Abstract
An activated sludge aeration basin (20 cm i.d., 140 cm height) equipped with either a coarse air diffuser (a plastic pipe perforated with 56 orifices of 2 mm in diameter) or a fine diffuser (porous plastic type with 100-micrometer pores) was utilized to treat an air-borne hydrophobic VOC (toluene, 700-800 mg/m^3). The purposes of this study were to test the influences of both MLSS and diffuser type on the VOC removal efficiency. Results show that higher MLSS (mixed liquor suspended solids) such as 10,000-40,000 mg/L in the mixed liquor did not enhance greatly the transfer and removal of the introduced toluene. Instead, activated sludge basins with a normal MLSS (e.g., 2000-4000 mg/L) in the mixed liquor and an efficient gas diffusion system with volumetric VOC transfer coefficient of around 10-15 1/h can be used for the removal of hydrophobic VOCs from the introduced gas. For achieving a removal of over 95% of the introduced toluene or similar hydrophobic VOCs, commercial air diffusers for aerobic biological wastewater treatment basins can be used with a submerged liquid depth of over 0.40 m over the diffusers and an aeration intensity (air flow rate/basin cross-sectional area) of lower than 5.0 m^3/m^2.h.
Highlights
Volatile organic compounds (VOCs) or odors in air streams at room temperatures can effectively be removed by activated carbonOne of the processes is the bio-scrubbing one which scrubs water-soluble contaminants with the waste gas being introduced into an activated sludge mixed liquor through gas diffusers and biologically degrades them (Bielefeldt and Stensel, 1998 and 1999; Hammervold et al, 2000). Bielefeldt and Stensel (1998 and 1999) stated that some researchers had reported the successful treatment 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
Where C and Co represent the VOC concentrations in the exit and the influent gas streams, respectively; H’ is the dimensionless apparent Henry’s law coefficient; x is the VOC concentration in the liquid phase; KLaVOC is the overall mass transfer coefficient of the VOC in clean water (1/h); D is a constant that corrects the mass transfer in activated sludge liquid as opposed to clean water; Z is the depth of the liquid over the sparger (m); G is the influent gas flow rate (m3/h); and A is the cross-sectional area of the activated sludge basin (m2)
Effect of Activated Sludge Concentration Using the coarse diffuser for introducing toluene-containing air into the basin, variations of fractional residual toluene concentration (C/Co) with the submerged liquid depth (Z) are shown in Fig. 2 for the three mixed liquor suspended solid (MLSS) concentrations
Summary
Volatile organic compounds (VOCs) or odors in air streams at room temperatures can effectively be removed by activated carbonOne of the processes is the bio-scrubbing one which scrubs water-soluble contaminants with the waste gas being introduced into an activated sludge mixed liquor through gas diffusers and biologically degrades them (Bielefeldt and Stensel, 1998 and 1999; Hammervold et al, 2000). Bielefeldt and Stensel (1998 and 1999) 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. Bielefeldt and Stensel (1998 and 1999) 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 stated that odorous gases can be treated in activated sludge gas treatment reactors at much lower depths of 0.6-1.3 m.
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