A QUANTITATIVE PROCEDURE TO SELECT MF/UF MEMBRANE DESIGN FLUX BASED UPON PILOTING PERFORMANCE. Introduction

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1 A QUANTITATIVE PROCEDURE TO SELECT MF/UF MEMBRANE DESIGN FLUX BASED UPON PILOTING PERFORMANCE Qigang Chang, Ph.D., PE, Advanced Engineering and Environmental Services, Inc. (AE2S), 3101 South Frontage Road, Moorhead, MN Phone: Brian R. Bergantine, PE, AE2S, Moorhead, MN Introduction Filtration remains a cornerstone of drinking water treatment; and for many conventional water treatment plants this is in the form of granular media depth filters. Although granular media filters can produce high quality water, they represent a probabilistic rather than an absolute barrier; consequently, pathogens can still pass through the filters and pose a health risk. The commercialization of hollow-filter microfiltration/ultrafiltration (MF/UF) membrane, which can be backwashed, for the removal of particulate in the early 1990s has had the most profound impact on the use, acceptance, and regulation of all types of membrane processes for drinking water treatment (USEPA, 2005). The design of an MF/UF membrane facility incorporates parameters and practices from traditional water treatment system design as well as other membrane system design. In general, these parameters include membrane flux, water quality, temperature compensation, crossconnection control, and system reliability. Membrane flux is one of the most fundamental considerations in the design of a membrane filtration system, which determines the total membrane area required to produce desired filtrate flow. Membrane flux is normally expressed in gallons per square foot per day of membrane area (GFD), equivalent to the loading rate in gallons per minute per square foot that is used the conventional media filtration. The selection of an appropriate membrane flux is a core question that design engineers have to answer. Due to the intrinsic characteristics of each water source, membrane pilot studies are strongly recommended to determine appropriate operation and design criteria prior to a large investment. Usually, large volumes of complicated data were amassed in the pilot study. As such, it can be a challenge to fully and appropriately analyze pilot performance for a sound engineering design. A new quantitative procedure was developed to assist investigators and design engineers to step bystep break down massive data in a meaningful and clear way to achieve a reliable membrane system design. New MF/UF Membrane Design Flux Selection Method In a sound engineering design, membrane permeability progressively decreases during normal operation. A 10-15% decrease of membrane permeability generally triggers a clean-in-place (CIP) to recover membrane permeability. Due to membrane module deterioration and irreversible fouling, membranes slowly lose permeability through the design life time. 1

2 This new quantitative MF/UF membrane design flux selection procedure defines and calculates a minimum temperature corrected membrane permeability (mtcmp) to evaluate membrane pilot study performance to select an appropriate design flux for a sustainable design, as shown in Equation 1. The formula inputs include design flux (to be evaluated), annual membrane permeability loss allowed, membrane design life time, maximum transmembrane pressure (TMP), and the lowest water temperature. Where: mtcmp = Flux TMPm 1.03(20 T) (1 A) n Equation 1 mtcmp: minimum temperature corrected membrane permeability at 20 C (gfd/psi) Flux: design flux (gfd) A: annual membrane permeability loss (percent) n: membrane design life time (year) TMPm: maximum operation transmembrane pressure (psi) T: lowest membrane influent water temperature ( C) The maximum TMP will be provided by membrane vendors based upon challenge tests. The annual membrane permeability loss allowed and membrane design life time should be determined by owner/engineer and membrane vendors in conjunction with the membrane performance observed in pilot studies. The lowest water temperature can be determined with review of historical raw water quality data. Because of the impact on water viscosity, higher TMP is generally required in the winter to produce the same flow as in the summer (AWWA, 2005). Each of the membrane fluxes tested in the pilot study can be entered into the formula to calculate a corresponding mtcmp, which will be against the observed TCMP in the pilot study. As shown in Figure 1, the selection of membrane flux A is likely under-designed while the selection of membrane flux C is likely over-designed. The membrane flux B is likely the appropriate selection to meet the design criteria. A Case Study The City of Fargo, North Dakota, currently operates a lime-softening water treatment plant (WTP) with a rated capacity of 113,562 m 3 /day (30 million gallons per day (MGD)). The City obtains its raw water from both the Red River and Sheyenne River. In recent years, there has been growing concern over the water quality in the Sheyenne River, primarily associated with elevated sulfate concentrations as a result of flood mitigation in the Devils Lake Basin and discharges from the Devils Lake Emergency Outlets. A study by the United States Geological Survey (USGS) predicted that the sulfate concentration in the Sheyenne River will reach 750 mg/l in the lower Sheyenne River at Fargo with the two emergency outlets operating at a combined 17 m 3 /s (600 cubic feet per second) (USGS, 2011). This significant water quality change in the Sheyenne River has caused concerns for the City of Fargo because its existing conventional lime-softening WTP relies on the Sheyenne River source 40 percent of the time and does not remove sulfates. In 2011, Advanced Engineering and Environmental Services (AE2S) and Black & Veatch prepared a master plan for the City of Fargo that recommended reverse osmosis (RO) as the preferred process 2

3 TCMP for sulfates removal (AE2S, 2011). A comprehensive membrane pilot study confirmed the viability of membrane technology to remove sulfate as well as other impurities for the City of Fargo. The City of Fargo s 15 MGD integrated dual membrane water treatment plant is currently under construction and anticipated to be operational in A B C Time Figure 1. Theoretical MF/UF Membrane TCMP Curves GE submersible ZeeWeed 1000, Siemens encased MemCor L20N, and Pall encased UNA-620A membranes were evaluated in the membrane pilot study. Each of the three membranes were tested at various membrane fluxes based upon review of the feed water quality and vendor s past experiences. The pilot study compiled an enormous volume of data which became a challenge and the responsibility of engineers to analyze and provide a set of sound and appropriate design parameters for the proposed 15 MGD integrated dual membrane WTP expansion. This new quantitative procedure was developed and successfully utilized to evaluate the three MF/UF membranes performance. Membrane A Three membrane fluxes were piloted for membrane A in the pilot study and the flux of 35 gfd was not further evaluated due to the short run. Table 1 summarizes design assumptions for calculation of the mtcmp. As shown in Figure 2, the mtcmp of flux 34 gfd is 4.66 gfd/psi and approximately half of the observed TCMP at 34 gfd is lower than mtcmp, which implies that 34 gfd will likely under-design the membrane system. The mtcmp is 4.11 gfd/psi for flux 30 gfd and is slightly lower than the majority of TCMP observed at 30 gfd, which indicates that design flux of 30 gfd is an appropriate design flux for membrane A. Membrane A manufacturer agreed that the selection of design membrane flux was appropriate for their membrane. 3

4 20 C (gfd/psi) gfd 4 gfd /7 1/18 3/1 4/12 5/24 Date Membrane A - 30 gfd Membrane A - 34 gfd Membrane A - 35 gfd Figure 2. Membrane A Design Flux Evaluation Table 1. Membrane A mtcmp Calculations Parameter Flux 1 Flux 2 Design Flux (gfd) A N TMP m(psi) T ( C) mtcmp (gfd/psi)

5 20 C (gfd/psi) Membrane B Membrane B was tested for five different membrane fluxes through the course of pilot study. Table 2 summarizes the calculation of mtcmps for each of five membrane fluxes tested. As presented in Figure 2, membrane fluxes of 35 and 40 gfd appear over-designed because mtcmp is higher than half of TCMP observed in the pilot study. Oppositely, membrane flux of 26.5 gfd appears under designed. Eventually, a flux of 30 gfd was selected for the membrane system procurement. Membrane B manufacturer was the consensus on the selection of design membrane flux for their membrane. The analysis on Membrane C was not presented within this manuscript, but is similar to the analyses of Membranes A and B. All three membranes tested in the pilot study were allowed to bid the project at different membrane fluxes based upon their performance. Eventually, GE submersible ZeeWeed 1000 won the project. This quantitative MF/UF membrane flux design procedure was also successfully employed in the City of Grand Forks, ND, water treatment plant project in gfd gfd gfd gfd gfd 2 0 8/29 10/10 11/21 1/2 2/13 3/27 5/8 6/19 7/31 9/11 Date Membrane B - 25 gfd Membrane B gfd Membrane B - 30 gfd Membrane B - 35 gfd Membrane B - 40 gfd Figure 3. Membrane B Design Flux Evaluation 5

6 Table 2. Membrane B mtcmp Calculations Parameter Flux 1 Flux 2 Flux 3 Flux 4 Flux 5 Design Flux (gfd) A n TMP m(psi) T ( C) mtcmp (gfd/psi) Acknowledgement The authors would like to acknowledge the City of Fargo Water Treatment Plant, the City of Grand Forks Water Treatment Plant, and individuals who contributed to the projects and this paper. Your support is highly appreciated. References AE2S and Black & Veatch. (2011), Fargo WTP Facility Plan. USEPA. (2005), Membrane Filtration Guidance Manual. AWWA. (2005) Microfiltration and Ultrafiltration Membranes for Drinking Water. M53. USGS (2011), Simulation of the Effects of Devils Lake Outlet Alternatives on Future Lake Levels and Downstream Water Quality in the Sheyenne River and Red River of the North Scientific Investigations Report