Saltwater Desalination

by Bennett Thomas and Michael Cuccinello

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Contents

• Introduction
• Reverse Osmosis
• Distillation
• Wastewater from Treatment Process
• Environmental Impacts
• Cost Analysis
• Legal Aspects

Introduction

Water is indisputably the most essential resource the earth has to offer to the human race. Unfortunately, it is distributed throughout the world as follows:

Of these categories,agulation and filtration are the pretreatment processes which can be used to remove suspended solids and other particles in the feedwater. After pretreatment, the water is pressurized and sent through semipermeable membranes which separate the salt from the water. In general, the membranes do not allow ions, large organics, partcicles, and bacteria to pass through them. RO membranes can even retain small ions such as sodium, chlorine, calcium, and magnesium (Gabler, 1988). The potable water then enters a post-treatment process and is sent through the distribution system. The highly concentrated salt solution which is separated by the membrane is discarded.

RO membranes do vary somewhat in their ability to remove impurities from water. An excellent membrane, operating in a well functioning plant, is capable of removing as high as 99% of the bacteria and up to 90% of simple inorganic ions. However, RO membranes are not as effective at removing organic compounds. This may be a problem if the feedwater contains high levels of trihalomethanes (THM's) (Gabler, 1988).

The efficiency of the reverse osmosis system is variable. It depends primarily on the quality of the feedwater, the pressure of the water as it is pushed through the membranes, and the the porosity of the membranes. Sometimes as high as 90% of the feedwater can be wasted. The following figure shows a standard Reverse Osmosis treatment system:

Distillation

The distillation process consists of heating the influent saltwater until it boils. This will separate out the dissolved minerals resulting in a purified and salt-free product. This product is then captured in its gaseous state, with high efficiency, and piped out to the distribution system. The three main distillation processes are separated according to their heat source. These processes are multistage flash distillation (MSF) in which the latent heat comes from the cooling of the liquid being evaporated, multiple-effect distillation (MED) in which the latent heat comes from a solid surface, and vapor compression distillation (VC) in which the latent heat is obtained regeneratively (van der Leeden, et al., 1990). Each process results in the same product, just by different means. The following schematic highlights the basic process of distillation:

Wastewater from Treatment Process

The picture below is of a saltwater desalination plant located on the Outer Banks of North Carolina (Photo by Dr. Daniel Gallagher). The plant can be seen at the top of the picture. On the bottom of the picture is the wastewater effluent leaving the plant by way of a creek which leads out into the bay.

In general, wastewater effluent from desalination plants may have the following types of potentially adverse constituents and qualities:

• Increase in salt concentrations from influent (influent concentration is about 35,000 ppm, desalination plants discharge brine ranging from 46,000 to 80,000 ppm).
• Increase in temperatures from influent (about 5° F increase at the point of discharge) for discharges from distillation plants.
• Increase in turbidity levels from influent water levels.
• Decrease in oxygen levels (distillation plants only).
• Chemicals from pre-treatment of the feedwater (these may include biocides, sulfur dioxide and coagulants).
• Organics and metals that are present in the feedwater and concentrated in the desalination process.
• Metals that are picked up by the brine in contact with plant equipment and pipelines.

Environmental Impacts

The use of desalination plants, by both Reverse Osmosis and Distillation, has the potential for adversely affecting the environment. Impacts on the environment can result from the discharge of chemicals used in the desalination process. Certain plants, for example, may use biocides such as chlorine to clean pipes or to pretreat the water. These chemicals must be treated before released to the ocean. Besides chemicals used in the plant, the wastewater from desalination plants is another concern because the effluent is a heavily concentrated brine solution. The brine solution, after it is discharged, has the potential to kill marine organisms. Although there are adverse environmental impacts from desalination plants, they can be avoided. For example, by using source water of a higher quality (i.e. from beach wells or infiltrat two desalination systems, one must examine the different costs involved. Both the Reverse Osmosis system and the Distillation system are expensive. The membranes used in the Reverse Osmosis process have a short life and the cost for replacement of these membranes can account for approximately half the cost of desalinating seawater. Because of this, it is suggested that reverse osmosis be used primarily with brackish waters as opposed to seawater, thus increasing the life of the membranes. Distillation plants consume more energy than Reverse Osmosis plants and are therefore more expensive to operate. The high energy cost for both types of plants can be alleviated by using solar energy or by using a cogeneration process. Cogeneration is a process in which the waste products from one system are used as a power source for another system. In this case, the exhaust steam from power plants could be used as a power source in both Reverse Osmosis and Distillation plants in order to reduce the energy needed. It should be noted that through the advancement of technology, desalination processes have become more efficient over the years, thereby requiring less energy. The following table represents desalination costs in the United States in 1985 dollars (van der Leeden, et al., 1990).

Legal Aspects

Within the planning process of a desalination plant, there are three main federal acts with which one must comply. They are the Coastal Zone Management Act, the Clean Water Act, and the National Environmental Policy Act.

The Coastal Zone Management Act states that any state with a coastal zone (including the Great Lake states) shall develop a management program to address all federal coastal water projects. If the program is approved by the Secretary of Commerce, the state can then determine if the proposed project is or is not compatible with their program. This gives the state a potential chance to veto the project. However, this act is usually utilized on a commenting level as opposed to a vetoing level. So in the case of implementing a desalination plant, the agency involved with the planning must check to see if there exists any incompatability. However, it is very unlikely for a desalination plant to be incompatible with a state's management program.

The Clean Water Act applies in several ways. Its major method of application is through the National Pollutant Discharge Elimination System (NPDES). This system applies to any project through which there is a discharge from a point source into navigable waters. It is administered by the Environmental Protection Agency (EPA). It is the EPA's responsibility to decide if a permit will or will not be granted for the proposed project. While this section of the Clean Water Act usually deals with wastewater treatment facilities and dams, a desalination plant would also require a permit. However due to its mild environmental impacts, resistance in obtaining a permit is highly unlikely.

The National Environmental Policy Act (NEPA) is one that had a massive effect on all water resource projects. It created Environmental Impact Statements (EIS's). An EIS requires the federal agency to put into writing all of the environmental impacts of their project. It must include all other possible alternatives, short term needs versus long term productivity, and sustainability. It must also include all of the adverse effects on the environment that could not be avoided if the projects it built. The discharge of the waste brine solution is the main area of interest for a desalination plant. Perhaps the waste might threaten the habitat of an endangered species of fish. However, concerns like this must be investigated for each and every project.

References

Gabler, Raymond. Is Your Water Safe to Drink?. Consumers Union. 1988.

Peavy, Howard and Rowe, Donald and Tchobanoglous, George. Water Resources and Environmental Engineering. McGraw-Hill, Inc. 1985.

van der Leeden, Frits and Troise, Fred L. and Todd, David Keith. 2nd Ed. Geraghty & Miller Groundwater Series: The Water Enclyclopedia. Lewis Publishers, Inc. 1990.

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Send comments or suggestions to:
Student Authors: Bennett Thomas bethomas@vt.edu and Michael Cuccinello mcuccine@vt.edu
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1997 Daniel Gallagher
Last Modified: 02/24/1998