Monitoring the influence of toxic compounds on microbial denitrifying biofilm processes J. Li* and P.L. Bishop** * Department of Civil Engineering and Mechanics, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201, USA ** Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, Ohio 45221- 0071, USA Abstract Microelectrode measurements were conducted to obtain nitrate, pH and redox potential profiles within anoxic denitrifying biofilms. The influence of a toxic organic compound (acid orange 7) on biofilm microprofiles was also monitored using microelectrodes. The data provide evidence that the denitrifying biofilms were stratified into an anoxic layer and an anaerobic layer. The anaerobic zone might provide a niche for the biodegradation of recalcitrant organic compounds in biofilms. It was found that acid orange 7 and its biodegradation byproducts had only a slight impact on biofilm nitrate, pH and redox potential profiles. Keywords Azo dye; biofilm; denitrification; microelectrode Introduction Biofilm reactors are widely used for nitrogen removal in wastewater treatment processes due to their long biomass retention times. Two sequential processes, aerobic nitrification and anoxic denitrification, are generally designed for the biological elimination of nitrogen from wastewater. The anoxic denitrification process removes nitrogen by reducing nitrate (NO 3 – ) and/or nitrite (NO 2 – ) to nitrogen gas (N 2 ) and nitrous oxide (N 2 O). In this process, nitrogenous oxides serve as respiratory electron acceptors and are reduced by a unique suite of complex enzymes. With an increasing number of organic chemicals introduced into the environment, the performance of biofilm reactors is affected by the toxic organic compounds present in the wastewater. Many of these compounds are recalcitrant under aerobic environments. Azo dyes are a group of synthetic colorants, not readily biodegradable in aerobic activated sludge or biofilm processes (Ganesh et al., 1994). However, the electron-withdrawing characteristic of azo bonds (–N=N–) makes it possible for them to serve as electron accep- tors under anoxic conditions, thus interfering with the denitrification process. At the microscale, toxic compounds can affect biofilm performance by shaping the biofilm microbial community structure and function, which are determined by physico- chemical factors and genetic mechanisms (Wimpenny, 2000). The development of micro- electrode techniques is a revolutionary advance in the measurement of both absolute levels and changes in chemical species in biofilms. Microelectrodes provide in situ measurements of microbial activities within biofilm with very small disturbance. During the last decade, microelectrodes have been widely applied in the field of microbial ecology, giving valu- able information on the microscale distribution of oxygen consumption (Fu et al., 1994; Horn, 2000; Lewandowski, 1993; Revsbech, 1989), sulfate reduction (Yu and Bishop, 2000), nitrification and denitrification (de Beer et al., 1997). More microelectrodes rele- vant to microbial ecology, such as CH 4 , NO 2 – , and CO 2 , have been developed recently indi- cating the dynamics of this field. The goal of the present study is to characterize the microprofiles of nitrate, pH and redox potential for anoxic denitrifying biofilms and monitor in situ the effect of acid orange Water Science and Technology Vol 47 No 5 pp 211–216 © IWA Publishing 2003 211 Downloaded from http://iwaponline.com/wst/article-pdf/47/5/211/422477/211.pdf by guest on 06 March 2021