(126e) The Impact of Metabolic Uncouplers on the Elimination Capacity of Toluene in a Gas-Phase Biofilter

A series of metabolic uncouplers were tested in a lab-scale biofilter to determine the effect on the elimination capacity (EC) of toluene.  Uncouplers are lipophilic weak acids capable of carrying protons across the cell membrane, thereby influencing ATP production.  Uncouplers have been used successfully in activated sludge systems to lower the sludge yield but their use has not been reported in gas phase biofilters.  The hypothesis was that as biofilters (without supplemental nutrient addition) are operated in an energy excess environment driven by maintenance energy, uncouplers could potentially increase the specific removal rate per cell thereby increasing the volumetric removal rate, EC (g m^-3 h^-1).

The lab-scale biofilter system combined a differential reactor design with a hanging water column to give good control over most environmental parameters including water content of the unsaturated soil.  It also allowed easy addition and removal of the uncouplers to the packing material. Soil was the packing material tested at 30 ^oC and -10 cm_H2O matric potential buffered at pH 7.  Toluene in air was used as the feed gas at 25 ml min^-1and the inlet load was varied by adjusting the inlet toluene concentration between 0 – 300 ppm using a diffusion tube system. The metabolic uncouplers tested included benzoic acid, pentachlorophenol (PCP), m-chlorophenol, trichlorophenol, malonic acid, CCCP and FCCP.  These uncouplers were shown to have a positive influence in activated sludge systems and also did not contain nitrogen, which was the limiting nutrient in the soil system.

The system response varied between the uncouplers. The concentrations tested were based on effective concentrations in the activated sludge literature. Benzoic acid (10 mM) and m-chlorophenol (160 mM) irreversibly decreased the EC by approximately 60% and 50% respectively. CCCP (0.002 mM) reversibly decreased the EC by 30% but the effect was reversible.  FCCP (0.01 mM), malonic acid (25 mM) and trichlorophenol (4.1 mM) had slight impacts on the EC.  PCP (140 mM) increased the EC by 60% albeit irreversibly. 

As only the CCCP response was reversible, the uncoupler mechanism in these mixed culture soil microcosms was unclear.  The mechanism is being investigated in pure cultures to try to elucidate the mechanism.  Also as most of these compounds are considered water pollutants, even the 60% increase in the EC for PCP is of questionable benefit in a full-scale biofilter.  Even though the water release from a traditional biofilter is low, the addition of PCP would greatly complicate the water disposal process. The increase in EC would need to be much greater to justify the added complexity.