WHAT TO DO WHEN THE DISINFECTION SYSTEM DOES NOT WORK: TROUBLESHOOTING CHLORINE DISINFECTION SYSTEMS

The 180 MGD 91st Avenue Multi-Cities Wastewater Treatment Plant (WWTP), operated by the City of Phoenix, uses chloramination for disinfection. During the summer of 2007, the plant saw record low influent flows with increased reuse water demands. During the plant's diurnal low flow period, automatic control strategies became unstable which led to breakpoint chemistry problems. Unusual solids issues, from the secondary effluent channels through discharge, were also present. Coliform counts began to increase in late June with exceedances during July, August and September. Throughout the summer, Operations staff conducted additional monitoring, developed alternatives, and implemented improvement projects. Coliform results returned to historical levels by mid- September, 2007. Flow forecasts for summer 2008 were even lower than 2007, and with increasing reuse water demands. To reduce the risk of future disinfection compliance issues, Plant staff worked to improve low flow conditions, constructed a temporary pump station to meet reuse water demand, added step-disinfection capability to maintain a higher residual, and implemented accelerated housekeeping. These improvements have resulted in the plant remaining in compliance during 2008.