Parameters used for anaerobic process control include volatile acids,
COD, gas volume and composition. Measuring headspace trace gases (hydr
ogen and carbon monoxide) are becoming more common in anaerobic resear
ch. For economy, it is important to know whether a single parameter (h
ydrogen, carbon monoxide or volatile acids) can be sufficient for proc
ess control. In this research, 4-nitrophenol (4-Np) inhibited, anaerob
ic, propionate fed, chemostats were used to study the limitation of us
ing (i) headspace hydrogen (or carbon monoxide) or (ii) effluent volat
ile acids (acetate and propionate) as monitoring parameters. It was fo
und that hydrogen utilizing methanogens were not affected with up to 6
6 mg/l 4-Np at 16-day SRT and with up to 33 mg/l at 10-day SRT. This l
ed to no hydrogen accumulation even though acetate and propionate accu
mulated 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 bo
th volatile acids and hydrogen is recommended unless prior comparative
knowledge is available on the effects of a toxicant on propionate-, a
cetate- and hydrogen-utilizers. CO went up to 3 ppm from 0.8 ppm with
a spike of 66 mg/l 4-Np at 16-day SRT. In most cases CO was not very u
seful as a monitoring parameter.