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LESSONS LEARNED FROM TWO MEMBRANE APPLICATIONS ON

WASTEWATER REUSE

Qigang Chang, Ph.D., PE, Advanced Engineering and Environmental Services, Inc. (AE2S),

3101 South Frontage Road, Moorhead, MN 56560

Email: qigang.chang@ae2s.com Phone: 218-299-5610

Jordan Grasser, PE, AE2S, Grand Forks, ND

Matt Erickson, AE2S, Grand Forks, ND

Brian R. Bergantine, PE, AE2S, Moorhead, MN

Jeff Hoff, Fargo Wastewater Treatment Plant

Abstract

Both the City of Fargo and the City of Grand Forks, North Dakota, draw water from the Red River of North as their primary drinking water source and discharge their wastewater to the same water body. Due to the limited available capacity of potable water, wastewater reuse became the solution to meet the growing demand from industries in the region. A 1.4 mgd Effluent Reuse Facility (ERF) was constructed in the fall of 2008 to supply water to an ethanol plant. The Fargo ERF is a dual membrane system comprised of three Memcor CP 120 UF membrane skids followed by four two-stage (10:5) RO skids with 8-inch Hydranautics elements installed. During the summer of 2014, a 4-month pilot study was performed to investigate an integrated membrane system that treats the City of Grand Forks WWTP secondary effluent and supplies high quality water to a proposed $2 billion nitrogen fertilizer plant. The PVC UF membrane was operated as cross-flow and inside-out mode to minimize membrane fouling. Due to the limit of the testing equipment, the UF membrane ran as constant feed pressure instead of constant flow. The membrane flux varied between 20 and 65 gfd during a 60-minute production cycle. Severe UF membrane fouling occurred when chlorine was fed to the UF feed as a measure to control bio-fouling. Similarly, when acid was fed to the RO feed, the RO membrane experienced unexpected fouling. The speculation is that organic matters were altered with chemical additions and they tended to plug membranes.

Introduction

Facility Summary

The City of Fargo, North Dakota, owns and operates a 25 mgd wastewater treatment plant (WWTP) with an average daily flow of 12 mgd. The Fargo WWTP treatment process consists of headworks, screen, trickling filters, denitrification, and chlorine disinfection and discharges effluent to the Red River of the North, a river shared by North Dakota and Minnesota. Fargo Effluent Reuse Facility (ERF) was built in 2008 to treat the Fargo WWTP effluent and supply the treated water to a 100 million gallon ethanol plant in Casselton,North Dakota.

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The Fargo ERF treatment process consists of UF membranes and RO membranes with a 1.4 mgd RO permeate production capacity (Figure 1 ). The UF membrane system consists of two inline 500 micro automatic strainers, three Siemens Memcor CP120 skids, and CIP system. The RO system consists of two sets of 5 micro cartridge filters, four RO skids, and CIP system. Two-stage (10:5) RO skid has six 8-inch elements per pressure vessel and is installed with Hydronautics ESPA 2 module. The RO membrane design flux is 11 gfd and design recovery is 70%. The overall recovery of RO system has been running around 65% in the last seven years with each skid producing 250 gpm RO permeate. The average RO concentrate flow is approximately 428 gpm and can be discharged to the Red River or recycled back to the head works of the Fargo WWTP depending on the seasons.

Figure 1. Fargo Wastewater Reuse Project Schematic

Figure 2 presents the Grand Forks wastewater treatment plant (GWWWTP) pilot study flow

diagram. The UF membrane system piloted was a model UF-2-HF system manufactured by

OSMO Asia Pacific, LTD. The system contained two (2) PVC crossflow membranes identified

as OAPmod48-6 with a nominal capacity of 8.8 gpm. The RO membrane skid consists of a

booster pump, a 5 micro cartridge filter, a high pressure pump, and two 4-inch RO elements in

series. Sulfuric acid was fed to a 275 gallon RO feed tank to maintain a constant pH of 6.2.

Antiscalant was not fed during the piloting effort.

Figure 2. GFWWTP pilot study process flow diagram

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Membrane System Performance

Fargo ERF Membrane Performance

Figure 3 indicates the UF membrane quickly lost a portion of its permeability during the break-in period and then stabilized between 2-6 gfd/psi. Due to unique and rapid UF membrane fouling, the CIP interval was shortened from three weeks (design) to seven days. UF membrane life time varied between five and seven years. A new UF membrane with more hydrophilic surface chemistry was installed during membrane replacement and initially a much higher membrane permeability was observed. However, the permeability of the new membrane continuously decreases with aging and drops to nearly the same level as the original UF membranes.

Figure 3. Fargo ERF UF Membrane Permeability

The four RO skids have been operating in a parallel fashion under the same operational parameters and feed water quality. These four skids exhibit almost identical performance; therefore, the RO performance was evaluated based on one of the four skids. The RO system has been running smoothly since the first day of operation and CIP generally was performed as a routine at a 6-12 month interval (Figure 4). The RO feed pressure varies between 150 psi and 200 psi and generally follows the pattern of the water temperature, as shown in Figure 5. The salt passage slightly increased in the eight years of operation from 0.5% to 1.5% (Figure 6). Table 1 summarizes the water quality for UF and RO membranes. It is anticipated that the RO membranes may reach ten years life time at the current operation conditions.

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Figure 4. Fargo ERF RO Normalized Permeate Flow

Figure 5. Fargo ERF RO Feed Pressure

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Figure 6. Fargo ERF RO Salt Passage

Table 1. Fargo ERF Water Quality (average)

Parameters UF Feed RO Feed

Chloride (mg/L) 114 119

Calcium (mg/L) 76.3 74.6

Silicon (mg/L) 6.44 6.54

BOD5 (mg/L) 7.2 < 2.0

pH 7.65 7.93

TDS (mg/L) 977 968

TSS (mg/L) 11.8 < 1.0

Turbidity (NTU) 7.21 0.295

Conductivity (µS/cm) 1,456 1,491

TOC (mg/L) 10.8 9.0

Magnesium (mg/L) 49.6 48.9

Sodium (mg/L) 138 137

Potassium (mg/L) 17.0 16.6

Iron (mg/L) 0.417 0.044

Manganese (mg/L) <0.020 <0.020

Sulfate (mg/L) 340 301

Nitrate - N (mg/L) 21.9 22.0

Ammonia as N (mg/L) 0.16 1.54

Total Phosphorus as PO4 (mg/L) 8.95 8.15

Alkalinity as Bicarbonate (mg/L) 154 156

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To date, the Fargo ERF pumped a total of 2 billion gallons of RO water to the ethanol plant and generated more than $5 million revenue for the City of Fargo. The O&M cost can be broken down into several components that consist of staff, engineering services, technical services, general maintenance, insurance, general supplies, energy, chemicals, education/training, and miscellaneous expenses. These cost categories are summarized into four main categories and presented in Figure 7. The staff and engineering services, general maintenance, energy, and chemical cost are $0.31, $0.12, $0.44, and $0.69 per 1,000 gallons produced, respectively. The success of the Fargo ERF promotes the regional interests of wastewater reclamation projects.

Figure 7. Fargo ERF Operation and Maintenance Cost

GFWWTP Pilot Study Membrane Performance

Unlike the Fargo ERF, no coagulant was fed to the secondary effluent prior to the UF membrane for GFWWTP pilot study. The UF membrane was operated as constant feed pressure mode rather than constant flow; therefore, the filtrate flow decreased significantly within the 60-minute backwash interval. Figure 8 shows that the temperature corrected UF membrane permeability varied widely from 1 gfd/psi to 8 gfd/psi in 8 hours operation. The UF membrane can be adequately cleaned through sodium hypochlorite cleaning, which implies the major fouling was a combination of organic and microbial. The UF membrane ran as cross-flow mode rather than dead-end filtration, and the concentrate was dumped to sanitary drain. The overall recovery of the UF membrane process was 80-90%.

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Figure 8. GFWWTP Pilot Study Temperature Corrected UF Membrane Permeability

The RO skid can be considered as a two-stage skid with a single RO element for each stage. A portion of RO concentrate was recycled to increase RO membrane cross flow as a measure to mitigate membrane fouling. At RO membrane flux of 18 gfd, recovery of 65%, and RO feed

conductivity 2000 µS/cm, RO permeate conductivity varied between 40 and 50 µS/cm, which

implies that the RO salt passage was less than 5%. During the course of piloting, both acid and caustic CIPs were performed to restore RO membrane permeability. It appears that RO membrane responded well to the caustic CIPs, not the acid CIP. This implies that RO membrane likely experienced organic fouling.

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Figure 9. GFWWTP Pilot Study RO Permeate Conductivity

Figure 10. GFWWTP Pilot Study RO Normalized Permeate Flow

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Lessons Learned

1. Strainer: the strainer drain was relocated from the side of the vessels to the bottom to drain the vessel completely.

2. Static mixer: the PVC static mixer was destroyed in two weeks of operation. A stainless steel mixer was installed and has been in operation since 2009.

3. Materials compatibility: the selection of equipment materials has to be compatible with the chemicals. It was found that Viton fittings and O-rings were destroyed by sodium hydroxide and ammonia hydroxide. It seems EPDM is a better choice for caustic chemicals. Coagulant ferric chloride slowly erodes out aluminum parts through the course of operation.

4. It was well known that spiral wound RO element is not compatible with chlorine and sodium bisulfite is commonly used to quench oxidants in RO feed. However, it was found that a trace chlorine in RO feed actually enhances the membrane performance. RO membrane performance obviously deteriorated after stopping feed free chlorine and the performance came back with the resumption of the chlorine feed. It is speculated that the trace chlorine reacts with the thin layer of organics and microbial on the membrane surface to mitigate fouling. Since chlorine is consumed by the thin layer, RO membrane maintains its integrity through the operation.

5. Overfeeding of chemicals not only increases the cost but also worsens membrane performance. When the pretreatment chemical ferric chloride dose was reduced from 3 mg/L to 0.5 mg/L, significant improvement was observed on both UF membrane and RO cartridge filters.

6. Proper chemical mixing is critical to achieve the goal of chemical feeding. The installation of the stainless steel static mixer extends the duration of RO cartridge filter from one month to six months. It was observed that the color of dirty cartridge filters changed from dark green to light brown after the installation of the new static mixer, which implies better coagulation in the direct filtration.

7. The standard cleaning regime may not work for every single project. It was found that citric acid and sodium hypochlorite cleaning couldn’t restore UF membrane permeability. Membrane autopsy indicates the major organic fouling was sodium petroleum sulfonate. A total of more than twenty different chemicals were experimented with and Micro-90, a detergent, was identified to clean up UF membrane adequately. A further study may be deserved to investigate the mechanism of Micro-90 to remove foulants from the UF membranes.

8. Chemical addition may cause unexpected issues. Sulfuric acid was fed to the UF feed in order to improve RO membrane performance. Unfortunately, the addition of sulfuric acid dramatically worsened the UF membrane performance, which forced the relocation of acid feed to downstream of the UF membranes. It is speculated that the acid altered the properties of a portion of organics and then increased the organic fouling potential. Similarly, sodium hypochlorite addition deteriorated the UF membrane performance, even though it is commonly used to mitigate UF membrane bio-fouling.

Acknowledgement

The authors would like to acknowledge the City of Fargo, the City of Grand Forks, and individuals who contributed to the projects and this paper. Your support is highly appreciated.