Holistic Approach to Residuals Handling at the F. Wayne Hill Water Resources Center

Authors:

  • Scott Hardy, Ron Latimer, Vivi Nguyen, Eddie McCallum, Michael Bullard - Hazen and Sawyer
  • Brandon Brown, Robert Harris, Eric Hancock, JC Lan - Gwinnett County DWR

Primary clarifier performance has shown excellent performance with consistent removal of greater than 50 percent total suspended solids for over two years.

The thickening performance has shown good control of thickened sludge concentration to the digesters, however, consistent thickening performance has been challenging with recent thickened sludge concentration falling from 5.5% TS to 4.5% TS.

The existing thickened sludge sample was underestimating the combined thickened sludge concentration as shown by the solids balance.

The F. Wayne Hill Water Resources Center (FWHWRC) is Gwinnett County’s largest and most advanced wastewater treatment facility with a permitted capacity of 60 mgd. The FWHWRC uses enhanced biological phosphorus removal and chemical trim to meet a stringent effluent TP limit of 0.08 mg/L. The current solids handling processes consist of co-thickening of primary sludge and WAS with rotary drum thickeners, mesophilic anaerobic digestion with FOG and High Strength Waste co-digestion, combined heat and power digester gas utilization, centrifuge dewatering, and side-stream nutrient recovery.

In 2010, Hazen performed a study at the F. Wayne Hill Water Resources Center (FWHWRC) to assess the plant capacity to determine the capacity impacts from receiving solids generated at the county’s Yellow River Water Reclamation Facility. The study suggested improvements to the primary clarifiers to achieve reliable removals to maintain secondary process capacity and implement co-thickening of primary sludge and WAS to delay the expansion of the anaerobic digestion facility, which was limited on capacity.

Since February 2011 the FWHWRC started receiving Yellow River WRF solids and has completed the following capital improvement projects:

  • Combined Heat and Power System: November 2011
  • FOG/HSW Receiving Facility for Co-Digestion: April 2012
  • Rotary Drum Co-Thickening Facility: April 2013
  • Primary Clarifier Rehabilitation: November 2012 through January 2014
  • Nutrient Recovery System: July 2015

In 2015, Hazen performed a follow-up study to confirm that the above capital improvement projects were performing as intended and to optimize the processes to obtain the performance criteria, if required.

Results
Primary clarifier performance has shown excellent performance with consistent removal of greater than 50 percent total suspended solids for over two years. Plant operators are keeping sludge blankets low to prevent septic sludge conditions and operating a sufficient number of basins to keep the surface overflow rate under 1,000 gpd/sf during dry weather conditions.

The thickening performance has shown good control of thickened sludge concentration to the digesters, however, consistent thickening performance has been challenging with recent thickened sludge concentration falling from 5.5% TS to 4.5% TS. This initiated a new polymer study by the plant to obtain greater solids capture and more consistent thickened sludge concentrations, especially with implementation of the WASSTRIP nutrient recovery process (combining primary sludge and WAS for P-release) for struvite harvesting. This study also showed that the existing thickened sludge sample was underestimating the combined thickened sludge concentration. A new sampling location for thickened sludge for better representative sample was selected.

The better than anticipated performance from the primary clarifiers is increasing the solids loading to the anaerobic digesters. To compensate for the additional loading, the thickened sludge key performance indicator (KPI) was increased from 5.5 to 6.5% total solids for increasing digester performance.

This presentation showcases the benefit of a whole-plant approach to optimization and the value of evaluating and optimizing the process after implementing upgrades. Results will also be shared related to benefits on digester gas production from increased primary clarifier performance and receipt of high strength organic wastes (FOG/HSW) receiving and how strategies for digester gas utilization have generated significant operational cost savings through on-site power generation.

For more information, please contact the author at shardy@hazenandsawyer.com.

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