DESALINATION OF BRACKISH AND MARGINAL WATER SOURCES IN ISRAEL: PAST PRESENT AND FUTURE

Transcription

1 DESALINATION OF BRACKISH AND MARGINAL WATER SOURCES IN ISRAEL: PAST PRESENT AND FUTURE P. Glueckstern, and M. Priel Mekorot Water Co., Desalination & Special Projects Div., POB 20128, Tel Aviv 61201, Israel Tel.: ; Fax.: Abstract In Israel, desalted brackish water sources are used for water supply for about three decades. Due to the current severe water supply problem, and following the recent decision of the Israeli Government to augment water resources, the existing brackish water desalination capacity will be doubled in the next coming years. In this paper an overview of the existing systems is presented along with a description of the integrated technological improvements implemented by Mekorot in the last two decades. Problems encountered in the ongoing projects design phases will be analyzed. Efforts and results of a multi-national research study, most important to facilitate utilization of marginal polluted surface water as a partial alternative to the much more energy intensive seawater desalination, are also presented. 1. Introduction The projected desalinated water demand of Israel and its neighbors, PNA and Jordan, in the next years, will amount to some 1,000 MCM/yr. This demand could most easily be met by water desalted through large desalination plants, located on the Mediterranean shore. However, this option imposes a heavy economic burden and furthermore, it is not the most adequate solution in respect to the environment, considering usage of energy and its negative local and global effects, and the need to remove excess of the gradually increasing salt content from the aquifers. A much more economic option, as well as environmentally right, is the option to provide a large part of the needed desalinated water by desalting marginal water sources such as available difficult, both ground and polluted surface water, including wastewater. This can be done by using new technologies such as backwashable capillary MF and membranes and by implementing other new developments. Israel, as many other countries worldwide, started with seawater desalination before desalination of brackish water. However, in the very early 1970s, Mekorot, Israel s national water company, initiated a comprehensive field testing program followed by implementation of brackish water desalination at remote locations lacking potable water [1-4]. In 1978, Mekorot started a gradual replacement of the seawater desalination capacity in Eilat, by brackish water desalination, using various RO membrane technologies [5]. In 1982, the total 11,000 m 3 /d of seawater capacity was fully replaced, thus achieving a very large saving in energy (over 20,000 tons of fuel-oil per year). Currently, Mekorot operates more than 30 BWRO units with a combined capacity of more than 40,000 m 3 /d. The ongoing brackish water desalination projects will double this capacity in the next 3-5 years.

2 In the first part of this paper an overview of the existing systems is given, emphasizing implementation of technological improvements meant to increase reliability and to reduce costs. In the second part, some of the problems encountered in the design of brackish water desalination systems, such as brine disposal at inland locations and uncertainty regarding stability of the source s salinity. In such cases, incorporating flexibility in the design in essential to reduce frequent and costly modifications to cope with increasing salinity of the raw water sources. Such a design is implemented in a BWRO plant, currently built to supply water to hotels located at the southern part of the Dead-Sea. In the last part of the paper, R&D efforts and results, aimed to cope with the problems of desalination of polluted surface water will be presented. In 1998 Mekorot initiated a field-testing program to investigate operation of integrated - RO systems to desalt polluted brackish surface water, including fish ponds effluents. The research program was later (Dec. 1998) integrated in a multi-national research project: «Improved energy efficiency in membrane processes for water purification and desalination», supported by the European Union. Other partners to this project, coordinated by Suez- Lyonnaise des Eaux (France) are the Technion - Israel Institute of Technology, CPERI (Greece) and AGBAR (Spain). Mekorot s contribution to the project is the investigation of pre-treatment methods for desalination of polluted brackish surface water, formulation of criteria for treatment selection and optimization of the integrated -RO systems. Two raw water types were investigated: polluted river water before entering the fish ponds and the fish ponds effluent. The pilot units are located in a national nature resort, adjacent to Kibutz Maagan Michael, about 30 km south of Haifa. The pilot plant s results are based on two and a half years of on-site experience. The pilot-scale tests proved that -RO coupled membranes processes are economically viable and much more cost effective for treatment of the heavily polluted surface water, in comparison with conventional coagulation and media filtration technology - currently used for pre-treatment of RO desalination. Long-term pilot studies on two difficult water types: Nahal Taninim river and fish ponds effluents, using several types of and RO membranes and various coagulants and antiscalants, provided relevant performance results needed to design and assess economically viable desalination plants, to exploit unused polluted brackish sources in Israel. 50 million c.m. per year of these water sources could be used as a partial alternative to the much more capital and energy intensive seawater desalination. Apart from this exploitable result, a basic know-how regarding criteria for selecting pre-treatment and optimization of polluted brackish sources was developed, formulated and documented. The research results bear a strong effect on energy efficiency by reducing the required energy where desalinated water is needed. In Israel alone, about 200 million kwh/yr could be saved by replacing seawater desalination by the less energy intensive alternative of desalting the not yet exploited polluted brackish sources. In terms of fossil fuel used for electricity generation, approx. 50,000 TOE/yr could be saved, exerting an important effect on local and global environment.

3 In this paper, the main results of the study, including a -RO system for 1000 m 3 /hr of polluted brackish surface water is presented. It also includes a comparative cost analysis with a BWRO system using conventional pre-treatment. 2. Overview on the existing systems 2.1. General Most of the existing desalination capacity is located in the southern part of Israel (Fig. 1), more than 90% at the Eilat-Sabha desalination center. The existing BWRO units include three different plant types: Large 5,000-10,000 m 3 /d units fed by brackish water (5,000-7,000 ppm TDS) wells, located up to 20 kms from the desalination site (Eilat-Sabha). Medium size (1,200 m 3 /d) units fed by a single brackish water (2,500 ppm TDS) well. Small size ( m 3 /d) units used for water quality improvement at remote locations, where general water supply, except for drinking and cooking, use low quality sources of up to 2,500 ppm TDS. PILOT PLANTS NAHAL TANINIM (1997) BRACKISH (SURFACE) WATER ASHDOD (1988) MEDITERRANEAN SEA EILAT (1994) RED SEA RO DESALINATION PLANTS FOR WATER QUALITY IMPROVEMENT KFAR DAROM (1989) NAHAL MORAG (1991) BEER ORA (1983) EILOT (1986) RO DESALINATION PLANTS FOR WATER SUPPLY MAAGAN MICHAEL (1994) 1,200 m 3 /day BW - SABHA "A" (1978) 25,500 m 3 /day BW - SABHA "B" (1993) 10,000 m 3 /day SW - SABHA "C" (1997) 10,000 m 3 /day Haifa Ceasarea Tel Aviv Jerusalem Ashdod BeerSheva Eilat RO DESALINATION PLANTS FOR WATER QUALITY IMPROVEMENT MIZPE SHALEM (1983) EIN BOKEK (1988) NEVE ZOHAR (1986) NEOT HAKIKAR (1982) EIDAN (1983) EIN YAHAV (1992) LOTAN (1983) YAHEL (1979) KTURA (1983) GROFIT (1974) YOTVATA (1973) MAALE SHACHARUT (1985) ELIPAZ (1983) SAMAR (1979) SDE UVDA 1 (1979) 2 (RESERV.) SDE UVDA 2 (1980) 500 m 3 /day (RESERV.) Figure 1: MEKOROT S Desalination Plants (1999)

4 2.2. Eilat-Sabha desalination center The Eilat - Sabha BWRO center is composed of two plants - Sabha A and Sabha B - with a combined capacity of about 38,000 m 3 /d. The two plants are located near the tourist town Eilat, and supply about 70% of the town water consumption. The first phase, consisting of one unit with a capacity of 700 m 3 /d, was completed in March This plant was extended and retrofitted in several phases. Following the last development phase, completed in 1999, Sabha A consists of five units with a combined capacity of about 28,000 m 3 /d. In July 1993 an additional BWRO plant (Sabha B) was put into operation. This plant started with an initial capacity of 6,300 m 3 /d and was extended in April 1996 to a capacity of 10,000 m 3 /d (Fig. 2). FROM BRACKISH WATER MULTIPLE WELLS (15-20) FEED WATER RESERVOIR RODED SAND SABHA A 28,000 m 3 /day MICRON RO UNITS 7,200 m 3 /day CHEMICALS INJECTION SAND SABHA B 10,000 m 3 /day MICRON CHEMICALS INJECTION M T H.P. PUMP/RECOVERY TURBINE 8,300 m 3 /day 8,000 m 3 /day BRINE FED UNITS 2,200 m 3 /day 2,300 m 3 /day BOOSTERS 10,000 m 3 /day TO EILAT BOOSTERS TO EILAT TO SEA TO SWRO PLANT Figure 2: Simplified flow diagram of the Eilat Sabha BWRO plant During the various development phases of the Sabha plants, efforts were directed toward decreasing the operating costs, most significantly by reducing the specific energy consumption (Fig. 3). This was achieved by the implementation of the following technological improvements: Use of more efficient membrane elements (regarding productivity and product salinity) which enable the units to perform at a lower feed pressure than originally designed [5]. Optimized membrane assembly configuration. Gradually product recovery increase (Fig. 4), up to a limit of 250% of calcium-sulfate over-saturation concentration [6]. Application of a highly efficient energy recovery turbine powered by the reject brine of all units of Sabha A and Sabha B (see Fig. 2). Installation of a computer-aided control system interconnected with the regional water supply central control system [7].

5 Improved feed pretreatment control. Improved membranes cleaning and restoration techniques [8]. By the implementation of the above improvements, substantial saving in operation and maintenance costs was achieved. ENERGY CONSUMPTION, kwh/m /79 80/81 82/83 84/85 86/ Figure 3: Evolution of Sabha BWRO plant s specific energy consumption 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% BRINE /79 80/81 82/83 84/85 86/ Figure 4: Evolution of Sabha BWRO plant s product recovery ratio 3. Typical design problems Two typical design problems encountered in the design of brackish desalination systems are brine disposal at inland locations [9] and uncertainty regarding possible increase in source salinity [10]. These problems have to be faced in some of the ongoing projects. It is well known that in locations with limited brackish water and high cost of the source s development and/or high cost of brine disposal, the most cost affecting parameter is the

6 product recovery ratio. Thus, desalination of sources with large content of sparingly soluble salts such as CaSO 4 and SiO 2 requires use of adequate and proven antiscalants. Also the systems design is somewhat more complicated, using two or more RO stages including interstage pumps. In some cases with pertaining uncertainty regarding the brackish source stability, such as in the case of an ongoing project in the Dead Sea area, incorporation of flexibility elements in the BWRO design is essential to avoid frequent and costly modifications needed to cope with the increasing salinity of the raw water source. An example of such a design is given in Fig. 5 which shows a 6,500 m 3 /d BWRO unit, designed to enhance water supply to the hotels at the southern part of the Dead Sea. A variable speed process pump and allocation of reserve area for additional membranes module, including an option to replace the membranes of the second brine stage by high-pressure seawater membranes, are the elements that give the flexibility to cope with the problem of a possible salinity increase of up to 50%. 1 from WELLS 2 SAND MICRON 3 RO STACK - I.st. BW 8-26 x 7 BW 8-32 x 7 5 FEED AND SEDIMENTATION HIGH PRESSURE PUMP (Variable Speed) 4 RO STACK - II.st. BW 8-13 x 7 SW 8-14 x7 6 BRINE OUT 7 - INITIAL SALINITY * - 50% SALINITY INCREASE Modifications for High Salinity: TO USER 8 RESERVOIR 1. Add 6x7 8 BW membranes in stage Replace Interstage Pump 3. Replace second stage BW Membranes by 14x7 8 SW Membranes. GENERAL DATA LINE NUMBER SALINITY, ppm TDS (123 Cl ) (*) (168 Cl ) FLOW, m 3 /hr PRESSURE, bar ph RECOVERY, % 83.38% TEMPERATURE, C 0 30 Figure 5: Typical BWRO designed to cope with future ground water salinity increase (water supply to hotels in the Dead Sea region) The remarkable cost effect of product recovery ratio is demonstrated in Fig. 6, where evaporation ponds have to be used for brine disposal in a project at an inland location. According to the case study of exploiting 10 M m 3 /yr of a brackish source, containing high concentration of CaSO 4, an increase of CaSO 4 concentration in the brine to over-saturation of 300%, reduces the cost of brine disposal from 1.92 M $/y to 1.28 M $/yr (including the value of brackish water), i.e. a decrease of more than 33%.

7 Currently, Mekorot is conducting an applied research to increase, as much as possible, the over-saturation of CaSO 4 and SiO 2, testing various membrane types and various advanced antiscalants. Operation with 300 ppm SiO 2 in the brine and over-saturation of 300% CaSO 4 was already demonstrated. RAW WATER SOURCE = 10 M m 3 /year NEGEV SITE Brine disposal by evaporation ponds: 1.2 $/m 3 -brine Brackish water shadow price: 0.4 $/m CaSO 4 Saturation, % SiO 2 Saturation, % 500 Brine Disposal + BW Value, M $/yr Oversaturation, % Product Recov ery, % 0 Figure 6: Effect of product recovery (limited by CaSO4 oversaturation) on brine disposal cost by evaporation ponds and loss of brackish water 4. Desalination of polluted brackish surface water In the framework of Joule Project JOE3-CT , Mekorot conducted pilot-scale tests of two polluted brackish sources: Nahal Taninim water and fishponds effluents. Performance of integrated -RO systems and RO systems using conventional pretreatment was recorded for about two and a half years ( ). Optimized performance results such as module downtime, water losses, specific energy and chemicals consumption were used to make an economic evaluation of alternative system designs. The method of approach adapted by the partners of this project is illustrated in Fig. 7. The comparative results indicate a superiority of pre-treatment over the conventional pretreatment. The energy consumption and unit water cost of the pretreated polluted surface water desalination is much lower in comparison with seawater desalination. Samples of the pilot-scale optimization results are shown in Fig. 8. General criteria for pre-treatment selection: conventional pre-treatment or backwashable membranes operating in dead end mode are shown in Fig. 9.

8 PROJECT DEFINITION / RESEARCH PROGRAM Standard Characterization Methods Raw Water Characterization Laboratory Investigation Economic Parameters: Interest Rate Project Lifetime Electric Power Cost Local Labor Cost Cost of Chemicals Projected Inflation Rates (labor, energy, materials) Pilot Scale Tests Performance Studies Definition of Alternatives Economic Evaluation of Alternatives membranes types RO membranes types Antiscalants Conventional pretreatment System operating modes e.g. dead-end Vs. cross flow () RO Product Recovery & RO Membrane Flux Water Losses Module Downtime Energy Consumption Chemicals Consumption Alternative available water sources System Capacities Basic Treatment Processes Pretreatment: CONV., MF/ Desalination: NF, RO, ED Equipment and Constr. Cost & RO Membr. Repl. Cost Environmental Aspects Externalities such as local and global cost of environmental issues (air and water contamination). Selection of Optimal Design Alternatives FORMULATION OF CRITERIA FOR SELECTION OF OPTIMAL SYSTEM Figure 7: Research program for formulation of criteria for optimal system selection Comparative design details of a -RO system and a RO system using conventional pretreatment are summarized in Table 1, and a flow diagram and energy balance of the -RO system is presented in Fig. 10. Chemical consumption and costs are summarized in Table 2. The resulting investment and unit water cost to produce desalted water from 1000 m 3 /hr Nahal Taninim water is shown in Fig. 11.

9 Performance results, especially for the fishponds effluents were presented at the recent EDS conference held in September 2000 in Paris [11] Filtrate losses,% Raw water losses,% Down time,% ( % ) Basic mode Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Step 7 Step 8 Step 9 Step 10 Figure 8: Water losses and down time (%) at the optimization steps Nahal Taninim brackish surface water (Source1) POLLUTED BRACKISH SURFACE RAW WATER < 60 NTU > 60 NTU < 45 NTU NTU < 5 NTU < 15 NTU 5-15 NTU NTU COAGULANT SAND FILTER COAGULANT CLARI- FIER 120 year 400 hr year 300 hr hr 48 hr hr 48 hr Flux, l/m 2- hr Chemical cleaning freq HCl (ph=2) addition freq Filtration time, min Disinfection freq, hour Disinf. dose - NaOCl, ppm hr 24 hr Figure 9: General criteria for pre-treatment selection

10 Table 1: Main design data of comparative RO plants using and conventional pretreatment System s Capacity 1,000 m 3 /hr raw water PRE-TREATMENT - Type 2 st. Sand Filter - Filter Area, m No. of Units & Module 11 x 26 2 x 8 RO SYSTEM - Flux, l/m 2 -hr Product Recovery, % 88% 82% - No. of Trains No. of Pressure Vessels/Membranes st. I. 54 x 7 st. II. 28 x 7 st. III. 14 x 7 st. I. 52 x 7 st. II. 26 x 7 st. III. 13 x 7 - Flow, m 3 /hr - Raw water 1,000 - Filltrate RO Product Operating hours 8,000 7,000 - Yearly Production, Mil. m 3 /year System Capacity, m 3 /day 20,592 16,767 - Specific Energy Consumption, kwh/m NAHAL TANINIM SOURCE 1: NAHAL TANINIM RIVER PRETREATMENT: 400 m 3 P(6) TO DRAIN P(1) SYSTEM 11 UNITS x 38 MODULES CARTRIDGE HP PUMPS 1,425 m 2 P(4) INTERIM 500 m 3 P(3) FILTERED WATER P(5) PUMPS TO DRAIN ENERGY BALANCE PUMPS' FLOW PRESSURE EFFICIEN. ENERGY, Kw SP. ENERGY NO m 3 /hr BA R P & M PUM P TOTA L kw h/m 3 P(1) Raw water to tank P(3) Pumps P(4) HP Main Pumps P(5) HP 3 rd stage Pumps P(6) Product Pumps P(7) Lighting & Auxil Total P(7) BACKWASH 500 m 3 P(4) RO SYSTEM 2 x 10,296 m 3 /day P(5) Figure 10: Principle flow diagram 1,000 m 3 /hr brackish surface water desalination in Nahal Taninim - pre-treatment

11 Table 2: Chemicals'consumption and cost - Nahal Taninim (source 1) CHEMICALS PRICE $/ton PERMATREAT 191 2,800 SULFURIC ACID H 2SO 4-98% 94 IRON CHLORIDE FeCl 3 1,700 SODIUM HYPOCHLORITE 100% 1,790 SODIUM BISULPHIT 100% 625 HYDROCHLORIC ACID 33% 121 HYDROCHLORIC ACID 100% 364 HYDROGEN PEROXID H 2 O Pretreatment Type RO Recovery, % 88% 2 st. Sand Filter 82% CHEMICALS INJECTION DOSING COST INJECTION DOSING COST PRETREATMENT: POINT g/m 3 -feed cent/m 3 -prod POINT g/m 3 -feed cent/m 3 -prod SODIUM HYPOCHLORITE disinfection before SF HYDROCHLORIC ACID - 33% cleaninig ph= SULFURIC ACID before SF IRON CHLORIDE before SF RO: FLOCON - for ph= before membr. before membr. PERMATREAT for ph= SODIUM BISULPHIT before membr before membr SULFURIC ACID (ph=7.2) before membr TOTAL COST Water Type: BRACKISH SURFACE WATER NAHAL TANINIM - SOURCE 1 System Capacity: 1,000 m 3 /hr raw water 7% 3% 19% 19% 3% 24% $/m 3 -year Conventional INVESTMENT 41% CENT/m % 17% 10% FIXED O&M Labour Maintanence Membr. Repl. Overhead Energy Chemicals Overhead VARIABLE O&M 32% Conventional UNIT WATER COST O&M CAPITAL 3% 9% CONVENTIONAL % Figure 11: Comparative annual cost and unit water cost of and conventional pretreatment

12 5. Summary and Conclusions Efficient utilization of brackish water to supply desalted water is demonstrated in Israel for about three decades. Ongoing projects will double the existing capacity in the next coming years. Recent R&D projects aimed to enable efficient utilization of marginal water sources yielded promising results, indicating the possibility to use them as less energy intensive and lower cost sources in comparison with seawater desalination. 6. REFERENCES [1] P. Glueckstern, M. Greenberger, Technological and economical analysis of various reverse osmosis units, Proc., Eleventh National Symposium on Desalination, Ohalo, Israel, [2] P. Glueckstern, M. Greenberger, Proc., Fifth International Symposium, on Fresh Water from the Sea, Athens, 4 (1976) 301. [3] P. Glueckstern, Y. Kantor and M. Wilf, Desalination, 24 (1978) 365. [4] P. Glueckstern, Y. Kantor and Y. Mansdorf, Proc., 6 th International Symposium on Fresh Water from the Sea, 3 (1978) 297. [5] P. Glueckstern, Y. Kantor, S Kremen, and M. Wilf, Desalination, 58 (1986) 55. [6] M. Wilf, and J. Ricklis, Desalination, 47 (1983) 209. [7] P. Glueckstern, M. Wilf, J. Etgar, and J. Ricklis, Desalination, 55 (1985) 469. [8] M. Wilf, P. Glueckstern, Desalination, 54 (1985) 343. [9] P. Glueckstern and M. Priel, Desalination, 108 (1996) 19. [10] P. Glueckstern, Desalination, 122 (1999) 123 [11] P. Glueckstern, M. Priel, A. Thoma and Y. Gelman Desalination of brackish fishponds effluents, EDS conference. Paris, France, 2000.