A trickling filter sewage treatment plant was designed and constructed in the Borough of New Providence, New Jersey to alleviate local sewage treatment plant hydraulic overloading and resultant loss of treatment efficiency caused by excessive infiltration. The plant uses two high rate trickling filters, one with rock media, the other with plastic media, operating in parallel to treat wet weather flow. During dry weather periods the plant is operated in series with a controlled flow to maintain an active biological slime on the filters. The plant also consists of a primary clarifier-leveling reservoir, secondary clarifier and chlorine contact tank. This investigation has shown that it is both technically feasible and economical to design, construct and operate a treatment plant to process both the controlled dry weather flow and the higher flows encountered during periods of excessive infiltration using a combination of series-parallel high rate trickling filters.