To maintain the biological stability of drinking water during distribution in large, complex networks, high standards have to be met—namely, low bacterial densities and low levels of biodegradable organic carbon. Second‐stage granular activated carbon (GAC) filtration (without regeneration of carbon) is used for this purpose at the Choisy‐le‐Roi, Paris, France, treatment plant. Effective removal of dissolved organic carbon has been observed with such filtration—mainly because of a reduction in the biodegradable organic carbon. To study the microbial processes involved in this removal, new methods based on the use of radio‐labeled tracers have been developed in order to measure the bacterial biomass and activity associated with GAC.

A single dual-layer filter, combining sand and granular activated carbon (GAC) and maintaining the mechanisms of biological treatment, was studied in France as a possible means of achieving high-rate filtration. Initial results attest that the dual layer far outperforms a single-layer sand unit, particularly with respect to the removal of suspended solids and ammonia. Using GAC for the top layer of the double filter ensures the removal of appreciable quantities of assimilable carbon and consequently has a positive effect on final chlorination and the palatability of the water.

No abstract

No abstract