The Passaic Valley Water Commission's Little Falls Water Treatment Plant is currently undergoing a major upgrade. The upgrade includes replacing the existing conventional treatment train with ballasted flocculation clarification (Actiflo), intermediate ozonation and biological filtration. As part of the design scope of this upgrade, Black & Veatch constructed and operated for one year a pilot plant that was identical to the proposed full-scale design. The purposes of the pilot plant were: to confirm the proposed unit process operating rates, which were higher than the manufacturers recommended loading rates; to obtain state regulatory approval of the proposed processes and loading rates process which had not previously been approved in that state; and, gather useful operational guidelines for the full scale plant. The scope also required that a permanent pilot plant be provided in another location as part of the full scale plant construction, so most of the initial temporary pilot plant will become a part of this permanent facility. The plant had an existing custom fabricated 10-gpm plate settler, ozone and filtration pilot plant, which the client made available for our use. However to simulate the new process train, the smallest size unit available was a 140 gpm Actiflo clarification unit, which is much larger than usually used for pilot studies was required. Multiple stage intermediate ozonation and GAC bio-filters had to be provided; these required extensive modifications to the existing equipment. Finding a suitable location for the Actiflo unit with access to the raw water conduits and a drain that could take the large excess flow presented some initial difficulty. Black & Veatch developed a formal pilot study protocol for review by the client and approval by the NJDEP. The test requirements developed in this protocol had a significant impact on the instrumentation requirements for the pilot as well as the sampling/operating labor and laboratory costs. The client's staff performed some of the modification work; however, much of the work had to be self-performed or done through sub-contractors. Similarly, the plant laboratory was available to perform some of the analytical testing, but for a number of parameters an outside testing laboratory had to be used. The lessons learned, including the decisions, manpower effort, time schedule and costs involved in the design, construction, startup and operation of the pilot plant are presented and discussed, as well as recommendations for future efforts involving a pilot plant study of this size and magnitude. Includes figure.