Development of a feedback control system for a differential biofilter degrading toluene contaminated air

In this study, a proportional - integral feedback control system was implemented on a lab-scale differential biofilter to control the gas phase toluene concentration in the soil bed through online manipulation of the inlet toluene concentration. The feedback control system was based on a cascade controller that manipulated the setpoint of an air bath diffusion system to manipulate the inlet toluene concentration. The controller performed well for toluene concentrations in the reactor of 10 - 300 ppm for both setpoint changes and disturbance rejections; however, the system was nonlinear requiring different tuning parameters at different outlet concentrations. Feedback control of the toluene concentration in the differential reactor was used to explore the impact of concentration on start-up and long-term biofilter operation in a rigorous fashion. Starting at an reactor concentration of 20 ppm and then increasing to 65 ppm increased the toluene removal rate (33 ± 1.6 g m^-3h^-1) compared to starting the reactor at an outlet concentration of 81 ppm before settling at 65 ppm (42 ± 0.9 g m^{- 3}h^-1). The toluene removal rate increased with increasing outlet toluene concentration and then eventually decreased when reaching the inhibitory toluene concentration (ranged from 80 to 250 ppm).