Knock Out, Drag Out Fight: Centrifuge vs. Belt Filter Press - Title for Best Dewatering Technology to Squeeze the Poop at William. E. Dunn Water Reclamation Facility!

BACKGROUND The William E. Dunn Water Reclamation Facility (WEDWRF) is in Pinellas County, Florida and is operated by Pinellas County Utilities to treat wastewater collected from the County's northern service area, and to generate reclaimed water supply for the County's reuse distribution system. The WEDWRF is a 9.0 MGD capacity advanced 5-Stage Bardenpho process wastewater treatment facility that currently operates at an annual average flow of approximately 6.5 MGD. The existing solids management system for the WEDWRF includes two (2) rotary drum thickeners (RDT) for thickening waste activated sludge (WAS), two open top sludge holding tanks, two (2) belt filter presses (BFP) and truck loading bay. The existing BFP are old and require replacement. A pilot scale dewatering study was conducted with the following objectives: 1.Evaluate the dewatering performance of new belt filter press and centrifuge. 2.Optimize the polymer dosage to maximize cake solids and solids capture. 3.The efficacy of generating thickened waste activated sludge (TWAS) using RDT to thicken WAS prior to dewatering or bypass the RDT to directly dewater WAS as an operational, energy and polymer cost saving measure. 4.Analyze composite samples of centrate/filtrate stream for Chemical oxygen demand (COD), Ammonia and Phosphorous to determine the quality of the reject stream back to the headworks. 5.Perform life cycle cost analysis of dewatering operation and maintenance for BFP and Centrifuge. PILOT TESTING SCHEDULE The pilot testing was conducted between April 18, 2023 - June 6, 2023. Alfa Laval's BFP was tested between April 18 - April 20 for WAS, April 25 - April 27 for TWAS. Andritz centrifuge was tested between May 9 - May 11 for WAS, May 15 - May 17 for TWAS. Alfa Laval's centrifuge was tested between May 23 - May 25 for WAS, June 5 - June 6 for TWAS. A filtrate/centrate stream composite sample was collected on each day of pilot testing for both WAS and TWAS. RESULTS 1.Waste Activated Sludge The percent cake solids (%TS) obtained for WAS in Alfa Laval BFP ranged between 16.8-18.5% for the polymer dosage ranging between 10.0-16.8 lb/Ton. The solids capture for BFP was between 95 - 99%. Similarly, the percent cake solids (%) obtained for Andritz and Alfa Laval Centrifuge ranged between 20.0-24.5% for polymer dosage ranging between 14.5-62.9 lb/Ton for Andritz centrifuge and 13.2-35.7 lb/Ton for Alfa Laval centrifuge. The solids capture for Andritz centrifuge ranged between 89-96% and Alfa Laval centrifuge ranged between 94-99%. 2.Thickened Waste Activated Sludge The percent cake solids (%TS) obtained for TWAS in Alfa Laval BFP ranged between 15.8-17.7% for the polymer dosage ranging between 7.3-11.4 lb/Ton. The solids capture for BFP was between 96-99%. Similarly, the percent cake solids (%) obtained for Andritz and Alfa Laval Centrifuge ranged between 18.7-22.7% for polymer dosage ranging between 18.7-44.8 lb/Ton for Andritz centrifuge and 27.0-32.0 lb/Ton for Alfa Laval centrifuge. The solids capture for Andritz centrifuge was between 84-99% and Alfa Laval centrifuge ranged between 94-99%. 3.Centrate/Filtrate Analysis A composite sample of centrate/filtrate was collected on each day of pilot testing for both WAS and TWAS. For WAS centrate stream, the concentrations of COD ranged between 60 - 470 ppm, ammoniacal nitrogen between 0.1 - 0.3 ppm, total phosphorus between 2 - 48 ppm. For TWAS centrate stream, the concentrations of COD ranged between 102 - 2,440 ppm, ammoniacal nitrogen between 0 -150 ppm, and total phosphorus between 133 - 490 ppm. It is noteworthy to mention that the concentration of analytes reported here as received from the contracted laboratory. The key observations in the filtrate/centrate composite sample analysis are: 1.Concentration of the analytes appear to be lower during the belt filter press dewatering operation when compared to the centrifuge dewatering operation. 2.Concentration of analytes for TWAS filtrate/centrate stream appear to be significantly higher than for WAS. This indicates that there is release of nutrients in the holding tank. 3.The P concentration in the filtrate for TWAS appears to be in high enough concentrations that it could lead to the formation of struvite in the facility. This could decrease the dewaterability of the sludge and create maintenance issues, due to buildup in pipes and valves, at the facility. KEY TAKE-AWAYS The pilot testing results indicate that both BFP and centrifuge technology could be suitable for WEDWRF dewatering improvements. The BFP consistently achieved a cake solids concentration of 17.5 (%TS) for WAS. In the case of TWAS, the average percent cake solids were about 17.0 (%TS). Similarly, both the centrifuges performed better than BFP producing cake solids at an average of 22.0-23.0 (%TS) for WAS and 20.0 (%TS) for TWAS. However, the polymer consumption in the case of centrifuge was at least twice as much as BFP polymer consumption for both WAS and TWAS to achieve a 3-5% increase in percent cake solids. The centrate/filtrate composite analysis samples showed interesting observations with high concentrations of nutrients for TWAS. This could be attributed to the release of nutrients in the sludge holding tank. A determination of the exact duration and conditions that would cause this release of nutrients and soluble COD is something the pilot did not focus on. An understanding of the impacts of sludge degradation in sludge holding tanks especially in hot summer months is important for this and many other plants that use sludge holding tanks as buffers to limit plant dewatering operations schedules to only a few days a week. This could also impact the dewaterability of TWAS which was evident as the cake solids (%) obtained in TWAS was relatively lower than WAS. Furthermore, higher nutrient and COD loading in the centrate stream recycled to the headworks of the plant could lead to increase in aeration demands to treat the increased ammonia load . Similarly, there could be an impact in disinfection chlorine demand. To conclude, both BFP and centrifuge should be evaluated for both WAS and TWAS to have a complete understanding of overall life-cycle cost implications on each of those equipment. Life cycle cost analysis will be performed and presented by the time of Residuals and Biosolids Conference in 2024. This presentation will cover the pilot testing equipment operating parameters, feed characteristics, relationship between wastewater sludge cake solids and polymer consumption of dewatering WAS and TWAS, important parameters for plant operations decision making. Furthermore, the presentation touches upon nutrient and COD impacts of dewatering recycle streams and their potential impacts on plant effluent limits. ACKNOWLEDGEMENTS The presenters thank Pinellas County Utilities, Alfa Laval, and Andritz for providing pilot testing equipment trailers and for assisting during the pilot testing phase.