Guide to Water Desalination: Engineering, Planning and Design

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Above: Desalination plant in Singapore.

Contents:

0. Introduction to this Guide

1. Desalination Engineering: Overview

2. Source Water Quality Characterization

3. Fundamentals of Reverse Osmosis Desalination

4. Planning Considerations

5. Environmental Review and Permitting

6. Intakes for Source Water Collection

7. Intake Pump Stations

8. Source Water Screening

9. Source Water Conditioning

10. Sand Removal, Sedimentation, and Dissolved Air Flotation

11. Pretreatment by Granular Media Filtration

12. Pretreatment by Membrane Filtration

13. Comparison of Granular Media and Membrane Pretreatment

14. Reverse Osmosis Separation

15. Post-Treatment of Desalinated Water

16. Desalination Plant Discharge Management

17. Desalination Project Cost Estimates


Introduction

Over the several years, water scarcity, changes in global climate patterns, and urban growth have led to a great shift in the paradigm of municipal water resource management. Low-cost surface and/or groundwater sources are practically depleted in many highly urbanized regions of the world. Therefore, the water supply planning paradigm is evolving from almost exclusive reliance on traditional freshwater resources toward building an environmentally sustainable diversified water portfolio in which low-cost conventional water sources are balanced with more costly but also more reliable and sustainable water supply alternatives such as desalination.

While only 0.5 percent of the world's available water resources are brackish in nature, brackish water desalination has found widespread application because it allows the production of freshwater at reasonably low cost and energy expenditures. At present, over 78 percent of the existing desalination plants in the United States are brackish water desalination facilities. Approximately 230 brackish water desalination plants produce freshwater for municipal water supplies in states such as Florida, Texas, California, New Mexico, and Virginia. Worldwide, brackish water desalination also contributes to municipal and agricultural water supplies in many arid regions, such as southern Spain, the Middle East, Australia, South America, and southern Israel.

Seawater desalination, while more costly at present, allows access to the world's ultimate water resource-the ocean. This water supply alternative has experienced a continuous exponential growth over the last 25 years, a pattern that is projected to continue well into the next decade.

This guide provides detailed background information on the planning and engineering of brackish and seawater desalination projects for municipal water supply.

While it includes a brief overview of key widely used desalination technologies, it focuses on reverse osmosis (RO) desalination, which at present is the most widely used technology for the production of freshwater from saline water sources.

The guide's sections address practically all aspects of brackish and seawater desalination, from basic principles to planning and environmental review of projects to the design of key desalination plant components such as intake, pretreatment facilities, the reverse osmosis system, post-treatment of desalinated water, and concentrate management.

The guide also provides guidance and examples for sizing and cost estimation of desalination plant facilities.

It is important to note that the facility and equipment sizing procedures presented in this guide are not intended to serve as standard all-inclusive design procedures; their main purpose is to illustrate typical methodologies and approaches used by desalination professionals. References to particular technologies, equipment, and membrane manufacturers should not be construed as endorsement by the author or a recommendation for preferential use or consideration. Cost graphs included in the guide are recommended for use in preparing initial order-of-magnitude estimates of a project's construction costs.

Site-specific project conditions may result in significant differences from the values determined based on the cost curves.

The guide includes a total of 17 sections, which follow a typical process of project planning, environmental review, and selection and sizing of key desalination plant components. Section 1 ("Desalination Engineering: An Overview") provides a brief review of the most commonly used desalination technologies at present, including thermal desalination, electrodialysis, and reverse osmosis separation. This section mainly emphasizes the basic principles of and differences between these technologies.

All other sections are focused only on reverse osmosis desalination processes, equipment, and technologies.

Section 2 ("Source Water Quality Characterization") provides guidance for the characterization of saline source water quality. The section describes the main source water constituents that can impact the performance of RO system membranes and identifies commonly used techniques for their measurement. It also provides practical direction on how the source water quality data can be applied in selecting desalination treatment processes and technologies.

Section 3 ("Fundamentals of Reverse Osmosis Desalination") features an overview of RO membrane structures, materials, and configurations that have found practical application for desalination. It presents basic theoretical principles and models for water and salt transport through membranes, with an emphasis on the nonporous solution-diffusion transport model, which is the most widely used model in practice at present. The section introduces desalination system performance parameters applied in planning and designing RO systems and describes phenomena that influence desalination efficiency, including concentration polarization, membrane fouling, and flux distribution in membrane vessels. It discusses the effects of salinity, recovery, feed pressure, and permeate back pressure on RO membrane performance.

Section 4 ("Planning Considerations") discusses the process of planning for a new desalination project. It describes factors, issues, and alternatives to consider in project planning, such as plant's service area and site; intake type and location; source water type and quality; product water quality; plant discharge; selection of key plant treatment processes, configuration, and layout; project implementation schedule and phasing; project economics; contractor procurement alternatives; and project funding considerations.

Section 5 ("Environmental Review and Permitting") provides an overview of the next step in the process of implementing a desalination project-assessing its potential impacts on the surrounding environment and developing measures to mitigate such impacts if they are found to be significant. The section describes the main environmental challenges associated with the impact of desalination plant intakes and discharges and identifies proven practical solutions to quantify and address these challenges. In addition, it presents the typical set of permits and permitting conditions for source water intakes, discharges, and product water quality.

Section 6 ("Intakes for Source Water Collection") focuses on the type and configuration of intakes used for brackish and seawater desalination, as well as design considerations for open and subsurface intakes. This section contains construction cost curves for onshore and offshore open intakes as well as for vertical well intakes.

Section 7 ("Intake Pump Stations") presents key advantages and disadvantages of alternative intake pump station configurations, including wet-well, dry-well, and canned pump systems, and provides guidance for pump station sizing and cost estimation.

Section 8 ("Source Water Screening") discusses alternative types of intake screens (including bar, band, and drum screens), microscreens, and cartridge filters. It also describes design and cost considerations for the selection of desalination plant screening facilities.

Section 9 ("Source Water Conditioning") is dedicated to systems for chemical conditioning of saline source water prior to its further pretreatment or direct application of membrane separation. The section addresses commonly used chemicals such as coagulants, flocculants, scale inhibitors, biocides, and pH adjustment compounds.

Section 10 ("Sand Removal, Sedimentation, and Dissolved Air Flotation") presents alternative pretreatment technologies that are commonly used for removing relatively large particulate solids from the source water. The section includes construction cost curves for lamella settlers and dissolved air flotation clarifiers.

Section 11 ("Pretreatment by Granular Media Filtration") is dedicated to the most commonly used type of technology for removing fine solid particles from the source water-granular media filtration. It discusses alternative filter configurations and their performance and applicability. The section includes cost curves for dual-media gravity and pressure filters.

Section 12 ("Pretreatment by Membrane Filtration") discusses the use of microfiltration and ultrafiltration membrane systems for pretreatment of saline source water. The section presents key design and planning considerations for most commonly used commercially available membranes, from Norit, Hydranautics, Filmtec, GE Zenon, and Memcor/Siemens. It contains design examples for submerged and pressure-driven ultra-filtration systems and cost curves for membrane pretreatment.

Section 13 ("Comparison of Granular Media and Membrane Pretreatment") provides a comparative evaluation of granular media and membrane pretreatment systems in terms of effect of source water quality on their performance, surface area requirements, generated residuals, chemical and power use, and overall water production costs. The section also includes guidelines for selecting a pretreatment system.

Section 14 ("Reverse Osmosis Separation") features a detailed overview of key RO system components-high-pressure pumps, RO racks, energy recovery system, RO membrane cleaning system, and instrumentation and controls. The section discusses the performance and configuration of state-of-the-art RO membrane elements used for nanofiltration, brackish water desalination, and seawater desalination. This section also addresses commonly applied RO system configurations and discusses full scale applications of such systems. It includes design examples and provides guidance information for cost estimation.

Section 15 ("Post-Treatment of Desalinated Water") describes commonly applied technologies for re-mineralization of permeate produced by desalination systems, including lime-carbon dioxide feed systems and limestone (calcite) contactors. It includes guidelines for the application, configuration, and design of such systems, as well as example cost estimates for lime-carbon dioxide and calcite-carbon dioxide conditioning systems. The section also features an overview of alternative disinfection systems and guidance for their use in desalination applications.

Section 16 ("Desalination Plant Discharge Management") includes a comprehensive overview of commonly applied technologies and systems for disposal of concentrate and other waste streams generated in desalination plants. The featured concentrate disposal alternatives are near-shore and offshore concentrate discharge, disposal to a sanitary sewer, deep injection wells, land application, evaporation ponds, and zero liquid discharge systems. Construction cost curves are presented for each of these systems.

Section 17 ("Desalination Project Cost Estimates") describes the main components of the capital and operation and maintenance costs of brackish and seawater desalination plants and provides guidelines for their assessment based on site-specific project conditions and components. This section also includes an example cost estimate of a 40,000 m3/day (10.6 mgd) seawater desalination plant.

This guide is intended for water utility engineers, managers, and planners; consulting engineers and designers; students and teachers in the desalination field; and staff members of federal and state regulatory agencies involved in the permitting of desalination projects.


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