Effects of substrate and oxygen loading rates on gas-phase toluene removal in a three-phase biofilm reactor

A three-phase, circulating-bed biofilm reactor using macroporous carriers removed toluene from a gas stream continuously for more than six months. Three steady states were established with toluene loading rates varying from 0.030 to 0.059 mol/m²·d. For each steady state, short-term experiments evaluated the effects of toluene and oxygen loading rates. At least 99% of the biomass in the system was accumulated inside the carrier macropores, and the total biomass was proportional to the toluene loading rate. Toluene removal ranged from approximately 100 to 55%. The lower toluene removals were associated with oxygen limitation, which also resulted in the accumulation of an intermediate (3-methylcatechol) and nontoluene chemical oxygen demand. The results suggest that excessive biomass accumulation hurt process performance by depleting oxygen within the biofilm because increased endogenous respiration consumed more oxygen, while increased biomass density may have slowed oxygen diffusion.