Sorption of pharmaceuticals to foam and aerobic granular sludge with different morphologies

In biological wastewater treatment, the sorption process is an important removal pathway of organic micropollutants from the aqueous phase. Beyond the conventional sorption to biomass and particulate matter, organic molecules can also partition to gas bubbles commonly present in aerated biological processes. This study investigated the partitioning behavior of 21 selected pharmaceuticals to two types of aerobic granular sludge, and the foam generated by aeration. Batch sorption experiments were performed with biologically inactive granules of controlled diameters (0.5-1, 1-2, and >2 mm). Removal during sorption tests was observed for four positively charged micropollutants (sertraline, citalopram, clarithromycin, and erythromycin), four neutral compounds (levonorgestrel, estradiol, ethinylestradiol, and ketoconazole), and one negatively charged pharmaceutical (losartan). This highlights the importance of electrostatic interactions and lipophilic affinity with the solids. For some compounds, the removal increased with time, suggesting that sorption in thick biofilm is limited by molecular diffusion into the biofilm matrix. Furthermore, partitioning of pharmaceuticals to aeration-induced foam was confirmed in separate batch tests. Clarithromycin, erythromycin, ketoconazole, losartan, levonorgestrel, and ethinylestradiol exhibited concentrations in the foam 1.0-5.3 times higher than the initial test values, indicating potential adsorption at the liquid/gas interface for these compounds.