First hydraulically optimized nanofiltration plant for Water Supply Company Overijssel, The Netherlands

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1 First hydraulically optimized nanofiltration plant for Water Supply Company Overijssel, The Netherlands Jacques van Paassen, Water Supply Company Overijssel Walter van der Meer, Water Supply Company Friesland Peter Wessels, DHV Consulting Engineers Ger Vos, Water Supply Company Oost-Brabant Summary Water Supply Company Overijssel has tested a new hydraulic design for spiral wound nanofiltration elements. The new design minimizes the hydraulic losses over the feed spacers by placing less membrane elements in series in a pressure vessel. Savings upto 20% of the membrane surface is possible with the new design [references 1,2 and 3]. Water Supply Company Overijssel tested a 2 stage nanofiltration with 3 (4") membrane elements in series in both stages. The recovery was held constant at 81%. Although the cross flow velocities were lower than in conventional installations with 6-7 elements in series per stage, no severe scaling or fouling of the membrane surface occurred. Based on the test results, Water supply Company Overijssel is planning to design and build the first full scale installation with a new type of pressure vessels designed by Water Supply Company Friesland, Water Supply Company Oost-Brabant and DHV. Introduction Water Supply Company Overijssel is planning to build several nanofiltration installations for hardness and colour reduction. Because of the cost saving potential, Water Supply Company Overijssel wanted to test a new hydraulically optimized design. A pilot program was started to investigate this new design. The pilot study is carried out in cooperation with Water Supply Company Overijssel, Water Supply Company Friesland, DHV Consulting Engineers, Kiwa Consulting and Demitec Membrane Filtration. Figure 1: The most important question to be answered was: is the new hydraulic concept working and does it not lead to severe scaling and/or fouling?

2 Background of this question is to be found in the cross flow velocity. A certain cross flow velocity is needed to keep the concentration polarisation at the membrane surface low enough. When the concentration polarisation is to high, scaling can occur. In current projection programs, a minimum cross flow velocity is recommended. When a projection calculation is made with the input of table 1, the current projection programs will judge the design as inadequate. This is because of the fact that the ß-factor is calculated as being to high. The minimum cross flow velocity however is asumed to be the same for seawater desalting as well as for nanofiltration of fresh groundwater. Traditional configurations with 6 to 7 membrane elements in series have high pressure losses (upto 25% of the feed pressure when nanofiltration membranes are used). By placing 3 membrane elements in series in every stage, the membrane surface can be decreased with 15-20% at the same feed pressure. However this design has never been judged as accurate by the current projection programs. Water Supply Company Overijssel however was willing test a new design, developed by Water Supply Company Friesland, Water Supply Company Oost-Brabant and DHV, because of the potential savings involved. The new design however should not give rise to severe scaling or fouling of the membrane surface. Pilot study In the pilot study a configuration was tested with 3 elements in series in a staging 2:1. The specifications of the installation used and the most important operational parameters are listed in table 1. Feedwater for the pilot installation was anaerobic groundwater. As pretreatment a cartridge filter and anti-scalant dosing was used. No acid dosing has been applied. Table 1: Specifications pilot installation and operational parameters NF WTP Boerhaar Pilot study Remarks Installation: - membrane elements in series - staging - total number of membranes - membranes used - measurements Operational parameters: - dosing - feed pressure - feed flow - recovery - average flux - pressure drop stage 1 - pressure drop stage 2 3 2:1 9 TS-80 (4 inch) manual instruments permatreat 191 8,4 bar / 125 psi 2,1 m 3 /h 81% ca. 23,8 l/(m 2.h) 0,33 bar / 5 psi 0,3 bar / 5 psi current practice: 6-7 elements in series no acid dosed common practice: approx. 20 l/(m 2.h) at 8,5 bar common practice: approx. 1 bar / 15 psi common practice: approx. 1 bar / 15 psi The pilot installation was equipped with 3 PVC pressure vessels (diameter 160 mm or 6,35 inch, suitable for housing 3 elements) in which the 4" membrane elements were placed according to a new developed concept. Feed, concentrate and permeate connections in this new concept are situated at one side of the pressure vessels (figure 2, 3 and 4). The concentrate flows between pressure vessel and membrane elements and is discharged at its connection at one side of the pressure vessel. In full scale plants pressure vessels for 6 elements can be used with a feed on both ends [references 1 and 3].

3 feed product concentrate Figure 2: Schematic view of the pressure vessels used in the pilot study Figure 3: Figure 4: PVC pressure vessels with concentrate discharge Feed, product and concentrate connections at between pressure vessel and membrane elements one side of the pressure vessels Results of the pilot study The pilot installation was in operation continuously for 8 weeks. One emergency shut down occurred on day 20 and over the period of day 30 through day 35, there was a lower dosing of anti-scalant than intended. Instead of the 3 mg/l anti-scalant, only approximately 1,5 mg/l was dosed in that period. All measurements (pressure, flows, etc.) were carried out manually. Results operational parameters Flux and recovery Average flux (figure 5) was 23,8 l/(m 2.h) during the 8 weeks investigation. Recovery was held constant at 81%. Flux stage 1 and Flux (l/(m 2.h) Time (days) Flux stage 1 Flux stage 2 Average flux stage 1 and 2 Figure 5: Flux

4 Feed pressure and pressure drop feedspacers In figure 6, the feed pressures of the first and second stage and the concentrate pressure (before concentrate valve) are given. From figure 6 it can be seen that no significant increase in feed pressure or pressure drop over the feedspacers was found during testing. This means that severe fouling of the feedspacers did not occur. Feed pressures stage 1 and 2 and concentrate pressure Pressure (kpa) Time (days) Feed pressure stage 2 Concentrate pressure stage 2 Feed pressure stage 1 Figure 6: Feedpressures stage 1 and 2, concentrate pressure Mass Transfer Coefficiënt (MTC) The Mass Transfer Coëfficient (MTC) gives an indication of the mass transport through the membranes. When the membrane surface is scaled or fouled, MTC will decrease. Figure 7 shows the MTC of both stages during the investigation. From figure 7 it can be concluded that MTC was fairly constant during the 8 weeks testing. Only around day 20 a sudden decrease in MTC was found, but this was an incidental decrease and after day 20 MTC remained constant again. The sudden MTC decrease is most likely caused by the emergency shut down on day 20. During this shut down, the concentrate was not flushed out of the membranes well enough. Because of the fact that the antiscalant is only effective for a certain period (approximately half an hour), scaling could be expected. Mass Transfer Coefficient stage 1 and 2 1 MTC m/s.kpa at 10 o C 0,95 0,9 0,85 0,8 0,75 0, Time (days) MTC stage 1 MTC stage 2 Figure 7: MTC stages 1 and 2

5 Water quality parameters Table 2 lists the average values of the most important water quality parameters. Water Supply Company Overijssel wants to apply nanofiltration to bring down both the calcium (hardness) and the high color. The groundwater has also fairly high concentrations of barium and sulfate. Bariumsulfate scaling was considered the most important problem that might occur during the pilot study. Table 2: Water quality parameters pilot study nanofiltration WTP Boerhaar ph HCO 3 calcium sulfate barium iron manganese color - mg/l mg/l mg/l µg/l mg/l mg/l mg Pt/l feed 7, ,3 0,28 19 product 6,1 25 <2,5 <2,5 4 0,08 <0,005 2 concentrate 7, ,2 56 retention - 92% 98% 96% 98% 98% 98% 89% When using TS-80 membranes (Trisep), about 98% of all bivalent ions could be retained. Membrane autopsy After the pilot testing the last membrane of stage 2 was investigated with a scanning electron microscope (SEM). The membrane surface was visually colored. The slightly brown-grey scaling on the membrane surface was analysed as a scaling of calcium, iron and phosphorus. Barium was not found. Scaling of calcium and iron can be caused by: 1. normal operation 2. the emergency shut down (day 20) 3. start up During the emergency shut down on day 20, the installation was not flushed properly. This can be the explanation for the scaling with calcium. Start up of the installation failed the first time because of technical problems with the unit. The membranes however had at that time already been in contact with the feedwater (which contained disolved iron). It then took a few days to final start up and during these days, the membranes might be scaled with iron. Conclusion The pilot study proved that the new developed hydraulic design works: = during pilot testing no significant increase in feed pressure and pressure drop over the feed spacers was found; = Mass Transfer Coefficient (MTC) was constant during the 8 weeks testing; = Scaling found in the autopsy is most likely explaned by the emergency shut down (day 20) and the start up = (limited) scaling of calcium, iron and phosphorus is acceptable and removable (chemical cleaning) Based on the good results during pilot testing, Water Supply Company Overijssel has decided to implement the new design in their nanofiltration installations to be built. A new pressure vessel design is worked out in detail for large scale production. Literature [1] L.P. Wessels, W.G.J. van der Meer, W.C. van Paassen, G. Vos Innovative design of nano- and ultrafiltration plants Desalination 118 (1998), pages

6 [2] W.G.J. van der Meer, M. Riemersma, J.C. van Dijk Only two membrane modules per pressure vessel? Hydraulic optimization of spiral-wound membrane filtration plants Desalination 119 (1998), pages Paper presented on the IWSA Conference "Membranes in drinking and industrial water production", September 1998 Amsterdam [3] B. van Efferen, L.P. Wessels, W.G.J. van der Meer, W.C. van Paassen, G. Vos Innovative design of nano- and ultrafiltration plants, a new approach in membrane system design Paper presented on the AWWA Membrane Technology Conference, March 1999 Long Beach, California