Water Desalination

1-SiteShaftAerialThe stated benefit of Desal is primarily to provide a reliable new source of freshwater to supplement finite or dwindling existing sources that cannot meet projected water demand. Desalination may also help reduce our unsustainable reliance and over-drafting of rivers, streams and aquifers. A constant flow of fresh water from desalination could avoid the environmental disruption of building more dams for storage. However, to ensure that these potential benefits come to fruition could require complex legal mechanisms and each project proposal involve different complications.

Desal facilities can have several negative environmental impacts, depending upon where they are located, how they are designed and operated, and the end use of the produced water. In some cases, there are also concerns about “privatizing” what has always been a public resource, as well as additional complications if a private owner is a foreign entity. A water supply system operated by a private company (perhaps a multinational company) may not be subject to restrictions of regulatory government agencies, or the general public. Their profit goals may encourage rate increases, reductions in quality, and promotion of more water use, as opposed to calls for water conservation and recycling.

pdf report: Desal-Not a good alternative

Water Desalination Plants

General Environmental Impacts

Most environmental concerns that are raised relate to both air and saltwater emissions. Air emissions are due to the desalination industry’s heavy energy consumption and involve the commonly named pollutants carbon dioxide and sulfur dioxide. In light of their substantially higher energy consumption, thermal processes are inferior to membrane processes when it comes to air pollution.

The other form of emission from desalination raising environmental concerns comprises the discharge of concentrated saltwater after the desalination process is completed. The effluent is approximately twice as concentrated as the original sea water solution. Additionally, it contains chemicals used in the pretreatment of feed water, such as anti-scalants, surfactants, and acid. Speed of dilution, once brine is released into the ocean, depends largely on depth and flow rates at the release location. To our knowledge, no empirical results from comprehensive studies investigating the impact on sea life around the brine outlet have been published. Many experts argue that the amount of brine release is too insignificant to pose a burden on ocean ecology against prevailing opinion among environmental activists.

The left-over concentrate from desalinating brackish groundwater appears to pose greater disposal problems. Without access to the sea the brine may significantly augment groundwater salinity once released into the ground. Storage of the concentrate on the other hand requires large amounts of space and measures to prevent saltwater penetrating the earth. Compliance with environmental standards for inland disposal of brine may entail substantial expenses for the desalination industry. A similar outcome in terms of extra cost could arise for seawater desalination if results of pending scientific studies find detrimental effects on ocean ecology from brine release.

The third critical environmental concern pertaining to desalination besides air emissions and brine discharge consists of the use of valuable coastal lands (These areas are extremely valuable from both an economic and environmental perspective). Membrane processes take up less surface area than distillation plants.

Absolute environmental impacts of desalination plants and the respective processes are largely unknown due to still sporadic application and limited public attention. Since this lack of public exposure is in the process of changing considerably, more information will be available soon with regard to environmental impacts.

Operation phase

High-energy use and resulting green house gas production.

The energy used in the desalination process is primarily electricity and heat. Energy requirements for desalination plants depend on the:

  • The volume of water produced;
  • Salinity and temperature of the feed water;
  • The quality of the water produced; and
  • The desalination technology used. Desalination plants due to the high energy requirement produce
large amounts of greenhouse gasses. A desalination plant using reverse osmosis technology would require less energy than other desalination technologies such as distillation. 
Sydney Water has projected that a desalination plant that produces up to 500 Mega litres (ML) of water per day through reverse osmosis would require 906 Giga Watt hours (GWh) per year and would produce between 950,000 tons (using the existing energy grid) and 480,000 tons (a gas power station adjoining the desalination plant) of greenhouse gasses per year depending on the source of energy.

    Impacts to Marine Ecology


    Physical Destruction to Marine Environment


    Overseas research has suggested that one of the greatest ecological problems associated with desalination plants that use seawater is that organisms living with in the vicinity of the desalination plant are sucked into its equipment.

    Recent analyses have noted that the impacts to marine life associated with intake designs were greater than first considered, hard to qualify and may represent the most significant direct adverse environmental impact of seawater desalination. This issue would need to be fully addressed in a comprehensive Environmental Impact Report.

    Most Coastal wetlands and estuaries have been filled in/developed or degraded from pollution and unnatural sediment loading. Consequently this habitat is already threatened to the aquatic and terrestrial life that depends on these locations for some stage of their life history (e.g., birds, fish, invertebrates, etc). Therefore desalination facilities that rely on a coastal “Source” water should be viewed with heightened scrutiny in its impacts.

Waste

Environmental impacts associated with concentrated discharge have historically been considered the major environmental concern with desalination plants. By some estimates, a desalination plant at Kurnell could produce 1.5 billion litres of brine a day to be released back to the ocean.

Further, desalination plants produce liquid wastes that may contain all or some of the following constituents:

  • high salt concentrations, chemicals used during defouling of plant equipment and pre-treatment, and
  • Toxic metals (which are most likely to be present if the discharge water was in contact with metallic materials used in construction of the plant facilities).
  • 
Liquid wastes may be:
  • discharged directly into the ocean;
  • combined with other discharges (e.g., power plant cooling water or sewage 
treatment plant effluent) before ocean discharge;
  • discharged into a sewer for treatment in a sewage treatment plant, or dried out. 
The environmental impacts of liquid waste treatment will vary depending on factors including the location of a desalination plant and method of waste disposal. Potential environmental impacts resulting from the increased turbidity, reduced oxygen levels and increased density of any discharged waste water would need to be fully addressed in an Environmental Impact Statement. Desalination plants also produce a small amount of solid waste (e.g., spent pre- treatment filters and solid particles that are filtered out in the pre-treatment process) that would have to be disposed of in land fill.
  • Alternatives to desalination 
 Construction of a desalination plant in Sydney is estimated to cost more than $2 billion. Desalination is not a substitute for good water saving practices, nor would it effectively ‘drought proof’ a city. Restrictions, in combination with other water conservation and water supply options, are key aspects of effective management of drought.
  • Alternative options for boosting water supply, namely water reclamation and water transport. Water Sensitive Urban Design
Water Sensitive Urban Design in the Sydney Region http://www.wsud.org/
  • Comparison of construction costs for water sensitive urban design and conventional storm water design. http://www.wsud.org/downloads/Info%20Exchange%20&%20Lit/Danny%20B%20WSUD%20vs%20Traditional%20P aper.pdf
  • 
Urban Storm water Connections to Natural Systems (Commonwealth, Department of Environment Heritage) http://www.deh.gov.au/coasts/publications/stormwater/urban.html
  • NSW Stormwater Trust (NSW Department of Environment Conservation) http://www.epa.nsw.gov.au/stormwater/usp/index.htm
  • Treatment Techniques for Managing Urban Storm water (NSW Department of Environment Conservation) http://www.dec.nsw.gov.au/resources/treattech.pdf
  • Urban Storm water Device Guide and BASIX Assessment iPlan – NSW Department of Infrastructure Planning and Natural Resources) http://www.iplan.nsw.gov.au/basix/pdf/method_stormwater_full.pdf
  • Water Conservation and Reuse
Water Smart Guidelines
(Master Plumbers Association of Australia) http://www.plumber.com.au/consumer/watersmart.asp
  • BASIX guide to selecting and installing water-efficient fittings and fixtures (NSW Department of Infrastructure Planning and Natural Resources http://www.basix.nsw.gov.au/information/tips.jsp
  • Wastewater reuse (Water Sensitive Urban Design in the Sydney Region) http://www.wsud.org/downloads/Planning%20Guide%20&%20PN%27s/09-Wastewater.pdf
  • Australian literature on grey water from WSUD.org http://www.wsud.org/literature.htm#fifth
  • Review of National & State Plumbing Codes to facilitate Domestic Water Reuse(Commonwealth Scientific and Industrial Research Organization (CSIRO) http://www.clw.csiro.au/priorities/urban/awcrrp/stage1files/awcrrp_7_final_23apr2004.pdf
  • Experiences with the Tampa Bay Desalination Project The various contractors ran into a number of difficulties both during the construction period and the project’s initial operating phase. These issues were of technical as well as financial nature. In fact, three firms were forced to declare bankruptcy and cease involvement in the project. In May 2003, two months after production had started, a performance test uncovered 31 deficiencies in the plant allowing the plant to run only “intermittently” (Tampa Bay Water, 2004) since then. Publicized major problems involve the cartridge filters used to catch large particles before the water permeates the delicate reverse-osmosis membranes. These were clogged after just a week – instead of the expected 90 days. Also, the 10,000 reverse-osmosis membranes, used in the final steps of water treatment to filter out the finest of salts and minerals, had to be cleaned of algae and bacteria every two weeks compared to an anticipated cleaning interval of two to six times a year (St. Petersburg Times, 2004). The current time table projects the plant to be fixed and taken off its current stand-by mode by spring 2006, roughly three years after the plant’s first run.
  • It is also important to note that many proposed current projects are located in areas with dramatic surface run-off problems and limited sewage treatment capacity.

 How Desalination Compares to Alternative Sources of Water

Continued demographic and economic growth result in increasingly strong competition for the available water supply. Making matters worse, water supply is shrinking for reasons like surface water pollution, groundwater depletion, or saltwater intrusion. Of two options to deal with the problem, increasing supply or curtailing demand, the former is pursued with much greater intensity. But, the array of means to enlarge the amount of drinking water for public use is fairly limited in scope and in some cases not practicable; for instance, it is not feasible to deplete ground or surface water reservoirs above the rate of natural replenishment for years and decades to come without a readily available alternative at hand that could reliably sustain entire region’s demographic and economic needs. Thus enhanced depletion of existing reservoirs does not represent a prudent option to increase water supply.

  • This leaves two alternatives against which desalination can be compared. Practiced since thousands of years, predominantly in arid regions, water transport from places with excess supply to places in need represents the first alternative. Relatively little empirical work has been published on this subject although water transport is undertaken in many locations all over the world (Zhou, 2004). Costs vary enormously and are highly dependent on case specific conditions.
  • It is critical to keep in mind when comparing water transport and seawater desalination that the latter actually augments total available supply of freshwater whereas the former only shifts water from a location with excess supply to a location in need of water. Thus in the long run only desalination can be considered a viable source of additional fresh water in contrast to the intermediate solution water transport.
  • The other more promising option to augment the available water supply consists of wastewater reclamation. As with water transportation, published empirical data is limited. A current high-profile case of wastewater reclamation is the Orange County, California, Regional Water Reclamation Project. The project’s unit cost of water supply is estimated to be slightly lower at $ 0.0015/gallon than those of large scale desalination projects. The majority of studies on wastewater reclamation pertain to reclamation of water withdrawn by the largest water user, agriculture. Haruvy et. al. (2001) estimate the direct cost of agricultural effluent reuse at $0.001/gallon. Interestingly, there are substantial synergies between wastewater reclamation and the RO desalination process because both use membrane technology. Both approaches also increase total available water supply.
  • Outlook Rising water scarcity in many parts of the United States have begun to expose the potential of desalination to a larger audience. The 2004 Desalination Energy Assistance Act proposal entails incentive payments for qualifying desalination facilities to partially offset the cost of electricity. Despite its considerable energy dependence, desalination, particularly of seawater, is backed by a number of strong arguments. Seawater is available in sheer limitless supply, which is in stark contrast to ground and surface water supplies in many regions. Supported by the evidence of declining criticism regarding the industry’s cost competitiveness in producing additional water supplies, the desalination industry is capable of producing water on a commercial basis even at the industry’s still early development stage.
  • On the other hand, the case of the Tampa Bay plant shows that at the cutting edge of implementing new technologies many deficiencies still remain to be overcome. For instance, the mechanism of water transfer and salt rejection in RO membranes is not clearly understood. Better understanding at the molecular level, however, will lead to new membranes that may show higher fluxes and better salt rejection for it is critical to improve both water recovery and quality.
  • For both membrane and particularly for thermal processes, it is crucial to enhance energy efficiency and create partnerships with power plants. By using heat recovery from a nearby power plant, energy consumption of thermal desalination processes is reduced by a factor of eight compared to a single stage process without heat recovery.
  • However, the feasibility of partnerships between power plants and desalination plants presently faces major obstacles. First, presently in the US, facilities are typically owned and operated by separate entities contrary to Saudi Arabia, the country with the world’s largest desalination capacity. Second, power plants typically purge waste heat at temperatures of 30 degree Celsius, well below the temperatures required for thermal processes. However, Semiat (2000) has called for an entirely different approach. He suggests the construction of desalination-dedicated power plants as energy sources. Thermal processes would use the heat produced while membrane processes would employ electricity. This hybrid approach is very similar to hybrid processes already employed in the chemical industry. While this solution would clearly enhance efficiency, it would not solve the problem that volatile energy prices pose for long term desalination planning.