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The minimum substrate concentration required for growth, Smin, was measured for Pseudomonas sp. strain B13 with 3-chlorobenzoate (3CB) and acetate in a recycling fermentor. The substrates were provided alone or in a mixture. Smin values predicted with kinetic parameters from resting-cell batches and chemostat cultures differed clearly from the values measured in the recycling fermentor. When 3CB and acetate were fed as single substrates, the measured Smin values were higher than the individual Smin values in the mixture. The Smin in the mixture reflected the relative energy contributions of the two substrates in the fermentor feed. The energy-based maintenance coefficients during zero growth in the recycling fermentor were comparable for all influent compositions (mean +/- standard deviation, 0.34 +/- 0.07 J mg [dry weight]-1 h-1). Maintenance coefficient values for acetate were significantly higher in chemostat experiments than in recycling-fermentor experiments. 3CB maintenance coefficients were comparable in both experimental systems. The parameters for 3CB consumption kinetics varied remarkably with the experimental growth conditions in batch, chemostat, and recycling-fermentor environments. The results demonstrate that the determination of kinetic parameters in the laboratory for prediction of microbial activity in complex natural systems should be done under conditions which best mimic the system under consideration.

Soil column experiments were performed to obtain insight in the different biological and physico-chemical processes affecting biodegradation of halogenated compounds under natural conditions in a water infiltration site. Lower chlorinated aromatic compounds could be degraded under aerobic conditions, whereas highly chlorinated compounds and chlorinated aliphatic compounds were mainly transformed under anaerobic conditions. Microorganisms which derive energy from reductive dechlorination were enriched and characterized. It was found that microbes could adapt to using chlorinated benzenes by evolution of new enzyme specificities and by exchange of genetic material. For halogenated pollutants, which are generally hydrophobic, sorption processes control the concentration available for biodegradation. The effects of very low concentrations of halogenated compounds on their biodegradability are described. The use of isolated bacterial strains to enhance biodegradation was evaluated with respect to their temperature-related activity and to their adhesion properties.

Microorganisms play an important role in the removal of synthetic organic compounds from the environment. This chapter gives an overview of the evolution of biodegradation pathways and describes the strategies that microorganisms have evolved to transform important molecular structures. The actual effectiveness of biodegradation in the environment is determined by the bioavailability of the compounds. As a general rule, one could state that the release rates of synthetic compounds should not exceed the environment’s ability to degrade them.

The biotransformation of 3-chlorobenzoate (3CB) by attached cells of Pseudomonas sp. strain B13 was studied in percolation columns operated at various flow rates and biomass contents. Steady-state residual effluent concentrations were compared with predictions from spreadsheet models describing the combined action of microbial and mass transfer kinetics. The effluent concentrations were successfully predicted above a critical ratio of flow rate and biomass. Below this critical point the steady-state residual effluent concentrations were higher than predicted and this deviation increased with decreasing flow rate:biomass ratios. The results are discussed in the light of convection-diffusion processes limiting the transport of both substrate and oxygen to the attached cells.

The discussion in the session was based on the statement papers of the participants. The most important conclusions from the papers and the discussion are summarized below.

Microbial transformation of chlorinated organic contaminants in columns, packed with sediment from a dune infiltration area and from River Rhine, has been investigated under different redox conditions. Low redox potentials favour reductive dechlorination of chlorinated ethylenes and benzenes to lower or non-chlorinated compounds. Under aerobic conditions transformation of 1,2- and 1,4 dichlorobenzene, but not of 1,3-dichlorobenzene took place. Biotransformation of chlorinated organic substances under denitrifying conditions was hardly observed. Only a slight removal of 1,2dichloro,4-nitrobenzene could be detected. Results indicate that biotransformations may also occur under conditions which approach a field situation (lowered temperature, presence of extra carbon source).