The adverse water quality conditions and the resulting teatment complications in the El Nino winter of 1998 led the San Francisco Public Utility Commission (SFPUC) to investigate the limitations of direct filtration. Questions addressed by the study included: what regulatory or capacity limitations exist at the Harry Tracy Water Treatment Plant (HTWTP) when the water quality in Peninsula Reservoirs deteriorates as a result of extreme weather events; and, is there a better coagulation chemistry for treating high turbidity water at the HTWTP? To answer these questions, the SFPUC mobile pilot plant (MPP) was used to conduct a series of pilot tests from the spring to the fall of 1998. The MPP has two treatment trains and each is equipped with all of the unit operations available at the HTWTP including ozonation, coagulation, flocculation, sedimentation (not used at HTWTP) and filtration. For the study, one of the pilot trains was operated in direct filtration mode and the other was operated in conventional filtration mode. The study consisted of two parts: ambient (low) turbidity testing, and spiked (high) turbidity testing. The ambient turbidity testing was conducted in late spring/early summer 1998 as the water quality impacts of the El Nino storm events were subsiding. A turbidity spiking system was designed and constructed to test higher turbidity waters. The spiked turbidity testing was completed in the late summer/early fall timeframe. Includes 9 references, tables, figures.