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Btex Treatment in a Solid-Liquid Two-Phase Partitioning Bioscrubber: Comparison of Stirred Tank and Airlift Configurations

Jennifer V. Littlejohns, Department of Chemical Engineering, Queen's University, 19 Division St, Dupuis Hall, Kingston, ON K7L 3N6, Canada and Andrew J. Daugulis, Chemical Engineering, Queen's University, 19 Division St, Dupuis Hall, Kingston, ON K7L 3N6, Canada.

Benzene, toluene, ethylbenzene and o-xylene, collectively known as BTEX, are toxic VOCs that are present in many gaseous waste streams produced during site remediation, particularly soil vapor extraction (SVE). Traditionally, physical or chemical treatment methods have been used in order to stay within regulatory emission guidelines. However, biological treatment methods, such as biofilters, which provide a low-cost, energy efficient and effective alternative to the destruction of toxic volatile chemicals (Datta and Allen, 2005), have been shown to be successful in treating low concentrations of BTEX components in gaseous waste streams (Oh and Bartha, 1997; Torkian et al., 2003). Although biofilters are effective for the treatment of low concentrations of VOCs, there has been recent interest in biological BTEX treatment beyond the current abilities of biofilters; that is, the biological treatment of high and fluctuating loadings of BTEX (Stewart et al., 2001; Mason et al., 2000).

A two-phase partitioning bioscrubber (TPPB) has been used successfully to treat waste gases containing high and fluctuating concentrations of individual BTEX compounds (Nielsen et al., 2005a; Boudreau and Daugulis, 2006). TPPBs consist of a cell containing aqueous phase and a nontoxic, non-bioavailable second phase that can sequester high and fluctuating concentrations of toxic substrates and release them to the aqueous phase for subsequent degradation based on metabolic demand. This provides the ability to treat high concentrations of toxic compounds with low water solubility by alleviating toxic levels in the aqueous phase. Organic solvents have traditionally served as the immiscible phase in a TPPB, however, these systems are typically limited to combinations of single VOCs and pure strains of microorganisms due to the potential bioavailability of the organic solvent second phase. Significantly, a treatment system for waste gases containing BTEX must allow for the use of a bacterial consortium, as there is no known pure strain of bacteria with the ability to effectively degrade all BTEX components (Bielefeldt and Stensel, 1999). Recently, solid polymer beads have been used in place of the organic solvent in TPPBs which provide the ability to use a bacterial consortium as polymers are commonly nonbioavailable to a wide range of bacteria (Prpich and Daugulis, 2004).

This presentation will report performance of a bench scale 3L stirred tank TPPB consisting of an aqueous phase containing a bacterial consortium and a polymeric phase of small silicone rubber pellets (solid volume fraction 0.1) used to treat gaseous waste streams containing a mixture of high and fluctuating concentrations of BTEX. Such a mixture of VOCs has not been treated in a TPPB to date. Silicone rubber was systematically selected as the sequestering phase due to its superior affinity for BTEX as determined by its favourable diffusion and partition coefficients and its ability to enhance the oxygen transfer rate into a TPPB (Littlejohns and Daugulis 2007). Fluctuations imposed on the TPPB system in the form of inlet loading step changes reveal that the system can achieve overall removal efficiencies of greater than 95% while obtaining overall elimination capacities of up to 281.5 mg/Lh. TPPB operation, however, succumbs to toxic substrate levels between step changes of 6 and 10 times the nominal loading value (360 to 600 mg/Lh). Aqueous phase and polymer phase concentrations that were monitored during these step changes show that the polymer phase sequesters large amounts of BTEX (70% to 93% of total BTEX in the working volume), thus buffering the aqueous phase from toxic levels.

Scale up of mechanically agitated TPPBs may not be feasible due to their excessive energy inputs. Airlift bioreactors require less energy and ease design scale up, and therefore may be a potential practical replacement for traditional stirred tank configurations. Performance of a bench scale 10L airlift TPPB containing a bacterial consortium and a polymeric phase of silicone rubber pellets (solid volume fraction 0.1) will be discussed and performance comparisons will be made between both stirred tank and airlift configurations. In addition, several critical parameters affecting successful airlift TPPB operation will be discussed including solid phase suspension and VOC mass transfer.


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