(602ac) Adsorption of Fluoroquinolone Antibiotics Onto Activated Sludge: Implications for Membrane Bioreactor Operation

Given the increasing detection of pharmaceuticals in treated wastewater, the need to optimize wastewater treatment processes for pharmaceutical removal is considerable.  In conventional wastewater treatment plants, the activated sludge process contains the greatest potential for pharmaceutical treatment.  That potential can be harvested using novel treatment processes, such as membrane bioreactors (MBRs), which allow the operator to uncouple the solids retention time (SRT) and the hydraulic residence time.  By increasing the SRT, greater degrees of pharmaceutical removal can be attained through adsorption to biosolids and biological transformation mechanisms.  This research focuses on two aspects of such systems:  (1) development of analytical methods for detecting trace concentrations of fluoroquinolone antibiotics and (2) description of fluoroquinolone adsorption onto activated sludge solids.

A rapid, accurate, and sensitive method has been developed for the quantitative determination of six fluoroquinolone antibacterial agents (i.e., ciprofloxacin, norfloxacin, levofloxacin, sparfloxacin, sarafloxacin, and orbifloxacin) by online solid-phase-extraction high performance liquid chromatography with fluorescence detection (oSPE-HPLC-FLD).  That method was used as a template for development of an ultrahigh performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS) detection method.  The UHPLC-MS/MS method allows quantitation of fluoroquinolone antibiotics to the ng/L level.  With this method, the six fluoroquinolone antibiotics of concern were measured in raw wastewater, the activated sludge reactor, and treated wastewater from the Little Patuxent Water Reclamation Plant (LPWRP; Savage, MD).  These concentrations of fluoroquinolones were employed in laboratory-based investigations of the adsorption of fluoroquinolones onto activated sludge. 

Activated sludge was collected from LPWRP; the specific biomass concentration was determined by weighing the dry solids.  Biological degradation of fluoroquinolones was minimized through the addition of 0.1% w/v of sodium azide.  Batch adsorption experiments were designed to investigate the impact of pH and ionic strength on fluoroquinolone partitioning between the aqueous and solid phase.  Specific masses of biosolids were added to batch reactors containing synthetic wastewater containing environmentally-relevant (from LPWRP) concentrations of fluoroquinolones.  At specific times, samples were collected and analyzed using UHPLC-MS/MS to determine the fluoroquinolone concentrations in the aqueous phase; solid phase concentrations were determined via mass balance.  The equilibrium data (contact time = 36 hours) was fit to typical isotherms to fully describe adsorption of fluoroquinolones onto activated sludge.  These data will further understanding of how SRT affects adsorption of antibiotics in activated sludge / MBR operation.  Ultimately, the results of this research will be used to design and operate a laboratory-scale MBR to investigate fluoroquinolone removal through adsorption and biological transformation mechanisms.