張 淼,彭永臻,張建華,王 聰,王淑瑩,曾 薇

?

進(jìn)水C/N對(duì)A2/O-BCO工藝反硝化除磷特性的影響

張 淼,彭永臻*,張建華,王 聰,王淑瑩,曾 薇

(北京工業(yè)大學(xué)北京市污水脫氮除磷處理與過程控制工程技術(shù)研究中心,北京市水質(zhì)科學(xué)與水環(huán)境恢復(fù)工程重點(diǎn)實(shí)驗(yàn)室,北京 100124)

采用厭氧/缺氧/好氧與生物接觸氧化工藝組成的雙污泥系統(tǒng)(A2/O-BCO)處理實(shí)際生活污水.通過投加乙酸鈉調(diào)節(jié)進(jìn)水碳氮比(C/N=2.44~8.85),考察了系統(tǒng)的反硝化除磷特性.試驗(yàn)結(jié)果表明:進(jìn)水有機(jī)物主要是通過改變硝化性能(即缺氧段反硝化負(fù)荷)以及聚-β-羥基鏈烷酸脂(PHA)的貯存和利用,進(jìn)而影響系統(tǒng)的脫氮除磷效果.當(dāng)進(jìn)水C/N為4~5時(shí),COD、TN和PO43--P去除率分別達(dá)到88%,80%和96%,實(shí)現(xiàn)了有機(jī)物、氮和磷的同步高效去除.碳平衡分析表明,A2/O反應(yīng)器去除的COD占去除總量的71.86%~77.28%,BCO反應(yīng)器去除的COD僅占2%~12%,碳源的高效利用是A2/O-BCO工藝在低C/N條件下實(shí)現(xiàn)深度脫氮除磷的重要原因.此外,通過進(jìn)水C/N與曝氣量、硝化液回流比、厭/缺氧反應(yīng)時(shí)間等相關(guān)性的分析,提出了系統(tǒng)的優(yōu)化運(yùn)行策略.

A2/O-BCO；C/N；反硝化除磷；生活污水；優(yōu)化運(yùn)行

隨著水體富營(yíng)養(yǎng)化的日益嚴(yán)重以及排放標(biāo)準(zhǔn)的不斷提高,厭氧/缺氧/好氧(A2/O)工藝,分段進(jìn)水,多級(jí)厭氧/缺氧(A/O)串聯(lián),開普敦大學(xué)研發(fā)的UCT工藝以及改良UCT(MUCT)等多種脫氮除磷工藝應(yīng)運(yùn)而生[1-4].傳統(tǒng)脫氮除磷(BNR)工藝中存在異養(yǎng)菌、反硝化菌和聚磷菌對(duì)進(jìn)水碳源的競(jìng)爭(zhēng),在單污泥系統(tǒng)中聚磷菌和硝化菌還存在泥齡上的矛盾,使得污水處理中氮、磷很難同時(shí)穩(wěn)定達(dá)標(biāo)[5].

在實(shí)際工程應(yīng)用中,由于水質(zhì)波動(dòng)大、技術(shù)管理復(fù)雜、處理效果不穩(wěn)定等問題,使得有機(jī)碳源成為影響B(tài)NR工藝同步脫氮除磷效果的關(guān)鍵因素[6].有研究表明制革廢水實(shí)現(xiàn)完全反硝化,進(jìn)

水碳氮比(C/N)需要大于8[7],而實(shí)際生活污水的進(jìn)水C/N往往低于6[8].絕大多數(shù)污水處理廠主要考慮外加碳源(如甲醇、乙醇、乙酸和葡萄糖等)或者將富含可生物降解有機(jī)物的工業(yè)廢水投加到城市污水中,MacDonal等調(diào)查研究表明:采用甲醇作為外加碳源的處理成本占污水廠運(yùn)行管理成本的70%之多[9],大大增加了污水處理的運(yùn)行費(fèi)用.因此,對(duì)于低C/N(C/N<5)污水,傳統(tǒng)工藝往往不能滿足排放標(biāo)準(zhǔn)的要求,再加上低溫問題[10],許多污水處理廠的升級(jí)改造面臨著嚴(yán)峻的挑戰(zhàn)[11].

反硝化聚磷菌(DNPAOs)的發(fā)現(xiàn)使生物除磷和同步反硝化脫氮成為可能,為解決上述問題提供了新的思路[12].在厭/缺氧交替運(yùn)行條件下,DNPAOs與傳統(tǒng)聚磷菌(PAOs)有相似的代謝機(jī)理[13],可節(jié)省30%的曝氣能耗,同時(shí)減少50%的碳源和污泥產(chǎn)量[14],具有很好的應(yīng)用前景.反硝化除磷技術(shù)主要分為單污泥工藝和雙污泥工藝兩大類,雖然單污泥系統(tǒng)(如UCT、MUCT等工藝)也存在反硝化除磷現(xiàn)象,但研究表明DEPHANOX、A2N、A2/O-BAF、A2/O-生物接觸氧化(BCO)等雙污泥系統(tǒng)更有優(yōu)勢(shì)[15-18].筆者前期對(duì)改良型雙污泥A2/O-BCO工藝的容積分配比,硝化液回流比,基質(zhì)轉(zhuǎn)化利用規(guī)律以及微生物特性進(jìn)行了考察[19-21],與A2/O-BAF相比[22-23],該工藝是一種脫氮除磷效果穩(wěn)定,無污泥膨脹之虞,運(yùn)行管理方便并深度節(jié)能降耗的反硝化除磷系統(tǒng),一體化廊道式的構(gòu)造可用于新建污水廠及舊污水廠的改造.

A2/O-BCO系統(tǒng)通過長(zhǎng)短泥齡分離,實(shí)現(xiàn)“一碳兩用”,緩解了不同微生物菌群對(duì)碳源的競(jìng)爭(zhēng),但作為一種新工藝,碳源對(duì)系統(tǒng)長(zhǎng)期運(yùn)行的影響尚不明確.此外,由于環(huán)境因素,研究條件以及污泥特性的差異,碳源類型及碳源濃度對(duì)脫氮除磷影響的結(jié)論也并不統(tǒng)一,且目前對(duì)反硝化除磷的研究較少.Lee、葛士建等[24-25]研究發(fā)現(xiàn),采用乙酸或乙酸鈉作為外加碳源,具有較高的反硝化速率和較低的污泥產(chǎn)率,當(dāng)溫度為10℃,C/N為2:1時(shí)即可實(shí)現(xiàn)完全反硝化[26],遠(yuǎn)遠(yuǎn)高于甲醇、乙醇、葡萄糖等的比反硝化速率.Oehmen證實(shí)與乙酸相比,丙酸是實(shí)現(xiàn)PAOs富集的優(yōu)質(zhì)碳源[27],但是相比單純以丙酸作為碳源,少量乙酸的存在能提高60%的丙酸利用率[28].鑒于上述分析,綜合考慮選擇乙酸鈉作為外加碳源.

本文以實(shí)際生活污水為處理對(duì)象,在前期試驗(yàn)研究的基礎(chǔ)上,通過長(zhǎng)達(dá)137d的連續(xù)運(yùn)行,考察了投加乙酸鈉條件下的進(jìn)水C/N 對(duì)系統(tǒng)反硝化除磷性能的影響;同時(shí)考察了運(yùn)行策略,為A2/O - BCO系統(tǒng)的推廣應(yīng)用以及反硝化除磷性能優(yōu)化提供參考.

1 材料與方法

1.1 試驗(yàn)裝置

A2/O - BCO系統(tǒng)由原水水箱、A2/O反應(yīng)器、中間沉淀池(MS)、BCO反應(yīng)器以及沉淀區(qū)順序連接組成,其中主體反應(yīng)器材質(zhì)均為有機(jī)玻璃,工藝流程如圖1所示.A2/O反應(yīng)器有效容積42L,進(jìn)水流量126L/d,均分為7個(gè)格室,分別為厭氧區(qū)(An),缺氧區(qū)(A1~ A5)和好氧區(qū)(O),即容積分配比為1:5:1.厭氧區(qū)和缺氧區(qū)設(shè)有攪拌,好氧區(qū)停留時(shí)間較短,溶解氧1.0 ~ 1.5mg/L,進(jìn)一步吸磷的同時(shí)用于吹脫反硝化過程產(chǎn)生的氮?dú)?便于后續(xù)的泥水分離.反應(yīng)器平均污泥濃度為1800 ~ 2000mg/L,污泥齡(SRT)為(12±2)d. A2/O反應(yīng)器出水進(jìn)入中間沉淀池實(shí)現(xiàn)泥水分離,中間沉淀池有效容積15L,沉淀污泥回流到A2/O反應(yīng)器的厭氧區(qū),污泥回流比= 100%

中間沉淀池的上清液經(jīng)中間提升泵進(jìn)入BCO反應(yīng)器,實(shí)現(xiàn)氨氮的氧化,硝化液經(jīng)沉淀區(qū)回流到A2/O反應(yīng)器的缺氧區(qū),硝化液回流比= 400%.BCO反應(yīng)器3格串聯(lián)(記為N1,N2,N3),總有效容積18L,內(nèi)設(shè)聚丙烯懸浮填料,填料形狀為圓柱體(尺寸×: 25mm×10mm),填充率為45%;三格溶解氧為3.5~4.5mg/L,總氣量為0.18~ 0.20m3/h,生物膜平均生物量為1000~1200mg/L.

1.2 試驗(yàn)安排及污水水質(zhì)

試驗(yàn)用水取自北京工業(yè)大學(xué)化糞池的實(shí)際生活污水(Run1),屬于典型的低C/N污水.本試驗(yàn)連續(xù)運(yùn)行137d,水溫(20±2)℃.在監(jiān)測(cè)原水COD和TN濃度后,向原水水箱投加乙酸鈉來調(diào)節(jié)進(jìn)水C/N,使其在2.44~8.85范圍內(nèi)波動(dòng)(Run1~Run5),主要考察進(jìn)水C/N對(duì)系統(tǒng)同步脫氮除磷性能的影響.在調(diào)節(jié)進(jìn)水C/N的同時(shí),C/P也在隨之改變(25.36~84.15),具體的水質(zhì)特征見表1.

表1 進(jìn)水水質(zhì)Table 1 Characteristics of influent wastewater

1.3 常規(guī)項(xiàng)目監(jiān)測(cè)

COD的測(cè)定采用蘭州連華5B - 1型快速測(cè)定儀;NH4+-N、NO3--N、NO2--N、PO43--P在水樣經(jīng)0.45μm中速濾紙過濾后,由流動(dòng)注射分析儀(Lachat Quik-Chem8000,Lachat Instrument, Milwaukee, USA)測(cè)定;TN采用TN/TOC分析儀(MultiN/C3100,Analytik Jena, AG)測(cè)定;混合液懸浮固體濃度(MLSS)和揮發(fā)性懸浮固體濃度(MLVSS)根據(jù)國(guó)家標(biāo)準(zhǔn)方法測(cè)定[29];聚-β-羥基鏈烷酸脂(poly-β-hydroxyalkanoates, PHA)采用氣相色譜(Agilent 6890N)及DB-1型色譜柱檢測(cè)[30].

1.4 碳平衡分析方法

采用COD間接表示系統(tǒng)中的碳指標(biāo),以穩(wěn)態(tài)條件下的試驗(yàn)數(shù)據(jù)為基礎(chǔ),對(duì)不同工況下的A2/O-BCO系統(tǒng)進(jìn)行物料平衡分析.進(jìn)入系統(tǒng)COD(inf)的去除途徑主要包括以下幾個(gè)方面[31]: A2/O反應(yīng)器厭氧段去除(An),缺氧段反硝化和好氧段氧化(A-O),BCO反應(yīng)器氧化(BCO),剩余污泥排放(WS),出水殘留(eff).具體計(jì)算過程如下:

infinfinf(1)

AninfinfinfRS(1)infAn(2)

A-O= (1+)inf·An+·inf·eff-

(1++)·inf·O(3)

BCO(1)infMSinfeffeffeff(4)

WSWSWSCV(5)

effeffeff(6)

式中:inf,WS,eff分別是進(jìn)水流量,剩余污泥排放量,出水流量(即進(jìn)水流量),L/d;inf,RS,An,O,MS,eff分別是進(jìn)水COD濃度,回流污泥攜帶的COD濃度(即沉淀池COD濃度),厭氧段COD濃度,好氧段COD濃度,沉淀池COD濃度,出水COD濃度,mg/L;為污泥回流比,100%;為硝化液回流比,400%;WS為剩余污泥濃度,mg/L;為MLVSS/MLSS;CV為活性污泥中有機(jī)物COD的化學(xué)計(jì)量系數(shù),計(jì)算中取1.48mgCOD/mgMLVSS[32].

2 結(jié)果與討論

2.1 C/N對(duì)有機(jī)物去除性能的影響

不同進(jìn)水C/N條件下,系統(tǒng)各階段COD的去除情況如圖2(a)所示.當(dāng)C/N在2.44 ~ 4.28范圍內(nèi)波動(dòng)時(shí)(Run1,Run2和Run5),厭氧段出水的平均COD濃度為56.9 ~ 68.3mg/L,再經(jīng)過缺氧區(qū)的部分反硝化作用,最終出水的COD濃度可穩(wěn)定達(dá)城鎮(zhèn)污水處理廠的一級(jí)A排放標(biāo)準(zhǔn)(GB18918 - 2002).但隨著C/N的增加(Run3和Run4),厭氧段COD濃度偏高,平均值為108.31,126.42mg/L,導(dǎo)致A2/O反應(yīng)器出水的COD濃度也呈現(xiàn)上升的趨勢(shì);經(jīng)過BCO反應(yīng)器中異養(yǎng)菌的進(jìn)一步同化作用,系統(tǒng)最終出水的COD濃度為48.97, 59.93mg/L.

從圖2(b)可以發(fā)現(xiàn),Run1~Run5厭氧段COD去除率為63.63%~75.48%,與之前的研究結(jié)論相似[19],作為缺氧吸磷的能量?jī)?chǔ)備,厭氧段是有機(jī)物被吸收用于PHA合成的重要場(chǎng)所[33].同時(shí)5種工況下A2/O反應(yīng)器中COD的平均去除率分別為79.09%, 82.84%, 81.98%, 82.56%和79.05%. Cao等[1]研究表明,分段進(jìn)水工藝在進(jìn)水C/N<5時(shí)可達(dá)到74%的碳源利用率,而在改良UCT工藝中,當(dāng)進(jìn)水C/N為3.5~5.0用于反硝化除磷的COD僅占(62±6)%[34].相比之下,本研究的雙污泥系統(tǒng)在處理低C/N污水,強(qiáng)化碳源利用方面具有明顯的優(yōu)勢(shì).

然而,對(duì)于C/N為5.92和8.85,COD在厭氧段的去除率、A2/O反應(yīng)器的去除率和系統(tǒng)的總?cè)コ嗜呦嗖钶^大,部分易降解的COD是在A2/O反應(yīng)器的缺氧區(qū)和BCO反應(yīng)器中被去除的.上述2種工況對(duì)于系統(tǒng)的長(zhǎng)期穩(wěn)定運(yùn)行存在隱患:一方面,缺氧區(qū)反硝化菌會(huì)優(yōu)先利用外碳源進(jìn)行反硝化,使得DNPAOs的生長(zhǎng)受到抑制[35];另一方面,BCO反應(yīng)器中外碳源的剩余會(huì)刺激異養(yǎng)菌大量繁殖,削弱生長(zhǎng)速率較低的自養(yǎng)型硝化菌的競(jìng)爭(zhēng)能力,對(duì)硝化性能的穩(wěn)定是極其不利的[17,36].因此,從有機(jī)物去除的角度來說,進(jìn)水C/N不宜超過5.92.

表2 碳平衡分析Table 2 Material balance analysis of carbon

注: COD平衡百分比= (AnA-OBCOWSeff)/inf.

此外,根據(jù)COD的遷移途徑可知(表2),5個(gè)不同進(jìn)水C/N條件下,COD平衡百分比均超過95%,可能是由于數(shù)據(jù)測(cè)量誤差造成的部分碳損失,但該平衡值遠(yuǎn)遠(yuǎn)高于UCT工藝(67%~89.7%)和MUCT工藝(60.7%~87.6%)[37].其中厭氧段去除的COD占總量的百分比分別為49%, 61%, 59%, 64%, 62%.同樣在A2/O - BCO系統(tǒng)中,王聰?shù)萚31]考察了不同硝化液回流比條件下COD的物料平衡,得出厭氧段利用的COD占57.9%~78.5%,比本研究結(jié)果偏高.分析原因可能是由于本研究中厭氧停留時(shí)間僅為1.14h (小于文獻(xiàn)中的1.6h),較短的反應(yīng)時(shí)間來不及充分利用原水碳源,另外試驗(yàn)條件和經(jīng)驗(yàn)值取值的不同也可能導(dǎo)致數(shù)據(jù)的偏差.盡管如此,A2/O反應(yīng)器去除的COD占71.86%~77.28%,BCO反應(yīng)器氧化的COD僅占2%~12%,絕大多數(shù)碳源還是優(yōu)先被聚磷菌或反硝化菌利用,很大程度上減少了曝氣能耗同時(shí)強(qiáng)化了硝化菌的富集,這也是該工藝在低C/N條件下可實(shí)現(xiàn)一級(jí)A排放標(biāo)準(zhǔn)的原因所在[17].綜上所述,A2/O - BCO工藝可實(shí)現(xiàn)原水中有機(jī)碳源的高效利用,尤其適用于處理低C/N生活污水;而對(duì)于高C/N污水,為了強(qiáng)化COD去除效果,可適當(dāng)延長(zhǎng)厭/缺氧水力停留時(shí)間.

2.2 C/N對(duì)氮去除性能的影響

由圖3(a)可知,NH4+-N和TN的去除率密切相關(guān),這主要是由于硝化過程為反硝化除磷提供電子受體(NO--N=NO2--N+NO3--N,其中NO3--N占絕大比例),在硝化液回流比恒定的條件下,NH4+-N的完全氧化是實(shí)現(xiàn)TN高效去除的基礎(chǔ)和關(guān)鍵[38].Run1~Run3工況下,平均NH4+-N去除率為98.28%,98.55%和95.40%,且前2種工況下出水NH4+-N濃度基本小于1.0mg/L;在此基礎(chǔ)上,TN去除率也隨著C/N的增加呈上升趨勢(shì),當(dāng)C/N為5.92時(shí),出水TN濃度11.32mg/L,TN去除率高達(dá)82%.傳統(tǒng)SBR工藝中脫氮過程C/N往往大于8[7],Vaiopoulou等[39]采用改良UCT工藝,在進(jìn)水C/N為9.27時(shí)實(shí)現(xiàn)了最優(yōu)的脫氮效果(TN去除率83%),本研究結(jié)果再次體現(xiàn)了A2/O - BCO工藝節(jié)省碳源的優(yōu)勢(shì).

當(dāng)C/N進(jìn)一步增加到8.85,硝化效果出現(xiàn)惡化,出水NH4+-N濃度逐步累積,最高可達(dá)17.18mg/L,平均TN去除率僅為49.50%.結(jié)合COD的利用情況可知,氮去除效果的惡化,主要是由于進(jìn)水有機(jī)物負(fù)荷過高導(dǎo)致的硝化不完全,進(jìn)而引起TN去除率的下降.張為堂等[40]研究也發(fā)現(xiàn),進(jìn)水C/N增加到9.5時(shí),NH4+-N去除率迅速下降至70%,出水NH4+-N濃度最高可達(dá)30mg/L.隨后把進(jìn)水C/N降為3.71,經(jīng)過15d的恢復(fù),NH4+-N去除率上升到96.51%,TN去除率也穩(wěn)定在75%左右.上述現(xiàn)象表明,在一定范圍內(nèi),增加進(jìn)水C/N是改善TN去除率的有力措施,但是必須建立在NH4+-N氧化完全的基礎(chǔ)上;否則,過高的C/N不僅造成碳源的浪費(fèi)還會(huì)導(dǎo)致脫氮效果的惡化.這也意味著在高C/N條件下,對(duì)BCO反應(yīng)器的曝氣量、水力停留時(shí)間、NH4+-N負(fù)荷等運(yùn)行參數(shù)的控制尤為重要.

為了進(jìn)一步分析各工況下氮的去除特性,圖3(b)給出了NO--N的沿程變化情況.對(duì)于Run1(C/N為2.44),由于C/N相對(duì)較低,A2/O反應(yīng)器中缺氧段出水,好氧段出水,中間沉淀池出水的NO--N濃度普遍偏高(6.92~7.29mg/L),對(duì)比Run5(C/N增加為3.71),NO--N濃度降為3.09~ 3.85mg/L,說明碳源不足成為限制反硝化脫氮的主要因素.隨著C/N從4.28增加到8.85,反應(yīng)器沿程的NO--N濃度從0.95~2.98mg/L降為0.36~ 0.86mg/L,系統(tǒng)最終出水的NO--N也逐步降低,表明增加進(jìn)水C/N確實(shí)有利于強(qiáng)化反硝化作用.

同時(shí),通過對(duì)缺氧區(qū)反硝化負(fù)荷分析發(fā)現(xiàn),Run1 ~ Run5,反硝化負(fù)荷分別為0.136, 0.152, 0.128, 0.110, 0.121kgN/(kgMLVSS·d).在高C/N運(yùn)行條件下,Run3和Run4反硝化負(fù)荷的降低,表明電子受體不足限制了TN去除率的進(jìn)一步提高,這一結(jié)論與NH4+-N去除率的下降吻合.筆者前期在硝化液回流比300%,進(jìn)水C/N為3.50條件下,對(duì)不同容積分配比進(jìn)行了優(yōu)化[19],發(fā)現(xiàn)缺氧區(qū)的反硝化負(fù)荷為0.104kgN/(kgMLVSS·d)時(shí),系統(tǒng)達(dá)到了最優(yōu)的TN去除率(75.22% ~ 78.70%).本研究在硝化液回流比400%,進(jìn)水C/N為4.28時(shí),反硝化負(fù)荷達(dá)到了0.152kgN/ (kgMLVSS·d),進(jìn)一步激發(fā)了DNPAOs反硝化潛能,TN去除率提高為80.5%.因此,為了實(shí)現(xiàn)TN的高效去除,充足的碳源和電子受體缺一不可.

2.3 C/N對(duì)磷去除性能的影響

由圖4(a)可知,C/N較低時(shí)(<4.28)有利于實(shí)現(xiàn)PO43--P的穩(wěn)定去除,其中C/N為2.44和4.28時(shí),平均出水PO43--P濃度為0.22, 0.31mg/L,尤其在TN不能達(dá)標(biāo)的情況下(當(dāng)C/N為2.44,出水平均濃度18.13mg/L),PO43--P去除率超過95%.在A2N - SBR反硝化除磷系統(tǒng)中[41],同樣發(fā)現(xiàn)低C/N不會(huì)影響除磷效果,C/N為3.50時(shí),TN去除率只有66%,但PO43--P去除率可維持88%.與此不同的是,傳統(tǒng)脫氮除磷工藝中,反硝化菌、聚磷菌在利用碳源方面存在矛盾和競(jìng)爭(zhēng)[42],有限的碳源會(huì)優(yōu)先滿足反硝化再除磷,這也是傳統(tǒng)BNR工藝脫氮除磷效率難以同步提高的主要原因.

隨著C/N的繼續(xù)增大,C/N達(dá)到5.29時(shí),出水PO43--P濃度略有升高,當(dāng)C/N增加到8.85,除磷效果逐步惡化,最終出水PO43--P濃度高達(dá)3.30mg/L,去除率僅為46.89%.當(dāng)C/N降為3.71,僅經(jīng)過10d,PO43--P去除率恢復(fù)到82%,出水濃度小于1mg/L.與脫氮性能相比,除磷效果恢復(fù)相對(duì)較快,主要原因在于:①A2/O反應(yīng)器好氧區(qū)的存在有利于除磷性能的恢復(fù)[43];②DNPAOs的缺氧吸磷速率比好氧吸磷速率低[44],即意味著DNPAOs的恢復(fù)速率比PAOs慢;③反硝化菌在20℃時(shí)的最大比增長(zhǎng)速率為0.30d-1[45],Sun等[46]對(duì)反硝化聚磷菌屬進(jìn)行富集培養(yǎng),得出15℃時(shí)最大比增長(zhǎng)速率為0.71d-1,聚磷菌較大的比增長(zhǎng)速率使其在短期內(nèi)可得到快速恢復(fù).

除此之外,在試驗(yàn)期間的23d (Run1),56d (Run2),77d (Run3),119d (Run5)分別考察了除磷過程中內(nèi)碳源PHA的利用情況(鑒于除磷效果不穩(wěn)定,Run4期間未進(jìn)行PHA的測(cè)定),見圖4(b).當(dāng)C/N為2.44時(shí),厭氧段合成的PHA含量(An-PHA)為75.16mgCOD/gMLVSS,缺氧段剩余的PHA含量(A-PHA)為22.89mgCOD/gMLVSS, PHA的利用率為69.54%;當(dāng)C/N增加到4.28, An-PHA和A-PHA分別為92.44, 22.02mgCOD/ gMLVSS, PHA利用率達(dá)到76.18%;當(dāng)C/N繼續(xù)上升到5.92, An-PHA增加為109.86mgCOD/ gMLVSS,但A-PHA仍殘留25.89mgCOD/ gMLVSS, PHA利用率(76.43%)已沒有太大的變化.PHA利用率的分析進(jìn)一步驗(yàn)證了TN的去除特性.葛士建等[47]研究發(fā)現(xiàn),在分段進(jìn)水工藝中,約38%的PHA在缺氧段用來吸磷,對(duì)比結(jié)果表明本系統(tǒng)中存在較明顯的反硝化除磷現(xiàn)象.此外,在脫氮除磷性能尚未恢復(fù)的119d, An-PHA和A-PHA分別為66.13, 30.63mgCOD/gMLVSS, PHA利用率僅為53.68%,表明DNPAOs的厭氧釋磷和缺氧吸磷的代謝性能均有大幅度下降,因此導(dǎo)致了除磷效果的惡化.

圖4(b)中PO43--P沿程變化,更加具體地闡釋了不同進(jìn)水C/N對(duì)除磷性能的影響.首先,隨著C/N增加(2.44到4.28),與An-PHA變化趨勢(shì)相同,厭氧段釋磷量從19.85mg/L上升為32.31mg/L,缺氧吸磷占磷總?cè)コ康陌俜直葟?2.34%上升為97.14%,表明有機(jī)物含量的增加強(qiáng)化了DNPAOs的代謝活性.但是當(dāng)C/N為5.92時(shí),厭氧釋磷量高達(dá)41.13mg/L,電子受體不足使得缺氧段出水PO43--P濃度為3.24mg/L,殘留的PO43--P在好氧段被進(jìn)一步去除,缺氧吸磷百分比下降為90.12%.隨后當(dāng)C/N繼續(xù)增加到8.85,厭氧釋磷效果進(jìn)一步惡化,到運(yùn)行的110d,厭氧釋磷量?jī)H為14.91mg/L,到好氧吸磷結(jié)束仍有2.28mg/L剩余,除磷性能瀕臨崩潰.

此外,當(dāng)C/N為5.92和8.85時(shí),發(fā)現(xiàn)中間沉淀池有不同程度的二次釋磷現(xiàn)象,結(jié)合系統(tǒng)的脫氮除磷機(jī)理[20],可知PO43--P與NO--N的去除特性密不可分.上述2種工況下,中間沉淀池的NO--N濃度為0.79, 0.36mg/L,較低的NO--N濃度促進(jìn)了二次釋磷現(xiàn)象的發(fā)生.結(jié)合圖4(b)中PHA的變化過程可知:增加進(jìn)水C/N促進(jìn)了厭氧段PHA的合成,但是缺氧段NO--N不足是導(dǎo)致PHA利用率和除磷效率不高的根本原因,因此調(diào)節(jié)硝化液回流比保證反硝化過程中充足的電子受體顯得很有必要.但是較高的NO--N濃度殘留(Run1和Run5)又難以實(shí)現(xiàn)TN的高效去除.因此,為了實(shí)現(xiàn)氮、磷的同步高效去除,中間沉淀池的NO--N濃度應(yīng)控制在2.20mg/L左右,該結(jié)果與前期的研究結(jié)論(中間沉淀池的NO--N濃度保持在1.95 ~ 2.75mg/L)[19]相吻合.

2.4 不同C/N下的同步脫氮除磷效果

綜上分析,氮、磷的去除與進(jìn)水C/N有明顯的相關(guān)性,主要原因是由于在A2/O-BCO系統(tǒng)中,TN和PO43--P是在反硝化除磷這一單元同步完成的,均需要碳源的參與.本研究中發(fā)現(xiàn),進(jìn)水C/N的改變,對(duì)COD去除效果影響不大,但是受硝化性能的影響,氮磷的同步高效去除必須建立在電子受體充足的基礎(chǔ)上.

如圖5所示,從C/N和C/P的角度出發(fā),對(duì)TN和PO43--P去除率的相關(guān)性進(jìn)行了比較.當(dāng)C/N小于4或者C/P小于40時(shí),TN去除率不超過70%,但PO43--P的去除率基本保持在95%左右;當(dāng)C/N超過7或者C/P超過80,TN和PO43--P的去除率均呈現(xiàn)下降趨勢(shì),去除率不超過60%.當(dāng)C/N=4 ~ 5或者C/P=30 ~ 50范圍內(nèi),TN和PO43--P去除率分別達(dá)到80%和96%以上,實(shí)現(xiàn)了氮和磷的同步高效去除.

考慮到工藝系統(tǒng)和運(yùn)行條件的差異,不同的微生物菌群結(jié)構(gòu)和代謝特性導(dǎo)致脫氮除磷效果千差萬別[48-49],有關(guān)最佳的C/N和C/P,目前尚沒有統(tǒng)一的定論.Wang等[41]研究發(fā)現(xiàn)在A2N - SBR系統(tǒng)中,C/N和C/P為5.8和30.8時(shí),最優(yōu)的TN和PO43--P去除率分別為79.4%和97%.在MUCT分段進(jìn)水工藝中,C/N為7.42時(shí),TN去除率高達(dá)89.2%;C/P>35時(shí),系統(tǒng)可保持90%以上的磷去除率[36].Chen等[50]考察了進(jìn)水C/N和硝化液回流比對(duì)A2/O-BAF系統(tǒng)的影響,C/N為5.5時(shí),TN去除率最高為90.1%.盡管結(jié)論不盡相同,有關(guān)進(jìn)水C/N對(duì)其他工藝的優(yōu)化運(yùn)行仍具有一定的借鑒意義和參考價(jià)值.

2.5 系統(tǒng)的控制策略和優(yōu)化運(yùn)行

本試驗(yàn)研究條件下,當(dāng)C/N=4 ~ 5或者C/P=30 ~ 50,TN和PO43--P去除率最高可達(dá)80%和96%,實(shí)現(xiàn)了氮、磷的同步高效去除.基于不同進(jìn)水C/N對(duì)A2/O-BCO系統(tǒng)反硝化除磷性能的研究,對(duì)工程實(shí)踐具有指導(dǎo)意義:(1)進(jìn)水C/N對(duì)COD的去除效果影響不大,厭氧段基本上實(shí)現(xiàn)了對(duì)絕大多數(shù)碳源的利用;在低C/N運(yùn)行條件下,A2/O反應(yīng)器可適當(dāng)縮短水力停留時(shí)間,進(jìn)一步減少基建費(fèi)用;(2)強(qiáng)化硝化效果是提高系統(tǒng)氮、磷去除率的前提和保證,研究BCO反應(yīng)器的抗NH4+-N沖擊能力,探索 NH4+-N負(fù)荷與曝氣量、水力停留時(shí)間、填料填充率等相關(guān)性,對(duì)進(jìn)一步降低能耗很有必要;(3)低C/N條件下, PO43--P的去除效果相對(duì)穩(wěn)定,但為了實(shí)現(xiàn)TN的高效去除,可延長(zhǎng)厭氧反應(yīng)時(shí)間,強(qiáng)化內(nèi)碳源的貯存;同時(shí)減少硝化液回流比,防止過高的硝態(tài)氮負(fù)荷抑制了DNPAOs的活性;(4)高C/N條件下,可通過延長(zhǎng)BCO反應(yīng)器的水力停留時(shí)間或增大曝氣量來強(qiáng)化硝化效果,增大硝化液回流比的同時(shí)調(diào)整缺氧反應(yīng)時(shí)間,保持缺氧段合適的硝態(tài)氮負(fù)荷;同時(shí)為了保證磷的穩(wěn)定去除,可適當(dāng)增加A2/O中好氧區(qū)的曝氣量.

3 結(jié)論

3.1 碳平衡分析表明系統(tǒng)絕大多數(shù)碳源是在A2/O反應(yīng)器中去除,COD利用率達(dá)79.09%~ 82.84%,但為了防止過多的易降解有機(jī)物在缺氧區(qū)和BCO反應(yīng)器被反硝化菌和異養(yǎng)菌利用,進(jìn)水C/N不宜超過5.92.

3.2 進(jìn)水C/N超過5.92時(shí),NH4+-N去除率的下降導(dǎo)致電子供體不足,脫氮除磷效率均發(fā)生惡化;進(jìn)水C/N低于3.71時(shí),碳源不足限制了TN去除率的提高.進(jìn)水C/N為4~5或者C/P為30~50時(shí), TN和PO43--P去除率分別達(dá)到80%和96%.

3.3 進(jìn)水C/N的優(yōu)化與污泥特性,硝化液回流比,BCO反應(yīng)器的硝化特性,各反應(yīng)區(qū)的水力停留時(shí)間等多種因素密切相關(guān).實(shí)際工程應(yīng)用中,各運(yùn)行參數(shù)的優(yōu)化控制應(yīng)因地制宜.

Cao G, Wang S, Peng Y, et al. Biological nutrient removal by applying modified four step-feed technology to treat weak wastewater [J]. Bioresource Technology, 2013,128:604-611.

Ge S, Peng Y, Wang S, et al. Enhanced nutrient removal in a modified step feed process treating municipal wastewater with different inflow distribution ratios and nutrient ratios [J]. Bioresource Technology, 2010,101(23):9012-9019.

Zeng W, Wang X, Li B, et al. Nitritation and denitrifying phosphorus removal via nitrite pathway from domestic wastewater in a continuous MUCT process [J]. Bioresource Technology, 2013,143:187-195.

Zhu G, Peng Y, Zhai L, et al. Performance and optimization of biological nitrogen removal process enhanced by anoxic/oxic step feeding [J]. Biochemical Engineering Journal, 2009,43(3):280- 287.

Baeza JA, Gabriel D, Lafuente J. Improving the nitrogen removal efficiency of an A2/O based WWTP by using an on-line Knowledge Based Expert System [J]. Water Research, 2002, 36(8):2109-2123.

Coats ER, Mockos A, Loge FJ. Post-anoxic denitrification driven by PHA and glycogen within enhanced biological phosphorus removal [J]. Bioresource Technology, 2011,102(2):1019-1027.

Carucci A, Chiavola A, Majone M, et al. Treatment of tannery wastewater in a sequencing batch reactor [J]. Water Science & Technology, 1999,40(1):253-259.

Kim D, Kim T-S, Ryu H-D, et al. Treatment of low carbon- to-nitrogen wastewater using two-stage sequencing batch reactor with independent nitrification [J]. Process Biochemistry, 2008, 43(4):406-413.

Stare A, Vrecko D, Hvala N, et al. Comparison of control strategies for nitrogen removal in an activated sludge process in terms of operating costs: A simulation study [J]. Water Research, 2007,41(9):2004-2014.

Szpyrkowicz L, Kaul S N. Biochemical removal of nitrogen from tannery wastewater: performance and stability of a full‐scale plant [J]. Journal of chemical technology and biotechnology, 2004, 79(8):879-888.

Ge S, Zhu Y, Lu C, et al. Full-scale demonstration of step feed concept for improving an anaerobic/anoxic/aerobic nutrient removal process [J]. Bioresource Technology, 2012,120:305-313.

Kuba T, Van Loosdrecht M C M, Brandse F A, et al. Occurrence of denitrifying phosphorus removing bacteria in modified UCT-type wastewater treatment plants [J]. Water Research, 1997,31(4): 777-786.

Wang Z, Meng Y, Fan T, et al. Phosphorus removal and N2O production in anaerobic/anoxic denitrifying phosphorus removal process: Long-term impact of influent phosphorus concentration [J]. Bioresource Technology, 2015,179:585-594.

Kuba T, van Loosdrecht M C M, Heijnen J J. Phosphorus and nitrogen removal with minimal COD requirement by integration of denitrifying dephosphatation and nitrification in a two-sludge system [J]. Water Research, 1996,30(7):1702-1710.

Bortone G, Saltarelli R, Alonso V, et al. Biological anoxic phosphorus removal - the dephanox process [J]. Water Science & Technology, 1996,34(s1/2):119-128.

Slhgmcmjjk T. Biological phosphorus removal from wastewater by anaerobic-anoxic sequencing batch reactor [J]. Waterence & Technology, 1993,27(5/6):241-252.

Chen Y, Peng C, Wang J, et al. Effect of nitrate recycling ratio on simultaneous biological nutrient removal in a novel anaerobic/ anoxic/oxic (A2/O)-biological aerated filter (BAF) system [J]. Bioresource Technology, 2011,102(10):5722-5727.

Zhang M, Peng Y, Wang C, et al. Optimization denitrifying phosphorus removal at different hydraulic retention times in a novel anaerobic anoxic oxic-biological contact oxidation process [J]. Biochemical Engineering Journal, 2016,106:26-36.

張 淼,彭永臻,王 聰,等.容積分配比對(duì)A2/O-生物接觸氧化工藝反硝化除磷特性的影響 [J]. 東南大學(xué)學(xué)報(bào)(自然科學(xué)版), 2015,45(3):531-538.

Zhang M, Wang C, Peng Y, et al. Organic substrate transformation and sludge characteristics in the integrated anaerobic anoxic oxic–biological contact oxidation (A2/O–BCO) system treating wastewater with low carbon/nitrogen ratio [J]. Chemical Engineering Journal, 2016,283:47-57.

王 聰,王淑瑩,張 淼,等.硝化液回流比對(duì)A2/O-BCO工藝反硝化除磷特性的影響 [J]. 中國(guó)環(huán)境科學(xué), 2014,34(11):2844- 2850.

張 勇,王淑瑩,趙偉華,等.低溫對(duì)中試AAO-BAF雙污泥脫氮除磷系統(tǒng)的影響 [J]. 中國(guó)環(huán)境科學(xué), 2016,36(1):56-65.

呂冬梅,彭永臻,趙偉華,等.A2O-BAF反硝化除磷工藝絲狀菌膨脹的發(fā)生及控制 [J]. 中國(guó)環(huán)境科學(xué), 2015,35(10):3026-3031.

葛士建,李夕耀,彭永臻,等.改良UCT分段進(jìn)水深度脫氮除磷工藝反硝化動(dòng)力學(xué)性能 [J]. 北京工業(yè)大學(xué)學(xué)報(bào), 2011,37(3):433- 439.

Lee N M, Welander T. The effect of different carbon sources on respiratory denitrification in biological wastewater treatment [J]. Journal of fermentation and bioengineering, 1996,82(3):277-285.

Elefsiniotis P, Li D. The effect of temperature and carbon source on denitrification using volatile fatty acids [J]. Biochemical Engineering Journal, 2006,28(2):148-155.

Oehmen A, Saunders A M, Vives M T, et al. Competition between polyphosphate and glycogen accumulating organisms in enhanced biological phosphorus removal systems with acetate and propionate as carbon sources [J]. Journal of Biotechnology, 2006, 123(1):22-32.

Dai Y, Yuan Z, Wang X, et al. Anaerobic metabolism of Defluviicoccus vanus related glycogen accumulating organisms (GAOs) with acetate and propionate as carbon sources [J]. Water Research, 2007,41(9):1885-1896.

Apha A W P C F. Standard methods for the examination of water and wastewater [Z]. 19th edition. American Public Health Association. Method E Apha – Awwa – Wef, 1995.

Oehmen A, Keller-Lehmann B, Zeng RJ, et al. Optimisation of poly-β-hydroxyalkanoate analysis using gas chromatography for enhanced biological phosphorus removal systems [J]. Journal of Chromatography A, 2005,1070(1):131-136.

王 聰,王淑瑩,張 淼,等.厭氧/缺氧/好氧生物接觸氧化處理低碳氮比污水的物料平衡 [J]. 農(nóng)業(yè)工程學(xué)報(bào), 2014,30(19): 273-281.

Chuang S-H, Ouyang C-F. The biomass fractions of heterotrophs and phosphate-accumulating organisms in a nitrogen and phosphorus removal system [J]. Water Research, 2000,34(99): 2283–2290.

Wei Y, Wang S, Ma B, et al. The effect of poly-β- hydroxyalkanoates degradation rate on nitrous oxide production in a denitrifying phosphorus removal system [J]. Bioresource Technology, 2014,170:175-182.

Peng Y, Ge S. Enhanced nutrient removal in three types of step feeding process from municipal wastewater [J]. Bioresource Technology, 2011,102(11):6405-6413.

Wang X, Peng Y, Wang S, et al. Influence of wastewater composition on nitrogen and phosphorus removal and process control in A2O process [J]. Bioprocess and Biosystems Engineering, 2006,28(6):397-404.

葛士建,彭永臻,曹 旭,等.改良UCT分段進(jìn)水工藝處理生活污水性能優(yōu)化研究 [J]. 環(huán)境科學(xué), 2011,32(7):2006-2012.

Nowak O, Franz A, Svardal K, et al. Parameter estimation for activated sludge models with the help of mass balances [J]. Water Science & Technology, 1999,39(4):113-120.

張為堂,侯 鋒,劉青松,等.HRT和曝氣量對(duì)AAO-BAF系統(tǒng)反硝化除磷性能的影響 [J]. 化工學(xué)報(bào), 2014,65(4):1436-1442.

Vaiopoulou E, Aivasidis A. A modified UCT method for biological nutrient removal: Configuration and performance [J]. Chemosphere, 2008,72(7):1062-1068.

張為堂,薛曉飛,龐洪濤,等.碳氮比對(duì)AAO-BAF工藝運(yùn)行性能的影響 [J]. 化工學(xué)報(bào), 2015,66(5):1925-1930.

Wang Y, Peng Y, Stephenson T. Effect of influent nutrient ratios and hydraulic retention time (HRT) on simultaneous phosphorus and nitrogen removal in a two-sludge sequencing batch reactor process [J]. Bioresource Technology, 2009,100(14):3506-3512.

陳永志,彭永臻,王建華,等.A~2/O-曝氣生物濾池工藝處理低C/N比生活污水脫氮除磷 [J]. 環(huán)境科學(xué)學(xué)報(bào), 2010,30(10): 1957-1963.

張為堂,薛同來,彭永臻,等.AAO-BAF反硝化除磷系統(tǒng)的二次啟動(dòng)特性 [J]. 化工學(xué)報(bào), 2014,65(2):658-663.

Ahn J, Daidou T, Tsuneda S, et al. Characterization of denitrifying phosphate-accumulating organisms cultivated under different electron acceptor conditions using polymerase chain reaction- denaturing gradient gel electrophoresis assay [J]. Water Research, 2002,36(2):403-412.

池勇志,李 毓,吳麗萍.城市污水前置反硝化生物脫氮工藝的動(dòng)力學(xué)計(jì)算 [J]. 天津城市建設(shè)學(xué)院學(xué)報(bào), 2004,10(2):122-124.

Sun L, Zhao X, Zhang H, et al. Biological characteristics of a denitrifying phosphorus-accumulating bacterium [J]. Ecological Engineering, 2015,81:82-88.

葛士建,王淑瑩,曹 旭,等.分段進(jìn)水脫氮除磷工藝中反硝化除磷的實(shí)現(xiàn)與維持 [J]. 化工學(xué)報(bào), 2011,62(9):2615-2622.

曾 薇,李博曉,王向東,等.MUCT短程硝化和反硝化除磷系統(tǒng)中Candidatus Accumulibacter的代謝活性和菌群結(jié)構(gòu) [J]. 中國(guó)環(huán)境科學(xué), 2013,33(7):1298-1308.

戴 嫻,王曉霞,彭永臻,等.進(jìn)水C/N對(duì)富集聚磷菌的SNDPR系統(tǒng)脫氮除磷的影響 [J]. 中國(guó)環(huán)境科學(xué), 2015,35(9):2636-2643.

Chen Y, Li B, Ye L, et al. The combined effects of COD/N ratio and nitrate recycling ratio on nitrogen and phosphorus removal in anaerobic/anoxic/aerobic (A2/O)-biological aerated filter (BAF) systems [J]. Biochemical Engineering Journal, 2015,93:235-242.

* 責(zé)任作者, 教授, pyz@bjut.edu.cn

Effect of influent C/N ratios on denitrifying phosphorus removal characteristics in the A2/O-BCO process

ZHANG Miao, PENG Yong-zhen*, ZHANG Jian-hua, WANG Cong, WANG Shu-ying, ZENG Wei

(Engineering Research Center of Beijing, Key Laboratory of Beijing for Water Quality Science and Water Environment Recovery Engineering, Beijing University of Technology, Beijing 100124, China)., 2016,36(5)：1366~1375

A two-sludge system combined anaerobic/anoxic/oxic with biological contact oxidation process (A2/O-BCO) was used to treat domestic wastewater. By adding sodium acetate to adjust influent carbon/nitrogen ratio (C/N=2.44 ~ 8.85), the denitrifying phosphorus removal characteristics of the system were investigated. The results showed that nitrification performance (anoxic denitrifying loading) and poly-β-hydroxyalkanoates (PHA) storage and utilization were mainly influenced by organic matter, which further effected the nitrogen and phosphorus removals. When the influent C/N ratio was 4 ~ 5, COD, TN and PO43--P removals reached to 88%, 80% and 96% respectively, which achieved high-efficiency removal of organic matter, nitrogen and phosphorus simultaneously. The material balance analysis of carbon revealed that COD removal in the A2/O reactor was 71.86% ~ 77.28% of the total COD removal, and COD removal in the BCO reactor only accounted for 2% ~ 12%, where the efficient utilization of carbon source in the A2/O- BCO process was the main reason to achieve deep denitrification and phosphorus removal under the condition of low C/N. In addition, through the correlation analysis of C/N with aeration rate, nitrate recycling ratio and anaerobic/anoxic reaction time, the optimal operation strategy of the A2/O-BCO process was proposed.

A2/O-BCO；C/N；denitrifying phosphorus removal；domestic wastewater；optimal operation

X703.5

A

1000-6923(2016)05-1366-10

張 淼(1989-),女,江蘇徐州人,北京工業(yè)大學(xué)博士研究生,主要從事污水生物處理理論與應(yīng)用研究.發(fā)表論文4篇.

2015-11-02

國(guó)家自然科學(xué)基金(51578014);北京市教委資助項(xiàng)目