The Fox River Water Reclamation District (FRWRD) provides wastewater treatment services to over 200,000 people within the Illinois communities of Elgin, South Elgin, West Dundee, portions of Sleepy Hollow, Streamwood, Hoffman Estates, unincorporated St. Charles Township, and surrounding areas in Kane and Cook Counties. FRWRD owns and operates three water reclamation facilities (WRFs): the Albin D. Pagorski WRF (ADP WRF), the North WRF, the West WRF, and 12 remote pump stations in the collection system.
The ADP WRF, North WRF, and West WRF have design average rated capacities of 25 million gallons per day (mgd), 7.75 mgd, and 5 mgd, respectively, which provides a total combined design capacity of 37.75 mgd for the three facilities. The ADP WRF serves as a regional biosolids facility, receiving the solids generated at the North WRF and the West WRF via the Southwest influent interceptor sewer. Treated effluent from each of the WRFs discharges into the Fox River.
In 2014, the Fox River Study Group along with the Illinois EPA negotiated special conditions of National Pollutant Discharge Elimination System (NPDES) permits for the district. Renewal conditions required that an Implementation Plan be amended to address an annual average limit of 1.0 mg/L total phosphorus with an implementation schedule. A future condition in the NPDES permits includes an effluent limit of 0.5 mg/L total phosphorus annual geometric mean by 2030.
Evaluation Process to Meet Required Nutrient Discharge Limits
To comply with the conditions of the NPDES permits, FRWRD began an extensive technologies evaluation process to meet the required nutrient discharge limits. The feasibility study was developed by Black and Veatch and included the three FRWRD WRFs. The study looked at meeting phosphorus effluent limits from 1.0 mg/L, 0.5 mg/L, .01mg/L TP, and TN = 8-10 mg/L. It evaluated both chemical and biological nutrient removal processes and considered the present worth of capital and O&M costs. At the conclusion of the feasibility study, Biological Phosphorus Removal (Bio-P) was chosen as the most cost-effective solution to achieve 1.0 mg/L for the three plants.
Evaluating Approach to Reduce Phosphorus LoadingThe next step was to evaluate the optimal approach for reducing phosphorus loading brought on by the Bio-P process in the post-dewatering recycle stream and its potential for struvite recovery, including (1) pre-dewatering phosphorus removal and recovery, and (2) post-dewatering phosphorus removal and recovery.
Four technologies were considered:
Pre-dewatering: 1) MagPrex® (Centrisys/CNP) 2) NuReSys™ (Schwing BioSet)
Post-dewatering: 3) Multiform Harvest™ (MHI) 4) Pearl Process™(Ostara)
Information provided by vendors was supplemented with full-plant process modeling, thermodynamic modeling, pilot experiments, historic FRWRD data, comparable case studies, and FRWRD input on the anticipated environmental regulatory needs as well as the business case evaluation.
The MagPrex phosphorus precipitation system, a Centrisys/CNP sludge optimization process, is typically installed between the anaerobic digestion and dewatering processes in the biosolids handling stage of wastewater treatment systems. MagPrex reduces soluble reactive phosphorus from the anaerobically digested biosolids by chemically precipitating struvite, a magnesium-ammonia-phosphate compound (NH4MgPO4·6H2O). The MagPrex process achieves this precipitation through the addition of magnesium chloride (MgCl2), as magnesium is commonly the limiting reagent found in anaerobically digested biosolids from Bio-P processes. In addition, the MagPrex system incorporates diffused air to strip out carbon dioxide (CO2), which raises the pH to the optimal precipitation environment for struvite formation.
Once the struvite precipitate is formed, it can be removed via settling to recover the struvite product for potential beneficial reuse (harvesting) or remain in the biosolids via the dewatering process (sequestration) to prevent the need for storage and handling of additional struvite byproduct. FRWRD considered the sequestration of the struvite material as their primary goal to prevent soluble phosphorus from returning to the main treatment process in the recycle flow stream and included future buildout of struvite harvesting equipment in the design plans.
Pilot Testing of MagPrex Phosphorus Precipitation System
Pilot testing of Centrisys/CNP’s MagPrex phosphorus precipitation system was conducted at the ADP WRF in 2015 to demonstrate the reduction of the soluble phosphorus concentration in the anaerobically digested sludge from the biosolids stream, as well as evaluate process parameters such as energy consumption and chemical dosage to better estimate full-scale operating costs. A Centrisys/CNP CS10-4 pilot scale centrifuge was incorporated in the pilot test to demonstrate improvements in sludge dewatering associated with the MagPrex process.
Soluble Phosphorus Reduction
During the 4-week pilot test, the MagPrex system consistently demonstrated soluble phosphorus reduction above 90 percent, while achieving an average increase in dewatered cake total solids (%TS) of 1.3 percent and an average decrease in polymer consumption of 16 percent using the pilot scale CS10-4 centrifuge. The dewatering results indicated that additional operational savings could be achieved through reduced polymer consumption or reduced biosolids hauling requirements.
Along with the other technologies evaluated, it was determined that pre-dewatering phosphorus recovery alternatives offer a greater net present value than post-dewatering phosphorus removal and recovery alternatives due to their simplicity, operating costs, and benefits in dewatering. Other considerations included downstream maintenance prevention that could be caused by uncontrolled struvite formation in process equipment and piping without pre-dewatering phosphorus removal. FRWRD recommended MagPrex based on results from the on-site pilot test, which demonstrated dewatering benefits resulting in lower estimated capital and operating costs. The single reactor setup allows FRWRD the flexibility to add a second reactor to harvest struvite should the regulatory conditions on phosphorous change or should market conditions for the sale of struvite determine that this would be a viable economic option in the future.
Construction of the full-scale MagPrex system at the FRWRD began in Fall 2019 and completed startup and commissioning in Spring 2021. The phosphorus precipitation process was installed prior to the district’s full-scale implementation of the Bio-P process for a couple of reasons. First, it would allow the FRWRD staff time to familiarize themselves with a system that is new to the operations staff. Secondly, once the new Bio-P system is complete, it would allow time to optimize the MagPrex operation in preparation for the anticipated increase in orthophosphate loading in the biosolids stream.
Results: 90 Percent Orthophosphate Reduction
Initial concentrations of orthophosphate at start-up were less than 100 mg/L PO4-P but increased to more than 400 mg/L by the Fall of 2022, when the full Bio-P at the North and ADP WRFs and the interplant sludge force main were completed.
Since the system has been in operation, MagPrex has reliably removed more than 90 percent of orthophosphate from the filtrate recycle loop which is discharged back to the primary treatment process.
Without the ability to remove the orthophosphate from the filtrate recycle, the FRWRDs Bio-P system alone would not be able to meet the lower phosphorous discharge limits set by the NPDES permit. This demonstrates the importance of the MagPrex system as part of FRWRDs overall multi-plant biosolids and phosphorous removal strategy.
As an added benefit, MagPrex has also shown the ability to improve the dewatering performance of the belt filter presses, by increasing cake total solids from 13 to 16 percent TS, which has a direct impact on operating costs for FRWRD by reducing the biosolids hauling.
Reducing Struvite Formation in Downstream Equipment
MagPrex will reduce the nuisance of struvite formation in downstream equipment, decrease costly chemical addition, decrease sludge hauling requirements, and increase dewatering performance.
“We are very happy with the MagPrex’s performance as it removes over 90 percent of the phosphorous in our digested sludge,” said Ed Brown, Operations Superintendent at FRWRD. “Reducing the soluble phosphorus loading in our filtrate recycle stream is critical for meeting phosphorus limits at the plant. Additionally, dewatering performance has been maintained even with all plants operating biological phosphorus removal.”
To learn more about Fox River Water Reclamation District's (FRWRD) NPDES implementation plan, technologies evaluation process, pilot test, and MagPrex installation, contact the experts at Centrisys/CNP today!