A study was conducted to examine the influence of hydraulic retention time (HRT) and solid retention time (SRT) on the removal of tetracycline in the activated sludge processes. Two lab-scale sequencing batch reactors (SBRs) were operated to simulate the activated sludge process. One SBR was spiked with 250 microg/L tetracycline, while the other SBR was evaluated at tetracycline concentrations found in the influent of the wastewater treatment plant (WWTP) where the activated sludge was obtained. The concentrations of tetracyclines in the influent of the WWTP ranged from 0.1 to 0.6 microg/L. Three different operating conditions were applied during the study (phase 1-HRT: 24 h and SRT: 10 days; phase 2-HRT: 7.4 h and SRT: 10 days; and phase 3-HRT: 7.4 h and SRT: 3 days). The removal efficiency of tetracycline in phase 3 (78.4 +/- 7.1%) was significantly lower than that observed in phase 1 (86.4 +/- 8.7%) and phase 2 (85.1 +/- 5.4%) at the 95% confidence level. The reduction of SRT in phase 3 while maintaining a constant HRT decreased tetracycline removal efficiency. Sorption kinetics reached equilibrium within 24 h. Batch equilibrium experiments yielded an adsorption coefficient (Kads) of 8400 +/- 500 mL/g and a desorption coefficient (Kdes) of 22 600 +/- 2200 mL/g. No evidence of biodegradation for tetracycline was observed during the biodegradability test, and sorption was found to be the principal removal mechanism of tetracycline in activated sludge.

The purpose of this research was to study the changes in polychlorinated biphenyl (PCB) availability as measured by desorption equilibrium and kinetics from industrial lagoon sediments collected at different times during a 24-month period of pilot-scale land biotreatment. During biotreatment, reductions of the lower chlorinated PCB congeners in the industrial lagoon sediments were observed. On the basis of past work on soils and sediments, it was originally hypothesized that these reductions in PCB concentration would result in reduced PCB availability. To evaluate this hypothesis, equilibrium partitioning studies and desorption kinetic studies were conducted with the industrial lagoon sediments (containing 0.91% oil and grease) as a function of biotreatment duration. Contrary to initial expectations, equilibrium aqueous total PCB concentrations increased with PCB loss during land biotreatment. This behavior was attributed to the association of PCBs with a waste oil phase in the lagoon sediment and an oil phase loss rate greater than the PCB loss rate during the biodegradation study. Maximum PCB desorption rates for the lagoon sediments also changed with biotreatment. A two-phase desorption behavior characterized by fast and slow desorbing fractions were observed. The estimated fast pool fraction for each PCB homologue decreased with biotreatment time, suggesting preferential removal of the PCBs from the fast pool during the bioremediation process. Although PCB availability based on estimated fast pool fraction decreased with biotreatment, availability based on aqueous equilibrium measurements increased with biotreatment over the 24 months of study.

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