A 14 month pilot-scale coagulation optimization study was conducted at the Peterborough Water Treatment Plant (Ontario, Canada) to compare treatment performance resulting from the application of aluminum sulfate vs. polyaluminum chloride (PACl). This paper describes results obtained from applying a statistical analysis approach to evaluate impacts on pH, turbidity, total organic carbon (TOC), ultraviolet absorbance (UVA), particle counts, chlorine residuals, filter head loss, flow rate, trihalomethanes (THMs), and nine haloacetic acids (HAA9). To allow a direct comparison, parallel pilot trains were operated such that they achieved equal settled water TOC by adjusting PACl dose, (52–74% of the alum dose by weight). Settled water turbidity was significantly higher (on average 0.23 NTU) in the alum treated water when compared to PACl. For equivalent filter run-times, head loss was greater by 0.002–0.011 m h−1 when applying alum. An increase in pH by approximately 0.7 units when using PACl was observed to cause a significant increase in THM formation (10–30%).