Desalination A National Perspective (2008) / Chapter Skim
Currently Skimming:
5 Environmental Issues
5 Environmental Issues
Pages 108-146
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 108... ...
For each type of source water, there are distinct environmental considerations when that water is withdrawn. In coastal surface waters, issues of impingement and entrainment of marine organ 108
|
From page 109... ...
Intakes for desalination plants co-located with power plants are regulated under Section 316B of the Clean Water Act, although states may choose to apply these regulations to stand-alone plants as well (see Box 5-1)
|
From page 110... ...
If state regulatory programs meet the EPA requirements, the programs can be delegated to be administered by the states; therefore, regulations may vary somewhat from state to state. Effluent limits for desalination plants may specify pH, metaphosphates, chlorides, dissolved oxygen, conductivity, copper, iron, radium, total dissolved solids (TDS)
|
From page 111... ...
. Typically, a co-located desalination plant takes its source water from 2 http://www.epa.gov/safewater/sdwa/30th/factsheets/standard.html#4.
|
From page 112... ...
Brackish Groundwater Source Issues Some inland and coastal communities utilize brackish groundwater as a source for desalination, and withdrawal of brackish groundwater creates a quite different set of environmental concerns, including the physical sustainability of the aquifer and the potential for subsidence. Following a brief overview of brackish water resources in the United States, the potential environmental impacts from brackish groundwater withdrawal are discussed in more detail.
|
From page 113... ...
Coastal groundwater pumping can cause seawater intrusion that increases the thickness of the brackish water zone of dispersion. Brackish water from irrigation return flows can also be utilized as desalination source water, although the quantity and quality typically vary by season and region.
|
From page 114... ...
, will be necessary to assess the quantity and quality of water available for a given desalination facility. Nevertheless, a national compilation of existing data and regional evaluations of flow and solute boundary conditions, thickness, extent, and hydraulic conductivity of major brackish aquifer systems in the United States could provide the framework for a potentially greater utilization of brackish groundwater resources.
|
From page 115... ...
The source water would have TDS ranging between 1,200 and 1,500 mg/L TDS and produce a final blended water of 700 to 800 mg/L TDS, comparable to existing water quality. The desalination facility would require 17 wells for source water and 16 wells for blending.
|
From page 116... ...
. Excessive development of a brackish water resource, particularly one that possesses thick sections of at-risk lithologies (i.e., clay, silt, organic material)
|
From page 117... ...
, which adds the Pitzer coefficients for dealing with brackish water, is a computer program that is designed to perform a wide variety of low-temperature aqueous geochemical calculations. PHREEQC has capabilities for (1)
|
From page 118... ...
FIGURE 5-3. Areas where subsidence has been attributed to groundwater withdrawal.
|
From page 119... ...
It is worth noting here that the limited research to date on the environmental effects of concentrate management practices has primarily been focused on seawater desalination plants, because globally, seawater plants tend to be the largest desalination facilities. Currently, the majority of desalination facilities in the United States, both in the number of plants and in total capacity, use brackish source water (see Chapter 2)
|
From page 120... ...
, which, depending on the source water quality and discharge concentrations, could cause adverse impacts if discharged into sensitive environments. When desalination plants are co-located with power plants, desalination concentrate may also contain excess heat that may pose concerns for disposal into the environment.
|
From page 121... ...
Benzotriazole chemicals are persistent, but have low potential to bioaccumulate and are toxic only at very high concentrations. Desalination concentrate from any source water may also contain metals from corrosion that may be toxic to organisms if they are discharged into the environment.
|
From page 122... ...
These chemicals can also be considerably toxic to freshwater and marine organisms. The nature and volume of cleaning solutions and waste depend on the system design, the size of the system, and the frequency of cleaning.
|
From page 123... ...
discussed the toxicity and potential risk of several common membrane cleaning chemicals, considering typical discharge concentrations and dilution, focusing on desalination plants in the Arabian Gulf. Sodium dodecyl benzene sulfonate (Na-DBS)
|
From page 124... ...
In overall considerations of the contamination of the Arabian Gulf, where there are numerous MSF thermal and RO plants, Lattemann and Höpner (2003) considered copper and chlorine to be the most serious environmental threats from seawater desalination concentrate discharge.
|
From page 125... ...
Coastal Concentrate Discharge into Oceans, Seas, and Estuaries The salinity of seawater desalination concentrate can approach 2.5 times the salinity of seawater, and the impacts of this discharge on the marine environment will vary with discharge method, source water salinity and quality, site conditions, and ecosystem type. Without proper dilution, a plume of elevated salinity discharge may extend for a considerable distance beyond the mixing zone and can harm the ecosystem (Younos, 2005)
|
From page 126... ...
. Concentrate from brackish water desalination plants may have salinities that are similar to or lower than seawater, depending on the source water salinity and the recovery efficiency.
|
From page 127... ...
In general, larvae and young individuals are more sensitive to environmental stresses than adults of the same species. The following section describes what is known from research about the potential impacts of seawater desalination concentrate on various marine biota.
|
From page 128... ...
. A few studies have investigated sublethal effects of RO discharge water, but the effects and sensitivities differed from species to species.
|
From page 129... ...
exposed to RO concentrate discharge for over 6 years and compared the results with two undisturbed reference zones. The concentrate contained both elevated salinity and nitrogen because of high nutrient levels in the source groundwater -- not a typical situation for desalination plants.
|
From page 130... ...
(2006) found no significant impacts attributable to concentrate discharges from a small seawater desalination plant in the northwest Mediterranean on organisms present on the surface.
|
From page 131... ...
Diatom numbers and types did not change from prediversion conditions. Benthic foraminifera distribution and abundance varied considerably and reflected a "stressed habitat," but there were no differences that related to the presence of the concentrate discharge.
|
From page 132... ...
There is about a 50 percent overlap in species and abundant seagrass communities at both locations, though there was only a 10 percent similarity of benthic infauna. Clearly, however, the study could not have been done in Tampa Bay itself.
|
From page 133... ...
Through this process, a utility can minimize environmental impacts from concentrate discharge and also generate greater public acceptance of the technology. Additional environmental ecosystem monitoring is needed to better demonstrate the environmental sustainability of desalination using surface water concentrate discharge.
|
From page 134... ...
Because the source water typically has significantly lower salinity than seawater, the concentrate from brackish water desalination plants also has significantly lower salinities than seawater desalination plants, and inland facilities that use brackish groundwater as source water require fewer pretreatment chemicals. Nevertheless, despite these apparent advantages, finding cost
|
From page 135... ...
Depending on the source water composition, brackish groundwater concentrate may add toxic trace and radioactive constituents leached from the subsurface, such as selenium, arsenic, uranium, or radium. The potential for major ion toxicity, due to "imbalance" of ions in the concentrate, discussed previously, is also a concern when concentrate from brackish groundwater is discharged into freshwater ecosystems.
|
From page 136... ...
If the source water, and thus the concentrate, contains contaminants of concern such as arsenic, nitrate, or other harm
|
From page 137... ...
To prevent adverse impacts to surrounding aquifers, the volume, location, and solute composition of any displaced fluids and how they might influence the water quality of surrounding aquifers or surface waters should be well understood. This involves quantifying all flow boundaries and simulating groundwater flow dynamics using appropriate three-dimensional numerical transport and flow models (see Box 5-3)
|
From page 138... ...
WATER QUALITY ISSUES IN DESALINATED PRODUCT 4 WATERS Because desalination processes employ advanced water treatment techniques, it is commonly assumed that desalinated water is devoid of contaminants. In reality, although desalination technologies remove various constituents to a large extent, not all constituents are fully removed and some species are removed to a lesser extent than others.
|
From page 139... ...
.6 Based on its calculated boron reference dose and an assumption that 20 percent of total daily boron consumption would come from drinking water, the EPA developed a health reference level for drinking water as 1.4 mg/L boron. With different assumptions of the total amount of boron exposures from drinking 5 See http://epa.gov/safewater/ccl/ccl2.html#chemical.
|
From page 140... ...
Additional analysis of the human health effects of boron in drinking water, considering other sources of boron, are needed to support firm state-level water quality guidance for seawater desalination process design that is suitably protective of public health. If seawater desalination becomes a significant source for drinking water supply in the United States, additional regulatory attention or national guidance may be needed.
|
From page 141... ...
At the Tampa Bay Seawater Desalination Plant, chlorine dioxide is utilized to control biological growth in the pretreatment process. The plant had previously used free chlorine but the disinfectant was changed due to elevated disinfection byproduct formation.
|
From page 142... ...
Although the percentage of statewide energy use is likely too small for desalination, planners will need a clear understanding of the energy and climate implications of desalination relative to other water supply alternatives as the nation takes steps to address the issue of greenhouse gas emissions. Energy sources other than fossil hydrocarbons can provide energy for desalination and thus avoid or significantly reduce greenhouse gas emissions.
|
From page 143... ...
Thus, there are numerous alternative energy technologies available, and these technologies may be able to provide the right quality of energy for desalination while reducing overall greenhouse gas emissions. More research, however, is needed to analyze the alternatives for coupling desalination with alternative energy sources in both inland and coastal areas.
|
From page 144... ...
CONCLUSIONS AND RECOMMENDATIONS Knowledge of the potential environmental impacts of desalination processes is essential to water supply planners when considering desalination among many water supply alternatives. All components of the water-use cycle should be considered, including source water impacts, the likely greenhouse gas emissions from the energy requirements of the desalination process, potential impacts from concentrate management approaches, and environmental health considerations in the product water.
|
From page 145... ...
Once a number of rigorous site-specific studies are conducted, this information should be synthesized to develop an overarching assessment of the possible range of impacts from both seawater and brackish water desalination in the United States. A characterization of the volume, hydraulic properties, flow boundary conditions, and solute chemistry of the nation's brackish groundwater resources and a characterization of the spatial distribution, thickness, and hydraulic properties of aquifer systems suitable for concentrate injection, relying heavily upon existing data, would assist the financial and environmental planning process for inland desalination facilities.
|
From page 146... ...
Boron can be removed through treatment optimization, but that treatment could aversely affect the cost of seawater desalination. Further research and applications of technology should be carried out on how to mitigate environmental impacts of desalination and reduce potential risks relative to other water supply alternatives.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.