Role of reduced empty bed contact times and pre-treatment by coagulation with Fe(III) salts on the removal of trace organic compounds during sequential biofiltration

Sequential biofiltration (SBF) has been demonstrated to be a promising approach to achieve enhanced biological removal of various trace organic compounds (TOrCs) from wastewater treatment plant (WWTP) effluents by establishing oxic and carbon-limited conditions. This study investigated options to further advance the feasibility of SBF systems by reducing empty bed contact times (EBCTs). Additional experiments were conducted to investigate the pre-treatment of WWTP effluent by coagulation as an option to further decrease the organic carbon availability during SBF. Results indicated that an EBCT reduction in the first filter stage from 90 to 45 and 30 min adversely affected process performance regarding the removal of organic bulk parameters and several TOrCs over short-term. However, after an extended adaptation period of ten weeks comparable performance was observed in two SBF systems with first stage filters operated at EBCTs of 90 and 45 min. The pre-treatment of secondary effluent by coagulation, flocculation and sedimentation was not found to enhance the performance of an SBF system despite substantial removal of organic bulk parameters during the pre-treatment. However, despite the vast removal of total phosphorous during coagulation, nutrient limitation was not found to adversely affect the biological performance of the subsequent SBF system.