Long‐term performance of bioreactors cleaning mercury‐contaminated wastewater and their response to temperature and mercury stress and mechanical perturbation

Abstract

The long-term performance of bioreactors retaining mercury from contaminated industrial wastewater was analyzed at the laboratory scale, and its response to mechanical perturbations (gas bubbles and shaking) as well as to physical (increased temperature and hydraulic load) and chemical stresses (increased mercury concentration) likely to occur during on site operation was studied. Two packed-bed bioreactors with 80-cm(3) lava chips as biofilm carrier were inoculated with nine Hg(II)-resistant natural isolates of alpha- and gamma-proteobacteria. Chloralkali wastewater containing ionic mercury (3.0 to 9.7 mg/L Hg(2+)), amended with sucrose and yeast extract, flowed through the bioreactors at 160 mL/h. During the 16-month investigation the bioreactors showed no sign of depleted performance in terms of mercury-retaining capacity. After 16 months, both bioreactors still retained 96% of the mercury load. The performance of the bioreactors was sensitive to mechanical perturbations (e.g., sheer forces of gas bubbles). Shifts to higher Hg(2+) inflow concentrations initially decreased the mercury retention efficacy slightly. However, the bioreactors could adapt to Hg(2+) concentrations of up to 7.6 mg/L within several days. Old biofilms were less affected than the younger ones. The performance of the bioreactors was not affected by an increase in temperature up to 41 degrees C and an increased volumetric load (up to 240 mL/h). The bioreactors regained activity spontaneously after the stress had stopped. Recovery could be accelerated by increased nutrient concentration, although this may lead to blocking of the packed bed.

Spatially Oscillating Activity and Microbial Succession of Mercury-Reducing Biofilms in a Technical-Scale Bioremediation System

“…Compared to laboratory-scale bioreactors operated under constant conditions, the mercury-reducing microbial biofilms of the technical-scale bioreactor were exposed to oscillating mercury concentrations with peak values of 10 mg/liter and temperatures up to 50°C. In previous laboratory-scale bioreactor experiments, partial breakthrough of mercury at inflow concentrations between 7 and 9 mg/liter was observed (29). Here, we observed that mercury concentrations above 6 mg/liter caused an inhibition of respiration and mercury reduction.…”

“…In previous experiments the efficient and cost-effective use of Hg(II)-resistant bacteria in packed-bed bioreactors was demonstrated for the retention of mercury from wastewater of three European chloralkali plants, both in a laboratory-scale plant (27,29) and in a technical-scale pilot plant (31). This study elucidates the active part of the bioremediation process, the mercury-reducing biofilms, during the operation of the pilot plant as reported previously (31).…”

“…In a previous experiment, biofilm diversity and activity during operation were monitored by analyzing bioreactor effluent samples (28,29). In this study, the biofilm community was monitored directly by sampling both the solid and liquid phases of four bioreactor horizons during operation.…”

Developments in Bioremediation of Soils and Sediments Polluted with Metals and Radionuclides: 2. Field Research on Bioremediation of Metals and Radionuclides

“…Long-term performance of the reactors has been studied, with no loss of the entrapped Hg (0) from the system over 16 months (VON CANSTEIN et al 2001). Although the reactors were sensitive to mechanical and physical stresses (e.g.…”

Genetic Engineering of Bacteria for Environmental Remediation of Mercury

“…Bioremediation strategies including biotransformation, biosorption, and bioprecipitation of mercurials have been developed and rarely been applied to remediation of mercurials in the environment. [4][5][6] The adsorptive treatment and bioprecipitation process are generally sensitive to the ambient conditions, e.g., pH and the presence of other metals or metal chelators. In particular, they lack specificity, which may cause difficulties in the recovery of the desired metals.…”