對污泥進(jìn)行氧化預(yù)處理,再額外投加不同濃度梯度的FeCl3,探究不同形態(tài)的鐵在預(yù)處理過程中作用,及其形成的鐵離子對消化過程中固液相物質(zhì)變化的影響.同時(shí)探討厭氧消化后不同鐵存在的環(huán)境下污泥中微生物群落結(jié)構(gòu)的變化.

1 材料與方法

1.1 污泥來源與性質(zhì)

實(shí)驗(yàn)污泥取自蘇州某污水處理廠A2O工藝沉淀池的剩余污泥,污泥經(jīng)簡單過篩、洗滌、沉降后置于4℃的冰箱儲存.原污泥中值粒徑(Dx(50))為1749.37μm,其余基本特性見表1.

表1 剩余污泥濃度及其上清液基本理化性質(zhì)

1.2 實(shí)驗(yàn)方法

1.2.1 預(yù)處理過程 各取1L新鮮的污水廠剩余污泥于9個(gè)有效容積為1L的燒杯中,分別標(biāo)記為R1、R2、R3、R4、R5、R6、R7、R8、R9.其中R1、R2作為對照組,R3～R9作為實(shí)驗(yàn)組.分別配置4mol/L的KOH溶液和12mol/L的HCL溶液控制pH值.20000mg/L的K2FeO4溶液及2000mg/L的FeCl3溶液控制不同鐵投加量.本課題組李祥等[12]從經(jīng)濟(jì)性角度研究發(fā)現(xiàn)最佳pH值為11,K2FeO4投加量為20mg Fe/g SS時(shí)即可達(dá)到破壞污泥聚集體而不破壞微生物細(xì)胞的氧化極限,有利于后期厭氧消化.基于此,本實(shí)驗(yàn)設(shè)計(jì)不同鐵投加量的預(yù)處理參數(shù)如表2所示.預(yù)處理過程中,先調(diào)節(jié)pH值,再將9個(gè)燒杯分別放置在六連攪拌機(jī)上以120r/min的速度攪拌.攪拌過程中同時(shí)投加對應(yīng)量的K2FeO4和FeCl3.預(yù)處理2h后各組留200mL污泥混合液用作后續(xù)樣品分析.

1.2.2 消化過程 預(yù)處理后,將9組污泥轉(zhuǎn)移至9個(gè)1L血清瓶中,放于35℃,120r/min的恒溫?fù)u床振蕩,進(jìn)行中溫厭氧消化實(shí)驗(yàn).消化后期每天用4mol/L的KOH溶液和12mol/L的HCL溶液將pH值穩(wěn)定在6[17].此后每隔24h取各組污泥上清液測定液相物質(zhì)濃度變化,連續(xù)測量15d,每組做2個(gè)平行實(shí)驗(yàn).

1.3 分析項(xiàng)目與測定方法

1.3.1 常規(guī)指標(biāo)分析 SS、VSS、SV30、pH值采用國際標(biāo)準(zhǔn)法測定,NH4+-N采用納氏試

表2 各組預(yù)處理實(shí)驗(yàn)條件及藥劑投加量

劑分光光度法,總氮(TN)采用過硫酸鉀氧化分光光度法,PO43--P、總磷(TP)采用鉬酸銨分光光度法,化學(xué)需氧量(COD)采用重鉻酸鉀分光光度法,總鐵(TFe)、Fe2+、Fe3+采用林菲羅零分光光度法測定[18]; VFAs經(jīng)預(yù)處理后采用氣相色譜測定(GC7890B, Agilent,美國)[19];對胞外聚合物(EPS)分層提取,蛋白(PN)濃度采用Lowry-Folin分光光度法測定,多糖(PS)濃度采用苯酚-硫酸分光光度法測定[20]; Dx(50)采用激光粒度儀測定(MASTERSIZER 3000, Malvern,英國).

1.3.2 X-衍射 (XRD)分析 對消化產(chǎn)生的污泥晶體沉淀采用XRD分析[21].將消化后的污泥放入冷凍干燥機(jī)(FreeZone 2.5L, Labconco,美國)干燥24h,研磨后過200目篩子過篩,采用X衍射儀(Ultima IV, Rigaku,日本)檢測.掃描范圍為5～90°,掃描速度為1°/min.測得的數(shù)據(jù)經(jīng)Jade軟件分析,Origin作圖,對污泥晶體定性.

1.3.3 微生物分析 微生物采用高通量測序進(jìn)行分析[22].采用細(xì)菌 16S V4-V5 區(qū)通用引物,對消化前后的污泥樣品,進(jìn)行3個(gè)重復(fù)擴(kuò)增,同一樣品擴(kuò)增產(chǎn)物進(jìn)行混合;前端引物515F(5-GTGCCAGCM- GCCGCGG-3),后端引物907R (5-CCGTCAATTC- MTTTRAG TTT-3);對PCR產(chǎn)物進(jìn)行回收純化和Qubit定量,采用Illumina公司的Miseq PE300/ NovaSeq PE 250進(jìn)行測序.

2 結(jié)果與討論

2.1 預(yù)處理后污泥特性變化

2.1.1 預(yù)處理后污泥液相變化 經(jīng)不同預(yù)處理后污泥中NH4+-N、TN、PO43--P、TP、VFAs、COD、TFe的釋放量具有較大差異(圖1).與空白(R1)相比,堿預(yù)處理(R2)具有輕微破壞污泥絮體,釋放有機(jī)物的作用.加FeCl3后(R3)效果與R2相似,表明單獨(dú)Fe3+的投加對污泥預(yù)處理沒有影響.堿K2FeO4氧化預(yù)處理后(R4～R9),污泥絮體分解,大分子及難降解性有機(jī)物快速水解為小分子有機(jī)物并釋放到液相,COD加速上升.當(dāng)再額外投加FeCl3(R5～R9)后, COD上升更加明顯.NH4+-N主要來源于污泥中吸附氨和胞內(nèi)有機(jī)氮氧化后的釋放.堿K2FeO4預(yù)處理后,污泥胞內(nèi)NH4+-N、TN均出現(xiàn)釋放,但是此時(shí)液相大部分氮為有機(jī)氮.這進(jìn)一步說明此時(shí)污泥僅實(shí)現(xiàn)破碎,未出現(xiàn)大量有機(jī)物氧化.

與僅投加K2FeO4預(yù)處理(R4)相比,額外增加FeCl3的組分(R5～R9)的TN濃度明顯下降.推測是更多的鐵起到了絮凝作用將大分子有機(jī)物吸附并轉(zhuǎn)移到固相[23].與空白相比,堿K2FeO4預(yù)處理后,胞內(nèi)PO43--P、TP大量釋放,同時(shí)隨著鐵投加量的增加,PO43--P濃度下降,說明部分鐵起到了絮凝和除磷作用[8].預(yù)處理后,污泥有機(jī)物仍以大分子存在,胞內(nèi)VFAs少量釋放.因預(yù)處理鐵投加量不同,各組液相TFe濃度隨鐵投加量增加而增加.

圖1 預(yù)處理后污泥液相物質(zhì)變化

圖2 預(yù)處理后污泥固相性質(zhì)

2.1.2 預(yù)處理后污泥固相變化 原污泥(R1)和單獨(dú)堿預(yù)處理后的污泥(R2)的SS分別為9.82和9.24g/L,VSS分別為7.13和6.32g/L,可見堿預(yù)處理僅使污泥濃度小幅下降.這主要是OH-會破壞微生物細(xì)胞結(jié)構(gòu),釋放污泥胞內(nèi)物質(zhì)[24].堿FeCl3預(yù)處理后R3的SS、VSS與R2相比變化不大.而經(jīng)K2FeO4和不同濃度的FeCl3共同預(yù)處理后(R4～R9)污泥濃度SS較空白(R1)均出現(xiàn)約20%的下降,VSS較空白(R1)均出現(xiàn)約25%的下降.這說明K2FeO4對污泥聚集體發(fā)生了氧化作用,額外投加的FeCl3對其影響不大.SV30代表著污泥沉降性能.隨著預(yù)處理后污泥濃度的下降,SV30也呈下降趨勢.與R2(54%)相比,R3 (43%)明顯更低,表明鐵通過絮凝和混凝作用提高了污泥沉降性能[25].與R4(41%)相比, R3(43%)投加了相同量的鐵,但SV30更高,主要是缺少了K2FeO4對污泥的氧化效果.與R1相比,隨著鐵投加量的增多, R4～R9沉降性能逐步加強(qiáng),推測是K2FeO4氧化效果和鐵絮凝效果共同作用[15].與R1相比,R2的Dx(50)減小了23%,R3的Dx(50)減小了45%,在相同的堿預(yù)處理的前提下R3的鐵離子作為橋聯(lián)電子,促進(jìn)污泥顆粒化[26].K2FeO4氧化預(yù)處理后的污泥Dx(50)顯著下降,且污泥粒徑隨鐵的增加略微增大.

2.2 消化過程中污泥特性變化

2.2.1 消化過程中污泥液相變化 不同預(yù)處理后污泥在消化過程中的COD、NH4+-N、TN、PO43--P、TP釋放量具有較大差異(圖3).

COD反映了液相有機(jī)物濃度,厭氧環(huán)境下污泥水解,消化15d內(nèi)各組COD均先增大后減小.與堿空白(R2)相比,堿FeCl3(R3)在消化過程中的COD濃度略有上升,約是R2的1.5倍.這說明鐵在厭氧消化體系里促進(jìn)微生物對EPS的分泌,對消化過程COD釋放起到了一定的促進(jìn)作用[27].而堿K2FeO4預(yù)處理后的COD均是R2的7倍以上,說明堿性K2FeO4破壞污泥聚集體后,對COD釋放起到了加速作用.隨著FeCl3投加量由7mg Fe/gVSS增加至21mg Fe/g VSS,R7在消化第4d,COD峰值達(dá)到各組最大為8990mg/L.而隨著FeCl3的投加量由21mg Fe/g VSS增加至35mg Fe/g VSS, COD峰值開始下降.這說明厭氧消化系統(tǒng)最適總鐵含量為41mg Fe/g VSS,過量的鐵抑制了微生物活性[28]或與有機(jī)物如腐殖酸等的絡(luò)合作用[29].

消化15d內(nèi),隨著有機(jī)物水解,NH4+-N濃度不斷上升.各組釋放速率均快于R1,其中R7的釋放速率最快,是R1的2.48倍.與R2相比, R3僅額外投加一定濃度的FeCl3后NH4+-N濃度略微增加.而K2FeO4預(yù)處理(R4)明顯加速了NH4+-N釋放.魯丁南等[30]發(fā)現(xiàn)NH4+-N能抑制甲烷菌產(chǎn)甲烷,降低VFAs的消耗,有利于后期產(chǎn)酸.同時(shí),與R4相比, R5、R6、R7的NH4+-N釋放速率更快,R7、R8變慢.這表明適量的鐵能加快有機(jī)物消化,而高濃度的鐵抑制消化過程.TN與NH4+-N變化相同,均隨時(shí)間增加而增大,且主要是無機(jī)氮.這表明大量有機(jī)氮被快速水解為無機(jī)氮[23].

消化15d內(nèi),因剩余污泥在厭氧條件下積累了大量還原性物質(zhì),K2FeO4及FeCl3產(chǎn)生的Fe3+通過鐵還原菌作用被迅速還原為Fe2+,Fe3+最終下降到0mg/L.消化反應(yīng)初期, Fe2+含量較低,此時(shí)僅少量 PO43--P被固定.隨著Fe3+轉(zhuǎn)化為Fe2+,各組Fe2+開始增加.隨著厭氧反應(yīng)的進(jìn)行,液相中的Fe2+不斷和剛釋放的PO43--P結(jié)合,生成Fe-P沉淀,導(dǎo)致鐵濃度和磷濃度同步降低.污泥厭氧消化15d后,R3～R6的Fe2+基本消耗完,因R7～R9的初始鐵濃度較高,消化15d后還存在一些Fe2+.

圖3 不同預(yù)處理樣品消化過程中液相污染物隨時(shí)間變化情況

消化初期, R1、R2的PO43--P濃度迅速升高并保持不變,這是因?yàn)閰捬醐h(huán)境下聚磷菌(PAO)持續(xù)釋磷直到釋盡[31],且堿預(yù)處理加速污泥里無極磷的釋放[32].與R1、R2不同,R3～R9的PO43--P先上升后下降,消化前期PAO釋磷與液相鐵磷反應(yīng)同時(shí)進(jìn)行,且釋磷速度更快導(dǎo)致PO43--P濃度積累.K2FeO4和鐵預(yù)處理加速污泥PO43--P釋放,后期PO43--P釋放完畢后和液相中的鐵反應(yīng)生成Fe-P沉淀,將液相中的磷元素轉(zhuǎn)移至固相.隨著鐵濃度增加PO43--P峰值出現(xiàn)更早且濃度更低.消化15d后,僅R9的PO43--P濃度不斷降到0mg/L,R3～R6在反應(yīng)后期保持穩(wěn)定后不再下降,表明液相中的Fe2+濃度過低不足以消耗PO43--P.而R7,R8分別還存在一定濃度的PO43--P和Fe2+,推測后續(xù)PO43--P繼續(xù)和Fe2+生成Fe-P沉淀.TP的變化與PO43--P相似,PAO厭氧釋磷,空白組R1,R2的TP不斷上升,R4～R9因投加鐵均先增加后減少,而R3投加FeCl3后TP并未下降.說明R3釋磷速度始終快于Fe-P沉淀的形成.

VFAs是由產(chǎn)酸菌利用污泥中的蛋白質(zhì)、多糖等有機(jī)物水解酸化而來,經(jīng)不同預(yù)處理后污泥在消化過程中液相VFAs變化及6種酸占比不同(圖4).VFAs均呈先上升后下降趨勢,隨著預(yù)處理不同VFAs峰值濃度及出現(xiàn)時(shí)間不同.與R1(98.7mg COD/g VSS)相比,堿預(yù)處理(R2)提高了VFAs的峰值,有利于VFAs的釋放.堿FeCl3(R3)預(yù)處理提高峰值的同時(shí)加快了產(chǎn)酸速率.推測異化鐵還原過程優(yōu)于產(chǎn)甲烷過程,有利于產(chǎn)酸[33].經(jīng)K2FeO4和FeCl3預(yù)處理后的污泥(R4～R9)產(chǎn)酸效果更優(yōu).其中R7最先在第3d達(dá)到最高峰值436.1mg COD/g VSS,前3dVFAs釋放速率是R1的18.76倍.說明堿性K2FeO4強(qiáng)氧化提升了有機(jī)酸的產(chǎn)生量,而鐵離子的作用加速了有機(jī)酸的產(chǎn)生速率,使其濃度快速達(dá)到峰值.R4～R7隨著額外投加FeCl3濃度的增加,VFAs峰值增加,而R7～R9隨著FeCl3濃度的增加,VFAs峰值出現(xiàn)下降.說明過高的鐵濃度抑制了產(chǎn)酸菌活性[28].

圖4 消化過程中VFAs變化及消化峰值VFAs 6種酸占比情況

在污泥厭氧發(fā)酵過程中,VFAs主要由乙酸、丙酸、正丁酸、異丁酸、正戊酸和異戊酸6種酸組成[34],其中乙酸是應(yīng)用最廣泛、經(jīng)濟(jì)價(jià)值最高的有機(jī)酸[35].VFAs峰值時(shí)加鐵組分(R3～R9)乙酸占比均超55%,最高達(dá)64.3%(R7).這推測鐵通過降低氧化還原電位(ORP)優(yōu)化水解產(chǎn)酸的類型,使產(chǎn)物更多的為乙酸[36].

2.2.2 消化過程中污泥固相變化 厭氧消化15d后,各組污泥固相發(fā)生了變化(表3).與消化前的R1相比,消化15d后,R1～R9的SS和VSS均出現(xiàn)下降.加FeCl3后(R3)污泥濃度低于R2,且同鐵濃度以K2FeO4形式投加(R4)的污泥濃度低于以FeCl3形式投加(R3).與R4相比,額外投加FeCl3(R5～R9)后,污泥濃度均有所下降,且額外鐵投加量為7mg Fe/g VSS時(shí), SS、VSS下降最多,污泥減量效果最好.隨著鐵投加量繼續(xù)增大(R7～R9),污泥減量效果變差,推測過量的鐵抑制了微生物活性,從而影響了污泥中有機(jī)物水解進(jìn)程[37].與R1相比,消化15d后,9組SV30均下降,且預(yù)處理組比空白組SV30更低,污泥沉降性能更好.隨著厭氧消化反應(yīng)的進(jìn)行,被氧化破壞的污泥顆粒重新聚集,污泥中值粒徑x(50)變化趨于一致.一方面是因?yàn)槲⑸锉旧砭哂姓承?另一方面鐵離子具有絮凝性,促進(jìn)微生物和污泥絮體聚集[38].

表3 消化過程中污泥固相性質(zhì)

2.3 污泥XRD分析

圖5 消化15d后不同鐵濃度下污泥XRD圖譜

隨著消化反應(yīng)的進(jìn)行,剩余污泥水解產(chǎn)酸,環(huán)境pH值不斷下降到酸性,更有利于PO43--P釋放[39].研究表明,較高的pH值會由于Fe(OH)2的競爭而抑制藍(lán)鐵礦(Fe3(PO4)2·8H2O)的形成,常規(guī)污泥厭氧消化回收藍(lán)鐵礦的最優(yōu)pH值在6.0～8.0之間[16].當(dāng)pH=6.0時(shí),鐵元素主要以固相形式存在,其中有54.7%的沉淀物以藍(lán)鐵礦的形式存在[40].此外,當(dāng)pH<6.5時(shí)產(chǎn)甲烷菌活性遭到抑制,更利于有機(jī)酸積累[41].故反應(yīng)后期統(tǒng)一調(diào)節(jié)環(huán)境pH值為6,促進(jìn)VFAs積累及Fe-P沉淀的生成. 厭氧消化15d后污泥XRD分析,空白組(R1, R2)無明顯峰型,存在的小的雜峰推測為其它雜質(zhì)物質(zhì)(圖5).而投加鐵的污泥組分(R3～R9)污泥樣品均在 11.05°、13.05°、18.05°、21.75°、27.86°及29.91°等角度出現(xiàn)衍射峰,證實(shí)污泥中出現(xiàn)的Fe-P沉淀與藍(lán)鐵礦標(biāo)準(zhǔn)圖譜(PDF#83-2453)一致,為藍(lán)鐵礦晶體.但由于鐵投加量不同,圖譜的峰型有差異,與R3僅投加FeCl3及R4僅投加K2FeO4相比, R5～R9的XRD圖相對雜峰更少.這說明TFe投加量在27～55mg Fe/g VSS時(shí)生成的藍(lán)鐵礦更穩(wěn)定.

2.4 污泥微生物群落分析

2.4.1 門水平微生物分析 消化3d后,不同組門水平上微生物豐度不同(圖6(a)).變形菌門(Proteobacteria)、擬桿菌門(Bacteroidetes)、綠彎菌門(Chloroflexi)、放線菌門(Actinobacteria)和厚壁菌門(Firmicutes)5種門是傳統(tǒng)厭氧消化主要微生物[42].與R1相比,消化3d后R2～R9的5種微生物總占比分別提高了8%, 14%, 16%, 18%, 19%, 21%, 18%和7%.這說明預(yù)處理提高了污泥消化能力.而隨著鐵投加量的增加,R9的消化微生物占比(81.8%)比堿空白(82.3%)更低,這進(jìn)一步預(yù)示著過高的鐵濃度反而抑制了消化反應(yīng).其中, Actinobacteria、Firmicutes、Bacteroidetes和Chloroflexi這4種門在水解酸化階段起重要作用,與VFAs的生成息息相關(guān)[43]. Actinobacteria為難降解有機(jī)物的主要分解者[44], Firmicutes和Bacteroidetes都具有水解纖維素和蛋白質(zhì)的功能,使污泥中的有機(jī)物初步分解成為可溶性物質(zhì)[45],而Chloroflexi則能為微生物聚體提供三維骨架,直接水解簡單有機(jī)物產(chǎn)生VFAs[46].消化3d后,R2～R9的4種微生物總占比分別由R1的41%提高到了51%,65%,66%,72%,78%,77%,69%,65%.適量的鐵增加了水解酸化菌占比,而過高濃度的鐵(如R9,65%),其水解酸化菌占比與R3(65%)、R4(66%)相似,并沒有增加相應(yīng)微生物占比,因此沒有必要投加過量的鐵.此外,Firmicutes還是常見的鐵還原優(yōu)勢菌門[47],部分菌群可在厭氧過程中利用有機(jī)物作為電子供體進(jìn)行Fe3+的還原[48],加鐵后R3～R9的Firmicutes門占比均有提高.這說明鐵預(yù)處理能富集鐵還原菌促進(jìn)鐵的還原及Fe-P沉淀的生成,且隨鐵投加量增加鐵還原菌占比增加.

圖6 厭氧消化3d后微生物在門和屬水平群落豐度結(jié)構(gòu)

2.4.2 屬水平微生物分析 不同預(yù)處理組分消化3d后屬水平上微生物變化不同(圖6(b)).蛋白質(zhì)菌屬(沉降桿菌屬)屠場桿菌屬()和理研菌屬()是屬水平上主要的水解酸化功能菌.以蛋白質(zhì)作為底物,產(chǎn)生乙酸、丙酸等揮發(fā)性脂肪酸[49].是厭氧消化系統(tǒng)中具有協(xié)同代謝能力的主要細(xì)菌之一,不僅能代謝有機(jī)底物產(chǎn)乙酸,還具有直接電子傳遞的作用[50].隨著預(yù)處理鐵濃度增加,R3～R9的4種酸總占比變化與門水平水解酸化菌相似(10%,16%,21%,26%, 27%,34%,34%,26%,22%),先增大后減小.這進(jìn)一步表明適量的鐵能富集產(chǎn)酸菌,但過量的鐵會減少產(chǎn)酸菌.氫孢菌屬()為產(chǎn)氫菌,能夠利用多種糖類產(chǎn)H2[51].加鐵后微生物占比下降,會導(dǎo)致反應(yīng)過程中H2產(chǎn)量的減少.不動桿菌屬()在自然環(huán)境中廣泛存在,能夠?qū)崿F(xiàn)對Fe3+等氧化態(tài)金屬元素的還原[52].與類似,地桿菌屬()是自然環(huán)境中廣泛存在的一種異化鐵還原菌,在分解有機(jī)物的同時(shí),可以將其產(chǎn)生的電子傳遞給胞外不溶性三價(jià)鐵氧化物[53].梭菌屬()屬于Firmicutes門,作為Fe3+還原類菌,在有機(jī)物降解中同樣發(fā)揮著重要作用,能產(chǎn)生乙酸、丁酸等產(chǎn)物[48].鐵預(yù)處理后(R3～R9)3種屬分別從空白的0%不斷提高到10%,鐵還原菌占比提高,有利于還原Fe3+及生成Fe-P沉淀.

投加適量濃度的鐵不僅能提高相關(guān)水解酸化菌占比,促進(jìn)VFAs生成速率,還能富集大量鐵還原菌,促進(jìn)鐵還原與Fe-P沉淀的生成,但鐵濃度過高反而會抑制消化過程中相關(guān)微生物的活性.因此,在利用堿K2FeO4預(yù)處理后適量補(bǔ)充廉價(jià)的鐵對強(qiáng)化消化過程產(chǎn)酸具有重要意義.

3 結(jié)論

3.1 堿K2FeO4預(yù)處理后污泥VSS下降了26.79%, Dx(50)下降了90%.預(yù)處理過程中K2FeO4對污泥聚集體具有很好地氧化破碎作用.而經(jīng)堿性FeCl3預(yù)處理后的污泥液相有機(jī)物釋放與堿空白相似,FeCl3在預(yù)處理過程中未起到氧化作用;FeCl3在預(yù)處理過程僅具有絮凝作用.

3.2 堿性K2FeO4預(yù)處理增加了厭氧消化過程VFAs的產(chǎn)量,而鐵離子促進(jìn)VFAs的產(chǎn)生速率提升.預(yù)處理最適鐵投加量為20mg Fe/gVSS的K2FeO4及21mg Fe/gVSS的FeCl3(即TFe投加量為41mg Fe/gVSS).此時(shí)VFAs峰值第3就出現(xiàn),且濃度最高(436.1mg COD/g VSS),此時(shí)鐵離子對K2FeO4促進(jìn)厭氧產(chǎn)酸起協(xié)同作用.但過量的鐵會降低VFAs峰值濃度,減慢VFAs生成速率.

3.3 堿性K2FeO4預(yù)處理后的鐵和FeCl3在厭氧環(huán)境下轉(zhuǎn)化為Fe2+,并與PO43--P以藍(lán)鐵礦結(jié)晶體形式沉淀.在最適鐵投加量下,15d內(nèi)PO43--P去除率達(dá)69.8%.

3.4 投加適量的鐵不僅能提高相關(guān)水解酸化菌豐度,促進(jìn)VFAs生成,還能富集大量鐵還原菌促進(jìn)鐵還原與Fe-P沉淀的生成.TFe投加量為41mg Fe/gVSS時(shí),消化3d后,4種主要的產(chǎn)酸菌門(Actinobacteria、Firmicutes、Bacteroidetes和 Chloroflexi)占比由41%提高到77%;3種主要的鐵還原菌()占比由0%提高到10%.但過量的鐵不僅會浪費(fèi)資源,還會降低主要產(chǎn)酸菌門豐度(69%),抑制消化過程產(chǎn)酸.

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Study on enhanced anaerobic digestion of excess sludge with K2FeO4and FeCl3for organic acid production.

TIANMeng-jia1, LIU Feng1,2, LI Xiang1,2,3*, MA Jun3, WANG Jia-en1, ZHAO Wei-dong1

(1.School of Environmental Science and Engineering, Suzhou University of Science and Technology, Suzhou 215009, China；2.Jiangsu Collaborative Innovation Center of Technology and Material of Water Treatment, Suzhou University of Science and Technology, Suzhou 215009, China；3.State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China)., 2023,43(8)：4089～4098

The excess sludge was reduced by alkaline potassium ferrate (K2FeO4)-FeCl3pretreatment combined with anaerobic digestion. Under the goal of efficient recovery of organic acid, the effects of different forms of iron sources and iron concentrations on sludge reduction were discussed, and the optimal iron concentration was found. The results showed that alkaline K2FeO4played an important role in the destruction of sludge aggregates during pretreatment. Compared with the blank group, Volatile Suspended Solids (VSS) decreased by 26.79%, median diameter (Dx(50)) decreased by 90%, and sludge settling velocity of 30minutes (SV30) decreased by 33%. The sludge settling performance became better and the sludge reduction effect was obvious. FeCl3only had flocculation during pretreatment process. During anaerobic digestion, the acid production of sludge pretreated with alkaline K2FeO4was higher than that of blank group, and the increase of iron ions promoted the rapid production of organic acids. When the dosage of K2FeO4was 20mg Fe/g VSS and FeCl3was 21mg Fe/g VSS (that is, TFE dosage of 41mg Fe/g VSS), the sludge showed the maximum volatile organic acids (VFAs) accumulation. On the 3thof digestion, the VFAs reached 436.1mg COD/g VSS, which was 4.45times that of the blank group. Under this iron concentration, after 15days of digestion, the removal rate of PO43--P reached 69.8%, which was 2.03 times that of only K2FeO4pretreatment. In addition, adding appropriate iron could increase acid-producing bacteria (Actinobacteria phylum and Chloroflexi phylum), which will promote acid production, while the iron dosage was higher than 48mg Fe/g VSS, the activity of main acid-producing bacteria in anaerobic environment would be inhibited.

K2FeO4；FeCl3；excess sludge；anaerobic digestion；volatile fatty acids (VFAs)

X705

A

1000-6923(2023)08-4089-10

田夢佳(1996-),女,河南駐馬店人,碩士研究生,主要從事污泥減量及污泥資源化利用.發(fā)表論文1篇.1050094028@qq.com.

田夢佳,劉 鋒,李 祥,等.K2FeO4-FeCl3聯(lián)合強(qiáng)化剩余污泥厭氧消化產(chǎn)酸 [J]. 中國環(huán)境科學(xué), 2023,43(8):4089-4098.

TianM J, Liu F, Li X, et al. Study on enhanced anaerobic digestion of excess sludge with K2FeO4and FeCl3for organic acid production [J]. China Environmental Science, 2023,43(8):4089-4098.

2023-01-18

國家自然科學(xué)基金資助項(xiàng)目(51938010);江蘇省實(shí)踐創(chuàng)新計(jì)劃項(xiàng)目(SJCX21-1406)

* 責(zé)任作者, 副教授, anammox@126.com