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To increase the survival potential in wastewater, a selected flora should have the desired degradative ability, tolerance to cocontaminants, and a natural spatial and tem porary abundance (van der Gast et al., 2003). Another possibility would be to add organisms containing plasmids having a broad host range, permitting conjugation and DNA exchange between different species or genera of bacteria.

To enhance degradation of organic compounds in activated sludge or by a biofilm, selected specialized bacteria, genetically modified bacteria, or bacteria as plasmid donors for degradative pathways can be added (McClure et al., 1991; Frank et al., 1996). Plasmid exchange from donor to recipient cells would be advantageous to the recipient bacteria, because the plasmids harbor genes whose products function in pathways for xenobiotic degradation, which would thus extend the substrate spectrum of the recipients. The transfer of naturally occurring mercury-resistance plasmids between Pseudomonas strains in biofilms was shown to occur rapidly (Bale et al., 1988).

Selvaratnam et al. (1997) added a phenol-degrading strain of Pseudomonas putida to an activated sludge SBR reactor, which had removed 170 mg L-1 of phenol before being augmented with the Pseudomonas putida strain. Whereas the original phenol-degrading activity was partially lost in the nonaugmented reactor upon further operation, the augmented reactor almost completely degraded the phenol. A more convincing approach would have been to use non-phenol-degrading activated sludge for this experiment, although the survival of the catabolic plasmid dmpN of Pseudomonas putida and its expression in the reactor biomass was demonstrated for 44 d by molecular biology techniques under steady-state conditions in the laboratory.

Successful bioaugmentation experiments in an upflow anaerobic sludge blanket reactor were reported by Ahring et al. (1992), who introduced a suspension of a pure culture of Desulfomonile tiedjei or a three-member consortium into an UASB reactor. They observed a rapid increase in the dehalogenation of 3-chlorobenzoate, whereas nonamended parallel incubations had no dehalogenating activity. Even after several months at 0.5-d hydraulic residence time, which was shorter than the generation time of Desulfomonile tiedjei, dehalogenating activity was still observed, and Desul-fomonile tiedjei could be found within the biofilm by the use of antibody probes. More recently, an UASB reactor was supplemented with Dehalospirillum multivorans to improve its dehalogenating activity (Horber et al., 1998). In contrast, Margesin and Schinner (1998) found, for biological decontamination of fuel-contaminated wastewater, that stimulation of the autochthonous flora by adding a mineral mix enhanced biodegradation to a larger extent than bioaugmentation with a cold-adapted mixed inoculum containing Pseudomonas sp. and Arthrobacter sp.

Bioaugmentation with a complex inoculum, which naturally occurs in metalwork fluids and contains strains of Clavibacter sp., Rhodococcus sp., Methylobacterium sp. and Pseudomonas sp. is very effective in degrading COD in wastewater. The augmented consortium degraded 67% of the COD (48 g L-1) and was therefore 50%-60% more effective than the indigenous flora. In-situ analysis showed that 100 h after the introduction, the augmented consortium constituted more than 90% of the population (van der Gast et al., 2003).

Degradation of many xenobiotics in anaerobic or aerobic pure cultures or complex ecosystems has been demonstrated. Whereas for 'intrinsic sanitation' of polluted soil, the time is not limited, so long as the pollutants are adsorbed tightly to the soil matrix, degradation of xenobiotics in wastewater must be completed during the residence time in the treatment system. Thus, merely the presence of a degradation potential within a wastewater ecosystem is not sufficient, but degradation rates must be high enough, and degradation must be faster than the residence time of bacteria in suspended systems. Supplementation with microorganisms as a potential tool to increase the degradation speed or to increase the degradation potential in wastewater cannot in general be considered a state-of-the-art procedure yet.

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