1 Desalination Efficiency: Energy, Water and Other Resources Technical Paper Author: William Harvey Desalination Commerc...
Desalination Efficiency: Energy, Water and Other Resources Author: William Harvey Desalination Commercial Development Leader GE Water and Process Technologies Enviro 2008 Conference, May 2008 Melbourne, Australia
Abstract In water scarce regions, there are normally several solutions to address the water supply and demand imbalance. The desalination of wastewater, well water, and seawater have different optimization schemes that affect both the project economics and the sustainable use of scarce water and energy resources. This paper compares and contrasts the different tradeoffs in energy use and water recovery ratio for the desalination of wastewater, well water, and seawater. The use of new filtration technologies to minimize land impact and to increase plant reliability is also discussed.
Introduction With increasing pressures on scarce and valuable water, energy and land resources, government leaders and water industry participants need to weigh the many options to create infrastructure projects and the ongoing supporting services. This includes balancing the needs for the project on one resource type, mitigating possible negative impacts on another resource type. There are many parameters for economization and resource use efficiency to select of water resource to be developed and the technologies deployed to achieve the project objectives. In many water desalination and reuse projects, they often fall into the following areas:
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• Water resource use efficiency • Concentrate disposal minimization and environmental impact mitigation • Source watershed protection • Energy use minimization • Impact of supporting infrastructure such as interconnecting pipelines and availability of reliable electricity
Water Recovery The term Water Recovery represents the volumetric processing efficiency of a purification process. In general, a primary design objective in water desalination projects is to have the highest practical water recovery. This does two things: First, it maximizes the use of the scarce water resources. Secondly, high water recovery designs create less concentrate for disposal. Limitations on water recovery are normally from 1) the ability of water to keep the salts in solution (scale precipitation), and 2) power/materials limits in trying to keep concentrate salt water and the pure water stream separated.
Influence of Mineral Scale on Water Recovery For many brackish waters, one of the primary limiting factors in water recovery is the amount and type of salts present in the raw water. All desalination process concentrate the individual ions into a smaller stream. However, if the salts are concentrated to levels higher the allowable saturation level, solid mineral scales can form. In many well waters, the typical scale formations are calcium carbonate, calcium sulfate, barium sulfate. These are also common to water reuse projects, but reuse projects typically also have calcium phosphate as an additional design parameter.
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SEM Photos of Scaled Membranes Barium Sulfate
Figure 1: Scanning Electron Microscope image of mineral scale.
The common ways to avoid scale formation on desalination membranes is to dilute the minerals by operating at a lower water recovery (and hence using more raw water for the desired pure water objective), or to use small quantities of antiscalant chemicals. Antiscalants work by both reducing the rate of crystal growth, and distort the shape of the crystal growth, limiting the propagation of scale crystals. Because of their effectiveness, antiscalants is the typical solution in most water scare regions in order to maximize the efficient use of the raw water source and minimize the quantity of concentrate for disposal.
Influence of Salinity Level on Water Recovery Desalination technologies are somewhat specialized in nature, where they operate very efficiently but over a define range of situations. This is depicted in Figure 2, where the technologies of seawater reverse osmosis (SWRO), brackish water reverse osmosis (BWRO) and electrodialysis reversal (EDR) are shown with their approximate operating ranges.
Salt Removal and Water Recovery TDS Removed, mg/l 25,000
75% Water Recovery
Figure 2: Typical operating ranges of desalination technologies. Page 2
In general, it is more challenging to operate at a high water recovery while removing a large concentration of salt. Because of the high salt level in seawater, seawater desalination facilities operate at the lower end of water recovery (40-50%), while brackish water projects utilizing surface, well aquifer and water reuse waters have higher water recoveries (75-94%). In general, the water recovery of seawater desalination facilities are limited by the amount of energy required to keep pure water from flowing back toward the concentrated salt (osmotic pressure), and the component materials of construction for high water recoveries and high salt concentrate levels for seawater desalination.
Disposal of Concentrate from Brackish, Water Reuse and Seawater Desalination For brackish water desalination projects, there are various disposal venues, including deep well, ocean, surface discharge, and evaporation ponds. As with all alternatives, there are challenges to each. Finding a compatible aquifer for using a well method, the distance to the sea, the compatibility of surface water ecosystems, and the cost of land for evaporation ponds are some of the considerations. For seawater desalination, returning the concentrate back to the sea is currently the only practical method, and typical design parameters center on the proper diffusion of the concentrate to minimize aquatic life impact as well as subsurface and subterranean compatibility to pipe infrastructure. In today’s water scarce regions, water recovery is becoming less of a cost/benefit type of analysis than in the past. The value of brackish and fresh water in the future is becoming difficult to calculate, and high water recovery design of brackish and water reuse projects is becoming a institutionalized norm to secure long-term economic sustainability, political stability while reducing environmental impact.
Energy Demand from Water Treatment Even with all of the technological advances in desalination and water reuse over the last twenty years to reduce water and energy use, the technologies are not completely efficient processes. In the simple movement of water, there are normal hydrodynamic pressure losses as water flows Technical Paper
across pipes, through control valves, and inherent mechanical inefficiencies in pump technology - all contribute to energy consumption in water desalination and reuse projects – but they also serve to motivate technology firms to invest in research and development. One of the challenges of water reuse projects is that reclaimed wastewater is not acceptable for all water uses and customers. There is often a substantial distance between the source of the wastewater (e.g. a major city) and the ultimate consumer (industrial parks, agriculture region.) The implications are the cost of segregated pipe infrastructure and the energy to transport the reclaimed water often present political and energy challenges when compared to other local resource development options. This is especially true for coastal communities that have alternatives such as seawater and well water desalination, which have solutions that are relatively more independent from political and trans-geographical issues.
New Approaches to Reduce Desalination Energy Consumption The pumping energy used in desalination, especially seawater, contributes a large percentage of the operating expense in a facility. However, in a seawater desalination plant, a large percentage of the pumping power is not lost; the concentrate water is still highly pressurized, and it can be recaptured to increase the energy efficiency of desalination. Classically, this high pressure stream was introduced to a rotating device that transfers pressure energy through the use of Pelton Wheels that turned a shaft connected to high pressure RO feed pump motor. A second option includes the use of a turbo-like device, where the concentrate water energy is captured by a turbine and this rotating energy is delivered back into the raw water with a turbo to “pre-pressurize” the water, thereby reducing the amount of pumping done by the high pressure RO pump system. However, both of these techniques have multiple points of efficiency losses since each step of energy conversion – pressure energy to rotating energy to pressure energy – each of which are not 100% efficient. Page 3
The most modern approach is called “work exchange” where the pressure energy of the concentrate stream is transferred more directly using a positive displacement mechanism. Work exchangers typically can offer 5-10% net higher energy recovery efficiency than rotating equipment like Pelton wheels and turbos. Because of this higher efficiency, work exchangers have dominated the energy recovery device landscape in the last few years.
Figure 3: A work exchange energy recovery device.
The original work exchange devices were small in scale, and several were manifolded together to meet the system concentrate flow. However, the average size of desalination plants has increased dramatically in the last two years, where a 200,000 m3/day desalination plant is now a common plant under consideration. The water technology industry is well on its way to tackling the economy of scale optimization by designing larger devices. But since manifold piping is an extension of the work exchanger device and the manifold has different pressure/velocity attributes along the length of the manifold, this has presented an interesting challenge: What is the most energy efficient way to design and connect a multiple device work exchange system? To solve this, scientists turned to using fluid mechanics analysis to optimize multiple device and manifold configurations.
Figure 4: Arrangement of work exchange devices with flow manifolds
For the design of the recently commissioned 200,000 m3/day Hamma Desalination facility in Algiers, Algeria, advanced fluid computer modeling was performed by Energy Recovery Inc. and GE Water and Process Technologies to optimize the pressure transfer and energy recovery. The analysis centered on the influence on the direction of flow, the flow velocities and the corresponding pressure gradient across the inlet and outlet side of the device.
Figure 5: Computational fluid mechanics modeling of two work exchanger manifold designs for the Hamma Desalination Project.
The results of the analysis showed that the direction and the order the low pressure side manifold of the devices was introduced to the high pressure from the concentrate had an influence on the design optimization. It was found that a manifold operation using opposite flow directions (“C flow”) provides a more efficient and even flow distribution.
Land Costs and Use Efficiency for Desalination Seawater as a raw water for desalination systems is by its nature a dynamic water source, influenced by normal current, storms, aquatic life, shipping traffic, temperature and other characteristics. The role of a pretreatment step prior to the desalination step is to control and minimize the variation of conditions that are challenging to reverse osmosis membranes. This includes the reduction of fine sand and microorganisms that may clog reverse osmosis membrane elements.
multimedia filters can work on polishing the remaining suspended material in a more efficient manner. The modern approach to pretreating seawater for desalination is to use ultrafiltration membrane technology. UF membranes have pores specifically designed to produce a superior quality water than conventional clarification and multimedia filtration. In addition, UF produces high quality permeate water even under dynamic conditions, assuring long membrane life for the reverse osmosis membranes downstream.
Figure 6: An SEM image of bacteria and particulates on a 0.45 micron filter paper
Classic pretreatment uses a mixed media filtration approach of a granulated media such as graded sand and garnet to remove these suspended solids. In cases where the seawater is particularly challenging with very high levels of suspended matter, clarification is included to aggregate the suspended material and provide a means for a bulk of the material to sink the bottom of a basin so the
To reduce the high cost and impact of piping infrastructure costs, seawater desalination plants are customarily locate at or very near the sea. However, seafront property is, with almost no exception, coveted and very valuable. And as more people desire to live near the coast, land values will continue to increase. UF pretreatment technology has the added benefit of substantially reducing the land area required for seawater pretreatment. For a large (190,000 m3/day) project currently under development in USA, an engineering analysis concluded that the amount of land required by a UF system will be at least 50% less than the land required for conventional multimedia system design.
California Desalination Project Plant Layout
Conventional Pretreatment Area UF Pretreatment Area
Figure 7: Comparison of land required for conventional technology and ultrafiltration technology for sweater pretreatment. Page 6
For seawater desalination projects, ultrafiltration technology provides the most judicious and efficient use of valuable and scarce land to minimize environmental impact, while providing additional technical, reliability and operational cost advantages.
Conclusion In water scarce regions of the world, there are also other scarce resources that need to be carefully managed. Solutions such as desalination and water reuse are technically and economically proven solutions that government leaders and industry participants can use to solve localized water supply and demand imbalances. Often, there are several resources other than water that are part of the optimization of the water solution.
5. Irwin, Kenneth; Ramroop, Ian; “Seawater Pretreatment Operation: Three Years Operation Of The 27.6 MIGD (125,530 M³/Day) Desalination Plant, International Water Conference, Pittsburgh, Pennsylvania USA, October 2005. 6. Kloos, Steven D.; Chambers, Cameron; Vandenberg, Russell; “Comparison of Water Reuse Technologies, Including Electrodialysis Reversal (EDR) and Zero Liquid Discharge (ZLD)”; OZ Water Conference, Sydney, Australia, March 2007.
References 1. Harvey, William T.; Maria, Dionardo; A.; Morrell, Jorge A.; Hodgson, John W.; Marchena, Filomeno A.; “The Decision Process to Use Membrane Desalination after 75 Years of Successful Thermal Desalination”, International Desalination Conference, Oranjestad, Aruba, June 2007. 2. Harvey, William T.; Mercusot, Michel; "Desalination Strategies In South Mediterranean Countries", EuroMed Conference: Cooperation Between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean, Montpellier, France, May 2006. 3. Stover, Richard L.; Martin, Jeremy; Nelson, Michael, “The 200,000 m3/day Hamma Seawater Desalination Plant – Largest Single-Train SWRO Capacity in the World and Alternative to Pressure Center Design”, International Desalination Association World Congress, Grand Canaria, Spain, October 2007. 4. Meyer-Steel, Shawn; von Gottberg, Antonia; Talavera; Jose Luis; “New Seawater Reverse Osmosis Plants for the Caribbean – Energy Recovery, Brine recovery and Cost Reduction”, Caribbean Water and Wastewater Association Annual Conference, Grand Cayman, October 2001.