It is anticipated that the City of Windsor’s West Windsor plant will be required to be upgraded from its current chemical primary treatment to full secondary treatment, while maintaining phosphorus removal. A Steering Committee established effluent design objects for effluent SS (ESS) and BOD5 of 15 mg/l, total phosphorus of 0.5 mg/l, and ammonia nitrogen of 3 mg/l. In initial testing, the Trickling Filter/Solids Contact (TF/SC) process failed to meet its objectives. A detailed diagnosis showed that the best process configuration had not been tested and that the pilot design was flawed in several respects. The inclusion of a sludge reaeration tank, the substitution of a flocculate-clarifier for a conventional scraper clarifier and the elimination of head losses causing excessive floc breakup fixed the problem. In testing over a year long period, the TF/SC process consistently met the target objectives, moving it up from fourth place to a tie with the other first ranked process, the BAF. Capital costs were found to be lower than for the activated sludge process and the BAF processes.