Influencing Biological Activity for the Removal of Organic Micropollutants in Granular Activated Carbon Filters

The biological activation of granular activated carbon (GAC) filters extends their service life until a threshold of 80% is reached for the removal of organic micropollutants (OMP). To quantify the influence of biological activity on OMP removal, the 12 indicator substances of the 2024 revised EU Urban Wastewater Treatment Directive were used. Five years of pilot testing were examined, presenting data from three GAC filters under various process settings. Hydraulic parameters (empty bed contact time (EBCT) and filtration velocity (vfiltr)) and the backwash frequency were investigated. The pilot-scale GAC filters were fed with effluent from the wastewater treatment plant Nette (86,000 p.e.), which was subsequently further treated by a pilot-scale ultrafiltration system. The suspended solids-free feed was, alongside the process settings and temperature, a further influencing factor that was investigated regarding its effects on biological activity and thus on OMP removal.The results show that biologically active GAC filters achieve consistently high removal efficiency (>90%) for highly biodegradable OMPs even after >115,000 BV (CUR <4.0 mgAC/L). However, biological activation also extends the filter service life for poorly biodegradable OMPs (e.g. carbamazepine). This could indicate a partial bioregeneration of the GAC. With appropriate process settings, biodegradation begins to compensate for the declining adsorption capacity at around 10,000–15,000 BV. The effect of backwashing diminishes as the filter service life increases and is expected to play a crucial role during the early growth phases. EBCT and vfiltr can limit the biodegradation of OMPs, thereby preventing the biological activity from being fully exploited.