Feeding composition and sludge retention time both affect (co-)metabolic biotransformation of pharmaceutical compounds in activated sludge systems

The role of heterotrophic and nitrifying microorganisms in the (co-)metabolic biotransformation of 10 pharmaceutically active compounds (PhACs) was investigated. To this aim, biotransformation assays were performed with heterotrophic and nitrifying sludge developed separately in a two-stage full-scale activated sludge system. Each stage was operated at different inflow wastewater characteristics and sludge retention times (on average 8 d and 35 d). The biotransformation capacity of each sludge was evaluated in the absence of primary substrate and in the presence of acetate and ammonium, to independently elucidate the co-metabolic role of heterotrophs and nitrifiers present in both sludges. Trimethoprim, diclofenac and carbamazepine were recalcitrant (removal < 5% after 1 d; biotransformation rate < 50 µg/g VSS·d) under all the tested conditions. High concentrations of caffeine, acetaminophen and iopromide were quickly biotransformed (> 80% after 1 d; > 800 µg/g VSS·d) in the absence of primary substrates. The heterotrophic sludge only showed a co-metabolic effect towards erythromycin, which increased its biotransformation rate between 43% and 53% when acetate and ammonium were supplied. In contrast, when stimulated, nitrifiers and slow-growing heterotrophs present in the nitrifying sludge co-metabolically biotransformed acetaminophen, ibuprofen and naproxen to a significant extent. Sulfamethoxazole was recalcitrant, except when the nitrifying sludge was fed with acetate (> 800 µg/g VSS·d), suggesting that slow-growing heterotrophs co-metabolically biotransformed it. This study provides evidence that biotransformation of PhACs depends on several metabolic activities, as the heterotrophic activity of the nitrifying sludge, which are not only determined by the SRT but also by the feeding composition.