Treatment of Gaseous Volatile Organic Compounds in Activated Sludge System

An activated sludge aeration tank (W × L × H = 40 × 40 × 300 cm) with a set of 2 mm orifice air spargers was utilized to treat gas-borne volatile organic compounds (VOCs, toluene, p-xylene, and dichloromethane) in air streams. Effects of liquid depth, aeration intensity (G/A), overall mass transfer coefficient of oxygen in clean water (KLaO2), dimensionless Henry’s law coefficient of the tested VOC (H), and the influent gaseous VOC concentration (C0) on the efficiency of the VOC removal were examined and compared with a literature-cited model. Results show that the measured VOC removal efficiencies and those predicted by the model were comparable at a G/A of 3.75-11.25 m m h and C0 of around 1-6 mg L. Experimental data also indicate that the reactor with KLaO2 = 5-15 h could achieve > 85% removal of VOCs with H = 0.24-0.25 at an aerated liquid depth of 1 m, and > 95% removal of dichloromethane with H = 0.13 at a liquid depth of 1 m. Corresponding author Email: [email protected] INTRODUCTION The bio-treatment of volatile organic compounds (VOCs) or odors in air streams provides an inexpensive alternative to conventional technologies such as catalytic and thermal oxidation, wet scrubbing, ozonation, and activated carbon adsorption [1,2]. One of air pollution control systems is a bio-scrubbing system, which scrubs water-soluble contaminants from the waste gas in an activated sludge aeration tank and then biologically degrades them [3,4]. Bielefeldt and Stensel [3] stated that some researchers had reported the successful treatment (up to 99%) of air-borne odorous compounds in full-scale activated sludge basins (2.4-5.5 m deep) for treating municipal wastewater, when the air obtained from the odor-producing areas was purposefully conducted to the aeration blowers. They also stated that odorous gases can be treated in activated sludge gas treatment reactors at much lower depths of 0.6-1.3 m. Bielefeldt and Stensel further developed a successful mechanistic model to evaluate the design and performance of the process. The model accurately predicted the removal characteristics of benzene, toluene, ethyl benzene and xylenes (BTEX) from air diffused into a 2 L, 40 cm deep, lab-scale reactor. The model also predicted that a 2 m deep gas treatment reactor should provide > 80% treatment efficiency for VOCs with dimensionless Henry’s law coefficient H < 0.35, when the reactor is operated with an influent gaseous VOC concentration of 10 mg L, a liquid VOC concentration of below 0.1 mg L, and a volumetric oxygen-transfer coefficient of 40 h at an air application rate of 55 m m h. However, limited experimental data are available to validate the model. This research attempts to verify the model based on the results of tests on the removal efficiencies of toluene, p-xylene, and dichloromethane in a pilotscale activated sludge basin operated at a range of influent VOC concentrations, liquid depths, and air application rates. Results of this study allow the verification of the model and provide a design basis for treating gases contaminated with a range of volatile compounds.