Biotreatment of industrial wastewater is often challenged by operation under transient states with respect to organic loads, pollutants, and physical characteristics. Furthermore, the potential presence of inhibitory compounds requires careful monitoring and adequate process design. This review describes difficulties encountered in biological treatment of wastewater with highly variable influent characteristics. Typical design aspects of biological processes are presented and discussed with respect to their success in treating highly fluctuating wastewaters. In general, biomass retention is a key factor for dealing with highly fluctuating and/or inhibitory wastewater, but the how it operates also affects the stability of performance, as it was shown that dynamic operation instead of operation at a constant flow enhances biodegradation onset and more evenly distributed activity. Although ultimately stable effluent quality must be achieved, the microbial population stability is not necessarily high, as it was shown that microbial diversity and flexibility may play a critical role in functional stability.

Three upflow fixed bed bioreactors treating an aqueous stream containing 2-fluorobenzoate were operated for a period of 7 months, during which they were exposed to high organic loading rates and starvation. The reactors contained granular activated carbon (GAC), polyethylene (PE) particles and expanded clay (EC) respectively as growth support for microbial biofilms. The performance of the reactors was compared and the biofilm microbial population was followed by cell counting and denaturing gradient gel electrophoresis (DGGE). The reactor containing GAC always had 100% removal efficiency owing to the adsorption properties of the material combined with biodegradation. The GAC reactor also recovered better after starvation periods in the sense that it showed more stable behaviour than the reactors containing EC and PE. The highest biological elimination capacity was observed for the reactor containing EC, which reached 200 mg day −1 L −1 during reactor start-up, but during long-term operation the reactor containing GAC showed the highest biological elimination capacity, 140 mg day −1 L −1 . DGGE analysis indicated that starvation periods seemed to be responsible for shifts in the microbial population.

Two up-flow fixed bed reactors (UFBR) were operated for 8 months treating a model synthetic wastewater containing 2-fluorobenzoate (2-FB) and dichloromethane (DCM). The stability of the reactors under dynamic conditions, that is, sequentially alternating pollutants (SAP), shock loads, and starvation periods was assessed. Two support materials were used: expanded clay (EC) that does not adsorb 2-FB or DCM, and granular-activated carbon (GAC) that adsorbs 180 mg g(-1) of 2-FB and 390 mg g(-1) of DCM. The reactors were inoculated with a 2-FB-degrading strain (FB2) and a DCM degrader (TM1). 2-FB was fed at organic loads ranging from 0 to 800 mg L(-1) d(-1), while DCM was fed at 0-250 mg L(-1) d(-1). 2-FB or DCM were never detected at the outlet of the GAC reactor, while in the EC reactor outlet small amounts were observed. Nevertheless, the highest biological elimination capacity was observed in the EC reactor (over 700 mg L(-1) d(-1) of 2-FB). DGGE analysis revealed a fairly stable bacterial community with the largest shifts occurring during starvation periods and changes in feed composition. Several bacterial strains isolated from the reactors showed capacity for 2-FB degradation, while only strain TM1 degraded DCM.

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A bacterial strain able to degrade dichloromethane (DCM) as the sole carbon source was isolated from a wastewater treatment plant receiving domestic and pharmaceutical effluent. 16S rDNA studies revealed the strain to be a Xanthobacter sp. (strain TM1). The new isolated strain when grown aerobically on DCM showed Luong type growth kinetics, with 1(max) of 0.094 h(-1) and S (m) of 1,435 mg l(-1). Strain TM1 was able to degrade other aromatic and aliphatic halogenated compounds, such as halobenzoates, 2-chloroethanol and dichloroethane. The gene for DCM dehalogenase, which is the key enzyme in DCM degradation, was amplified through PCR reactions. Strain TM1 contains type A DCM dehalogenase (dcmAa), while no product could be obtained for type B dehalogense (dcmAb). The sequence was compared against 12 dcmAa from other DCM degrading strains and 98% or 99% similarity was observed with all other previously isolated DCM dehalogenase genes. This is the first time a Xanthobacter sp. is reported to degrade DCM.