This pilot-plant study was initiated to evaluate biological filtration for the removal of aldehydes formed during ozonation. An additional objective of this testing was to demonstrate that aldehyde measurements could be used as a surrogate for analysis of assimilable organic carbon (AOC). The use of granular activated carbon (GAC) as an alternative to anthracite coal as the filter medium was also investigated, and it was observed that GAC filters developed biological activity sooner and showed longer-term stability. Although biological activity was established sooner on slow-rate filters, the high-rate filters in time achieved a comparable capability. Data for formaldehyde and glyoxal provide information on removal of readily biodegradable and more recalcitrant ozone by-products, respectively, and demonstrate trends similar to those for the removal of AOC. Includes 31 references, figures.