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含氮废水的处理方法主要有物理法、化学法、生物法等,而生物法是目前污水处理厂最常用的方法[1]。传统的生物法脱氮是利用微生物将水体中的有机氮、氨态氮和硝态氮转化成氮气的过程,包括氨化、硝化和反硝化的3个步骤。其中反硝化作用是由反硝化细菌在厌氧条件下将硝酸盐还原为氮气或氮氧化物的过程,是去除污水中含氮污染物的关键步骤。目前反硝化作用作为一种重要的脱氮工艺已得到广泛应用,但在实际应用中发现存在反硝化速率低的问题,从而影响了生物系统脱氮效率。反硝化过程需要有机物作为电子供体,研究表明我国城市污水处理厂普遍存在进水碳氮比低的问题,碳源不足已经成为制约污水处理厂脱氮效率的主要因素[2]。有很多学者采用新型生物脱氮工艺来提高脱氮效率,例如短程硝化反硝化[3]、厌氧氨氧化[4]和同步硝化反硝化[5]等。但新型工艺受到众多因素的影响,目前在实际工程中难以大规模推广应用。在实际污水处理过程中,碳源不足是制约反硝化脱氮效果的主要原因,但也有研究表明反硝化的电子传递速率是影响反硝化效率的因素之一[6-7]。反硝化反应涉及多重电子传递过程,电子传递速率是影响反硝化速率的重要因素。氧化还原介体是一类能够在电子受体和电子供体之间可逆地被氧化和还原,从而加速电子转移的化合物[8]。通过投加氧化还原介体提高反硝化中电子传递速率是提高生物脱氮速率的有效措施,因此成为目前的研究热点。
研究表明投加氧化还原介体可以提高反硝化过程电子传递速率、降低反应活化能从而加快反硝化脱氮速率[7, 9],通过提高生物反硝化性能减少缺氧池容积、节约投资成本,具有显著的经济效益。氧化还原介体-反硝化技术与新型生物脱氮工艺相比,其对水质、运行环境、操作条件等要求低,易于实施,管理方便。并且氧化还原介体种类多,来源较广泛,因此具有很好的应用前景。该技术的应用为解决生物转化速率慢的问题和高效脱氮提供了新思路。为此,本文对现阶段氧化还原介体强化生物反硝化脱氮研究进展进行了综述。
氧化还原介体强化生物反硝化脱氮研究进展
Enhanced biological denitrification by redox mediators: A review
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摘要: 传统的生物脱氮包括氨化、硝化、反硝化,而反硝化过程是去除污水中含氮污染物的关键步骤。在实际污水处理过程中,低反硝化速率影响了生物系统脱氮效果。研究发现反硝化反应涉及多重电子传递过程,电子传递速率是影响反硝化速率的重要因素。氧化还原介体是一类能够在电子供体和电子受体之间可逆地被氧化和还原,加速电子传递的化合物。研究表明,氧化还原介体可以促进反硝化的电子转移速率,降低反应的活化能从而提高脱氮效果。本文总结了反硝化作用中电子传递过程,在此基础上,论述了氧化还原介体定义及作用,重点分析氧化还原介体的种类及其在反硝化过程中的应用,并指出了今后的研究方向,以提高氧化还原介体在反硝化过程中的应用。Abstract: The traditional biological nitrogen removal includes ammoniation, nitrification and denitrification. Denitrification is the key step in nitrogen removal. Slow denitrification rate is the bottleneck for nitrogen removal in practical wastewater. Denitrification involved multiple electron transfer processes, and electron transfer rate was the important factor for denitrification rate. Redox mediators were compounds that can be reversibly oxidized and reduced between an electron donor and an electron acceptor, thus electron transfer can be accelerated. The studies showed that the redox mediators can improve the denitrification by increasing the electron transfer rate and decreasing the activation energy of the reaction. This paper summarized the electron transfer process of denitrification and discussed the definition and functions of redox mediators. The kinds of redox mediators and their application in the denitrification process was reviewed. Moreover, the future research directions were pointed out to improve the application of redox mediators in denitrification.
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Key words:
- nitrogen removal /
- redox mediators /
- denitrification /
- electron transfer
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表 1 反硝化脱氮步骤及作用酶的特点
Table 1. The steps of denitrification and the characteristics of enzymes
步骤
Step作用酶
Enzyme酶的特点
Characteristics of enzymeNO3-+2H++2e-→NO2-+H2O NaR 存在膜结合硝酸盐还原酶和周质硝酸盐还原酶,这两种酶在表达条件、结构和编码基因有很大差异[11]。 NO2-+2H++e-→NO+H2O NiR 细胞周质酶,含有两种不同类型的细胞色素:Cyt c和Cyt d1[12]。 2NO+2H++2e-→N2O+H2O NoR 膜结合的细胞色素bc型酶,含有非血红素铁;该酶很不稳定,对NO有着极高的亲和力,可使NO浓度维持在低于毒害临界值的水平[11-12]。 N2O+2H++2e-→N2+H2O N2OR 细胞周质酶,易受pH的抑制,对氧的敏感度高于其他脱氮酶,该酶的每个亚基具有6个Cu原子[13],还含有细胞色素。 表 2 反硝化电子传递链中电子载体和常见RMs的ORP
Table 2. ORP of electron carriers in denitrifying electron transport chains and common RMs
反硝化中电子载体[13, 21-22]
Electron carriers for denitrification[13, 21-22]ORP/mV 氧化还原介体
Redox mediatorORP/mV 烟酰胺腺嘌呤二核苷酸(NADH) −320 黄素腺嘌呤二核苷酸(FAD)[23] −219 黄素蛋白(FP) −300 核黄素(RF)[24] −208 黄素单核苷酸(FMN) −219 2-羟基-1,4-萘醌(LAW)[25] −139 铁硫蛋白(FeS) −180 甲萘醌(ME)[25] +203 细胞色素b(Cyt b) +30 2-蒽醌二磺酸钠(AQS)[26] −218 辅酶Q(CoQ) +100 2,6-蒽醌二磺酸钠(AQDS)[25, 27] −184 细胞色素c(Cyt c) +254 绿脓菌素(PYO)[28] −32 硝酸盐(NO3−) +433 吩嗪-1-羧酸(PCA)[29] −114 亚硝酸盐(NO2−) +320 叶绿素a[30] −35 表 3 外源性RMs种类及其在反硝化过程中的应用
Table 3. Exogenous RMs species and applications in denitrification
种类
Kind氧化还原介体
Redox mediator强化效果
Enhanced effectiveness参考文献
Reference腐殖质 富里酸 总氮去除率提高1.34倍,降低亚硝酸盐累积和减少N2O排放 [7] 腐殖酸 与对照组比,HS-A2O可将总氮去除率提高10%,同时污泥产量减少30% [32] 可溶性醌类物质 AQDS 反硝化速率提高1.5倍 [20] AQ、ME和LAW 反硝化速率分别提高1.60、1.25、2.08倍 [25] NQS,AQDS,LAW 显著提高N和S元素的去除 [33] 2-羟基-1,4-萘醌 使硝酸盐还原率提高了1.38倍 [34] NQS 在低温下,NQS使硝酸盐去除率提高1.5倍,总氮去除率提高1.74倍 [35] 固定醌类物质 海藻酸钙固定蒽醌 反硝化速率提高约2倍 [36] 醋酸纤维素包埋醌类介体 硝酸盐去除率提高1.84倍 [37] 固定醌类物质 醌基功能型高分子生物载体
(PET-1,8-DCA)反硝化速率提高1.2倍 [38] 聚吡咯固定化介体
(AQS/PPY/ACF)减少亚硝酸盐的累积,反硝化速率提高约35%,具有很好的重复性 [39] 碳质材料 生物炭 TN去除率提高415%,N2O排放量减少78% [40] 生物炭 低温热解的生物炭强化了反硝化效果,高温热解的生物碳减少74.1%—99.9%的N2O排放 [41] 石墨微粒(MGPs) MGPs浓度为0.16 g·L−1,反硝化速率提高83.4% [42] 卟啉类化合物 氯化血红素 硝酸盐还原率提高2—3倍,反应活化能降低87% [31] 叶绿素 硝酸盐和亚硝酸盐还原率提高7.26倍和7.31倍 [30] 无机类 磷钼酸 硝酸盐还原速率提高3.93倍 [9] -
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