Effects of 4-nitrophenol on H2 and CO levels in anaerobic propionate systems

Parameters used for anaerobic process control include volatile acids, COD, gas volume and composition. Measuring headspace trace gases (hydrogen and carbon monoxide) are becoming more common in anaerobic research. For economy, it is important to know whether a single parameter (hydrogen, carbon monoxide or volatile acids) can be sufficient for process control. In this research, 4-nitrophenol (4-Np) inhibited, anaerobic, propionate fed, chemostats were used to study the limitation of using (i) headspace hydrogen (or carbon monoxide) or (ii) effluent volatile acids (acetate and propionate) as monitoring parameters. It was found that hydrogen utilizing methanogens were not affected with up to 66 mg/1 4-Np at 16-day SRT and with up to 33 mg/1 at 10-day SRT. This led to no hydrogen accumulation even though acetate and propionate accumulated significantly showing inhibition of both propionate-utilizing acetogens and acetate-utilizing methanogens. This shows that hydrogen measurements alone would be misleading in this case. Measurement of both volatile acids and hydrogen is recommended unless prior comparative knowledge is available on the effects of a toxicant on propionate-, acetate- and hydrogen-utilizers. CO went up to 3 ppm from 0.8 ppm with a spike of 66 mg/1 4-Np at 16-day SRT. In most cases CO was not very useful as a monitoring parameter.