After administration, pharmaceuticals are excreted and released into the aquatic environment via wastewater effluent. Unused drugs are sometimes disposed of down drains, and, unless they are biodegraded or eliminated during sewage treatment, traces may enter the aquatic environment and eventually reach drinking water. It is also hypothesised that antibiotics and disinfectants disturb the wastewater treatment process and the microbial ecology in surface waters. Furthermore, resistant bacteria may be selected in the aeration tanks of STPs by the antibiotic substances present.