Nutrient Recovery From WAS Streams for Struvite Control and Recycle Load Reduction

Authors:

  • Joe Rohrbacher - Hazen and Sawyer

The F. Wayne Hill Water Resources Center (FWHWRC), Gwinnett County’s largest and most advanced wastewater treatment facility with a capacity of 60 mgd, uses enhanced biological phosphorus removal and chemical trim to meet a stringent TP limit of 0.08 mg/L.

The F. Wayne Hill Water Resources Center (FWHWRC) is Gwinnett County’s largest and most advanced wastewater treatment facility with a capacity of 60 mgd. The FWHWRC uses enhanced biological phosphorus removal and chemical trim to meet a stringent TP limit of 0.08 mg/L. Solids handling consists of anaerobic digestion of combined primary sludge and WAS in egg-shaped digesters. In 2009, Gwinnett County began adding magnesium hydroxide (Mag) to their collection system for odor and corrosion control. With the addition of Mag in the collection system, struvite precipitation in the solids handling process has become a major issue at the FWHWRC. However, the digesters at the FWHWRC are now phosphorus limited with respect to struvite formation due to the Mag addition in the collection system, which results in a benefit of very low P concentrations in the dewatering centrate. Therefore if Mag addition is halted, the P-recycle load (most P removal is now by EBPR) will become a major concern that must be addressed with additional metal salt addition and if Mag addition continues, the plant must address the struvite formation issues of clogged centrate pipes, centrifuge impacts and potential catastrophic failures. This project evaluated nutrient recovery options compared with convention metal salt addition to the digesters or centrate for both struvite control (if Mag continues) or recycle P load reduction (if Mag is stopped).

Objectives
This paper details the comprehensive investigation which was performed to evaluate nutrient recovery as an option for phosphorus control in the solids handling processes. The investigation included bench and pilot scale studies as well as full scale process stream characterization, BioWin modeling, and a business case evaluation.

Methodology
Bench scale reactors were operated to optimize operational parameters (reaction time and WAS:Primary Sludge (PS) ratio) for the pilot scale work, and to determine the potential for P-release from WAS. P-release was studied under three different conditions: endogenous release (WAS only), ideal volatile fatty acid (VFA) conditions (WAS + sodium acetate), and actual proposed plant conditions (WAS and primary sludge blend).

Pilot scale systems from two different nutrient recovery technology vendors (Ostara and Multiform Harvest (MH)) were tested to determine potential for P and N recovery at FWHWRC. Ostara’s WASSTRIP and Multiform Harvest’s MH P-release processes were tested along with the nutrient recovery systems. The WASSTRIP option, which releases P and Mg from the WAS prior to digestion, was selected to control the struvite formation potential in the solids handling process. The pilots were run for 3-4 weeks, and samples were collected over each process three days per week.

BioWin modeling, including struvite precipitation modeling, was used to compare the effectiveness of several nutrient control strategies against conventional ferric treatment for P-removal and struvite precipitation.

Results
The results show approximately 90% PO4-P release after 3 hours for non-endogenous reactors and that the best WAS:PS ratio was a 1:1 by mass mix, which is approximately the current production ratio.

The WASSTRIP pilot setup for the P release reactor confirmed the detention time of 2 to 3 hours was sufficient to release to achieve full P release of the Bio-P WAS, in line with the bench scale testing. Due to the high Mg concentration in the centrate, magnesium chloride addition was not required for either the Ostara or MH reactors.

Sampling and mass balances of Mg over the plant shows Mg in the dewatered cake corresponding to an estimated 7,000 lb/day of struvite in dewatered sludge produced currently at the plant. P-removal in sludge cake through struvite explains the very low P concentration seen in FWHWRC’s centrate stream.

Discussions and Conclusions
• The FWHWRC’s PS is a viable source of VFAs to induce rapid P-release from the WAS PAOs.
• Modeling results show that applying Ostara’s WASSTRIP process while continuing Mg(OH)2 use in the collection system can reduce the amount of struvite formed in the sludge cake to levels comparable with ferric addition in the digesters with a payback less than 7 years compared to ferric addition.
• If the use of Mg(OH)2 is discontinued, applying Ostara WASSTRIP or Ostara on centrate feed can drastically reduce the P-recycle load and had a payback period of less than 8 years compared with metal salt addition.
• The business case evaluation determined nutrient recovery was the best option for struvite and phosphorus recycle load control.
• Both pilots were capable of achieving +75% P-removal rates and 15 to 20% NH3-N removal. Nutrient recovery with the WASSTRIP process is a viable technology for reducing struvite formation and/or reducing P-recycle loads at Bio-P WWTPs.
• The county has preselected the nutrient recovery process vendor. The full scale system is currently under final design.

To request a copy of the full paper, contact the author at jrohrbacher@hazenandsawyer.com.

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