Construction scheduling for the expansion of the City of Greeley (Colorado) Water Pollution Control Facility from 12 to 16 mgd allowed two unique opportunities for a side-by-side performance comparison of different final clarifier designs. In the first comparison, the performance of a final clarifier constructed in the 1970s was compared to the performance of a new final clarifier, which incorporated state-of-the-art design concepts. In the second comparison, the performance of the older clarifier, retrofitted with a number of performance enhancing modifications, was compared to the performance of the new clarifier. These side-by-side comparisons were unique in the respect that they removed sludge quality as a variable since both clarifiers received the same activated sludge during the performance evaluations. In the first comparison, the new final clarifier performed significantly better than the old clarifier. In addition, the variability in effluent quality from the new clarifier was significantly less than the variability in effluent quality from the old clarifier. In the second comparison, the new clarifier performed better than the old clarifier but the variances in the data from the two clarifiers was not significantly different, indicating the modifications made to the old clarifier improved effluent quality. Relatively inexpensive modifications to final clarifiers such as those incorporated into the old final clarifiers at Greeley improve performance and unlock capacity.