| This study involved the assessment of various physical and chemical properties of activated sludge. Floc size, porosity, structure, and activity of activated sludge developed in laboratory sequencing batch reactors (SBR's) were investigated. The influence of varying the settling time of the SBR's, on the biodegradation of a toxic nitroaromatic compound, 2,4-dinitrophenol (DNP) kinetics, floc activity, biomass characteristics and the SBR environment were examined. In an effort to use the modified Stoke's law, the porosity of the activated sludge was calculated using the centrifuge method.; The SBR's were operated at five different settling times. Settling time was increased incrementally from 1-hour to 4 and 8-hour and then reduced to 20-minute. The 8-hour settling reactor was switched to 1-hour. Surprisingly, it was found that increasing settling time from one hour significantly reduced the rate of degradation of DNP, from 7.2 mg-DNP/L/hr for the 1-hour settling time, to 4.2 mg-DNP/L/hr and 3.3 mg-DNP/L/hr for the 4 and 8-hour settling times, respectively. It was also observed that the longer-settling activated sludge cultures were subject to longer anoxic periods and were dominated by smaller, and less-porous flocs. There was no significant difference in the number of DNP-degrading bacteria in the activated sludge populations, as measured by the most probable number (MPN) method. In addition, the 8-hour settling flocs were associated with significantly less specific biological activity than the 1-hour settling flocs.; Reduction of the settling time from 1-hour to 20-minute was not associated with a decrease in DNP biodegradation rate. The mixed liquor suspended solids concentration was significantly lower with the short settling period, and the effluent suspended solids concentration was higher, however, washout of the DNP-degrading bacteria was not observed.; Switching the 8-hour reactor to a 1-hour settling period was associated with an increase in DNP biodegradation rate but this rate never reached the steady state DNP degradation rate of the control reactor (1-hour settling). Both floc porosity and average diameter also increased, although only slightly.; In summary, the effect of settling time on SBR biodegradation of a xenobiotic substrate, DNP, was found to be complex. Very long settling periods of 4 and 8-hour resulted in reduced DNP biodegradation rates compared with 1-hour settling time (control SBR). Apparently, longer periods of anoxia were accompanied by changes in the activated floc structure which in turn may have reduced the transport of substrate, oxygen and nutrients to a significant portion of bacteria in the flocs. The results of this study indicate that these changes are principally the result of anoxia stress. It has been recommended that the SBR settling period be limited to the time necessary to prevent bacterial washout, and not be extended unnecessarily. |
