Biodegradation of phenol in a continuous process: comparative study of stirred tank and ¯uidized-bed bioreactors G. Gonz alez * , M.G. Herrera, M.T. Garc  õa, M.M. Pe~ na Department of Chemical Engineering, University of Valladolid, 47011 Valladolid, Spain Received 12 November 1999; received in revised form 15 June 2000; accepted 21 July 2000 Abstract The paper presents the main results obtained from the study of the biodegradation process of phenol by a pure culture of Pseudomonas putida ATCC 17484. The experimental work was carried out in two dierent systems: a stirred tank where cells grew as a suspended culture and a ¯uidized bed where cells were immobilized within calcium alginate gel beads. The in¯uence of the hy- draulic residence time (HRT) and organic loading rate on the removal eciency of phenol was determined for both bioreactors. Also, the stability of the ¯uidized-bed bioreactor (FBB) in terms of its ability to withstand sudden phenol overdoses is also reported. Experimental values indicated that both bioreactors showed high phenol degradation eciencies, higher than 90%, even for a phenol loading rate in the in¯uent as high as 4 g phenol/l day. The FBB showed better performance than the suspended-culture bioreactor due to its better control and because it could operate with lower HRT. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Phenol degradation; Pseudomonas putida; Immobilized cells; Fluidized bed 1. Introduction Phenol is a highly toxic organic compound even at low concentrations and its presence in aqueous media is severely limited by regulations, the maximum permitted concentration level being 0:5 mg=l for drinking water and 0.5±1 mg/l for industrial wastewater, according to the actual Spanish legislation (BOE 14-9-90). Most of the industrial euents with high phenol concentrations come from re®neries, the plastic indus- try, and phenolic resins production processes. Ecient treatment of these wastewaters can be conducted by ei- ther conventional physical±chemical (Zilli et al., 1993; Lin and Chuang, 1994) or biological techniques (Sin- glenton, 1994; Jianmin et al., 1993; Autenrieth et al., 1991). However, these treatments are very complex and expensive. This situation is triggering the development of new treatment technologies for these wastewaters with high phenol concentrations, especially technologies claiming a small production of additional wastes. In this sense, the application of biodegradation pro- cesses using pure cultures of microorganisms specially adapted to metabolize the contaminant appears to be an alternative to these conventional processes (Dluh y et al., 1993; Buitr on and L opez-Marin, 1998; Kapoor et al., 1998; Hannaford and Kuek, 1999). The aim of this work was to study the performance of two continuous biodegradation processes treating phe- nol using microorganisms specially adapted to phenol such as Pseudomonas putida. Both bioreactors, the stir- red-tank bioreactor (CSTB) with free cells, and the ¯u- idized-bed bioreactor (FBB) with immobilized cells, were operated at dierent operation conditions. 2. Methods 2.1. Culture and growth medium A strain of P. putida ATCC 17484, biotype B, from the Laboratory of Microbiology Voor (Gante, Belgium) was used as pure culture. The bacteria were maintained on slants of agar containing (in g/l): beef extract, 1; yeast extract, 2; peptone, 5; European bacteriological agar, 15 (Pronadisa, Spain); NaCl, 5; KH 2 PO 4 ; 2:39; as was re- ported in a previous work (Gonz alez and Herrera, 1995). The cultures were grown aerobically in 250 cm 3 ¯ask ®lled to 150 cm 3 with growth medium and stirred Bioresource Technology 76 (2001) 245±251 * Corresponding author. Tel.: +34-983-42-31-70; fax: 34-983-42-36- 16. E-mail address: gerardo@siq.iq.cie.uva.es (G. Gonza Âlez). 0960-8524/01/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 0 0 ) 0 0 0 9 2 - 4