Phenol degradation by Ralstonia eutropha: Colorimetric determination of 2-hydroxymuconate semialdehyde accumulation to control feed strategy in fed-batch fermentations

Phenol biodegradation by Ralstonia eutropha was modeled in different cultur e modes to assess phenol feeding in biotechnological depollution processes. The substrate-inhibited growth of R. eutropha was described by the Haldane equation with a K-s of 2 mg/L, a K-i of 350 mg/L and a mu(max) of 0.41 h(- 1). Furthermore, growth in several culture modes was characterized by the a ppearance of a yellow color, due to production of a metabolic intermediate of the phenol catabolic pathway, 2-hydroxymuconic semialdehyde (2-hms) whic h was directly correlated to the growth rate and/or the phenol-degradation rate, because these two parameters are coupled (as seen by the constant gro wth yield of 0.68 g biomass/g phenol whatever the phenol concentration). Th is correlation between color appearance and metabolic activity was used to develop a control procedure for optimal phenol degradation. A mass-balance equation modeling approach combined with a filtering step using an extended Kalman filter enabled state variables of the biological system to be simul ated. A PI controller, using the estimation of the phenol concentration pro vided by the modeling step, was then built to maintain the phenol concentra tion at a constant set-point of 0.1 g/L which corresponded to a constant sp ecific growth rate of 0.3 h(-1) close to the maximal specific growth value of the strain. This monitoring strategy, validated for two fed-batch cultur es, could lead, in self-cycling fermentation systems, to a productivity of more than 19 kg of phenol consumed/m(3)/d which is the highest value report ed to date in the literature. This system of monitoring metabolic activity also protected the bacterial culture against toxicity problems due to the t ransient accumulation of phenol. (C) 1999 John Wiley & Sons, Inc.