A Proposed Transient Model for Cometabolism in Biofilm Systems Pascale Champagne, Paul J. Van Geel, Wayne J. Parker Department of Civil and Environmental Engineering, Carleton University, Ottawa, Ontario, telephone: 613-520-2600, ext. 4628; fax: 613-520-3951; e-mail: pchampag@ccs.carleton.ca Received 11 November 1997; accepted 7 May 1998 Abstract: A dynamic model was developed to describe the behaviour of primary and secondary substrates in a biofilm reactor. The model incorporates structured ki- netics to describe the generation and consumption of reducing power in the catabolic and respiratory sub- systems, respectively. Secondary substrate transforma- tion through oxygenolytic or reductive mechanisms can be modelled under either single or dual limitation of the electron donor and electron acceptor substrates. An ex- ample simulation of a theoretical biofilm system was performed. © 1998 John Wiley & Sons, Inc. Biotechnol Bioeng 60: 541–550, 1998. Keywords: biofilm; dual substrate limitation; cometabo- lism; secondary substrate; biofilm modeling INTRODUCTION In biological wastewater treatment, many contaminants that are of concern, if released into the environment, may be subject to biodegradation through cometabolic processes (e.g., quinoline (Malmstead et al., 1995), carbon tetrachlo- ride (Bae and Rittmann, 1990)). Cometabolism refers to the transformation of a secondary substrate by metabolic reac- tions that do not contribute to the growth of microorgan- isms. These reactions are significant in terms of secondary degradation because the secondary substrates may be pre- sent at concentrations below those required to sustain a microbial population, or because the rate-limiting reactions require energy that must be provided by a primary substrate. Biodegradation of these compounds will hence be linked to the presence and utilization of the primary substrates, which consist of electron donors and electron acceptors. These primary substrates influence the quantity of active biomass that is present, and may either provide energy for cometa- bolic reactions when metabolized, or act as a reactant in the transformation reaction. Numerous studies have demonstrated the relationship be- tween primary substrate utilization and biomass growth un- der single substrate limiting conditions (Annachhtre and Khanna, 1987; Atkinson and Davies, 1974; Belhadir et al., 1988; Capdeville et al., 1988; Hamoda, 1989; Kim and Suidan, 1989; Lawrence and McCarty, 1970; Rittman, 1984; Rittmann and Manem, 1992; Rittmann and McCarty, 1978, 1980, 1981; Sa ´ez and Rittmann, 1988, 1990; Sa ´ez et al., 1984; Shamat and Maier, 1980; Skowlund, 1990; Suidan and Wang, 1985; Suidan et al., 1987; Tabak et al., 1990; Trulear and Characklis, 1982; Wanner and Gujer, 1984). In general, the Monod relationship has been found to ad- equately represent this relationship. Fewer studies have ex- amined growth when both the electron donor and electron acceptor are limiting (Barton and McKeown, 1986; Borden and Bedient, 1986; Howell and Atkinson, 1976; Kissel et al., 1984; Lau et al., 1984; Odencrantz et al., 1990; Ritt- mann and Dovantsiz, 1983; Sinclair and Ryder, 1975; Sykes, 1973; Williamson and McCarty, 1976a,b). However, recent studies (Bae, 1992; Bae and Rittmann, 1996a,b) have indicated that a multiplicative double Monod relationship adequately represents this behavior. Several relationships between primary substrates and the degradation of secondary substrates through oxygenolytic and reductive reactions have been described in the litera- ture: Hooker et al. (1994), Bae and Rittmann (1995), and Wrenn and Rittmann (1995, 1996). The model of Hooker et al. (1994) was derived through responses observed in ex- perimental studies on the biodegradation of CCl 4 by deni- trifying bacteria. The models developed by Wrenn and Ritt- mann (1995, 1996) and Bae and Rittmann (1995) relate the rate of transformation of a secondary substrate to the inter- nal reducing power of the biomass, as indicated by the ratio of reduced to oxidized nicotinamide adenine dinucleotide (NADH/NAD) and the ratio of adenosine triphosphate to adenosine diphosphate (ATP/ADP). These ratios are influ- enced by the rate of metabolism of the electron donor and the relative abundance of the electron acceptor. In suspended growth systems, the bulk-phase concentra- tions of the primary substrates provide the main driving force for secondary substrate transformation. However, in biofilm systems, mass transfer and diffusive limitations within the biofilm will influence the relative concentrations of primary and secondary substrates, as well as the level of biomass activity. In addition, the concentrations of electron donor and electron acceptor relative to each other will also change through the biofilm depth. Thus, the rate of cometa- bolic biodegradation of a secondary substrate can also be expected to vary through the depth of the biofilm. Hence, Correspondence to: Pascale Champagne Contract grant sponsors: The Natural Sciences and Engineering Re- search Council of Canada © 1998 John Wiley & Sons, Inc. CCC 0006-3592/98/050541-10