All three isomers of trichlorobenzene were reductively dechlorinated to monochlorobenzene via dichlorobenzenes in anaerobic sediment columns. The dechlorination was specific: 1,2,3‐ and 1,3,5‐trichlorobenzene were solely transformed to 1,3‐dichlorobenzene, while 1,4‐dichlorobenzene was the only product of 1,2,4‐trichlorobenzene transformation. Microorganisms were responsible for the observed transformations. Since monochlorobenzene and dichlorobenzene are mineralized by bacteria in the presence of oxygen, the process of reductive dechlorination may be an important initial step to obtain complete mineralization of otherwise recalcitrant trichlorobenzenes. This is especially true for the 1,3,5‐isomer, which seems to resist biodegradation in oxic environments.

The transformation of 3-chlorobenzoate (3CB) and acetate at initial concentrations in the wide range of 10 nM to 16 mM was studied in batch experiments with Pseudomonas sp. strain B13. Transformation rates of 3CB at millimolar concentrations could be described by Michaelis-Menten kinetics (K m , 0.13 mM; V max , 24 nmol ⅐ mg of protein ؊1 ⅐ min ؊1). Experiments with nanomolar and low micromolar concentrations of 3CB indicated the possible existence of two different transformation systems for 3CB. The first transformation system operated above 1 M 3CB, with an apparent threshold concentration of 0.50 ؎ 0.11 M. A second transformation system operated below 1 M 3CB and showed first-order kinetics (rate constant, 0.076 liter ⅐ g of protein ؊1 ⅐ min ؊1), with no threshold concentration in the nanomolar range. A residual substrate concentration, as has been reported for some other Pseudomonas strains, could not be detected for 3CB (detection limit, 1.0 nM) in batch incubations with Pseudomonas sp. strain B13. The addition of various concentrations of acetate as a second, easily degradable substrate neither affected the transformation kinetics of 3CB nor induced a detectable residual substrate concentration. Acetate alone also showed no residual concentration (detection limit, 0.5 nM). The results presented indicate that the concentration limits for substrate conversion obtained by extrapolation from kinetic data at higher substrate concentrations may underestimate the true conversion capacity of a microbial culture.

The minimum substrate concentration required for growth, S min , 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. S min 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 S min values were higher than the individual S min values in the mixture. The S min 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.

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.