CERAMIC MEMBRANE PILOT TESTING ON LAKE MICHIGAN. Abstract. Introduction. Background. Objectives

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1 CERAMIC MEMBRANE PILOT TESTING ON LAKE MICHIGAN Holly Shorney-Darby, PhD, PE PWN Technologies, Dijkweg 1, 1619ZH Andijk, Netherlands, +31(0) Rob Michaelson, Manitowoc Public Utilities, Wisconsin Paul Aumann, Manitowoc Public Utilities, Wisconsin Gilbert Galjaard, PWN Technologies, Andijk, Netherlands Jumeng Zheng, PhD, PWN Technologies, Andijk, Netherlands Abstract Results from a twelve-month pilot study show that ceramic membranes can treat Lake Michigan water in a sustainable manner with a flux of 103 gfd (175 lmh), with chlorinated and acidic chemically enhanced backwashes, and with pre-treatment with polyaluminum chloride coagulation. The most recent, long-term test run data was used to estimate that the cleaning interval could be up to 10 months. Although the water quality of Lake Michigan water is characterized by low turbidity and low organic content, on average, it was necessary to apply pre-coagulation upstream of the ceramic membranes for sustainable operation. Introduction In November 2014, a pilot study of ceramic membranes began to evaluate treatment of a surface water from Lake Michigan. The pilot is located at Manitowoc Public Utilities (MPU) water treatment plant (WTP), which is located on the western shore of Lake Michigan, north of the city of Milwaukee. Lake Michigan water has low turbidity and organic content, but can have Cryptosporidium and/or Giardia, thus the robust barrier of membrane filtration has been implemented in the form of pressurized and submerged polymeric membranes in the existing WTP. MPU is, however, investigating other membrane treatment options for the potential future upgrade and/or replacement of the existing system. Background The Lake Michigan source water is characterized as a low turbidity water with low organics. Table 1 shows typical raw water quality at the MPU water treatment plant (WTP). Objectives The aim of the pilot tests was to show the performance of ceramic membranes for filtering Lake Michigan water. MPU desired a clean-in-place (CIP) interval of 30 to 45 days, but put no restrictions on daily maintenance washes (referred to as enhanced backwashes; EBW, herein). 1

2 Table 1 Typical Raw Water Quality for MPU WTP (mg/l unless noted otherwise) Parameter Value Calcium 38 Chloride 9.2 Copper, ppb 3.2 Fluoride Hardness (calculation) as CaCO3 140 Iron Lead, ppb 2 Magnesium 11 Nitrate 0.1 ph, units Silica/silicate 0.29 Sodium 6.1 Sulfate 21 Total organic carbon Turbidity (NTU) Total coliform (count per 100 ml) Cryptosporidium ND Giardia ND ND= non-detect From: Lozier et al. (2008) Description of pilot plant A skid-mounted mobile ceramic pilot plant (Figure 1) was housed in a building at the MPU facility. Raw water was strained with a 500 micron strainer, which was part of the existing MPU facility. The flow diagram is shown in Figure 2. Water was fed into a three-chamber coagulation/flocculation system (i.e., the first tank for ph adjustment, the second tank for rapid mixing of the coagulant, and the third tank for flocculation) and then pumped into the ceramic membrane vessel. The membrane was a new ceramic membrane (Metawater, Japan), with a surface area of 269 square feet (25 m 2 ). There was a dedicated backwash (BW) water (i.e., filtrate), and this tank also held the EBW solutions prior to them being used to clean the membrane. Clean-in-place (CIPs) events were initially performed with heated water (between 30 and 35 C) that was recirculated for two hours with 500 mg/l sodium hypochlorite. In later test runs with coagulant, a second cycle of cleaning was added: approximately ph 2 with sulfuric acid. In this study, test runs are defined as the operational period between CIPs. 2

3 Figure 1 Photograph of the Pilot Installation at MPU In one set of tests (test run 9), a low ph with peroxide (100 mg/l) EBW was trialed, but the performance was not deemed to be improved over a low ph EBW, so it was discontinued prior to the next test run (test run 10). The EBW regime changed after test run 6: initial runs had four chlorine EBWs then one low ph EBW, with nine BWs between each EBW; whereas runs 7 onwards had back-to-back EBWs, meaning there were no filtration cycles between them, and one EBW with chlorine was followed by one EBW at low ph. Lake Michigan Raw Water Sieve (at WTP; 500 micron) Sulfuric acid Coagulant Chemical Mixing Rapid Mixing Mixing for Flocculation Ceramic membrane Filtered Water Tank Backwashes and Enhanced Backwashes Figure 2 Process Flow Diagram of the Pilot Plant 3

4 The coagulant used for test runs 10 to 17 was Sumalchlor 50 (Summit Research Labs; Huguenot, New York, USA), which is a polyaluminum chloride (PACl) coagulant. The details of this coagulant are shown in Table 2. All dosages in this report are as product, but for reference, a 20 mg/l as product dose is about 2.5 mg Al 3+ /L. Waste flows were directed to a sewer connection on-site. Table 2 Chemical Information About Sumalchlor 50 Coagulant Characteristic Value % aluminum % Al 2 O C (g/ml) Experimental Program Pilot testing began with critical flux tests (test runs 1 to 7), which were followed by two additional test runs without coagulant. Since test run 10, coagulation pre-treatment has been used, and there have been some adjustments to the BW interval and EBW sequencing. Table 3 shows a general overview of the testing performed thus far. Table 3 Overview of Pilot Tests at MPU Test Flux Flux BW Interval No. BW until Pre-treatment Run (gfd) (lmh) (min) EBW 1 None None None None None None ** None None None mg/l mg/l then 20 mg/l mg/l mg/l mg/l mg/l * 20 then 10 mg/l * test run 17 is on-going ** the EBW regime changed at the start of run 7, with one chlorinated EBW followed by one low ph EBW, then filtration runs and 18 backwashes before repeating the EBW. 4

5 Results A summary of the critical flux testing is provided followed by data from test runs with and without coagulant. Summary of critical flux tests All critical flux testing was performed without pre-coagulation. These tests included varied filtration times (i.e., BW, intervals; ranging from 17 to 30 minutes) to allow for a comparison of test runs which had filtered the same amount of water. Critical flux testing (i.e., test runs 1 to 7) was conducted at 59 to 118 gfd (100 to 200 lmh), and it was decided that the critical flux for further evaluation would be 103 gfd (175 lmh). Performance without coagulant Figure 3 shows the performance at a flux of 59 gfd (100 lmh) during the critical flux trials. A stable operation was observed, but this flux was deemed too low for a cost-effective full-scale system. In test run 6, NaOH was tested as a replacement to the chlorine for EBWs. The NaOH EBW did not provide a performance benefit, and chlorine was used for the remainder of the study. In test run 7, the EBW regime was changed from four chlorine EBWs then one low ph EBW, with nine BWs between each EBW, to operation with backwashes, and after 18 BWs, a chlorine EBW was performed followed by an acidic EBW, then begin filtration cycles again. The TMP development of test runs 6 and 7 are shown in Figure 4. Figure 3 TMP Development and Turbidity for Test Run 1 (no coagulation pre-treatment) 5

6 Figure 4 TMP Development and Turbidity for Test Runs 6 and 7 (no coagulation pre-treatment and different EBW regimes) After completion of the critical flux tests, test runs 8 and 9 were conducted without coagulation pre-treatment (Figures 5 and 6), but at 104 gfd (175 lmh). It is clear that there was significant transmembrane pressure (TMP) development over these short test runs, and this would not lead to a long CIP interval. It was decided to begin pre-treatment with PACl for test run 10. Figure 5 TMP Development and Turbidity for Test Run 8 (no coagulation pre-treatment) 6

7 Figure 6 TMP Development for Test Run 9 (no coagulation pre-treatment) Performance with coagulation pre-treatment Coagulation was used as a pre-treatment to the ceramic membrane since test run 10. Several equipment issues initially caused short or un-finished test runs: during test run 10, it was determined that the EBW sequence was not functioning properly due to a solenoid malfunction, and the test run was terminated, and in test run 11, a level sensor was found to malfunction and was replaced. Test run 12 was operated with 10 mg/l as product PACl, and this was increased to 20 mg/l mid-way through the test run (Figure 7). With the lower coagulant of 10 mg/l, the fouling rate was 0.52 psi/d (3.6 kpa/day), and with the higher coagulant dose of 20 mg/l, the fouling rate was actually negative, meaning lower initial TMP during filtration after each EBW sequence. Stable performance with 20 mg/l PACl pre-coagulation continued in test run 13 (Figure 8), with a fouling rate of 0.05 psi/d (0.32 kpa/d). This equated to a CIP frequency of nearly 400 days. In test run 14, a slightly lower coagulant dose was tested (i.e., 15 mg/l as product). As shown in Figure 9, this coagulant dose was not sufficient, and the fouling rate was elevated. Also, it appeared that the chlorine for EBW was not being dosed properly, and this accelerated the fouling at the end of the test run. Due to the problems with the sodium hypochlorite dosing in test run 14, the test was repeated in test run 15 (Figure 10); however, there were other problems with the ph sensor and the coagulant mixing, and the TMP rose dramatically. This was in such contrast to results observed in test run 13, that test run 16 was started after repair of the equipment and testing was with 20 mg/l PACl (Figures 11 and 12). A very stable operation of the ceramic membrane was achieved again with 20 mg/l as product PACl at a flux of 103 gfd (175 lmh) during test run 16. In Figure 12, there is a larger increase in TMP over a run cycle between EBWs, and this seemed to coincide with a rise and then decrease in temperature, perhaps due to a lake turnover effect. The membrane performed well, with recovered TMP after EBWs at the 20 mg/l as product coagulant dose. 7

8 Figure 7 TMP Development for Test Run 12, Showing Improved Operation with a Higher Coagulant Dose Figure 8 TMP Development for Test Run 13, Showing Stable Performance with a Low Fouling Rate when Using 20 mg/l PACl, as Product 8

9 Figure 9 TMP Development for Test Run 14 Figure 10 TMP Development in Test Run 15 (Note: ph sensor error and coagulation tank mixing malfunction) 9

10 Figure 11 TMP Development in Test Run 16 (part a) with a Very Low Fouling Rate Figure 12 TMP Development in Test Run 16 (part b) with a Very Low Fouling Rate 10

11 Discussion The data presented herein shows the most stable performance can be achieved with up to 20 mg/l as product PACl to the ceramic membrane. This resulted in an estimated CIP interval of over 10 months. In practice, the organic content of the Lake Michigan feed water will vary, and slightly higher or lower coagulant dosages would need to be used. Also, at the start of run 17, a new mixing regime with static mixers was installed, and the coagulant dose was reduced to 10 mg/l as product. The impact of these changes will be observed to see if the mixing leads to a stable operation and TMP for test run 17. Impact of coagulation Coagulation works to capture organics and alter their charge so that they are not attracted to the membrane surface, which would cause fouling. Coagulated micro-flocs on the membrane surface act as a cake layer, which provides additional protection to the membrane from fouling organics. Although these micro-flocs cause a TMP build-up during the filtration cycle, these micro-flocs are easily backwashed away and the TMP is restored to allow for continued operation without frequent cleans. The impact of coagulant dose on the fouling rate is shown in Figure 13. It is important to recognize that the coagulation ph is important. In tests conducted in November 2015, it was found that the ph of coagulation with Sumalchlor 50 coagulant needed to be at least ph 7.0 to maintain residual aluminum levels below the secondary maximum contaminant level (SMCL) of 0.2 mg/l. Previous test runs with coagulant had been at ph 6.6 (i.e., near the point of minimum solubility for aluminum), and this led to aluminum residuals in excess of the SMCL. Filtered water quality Filtered water turbidity was proven to be low throughout the study, with daily average turbidities being less than 0.02 NTU. Figure 14 shows the ultraviolet light transmission (UVT) of manual samples taken during the pilot study. In the first test runs, it is clear that the raw and filtrate are nearly the same (e.g., the average raw UVT was 95.8 percent and the average filtrate UVT was 96.0 percent); however there are some times when the raw UVT is lower than the filtrate UVT (on average, the raw UVT was 0.4 percent lower than the filtrate in the tests without coagulation). This would indicate a retention of some organics (i.e., 0.4 percent as UVT, on average) on the membrane, and those contribute to fouling. As expected, there is difference in the filtrate UVT after coagulation began. The filtrate UVT averaged 97.9 percent, and the raw water UVT averaged There was nearly a 2 percent increase in UVT by coagulation. These data are for filtered UVT samples. Average TMP The average TMP is an important parameter, because it is used to calculate the energy use for the membrane system. The lower the TMP, the lower the energy consumption for pumping through the membrane. For test run 16, the average TMP was 6.1 psi (42 kpa), which is very low after nearly three months of operation at 103 gfd (175 lmh) without any CIPs. 11

12 Figure 13 Impact of Coagulant Dose on Fouling Rate (at 103 gfd; 175 lmh) Figure 14 UVT of Raw and Filtrate Comparison Between Test Runs with and without Coagulant 12

13 Fouling rate The fouling rate of operation influences the cleaning (i.e., CIP) interval. For this study, the aim was to have a CIP interval greater than 45 days. Figure 15 shows the calculated fouling rate for each of the first 16 tests runs. The fouling rate after coagulant was added in test run 10 is dramatically lower than without coagulant (note that test run 15 had some equipment problems that contributed to fouling). Specific flux recovery For the majority of the pilot study, only a two-hour 500 mg/l chlorine CIP was performed, and this recovered the initial specific flux well. The average specific flux after chlorine CIPs in test runs 1 to 7 was 30 gfd/psi (750 lmh/bar). After test run 15, a heated acid CIP (i.e., ph 1.5 with sulfuric acid, 19 hours) after a chlorine CIP was performed, and the specific flux rose from 19.5 gfd/psi (479 lmh/bar) before the CIP to 57.8 gfd/psi (1443 lmh/bar) after the CIP. The 57.8 gfd/psi (1443 lmh/bar) is actually better than the new membrane specific flux, so cleaning regime that included both chlorine and low ph provideds excellent specific flux recovery. The initial specific flux after each CIP for the 16 test runs is shown in Figure 16. It is clear that the final CIP with chlorine followed by acid performed best. Figure 15 The Fouling Rate for 16 Test Runs 13

14 Figure 16 Initial Specific Flux after Cleaning for Test Runs 1 to 16 System recovery The system recovery is calculated to be greater than 92 percent, with the operational settings from test run 16, and based on using the net produced (i.e., gross produced less losses for BW) compared to the total feed flow. When comparing the net produced to gross produced, that recovery is greater than 94 percent. This may be further increased with additional testing to optimize system recovery. Conclusions Pilot testing to treat Lake Michigan surface water at the MPU WTP has shown the following: 1. A sustainable flux of 103 gfd (175 lmh) can be maintained with a CIP interval of up to 10 months, and possibly longer with adequate coagulation pre-treatment (e.g., 20 mg/l as product Sumalchlor 50 coagulant was required in the summer/autumn months of testing, and other seasons may require more or less coagulant). 2. The ph for coagulation should be maintained near ph 7.0 to maintain low levels of residual aluminum in the filtrate. 3. The average TMP in the latest available test run under these stated conditions has been 5.4 psi (37 kpa). 4. Having back-to-back EBWs of chlorine and then low ph yielded better TMP recovery during operation than when operating with individual EBWs after each nine BW cycles. 5. Cleaning with a heated (i.e., 30 to 35 C) 500 mg/l chlorine solution for at least two hours has adequately restored the specific flux on many occasions, but during the 14

15 coagulation test runs, an acid CIP recirculatiohn after the chlorine recirculation was needed, and this combination restored the specific flux to like-new membrane specific flux. 6. At lower fluxes, it may be possible to operate without coagulant, but the CIP frequency might be unacceptable. References Lozier, J., Cappucci, L., Amy, G., Lee, N., Jacangelo, J., Huang, H., Young, T., Mysore, C., Emeraux, C., Clouet, J., Croué, J.P., Heijmann, B. (2008) Natural Organic Matter Fouling of Low-Pressure Membrane Systems, AWWA Research Foundation, Denver, Colorado. Acknowledgements MPU staff also assisted with monitoring and pilot operation assistance, and the authors want to thank them for their efforts. The authors would like to acknowledge the contributions of equipment, support, and assistance from METAWATER. They would also like to thank those responsible for the operation of the pilot plant, including Rob Kooijman and Harry Scheerman. 15