ORIGINAL PAPER G. A. Hill á A. J. Daugulis Phenol inhibition kinetics for growth of Acetobacter aceti on ethanol Received: 6 November 1998 / Received revision: 8 February 1999 / Accepted: 12 February 1999 Abstract Acetobacter aceti have been grown on ethanol under inhibitory conditions created by high concentra- tions of phenol. A de®ned medium with no vitamin or amino acid supplements has been used such that ethanol was the sole carbon substrate. The culture temperature was maintained at 30 °C while the pH was manually controlled to fall within the range 4.5±6.0 during ethanol consumption. Growth on ethanol at a few thousand milligrams per litre (below the known inhibitory level) resulted in a maximum speci®c growth rate of 0.16 h )1 with a 95% yield of acetic acid, followed immediately by acetic acid consumption at a growth rate of 0.037 h )1 . Phenol was found to inhibit growth by decreasing both the speci®c growth rate and the biomass yield during ethanol consumption. On the other hand, the yield of acetic acid during ethanol consumption and the yield of biomass during acetic acid consumption remained con- stant, independent of phenol inhibition. A model is presented and is shown to represent the phenol-inhibited growth behaviour of A. aceti during both ethanol and acetic acid consumption. Introduction Industrial wastes often contain mixtures of organic compounds that are emitted as liquid (wastewater), gas (chimneys) or solid (sludge) discharges. The range of organic compounds emitted to the environment is vast and includes aliphatic (including alcohols), alicyclic and aromatic (including phenols) structures (Schwarzenbach et al. 1993). Ethanol and phenol are both water-soluble compounds found in the discharges of several manu- facturing and food processes. They can be found in combination in wastewater collection systems. Biologi- cal treatment of industrial wastewater is a simple and economical technology, but at the microbiological level it is a very complex process. This is due to a wide di- versity of microorganisms that interact in complex ways to consume the organic compounds. Models of these systems are always much simpler than the complex conditions of the treatment lagoon, sometimes involving only one lumped biomass species (X) and one lumped organic species (S). We have previously used both Pseudomonas putida and Alcaligenes eutrophus for bioremediation of pure phenol and mixtures of chlorinated phenols and phenol (Dapaah and Hill 1992; Hill et al. 1996). Models have been developed that treat the chemical species individu- ally as well as predict the inhibition eects of the com- bined substrates. However, both microbial species are unable to metabolize ethanol. In this work, we have studied the consumption of ethanol by a very common aerobic microorganism, Acetobacter aceti. Although this bacterium is not normally found in wastewater treatment lagoons, it does provide a unique feature in that it can metabolize ethanol but not phenol (De Ley et al. 1984). A great deal is known about A. aceti because of its use in the commercial production of vinegar (Asai 1968). The metabolism of ethanol to acetic acid involves the production of acetaldehyde as an intermediate. In the absence of ethanol, Acetobacter are capable of oxidizing acetic acid to produce formaldehyde and carbon diox- ide. Asai (1968) also reports that A. aceti require no vitamins and are able to assimilate ammonium salts as their sole source of nitrogen. In spite of this, most ex- perimental studies and all industrial applications involve the utilization of complex nutrient sources such as yeast extract or fruit juices. This leads to reproducibility concerns since such nutrients can vary from day to day or batch to batch. Appl Microbiol Biotechnol (1999) 51: 841±846 Ó Springer-Verlag 1999 G. A. Hill (&) Department of Chemical Engineering, 110 Science Place, University of Saskatchewan, Saskatoon, SK, S7N 5C9, Canada e-mail: hill@engr.usask.ca Tel.: +1-306-966-4765 Fax: +1-306-966-4777 A. J. Daugulis Department of Chemical Engineering, Dupuis Hall, Queen's University, Kingston, ON, K7L 3N6, Canada