Treating VOC-Contaminated Gases in Activated Sludge: Mechanistic Model To Evaluate Design and Performance

Abstract

A mechanistic model based on independently measurable mass transfer and biokinetic parameters was developed to describe the removal of volatile organic compounds (VOCs) contained in air sparged into activated sludge (suspended growth) gas treatment reactor. The critical mass transfer parameters are the VOC mass transfer coefficient (Klavoc), VOC Henry's coefficient (H) and diffusion coefficient in water, gas flow rate per unit reactor area, and liquid depth. The Klavoc is equal to the oxygen Kla (KlaO2) multiplied by the ratio of the VOC to oxygen diffusivity coefficients in water raised to the power n. Depending on the system power intensity, n ranges from 0.5 to 1.0; 1.0 provides a conservative design. Biokinetic parameters of importance include the Monod coefficients, biomass yield and endogenous decay coefficients, and solids retention time (SRT). The model accurately predicted BTEX removal from air diffused into a 2-L, 40-cm deep lab-scale reactor. Based on the model, a 2-m deep gas treatment reactor should provide >80% gas treatment efficiency for VOCs with H < 0.35, when the reactor is operated at an SRT which maintains the VOC liquid concentration below 0.1 mg/L, with a KlaO2 of 40 h-1 at an air application rate of 55 m3/m2-h.