COD/N與pH值對(duì)短程硝化反硝化過(guò)程中N2O產(chǎn)生的影響

李鵬章,王淑瑩,彭永臻,劉 越

(北京工業(yè)大學(xué)環(huán)境與能源學(xué)院,北京100022)

COD/N與pH值對(duì)短程硝化反硝化過(guò)程中N2O產(chǎn)生的影響

李鵬章,王淑瑩*,彭永臻,劉 越

(北京工業(yè)大學(xué)環(huán)境與能源學(xué)院,北京100022)

利用SBR反應(yīng)器,通過(guò)投加乙醇控制COD/N為0、1.5、3、4.5,調(diào)節(jié)pH值分別在6、7、8,反硝化初始投加NO-N為30mg/L,考察了缺氧條件下COD/N與pH值對(duì)短程硝化反硝化過(guò)程中N2O產(chǎn)量的影響.結(jié)果表明:低COD/N可以造成N2O持續(xù)較高的逸出,N2O最大產(chǎn)生量為2.35mg/L;低pH值條件下增加了N2O的積累,pH值在6時(shí)的N2O積累量是pH在7、8時(shí)的800倍;高COD/N和高pH值下的N2O產(chǎn)生速率最小,而當(dāng)pH=6,COD/N=0時(shí),N2O產(chǎn)生速率最大,為2.35×10-3mgN/(mgMLSS·L·h).其原因是:N2O還原酶爭(zhēng)奪電子的能力較弱,充足的電子供體有利于N2O的還原;低pH值可影響微生物的代謝,且在H+存在時(shí)產(chǎn)生的游離亞硝酸(HNO2)對(duì)N2O還原酶具有抑制作用.充足的碳源和堿性條件,是降低短程硝化反硝化過(guò)程中N2O產(chǎn)量的關(guān)鍵因素.

碳氮比；pH值；短程硝化反硝化；N2O

N2O是一種強(qiáng)力溫室氣體,其溫室效應(yīng)大約是 CO2的300倍,在大氣層中存留時(shí)間為114a.N2O的釋放量每年正以0.3%的趨勢(shì)增長(zhǎng),對(duì)全球溫室效應(yīng)的貢獻(xiàn)已經(jīng)增至5%～6%[1].污水生物脫氮過(guò)程是 N2O的潛在人為源之一[2-5].隨著水體富營(yíng)養(yǎng)化的日益嚴(yán)重,污水廠需要達(dá)到脫氮要求,將增加 N2O的釋放量,因此,污水脫氮處理過(guò)程中,也要控制N2O的釋放.

在污水生物脫氮技術(shù)中,反硝化過(guò)程是氮循環(huán)中的關(guān)鍵步驟.完整的反硝化過(guò)程是在異養(yǎng)型微生物的作用下,在缺氧條件下,把 NO3-依次經(jīng)過(guò) NO2-,NO,N2O還原為 N2的過(guò)程.在此過(guò)程中,NO3-還原酶(Nar)、NO2-還原酶(Nir)、NO還原酶(Nor)、N2O還原酶(Nos)參與其中[6-8],每種酶的缺失均會(huì)影響相應(yīng)基質(zhì)的還原,即導(dǎo)致這種基質(zhì)在污水中的積累.短程硝化反硝化是污水生物脫氮的新技術(shù),其脫氮原理是將污水中NH4+經(jīng)AOB的作用氧化為NO2-,不再繼續(xù)將NO2-氧化為 NO3-,然后以 NO2-作為電子受體進(jìn)行反硝化.與傳統(tǒng)全程硝化反硝化相比,短程硝化反硝化具有以下優(yōu)點(diǎn)[9]:硝化過(guò)程節(jié)約25%的O2消耗;反硝化過(guò)程節(jié)約40%的外加碳源(以甲醇計(jì)).但是,已有研究認(rèn)為,不同電子受體(NO3-、NO2-)作為反硝化基質(zhì),其在反硝化過(guò)程中 N2O 產(chǎn)生量不同:Zhao等[10]考察了不同鹽度沖擊下 NO-,NO-32分別作為電子受體時(shí)N2O的產(chǎn)量,發(fā)現(xiàn)在碳源受限時(shí),NO2-作為電子受體 N2O的產(chǎn)生率更高;王莎莎等[11]比較了NO3-,NO2-作為電子受體時(shí)N2O產(chǎn)量,發(fā)現(xiàn)亞硝態(tài)氮反硝化時(shí)產(chǎn)生N2O的量是硝態(tài)氮反硝化時(shí)的9.12倍.因此,對(duì)以 NO2-為電子受體的短程硝化反硝化過(guò)程中N2O控逸具有重要意義.

作為電子供體,碳源是影響反硝化過(guò)程中N2O產(chǎn)量的關(guān)鍵因素,而在污水廠實(shí)際運(yùn)行中,普遍存在反硝化碳源不足的情形.pH值是污水廠實(shí)際運(yùn)行中的實(shí)時(shí)控制因素,pH值不僅能改變微生物代謝途徑,還會(huì)對(duì)反硝化過(guò)程中某些物質(zhì)(HNO2等)的存在及濃度產(chǎn)生影響.作為反硝化過(guò)程中兩個(gè)可調(diào)因素,考察COD/N,pH值對(duì)反硝化N2O產(chǎn)量共同影響具有實(shí)際意義.本實(shí)驗(yàn)采用SBR反應(yīng)器,以實(shí)際生活污水為對(duì)象,首次研究了不同COD/N與pH值對(duì)短程反硝化過(guò)程中N2O產(chǎn)生量的協(xié)同影響,為短程硝化反硝化運(yùn)行中N2O的減量,提供了理論依據(jù).

1 材料與方法

1.1 實(shí)驗(yàn)用污泥、水質(zhì)、控制參數(shù)

實(shí)驗(yàn)污泥取自12L的SBR反應(yīng)器.此SBR反應(yīng)器進(jìn)水為北京某高校家屬區(qū)生活污水,采用傳統(tǒng)進(jìn)水-好氧-缺氧-靜沉-排水的運(yùn)行方式.經(jīng)過(guò)3個(gè)月270周期的連續(xù)培養(yǎng),NO2-積累率達(dá)97%,成功實(shí)現(xiàn)了穩(wěn)定短程硝化.出水 NH4+-N、NO2--N、NO3--N均在1mg/L以下.污泥取出后先曝氣12h,然后用去離子水反復(fù)清洗,沉淀濃縮后待用.

實(shí)驗(yàn)用水為該反應(yīng)器反硝化結(jié)束出水,出水NH4+-N、NO2--N、NO3--N均在1mg/L以下.出水取出后,充分曝氣12h,此時(shí)COD約為30mg/L,均為難降解有機(jī)物.批次實(shí)驗(yàn)反應(yīng)初始投加NO2--N為30mg/L,根據(jù)此NO2--N投加量按COD/N0、1.5、3、4.5算得乙醇投加量.pH值控制在6、7、8,為典型的污水處理過(guò)程中pH值的變化范圍[12],采用0.1mol/L的鹽酸溶液和0.1mol/L的氫氧化鈉調(diào)節(jié).實(shí)驗(yàn)分3個(gè)批次,1個(gè)批次分別采用1個(gè)pH值梯度,4個(gè)COD/N梯度.實(shí)驗(yàn)溫度為即時(shí)室溫:25℃.

1.2 實(shí)驗(yàn)裝置及運(yùn)行

實(shí)驗(yàn)用批次反應(yīng)器如圖1所示.此反應(yīng)器有效容積3L,反應(yīng)開(kāi)始先加0.6L濃縮后的污泥,然后加入2.4L的經(jīng)處理過(guò)的出水,此時(shí)將泥水混合液(污泥濃度為2000mg/L)調(diào)到批次實(shí)驗(yàn)所需pH值,再依次加入30mg/L的NO2--N、無(wú)水乙醇,實(shí)驗(yàn)開(kāi)始運(yùn)行,運(yùn)行時(shí)間為4h.整個(gè)反應(yīng)過(guò)程為缺氧運(yùn)行,DO含量均在0.05mg/L以下.

圖1 批次實(shí)驗(yàn)SBR裝置Fig.1 Schematic diagram of batch-mode SBR systems

1.3 測(cè)試方法

試驗(yàn)中COD、NO2--N的測(cè)定均采用標(biāo)準(zhǔn)方法[13].pH值、DO分別使用Multi340i型便攜式多功能 pH 值、DO 測(cè)定儀測(cè)定.N2O 使用UNISENSE(Picoammeter PA2000,檢 測(cè) 下 限0.01μmol/L)測(cè)定儀在線測(cè)定.

2 結(jié)果

2.1 相同pH值下COD/N對(duì)NO2--N和N2O的影響

圖2為相同pH值下不同COD/N對(duì)NO2--N降解的影響.當(dāng)COD與NO2--N之比為0(反硝化過(guò)程無(wú)外加碳源加入)時(shí),微生物利用自身物質(zhì)提供電子進(jìn)行內(nèi)源反硝化,NO2--N降解變化緩慢,整個(gè)反硝化過(guò)程需要240min才能完成.當(dāng)加入乙醇作為碳源時(shí),NO2--N 降解速度加快,在120min之內(nèi)便可完成.當(dāng)COD/N為4.5時(shí),微生物在60min之內(nèi)便可完成反硝化過(guò)程.說(shuō)明在反硝化過(guò)程中,NO2--N的降解速度隨COD/N的增加而增大.

圖2 相同pH值不同COD/N下NO2--N的變化Fig.2 Variations of NO2--N at different COD/N ratios with pH kept consistent

圖3 相同pH值不同COD/N下N2O的變化Fig.3 Variations of N2O at different COD/N ratios with pH kept consistent

圖3為相同pH值不同COD/N下的N2O產(chǎn)生量變化圖.由此圖可知,當(dāng)外加碳源為0(無(wú)外加電子供體)時(shí),微生物靠自身內(nèi)源物質(zhì)進(jìn)行反硝化,N2O產(chǎn)生量較有外加碳源時(shí)多.有外加碳源時(shí),N2O 產(chǎn)量很快達(dá)到峰值,然后再依次降低,COD/N為0時(shí)N2O產(chǎn)量先緩慢增加,再緩慢下降.在COD/N設(shè)定范圍內(nèi),N2O的產(chǎn)生量隨著COD/N的增加而減少,當(dāng)COD/N分別為3和4.5時(shí),N2O 有較為相近的變化趨勢(shì),這是因?yàn)镃OD/N已經(jīng)接近和達(dá)到使NO2--N完全還原的理論值.

2.2 相同COD/N下pH值對(duì)NO2--N和N2O的影響

圖4為相同COD/N不同pH值下NO2--N的變化趨勢(shì)圖.如圖所示,在pH=8時(shí)NO2--N的降解速率明顯大于pH=6、7時(shí)NO2--N的降解速率.當(dāng) pH=7時(shí),NO2--N 的降解速率大于或者與pH=6的降解速率相近.這表明,在反硝化過(guò)程中NO2--N的變化同樣受pH值的影響,pH=6、7時(shí),降解速率相近,但當(dāng)pH=8時(shí),NO2--N有著相對(duì)較快的降解速率.

圖5為相同COD/N不同pH值下的N2O產(chǎn)生變化圖.其中,當(dāng)pH=6時(shí),在短程反硝化過(guò)程中N2O產(chǎn)生量較大,N2O產(chǎn)生最大值在2.4mg/L.與pH值較低時(shí)相比,pH值為7和8時(shí)的N2O產(chǎn)生量相近且較少,均在0.03mg/L以下.這表明,污水中較低的pH值不利于N2O的繼續(xù)還原,當(dāng)污水為中性或堿性時(shí),N2O可以較快的還原為 N2,不利于自身的積累.

2.3 COD/N和pH值的共同作用對(duì)NO2--N降解速率和N2O生成速率的影響

圖6為不同COD/N和pH值下的NO2--N降解速率變化圖.其中NO2--N的降解速率由圖2中COD/N與pH值對(duì)應(yīng)的NO2--N變化曲線斜率算得.由圖6可見(jiàn),當(dāng)pH值為6、COD/N=0時(shí), NO2--N 的降解速率最低,為0.002878mgN/ (mgMLSS·L·h),但是當(dāng)pH=8且COD/N=4.5時(shí), NO2--N 還原速率迅速增至0.02875mgN/ (mgMLSS·L·h),是 pH=6、COD/N=0時(shí)的10倍.NO2--N的降解速率隨著pH值的升高、碳氮比的增加而增大.

圖4 相同COD/N不同pH值下NO2--N的變化Fig.4 Variations of NO2--N at pH with COD/N ratio kept consistent

圖5 相同COD/N不同pH值下N2O的變化Fig.5 Variations of N2O at different pH with COD/N kept consistent

圖6 不同COD/N和pH值下的NO2--N降解速率變化Fig.6 Variations of NO2--N reduction rate at the different COD/N and pH

圖7為不同COD/N和pH值下的N2O產(chǎn)生速率變化圖.N2O變化速率由圖3中的 COD/N和pH值對(duì)應(yīng)的N2O變化曲線增長(zhǎng)段的斜率算得.由此圖可知,當(dāng)pH=7和8時(shí),COD/N在0、1.5、3、4.5時(shí)均有較小的N2O產(chǎn)生速率,N2O產(chǎn)生速率均在2.8×10-5mgN/(mgMLSS·L·h)以下.但是,當(dāng)pH值降至6,無(wú)外加碳源時(shí),N2O的產(chǎn)生速率陡 增 至 2.35×10-3mgN/(mgMLSS·L·h).這 說(shuō)明,COD/N的減少和pH值的降低均對(duì)N2O產(chǎn)生速率產(chǎn)生影響,均會(huì)導(dǎo)致 N2O較快的產(chǎn)生,但是N2O的產(chǎn)生速率隨著pH值的降低變化更快,即在酸性條件下,不論碳源是否充足,N2O均有相對(duì)較大的產(chǎn)生速率.

圖7 不同COD/N和pH值下的N2O產(chǎn)生速率變化Fig.7 Variations of N2O production rate at the different COD/N and pH

3 討論

Letey等[14]研究認(rèn)為,在缺氧條件下,Nos的合成滯后于 Nar,在反硝化過(guò)程的初期,由于污水中Nos的缺乏,反硝化過(guò)程中產(chǎn)生的N2O不能及時(shí)被還原為 N2,從而導(dǎo)致 N2O的積累.與此研究類(lèi)似,在本實(shí)驗(yàn)中,無(wú)論碳源是否充足,在反硝化初始階段,均有N2O的積累,這表明,在以NO2-為電子受體的反硝化過(guò)程的初始階段,Nos的合成可能同樣滯后于 Nir.此外,反硝化初始時(shí)刻較高的 NO2-濃度可能對(duì) Nos具有毒性,使得產(chǎn)生的N2O無(wú)法迅速被還原成N2.

本研究表明,較高的COD/N有利于N2O的還原,當(dāng)碳源不足,微生物利用內(nèi)碳源進(jìn)行反硝化時(shí),系統(tǒng)出現(xiàn)了持續(xù)的N2O積累.Schalk-otte等[15]的研究也表明,在反硝化階段,各種還原酶對(duì)電子的競(jìng)爭(zhēng)能力不同,其中以Nos的電子競(jìng)爭(zhēng)能力最弱,當(dāng) NO2-存在且積累時(shí),多種還原酶之間存在電子競(jìng)爭(zhēng),Nos對(duì)電子的親和力較弱,從而在系統(tǒng)內(nèi)發(fā)生N2O積累.本實(shí)驗(yàn)中,外加碳源為0時(shí),在限制性電子供體的條件下,出現(xiàn)了較多的N2O的持續(xù)累積,表明Nos電子競(jìng)爭(zhēng)能力較弱.當(dāng)有外加碳源時(shí),系統(tǒng)內(nèi)的N2O產(chǎn)量隨著外加碳源增加而下降(圖2).與本研究結(jié)果類(lèi)似,Meyer等[16]對(duì)同步脫氮除磷系統(tǒng)的研究表明,在較低的 COD/N下有著較高的N2O產(chǎn)量,當(dāng)COD/N較低時(shí),反硝化細(xì)菌利用PHA作為電子供體進(jìn)行反硝化,即使有少量NO2-,也會(huì)導(dǎo)致N2O的積累.Itokawa等[17]利用SBR考察了低COD/N下N2O的產(chǎn)量,發(fā)現(xiàn)COD/N在2.4和3.5時(shí),進(jìn)水中20%～30%的氮被轉(zhuǎn)化為N2O,當(dāng)COD/N較高時(shí),只有1%的氮轉(zhuǎn)化為N2O.Pan等[18]則認(rèn)為,無(wú)論碳源是否充足均會(huì)發(fā)生電子競(jìng)爭(zhēng),電子在4種氮素還原酶的分布主要受碳源負(fù)荷的影響,較低的碳源負(fù)荷會(huì)降低電子在Nos上的分布,當(dāng)流向Nir的電子比流向Nos多時(shí)就會(huì)造成N2O的累積.

pH值是反硝化過(guò)程中一個(gè)重要的影響因素.本實(shí)驗(yàn)中,在碳源一定的情況下,當(dāng)pH值在7、8時(shí)有少量的N2O積累,而當(dāng)pH值為6時(shí),N2O出現(xiàn)大量積累,積累量是pH值在7、8時(shí)的800倍(圖5).Marina等[19]研究發(fā)現(xiàn),當(dāng)pH<6.8時(shí),反硝化過(guò)程中產(chǎn)生N2O,pH=5、6時(shí),N2O產(chǎn)量最多.Hanaki等[20]的研究也表明,pH值由8.5降至6.5時(shí),系統(tǒng)反硝化過(guò)程 N2O產(chǎn)量增加,其原因可能是低 pH值有利于以N2O為終產(chǎn)物的菌種生長(zhǎng),也可能是低pH值改變了反硝化的代謝途徑,導(dǎo)致N2O積累.Zhou等[21]則認(rèn)為在低pH值、NO2-積累時(shí)形成的HNO2會(huì)對(duì)Nos產(chǎn)生抑制作用是造成N2O積累的原因,并揭示了HNO2對(duì)N2O還原的抑制濃度為0.0007～0.001mg/L.本實(shí)驗(yàn)中,當(dāng)pH=6時(shí), HNO2(25℃)濃度最高為0.067mg/L,已有足夠濃度對(duì)Nos發(fā)生抑制.圖7所示,當(dāng)pH＜7時(shí),N2O的產(chǎn)生速率隨pH值的變化比隨COD/N的變化更大,即在酸性條件下,不論碳源是否充足,N2O均有相對(duì)較大的產(chǎn)生速率,而在pH=6時(shí),NO2--N降解速率的下降幅度并不大(圖6).這表明,Nos還原酶在酸性條件下的活性比 Nir更低,Pan等[22]的研究也有類(lèi)似結(jié)果.

由圖6可知,在COD/N和pH值的設(shè)定范圍內(nèi),增加COD/N而不提高pH值或者提高pH值而不增加COD/N均能提高NO2--N的還原速率,但同時(shí)增加COD/N和提高pH值時(shí),將能更加顯著地提高NO2--N的還原速率.圖7表明,分別增加COD/N和提高pH值均能降低N2O的產(chǎn)生速率,但是同時(shí)提高COD/N與pH值更能有效減小N2O的產(chǎn)生速率,這是因?yàn)樵黾覥OD/N會(huì)使Nos得到更多的電子用于N2O的還原,而提高pH值則能減少HNO2對(duì)Nos抑制作用.

Sommer等[23]指出,冬季時(shí),污水處理廠中反硝化段溶解在污水中N2O濃度較高,這些N2O會(huì)在曝氣段被吹脫出去.污水生物脫氮反硝化過(guò)程是 pH不斷升高的過(guò)程(長(zhǎng)期監(jiān)測(cè)數(shù)據(jù)約在6.3～8.5之間),如果把反硝化過(guò)程始終控制在堿性條件,并提供適宜的碳源,將會(huì)極大的減少N2O的排放量.

4 結(jié)論

4.1 碳源是影響短程脫氮反硝化過(guò)程N(yùn)2O產(chǎn)生的重要因素.N2O的釋放隨著COD/N的增加而降低.Nos在碳源匱乏時(shí)對(duì)電子的競(jìng)爭(zhēng)處于不利地位是反硝化過(guò)程中N2O積累的主要原因.

4.2 pH值也是N2O產(chǎn)生的影響因素, pH<7時(shí), N2O產(chǎn)生量較大且產(chǎn)生速率變化較快, pH值在7、8時(shí), N2O積累量較小.在低pH值下, Nos較Nir有著較小的活性.

4.3 在酸性條件下,不論是碳源缺少還是相對(duì)充足,N2O均有較高的產(chǎn)生速率.提供充足的碳源和堿性條件,是減少短程脫氮反硝化過(guò)程中 N2O排放的有效途徑.

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Effect of COD/N ratios and pH on N2O production during nitrite denitrification process.

LI Peng-zhang, WANG Shu-ying*, PENG Yong-zhen, LIU Yue
(College of Environmental and Energy Engineering, Beijing University of Technology, Beijing100022, China). China Environmental Science,2014,34(8)：2003～2009

The effect of COD/N ratios and pH on N2O production was studied in sequencing batch reactors (SBR). The initial NO2--N concentration of30mg N/L was obtained by adding NaNO2, the COD/N ratios of0,1.5,3 and4.5 were reached by adding ethanol, and the pH were controlled at6,7, and8. The experimental results showed that low COD/N produced more N2O with the maximum N2O generating capacity of2.35mg/L, and low pH conditions increased the accumulation of N2O. The production rate of N2O reached2.35×10-3mgN/(mgMLSS·L·h) at low COD/N and low pH. There were two reasons. The ability of N2O reductase for competing electrons was weak, and sufficient electron donors were favorable for the reduction of N2O. Low pH affected microbial metabolism, and the N2O reductase were inhibited by HNO2(produced by H+and NO2-). The study showed that sufficient carbon sources and alkaline conditions were the key factors to reduce the accumulation of N2O in nitrite denitrification processes.

t：COD/N ratios；pH；nitrite denitrification；N2O

X703

：A

：1000-6923(2014)08-2003-07

李鵬章(1985–),男,江蘇徐州人,博士研究生,主要從事生活污水生物脫氮過(guò)程中N2O產(chǎn)生機(jī)理的研究.

《中國(guó)環(huán)境科學(xué)》獲評(píng)“百種中國(guó)杰出學(xué)術(shù)期刊”

《中國(guó)環(huán)境科學(xué)》編輯部

2013-11-18

國(guó)家“863”項(xiàng)目(2011AA060903-02);國(guó)家自然科學(xué)基金項(xiàng)目(51008005)

* 責(zé)任作者, 教授, wsy@bjut.edu.cn

《中國(guó)環(huán)境科學(xué)》2012年被中國(guó)科學(xué)技術(shù)信息研究所評(píng)為“2011年度百種中國(guó)杰出學(xué)術(shù)期刊”.“百種中國(guó)杰出學(xué)術(shù)期刊”是根據(jù)中國(guó)科技學(xué)術(shù)期刊綜合評(píng)價(jià)指標(biāo)體系進(jìn)行評(píng)定的,包含總被引頻次、影響因子、基金論文比、他引總引比等多個(gè)文獻(xiàn)計(jì)量學(xué)指標(biāo).