Development of Two Biomass Control Strategies for Extended, Stable Operation of Highly Efficient Biofilters with High Toluene Loadings

Abstract

For stable long-term continuous operation of highly loaded trickle bed air biofilters, the prevention of plugging due to accumulating biomass is essential for avoiding biofilter failure. Two biomass control strategies were evaluated to maintain high VOC removal efficiencies at high toluene loadings for over 200 days. A sustained toluene removal efficiency of over 99% was achieved. Backwashing with medium fluidization was found to be very effective in preventing accumulation of excess biomass. A specific flow rate of 190 m/h (for a bed expansion of 40%) for 1 h twice per week was found to be adequate. The use of nitrate (NO3-N) instead of ammonia (NH3-N) as the sole source of nutrient-nitrogen (N) was very effective in reducing the observed biomass yield. A pilot-scale evaluation was made of the performance of two similarly operated, backwashed biofilters receiving influent air with 250 ppmv toluene and varying only the VOC loadings and the form of nutrient-N. The biofilter receiving NO3-N performed significantly better overall. A comparative bacterial enumeration of the two biofilter microbial consortia was performed. This enumeration revealed that the populations of toluene degraders in both biofilters were about the same; however, the populations of total heterotrophs were starkly different.