Abstract
Vertical flow filters (unplanted) and vertical flow constructed wetlands (planted), simple and inexpensive technologies to treat effectively volatile organic compounds (VOCs) contaminated water, consist of containers filled with granular material which is intermittently fed with contaminated water. Contaminants are removed from the water percolating through the filter due to a
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combination of biodegradation, volatilization and root uptake as the main removal processes. While biodegradation is the desired process, volatile emission might restrict the application of such systems. The focus of this thesis is on the assessment and quantification of individual removal processes in pilot-scale outdoor vertical filters and wetlands, located in Leuna, Germany used for VOCs contaminated groundwater treatment.
Numerical reactive transport simulations were performed to extrapolate findings from a pilot-scale intermittently loaded system used for domestic waste water treatment to predict the performance of such a system for treatment of water containing ammonium and two VOCs: benzene and methyl tertiary butyl ether – MTBE. Model simulations considered aerobic degradation and volatilization assuming different scenarios. Model results predicted depth and frequency of the intermittent water loading, as well as the degradation kinetics and material properties are relevant for the contribution of the individual processes to total VOCs removal. An appropriate combination of injection depth and composition of the filter material would result in increasing biodegradation enhanced by oxygen recharge, at a limited emission of volatiles.
For a selected pilot-scale vertical flow filter in Leuna, benzene and MTBE volatilization and biodegradation were investigated by a combination of field measurements and conservative numerical model simulations. Non-reactive tracer tests and flow rate measurements showed highly transient flow and heterogeneous transport regime in this filter system. Radon-222 (naturally occurring in the treated water and measured at the in- and out-flow of the filter) behavior reproduced by numerical simulations and extrapolated for benzene and MTBE, indicated high volatilization potential. In contrast, relatively low volatile concentrations were detected by passive samplers on top of the filter. Qualitative assessment of biodegradation by catabolic genes analysis at different depths of the filter; and semi-quantitative assessment by stable isotope fractionation analysis indicated high potential for aerobic degradation. Thus, despite the high volatilization potential, biodegradation is dominating the contaminant mass removal: VOCs entering the soil air are subject to subsequent biodegradation in the upper filter layer acting, which is acting as a biofilter and reducing atmospheric VOCs emissions.
Removal processes in a pilot-scale vertical flow filter and a vertical flow constructed wetland showing high VOCs removal, were analyzed by numerical reactive transport model simulations. Model calibration was based on porous material analyses, flow rate measurements, conservative solute and gas tracer tests, and in- and out-flow contaminant concentrations measurement. Numerical modeling results suggested very high biodegradation rates at both filters avoided volatile atmospheric emissions attributing the observed removal to biodegradation only. The simulation study thus supports the use of both of these vertical flow systems for the treatment of contaminated groundwater and the use of reactive transport modeling for the assessment of volatile organic compound removal in these high performance treatment systems.
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