The impact of metabolic uncouplers on the performance of a toluene-degrading biotrickling filter

Type of content
Theses / Dissertations
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Thesis discipline
Chemical Engineering
Degree name
Doctor of Philosophy
Publisher
University of Canterbury
Journal Title
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Volume Title
Language
English
Date
2019
Authors
De Vela, Roger Jay L.
Abstract

This study investigated the impact of metabolic uncouplers as a biomass control strategy in a biotrickling filter (BTF) with toluene as the model pollutant. Classical metabolic uncouplers such as carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (FCCP) and carbonyl cyanide m-chlorophenylhydrazone (CCCP), and the more common m-chlorophenol (m-CP), previously tested in activated sludge systems were tested for the first time in BTFs with the objective of minimising biomass accumulation. Two known mechanisms by which uncouplers control biomass accumulation were tested: (1) true metabolic uncoupling which limits the energy production for growth, and (2) weakening the biofilm causing excess biomass to be released from the bed.

The experiments employed a column reactor and a novel differential BTF (DBTF) reactor, both packed with 5-mm glass beads at a working volume of 0.45 L and 0.08 – 0.39 L, respectively. Toluene concentrations were varied between 140 ±10 ppm and 230 ±15 ppm and operated at empty bed residence times (EBRT) of 32 and 6 - 28 seconds, respectively. Liquid trickling rate in the column reactor was 24 ± 1 mL⸱min-1 (0.72 ± 0.03 m·h-1) while that of the differential BTF was in the form of aerosol at a rate of 0.09 to 0.70 mL·min-1 (0.0007 to 0.005 m·h-1). The differential BTF recycled the gas at 20 to 80 L⸱min-1 (17 to 612 m·h-1) depending on the level of biomass accumulation in the bed. Various concentrations of the uncouplers were tested: 5 to 200 µM FCCP, 50 to 800 µM CCCP and 0.4 to 4.0 mM m-CP.

In general, the EC (~ 33 g⸱m-3h-1 for column reactors and ~ 600 g⸱m-3h-1 for DBTF when uncoupler test was done) decreased by 15 to 97%, in an uncoupler-dose dependent fashion. However, the EC completely recovered in the column reactor, within 3 to 13 days depending on the concentration of the uncoupler, but only partially recovered (only up to 40% of the original) in the DBTF.

True metabolic uncoupling was exhibited by the uncouplers immediately after their application to the systems, as indicated by the 20% to 160% increase in %CO2 recovery (which was typically in the 55 – 70% range prior to uncoupler treatment) that lasted for one to three days, depending on the uncoupler concentration. Although not measured, the %CO2 recovery exceeding 100% could be due to the CO2 generated from degradation of polyhydroxybutyrate (PHB) which could have accumulated in the biomass. On the basis of TOC and pressure drop stability, FCCP and CCCP did not exhibit a potential for sustained biomass control in a column BTF, but rather caused further increase in pressure drop potentially due to increased EPS production. The 4.0 mM m-CP weakened the biofilm in the BTF bed as shown by up to 130% increase in the total organic carbon (TOC) in the liquid sump of the column reactor and up to 500% increase in TOC of the DBTF’s liquid sump. The effect of the uncoupler on extracellular polymeric substances (EPS) production which in turn influenced biomass release, was also measured. The amount of EPS released from the bed was initially reduced by 10% with m-CP treatment, but eventually increased again by up to 125% with sustained exposure to the uncoupler. The production of more EPS by the microbes, as a protective response against the uncoupler, could have contributed to the full and partial EC recovery in the column and in the DBTF, respectively.

Although m-CP has potential to reduce biomass in the bed, its long-term use as a biomass control strategy is not feasible because 40% was lost in 20 days, most likely due to microbial degradation. The liquid phase turned deep black indicating the potential accumulation of chlorocatechol, an intermediate in the bacterial degradation of m-CP. Although FCCP degradation was not measured, its analogue, CCCP was shown to be degraded within 24 hours after its application in the BTFs.

Meanwhile, the novel differential BTF which employed aerosol as a means to deliver nutrients to the bed and hence minimizing gradients across the bed, was a better research tool than a column reactor. This was primarily because it employed single-pass-liquid phase flow, hence eliminating recirculation which returned the released biomass to the bed, and eventually contributed to the recovery of EC in the column reactors. While biomass recirculation contributes to EC recovery, it makes understanding the effect of uncouplers on TOC and EPS a challenge.

Even at a high loading rate of 420 g⸱m-3h-1, the differential BTF still showed a remarkably high removal efficiency of about 100%, hence EC was also at 420 g⸱m-3h-1. Further increasing the loading rate (470 to 660 g⸱m-3h-1) decreased the EC implying inhibitory effect of high concentration of toluene to the biofilm. This high EC was potentially due to the low but uniform liquid supply, which minimised the mass transfer resistance between the gas phase and the biofilm and potentially minimising biofilm non-uniformity in the bed. In addition, nutrients and substrate were more or less uniformly distributed across the DBTF bed therefore leading to better performance of its microbial community.

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