The effect of dissolved oxygen concentration on the activated sludge process

Graduation date: 1982Since the introduction of pure oxygen activated sludge systems in\ud the late 1960's, proponents have claimed several advantages resulting\ud from the elevated dissolved oxygen (DO) levels maintained. However,\ud numerous comparative studies have failed to produce a consensus of\ud professional opinion regarding the validity of these claimed advantages.\ud The purpose of this study was to evaluate fundamental theories\ud that might explain the effects of elevated DO levels, to assess the\ud validity of claimed advantages for pure oxygen systems, and to provide\ud guidance regarding appropriate DO levels in activated sludge systems.\ud A literature review revealed that dispersed bacterial cells\ud respond to DO, as measured by respiration rate or intracellular\ud concentrations of metabolites, only at very low concentrations (less\ud than about 0.1 mg/1). Consequently, the occurrence of oxygen diffusional\ud limitations in large bacterial floc and the development of an\ud anoxic core was preliminarily identified as the only reasonable\ud theory for explaining effects of high DO levels.\ud The first phase of this study involved the development of\ud dispersed bacterial cultures in chemostats. Results indicated no\ud significant difference in yield, kinetic coefficients, or specific\ud ATP content between chemostats operated at a low DO level (2.0 mg/1)\ud and a high DO level (15 mg/1). These results confirmed that differences\ud in activated sludge characteristics that apparently result from the\ud maintenance of high DO levels can only be attributed to diffusional\ud limitations.\ud The second phase of this research utilized laboratory-scale\ud activated sludge systems. A simple respirometric procedure was\ud utilized to evaluate the response of respiration rate to DO concentration\ud for these flocculant cultures and plots of specific respiration\ud rate versus DO concentration were developed. These curves were\ud subsequently described in terms of the DO at which the respiration\ud rate was 50 percent of the maximum (C° 5) and the DO at which the\ud respiration rate first reached the maximum (Co). Values for C\ud o\ud 0.5\ud ranged up to about 1.0 mg/1 and for Co up to about 10 mg/1 for culture\ud samples with maximum floc diameters of 1200 microns and respiration\ud rates up to 200 mg 02/g TSS-hr.\ud The final phase of this research involved respirometric tests on\ud mixed liquor samples obtained from three full-scale activated sludge\ud plants, Results indicated that the critical DO (Co) was typically less\ud than 1.5 mg/1, even at higher respiration rates resulting from the\ud treatment of high-strength industrial wastewaters. Floc size\ud measurements indicated that flocs in full-scale plants rarely exceed\ud 300 microns in diameter. It was concluded that oxygen diffusional\ud limitations typically do not occur at full-scale activated sludge\ud plants if aeration basin DO concentrations are maintained above about 1.0 mg/l