Odor Control System Evaluation and Optimization at F. Wayne Hill WRC

The 60-mgd F. Wayne Hill Water Resources Center (FWHWRC) is Gwinnett County's largest and most advanced wastewater treatment plant. The FWHWRC utilizes enhanced biological phosphorus removal (EBPR) and chemical polishing to meet a stringent effluent total phosphorus limit of 0.08 mg/L. The liquid treatment train is comprised of influent screenings, grit removal, primary clarification, activated sludge (biological reactor basins (BRBs)), secondary clarification, tertiary filtration, pre-ozonation, biological activated carbon (BAC), and post-ozonation. Solids handling includes co-thickening of primary sludge and WAS, anaerobic digestion, and sludge dewatering. The facility also utilizes the OSTARA Pearl process with WASSTRIP (i.e. Nutrient Recovery) for controlled harvesting of struvite and the reduction of phosphorus recycle loadings on the liquid treatment train. The FWHWRC is a high-profile, complex facility with numerous process components and five operating odor control systems, the largest of which is a 256,000-cfm chemical scrubber system. Despite GCDWR's attempts to work with its on-call contractors and engineers to identify and remedy several known odor sources, odor complaints persisted. An odor study was therefore recommended to fully characterize odors from various parts of the plant. This paper discusses the recommendations from the odor study conducted at the treatment plant, as well as the outcome from implementing the recommended changes in full-scale. Optimization Effort The odor study included an odor source survey, odor sampling and sample analysis, and development of an Immediate Action Plan; odor source prioritization, and development of a Long-Term Action Plan. The Immediate Action Plan was intended to identify issues that could be addressed in the short term to help alleviate any odor complaints originating within the boundary of the treatment plant. This task began with a site visit by a team of odor and process experts, who conducted an operational review of the facilities in the plant. The site visit included visual inspection of all existing odor control systems to identify improvements that can be made to the operation of the systems, in addition to a review of operational practices that may be contributing to off-site odors. As part of the site visit, all potential odor sources were identified, and a detailed sampling plan was developed with more than 40 sample locations. Grab samples were collected and analyzed for sensory samples (odor threshold, intensity, and dose-response), sulfur speciation, and ammonia/amines (at select locations). In addition, several OdaLog units were installed to determine diurnal trends for hydrogen sulfide gas. Results D/T Highest D/T values were observed near the influent distribution box, followed by the solids odor control system. Odors observed near the solids disposal truck bay were determined to be stationary. D/T values measured near the FOG receiving station were also observed to be low. D/T values measured at the exhaust of all other odor control systems were lower than 250 o.u. Sulfur compounds- The highest concentrations of reduced sulfur compounds were detected at the Solids Odor Control System inlet (350 ppb H2S) and exhaust (210 ppb Dimethyl Sulfide); and the Influent Distribution Box (180 ppb Dimethyl Disulfide). The remaining sample locations had concentrations of reduced sulfur compounds that were either non-detectable (ND) or at low ppb levels. H2S concentrations across the range of sample locations were considerably low or not detected, except for occasional spikes at the Influent Distribution Box due to diurnal flow variations at noon and midnight. Amines and Ammonia- With the exception of one sample at the solids disposal truck (17.5 ppm), all ammonia concentrations detected were below the detection threshold (17 ppm). The highest concentration of trimethyl amine (TMA) compounds was detected in the Truck Loading Bay, which had a high concentration of 11 ppb. All other locations sampled and tested, including the Nutrient Recovery Facility and its associated odor control system and solids disposal trucks had relatively low levels of TMA. Odor Emissions The primary odor source on site was determined to be the large chemical scrubber system. Although the measured H2S values were low (0.09 0.9 ppm), D/T values in the scrubber exhausts were higher than expected, ranging from 250 to 1,900. Combined with the high exhaust rate of 256,000 cfm, these scrubbers were determined to be a clear source for off-site odors. In addition, the removal efficiencies for D/T through the chemical scrubbers averaged only about 90%, which is below the expected removal efficiency of 99%. Based on the sampling and emissions rate data, this source was deemed a high-priority source. The Immediate Action Plan helped identify and prioritize odor improvements into three categories: high, medium, and low priority. The highest priority items included refurbishment of the existing 256,000 cfm chemical scrubber system, including: rebalancing of the ductwork system-a 2-month long process that included over 250 individual take-off points, adjustment of chemical feed setpoints, adjustment of chemical feed/sampling locations, operational procedure improvements (including packing cleaning procedures, fan operation, etc.), fine-tuning of automatic controls for fans and chemical feed pumps, and replacement of scrubber packing and spray nozzles for increased efficiency. In addition, other operational improvements were identified, including: adjustment of operation of splitter boxes to minimize fugitive emissions, addition of a new carbon adsorption system to serve the equalization splitter box, recommendations for improvements to the operation of the solids process, and reducing sludge blanket levels in the secondary clarifiers. Conclusion Based on the results of the odor study, many of the Immediate Action Plan recommendations were successfully implemented by GCDWR which in turn has led to mitigating any odor complaints since. Modifications to sampling location and operating parameters such as pH and ORP set points on the chemical scrubbers has also led to significant cost savings from reduced chemical usage (>50% reduction in chemical usage). The other modifications implemented for the chemical scrubbers have increased scrubber removal efficiencies, reduced fugitive emissions from the plant, and reduced scaling in the chemical scrubbing process. In addition, the recommended improvements have increased the reliability of the existing odor control systems and reduced odor emissions from the facility. Completion of the odor study allowed GCDWR to prioritize odor improvements and zero in on the most problematic areas of the plant prior to making any major capital investments.