PLANNING FOR pH CHANGE: MAINTAINING OPTIMAL CORROSION CONTROL TREATMENT AFTER CHANGES TO WATER QUALITY TREATMENT PROCESSES

摘要

Two process changes were proposed at the Dalecarlia and McMillan water treatment plants operated by Washington Aqueduct, a United States Army Corps of Engineers organization. At both plants, it has been proposed to switch from alum to polyaluminum chloride (PACl) and from chlorine gas to sodium hypochlorite for coagulation and disinfection, respectively. However, throughout the proposed process changes, both plants must maintain a finished water pH of 7.7±0.1 to meet their optimal corrosion control treatment (OCCT) targets. This paper addresses the analysis conducted to determine the changes that will be needed to both plants’ corrosion control processes to maintain the OCCT target pH of 7.7±0.1. The analysis include computer modeling work, which was then evaluated with bench-scale testing. Projections show that, following the switch to hypochlorite, it will be necessary to add acid feed capabilities at McMillan to stay within the target range. While acid will not be needed at Dalecarlia, significantly lower doses of lime will be needed. Lime, caustic soda, and acid dose projections were made for each plant.