Rensning af MTBE forurenet grundvand i bioreaktor med MTBE som primært substrat

Biological Degradation of MTBE as a Primary Substrate

Summary

In Denmark, an increasing number of contaminated sites at gas stations have been discovered as a result of leaking installations. The contaminants consist of different hydro-carbonates and MTBE. However, MTBE has only been applied for a limited number of years, and it is therefore likely that a considerable number of contaminations have yet not reached the groundwater aquifer. Groundwater contaminated with gasoline has traditionally been treated by conventional methods such as air stripping or by filtration through activated carbon. The main purpose was to remove aromatic compounds (BTEX), though it is expected that in the future treatment should include MTBE. The capacity for removal of MTBE by activated carbon filtration is limited compared to BTEX, and the operational costs at the treatment plants will, therefore, be higher.

Various bacterial cultures that can degrade MTBE have been isolated. In this project, the potential for biological degradation of MTBE as a primary substrate is studied. It is the purpose of the project to apply biological processes for the treatment of MTBE contaminated groundwater. For a range of other organic pollutants, biological treatment was shown to be economically competitive compared to the conventional water treatment methods.

In this project, the growth kinetics of the MTBE degrading bacteria was studied. The bacteria was studied in suspension and attached to filter material with the purpose of determining the most appropriate reactor type. Two different bench-scale reactors were designed based on the results from the laboratory experiments: a) A sequencing batch reactor based on a suspended bacterial culture, and b) a biological filter based on an attached bacterial culture.

The experiments show, that MTBE is biodegradable under aerobic conditions, where MTBE acts as primary substrate. MTBE could be degraded from high concentrations (mg/L) to a concentration of a few μg/L in the biological reactor. The kinetics of MTBE degradation was quantified for a suspended culture using radioactive labelled MTBE. The modelling of the results enabled the estimation of the size of MTBE degrading biomass, the growth rate, the yield coefficient, and the half saturation constant.

The experiments show that the degradation could take place at low temperature (10°C), though the degradation rate was much higher at 20°C.

No toxicity of water was found (Microtox) after degradation of 20 mg MTBE/L.

The growth rate of the MTBE degrading biomass was slow, which means that a significant amount of time is required to start-up a full-scale plant. In addition, the full-scale treatment plant will be vulnerable to operational stops and variations in the MTBE load.