LOWER ENERGY SEAWATER DESAL VIA A DUAL NANOFILTRATION/REVERSE OSMOSIS SYSTEM

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1 Abstract LOWER ENERGY SEAWATER DESAL VIA A DUAL NANOFILTRATION/REVERSE OSMOSIS SYSTEM Brian Bolto, Manh Hoang and Thuy Tran EP /9/10 In seawater desalination it makes sense to have a low pressure nanofiltration (NF) stage before reverse osmosis (RO), taking out the multivalent ions, plus some sodium chloride and organics, leaving a feed for the following RO system that is of much lower ionic strength than the original raw water. The consequence is that there is a smaller osmotic pressure effect and the applied pressure requirement and hence energy need overall is much less. Also, the yield of product water is higher. Unfortunately, the total costs for an NF/RO system are usually about 10% more than for an RO only system. However, there are certain situations where the approach is justified on economic grounds, because of the organics removal by NF resulting in a marked decrease in RO membrane fouling and a significantly enhanced membrane life. A full scale plant has been operating on this basis in Saudi Arabia which lowers the pressure used in the RO plant by 17% and gives a 30% increase in water yield. Other examples show a 25-30% lower energy need and a % higher yield. Introduction Nanofiltration (NF) membranes are used for softening and the removal of organic compounds. The pore size for such membranes is nm, ranging between that of ultrafiltration (UF) and RO membranes (Schäfer et al., 2005). Their most distinctive characteristics are the lower pressure need of MPa versus 4-7 MPa for RO, and their unique rejection properties, in that they can take out virtually all multivalent anions such as sulphate and phosphate 0-70% of sodium chloride, plus uncharged and positively charged species, depending on their size and shape. NF/RO Combinations Sea water desalination Pre-treatment of seawater with NF provides an excellent feed water for RO, allowing operation at much higher fluxes and recovery rates, and also at lower pressure as the osmotic pressure to be overcome in the RO stage is lower because of the reduced salts content (Hassan et al., 2000). In a seawater desalination demonstration plant of capacity 72 kl/day it was found that NF removed material that had MW >200 Da, reduced the CaSO 4 content by about 90%, and lowered the NaCl level by about 50%. There was a doubling of the RO product water yield following this pretreatment, plus an improvement in the final water quality to <200 mg/l of total dissolved solids (TDS). Another study has shown an increase in the water recovery of 50% (Drioli et al., 2002). A full scale plant has now been operated along these lines in Umm Lujj, Saudia Arabia, following experiments to determine conditions under which economic operation was possible (Eriksson et al., 2005). The plant treats 8.6 ML/day, with the NF component reducing hardness from 7,500 to 220 mg/l lowering TDS from 45,460 to 28,260 mg/l rejecting sulphate to >99%

2 reducing divalent cations by 80-95% lowering the pressure used in the RO plant by 17%. The NF stage operated at a 65% conversion rate and the RO stage at a 56% conversion rate, giving an overall conversion of 36%. This compared favourably with a parallel RO-only plant which had a conversion rate of 28%, thus showing a 30% increase in overall recovery for the NF/RO system. Other estimates of the energy saving are 25-30% (Tanninen et al., 2005). A system designated as a two-pass NF process, known as NF 2, has been tested by the Long Beach Water Department (Leung and Rohe, 2006; Le Gouellec et al., 2006). It makes use of an initial loose membrane, followed by a tighter second membrane. Pilot testing has shown that the pretreated seawater of TDS 37,500 mg/l can be lowered to 3,250 mg/l after the first pass, the residue being mainly NaCl, and then to 218 mg/l in the final product water. It has been demonstrated on the pilot scale that a water quality equivalent to that from RO treatment alone can be achieved at a ~20% lower operating pressure ( MPa versus MPa). A 568 kl/day prototype plant is planned, together with a parallel RO plant of similar size. However, the recovery was only 40%, versus 50% for a typical seawater RO system, which needs further explanation (Gouellec et al., 2006). A summary of the advantages of the dual systems is given in Table 1. Table 1. Dual NF/RO systems with seawater feed Energy Saving, % Water Recovery Reference Increase, % Hassan et al., Drioli et al., Eriksson et al., Tanninen et al., Le Gouellec et al., 2006 The cost advantage of pre-treatment with NF is not widely achieved (Fritzmann et al., 2007). Higher recoveries can in special circumstances make up for the additional investment cost, but these are stated to be unlikely to occur in the practical operation of a desalting plant. The operating costs for an NF/RO system are usually about 10% more than for a two-pass RO system. An important finding at the Umm Lujj plant was that the NF membranes were less prone to fouling than polyamide RO membranes (Eriksson et al., 2005). This was partly ascribed to their lower salt rejection, as with the better salt rejection of the RO membranes there is a higher ionic strength at the membrane/feed interface, which reduces the repulsion force between the charged colloidal particles so that it is easier for them to aggregate and form larger agglomerates. These are more prone to deposit on the membrane surface. The higher costs for dual systems are based on the assumption that there is minimal membrane fouling, which is not the case at Umm Lujj. Here the NF pretreatment for RO makes good economic sense because of the marked reduction in the RO fouling rate. It has been noted that biofouling is a serious operational problem in NF and RO installations, with membrane autopsies revealing biofouling in 12 of 13 pilot plants investigated (Vrouwenvelder and van der Kooij, 2001). Autopsy of the Umm Lujj NF membranes found that there was no biofouling after 9000 h of operation, with the membrane deposits found to be mainly organic matter, iron, chromium and fungus (Al-Amoudi and Farooque, 2005). Fouling of NF membranes has been reviewed recently (Al-Amoudi, 2010). The application of NF before RO has been investigated also as a means of reducing the concentration of divalent ions that are responsible for membrane scaling. Another viewpoint 2

3 however considers that the approach merely transfers the scaling problem further upstream (El- Maharawy and Hafez, 2000). Brackish water desalination Similar improved water recoveries have been obtained in brackish water applications when NF precedes RO (El-Zanati and El-Khatib, 2007). The overall cost with the more dilute feed waters can be less than half that for seawater desalination. Scaling by silica can be prevented if treatment is carried out under alkaline conditions, when the silica is present mainly in the anionic form, with water yields of 98% being claimed from river water of TDS mg/l when the operation is at ph 10.8 on raw water containing mg/l of silica (Mavrov et al., 1999). At that ph level the silica is 94% anionic (Albert and Serjeant, 1962). However, precipitation of carbonates may become an issue (Her et al., 2000). The usual practice is to operate at ph 8.5 or higher (AWWA, 2007). 14% of the silica is then anionic. Two distinct mechanisms of scale formation, surface and bulk crystallisation, have been identified and explored (Lee and Lee, 2005). Attempts to decrease scale formation include increasing the fluid velocity adding antiscalants chemical pre-treatment ion exchange as a pre-treatment combining a crystalliser with NF/RO. Silica can form an amorphous inorganic deposit on membranes (AWWA, 2007). Because of its often polymeric form, scale inhibitors have not been very effective in stabilising supersaturated silica solutions. There are, however, dispersants which are successful in controlling silica deposition. Silica has a changeable nature, being generally considered to exist as undissociated silicic acid in most naturally occurring waters having a ph level of up to 8. The structure of precipitated particles is dependent on the silica concentration, the solution ph, the presence of ions such as calcium and especially magnesium, and the temperature (Koo et al., 2001). The reverse coupling of RO and NF for the desalination of brackish water is reported, with the emphasis on the effects of di- and monovalent cations on operation (M nif et al., 2007). At a pressure of only 0.6 MPa when run in sequence with RO before NF, the brine reject from the RO unit can be desalted with respect to divalent ions, so that the recovery of 40% for RO is increased to 80% with the coupled system. Industrial water desalination Wastewaters from a wide range of industries have been subjected to NF treatment. The industries include (Schäfer et al., 2005) Food production Chemical processing Pulp and paper Textiles Metal separation Petroleum (Ashaghi et al., 2007) Electroplating (Mohammad et al., 2004). Of the various industries, that of most interest is NF/RO seawater desalination where brine is isolated as a raw material for the inorganic chemical industry (Kyburz and Meindersma, 2005). NF 3

4 prior to RO or multi-stage distillation is used to improve the recovery from a 45,000 mg/l feed by decreasing its scaling intensity. Also, treatment of the RO concentrate containing mainly NaCl produces a purer form for chlorine production in the chlor-alkali industry. Another option is for the NF concentrate, mainly salts like MgSO 4, to be used as a source material for the magnesium metal industry. Typical result for various ion rejections by spiral wound units of an Osmonics DS5 DK membrane at a large NF plant in Bahrain are given in Table 2. Table 2. Rejection of ions in seawater treatment by NF (Kyburz and Meindersma, 2005) Ion Rejection, % 99.9 Mg Ca HCO 3 56 TDS 37.7 SO 4 2- Conclusions In seawater desalination the use of NF to remove multivalent ions, plus organics and some sodium chloride before RO will result in a lower pressure requirement for the RO stage and a 17-30% lower energy need overall. Also, the yield of product water is % higher. However, this is only economic for seawater desalination where serious fouling of the membranes occurs in an RO-only system. Then the organics removal by NF results in a marked decrease in RO membrane fouling and a significantly enhanced membrane life. Similar improvements are possible in brackish water applications where the overall cost of desalting the more dilute feed waters can be less than half that for seawater desalination, and water yields can be doubled. Fouling and scaling are particular issues. Silica can be tolerated by operating at high ph levels of <8.5. References Albert, A. and Serjeant, E. P. (1962). Ionization Constants of Acids and Bases. Methuen, New York, pp Al-Amoudi, A. S. (2010). Factors affecting natural organic matter (NOM) and scaling fouling in NF membranes: A review. Desalination 259, Al-Amoudi, A. S. and Farooque, A. M. (2005). Performance restoration and autopsy of NF membranes used in seawater pretreatment. Desalination 178, Ashaghi, K. S., Ebrahimi, M. and Czermak, P. (2007). Ceramic ultra- and nanofiltration membranes for oilfield produced water treatment: a mini review. The Open Environmental Journal 1, 1-8. AWWA (2007). Reverse osmosis and nanofiltration. Manual of Water Supply Practices M46, 2nd Edition, American Water Works Association, Denver. Drioli, E., Criscuoli, A. and Curcioa, E. (2002). Integrated membrane operations for seawater desalination. Desalination 147, El-Maharawy, S. and Hafez, A. (2000). Technical management of RO system. Desalination 131, El-Zanati, E. and El-Khatib, K. M. (2007). Integrated membrane-based desalination system. Desalination 205, Eriksson, P., Kyburz, M. and Pergande, W. (2005). NF membrane characteristics and evaluation for sea water processing applications. Desalination 184,

5 Fritzmann, C., Löwenberg, J., Wintgens, T. and Melin, T. (2007). State-of-the-art of reverse osmosis desalination. Desalination 216, Hassan, A. M., Farooque, A. M., Jamaluddin, A. T. M., Al-Amoudi, A. S., Al-Sofi, M. A. K., Al- Rubaian, A. F., Kither, N.M., Al-Tisan, I. A. R. and Rowaili, A. (2000). A demonstration plant based on the new NF-SWRO process. Desalination 131, Her, N., Amy, G., Jarusutthirak, C. (2000). Seasonal variations of nanofiltration foulants: identification and control. Desalination 132, Koo, T., Lee, Y. J. and Sheikholeslami, R. (2001). Silica fouling and cleaning of reverse osmosis membranes. Desalination 139, Kyburz, M. and Meindersma, G. W. (2005). Nanofiltration in the chemical processing industry. In: Nanofiltration Principles and Applications, A. I. Schäfer, A. G. Fane and T. D. Waite (Eds.), Elsevier, Oxford, pp Le Gouellec, Y. A., Cornwell, D. A., Cheng, R. C., Tseng, T. J., Vuong, D. X., Wattier, K. L., Harrison, C. J. and Childress, A. E. (2006). A novel approach to seawater desalination using dual-staged nanofiltration. Awwa Research Foundation, American Water Works Association, Denver. Lee, S. and Lee, S. H. (2005). Scale formation in NF/RO: mechanism and control. Water Sci. Technol. 51(6-7), Leung, E. and Rohe, D. L. (2006). Prototype testing facility for two-pass nanofiltration membrane seawater desalination process. In: Membrane treatment for drinking water and reuse applications: A compendium of peer-reviewed papers, K. J. Howe (Ed.), American Water Works Association, Denver, pp Mohammad, A. W., Othaman, and Hilal, N. (2004). Potential use of nanofiltration membranes in treatment of industrial wastewater from Ni-P electroless plating. Desalination 168, Mavrov, V., Chmiel, H., Heitele, B. and Rögener, F. (1999). Desalination of surface water to industrial water with lower impact on the environment. Part 3. Desalination under alkaline conditions. Desalination 123, M nif, A., Bouguecha, S., Hamrouni, B. and Dhahbi, M. (2007). Coupling of membrane processes for brackish water desalination. Desalination 203, Schäfer, A. I., Fane, A. G. and Waite, T. D. (2005). Nanofiltration Principles and Applications, A. I. Schäfer, A. G. Fane and T. D. Waite (Eds.), Elsevier, Oxford, Chapter 1. Tanninen, J., Kamppinen, L. and Nyström, M. (2005). Pretreatment and hybrid processes. Nanofiltration Principles and Applications, A. I. Schäfer, A. G. Fane and T. D. Waite (Eds.), Elsevier, Oxford, pp Vrouwenvelder, J. S. and van der Kooij, D. (2001). Diagnosis, prediction and prevention of biofouling in NF and RO membranes. Desalination 139, The authors: Dr Brian Bolto, Dr Manh Hoang and Dr Thuy Tran and ( brian.bolto@csiro.au; manh.hoang@csiro.au; thuy.tran@csiro.au) work for CSIRO Materials Science and Engineering, Clayton, Victoria. 5