WASTE-WATER DISINFECTION WITH OZONE-PROCESS CONTROL AND OPERATING RESULTS

The City of Indianapolis, Indiana operates two 125 mgd advanced wastewater treatment plants with ozone disinfection. The rated capacity of the oxygen-fed ozone generators is 6,380 lb/day, which is used to meet geometric mean weekly and monthly disinfection permit limits for decal coliforms of 400 and 200 per 100 mL, respectively. Since 1989, a disciplined process monitoring and control program was initiated. Records indicate a significant effect on process performance due to wastewater flow, contactor influent fecal coliform concentration, and ozone demand. Previously, ozone demand information was unknown. Several tasks/studies were performed in order to better control the ozone disinfection process. These include the recent installation of a pilot-scale ozone contactor to allow the plant staff to measure ozone demand on a daily basis. Also, tracer tests were conducted to measure contactor short-circuiting potential. Results demonstrated a noticeable benefit of adding additional baffles. Results also indicated operating strategies that could maximize fecal coliform removal, such as reducing the number of contactors in service at low and moderate flow conditions.