CONDUCTIVITY TRACER STUDIES FOR A FLUIDIZED-BED BIOREACTOR
|S.Y. Leung and R. L. Segar, Jr., Civil Engineering, University of Missouri-Columbia, Columbia, MO 65211.||
The study of bioremediation processes with tracer testing is pertinent because the process performance is governed physically by mass transport conditions coupled with the intrinsic biokinetics. Fluid hydrodynamics are particularly important when a high degree of contaminant removal is required. For instance, simplified rate equations describing chlorinated solvent degradation at Mg/L concentrations are typically first-order in substrate concentration.
Reactor theory predicts for simple first-order kinetics that removal in a bioreactor will increase as the dispersion decreases. However, to apply this concept to develop full-scale bioreactor designs, expedient methods for measurement of hydrodynamic characteristics for bench- and pilot-scale bioreactors is needed. Traditionally, tracer studies with various types of non-reactive chemicals have been used to quantify dispersion.
In this research, an automated conductivity tracer test was developed to measure the residence time distribution (RTD) of a cometabolic fluidized-bed bioreactor (FBBR). The FBBR contained sand-core bioparticles grown with phenol and it provided high removals of trichloroethene (TCE) at short detention times. Measurement of the hydraulic residence time, dispersion number, and bioparticle volume was needed for bioreactor characterization and optimization. Also, an understanding of the dispersive behavior of concentrated phenol pulses used in periodic feeding strategies was needed.
The tracer test apparatus was constructed with "off-the-shelf" components including a basic PC data acquisition system, a flow-through conductivity probe, a conductivity meter/transmitter, a peristaltic pump for sample withdrawal, and a diaphragm pump for brine pulse injection. This system allowed for non-disruptive testing during normal operation of the FBBR.
Brine pulses were monitored at the reactor inlet and outlet. The response of the conductivity sensor to sample variations was rapid and tracer curves were recorded to data files for subsequent analysis. Dispersion numbers and detention times were computed by fitting the advection-dispersion model to the tracer curves.
Tracer tests were performed with several types of sand and different operating conditions for biofilm growth. Typical hydraulic residence times were in the range 0.5 to 2.0 minutes and dispersion numbers ranged from 0.005 to 0.01. Consideration of the input pulse shape was found to impact the results of data analysis.
Some difficulties were encountered measuring conductivity in the presence of biofilm particulates and in the analysis of tracer data when thick biofilms caused tailing of the tracer. Based on the behavior of brine pulses, it was determined that phenol pulses at inhibitory concentrations over 2 g/L would be rapidly dispersed in the biological bed. The automated brine conductivity tracer method was shown to be useful for obtaining the RTD in the FBBR.
Key words: tracers, fluidized-bed, biofilm, dispersion, residence time distribution
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