In the present study, we examined seawater biofiltration in terms of adenosine triphosphate (ATP) and turbidity. A pilot biofilter continuously fed with fresh seawater reduced both turbidity and biological activity measured by ATP. Experiments operated with an empty bed contact time (EBCT) of between 2 and 14 min resulted in cellular ATP removals of 32% to 60% and turbidity removals of 38% to 75%. Analysis of the water from backwashing the biofilter revealed that the first half of the biofilter concentrated around 80% of the active biomass and colloidal material that produces turbidity. By reducing the EBCT, the biological activity moved from the first part of the biofilter to the end. Balances of cellular ATP and turbidity between consecutive backwashings indicated that the biological activity generated in the biofilter represented more than 90% of the detached cellular ATP. In contrast, the trapped ATP was less than 10% of the overall cellular ATP detached during the backwashing process. Furthermore, the biological activity generated in the biofilter seemed to be more dependent on the elapsed time than the volume filtered. In contrast, the turbidity trapped in the biofilter was proportional to the volume filtered, although a slightly higher amount of turbidity was found in the backwashing water; this was probably due to attrition of the bed medium. Finally, no correlations were found between turbidity and ATP, indicating that the two parameters focus on different matter. This suggests that turbidity should not be used as an alternative to cellular concentration.