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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.

The trial at OCSD was on a headworks scrubber serving as a second-stage treatment with typical incoming H2S levels of 3-4 PPM. The Sycamore CEM and controller used an outlet hydrogen sulfide concentration set point of 0.010 PPM for one part of the test and 0.005 PPM for another. Operating under both of these scenarios kept hydrogen sulfide levels well below that desired (peak emitted hydrogen sulfide levels not to exceed 0.20 PPM) with averages of 0.007 PPM and no peak above 0.09 PPM. Although the existing OCSD control system achieved slightly lower average H2S levels out the stack (0.0007 PPM), that came at the cost of a far higher chlorine residual level of 0.80 PPM compared to the 0.35 PPM chlorine residual observed when the Sycamore system was controlling‥ Although usage was not directly measured in the trial, this lower chlorine residual would likely result in a roughly estimated 50% reduction in bleach usage with the Sycamore CEM controlling. There was inadequate time to optimize the chemical feed control on the Sycamore system during this trial, however further testing should permit control optimization to reduce or eliminate the few peaks observed.

The City of Glendale's existing Sycamore Two-Point CEM monitors a two-stage scrubber (with a carbon follower), treating foul air from the headworks. A switch was incorporated to facilitate selection between the existing ORP controller and that interfaced with the Sycamore CEM. Inlet hydrogen sulfide concentrations ranged from ∼0-40 PPM. For approximately four days, inlet and outlet levels were monitored with the Sycamore CEM/controller using a 30 ppb set point. Then for an additional four days, the inlet and outlet levels of H2S were monitored with the existing ORP system controlling. The data is summarized in the table below.

For comparable inlet levels, the Sycamore CEM with controller yields much tighter limits on the outlet H2S levels than the ORP control as indicated by the standard deviations and the maximum outlet levels observed. Tighter control should help reduce buildup of scale and sulfur in the scrubber. Furthermore, the difference between the set point on the CEM controller and the actual observed average of the outlet data over several days was only 7% indicating that the set point accuracy over time is quite good. This could provide the operator with some predictive capability regarding the life of the carbon follower.

With its capability of measuring outlet hydrogen sulfide concentrations down to 0.005 PPM, and its ability to control to set points at levels up from that limit, the Sycamore CEM and controller will prove especially useful as outlet hydrogen sulfide concentration values of 0.005-0.100 PPM are typical levels seen with most scrubbers systems across the country. Based upon this study, the Sycamore CEM with controller is a viable option to monitor and control bleach in chemical scrubbers and maintain outlet hydrogen sulfide concentration in the PPB range.

Document Type: Research Article


Publication date: 2007-01-01

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