Nutrient Recovery by Struvite Crystallization Process: Virginia Experience
Abstract:The Hampton Roads Sanitation District's (HRSD), Nansemond Treatment plant (NTP) services the cities of Chesapeake, Portsmouth and Suffolk, Virginia. The plants treated effluent is discharged into the Chesapeake Bay watershed. The NTP was designed as a 30 MGD biological nutrient removal (BNR) plant with seasonal nitrification and utilizes anaerobic digestion for solids stabilization and centrifugation for dewatering. Centrate containing high levels of phosphorus (up to 900 mg P/L) and ammonia (∼700 mg N/L) is returned to the plant headwork and contributes significantly to the BNR process loading. The high loading of these nutrients is a potential cause of instability to the treatment process leading to spikes in the secondary effluent phosphorus concentrations as well as reducing plant nitrification and denitrification capacities. During these periods, metal salts (FeCl3) have been intermittently required to precipitate phosphorus to achieve compliance. Precipitating phosphorus is quite effective, but this option is not considered as a sustainable solution for phosphorus removal due to the high costs of chemicals, solids handling and disposal.
The formation of struvite (MgNH4PO4 · 6H2O) has recently been commercialized as a treatment process for phosphorous and ammonia recovery from sludge dewatering sidestream and is being considered for treating centrate at the NTP. A pilot scale struvite crystallization process (Ostara Nutrient Recovery Technologies Inc.) has been operating at the NTP from October 2006 to March 2007 to determine the feasibility of the process for full scale implementation at this site. Centrate fed to the pilot reactor typically measures 300-700 mg/L phosphate-P and 820 mg/L ammonia-N. Over the course of the pilot demonstration, phosphorous removal averaged 85% while ammonia removal ranged from 35 to 50%. A full scale struvite recovery process, currently being evaluated, would reduce loading in the main liquid process train by approximately 500 lbs PO4-P/day and 250 lbs NH4-N/day in addition to providing diurnal centrate flow equalization.
Document Type: Research Article
Publication date: 2007-10-01
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