Website: www.nrri.umn.edu/ NRRI Duluth // Laboratories and Admin // 5013 5013 Miller Trunk Highway, Duluth, MN 55811 // (218) 788-2694 NRRI Coleraine // Laboratories // P.O. Box 188 // One Gayley Avenue, Coleraine, MN 55722 // (218) 667-4201 NRRI TECHNICAL REPORT MOBILE WATER TREATMENT DEMONSTRATION SYSTEM FOR SULFATE REDUCTION Submitted by: Meijun Cai, Shashi Rao, Sara Post, Adrian Hanson, Chan Lan Chun, Lucinda Johnson, George Hudak, Rolf Weberg Date: August 2022 Report Number: NRRI/TR-2022/12 Funding: Funding for this project was provided by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR). Recommended Citation: Cai, M., Rao, S., Post, S., Hanson, A., Chun, C.L., Johnson, L., Hudak, G., and Weberg, R. 2022. Mobile Water Treatment Demonstration System for Sulfate Reduction. Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/TR-2022/12. 33 p. Keywords: water treatment system, sulfate reduction, municipal wastewater, barite precipitation reaction, mobile water treatment demonstration system Natural Resources Research Institute University of Minnesota, Duluth 5013 Miller Trunk Highway Duluth, MN 55811-1442 Telephone: 218.788.2694 e-mail: nrri-reports@umn.edu Visit our website for access to NRRI publications. ©2022 by the Regents of the University of Minnesota All rights reserved. The University of Minnesota is committed to the policy that all persons shall have equal access to its programs, facilities, and employment without regard to race, color, creed, religion, national origin, sex, age, marital status, disability, public assistance status, veteran status, or sexual orientation. NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction i Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance EXECUTIVE SUMMARY The State of Minnesota adopted a sulfate standard of 10 mg/L for wild rice waters in 1973. Although under review, current technology for achieving this standard is a challenge for small industries and municipalities. Membrane-based technologies such as nanofiltration and reverse osmosis are capable of treating water to reach the Minnesota wild rice water sulfate standard; however, they typically require high capital and operation costs. Therefore, there is a need to develop cost-effective sulfate treatment alternatives. The Natural Resources Research Institute (NRRI) has developed a treatment system based on barite chemical precipitation reactions to reduce sulfate levels in water from 60-200 mg/L to below 10 mg/L. This system was demonstrated at bench-scale batch and continuous tests. The data collected from these lab tests were used to scale up the process to a trailer-based modular demonstration treatment system. This study highlights the outcomes of field pilot tests conducted by NRRI using this treatment system. The objectives of the field pilot trials were to: (1) Evaluate the efficacy of the chemical precipitation process when scaled up from 200 ml/min to 2 GPM; (2) Study the effect of co-existing chelating organics of the raw wastewater on barite precipitation reactions; (3) Optimize the chemical reagent dosage levels; (4) Investigate the potential of reusing process sludge to promote precipitation reactions; (5) Identify strategies to minimize scale formation on process equipment; and (6) Estimate the chemical reagent costs. The pilot tests were conducted using effluent from two municipal wastewater treatment plants (WWTP)—the Virginia WWTP and the Grand Rapids WWTP in northeastern Minnesota—from June 2021 until October 2021. The Virginia WWTP treats domestic wastewater, and the resulting effluent has relatively steady sulfate concentrations of 60 mg/L. The wastewater effluent was tested at 2 gallons/minute in the pilot system using a combination of precipitation, flocculation, sedimentation, and filtration. During the 2-month test period, the team evaluated the sulfate reduction efficacy at different chemical dosage levels and seed types. The pilot test results indicated that the chemical precipitation system consistently reduced the sulfate levels below 10 mg/L. The optimum reagent additions to achieve these results were 230 mg/L for BaCl2·2H2O and 20 mg/L or less for BaSO4. The Grand Rapids WWTP treats a mixture of domestic wastewater and industrial wastewater supplied from a regional paper mill. The sulfate concentrations in effluent varied from 115 to 85 mg/L from late August 2021 to early October 2021. The wastewater contains unknown chelating organic compounds, which negatively impede the barite precipitation reaction. A pretreatment process of flocculation with ferric chloride was designed to negate the impact of organic chelates on the barite precipitation reaction. Through the tests at different chemical dosage levels, the results suggested a need for a pretreatment process that included addition of ferric chloride prior to the addition of barium chloride and barium sulfate. The pilot test results indicated that the pretreatment process enabled the chemical precipitation reaction to consistently reduce the sulfate levels below 10 mg/L. The optimum reagent NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction ii Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance additions to achieve these results were 430 mg/L for BaCl2·2H2O, 20 mg/L or less for BaSO4 and 100mg/L or above for FeCl3. Based on pilot testing, the chemical costs to reduce sulfate concentrations to below 10 mg/L were estimated to be $2.27 and $5.50 per thousand gallons of effluent from Virginia and Grand Rapids wastewater treatment plants, respectively. Although pilot tests confirmed the feasibility of using barium chloride to reduce sulfate levels below 10 mg/L, scale generation on process equipment led to process interruption, which required periodic maintenance to remove the scale from the process equipment. The deposition of scale on tubes and vessels can result in loss of treatment capacity, equipment malfunctioning, equipment replacement, and increased maintenance costs. Further research through bench-scale tests and a 3-week indoor pilot trial explored the feasibility of utilizing ferric chloride pre-treatment to slow down scale generation. The morphology of the sludge samples collected from the tests with and without ferric chloride pretreatment was examined by a scanning electron microscope (SEM). The test results showed that ferric chloride changes the sludge morphology from small twinned flattened tabular crystal to large flattened tabular crystal, which probably helped reduce scale production and adhesion. The field pilot testing demonstrated the barite precipitation system’s ability to meet the sulfate standard of 10 mg/L from two different sources. The data gathered from the field pilot trial will help in designing an industrial scale treatment system and in retrofitting existing municipal wastewater facilities for sulfate removal. Acknowledgement The authors thank the Environment and Natural Resources Trust Fund for the financial, technical, operational, and administrative assistance in funding and managing the project. We gratefully acknowledge Alexis Ward and Matt Anthony, who were responsible for the field pilot trial by visiting the field sites, operating the system, refilling the chemicals, performing onsite measurement, collecting samples, and processing samples for lab measurement. The authors would like to recognize and give special thanks to Mr. Jeff Kinkel, Mr. Jack Grochowski, Mr. Tony Masching, and Mr. Kevin Jamsa in deploying, decommissioning, maintaining, repairing, and cleaning the pilot system. We would also like to thank Mr. Dave Sersha for setting up the electrical system of the pilot system. We would like to thank Mr. Kevin Kangas for the facility management. We appreciate and acknowledge the vital administrative and purchasing support provided by Ms. Julie Christopherson, Ms. Robin Oberton, and Ms. Tammy Thomasson-Ehrhart. The authors wish to acknowledge the gracious participation of Mr. Craig Maly, Ms. Jean Cranston, and Ms. Lisa Estepp for their assistance in setting up and modifying the safety and operation procedures. We would also like to acknowledge the support extended by the Virginia Wastewater Treatment Plant and Grand Rapids Public Utilities Commission (GRPUC) for providing the sites for the field tests and the electricity to run the pilot system. Finally, we would like to acknowledge the technical support extended by the Intuitech Inc., Widdes Trailer Sales, and Hart Electric. NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction iii Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance TABLE OF CONTENTS Executive Summary ........................................................................................................................................ i Acknowledgement .................................................................................................................................... ii List of Tables ................................................................................................................................................ iv List of Figures ............................................................................................................................................... iv 1. Background........................................................................................................................................... 1 2. Methods and Materials ........................................................................................................................ 3 2.1 Field Site Selection .............................................................................................................................. 3 2.2 Treatment Process .............................................................................................................................. 3 2.3 Chemical materials ............................................................................................................................. 7 2.4 Analytical Methods ............................................................................................................................. 7 2.5 Sampling Methods .............................................................................................................................. 8 2.6 Quality assurance and quality control .............................................................................................. 10 2.6.1 Comparing sulfate concentrations measured by spectrophotometer and Ionic Chromatography ................................................................................................................................ 10 2.6.2 Comparing sulfate concentrations measured by different labs ................................................ 10 3. Field pilot trial .................................................................................................................................... 12 3.1 Pilot Trial in the Virginia WWTP ....................................................................................................... 12 3.1.1 Water quality of the influent to the pilot system ..................................................................... 13 3.1.2 Conditions to be tested ............................................................................................................. 14 3.1.3.1 Pilot trial results ..................................................................................................................... 15 3.1.3.1 Sulfate treatment ................................................................................................................ 15 3.1.3.2 Effect of seed dosage rates ................................................................................................. 15 3.1.3.3 Sludge recycling as seed ...................................................................................................... 15 3.1.3.4 Chemical changes over the process .................................................................................... 17 3.1.3.5 Chemical cost of the pilot trial ............................................................................................ 18 3.1.3.6 Operational issues ............................................................................................................... 19 3.2 Pilot Trial in the Grand Rapids Wastewater Treatment Plant .......................................................... 19 3.2.1 Water quality of the influent to the pilot system ..................................................................... 21 3.2.2 Conditions tested in the Grand Rapids WWTP ......................................................................... 22 3.2.3 Pilot trial results ........................................................................................................................ 23 3.2.3.1 Sulfate treatment ................................................................................................................ 23 3.2.3.2 Chemical changes over the process .................................................................................... 25 3.2.3.3 Chemical cost of the pilot trial ............................................................................................ 26 3.2.3.4 Operational issues ............................................................................................................... 27 NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction iv Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance 3.3 Indoor Pilot Trial in NRRI Coleraine facility....................................................................................... 27 3.4 Design and Operation Instructions for a Full-scale System .............................................................. 29 3.4.1 Process design ........................................................................................................................... 30 3.4.2 Operation instruction ................................................................................................................ 30 4. Conclusion .......................................................................................................................................... 32 5. References .......................................................................................................................................... 33 LIST OF TABLES Table 1. Summary of baseline operating parameters for the Virginia and Grand Rapids municipal wastewater ................................................................................................................................................... 6 Table 2. Description of chemicals used in the study..................................................................................... 7 Table 3. Description of analytical methods .................................................................................................. 7 Table 4. Sample collection schedule ............................................................................................................. 9 Table 5. The water quality of the Virginia WWTP wastewater flowing into the pilot system .................... 14 Table 6. The conditions tested in the Virginia site ...................................................................................... 14 Table 7. Chemical cost estimate to treat total wastewater of 86,518.5 gallons ........................................ 18 Table 8. The water quality of the Grand Rapids WWTP wastewater flowing into the pilot system .......... 22 Table 9. The test conditions, runs were performed 24/7 ........................................................................... 23 Table 10. Chemical cost estimate for treating 60,049 gals of wastewater water in the Grand Rapids WWTP .............................................................................................................................................. 26 LIST OF FIGURES Figure 1. The process diagram of Virginia wastewater treatment ............................................................... 4 Figure 2. The pretreatment, reaction and flocculation process to treat the GR wastewater ...................... 5 Figure 3. The comparison of sulfate concentrations measured by HACH spectrophotometer and by IC............................................................................................................................................................. 10 Figure 4. The measurement results of sulfate concentrations for the same samples collected from (a) the Virginia site and (b) the Grand Rapids site ............................................................................. 11 Figure 5. The layout of the Virginia wastewater treatment plant. ............................................................. 12 Figure 6. The modified inflow uptake to the pilot system .......................................................................... 13 Figure 7. The chemical concentrations of the effluent after the chemical precipitation process.............. 16 Figure 8. The sulfate and chloride concentration gradient along the process flow direction .................... 17 Figure 9. Turbidity, conductivity and pH for the samples collected from different process tanks ............ 18 Figure 10. (a) The sludge scale on the wall of the reaction tank and (b) the sludge settled on the bottom of the flocculation tank .................................................................................................................. 19 Figure 11. The flow diagram of the treatment process in the Grand Rapids WWTP ................................. 20 NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction v Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance Figure 12. The layout of the Grand Rapids Wastewater Treatment Plant and the parking spot of the trailer system ........................................................................................................................................ 21 Figure 13. The chemical concentrations of the final effluent after the chemical precipitation process ........................................................................................................................................................ 24 Figure 14. Sulfate concentration changed from pretreatment, reaction, flocculation and filtration ........ 25 Figure 15. pH, conductivity and turbidity for the samples collected from different process tanks ........... 26 Figure 16. The sludge collected from the Grand Rapids WWTP ................................................................. 27 Figure 17. The sludge morphology measured by scanning electron microscope (SEM) (a) without FeCl3 pretreatment, (b) with FeCl3 pretreatment at 10 mg/L, and (c) with FeCl3 pretreatment at 100 mg/L ..................................................................................................................................................... 28 Figure 18. The scale built up on the wall of the second cell of the reaction tank after the system ran for four days with pretreatment FeCl3 at the dosage concentrations of (a) 100 mg/L, (b) 50 mg/L, and (c) 20 mg/L ................................................................................................................................. 29 Figure 19. Sludge morphology for the sludge collected from the indoor pilot trial with FeCl3 pretreatment at the concentrations of (a) 100 mg/L, (b) 50 mg/L, and (c) 20 mg/L.................................. 29 NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 1 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance 1. BACKGROUND Minnesota is one of the world’s largest wild rice producers, with more than 64,000 acres of wild rice over 1,200 lakes and rivers in the state (DNR, 2022). However, historic observations suggest that wild rice abundance is significantly reduced when sulfate concentrations exceed 10 mg/L, and the populations become uncommon when sulfate concentrations are above 50 mg/L (Moyle 1944, 1945). Based on Moyle’s research, the State of Minnesota set a 10 mg/L sulfate standard to protect wild rice in 1973. Minn. R. 7050.0224 specifies that the sulfate standard of 10 mg/L is applied to “water used for the production of wild rice during periods when the rice may be susceptible to damage by high sulfate levels” in class 4A water. That standard has proven to be challenging to implement, due in part to the current costs of sulfate treatment using conventional membrane technologies (Environmental Quality Board, 2019). Several potential sulfate treatment technologies are available. The best available techniques are continually changing as new technologies emerge, offering new unit processes for sulfate treatment or making currently used processes more efficient or economical. Understanding the treatment costs of these sulfate removal technologies is essential for municipal wastewater treatment plants that may need to reduce sulfate concentrations in their discharges. A recent technology survey study conducted by Barr Engineering and Bolton & Menk (Andrews & Richard, 2017) indicated that reverse osmosis and nanofiltration are the most well-developed and effective alternatives for sulfate removal. The primary concerns with the membrane-based approach for sulfate removal have been capital and operation costs surrounding management of the residuals from these technologies, which pose a significant impediment to their implementation. This concern has informed the investigation of cost-effective treatment alternatives to support municipal wastewater treatment plants and led to this project. To develop a cost-effective treatment method, the Natural Resources Research Institute (NRRI) studied the chemical precipitation process to reduce sulfate level to below 10 mg/L. In 2018 and 2019, NRRI demonstrated bench-scale tests of the barite precipitation reaction using municipal wastewaters from three wastewater treatment plants in northern Minnesota. The lab testing identified an acceptable treatment train protocol and an operational range for chemical dosing. NRRI designed a pilot scale treatment system based on data gathered from laboratory tests. In February 2020, NRRI awarded a contract to Intuitech (Salt Lake City, Utah) to design, fabricate, and supply a modular wastewater treatment system. The goal was to design a system that treated two gallons of wastewater per minute. Specifications called for the complete system to contain equipment for precipitation reaction by the addition of barium chloride, flocculation by the addition of ferric chloride, sedimentation supported by inclined plate settler, filtration aided by gravity media filters, and a sludge holding system. Two skid modules were delivered and installed in a trailer in August 2020. In late 2020, indoor pilot trials were performed using Coleraine, Minnesota tap water and effluent from the Grand Rapids, Minnesota municipal wastewater treatment plant (Rao and Cai, 2021). The indoor pilot trial identified baseline operating conditions, maintenance guidelines, and startup and shutdown procedures. To demonstrate the ability of chemical precipitation process to meet the sulfate standard of 10 mg/L at a large scale, NRRI deployed the mobile demonstration system at two municipal wastewater treatment plants in northern Minnesota. Field trials were performed at the flow rate of 1-2 gallons/minute from June to October 2021. The objectives of the field pilot trail were to: NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 2 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance (1) Evaluate the efficacy of the chemical precipitation process when scaled up from 200 ml/min to 2 GPM; (2) Study the effect of co-existing chelating organics of the raw wastewater on barite precipitation reaction; (3) Optimize the chemical reagent dosage levels; (4) Investigate the potential of reusing process sludge to promote precipitation reaction; (5) Identify strategies to minimize scale formation on process equipment; and (6) Estimate the chemical reagent costs. This report describes the results from field pilot trial that took place between June 2021 and October 2021. NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 3 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance 2. METHODS AND MATERIALS 2.1 Field Site Selection In 2019, NRRI hosted a sulfate treatment workshop with attendees from wastewater treatment facilities and Minnesota Pollution Control Agency (MPCA). In the workshop, NRRI introduced the chemical precipitation treatment system, collected the problems/concerns in current water treatment, and identified the plants interested in testing this method. The Virginia WWTP and the Grand Rapids WWTP expressed their interest to demonstrate our pilot trial. After the workshop, NRRI communicated with the Virginia City Council and Grand Rapids Public Utilities to explain the detailed treatment process, work design and fieldwork plan. With their support, our pilot system was successfully deployed in both plants to perform the pilot trial in 2021. 2.2 Treatment Process Field pilot trials were performed in the Virginia WWTP and the Grand Rapids WWTP from June 2021 to October 2021. The general process involved precipitation by the addition of barium chloride and barium sulfate, flocculation by the addition of ferric chloride, sedimentation by inclined plate settler and finally, filtration (Figure 1). This process was applied to the effluent from Virginia WWTP site at a flow rate of 2 gallons/minute. At this flow rate, the residence times were estimated at 30 minutes, 30 minutes, and 45 minutes for reaction, flocculation, and sedimentation tanks, respectively. The Grand Rapids wastewater receives both domestic wastewater and industrial wastewater from a paper mill. Previous research found that chelating agent concentrations of up to 40 mg/L have been identified in pulp mill final effluent (Virtapohja, 1998). The chelating organics in the wastewater likely hinder the precipitation reaction between barium and sulfate because of the dissolving ability (Luo et al., 2020). To overcome the hindrance caused by organic chelating agents, the water was subjected to flocculation pretreatment using ferric chloride before the precipitation reaction (Figure 2). Consequently, the addition points of barium chemicals and 2% ferric chloride (FeCl3) are moved to the third cell of the reaction tank and the second cell of the flocculation tank, respectively. The effluent from the flocculation tank flowed to the sedimentation tank and finally into the filter columns. In order to achieve a sufficient residence time for each process, the inflow rate was set at 1 gallon/minute, and the residence time was increased to 40 minutes each for pretreatment, precipitation reaction and flocculation. NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 4 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance Figure 1. The process diagram of Virginia wastewater treatment. Existing Wastewater Treatment Plant Reaction Tank (Chamber 1) Reaction Tank (Chamber 2) Reaction Tank (Chamber 3) Flow Rate 1 to 2 GPM BaSO4 BaCl2· 2H2O G=300 s-1 G=300 s-1 G=300 s-1 Reaction Tank, Tank size = 60 gallons Flocculation Tank (Chamber 3) Flocculation Tank (Chamber 2) Flocculation Tank (Chamber 1) G=25 s-1 G=40 s-1 G=75 s-1 Flocculation Tank, Tank size = 60 gallons Rapid Mix FeCl3 Reagent Metering Pump Tank Size = 2 gallons G=120 s-1 Plate Settler Sludge CollectionFinal Effluent Rapid Sand Filtration Effluent Sludge Recyling NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 5 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance Figure 2. The pretreatment, reaction and flocculation process to treat the GR wastewater. The operational settings of the pilot trials in Virginia and Grand Rapids are summarized in Table 1. The major difference in operations between these two plants included the inflow rates, filtration rates, dosing rates of barium chloride, mixing speed, and the sludge removal from the sedimentation tank. The ferric chloride pretreatment added in the Grand Rapids site generated a large amount of sludge, which required frequent removal. NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 6 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance Table 1. Summary of baseline operating parameters for the Virginia and Grand Rapids municipal wastewater. Parameter Units Baseline settings for Virginia wastewater Baseline settings for Grand Rapids wastewater Influent water flow rate gallons per minute (gpm) 2 1 Reagent dose Pretreatment ferric chloride (FeCl3) mg/L 0 300, 200, 100, 0 Barium chloride (BaCl2.2H2O) – Reactant mg/L 230, 200 430, 380 Barium sulfate (BaSO4) – Seed mg/L 100, 50, 20 100, 50 Ferric chloride (FeCl3) – Flocculant mg/L 2.5 2.5 Velocity gradient in precipitation basin Mixer 1310 s-1 G=350 G=60 Mixer 1320 s-1 G=300 G=60 Mixer 1330 s-1 G=300 G=120 Velocity gradient in rapid mix basin Mixer 2210 s-1 G=120 G=120 Velocity gradient in the flocculation basin Mixer 2310 s-1 G=75 G=120 Mixer 2320 s-1 G=40 G=60 Mixer 2330 s-1 G=25 G=25 Sludge removal from sedimentation tank Frequency to remove the sludge Once every four hours Once every 25 minutes Pump out percent 30% (~1 gallon) 50% Filtration Filtration X100, X200 Inflow gallons per minute (GPM) 0.6 0.3 Filtration rate GPM/ft2 3.0 1.5 NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 7 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance 2.3 Chemical materials Table 2 lists the chemicals used in this study. The solutions of barium chloride dihydrate and ferric chloride were prepared by dissolving/mixing the chemicals with tap water. The barium sulfate and tap water were weighed and mixed in the chemical tank directly. Table 2. Description of chemicals used in the study. Chemical Name Grade Manufacturer Concentration of stock solution (g/L) Barium Chloride Dihydrate Industrial grade solid Basstech International LLC 300 Barium Sulfate Industrial grade solid Excalibar Minerals LLC 300 Ferric Chloride 35% liquid Hawkins Water Treatment Group 20 2.4 Analytical Methods Table 3 lists the analytical methods used in this study. Table 3. Description of analytical methods. Parameter Method Standard Method No. Lab pH & Conductivity HACH HQ440d 4500 H+ Field Lab Turbidity HACH 2100Q 2130 Field Lab SO4, Cl Ion chromatography EPA 300.0 RMB Lab NRRI SO4 HACH DR1900 HACH 8051 Field Lab Ba Inductively Coupled Plasma-Atomic Emission Spectrometry EPA 200.7 RMB Lab Sediment characterization X-ray diffraction - UMD Campus Lab Sediment characterization Scanning Electron Microscope (SEM) - UMD Campus Lab NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 8 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance 2.5 Sampling Methods Table 4 lists the sample collection frequency and locations. The test samples were collected under steady state operation. The research team visited the site at least three times a week to collect samples, refill chemicals, perform onsite measurements and change the operation settings. The operational data were recorded by the SD cards every five minutes. The autosampler collected test effluent samples every hour from the backwash tank. Six consecutive samples were collected in one bottle to make one composite sample. One of every four composite samples and one influent sample every week were analyzed by the external lab (RMB Environmental Lab) for the measurement of SO4, Cl and Ba. For every condition tested, at least one bottle of sample was manually collected from the reaction tank, the flocculation tank and the sedimentation tank to check the chemical changes along the process. Other effluent samples and the process samples were filtered using a 0.45 µm membrane. NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 9 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance Table 4. Sample collection schedule. Location Frequency Sample Collection Parameters measured online Parameters measured in-situ Parameters measured in NRRI lab Parameters measured by external lab Inflow water Every week Manual collection - SO4 SO4, Cl, Ba Reaction tank, flocculation tank, sedimentation tank Every field visit Manual collection - pH, turbidity, conductivity SO4 - Sludge tank Once Manual collection for soil composition characterization - - Chemical element Backwash tank Every field visit Manual collection - pH, turbidity, conductivity - Hourly, composite six- hour samples in one bottle Remotely access, check daily Turbidity - SO4 SO4 for one sample every day Demanded by the project manager Pick from the composite sample - - - Ba, Cl NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 10 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance 2.6 Quality assurance and quality control The pilot trial tests were performed in multiple locations by multiple persons. To get reliable data and achieve successful results, quality control was essential. The research team controlled quality by developing operational procedures and process hazard analysis documents, training staff, and operating the system following the field operation checklist. For quality control and assurance, samples were analyzed in external certified laboratory (RMB Environmental Lab). 2.6.1 Comparing sulfate concentrations measured by spectrophotometer and Ionic Chromatography The influent sulfate concentrations determined the addition rates for barium chemicals. Typically, sulfate concentration is measured by ion chromatography (IC) in the laboratory. The fastest way to get sulfate data on site was through spectrophotometer with a relatively large measurement error (Figure 3). The measurement difference was usually below 10%. This relatively small error confirmed that the on-site sulfate measurement by spectrophotometer could provide a quick and sufficiently accurate measurement of sulfate concentration to adjust the dosing of barium chemicals. Figure 3. The comparison of sulfate concentrations measured by HACH spectrophotometer and by IC. 2.6.2 Comparing sulfate concentrations measured by different labs Sulfate concentrations of treated water were measured by both external lab (RMB Environmental Lab) and internal NRRI lab (Figure 4). Overall, the measurement difference could be up to 2.5 mg/L, but the concentrations were still below 10 mg/L. SO₄ measured by IC by RMB lab, mg/L 50 60 70 80 90 100 110 120 50 60 70 80 90 100 110 120 Grand Rapids Virginia NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 11 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance Figure 4. The measurement results of sulfate concentrations for the same samples collected from (a) the Virginia site and (b) the Grand Rapids site. The same sample was measured by the NRRI and RMB labs. Date 08/31/2021 09/10/2021 09/20/2021 09/30/2021 0 5 10 15 20 NRRI RMB Date 06/11/2021 07/01/2021 07/21/2021 0.0 2.5 5.0 7.5 10.0 12.5 NRRI RMB (a) (b) NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 12 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance 3. FIELD PILOT TRIAL Field pilot trials were performed in two municipal wastewater treatment plants (WWTP) located in northern Minnesota from June to October 2021, and in the NRRI Coleraine facility from late April to middle May 2022. 3.1 Pilot Trial in the Virginia WWTP The Virginia Wastewater Treatment Plant is located at 1204 Southern Drive, Virginia, Minnesota. The facility has primary, secondary, and tertiary waste treatment systems using an activated sludge process. The plant has a design capacity of 2.7 million gallons per day (MGD) with a peak flow of approximately 6 MGD (City of Virginia, 2021). NRRI’s pilot system was deployed next to the final effluent pond of the plant (Figure 5). The water from the effluent basin was pumped directly to the system as the inflow for the pilot trial. Figure 5. The layout of the Virginia wastewater treatment plant. The field pilot trial was performed between June 4th and August 2nd in the Virginia WWTP. The flow rate was set at 2 gallon/minute over the entire test period. Initially, tested water was pumped directly from the plant effluent pond by a submerged pump to the system. However, the water level of the effluent basin regularly dropped around 2 ft for around 30 minutes three times a day. This occurred when the effluent water was used for the backwash of the plant filters. The sudden decline of the water level significantly reduced the inflow rate to the pilot system. Particularly, during the 6am backwash the water inflow to the pilot system could drop to Effluent pond NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 13 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance 0 GPM. In order to get a stable inflow, a surge tank was added between the submerged pump and the trailer on June 17th (Figure 6). The addition of the surge tank avoided the sudden drop in the inflow rate but led to a steady decline in the inflow to around 1.3 GPM over two days. The decreased water flow rate was manually corrected during every field visit. Figure 6. The modified inflow uptake to the pilot system. 3.1.1 Water quality of the influent to the pilot system The influent samples of the pilot system were manually collected from the plant effluent basin weekly to measure the concentrations of sulfate and sometimes chloride and barium (Table 5). The sulfate and barium concentrations were stable, ranging from 55 to 65 mg/L for sulfate, and typically below 1 mg/L for barium. Chloride concentrations fluctuated from 170 to 326 mg/L during the test period. NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 14 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance Table 5. The water quality of the Virginia WWTP wastewater flowing into the pilot system. Date SO4, mg/L Cl, mg/L Ba, mg/L pH Conductivity Turbidity 6/3/2021 6.971 1351 1.09 6/9/2021 61.2 230 0.38 7.081 1374 0.54 6/16/2021 65.2 7.45 1395 0.29 6/21/2021 60.3 7.34 1313 0.63 6/29/2021 62 326 6.89 7.413 1704 1.14 7/6/2021 62.9 202 0.07 7.608 1363 0.63 7/14/2021 62.4 7.465 1432 0.27 7/23/2021 54.5 7.735 1361 0.4 7/26/2021 55.7 169 0.06 7.76 1273 0.35 3.1.2 Conditions to be tested During the two-month test, seven conditions were tested, including seeding rates, sludge recycling conditions and barium chloride dosage rates (Table 6). The test conditions were designed to evaluate feasibility of the treatment system, minimize the chemical dosage rates, and evaluate if the process sludge could be recycled as seed to promote the precipitation reaction. Table 6. The conditions tested in the Virginia site. Test duration Inflow sulfate concentration, mg/L BaCl2.2H2O dosage rate, mg/L Ba:SO4 molar ratio Seed Test time 6/4-6/16 61.2 230 1.48 BaSO4 =100 mg/L 9:00-14:30, workdays only 6/16-6/24 62.7 230 1.44 BaSO4 =50 mg/L 24 hours, workdays only 6/29-7/1, 7/13-7/19 62.2 230 1.45 Sludge recycles from sludge tank. Pump at 0.05 GPM (1%) for 1 minute every 6.5 minutes, average seed concentration = 39 mg/L 24/7 7/8-7/13 62.7 230 1.44 BaSO4 =20 mg/L 24/7 7/21-7/28 55.1 230 1.64 Sludge added from chemical tank, 50 mg/L 24/7 7/28-7/30 55.7 230 1.62 No seed 24/7 7/30-8/2 55.7 200 1.41 BaSO4 =20 mg/L 24/7 NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 15 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance 3.1.3.1 Pilot trial results 3.1.3.1 Sulfate treatment The primary objective of the field pilot trial was to test if the precipitation treatment system would reduce sulfate concentration to below 10 mg/L. Over the seven conditions tested, the effluent sulfate concentrations were below 10 mg/L except when the barium chloride dosage level was at 200 mg/L (Figure 7). The results indicated that the dosage level of the barium chemical must be at a Ba:SO4 molar ratio greater than 1.41 in order to reduce the effluent sulfate concentrations from 60 mg/L to below 10 mg/L. Although the effluent sulfate concentrations were reduced to below 10 mg/L, the addition of the barium chloride increased the effluent chloride and barium concentrations from around 200 mg/L to around 300-350 mg/L, and from <1 mg/L to around 40 mg/L, respectively. 3.1.3.2 Effect of seed dosage rates Seed was necessary to initiate the chemical precipitation reaction when the sulfate concentration was below 100 mg/L. Barium sulfate (BaSO4) was selected as the seed chemical and tested. The bench-scale test showed that the seed amount of 100 mg/L was sufficient to assist the precipitation reaction. The pilot trials tested the BaSO4 dosage rates at 100 mg/L, 50 mg/L, 20 mg/L and finally 0 mg/L (Figure 7). The effluent concentrations of sulfate did not show a significant difference when the dosage rate was 20 mg/L and above. When no seed was added, the sulfate concentrations at the effluent could be reduced to below 10 mg/L but significantly higher than in other seeding conditions. This indicated that the BaSO4 seed could promote the precipitation reaction at a very low level, 20 mg/L or lower. 3.1.3.3 Sludge recycling as seed Sludge was used to replace BaSO4 as the seed in two supplying modes, intermittently (one minute every 6.5 minutes, average concentration = 39 mg/L) and continuously (50 mg/L). In both modes, sulfate concentrations were successfully reduced to below 10 mg/L (Figure 7). This information suggests that sludge could be used as seed for the reaction. NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 16 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance Figure 7. The chemical concentrations of the effluent after the chemical precipitation process. BaCl₂ = 230 mg/L, BaSO₄ = 100 mg/L BaCl₂ = 230 mg/L, BaSO₄ = 50 mg/L BaCl₂ = 230 mg/L, BaSO₄ = 20 mg/L BaCl₂ = 230 mg/L, No seed BaCl₂ = 230 mg/L, Sludge recycle intermittently BaCl₂ = 230 mg/L, Sludge recycle continously BaCl₂ = 200 mg/L, BaSO₄= 20 mg/L 33 35 38 40 43 45 48 200 300 400 500 600 0 2 4 6 8 10 12 NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 17 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance 3.1.3.4 Chemical changes over the process The treatment process includes four steps from the start to the end: 1) precipitation reaction; 2) flocculation; 3) sedimentation; and 4) filtration. Generally, sulfate concentrations were reduced to below 10 mg/L in the last cell of the reaction tank and declined during the treatment process to below 5 mg/L at the final effluent (backwash tank) (Figure 8). Chloride concentrations remained in the range of 250 and 350 mg/L at all four sampling locations. Figure 8. The sulfate and chloride concentration gradient along the process flow direction. The treatment process was monitored by onsite measurement of pH, conductivity, and turbidity (Figure 9). The pH and conductivity values did not show significant changes along the downstream process. However, turbidity declined significantly from the reaction tank to the final effluent. Water turbidity was already reduced to below 10 NTU after sedimentation. The filtration process further reduced turbidity to below 1 NTU. Reaction tank Flocculation tank Sedimentation tank Backwash tank 150 200 250 300 350 400 0 5 10 15 Flow direction NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 18 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance Figure 9. Turbidity, conductivity and pH for the samples collected from different process tanks. 3.1.3.5 Chemical cost of the pilot trial One of the objectives of the field pilot trial was to estimate the reagent cost for this treatment system. The chemical cost accounted for the major chemicals used in the process but did not include the chemicals (pH buffer solutions, sulfate kits) used for onsite measurement. Table 7 summarized the amounts and costs of chemicals used. During this 2-month trial period, approximately 86,518.5 gal of wastewater was treated. For every thousand gallons of water treated, the chemical cost was estimated to be $2.27. About 74% of the cost was the cost of barium chloride. Table 7. Chemical cost estimate to treat total wastewater of 86,518.5 gallons. Item Amount Unit Price Cost Cost per thousand gallons treated water BaCl2·2H2O, kg 68.68 $2.11/kg $144.91 $1.67 BaSO4, kg 7.39 $6.82/kg $50.40 $0.58 FeCl3, liter 2.2 $0.59/kg $1.30 $0.02 Total $196.61 $2.27 Date 06/01/2021 06/11/2021 06/21/2021 07/01/2021 07/11/2021 07/21/2021 07/31/2021 0.2 0.3 0.5 1 2 4 10 20 40 100 1200 1300 1400 1500 1600 1700 1800 1900 2000 pH 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 Reaction tank Flocculation tank Sedimentation tank Backwash tank NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 19 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance 3.1.3.6 Operational issues The primary issue was the scale buildup in the reaction tank and the sludge settling in the bottom of the flocculation tank (Figure 10). The barite scale built up on the wall quickly and clogged the water passage holes easily. The scale was not able to be removed by simply rising with pressurized water. Every week, the operation team had to manually clean the wall once. In the flocculation tank, particularly in the third cell, a large amount of sludge settled on the bottom and was washed out by pressurized water once a week. Figure 10. (a) The sludge scale on the wall of the reaction tank and (b) the sludge settled on the bottom of the flocculation tank. The hard scale is potentially a serious issue for future operations. In order to mitigate scale formation, ferric chloride was added prior to the reaction process to investigate if this pretreatment could inhibit scale production. The results are presented in section 3.3. 3.2 Pilot Trial in the Grand Rapids Wastewater Treatment Plant The Grand Rapids wastewater treatment plant is located in Grand Rapids, MN. The Wastewater Treatment Facilities consist of the industrial screening/pumping station, the industrial primary treatment plant, the secondary treatment plant, and the industrial sludge landfill (Figure 11). This facility treats an average of 5.5 million gallons of waste effluent per day: 4.0 million gallons from UPM/Blandin Paper Company and 1.5 million gallons from domestic users (Grand Rapids Public Utilities, 2011). (a) (b) NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 20 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance Figure 11. The flow diagram of the treatment process in the Grand Rapids WWTP. NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 21 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance The trailer of the pilot system was parked in the parking lot next to the solid waste building, but the water supplied to the trailer was pumped from the effluent polishing pond (Figure 12). Figure 12. The layout of the Grand Rapids Wastewater Treatment Plant and the parking spot of the trailer system. On August 20th, the pilot system was deployed in the plant. The field pilot trial in the Grand Rapids WWTP was performed from August 26th to October 8th, 2021. The system was operated continuously for 24 hours and seven days. 3.2.1 Water quality of the influent to the pilot system Influent samples of the pilot system were manually collected weekly from the inflow pipe of the trailer. The samples were taken to measure the concentrations of sulfate, chloride, and barium (Table 8). Other parameters that were also measured included turbidity, conductivity, and pH. The sulfate concentrations were not stable, varying from 115 mg/L at the beginning of the trial to 85.1 mg/L at the lowest point. The chloride concentrations were typically around 100 mg/L and the barium concentrations were below 0.3 mg/L. Effluent polishing pond NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 22 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance Table 8. The water quality of the Grand Rapids WWTP wastewater flowing into the pilot system. Date Conductivity pH Turbidity SO4, mg/L Cl, mg/L Ba, mg/L 8/27/2021 1548 7.961 2.08 115 103 0.27 8/30/2021 1577 8.058 2.26 9/1/2021 1547 7.878 1.92 103 107 0.23 9/7/2021 1708 7.791 1.52 90.5 125 0.24 9/10/2021 1607 8.114 3.3 9/15/2021 1542 8 7.3 87.7 110 0.18 9/17/2021 1497 8.001 2.03 9/22/2021 1478 8.017 5.75 93.8 98 0.17 9/24/2021 1487 8.082 6.57 9/27/2021 1537 8.155 2.28 9/29/2021 1533 7.957 9.52 85.1 93.8 0.2 10/1/2021 1529 7.893 6.84 10/4/2021 1542 7.975 3.73 10/6/2021 1562 7.988 21.5 91.9 91 3.2.2 Conditions tested in the Grand Rapids WWTP The Grand Rapids WWTP was selected as one of the test sites because the wastewater of this plant comprises both industrial wastewater from a paper mill and domestic wastewater. The tests performed in the Grand Rapids WWTP were primarily to study the pretreatment ferric chloride dosage levels. The chelating agents used in paper mill were incorporated into the mixed wastewater and inhibited the precipitation reaction. Therefore, a pretreatment process was necessary to remove the chelating organics prior to the reaction process with barium chemicals. Lab bench-scale tests found that ferric chloride could efficiently remove the chelating organics prior to the precipitation reaction. Nine conditions, including two flow rates, two BaSO4 dosage rates, one sludge dosage rate, and four pretreatment ferric chloride dosage rates, were tested during the 1.5-month test period (Table 9). Because of the unstable inflow sulfate concentrations, barium chloride was overdosed most of the time at a Ba:SO4 molar ratio higher than 1.6. NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 23 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance Table 9. The test conditions, runs were performed 24/7. Test duration Inflow sulfate concentration, mg/L Flow rate, GPM BaCl2.2H2O dosage rate, mg/L Ba:SO4 molar ratio Seed Pretreatment FeCl3 dosage rate, mg/L 8/26-8/30 115 1 430 1.47 BaSO4 = 100 mg/L 300 8/30-9/2 103 2 430 1.64 BaSO4 = 100 mg/L 300 9/7-9/13 90.5 1 430 1.87 BaSO4 = 50 mg/L 300 9/13-9/20 87.7 1 430 1.93 BaSO4 = 50 mg/L 200 9/20-9/24 93.8 1 430 1.80 Sludge = 20 mg/L 300 9/24-9/27 93.8 1 430 1.80 Sludge = 20 mg/L 200 9/27-10/1 85.1 1 380 1.76 BaSO4 = 50 mg/L 200 10/1-10/6 91.9 1 380 1.63 BaSO4 = 50 mg/L 100 10/6-10/8 91.9 1 380 1.63 BaSO4 = 50 mg/L 0 3.2.3 Pilot trial results 3.2.3.1 Sulfate treatment Because of the presence of chelating organics in the effluent from Grand Rapids Wastewater Treatment Plant, the precipitation treatment process was slightly modified from that of the Virginia Wastewater Treatment Plant by adding a ferric chloride pretreatment process prior to any other processes (Figure2). The primary purpose of the test at the Grand Rapids WWTP was to identify the lowest level of ferric chloride dosage needed overcome the potential inhibition of the reaction process by chelating organics. Over the nine conditions tested, the effluent sulfate concentrations were mostly below 10 mg/L, except when the pretreatment ferric chloride dosage level was reduced to 100 mg/L or below (Figure 13). This result indicated that the Grand Rapids wastewater must be pretreated by ferric chloride at a dosage level above 100 mg/L. Similar to the findings from the tests in the Virginia site, the barium sulfate seed amount could be below 50 mg/L and sludge could replace barium sulfate as seed at the dosage level of 20 mg/L. NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 24 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance Figure 13. The chemical concentrations of the final effluent after the chemical precipitation process. Inflow rate = 1 GPM, BaSO₄ = 100 mg/L, FeCl₃ = 300 mg/L, BaCl₂·2H₂O = 430 mg/L Inflow rate = 2 GPM, BaSO₄ = 100 mg/L, FeCl₃ = 300 mg/L, BaCl₂·2H₂O = 430 mg/L Inflow rate = 1 GPM, BaSO₄ = 50 mg/L, FeCl₃ = 300 mg/L, BaCl₂·2H₂O = 430 mg/L Inflow rate = 1 GPM, sludge = 20 mg/L, FeCl₃=300 mg/L, BaCl₂·2H₂O = 430 mg/L Inflow rate = 1 GPM, sludge = 20 mg/L, FeCl₃=200 mg/L, BaCl₂·2H₂O = 430 mg/L Inflow rate = 1 GPM, BaSO₄ = 50 mg/L, FeCl₃=200 mg/L, BaCl₂·2H₂O = 430 mg/L Inflow rate = 1 GPM, BaSO₄ = 50 mg/L, FeCl₃=200 mg/L, BaCl₂·2H₂O = 380 mg/L Inflow rate = 1 GPM, BaSO₄ = 50 mg/L, FeCl₃=100 mg/L, BaCl₂·2H₂O = 380 mg/L Inflow rate = 1 GPM, BaSO₄ = 50 mg/L, FeCl₃=0 mg/L, BaCl₂·2H₂O = 380 mg/L 40 60 80 100 120 140 100 200 300 400 500 0.1 1 10 NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 25 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance 3.2.3.2 Chemical changes over the process The sulfate concentrations along the treatment process clearly indicated that the pretreatment with ferric chloride did not remove sulfate (Figure 14). Through the reaction process, sulfate concentrations were reduced to below 10 mg/L. After that, no significant changes in sulfate concentrations were observed in the flocculation and filtration process. Figure 14. Sulfate concentration changed from pretreatment, reaction, flocculation and filtration. Data were collected from 15 samplings. The treatment process was monitored by onsite measurement of pH, conductivity and turbidity (Figure 15). The pretreatment of ferric chloride reduced water pH from around 8 to 6.2-7.5 depending on the dosage amount of ferric chloride. Conductivity was significantly increased by around 200 µs/cm once the precipitation reaction happened. Most barium sulfate particles settled in the sedimentation tank, leading to a sudden drop of turbidity from around 100 NTU to below 10 NTU after the sedimentation tank. Pretreatment Reaction Flocculation Filtration 0.3 0.4 0.5 0.6 0.7 0.8 1 2 3 4 5 6 7 8 10 20 30 40 50 60 70 80 100 200 NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 26 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance Figure 15. pH, conductivity and turbidity for the samples collected from different process tanks. 3.2.3.3 Chemical cost of the pilot trial Experiments at the Grand Rapids WWTP treated approximately 60,049 gals of wastewater. The chemical cost to treat the wastewater containing chelating organics and with a sulfate concentration of 85- 105 mg/L was estimated to be around $5.88 per thousand gallons of treated water (Table 10). This chemical cost was more than two times the chemical cost estimated from the trial at the Virginia site. This difference resulted from the use of ferric chloride for the pretreatment and an increased amount barium chloride used to react with higher sulfate contents in the Grand Rapids wastewater. Table 10. Chemical cost estimate for treating 60,049 gals of wastewater water in the Grand Rapids WWTP. Item Amount Unit Price Cost Cost per thousand gallon treated water BaCl2·2H2O, kg 90.88 $2.11/kg $191.76 $3.19 BaSO4, kg 11.28 $6.82/kg $76.93 $1.28 FeCl3, liter 142.89 $0.59/kg $84.31 $1.40 Total $352.99 $5.88 Pretreatment Reaction Flocculation Sedimentation Filtration 1 10 100 1500 1600 1700 1800 1900 2000 2100 2200 pH 6.0 6.5 7.0 7.5 NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 27 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance 3.2.3.4 Operational issues Previous tests at the Virginia site raised the concerns of scale built-up in the reaction tank and sludge sedimentation in the flocculation. Interestingly, both issues identified at the Virginia site did not happen at the Grand Rapids site, probably because of the ferric chloride pretreatment. This difference implied that ferric chloride pretreatment may be one efficient way to inhibit scale production. However, the pretreatment produced a large amount of sludge, which amounted to around 5% of the inflow. The sludge was hard to condense by simple gravity settling (Figure 16). A simple gravity settling for two days increased particle content from 1% to 3%. Figure 16. The sludge collected from the Grand Rapids WWTP. 3.3 Indoor Pilot Trial in NRRI Coleraine facility One of the operational issues identified throughout the pilot trial was the scale production in the reaction tank. This issue was particularly observed in the Virginia site, where a thick and hard scale layer was easily built up in the reaction tank within one week. The scale created reduced the tank volume and clogged the passage holes on the walls. This white scale layer was hard to clean without manual scrubbing. In contrast, no scale was found in the reaction tank for the field test at the Grand Rapids site. The major difference of the tests in these two sites was the pretreatment of ferric chloride, which was performed at the Grand Rapids site but not at the Virginia site. Because of this, the research team suspected that ferric chloride pretreatment may have helped inhibit barium sulfate scale production. The project team first performed a bench-scale experiment to explore the sludge morphology with or without ferric chloride pre-treatment. Coleraine tap water with sulfate level around 120 mg/L was used as the tested water. To add the ferric chloride pretreatment process, ferric chloride at the designed concentrations was first added to the test sample and mixed for 1 minute. After that, the precipitation and flocculation reactions were performed following the process demonstrated in Figure 1. After the process, the sludge samples were collected and analyzed by using a scanning electronic microscope NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 28 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance (SEM). Without the ferric chloride pretreatment, the sludge was primarily composed of barite (BaSO4) in small twinned flattened tabular crystal structure (Figure 17). The crystal particles grew big with ferric chloride treatment at 10 mg/L and became mostly large flattened tabular crystals at 100 mg/L. Without FeCl3 pretreatment, only the barite mineral is produced in the water. The adjacent crystals of the same mineral are attached to form crystal twinning in a symmetrical manner. This results from an intergrowth of two separate crystals that are tightly bonded to each other. However, the addition of ferric chloride introduced other molecules or even minerals, leading to the reduced chance of intergrowth of two adjacent crystals of the barite mineral. Figure 17. The sludge morphology measured by scanning electron microscope (SEM) (a) without FeCl3 pretreatment, (b) with FeCl3 pretreatment at 10 mg/L, and (c) with FeCl3 pretreatment at 100 mg/L. As the different morphologies of the sludge collected from the process with and without ferric chloride pretreatment implied that this pretreatment may inhibit scale production, an indoor pilot was conducted at NRRI’s Coleraine facility for three weeks using the same Coleraine tap water as the water source to confirm this finding. Each week the test was started from Monday morning and ran continuously until Friday morning, when the system was shut down and the scale condition was visually examined (Figure 18). During this three-week test, three pretreatment ferric chloride concentrations were tested, and each was tested for one week. Visually, the scale growth was significantly inhibited when 100 mg/L ferric chloride was used as the pretreatment. When the ferric chloride pretreatment concentration was reduced to 50 mg/L or 20 mg/L, scale built up on the wall, but not as thick as observed in the Virginia site (Figure 8). The scale was only observed in the cell where the barium chemical was directly added. In addition, the scale layer was easily washed off by rinsing with pressurized water. This indicated that ferric chloride addition prior to the treatment process could reduce scale growth and scale adhesion. (a) (b) (c) NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 29 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance Figure 18. The scale built up on the wall of the second cell of the reaction tank after the system ran for four days with pretreatment FeCl3 at the dosage concentrations of (a) 100 mg/L, (b) 50 mg/L, and (c) 20 mg/L. The sludge samples were collected and analyzed by SEM to compare the morphology difference produced by three FeCl3 concentrations (Figure 19). The sludge particle shapes were mostly irregular, particularly when the FeCl3 concentration was at 100 mg/L. When the dosage amount of the FeCl3 chemical was reduced to 20 mg/L, some small twinned crystals were observed. Figure 19. Sludge morphology for the sludge collected from the indoor pilot trial with FeCl3 pretreatment at the concentrations of (a) 100 mg/L, (b) 50 mg/L, and (c) 20 mg/L. The addition of ferric chloride slightly reduced solution pH to approximately 6.5 but did not affect sulfate reduction. For all 12 effluent samples collected from this three-week test, sulfate concentrations were all below 10 mg/L. This test suggests that pretreatment with ferric chloride addition could significantly inhibit the scale growth when the ferric chloride concentration is 100 mg/L or above, but could reduce scale adhesive when the ferric chloride concentration is below 100 mg/L. 3.4 Design and Operation Instructions for a Full-scale System The field pilot trial was used to evaluate the feasibility of the precipitation treatment system, to estimate the operational cost, and to identify any operational issues or unpredicted results in order to properly design and implement a full-scale process. (a) (b) (c) (a) (c)(b) NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 30 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance 3.4.1 Process design The current system with the processes of precipitation reaction, flocculation, and sedimentation can successfully reduce the sulfate level below 10 mg/L. However, several issues were identified, including the scale growth on the tank wall and the sludge precipitation on the bottom of the reaction and flocculation tanks. To inhibit the scale growth, ferric chloride is recommended to be mixed with the water before the precipitation reaction. The addition of ferric chloride is recommended to be controlled at around 100 mg/L. The bottom of the reaction tank and the flocculation tank is suggested to be designed as a cyclone shape to collect the sludge regularly. The operational cost of the chemical precipitation system is positively related to the reduced sulfate level. Because of this, the chemical precipitation system developed during this study is particularly recommended for wastewater treatment plants having the following conditions: • Sulfate concentration at low concentrations, such as below 100 mg/L, to reduce the chemical cost and sludge production; • Wastewater does not contain chelating organics to avoid the pretreatment process; • Inflow sulfate concentration is stable, as an online sulfate measurement instrument is not available, and overdose of chemicals will increase chemical cost and add too many chemicals. 3.4.2 Operation instruction During our field pilot trial for three-and-a-half months, the team visited the Virginia site almost every weekday and visited the Grand Rapids site three times a week. The work experiences gained from the field operation generated operation instructions including daily, weekly, monthly, yearly, and intermittent tasks. It should be noted that our operation was performed only during the summer season, so the instructions may not cover additional tasks necessary for the other three seasons. Daily operation • Check pipes, mixers, and pumps for any leaking or abnormal noise. • Examine chemical tank volume and refill the tank when necessary. • Collect one 24-hr composite sample from the final effluent to determine chemical contents, including sulfate, chloride, and barium. • Measure turbidity for inflow, reaction tank, flocculation tank, sedimentation tank, and final effluent to check if the reaction is proceeding as designed. • Backwashing filtration columns/system regularly. Weekly operation • Clean the tanks or pipes if any clog is observed. • Collect one sample from each of inflow, reaction tank, flocculation tank, and sedimentation tank for water quality measurement. Monthly operation • Periodically lubricating bearings, motor, and other moving parts. NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 31 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance Yearly operation • Shut down the plant and thoroughly clean all tanks, mixers, and pipes. • Repair any leakage spots. • Train employees about safety and operation procedures. Intermittent tasks • Purchase chemicals when needed. • Develop an emergency response plan for onsite storage of chemicals. NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 32 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance 4. CONCLUSION The field pilot trial in two wastewater treatment plants for three-and-a-half months and the indoor pilot trial for three weeks provided important information for the future full-scale design and operation. The major findings include: • The sulfate treatment system based on barite precipitation is feasible to reduce sulfate levels to below 10 mg/L. However, the chemical cost of this process is positively related to the amount of sulfate reduced, therefore this technology is recommended to reduce sulfate level by 200 mg/L or less. • The barium reaction process produced a significant amount of scale in the reaction tank. Ferric chloride is recommended to mix with the influent wastewater prior to the reaction process to reduce scale growth and scale adhesion. • Sludge could be recycled to be used as seed. Further study is needed to identify how long the sludge could be used without an external seed supplement. • All applications of this treatment process should be piloted prior to design to verify chemical usage and mixing requirements. This research team will continue to perform pilot-scale tests to treat water with high sulfate concentrations. The results from the treatment at high sulfate concentrations will be compared with the results from the current study to identify any possible changes in the process design and operation instructions. NRRI/TR-2022/12 – Mobile Water Treatment Demonstration System for Sulfate Reduction 33 Natural Resources Research Institute Innovative Research • Minnesota Value • Global Relevance 5. REFERENCES Andrews, E. J. and Richard, D. E. 2017. Analyzing Alternatives for Sulfate Treatment in Municipal Wastewater Part 1: Feasibility Alternative Review, St. Paul: Minnesota Pollution Control Agency. City of Virginia. 2021. City of Virginia, MN utilities website, retrieved on Nov. 2, 2021. Environmental Quality Board 2019. Governor's Task Force on Wild Rice, retrieved on August 1, 2022. Grand Rapids Public Utilities. 2021. City of Grand Rapids, MN utilities website. Luo, Z., Zhang, N., Zhao, L., Wang, C., Wu, L., Liu, P. and Ji, H. (2020). A chelating agent system for the removal of barium sulfate scale. Journal of Petroleum Exploration and Production Technology, 10: 3069- 3079. MN Department of Natural Resources (2022). Wild rice management, retrieved on May 9, 2022. Moyle, J. B. 1944. Wild rice in Minnesota. Journal of Wildlife Management 8:177–184. Moyle, J. B. 1945. Some chemical factors influencing the distribution of aquatic plants in Minnesota. American Midland Naturalist 34:402–420. Rao, S., and Cai, M. (2021). Indoor Pilot Trial for Sulfate Reduction. NRRI technical report. Virtapohja, J., (1998). Fate of Chelating Agents Used in the Pulp and Paper Industries, Department of Chemistry, University of Jyväskylä, Research Report No. 67.