Packed-tower chemical scrubbers, with sodium hydroxide (caustic) and sodium hypochlorite (bleach), are commonly used to treat foul air from wastewater treatment processes. The available control mechanisms include a pH probe and controller for the sodium hydroxide feed, an oxidation reduction potential (ORP) probe with a controller to
establish the bleach feed, and proprietary continuous emission monitoring and control systems (CEMACS) that measure the vapor-phase constituents to determine the chemical feed. The ORP method often does not provide rapid response to system changes, and could use significantly more bleach than is necessary. The vapor-phase CEMACS show significant improvements over the ORP control method; however there are only two known systems available, which both utilize electrochemical hydrogen sulfide sensors. The sensor used in the Sycamore Continuous Emission Monitoring (CEM) offers benefits over those used in the currently available control systems.
The goal of this study was to determine the viability of the Sycamore CEM, in combination with a controller, to effectively control the feed of bleach to scrubbers. Effectiveness was determined through hydrogen sulfide and chlorine leaving the stack of the chemical scrubber and bleach usage. In 2006, two trials were conducted with the Sycamore CEM at two different wastewater treatment plants, the Orange County Sanitation District’s Plant 2 (OCSD) and the city of Glendale’s Arrowhead Ranch WRF (Glendale). These trials permitted a unique comparison as OCSD normally operates its chemical scrubbers with a vapor phase CEMACS and Glendale utilizes ORP for bleach control. The results of this study show the potential benefits of the Sycamore CEM with a controller over ORP and CEMACS bleach control.
Unlike the currently available CEMACS, the Sycamore CEM operates with a sensor that can accurately measure down to 0.005 parts per million (PPM) of hydrogen sulfide. OCSD initiated its trial to identify other available sensors and technologies for a scrubber monitoring and control application. Glendale conducted the trial to achieve better hydrogen sulfide removal than was currently accomplished through ORP control. The Sycamore controller is only designed to address bleach addition, as the standard pH probe and controller are more than adequate at controlling pH levels in the sump. The Sycamore CEM and a controller use outlet hydrogen sulfide concentrations to determine the amount of bleach to add to a scrubber system to maintain the H2S concentration in the scrubbed air at a preset level.