Desalination – water for the next generation

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Desalination – water for the next generation ven though the volume of the earth’s water is vast, less than 10 million of the 1,400 million cubic metres of water on the planet is of low salinity and suitable for use after applying conventional water treatment alone.The other 97.5% of the water on our planet is to be found in the oceans, where it is officially classified as seawater. Desalination provides a means of tapping this resource, and over the last 30 years, seawater desalination technology has made great strides in many arid regions of the world, such as the Middle East and the Mediterranean, and is starting to be considered viable even in cooler regions of the world not traditionally associated with water shortage (the UK for example.)

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· Filtration + Separation looks at the subject of desalination, starting with an overview of the current situation; we ask some experts in the field to point out the reasons why desalination has now become such an acceptable worldwide water treatment phenomenon (Factors behind the growth in desalination - below, and pages 15 – 18.) · On page 19, we briefly examine the different technical aspects of desalination available today (Desalination – the basics.) · We then publish two case studies; the first looks at Israel, which has embarked upon a large-scale seawater desalination program, most notably the Ashkelon project – a 100 million m3/year SWRO desalination plant (Case study: assessing the need for, and costs of, seawater desalination in Israel - pages 20 and 21.) · The second looks at Pretreatment - a vital part of any desalination process – using the US, California, Carlsbad seawater desalination pilot plant as an example (Case study: Pretreatment at Carlsbad seawater desalination pilot plant – pages 22 – 25.)

I. Factors behind the growth in desalination Today, over 15,000 desalination facilities operate in more than 120 countries worldwide producing in excess of 3,500 million gallons per day of potable water, a dramatic rise compared with 30 years ago, when virtually no desalination took place at all. Indeed, some countries, such as Spain, Saudi Arabia and the United Arab Emirates, now rely on desalinated water for more than 70% of their water supply. This increased acceptance of desalination for municipal, industrial and commercial applications has been driven by a reduction in cost brought about by newer and more efficient technologies, especially in the Reverse Osmosis (RO) field. And this has also brought about a modernisation in the way plants are delivered. Most of the large seawater desalination facilities built in the past 10 years (or currently undergoing construction) are delivered under public-private partnership arrangement, using the build-own-operate-transfer (BOOT) method of project implementation. The BOOT project delivery method is preferred by municipalities and public utilities worldwide because it allows

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cost-effective transfer to the private sector of the risks associated with the variables affecting the cost of desalinated water, namely: intake water quality and its effects on plant performance, which are difficult to predict; permitting challenges; start-up and commissioning difficulties; dealing with the fast-changing membrane technology and equipment market; and limited public sector experience with the operation of large seawater desalination facilities.

The difference between desalination and conventional water treatment processes People define desalination in different ways, says Antonia Von Gottberg, director of municipal technology at Koch Membrane Systems, a supplier of spiral wound RO membranes used to desalinate brackish water and seawater. ISSN 0015-1882/05 © 2005 Elsevier Ltd. All rights reserved

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Figure 1: typical Reverse Osmosis membrane system (courtesy of Poseidon Resources Corp)

“Most conventional water treatment plants use a combination of settling, filtration and disinfection to treat water sources,” she says. “In many cases these water sources are ‘fresh water’, and the main requirements are simply the removal of suspended solids, bacteria and viruses. In some cases, this might require precipitation chemistry to precipitate and dissolve species such as hardness and arsenic, followed by the clarifying or filtering of the water to remove them,” says Von Gottberg But these conventional technologies do not remove salts or most soluble non-organic and organic substances, and cannot be used to produce fresh water out of seawater or brackish water, adds Nikolay Voutchkov, senior vice president and corporate technical director for Poseidon Resources Corp. Desalination sees these dissolved salts removed through either thermal processes (where water is evaporated then freshwater is condensed) or membranes (where salts are retained by the membrane barrier). Nowadays, RO membranes are capable of making seawater fresh, and such RO processes are widely acknowledged to have overtaken thermal techniques in terms of cost effectiveness, and the trend is likely to continue (see figure 1 above). “RO plants can remove practically all soluble salts and fine soluble or insoluble inorganic and organic materials in the source water,” says Voutchkov. But it is important to recognise that depending on the specifics of a project, the desalination aspect (thermal or membrane) may only be one part of a plant’s process. “If the seawater desalination processes (both thermal and membrane based) are relatively standard, regardless of location/seawater type, it is the pre-treatment that tends to vary,” says Chris Howorth, market development manager for Aquious PCI Membranes (a division of ITT Advanced Water Treatment). “Pre-treatment can be performed by a vast array of technologies, including those that would be considered

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‘conventional’ for treating freshwater, together with others, such as pre-coat filtration, that are rarely used for drinking water applications. It is all about feed water quality and variability, particularly in relation to organics when RO is employed.” Voutchkov adds that “RO plants typically consist of two types of treatment facilities in series – a pretreatment system and RO membrane system. The purpose of the pretreatment system is to remove particulate matter (mainly suspended solids) from the source water. For that reason, a conventional water treatment plant and the pretreatment system of a typical desalination plant are very similar – i.e. they use the same treatment technologies such as sedimentation and granular media (sand/garnet) filtration. “The purpose of the RO membrane system, which is located downstream of the pretreatment system, is to remove soluble materials in the source water (salts, natural or manmade organic materials and soluble metals) and all very fine particulate materials (fine solids, silt, bacteria, viruses and other pathogens), which the pretreatment system was incapable of removing,” Voutchkov continues. “Brackish water RO membranes are used for low salinity (below 15,000 mg/L TDS) source water. Seawater RO membranes are used for desalination of ocean and seawater.”

What has driven the emergence of desalination? Apart from a general increased acceptance of desalination as an alternative means of generating useable water sources, what other drivers do those operating within the desalination field pinpoint? “Although every country, region and even specific water supply area will have its own unique set of circumstances,” explains Howorth, “the drivers of desalination often tend to be similar across the globe. The principal driver is limited freshwater – this is

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Figure 2: spiral-wound Reverse Osmosis membrane element and vessel (courtesy of Poseidon Resources Corp)

common to all projects, including the one in London, UK, though this was also driven by the fact that there was nowhere to put a large reservoir.” Accessible fresh water reserves are becoming harder to find and increasingly costly to exploit, Howorth says, partly because the environmental cost of their exploitation is being increasingly considered. Over-exploited groundwater reserves are also becoming progressively more saline due to seawater intrusion. “Where sufficient freshwater resources are not available and demand cannot be reduced to meet capacity, the only alternative to desalination is generally water transfer (using tankers at small scale or conveyance infrastructure at larger scales),” he explains. “Seawater desalination offers an inexhaustible water resource.” Along with limited freshwater, comes increasing demand. With populations growing and tourism to foreign climates becoming ever more affordable, demographic patterns are showing that dry coastal areas are growing particularly fast in many regions, and living standards improving around the world. This has made the demand for drinking water greater than ever. “Agricultural demand is also rising due to a trend of more intensive agriculture and the increasing use of irrigation,” Howorth adds. Then there is global warming: “The changing climate appears to be making droughts more severe and also raising average temperatures, increasing areas in which water is considered to be scarce.” Another important aspect driving desalination is regulation. “In the more developed regions the very high level of treatment provides the advantage of compliance with increasingly strict drinking water quality standards,” says Howorth. “The European Union’s Water Framework Directive, for example, is demanding a more holistic consideration of water supply, which will elevate the importance placed on protecting vulnerable and over exploited freshwater resources.” In other places such as China, adds Koch’s Von Gottberg,

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“regulations are preventing industries from using fresh water sources to meet their water demands, so industries need to find other water sources, which entails the recycling of industrial and municipal waste or seawater desalination”. Other factors include cost effectiveness – recent developments in desalination technology have made desalination more cost effective, and hence more economically viable, than ever before. And then there is politics. “Desalination at a significant scale is always intimately connected with politics, although this is a very subjective driver,” says Howorth. “The short-term mentality of some politicians (and indeed consumers) makes drought an extremely effective incentive for desalination installation – when consumers are facing usage restrictions, methods of alleviating the situation are likely to be attractive. Consumers’ perceptions of the value of water are also (slowly) changing, from a free resource that falls from the sky, to a precious resource that fundamentally underpins economic development. The United Nations (UN) suggests that 1500m3/year/capita of naturally renewable freshwater is required to support unhindered economic development. In Europe, both Malta and Cyprus are below this limit (74 m3 and 979 m3 respectively), as are specific regions of other countries, e.g. Spain’s Canary Islands.”

Cost reduction in desalination Historically, one of the key obstacles limiting the use of seawater desalination on a large scale has been the high cost of water production, explains Nikolay Voutchkov. “However, a number of cost saving innovations in seawater desalination technology over the last ten years are transforming this once-costly option of last resort into a viable water supply alternative.” A typical RO membrane desalination plant includes the following key components: source water intake system; pretreatment facilities; high-pressure feed pumps; RO membrane

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Trisep’s Spirasep membrane has received plaudits for its specific membrane properties. It is a spiral wound ultrafiltration membrane module, which uses backflushing and aeration scrubbing of the membrane to prevent fouling

trains; and a desalinated water conditioning system (see Figure 1). The ‘engine’ of every desalination plant that turns seawater into fresh potable water is the RO membrane element (see Figure 2). The most widely used type of RO membrane elements consist of two membrane sheets glued together and spirally wound around a perforated central tube through which the desalinated water exits the membrane element. A large seawater desalination facility usually has thousands of membrane elements connected into a highly automated and efficient water treatment system, which typically produces one gallon of fresh water from approximately two gallons of seawater. By and large, the membrane productivity, energy use, salt separation efficiency, cost of production and durability of the membrane elements all determine the cost of the desalinated water. Technological and production improvements in all of these areas in the last two decades are now rendering water supply from the ocean affordable. Membrane productivity, i.e. the amount of water that can be produced by one membrane element, has more than doubled in the last 20 years. Innovations such as the recent introduction of spiral wound membrane elements with a larger number of membrane ‘leaves’and denser packing also offer increased efficiency when compared to older designs (see image above). Today’s most efficient elements have more than twice as many membrane leaves compared to older designs. Higher productivity means that the same amount of water can be produced with significantly less membrane elements, which has a profound effect on the size of the membrane equipment, treatment plant buildings, and the footprint of the desalination facility – all of which ultimately reduce the cost of water production. Koch Membranes Systems, for example, has recently introduced the MegaMagnum RO element (pictured on page 18), which the company describes as the world’s largest spiral wound element. “This has seven times the membrane area of typical 8 inch x 40 inch elements, and so fewer elements and housings are needed for an application. This offers the customer potential savings due to fewer connections and smaller footprint,” explains Koch’s Antonia Von Gottberg. In seawater desalination facilities, salts are separated from the

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fresh water applying pressure to the seawater, which is 60 to 70 times higher than the atmospheric pressure. After the salt/water separation is complete, a great portion of this energy stays with the more concentrated seawater and can be recovered and reused to minimise the overall energy cost for seawater desalination. Dramatic improvements in the membrane element materials and energy recovery equipment over the last 20 years coupled with enhancements in the efficiency of RO feed pumps and reduction of the pressure losses through the membrane elements have allowed a reduction in the use of power needed to desalinate seawater to less than 14 kWh/1,000 gallons of produced fresh water. Taking into consideration that the cost of power is typically 20 to 30% of the total cost of desalinated water, these technological innovations have contributed greatly to the reduction of the overall cost of seawater desalination. Novel energy recovery systems working on the pressure exchange principle (pressure exchangers) are currently available in the market and use of these systems is expected to further reduce the desalination power costs by approximately 10 to 15%. The pressure exchangers transfer the high pressure of the concentrated seawater directly into the RO feed water with an efficiency exceeding 95%. Future lower-energy RO membrane elements are expected to operate at even lower pressures and to continue to yield further reduction in cost of desalinated water. Membrane performance tends to naturally deteriorate over time due to a combination of material wear-and-tear, and irreversible fouling of the membrane elements. Typically, membrane elements have to be replaced every five years to maintain their performance in terms of water quality and power demand for salt separation. Improvements in membrane element polymer chemistry and production processes over the last 10 years have made the membranes more durable and have extended their useful life. The use of elaborate conventional media pre-treatment technologies and ultra and micro-filtration membrane pre-treatment systems prior to RO desalination is expected to prolong the membrane’s useful life to seven years and beyond, thereby reducing costs for its replacement and therefore the overall cost of water. Today, RO membrane technology and elements are highly standardised in terms of size, productivity, durability and useful life. There are a number of manufacturers of high-quality seawater RO membrane elements that provide interchangeable products of excellent quality and with a proven track record and performance. Many of the leading membrane manufacturers are investing in R&D to support the water desalination market and its advancing membrane technology and science at a pace no other water technology can compare with. “It is expected that an oxidant resistant membrane will be developed in the future,” explains Aquious’ Chris Howorth, “overcoming the significant limitation of current polyamide films in terms of their inability to use

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industryfocus common biocides to control biofouling and their susceptibility to damage by very small concentrations of oxidants.” Another issue Howorth highlights is the growing importance of boron passage through membranes: “Acceptable product water concentrations are being progressively reduced. Product cost reduction is a major objective. Advances in membrane performance are also ongoing, in terms of flux rate, fouling resistance, longevity, and salt rejection.”

RO technology advancements compared to computer revolution Nikolay Voutchkov claims that the advances in RO desalination technology are closest in dynamics to those of computer technology. “While conventional technologies, such as sedimentation and filtration have seen modest advancement since their initial use for potable water treatment several centuries ago, new and more efficient seawater desalination membranes and membrane technologies – as well as equipment improvements – are released every few years. And, as with computers, the RO membranes of today are many times smaller, more productive and cheaper than the first working prototypes,” he says. “Although no major technology breakthroughs are expected to bring the cost of seawater desalination down further dramatically in the next few years, the steady reduction of desalinated water

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The Koch Membrane Systems Megamagnum is helping to reduce desalination costs

production costs coupled with increasing costs of water treatment – and driven by more stringent regulatory requirements – are expected to accelerate the current trend of increased reliance on the ocean as an environmentally friendly and competitive water source. This trend is forecasted to continue in the future and to further establish ocean water desalination as a reliable drought-proof alternative for many communities worldwide.”

Filtration + Separation would like to thank Nikolay Voutchkov of Poseidon Resource Corp, Antonia Von Gottberg at KMS, and Chris Howorth of ITT’s Aquious PCI Membranes.

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II. Desalination - the basics esalination is the production of fresh, low-salinity potable water from a saline water source (seawater or brackish water) via membrane separation or evaporation. Sea or brackish waters are typically desalinated using two general types of water treatment technologies – thermal evaporation (distillation) and membrane separation. Currently, approximately 43.5% of the world’s desalination systems use thermal evaporation technologies. This percentage has been decreasing steadily over the past 10 years due to the increasing popularity of membrane desalination.

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Desalination processes – thermal All thermal desalination technologies apply distillation (i.e. are based on heating of the source water) to produce water vapour that is then condensed into a low-salinity potable water. Thermal desalination is most popular in the Middle East, where seawater desalination is typically combined with power generation, which provides low-cost steam for the distillation process. Thermal desalination also requires large quantity of steam. The thermal desalination technologies most widely used today are multistage flash distillation (MSF), multiple effect distillation (MED) and vapour compression (VC). · In the MSF evaporator vessels (flash stages or effects) the highsalinity source water is heated while the vessel pressure is reduced to a level at which the water vapour ‘flashes’ into steam. This steam condenses into pure water (distillate) on the heat exchanger tubes and is collected in distillate trays from where it is conveyed to a product water tank. · In the MED process, the source water passes through a number of evaporators (effects or chambers) connected in series, and operating in progressively lower pressures. In MED systems, the steam vapour from one evaporator (effect) is used to evaporate water from the next effect. · The heat source for VC systems is compressed vapour produced by a mechanical compressor or a steam jet ejector, rather than a direct exchange of heat from steam. In these systems the source water is evaporated and the vapour is conveyed to a compressor. The vapour is than compressed to increase its temperature to a point adequate to evaporate source water sprayed over a tube bundle through which the vapour is conveyed. As the compressed vapour exchanges its heat with the new source water which is being evaporated, it is condensed into pure water.

Desalination processes – membrane separation Membrane desalination is a process of separation of minerals from the source water using semi-permeable membranes. Two general types of technologies are currently applied for membrane desalination – Reverse Osmosis (RO) and electrodialysis (ED). · RO is a process in which the product water (permeate) is separated from the salts in the source water by pressure-driven transport through a membrane. As a result of the RO process, desalinated water is transported under pressure through the membrane while the minerals of the source water are concentrated and retained by

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the membrane. Applying high pressure for desalination is mainly needed to overcome the naturally occurring process of osmosis, which drives the desalinated water back through the membrane into the water of more concentrated mineral content. Nanofiltration (NF) is a process similar to RO where membranes with order of magnitude larger pore size are used to remove largemolecular weight compounds causing water hardness (i.e. calcium and magnesium). · In ED-based treatment systems the mineral-product water separation is achieved by applying electrical direct current (DC) to the source water, which drives the mineral ions in the source water through membranes to a pair of electrodes of opposite charge. A commonly used desalination technology that applies the ED principle is electrodialysis reversal (EDR). In EDR systems, the polarity of the electrodes is reversed periodically during the treatment process. RO desalination is the most widely used membrane separation process today. Currently, there are over 2,000 RO membrane seawater desalination plants worldwide with total production capacity in excess of 3 million cubic metres per day (800 MGD). For comparison, the number of ED plants in operation is less than 300 and their total production capacity is approximately 0.15 million cubic metres per day (40 MGD). Typically, desalination plants using brackish source water can achieve 65 to 85 % recovery. Seawater desalination plants can only turn 40 to 60 % of the source water into potable water because seawater typically has an order of magnitude higher salinity than brackish water.

Energy Costs of RO systems After the RO salt/water separation is complete, a large portion of the feed water energy applied through the high-pressure RO pumps stays with the more concentrated seawater and can be recovered and reused to minimise the overall energy cost for seawater desalination. Dramatic improvements of the membrane element materials and energy recovery equipment over the last 20 years coupled with enhancements in the efficiency of RO feed pumps and reduction of the pressure losses through the membrane elements have made it possible to reduce the use of power to desalinate seawater to less than 3.5 kWh/m3 (13.5 kWh/1,000 gallons) of produced fresh water today. Taking into account that the cost of power is typically 20 to 30% of the total cost of desalinated water, these technological innovations have contributec greatly to the reduction of the overall cost of seawater desalination. Novel energy recovery systems working on the pressure exchange principle (pressure exchangers) are currently available in the market and use of these systems is expected to further reduce the desalination power costs with approximately 10% to 15%. The pressure exchangers transfer the high pressure of the concentrated seawater directly into the RO feed water with an efficiency exceeding 95%. Future lower-energy RO membrane elements are expected to operate at even lower pressures and to continue to yield further reduction in cost of desalinated water. Filtration + Separation would like to thank Nikolay Voutchkov of Poseidon Resource Corp.

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III. Case study: the need for, and costs of, seawater desalination in Israel Israel is a perfect example of a country that requires a long-term solution to increase and improve its supply of potable water. The country has recently embarked upon a large-scale seawater desalination program, most notably the Ashkelon project – a 100 million m3/year SWRO desalination plant scheduled for commissioning later in 2005. he background for the initiation of the Ashkelon project, and a full analysis of its total water costs as well as benefits, was presented in Limassol, Cyprus, in December 2004, within an international conference on Desalination Costing sponsored by the Middle East Desalination Research Center (MEDRC). The following excerpts are taken from the paper, which was written with the assistance of Daniel Hoffman of ADAN Technical & Economic Services (Tel Aviv), and delivered at the conference by Dr Yosef Dreizin, senior manager in the Israeli Water Commission.

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Israel’s growing water supply problems By the late 1990s it was clear to Israel’s water planners that the country’s water supply system was approaching a crisis point: · The utilisation of all natural potable water sources was almost complete; · The price-inflexible municipal water demand was growing continuously, in spite of conservation measures; · Despite price increases, allotment cutbacks, increased water usage efficiency and the growing reuse of treated municipal wastewater, agricultural irrigation with potable water had reached a critical minimum;

· Due to repeated cycles of multi-year droughts and usage beyond natural replenishment, water levels in all the major natural reservoirs (Sea of Galilee and aquifers) had reached all-time lows, way past their minimum safe-use “red-line” limits; · The resultant seawater and other underground saline water intrusions had threatened the quality of the major coastal aquifer; · Chloride, sodium and nitrate levels in all the aquifers were increasing at alarming rates due to agricultural fertilisation and irrigation, with higher salinity waters as well as the nitrate levels in at least one third of the coastal aquifer wells exceeding the drinking water limits set by the Israeli Ministry of Health; · Localised contamination of groundwater by pesticides, heavy metals and various other industrial pollutants was also on the increase.

The seawater desalination solution Looking at all alternatives, including brackish water desalination, rehabilitation of contaminated wells, advanced treatment, including desalination, and increased reuse of municipal wastewater, and potable water import (from Turkey), the Israeli Water Commission decided that the only practical solution would be large-scale seawater desalination. It was felt that:

The seventh annual conference of the Israel Desalination Society (IDS) saw the 220 participants take a grand tour of the 100 million m3/year Ashkelon seawater reverse osmosis (SWRO) desalination plant (pictured). At its planned capacity, the Ashkelon plant, scheduled for commissioning during the second half of 2005, will be the largest SWRO plant in the world. But more importantly, says Daniel Hoffman, in spite of Israel's relatively high energy costs, it will operate, within a longrange BOT contract, at a price lower than with any other seawater desalination plant worldwide. (Image courtesy of VID – Vivendi, IDE Technologies and Dankner Investments.)

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industryfocus Israel's projected water demand by water quality and user sectors - in million m3/year Year

2005

2010

2015

2020

Agricultural Potable water Brackish water Treated wastewater Total

530 160 300 990

530 140 500 1170

530 140 600 1270

530 140 700 1370

Industrial Potable water Brackish water Treated wastwater Total

85 40 0 125

90 40 5 135

95 40 13 148

100 40 15 155

Domestic Potable water

720

840

960

1080

Nature conservation

25

50

50

50

Aquifer rehabilitation Potable water

100

200

0

0

Neighbouring entities

100

110

130

150

2060

2505

2558

2805

Total demand

· Brackish water sources near Israel’s population centres and regional and national potable water grids were few and far between; · The rehabilitation of wells was a local quality improvement solution that only increased the efficiency of groundwater utilisation within its existing replenishment constraints; · Desalinating municipal wastewater would, likewise, reduce the quality deterioration problem, but, due to the already wide use of this resource for irrigation, would not add to the overall water balance; · Water imports proved to be prohibitively expensive. The Water Commission consequently developed and presented to the Israeli Government a ten-year program for the desalination of a total of 500 million m3/year by 2010, a quantity that will eventually represent about 25% of total potable water supply in Israel (28%, when added to the 80 million m3/year of desalinated brackish water that will also be included in the system.) The commission’s program demonstrated individual plants’ capacities, locations, time frame, budgets, etc. To improve the quality of municipal water supplies, with which the desalinated water will be preferentially blended, the ten-year plan dictates that all desalinated water in Israel must have a chloride concentration of only 10-80 ppm max. Boron concentration, which, likewise, threatens some sensitive and important crops, is also limited in the plan to 0.3 ppm. At this point in time, the Government has approved the construction of seawater desalination plants with a total output of 315 million m3/year by 2010. The Ashkelon 100 million m3/year plant was the first step in implementing this program.

Quantifying the costs, benefits and risks of the Ashkelon plant The contracted price for the Ashkelon plant’s desalinated water, at the plant’s boundary limits, was 52.5 US¢/m3. Since the various fixed and variable cost components that comprise this price were linked to their relevant cost escalation indices (e.g. energy, cost of living, currency

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and exchange rates, etc.), this price has escalated since the contract signing and as of September 2004 stood at 57.5 US¢/m3. To this cost one must add the government’s own project related self costs, which include: · Its initial investments for tender administration, plant construction supervision and out of plant infrastructure (product storage, pumping and interconnection to the regional water grid); · Annual running costs for product chlorination and pumping, out of plant infrastructure operation and maintenance, and project administration; · Allowances for costs related to government-assumed project risks, e.g. payment of fixed costs for water not received due to government instructions, i.e. inability to accept and reserves for costs related to uninsured events of Force Majeure and termination due to default by Seller. The sum total of these government-assumed costs has been calculated/estimated at 8.9 US¢/m3. Together with the contracted price of the water, the Total Water Cost (TWC) for the Ashkelon plant is currently about 66.4 US¢/m3. This is about 25 US¢/m3 higher than the current cost of water pumped south to the Ashkelon area from the Sea of Galilee, but when the benefits from the higher quality desalinated water are factored into the equation, the effective or “net” desalinated water costs to the Government reduce to about 51 US¢/m3 and the gap narrows to about 10 US¢/m3. These benefits for the Ashkelon project add up to 15 US¢/m3, and are due to: · Improvements in municipal water supply quality (softening, chloride and nitrate level reductions) after the blending of existing, hard local groundwater with the soft, low salinity desalinated water (about 11 US¢/m3 for the Ashkelon area); · Savings in pumping energy required to deliver water from the north (about 3 US¢/m3); · Increased water supply reliability (estimated conservatively, and based on incomes generated by the lowest paying economic activity not curtailed due to water shortages, income from the lowest value agricultural crops, at 1.2 US¢/m3). Though, under most scenarios, the Commission expects that the TWC from the Ashkelon plant to escalate faster than the current cost of delivering conventional water from the north, so will the value of the benefits from the use of the higher quality desalted water. The consumers of all additional, critically required water will soon be only households and industry, who currently pay for their existing and inferior-quality water almost twice the current total desalted water cost. The economic consequences of water shortages to these consumers will be significantly higher than the conservative values assigned to them above. Israel’s current, let alone future economic activities and standards of living require this additional high quality water, and can also afford it. Filtration + Separation would like to thank Daniel Hoffman of ADAN Technical & Economic Services, Dr Yosef Dreizin of the Israeli Water Commission, and Gustavo Kronenberg, manager of VID (Vivendi, IDE Technologies and Dankner Investments.)

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IV. Case study: pretreatment at Carlsbad seawater desalination pilot plant Prolonged drought, dwindling traditional water sources and new stringent regulatory requirements are driving the costs of conventional water supplies up and bringing seawater desalination back into the limelight in California. Nikolay Voutchkov focuses on some of the pretreatment options available. urrently, there are five large-scale projects with cumulative capacity of over 560,000 m3/day in various stages of development in Southern California, USA. Two of these projects, the Huntington Beach and the Carlsbad seawater desalination plants, are being developed in a public-private partnership between Poseidon Resources and local municipalities and utilities. These desalination plants will be located at existing coastal electrical power generation stations, and are projected to have an product water capacity of 190,000 m3/day (50 MGD). They may be developed in one or more phases. At present, the two projects are in the process of environmental feasibility review and permitting, and their construction is planned to start within one year. To support the development of the Carlsbad and Huntington Beach seawater desalination projects, in early 2003, Poseidon Resources initiated the operation of a 110 m3/day pilot plant located at the Encina Power Plant in Carlsbad, Southern California. The seawater desalination pilot plant consists of a source seawater intake feed pump station; two pretreatment filtration systems configured to operate in parallel; filtered water transfer pumps; one membrane system feed seawater storage tank; one 5 micron cotton cartridge filter per pretreatment system; one 245 m3/day high-pressure RO feed pump; a single-stage RO system; permeate lime conditioning system; and a UV disinfection system (see figure 1 below).

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Figure 1 – General view of the Carlsbad seawater desalination pilot plant 22 March 2005

The seawater pilot plant has a potable water sampling station that allows visitors to taste the desalinated water, and is equipped with a number of ports for collecting samples to illustrate water quality. In addition, this demonstration plant is fully automated and designed for remote monitoring via the internet. The source of feed seawater for the planned full-scale desalination plant and the pilot facility is the warm cooling water of the Encina power plant. This once-through power generation station withdraws cooling water from the Pacific Ocean via the Agua Hedionda Lagoon (see figure 2, page 24). After passing through the power plant intake structure, trash racks and traveling screens, the cooling water is pumped through the condensers of the power plant generation units. The power plant has a total of five power generators, and, depending on the number of units in operation, can pump between 0.8 and 3.1 million m3/day (200 MGD and 820 MGD) of cooling water through the condensers. The warm cooling water from all condensers is directed to a common discharge tunnel and pond connected to the ocean via a jetty. In the full-scale desalination facility, it is planned to tap this discharge tunnel for both desalination plant feed water, as well as for discharging high-salinity concentrate downstream of the intake area. The pilot desalination plant intake withdraws warm water from a small discharge pond located at the end of the power plant discharge tunnel. The power plant discharge cooling water is typically 3-10°C warmer than the ocean seawater. The intake seawater’s total dissolved solids (TDS) concentration varies between 33,000 milligrams per litre (mg/l) and 34,500 mg/l, and averages 33,500 mg/l. The pilot plant’s dry-weather intake water turbidity is usually between 1 and 4 nephelometric turbidity units (NTU). During wet-weather conditions, which are usually brief and occur mostly in the winter, raw seawater turbidity typically varies between 6 to 12 NTU, with occasional hourly spikes of up to 30 NTU. The intake seawater for the pilot plant is conveyed to a feed storage tank from where it is pumped to the pretreatment systems. Currently, the two pretreatment systems undergoing testing are Parkson’s two-stage, continuous backwash granular media filtration system and Hydranautics’ HydraSub immersed microfiltration (MF) system. The reverse osmosis system consists of two 4 element pressure vessels in series. This RO system

industryfocus configuration makes it possible to collect permeate from one or both ends of each vessel and to test a different number of RO membrane elements. The tested seawater reverse osmosis membrane elements are high salt-rejection units 8 inches in diameter, provided by Hydranautics. The RO system is designed to run in a range of 45% to 55% recovery and typically runs at 45% to 50% recovery. This system is operated to produce target permeate TDS concentration of 350 mg/l.

Granular media pretreatment system The granular media pretreatment system includes two Parkson Dynasand continuous backwash upflow filters in series, otherwise known as the D-2 system. The first filter has a coarse (0.9 mm) sand media bed, which is 2.03 m (80 inches) deep. The second filter contains finer (0.5 mm) sand media and its media depth is 1.02 m (40 inches). As the seawater passes through the sand filter media the sand granules travel from the bottom of the filter to the top and back to the bottom in a continuous motion. Seawater solids trapped by the sand media travel downwards with the sand particles to the bottom of the filter. From there, the removed solids are lifted upwards to the sand washer located on the top of the filter via an air lift, and are removed from the filter cell as a waste filter backwash (reject). Both first and second-stage filters have instrumentation for continuous turbidity monitoring and data logging. The second-stage filter is equipped with a particle counter as well. The source seawater is typically conditioned with ferric sulphate at a dosage of two to 15 mg/l and with a low dosage of chlorine (0.3 to 1.5 mg/l) before entering the first filter stage. In addition, the polymer is fed to the source water during heavy rain and red tide events to enhance the flocculation of the source water particles prior to filtration. Ferric sulphate and all other chemicals are fed upstream of a static mixer installed on the feed pipe at a distance of approximately 400 times the feed pipe diameter upstream of the entrance to the first-stage filter, to simulate the full-scale location and performance of the static mixer. No chemicals are added to the second stage feed water of the D-2 system. The ferric sulphate dosage varies and is dependent on the amount of solids in the source seawater. The amount of chlorine added to the source water is just enough to be consumed within the filter beds. The secondstage filter effluent hardly has any chlorine residual left prior to conveyance in the RO system. Chlorine is introduced at the dosage, which results in an oxidationreduction potential (ORP) of the second-stage filter effluent of below 200 mV. This ORP is acceptable for processing through the RO membranes and has no negative effect on membrane integrity, useful life and performance. The backwash water from the first stage filter is directed to a lamella settler and the settled water is conveyed to the feed water of the first-stage filter. The second-stage filter’s backwash water is conveyed to the first-stage filter feed line for processing. This makes it possible to minimise the overall waste filter backwash volume to less than 4% of the filter feed flow.

Filtration+Separation

Microfiltration pretreatment system The microfiltration system consists of a test vessel, which contains two completely submerged membrane modules with 12 membrane fibre bundles per module. Each bundle contains over 16,800 capillary fibres. The fibres are made of polypropylene and have an outside diameter of 0.31 mm and wall thickness of 0.035 mm. The fibre pore size is less than 0.2 µ and fibre porosity is 45%. The total active surface area of one module is 250 m2 (2,700 m2). The production capacity of one module is 102 m3/day at a flux of 17 lmh (18.7 gpm at 10 gfd). The permeate side of the modules is connected to a vacuum pump, which moves the source seawater across the MF membranes. As the membrane fibres foul during the filtration process, a higher vacuum is required for production of the same amount of filtered water (permeate). The pump vacuum level is regulated by a variable frequency drive controlling the pump motor speed. This system is also equipped with instrumentation for feed water and filtrate turbidity monitoring, and for automated data reporting and acquisition. The filtration process includes a 15-minute filtration cycle followed by 15-second backwash cycle. The membrane surface is kept clean by aeration. During the backwash cycles, filter effluent is forced in a reverse direction, i.e. from the inside of the membrane lumen through the fibre walls and out into the test tank. The reversal of flow direction results in improved removal of foulants deposited on the membrane fibre surface. The MF modules are periodically cleaned using chemically enhanced backwash (CEB) with a combination of chlorine and mineral acid.

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industryfocus

Figure 2 – Intake and discharge configuration of the full-scale Carlsbad Seawater Desalination Plant

Under normal operating conditions the CEB frequency is once a day. The cleaning-in-place (CIP) frequency of the MF system is typically 60 to 90 days. To date, the MF system has been operated without chemical conditioning of the source seawater. The MF system-feed seawater is prescreened by a self-cleaning disk filter with a nominal screening size of 80 µ. The self-cleaning disk filter was selected after a conventional micro-screen of the same size was tested for eight months and replaced because of the more reliable and consistent performance of the disk filter. Initially a 120 µ-mesh micro-screen was used for MF membrane feed seawater prescreening. Although this size micro-screen was adequate to protect the integrity of the MF fibres, it did not provide an effective removal of the larvae of the barnacles in the source seawater. As a result, barnacle growth was observed on the MF vessel’s inner walls after approximately six months of operation. Barnacle incrustations are very difficult to remove and their uncontrolled growth may interfere with the normal operations of the pretreatment system. While adult barnacles are quite large, their larvae are relatively small and can pass through 120-µ screen openings. Subsequent investigation by an expert marine biologist identified that the size of the barnacle larvae in question was only 85µ. Therefore, a screen larger than 80µ is inadequate to retain barnacle larvae effectively, and protect the MF reactors from barnacle growth. To address this issue, the 120 µ screens were replaced with 80 µ screening filters. After the installation of these finer screening devices over one year ago (and subsequently of the 80 micron disk filter) no barnacle growth in the MF reactors has been observed. Subsequent investigations of the MF influent indicate that barnacle larvae have been effectively retained by the prescreening equipment.

of the time, both of them perform well and produce effluent suitable for subsequent RO system treatment. The cartridge filter replacement cycle for both pretreatment systems is over 12 weeks, which is well within industry standards. Table 1 summarises key filter effluent water quality parameters for the two pretreatment systems under typical dry weather conditions. During these conditions, the granular media system operates at surface loading rates of the first and second stage of 12.7 m3/m2.h (5.2 gpm/sf) and 8.5 m3/m2.h (3.5 gpm/sf) respectively. It should be pointed out that during the first phase of the pilot testing efforts, the two stages of the D-2 system were operated at the same loading rate of approximately 13.4 m3/m2.h (5.5 gpm/sf) to simulate Tampa Bay desalination plant filter surface loading rate conditions. The operation at these rates produced inferior results (approximately 30% higher effluent turbidity and SDI values) than the optimum combination of loading rates indicated in Table 1. Second stage filter surface loading rate reduction to 8.5 m3/m2.h (3.5gpm/sf) resulted in improved stable performance of the granular media filtration system. The membrane pretreatment system typically operates at a flux of 10 to 17 lph (8.5 to 10 gfd)under typical dryweather conditions (see Table 1 and Figure 5). Membrane permeability at 20°C is 2.0 to 3.0 gfd/psig. The typical CEB cycle during dry weather conditions is once per day. Heavy rain events may require two CEB cycles to be performed per day in order to maintain consistent operation at the flux range indicated in Table 1. The analysis of the data in Table 1 leads to the conclusion that both pretreatment systems provide effective removal of source water particulates, and that the downstream cartridge filters operate as intended (i.e. serve as a protective equipment rather than as solids removal device).

Pretreatment system performance under dry weather conditions

Pretreatment operation under extreme source water conditions

The site-by-side comparison of the operation of the two pretreatment systems to date indicates that under typical dry weather conditions, which in southern California occur over 98%

The most challenging conditions for the performance of both pretreatment systems are red tide-events, heavy rains, and power plant intake area bottom dredging.

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industryfocus Red tides in the area of the power plant intake typically occur in late summer/early fall once every few years and their effect on the water quality usually lasts for a period of one to two months. The D-2 system had slightly inferior performance as compared to the MF system in terms of SDI during red tides (SDI of 3 to 4 versus. 2.5 to 3.5). In order to maintain consistent performance, the source water coagulant feed dosage of the D-2 system was increased from 2 to up to 15 mg/l, the polymer was fed at a dosage of 0.5 to 0.75 mg/l and the chlorine dosage was increased from 0.3 mg/l to 1.5 mg/l. Although the MF system effluent SDI was lower during the red-tide period, feeding this effluent to the RO membranes resulted in accelerated membrane bio-fouling in a very short period of time (two to three weeks). No RO membrane befouling was observed during the red-tide period when the RO system was fed with D-2 filter effluent. Rain events typically do not represent a significant challenge for both pretreatment systems. However, during late December 2004/early January 2005, when a two-week rain period coincided with the dredging of the source water intake area, the two pretreatment systems were exposed to the most significant challenge to date – elevated content of silt, bacteria and organics in the source seawater. While the conventional pretreatment system operated well and produced effluent suitable for RO system treatment (i.e. turbidity < 0.06 NTU and SDI < 3.5), the MF system had to be either operated at approximately 30% lower flux or shut down frequently due to accelerated plugging of the filters with silt and organics in the source water.

Comparison of granular media and membrane pretreatment systems Under typical dry-weather conditions, both the granular and membrane filtration pretreatment systems performed well. The pilot data collected to date does not indicate significant performance advantages of one pretreatment system over the other for the specific source water quality of the Carlsbad seawater desalination project. The key advantages of the MF pretreatment system over the granular media filters are that it does not use feed water conditioning chemicals (coagulant and polymer) and it requires less operator attention. Avoiding the use of conditioning chemicals has two key benefits; it results in reduction in the overall plant chemical costs and it yields less waste solids and therefore decreases solids handling and disposal expenditures. In addition, the full-scale membrane system may not require cartridge filtration for its effective operation, which would be advantageous in terms of both capital and operations and maintenance (O&M) costs. The footprint of the membrane pretreatment system needed for source water filtration is projected to be approximately 25% to 30% smaller that that of the pretreatment D-2 system. However, the use of the membrane pretreatment system would require an additional area for the installation of the prescreening system as well as for a second-stage membrane system to treat the backwash water from the main membrane filtration process, in order to produce backwash of volume comparable to that of the granular media filtration system. When these additional area requirements are taken into consideration, the

Filtration+Separation

Table 1: pretreatment filter effluent water quality (Dry weather conditions, based on October-December 2004 pilot data) Parameter Loading Rate

Turbidity, NTU

Two-stage Granular Media Filtration System (D-2) Surface Overflow Rate: - First Stage = 12.7 m3/m2.h (5.2 gpm/sf) - Second Stage = 8.5 m3/m2 .h(3.5 gpm/sf) 0.03 - 0.33 (avg. =0.05)

SDI Before Cartridge 1.1 - 3.1 (avg.=1.8) Filter SDI After Cartridge 1.4 - 3.3 (avg.=1.8) Filter

Microfiltration System (Hydrasub) Flux: 14.5 - 17.0 lph (8.5 to 10 gfd)

0.03-0.35 (avg.=0.07) 1.0 - 2.8 (avg.=1.8) 0.9 - 3.6 (avg. =1.7)

total footprint of the two pretreatment systems becomes comparable. Key advantages of the granular media filtration system are that it does not require source water pre-screening, a daily chemically enhanced backwash, or a periodic chemical cleaning of the filtration, which reduces plant operations and maintenance costs. The hydraulic headlosses through the membrane system prescreening filters are 20 to 40 psi, which for a full-scale 190,000 m3/day (50 MGD) seawater desalination plant would result in an additional energy use of 0.9 to 1.8 MW. For the site-specific conditions of this project, these headlosses yield US$0.5 MM to $1.0 MM/yr of additional annual energy/O&M costs as compared to the conventional filtration system, whose operation does not require the pre-screening of the source seawater. The granular pretreatment system’s sand media is a standard commodity that is available from a number of manufacturers and is easy to produce, deliver and replace. The estimated loss of granular media at the pilot system to date is approximately 5% to 10% per year, which corresponds to a complete media replacement of 10 to 20 years. The useful life of the MF membranes is expected to be approximately five years. Taking into consideration the numerous factors affecting the pretreatment costs of a full-scale seawater desalination plant, the selection of the most suitable pretreatment system for this project has to be completed based on a thorough life-cycle cost analysis which accounts for all expenditures associated with the installation and operation of the two systems.

Conclusion The ongoing seawater desalination pilot testing study at the Encina power plant confirms the viability of the use of both conventional granular media filtration and membrane filtration for pretreatment of the source water of the full-scale plant. The project results indicate that the most challenging conditions for sizing of the pretreatment system occur during prolonged heavy rain and red tide events. If a membrane pretreatment system is used, a prescreening system of nominal size of 80 µ or less is necessary to effectively protect membrane fibres from damage.

Contact: Nikolay Voutchkov Senior vice president, technical services Poseidon Resources Corporation Email: [email protected]

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