吕永涛,闫建平,张瑶,刘婷,曾玉莲,王磊
(西安建筑科技大学 环境与市政工程学院,陕西省膜分离重点实验室,西安 710055)
摘要:
为实现N2O的减量化控制或资源化利用,接种普通活性污泥,以乙酸钠和硝酸盐为基质,通过淘洗反硝化聚磷菌,在SBR中以厌氧/缺氧交替运行的方式启动內源反硝化,单周期内约50%的NO3--N转化为NO2--N,N2O释放速率随着亚硝酸的积累逐渐增大,N2O转化率(释放量占TN去除的比例)为2.04%.在此基础上,分别取缺氧末和厌氧末污泥,对比研究胞内聚合物PHB合成前后,外碳源投加量(碳氮比为0,0.75和2.50)对亚硝酸型反硝化过程N2O释放特性的影响,结果表明,外碳源存在时,N2O释放量随总碳源的增加呈略微减少的趋势,转化率在0.24%~1.61%;而当仅利用內源物质进行反硝化时,N2O的转化率高达15.90%,单位SS最大释放速率达71.29 μg/(min·g),释放量是其余条件下的14~26倍.表明单独利用PHB进行亚硝酸型反硝化会大幅增加N2O的释放.
关键词: 內源反硝化 碳氮比 N2O PHB 亚硝酸
DOI:10.11918/j.issn.0367-6234.201612121
分类号:X703.1
文献标识码:A
基金项目:国家自然科学基金 (51108367);陕西省建设厅科技发展计划项目(2015-K61)
Effect of external carbon source dosage on N2O emission from denitrification using nitrite as the electron acceptor
Lü Yongtao,YAN Jianping,ZHANG Yao,LIU Ting,ZENG Yulian,WANG Lei
(School of Environmental and Municipal Engineering, Key Laboratory of Membrane Separation of Shanxi Province,Xi’an University of Architecture and Technology, Xi’an 710055, China)
Abstract:
To eliminate N2O emission or utilize it for energy recovery, a laboratory-scale SBR inoculated with common activated sludge with acetate and nitrate additions decoupled, was conducted for endogenous denitrification running with anaerobic/anoxic cycles without DPAOs. During one cycle, approximately 50% of nitrate was converted to nitrite, N2O emission rate increased with nitrite accumulation, and 2.04% of removed nitrogen was emitted as nitrous oxide (N2O). Batch tests were conducted to compare the emission of N2O that was influenced by external carbon source dosage (C/N was 0, 0.75 and 2.50) using nitrite as the electron acceptor before and after poly-β-hydroxybutyrate (PHB) was synthesized.The results showed that, the N2O emission was slightly decreased with the increase of total carbon source, and the N2O conversion ratio was between 0.24% and 1.61%. 15.90% of N2O conversion ratio was obtained in the absence of external organic carbon, the maximum N2O emission rate was 71.29 μg/(min·g), and the N2O emission was 14-26 times higher than that of the other batch test results, indicating that when the PHB was the sole electron donor, the endogenous denitrification could produce more N2O using nitrite as the electron acceptor.
Key words: endogenous denitrification C/N nitrous oxide PHB nitrite
吕永涛, 闫建平, 张瑶, 刘婷, 曾玉莲, 王磊. 外碳源投量对亚硝酸型反硝化过程N2O释放影响[J]. 哈尔滨工业大学学报, 2018, 50(8): 39-44. DOI: 10.11918/j.issn.0367-6234.201612121.
![复制到剪切板](http://hit.alljournals.cn/html/hitxb_cn/2018/8/images/copy.gif)
Lü Yongtao, YAN Jianping, ZHANG Yao, LIU Ting, ZENG Yulian, WANG Lei. Effect of external carbon source dosage on N2O emission from denitrification using nitrite as the electron acceptor[J]. Journal of Harbin Institute of Technology, 2018, 50(8): 39-44. DOI: 10.11918/j.issn.0367-6234.201612121.
![复制到剪切板](http://hit.alljournals.cn/html/hitxb_cn/2018/8/images/copy.gif)
基金项目 国家自然科学基金(51108367);陕西省建设厅科技发展计划项目(2015-K61) 作者简介 吕永涛(1980—),男,副教授,硕士生导师;
王磊(1971—),男,教授,博士生导师 通信作者 王磊, wl0178@126.com 文章历史 收稿日期: 2016-12-25
Contents -->Abstract Full text Figures/Tables PDF
外碳源投量对亚硝酸型反硝化过程N2O释放影响
吕永涛, 闫建平, 张瑶, 刘婷, 曾玉莲, 王磊
![通讯作者](http://hit.alljournals.cn/html/hitxb_cn/2018/8/images/REcor.gif)
![wl0178@126.com](http://hit.alljournals.cn/html/hitxb_cn/2018/8/images/REemail.gif)
西安建筑科技大学 环境与市政工程学院,陕西省膜分离重点实验室,西安 710055
收稿日期: 2016-12-25
基金项目: 国家自然科学基金(51108367);陕西省建设厅科技发展计划项目(2015-K61)
作者简介: 吕永涛(1980—),男,副教授,硕士生导师;
王磊(1971—),男,教授,博士生导师
通信作者: 王磊, wl0178@126.com
摘要: 为实现N2O的减量化控制或资源化利用,接种普通活性污泥,以乙酸钠和硝酸盐为基质,通过淘洗反硝化聚磷菌,在SBR中以厌氧/缺氧交替运行的方式启动內源反硝化,单周期内约50%的NO3--N转化为NO2--N,N2O释放速率随着亚硝酸的积累逐渐增大,N2O转化率(释放量占TN去除的比例)为2.04%.在此基础上,分别取缺氧末和厌氧末污泥,对比研究胞内聚合物PHB合成前后,外碳源投加量(碳氮比为0,0.75和2.50)对亚硝酸型反硝化过程N2O释放特性的影响,结果表明,外碳源存在时,N2O释放量随总碳源的增加呈略微减少的趋势,转化率在0.24%~1.61%;而当仅利用內源物质进行反硝化时,N2O的转化率高达15.90%,单位SS最大释放速率达71.29 μg/(min·g),释放量是其余条件下的14~26倍.表明单独利用PHB进行亚硝酸型反硝化会大幅增加N2O的释放.
关键词: 內源反硝化 碳氮比 N2O PHB 亚硝酸
Effect of external carbon source dosage on N2O emission from denitrification using nitrite as the electron acceptor
Lü Yongtao, YAN Jianping, ZHANG Yao, LIU Ting, ZENG Yulian, WANG Lei
![Corresponding author](http://hit.alljournals.cn/html/hitxb_cn/2018/8/images/REcor.gif)
![wl0178@126.com](http://hit.alljournals.cn/html/hitxb_cn/2018/8/images/REemail.gif)
School of Environmental and Municipal Engineering, Key Laboratory of Membrane Separation of Shanxi Province, Xi'an University of Architecture and Technology, Xi'an 710055, China
Abstract: To eliminate N2O emission or utilize it for energy recovery, a laboratory-scale SBR inoculated with common activated sludge with acetate and nitrate additions decoupled, was conducted for endogenous denitrification running with anaerobic/anoxic cycles without DPAOs. During one cycle, approximately 50% of nitrate was converted to nitrite, N2O emission rate increased with nitrite accumulation, and 2.04% of removed nitrogen was emitted as nitrous oxide (N2O). Batch tests were conducted to compare the emission of N2O that was influenced by external carbon source dosage (C/N was 0, 0.75 and 2.50) using nitrite as the electron acceptor before and after poly-β-hydroxybutyrate (PHB) was synthesized.The results showed that, the N2O emission was slightly decreased with the increase of total carbon source, and the N2O conversion ratio was between 0.24% and 1.61%. 15.90% of N2O conversion ratio was obtained in the absence of external organic carbon, the maximum N2O emission rate was 71.29 μg/(min·g), and the N2O emission was 14-26 times higher than that of the other batch test results, indicating that when the PHB was the sole electron donor, the endogenous denitrification could produce more N2O using nitrite as the electron acceptor.
Key words: endogenous denitrification C/N nitrous oxide PHB nitrite
N2O是一种强温室气体,温室效应约为CO2的300倍[1],同时又是一种可再生能源物质,利用其替代O2与CH4混合燃烧可提高约37%的热值[2-3].探究N2O的释放机制可为污水脱氮过程中其减量化控制或资源化利用提供理论支持.N2O是反硝化的中间产物,因此,反硝化过程的释放几乎不可避免.研究表明,pH、DO质量浓度、亚硝酸质量浓度、碳氮比等因素均会影响反硝化过程中N2O的释放[4-7],其中,碳源是主要的影响因素之一.研究发现,当外部碳源受限时,微生物会利用PHB(poly-β-hydroxybutyrate)作电子供体进行内源反硝化(ED, endogenous denitrification),此过程中会增加N2O的释放[8-10].众所周知,反硝化聚磷菌(DPAOs, denitrifying phosphate-accumulating organisms)和反硝化聚糖菌(DGAOs, denitrifying glycogen-accumulating organisms)均可进行內源反硝化,且二者往往同时存在[2, 11],究竟哪种微生物是N2O释放的主要贡献者尚不清楚,尤其是在未涉及磷去除的情况下,內源反硝化过程N2O释放特性的研究尚不多见.
本课题借助SBR反应器,通过淘洗DPAOs获得了內源反硝化的效果,在此基础上分别探究胞内聚合物PHB合成前后,外碳源投量对PHB水平及亚硝酸型反硝化过程N2O释放特性的影响,旨在为N2O的释放控制或资源化利用提供依据.
1 实验 1.1 实验装置及运行实验采用圆柱形SBR反应器,有效容积3 L,排水比为2/3.通过先后分别投加乙酸钠和硝酸盐,形成厌氧/缺氧交替运行的环境启动SBR, 单周期时间为8 h,其中厌氧进水5 min,厌氧搅拌150 min,厌氧沉淀30 min,厌氧出水10 min,缺氧进水5 min,缺氧搅拌240 min,缺氧沉淀30 min,缺氧出水10 min.本实验阶段MLSS维持在3 000 mg/L左右(VSS与SS比约为0.7).反应器运行期间通过水浴加热维持温度在(26±1)℃.
1.2 接种污泥与实验废水实验污泥接种自西安市某污水处理厂A2/O回流污泥,接种量为3 L,初始污泥质量浓度为4 500 mg/L,实验采用人工配制废水,厌氧进水成份主要为:0.26 g/L CH3COONa,缺氧进水成份主要为0.29 g/L KNO3,其余配水基质均为1.0 g/L NaHCO3、0.01 g/L NH4Cl、0.1 g/L CaCl2、0.1 g/L MgSO4以及微量元素Ⅰ和Ⅱ[2],驯化过程中仅添加少量的KH2PO4 用以维持微生物的正常生长需求(ρ(PO43--P) ≈1 mg/L),所有药品均为分析纯.
1.3 批式实验装置及方案批式实验采用气密性良好的圆柱形反应器,有效容积1.0 L.采用高纯N2曝气,以多孔气泡石作为微孔曝气器,曝气量维持在0.4 L/min.
实验保持NO2--N质量浓度不变,通过投加不同量的无水乙酸钠设定不同的碳氮比.实验分为A、B组,其中A组实验污泥取自SBR母反应器缺氧末(此时微生物体内PHB含量最少),B组实验污泥取自SBR母反应器厌氧末(此时微生物体内储存的PHB最多).各组配水如表 1所示,A组实验开始前,取缺氧末1 L泥水混合液沉淀后用去离子水反复清洗实验污泥4次,然后弃去上清液将污泥等分成3份置于3个小型SBR装置中(此时MLSS约为1 000 mg/L),再分别向A1、A2、A3中加入配置好的基质,使混合液总体积为1 L,随后加盖并立即曝N2.B组实验操作与A组相同.
表 1
![点击查看表格内容](http://hit.alljournals.cn/html/hitxb_cn/2018/8/images/table-icon.gif)
COD NO2--N MgSO4 CaCl2 NaHCO3
A1、B2 0 40 100 100 1 000
A2、B2 30 40 100 100 1 000
A3、B3 100 40 100 100 1 000
表 1 批式实验配水组分 Table 1 Composition of the medium used for batch tests
1.4 水质分析NO2--N、NO3--N、PO43--P均按照国家环保总局颁布的标准方法[12]进行测定,其中NO2--N采用N-(1-萘基)-乙二胺光度法,NO3--N采用紫外分光光度法,PO43--P采用钼酸铵分光光度法;TOC采用multi N/C 2100s测定; MLSS和MLVSS采用标准重量法.
1.5 曝气阶段N2O气体采集分析与计算反应产生的气体先后经由饱和的亚硫酸钠溶液和98%的浓硫酸去除氧气和水蒸汽后,用气密性良好的气体采样针收集5 mL气样,然后用PE 600气相色谱仪对其中的N2O进行分析,所用色谱柱为Porapak Q填充柱,检测器为电子捕获检测器(ECD),以20 mL/min高纯N2作为载气,进样口、柱温箱和检测器的温度分别为150、50和380 ℃,所有气体样品均测定3次.采集时间分别在曝N2的第0、0.5、1、2、3、5、8、10、15、20、30、40、50、60、70、80、90、100、120、140、160、180分钟.
N2O释放速率与释放量分别通过下式计算:
${{w}_{{{\text{N}}_{\text{2}}}\text{O}}}=Q\cdot {{C}_{{{\text{N}}_{\text{2}}}\text{O}}}\cdot {{M}_{{{\text{N}}_{\text{2}}}\text{O}}}\cdot p/\left( R\cdot T\cdot {{V}_{\text{L}}}\cdot {{\rho }_{\text{MLSS}}} \right), ~$ (1)
${{m}_{{{\text{N}}_{\text{2}}}\text{O}}}=\sum\limits_{2}^{n}{\left[ \frac{\left( {{w}_{{{\text{N}}_{\text{2}}}\text{O},n-1}}+{{w}_{{{\text{N}}_{\text{2}}}\text{O},n}} \right)}{2}\text{ }\cdot \Delta t\cdot {{V}_{\text{L}}}\cdot {{\rho }_{\text{MLSS}}} \right].}$ (2)
式中:wN2O为单位MLSS N2O释放速率(μg/(min·g)),Q为曝气阶段的体积流量(L/min),CN2O为所测得的N2O体积分数(10-6 m3/m3),MN2O为N2O摩尔质量(44.02 g/mol),p为大气压强(101 kPa),R为气体常数(8.314 L·kPa/(K·mol)),T为温度(K),VL为反应器有效容积(L),ρMLSS为混合液悬浮固体质量浓度(g/L),mN2O为N2O释放量(mg),Δt为N2O气体采样时间间隔(min).
1.6 胞内聚合物PHB预处理及测定取反应器内60 mL泥水混合液经离心浓缩并去掉上清液,然后用超声波细胞破碎仪破碎,取3 mL破碎后的泥样于10 mL的哈希管中,依次加入2 mL 10%酸化甲醇和2 mL氯仿并立即密封,置于HACH消解仪中于100 ℃消解240 min,消解结束后于微量振荡器中剧烈震荡30 min并用冰水冷却,之后在4 ℃低温条件下离心10 min,用1 mL卡介苗注射器吸取底层的氯仿相并用0.22 μm有机滤头过滤于1.5 mL的进样瓶中.气相色谱分析条件:采用Agilent 6890N气相色谱仪测定泥样中的PHB,色谱柱型号为(19091M-133-HP-5,30 m×0.320 mm× 0.25 μm),检测器为氢火焰离子检测器(FID),进样量为1 μL,分流比为2.7:1,载气为高纯N2,采用恒流模式,流量为0.7 mL/min,进样口温度250 ℃,柱温箱升温程序:初始值50 ℃,停留1 min,之后以10 ℃/min升温至120 ℃,然后以120 ℃/min升温到270 ℃且停留6 min.检测器温度为300 ℃,氢气流量为45 mL/min,空气流量为450 mL/min.所有样品均测定2次取平均值.
2 结果与讨论 2.1 SBR驯化过程中的氮素变化特性图 1为内源反硝化启动过程中氮、磷的变化趋势.1~13 d,保持缺氧进水中NO3--N和PO43--P质量浓度分别为(50±1.0)和(15±1.0) mg/L,第6天后,缺氧出水中PO43--P质量浓度低于1 mg/L,系统TP和TN的去除率分别达93.33%和85%,SBR获得了良好的反硝化除磷效果.为淘洗DPAOs,从第15天开始,逐渐降低缺氧段进水中PO43--P质量浓度至1 mg/L(磷仅用于维持微生物的正常生长需求),第21天开始,缺氧出水中NO2--N开始积累并升至28 mg/L,导致TN去除率降至16%以下,这是由于DPAOs的活性受到抑制所致,与Zeng等[13]的研究结果相吻合.之后从第50天开始系统运行稳定,缺氧段NO3--N的去除率为(85±1)%,但约50%的NO3--N转化为NO2--N而非其他气态产物(NO、N2O和N2等),TN的去除率仅为(42±1)%.表明系统中已富集了反硝化聚糖菌.
Figure 1
![点击查看原图](http://hit.alljournals.cn/html/hitxb_cn/2018/8/PIC/hebgydxxb-50-8-39-1.jpg)
2.2 SBR典型周期内碳、氮、磷的转化特性图 2为第85天SBR反应器内各污染物的变化情况,由于整个周期中没有涉及磷的吸取与释放(Δρ(PO43--P)≤1 mg/L),几乎没有发生反硝化除磷.在厌氧段:TOC质量浓度由74.60 mg/L降低至16.21 mg/L,同时, 单位VSS PHB含量由4.52 mg/g逐渐升至38.02 mg/g,表明厌氧阶段DGAOs吸收乙酸钠合成了PHB.在缺氧段:NO3--N质量浓度由42.36 mg/L降至4.23 mg/L,同时PHB含量由39.52 mg/g降至4.34 mg/g,表明缺氧阶段发生了內源反硝化.其中NO2--N和N2O均呈先升高后降低的趋势,且N2O的最大释放速率基本发生在NO2--N质量浓度最高时,这是由于产生的高质量浓度游离亚硝酸(FNA)对N2O还原酶(Nos)产生抑制作用所致[9, 11, 14].单周期内N2O释放量仅占TN去除的2.04%,而Scherson等[2]研究发现,脉冲式投加NO2--N进行內源反硝化时富集了N2O,其转化率高达60%~65%.二者差别较大主要是电子受体不同所致,本实验以NO3--N作为电子受体,大部分被转化为NO2--N,即尚未进行到N2O累积的阶段.因此,利用內源物质PHB进行反硝化仅是N2O富集的条件之一.此外,还可能与反硝化的电子受体及投加方式有关,尚需进一步研究.
Figure 2
![点击查看原图](http://hit.alljournals.cn/html/hitxb_cn/2018/8/PIC/hebgydxxb-50-8-39-2.jpg)
2.3 PHB合成前外碳源投量对亚硝酸型反硝化过程N2O释放特性的影响图 3为PHB合成前,外碳源投量对亚硝酸型反硝化及N2O释放的影响.未投加外碳源时(图 3(a)),因缺乏电子供体无法发生反硝化作用,NO2--N几乎不发生变化,单位SS N2O产量仅为0.16 mg/g.
Figure 3
![点击查看原图](http://hit.alljournals.cn/html/hitxb_cn/2018/8/PIC/hebgydxxb-50-8-39-3.jpg)
当碳氮比为0.75:1(图 3(b)),即外碳源不足时,NO2--N还原主要发生在前30 min内.N2O的释放在0~10 min内呈先升高后降低的趋势,最大释放速率达到38.96 μg/(min·g),前10 min内N2O释放量占总释放量的67.05%.而PHB质量浓度在整个反应过程中几乎没有发生变化,说明外碳源不足时,主要被用于反硝化而非合成PHB.
当碳氮比为2.50:1时(图 3(c)),NO2--N与N2O的变化趋势与图 3(b)的变化趋势类似,N2O最大释放速率为22.25 μg/(min·g),56.03%的N2O释放量发生在前10 min.而PHB的趋势发生了变化,其含量在30 min后逐渐增加,到180 min时较初始值提高了3.1倍.表明外碳源充足时,除进行反硝化外,还同时合成了PHB,这也是反硝化速率逐渐降低的原因.与图 3(b)相比,N2O释放量减少,即低碳氮比会增加N2O释放,这与普通污泥进行反硝化所得结论一致[15].
2.4 PHB合成后外碳源投量对亚硝酸型反硝化过程N2O释放特性的影响图 4为PHB合成后,外碳源投量对亚硝酸型反硝化及N2O释放的影响.未投加外碳源时(图 4(a)),DGAOs进行內源反硝化,单位SS反硝化速率和单位VSS PHB降解速率分别为3.62 mg/(h·g)和8.99 mg/(h·g).N2O释放与PHB合成前的变化趋势相似,但最大释放速率远高于外源反硝化时,达到71.29 μg/(min·g),且N2O释放时间延长至前60 min,占总释放量的72.50%.
Figure 4
![点击查看原图](http://hit.alljournals.cn/html/hitxb_cn/2018/8/PIC/hebgydxxb-50-8-39-4.jpg)
B2相较于B1,单位SS NO2--N还原速率增大,达4.78 mg/(h·g),原因是提供的总碳源有所增加;但PHB的降解速率降低,仅为5.36 mg/(h·g).表明当内外碳源共存且外碳源不足时,外碳源被优先进行反硝化.此外,还利用部分内碳源进行反硝化.N2O主要在前15 min内产生,N2O释放呈先升高后降低的趋势,最大释放速率为19.11 μg/(min·g).
B3在外碳源充足时(碳氮比为2.50:1),反硝化速率为15.50 mg/(h·g),分别是B1、B2的4.28倍和3.24倍,脱氮效率分别是B1、B2的2.84倍和1.42倍,即投加足量的外碳源可以大幅提升反硝化速率和脱氮效率.相反,整个过程中N2O释放速率低于2.25 μg/(min·g),N2O释放量仅为0.23 mg/g,与B1、B2相比,PHB含量不降反升,在30~180 min内PHB合成速率为4.08 mg/(h·g),PHB含量提高了24.06%,与Qin等[16]的研究结果相似.表明外碳源充足时,即使体内有较高含量的PHB,DGAOs在利用外碳源进行反硝化的同时仍会继续合成PHB.
综上,当外碳源存在时,DGAOs会优先利用外碳源进行反硝化,至于是否进一步合成PHB,还与外碳源的投量有关.当外碳源不足以提供反硝化所需的电子时,PHB合成的现象不明显;反之,当外碳源充足时,才会被进一步合成PHB.
此外,N2O的释放主要发生在反硝化的初始阶段,是因为初始阶段较高质量浓度NO2-会抑制N2O还原酶(Nos)的活性导致N2O积累[11];随着反应的进行,NO2-质量浓度不断降低,抑制作用逐渐削弱,使N2O的产生与消耗不断趋于平衡,即降低了N2O的释放量.
2.5 胞内聚合物PHB合成前后外碳源投量对N2O释放影响结果对比理论上1 mg NO2--N完全反硝化转化为N2需要1.72 mg的COD,其中1 mg的PHB相当于1.67 mg的COD[17].由表 2可知,B1中NO2--N去除量为9.22 mg,根据理论计算需要9.50 mg PHB,但B1中PHB的实际消耗量(14.63 mg)大于理论计算量,原因是部分PHB用以补充糖原维持正常生长代谢.
表 2
![点击查看表格内容](http://hit.alljournals.cn/html/hitxb_cn/2018/8/images/table-icon.gif)
碳氮比为0 碳氮比0.75:1 碳氮比2.50:1 碳氮比为0 碳氮比0.75:1 碳氮比2.50:1
进水NO2--N/(mg·L-1) 40.20 ± 1.0 39.40 ± 0.5 39.90 ± 1.0 38.13 ± 1.5 40.77 ± 0.6 39.40 ± 0.5
出水NO2--N/(mg·L-1) 39.06 ± 0.6 28.62 ± 1.0 13.42 ± 0.5 28.91 ± 1.0 16.10 ± 0.5 9.07 ± 0.5
N2O释放量/(mg·g-1) 0.16 ± 0.1 0.30 ± 0.5 0.24 ± 0.1 4.08 ± 0.9 0.26 ± 0.6 0.23 ± 0.1
N2O-N转化率/% 4.47±0.5 1.61±1.0 0.42±0.5 15.90±1.1 1.15±0.8 0.24±0.8
N2O最大释放速率/(μg·min-1·g-1) 2.84 ± 1.1 38.96 ± 1.5 22.25± 2.0 71.29 ± 2.5 19.11 ± 2.0 2.25 ± 2.3
NO2--N还原速率/(mg ·h-1·g-1) 0.19 ± 0.5 2.71 ± 1.1 7.90 ± 0.9 3.62 ± 0.4 4.78 ± 0.9 15.50 ± 0.7
TN去除率/% 4.24 ± 1.0 28.49 ± 1.0 64.56 ± 1.0 27.66 ± 1.0 55.24 ± 1.0 78.67 ± 1.5
ΔPHB/(mg·g-1) -0.36 +0.54 +13.96 -27.5 -15.59 +7.88
表 2 胞内聚合物PHB合成前后外碳源投量对N2O释放结果汇总 Table 2 Characteristics of N2O emission influenced by external carbon source dosage before and after PHB was stored
当外碳源存在时,根据PHB与COD的理论计量关系,计算得到总碳源的大小关系为:A2<B2<A3<B3,而N2O释放量的关系为:A2>B2>A3>B3,说明亚硝酸型反硝化过程中N2O的释放量随着总碳源的增加呈现减小的趋势.主要是因为电子供体不足导致各反硝化酶之间的竞争,使Nos处于劣势,即N2O的生成速率大于消耗速率,造成N2O的积累.但是,N2O释放量变化幅度较小((0.23±0.1)~(0.30±0.5) mg/g),与Zhou等[18]的研究结果类似.
当PHB作为唯一碳源进行反硝化时(B1),N2O的转化率高达15.90%,N2O释放量是其余条件下的14~26倍,表明单独利用胞内聚合物PHB进行亚硝酸型反硝化会大幅增加N2O释放.因为以PHB为电子供体进行反硝化时,与其他碳源相比,Nos更难竞争得到电子,导致N2O释放量增加[18-19];且PHB为难降解有机物[19],相对较慢的反硝化速率导致溶液中剩余较高的NO2--N质量浓度,且维持更长的时间,这也是内源反硝化初期N2O释放时间延长的主要原因.
当内外碳源共存且外碳源不足时,反硝化过程中也消耗部分PHB,但此时N2O的释放量(B2)反而小于单独利用外碳源时的释放量(A2),主要原因还是与总碳源的投量有关.因为N2O主要在反应初期释放,二者在反应初期均利用外碳源进行反硝化,总碳源少(A2)时Nos竞争电子处于更为劣势的状态,导致其释放量高于总碳源多(B2)时.
3 结语利用批试实验研究了内源反硝化污泥在PHB合成前与合成后,外碳源投加量(碳氮比为0,0.75和2.5)对亚硝酸型反硝化过程N2O释放特性的影响,结果表明,外碳源存在时,N2O释放量随总碳源的增加呈略微减少的趋势,转化率在0.24%~1.61%;当仅利用PHB进行內源反硝化时,N2O的转化率高达15.90%,释放量是其余条件下的14~26倍,表明单独利用PHB进行亚硝酸型反硝化会大幅增加N2O释放.
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