Miljøprojekt Nr. 1173 2007 Teknologiudviklingsprogrammet for jord- og grundvandsforurening – Rensning af MTBE-forurenet grundvand vha. propanoxiderende mikroorganismer

Rensning af MTBE-forurenet grundvand vha. propanoxiderende mikroorganismer

Summary

Purpose and strategy

The purpose of this project is to investigate whether MTBE-contaminated groundwater can be treated cost-effectively in on-site bioreactors containing propane-oxidizing micro-organisms. The strategy was to perform a series of laboratory experiments in batch and bench-scale reactors in order to obtain detailed information on the strengths and weaknesses of the technology. Based on the results of these experiments, a series of technical and financial up-scaling calculations were made.

Batch experiments

A series of batch experiments examined an extensive series of factors that could play a major role in the success of starting and running an on-site reactor using the mixed culture of propane-oxidizers. It was found that it is relatively easy to select and grow a culture of MTBE-degrading propane-oxidizers. The present culture was shown to be relatively temperature sensitive, and this explains why degradation of MTBE at typical Danish groundwater temperatures of 10-15°C is relatively slow. Even though the culture is aerobic, it was shown to be relatively insensitive to oxygen concentration. Hence, 88% of the degradation capacity is retained at oxygen concentrations above 2 mg/L. The culture degrades benzene although the presence of benzene inhibits the degradation of MTBE.

Reactor experiments

The reactor experiments were performed in a so-called sequencing batch reactor, where the groundwater is pumped into a retention tank. At intervals, the water is pumped from the retention tank into the bioreactor, where it is treated. In the bioreactor, the treatment proceeds as in a closed batch experiment, and when the MTBE concentration reaches the specified outlet concentration, the cleansed water is let out of the reactor, and the reactor is refilled with polluted water from the retention tank. Based on the experiments, it is recommended that the initial propane concentration is set relatively high (~0,3 mg/L) in order to obtain a good balance between a permanently high MTBE degradation capacity, and minimal consumption of propane and oxygen. It is hypothesized that a good indicator of a reactor with a good balance between the degradation capacity and consumption of propane can be observed through a small constant increase in biomass concentration; even though the reactor is not run with the purpose of biomass production.

Prospects for on-site treatment

Based on the results of the batch and reactor experiments, six up-scaling scenarios, with different assumptions regarding the inlet and outlet concentrations and the temperature dependency of the degradation processes were constructed, and financial calculations were performed for three different hydraulic loadings (2, 5 and 10 m³/h) and two different write-off periods (5 and 10 years). For these scenarios, unit treatment prices of DKK 3.4 to 8.7 per m³ were calculated, and it was concluded that the technical-financial potential of the technology is as a mass removal unit (80-90% reduction of the MTBE concentration) in combination with another technology for reaching low outlet requirements; e.g. activated carbon.