Long term capacity of biological activated carbon filtration for organics removal

Since March 1995 Amsterdam Water Supply has applied biological activated carbon filtration (BACF) in the treatment process of the Leiduin plant. In this plant (capacity 70 × 106 m3/y) pretreated River Rhine water is infiltrated in the dune area, west of Amsterdam, for artificial recharge. Post treatment comprises rapid sand filtration, ozonation, hardness removal, biological activated carbon filtration and slow sand filtration. At the start the carbon reactivation frequency was set at 18 months, based on removal efficiencies for AOX (adsorbable organic halogens), DOC, pesticides and micropollutants. After four years of operation of a pilot plant (10 m3/hour) in parallel with the full-scale plant, the remaining removal capacity and the break-through profile of the carbon filters was investigated. In contrast to the full-scale plant, no carbon reactivation was applied in the pilot plant during the operation of 4 years. Spiking experiments were carried out after ozonation, in the influent of the biological activated carbon filtration with a cocktail of different pesticides after 1.5, 3 and 4 years. Influent concentrations varied between 2 to 10 μg/l. Without carbon reactivation the filter effluent still complies with the Dutch drinking water standards and guide lines, as well as with the Amsterdam Water Supply standards: DOC is less than 2 mg/l (actually 1.2 mg/l) and AOX remains below 5 μg/l. After four years, with spiking concentrations of 2 μg/l still no pesticide break-through was observed in the two-stage biological activated carbon filtration process. It can be concluded that a running time of 3 years between two reactivations in the two stage biological active carbon filtration is achievable, without negatively affecting the finished water quality. Average DOC concentrations will increase up to 1.2 mg/l, from 1 mg/l with running times of 2 years. After four years or 100,000 bedvolumes the AOC content is equal to or lower than 10 μg/l after biological activated carbon filtration. With slow sand filtration at the end and as polishing step AOC will be less than 10-5 μg/l. With every extension of six months duration time a saving of Euro 305,000 on reactivation costs is possible. With every 6 month extension of the running time of the carbon filters a saving of €610,000 is realised on the reactivation costs of the Leiduin treatment plant.