Resource Management Issues: Desalination

Introuction
Desalination is the process by which salts and other chemicals are removed from salt or brackish water and other impaired water resources. It is also known as desalinization or desalting or commonly referred to as “desal.” As traditional sources of fresh water continue to be depleted and degraded, society is increasingly looking toward desalination as an option for obtaining water for both private, and municipal freshwater supply. In the past it has not been used extensively in this country, primarily because the cost of the product water has been so much higher than that from conventional sources. With more efficient desalting technologies being able to produce the water cheaper, in conjunction with escalating costs of obtaining fresh water from conventional sources, desalination is likely to look more and more attractive as an option to many proponents.

While desalination refers to any technology that removes salt from water, it includes a wide range of technologies that fall into two main categories, with many variations on each. Distillation processes involve heating the intake water to produce steam, which is then condensed to produce water with a very low salt concentration. Reverse Osmosis (RO) refers to the processes in which intake water is pressurized and forced through a semi-permeable membrane. The water passes through the membrane, but the salt molecules do not. With either technology, after the desalting process both fresh water and concentrated saline brine are produced. RO is the predominant technology being used and proposed in the Sanctuary region.

Three desalination facilities currently operate within the boundaries of the Sanctuary; however there has recently been an increase in interest for both private and public desalination plants. Approximately ten facilities have recently been proposed.

Rather than utilizing a coordinated regional planning approach, each plant has been conceived and designed as a separate project. Due to population growth in the area, continuing shortages and degradation of conventional water supplies, and advances in desalination technology, the trend will likely continue. Desalination plants have the potential to negatively impact the marine environment through the introduction of brine waste effluent and other substances to Sanctuary waters. Additionally, the construction of desalination facilities and associated pipelines often causes alteration of the seabed.

This action plan lays out a framework for a regional approach to address desalination, aimed at reducing impacts to marine resources in the Sanctuary through consideration of regional planning, facility siting issues, on-site mitigation measures, modeling and monitoring, and outreach and information exchange. While the Sanctuary is concerned with potential growth inducing impacts associated with increased supplies of water from desalination facilities, decisions regarding water supply and use will ultimately fall to local governments (cities and counties), the California Coastal Commission, and water agencies, to address and resolve. It is also the responsibility of these agencies to ensure that all alternatives have been analyzed, and that desalination is a necessary option.

Potential Impacts of Desalination
Desalination impacts vary widely and typically vary based on the specifics of each site. The degree of the impacts in large part depends on overall plant design and operation, methods used for effluent disposal and specific physical and biological conditions in the vicinity of the plant. While desalination can cause adverse environmental effects, there are often effective mitigation measures that can be taken to reduce impacts. Furthermore, it is important to consider that all other methods of obtaining municipal fresh water also involve major environmental impacts. This is especially the case when salt-water intrusion, or damage to anadramous or endangered species habitat caused by over-drafting of water from aquifers, rivers, and streams, are an issue.

Construction of a desalination facility, especially if new offshore pipeline construction is involved, can have significant environmental impacts, including disturbances to seafloor, surf zone, and dune ecology. By using existing pipeline structures or alternative technologies such as injection wells or percolation galleries, benthic impacts can be minimized or eliminated altogether.

The Sanctuary is also concerned with the discharge of the hyper-saline water that remains as a byproduct from the desalination process. This brine effluent is generally about twice as salty as the ambient seawater, however this varies depending upon the specific technology being used, and can range anywhere between 46 and 80 parts per thousand (ppt) (typical salinity in the region of the MBNMS is around 33ppt). This effluent is denser than seawater and without sufficient mixing, tends to sink to bottom where it may become concentrated. Both high levels of concentration, and fluctuations in salinity levels may impact sensitive organisms near the outfall. While tolerances vary among organisms, more research is needed to determine the extent of impacts for various species found in the MBNMS. The impacts of the brine effluent vary widely as a function of the location of the outfall. Impacts are generally more severe in rocky substrate than sandy seafloor habitats. Other issues associated with the discharge are: increased turbidity; and concentration of organic substances and metals that are contained in the feed waters. Additional impacts specific to distillation facilities include concentration of metals picked up through contact with the plant components, thermal pollution and decreased oxygen levels.

While if unmitigated, the impacts caused by brine effluent can be severe, there are many existing measures that can be taken to minimize these impacts. Certain technologies such as injection wells or percolation galleries minimize the impact from the saline brine discharge due to adequate mixing of brine and ambient seawater. Diffusers of appropriate design and number, used with open ocean disposal structures also can facilitate mixing of desalination discharge with ambient seawater in a limited mixing zone. Certain plants, such as the one located in the City of Marina, and the proposed Sand City facility utilize brackish groundwater as a feed water source; this results in a reject that is lower in salinity than typical brine effluent from similar facilities that desalt seawater.

Intake of water directly from the ocean usually results in loss of marine species as a result of impingement and entrainment. Impingement is when organisms collide with screens at the intake, and entrainment is when species are taken into the plant with the feed water and are killed during plant processes. Impingement and entrainment impacts can be mitigated by the use of certain designs and technologies. Properly engineered intake structures can reduce the potential for entrainment and impingement, and in certain cases the need for chemicals. Structures such as onshore intake wells or infiltration galleries have been proven highly effective. Appropriately sized screens at the intake, as well as low velocity water flow are potential mitigation measures for open water intake structures.

Clearly the most contentious and controversial issue surrounding desalination is its potential to induce community growth. Along most of California’s central coast, fresh water supply is the limiting factor for community growth. With the addition of an unlimited source of freshwater, growth can be allowed to occur. While this issue is not addressed directly by Sanctuary regulations, it is of major concern. Increased development of the coastline adjacent to the MBNMS could lead to degradation of water quality and many other challenges to the protection of Sanctuary resources. It is up to local jurisdictions to ensure that a proliferation of desalination facilities does not lead to unsustainable community growth, through responsible planning, and limitations in plant capacities. This issue is addressed by many other agencies including the California Coastal Commission and local jurisdictions.

Sanctuary Regulations and Desalination
Sanctuary management is concerned with desalination, because it has the potential to negatively impact the marine environment through the introduction of brine waste effluent and other substances to Sanctuary waters. Additionally, the construction of desalination facilities and associated pipelines often causes alteration of the seabed. Three of the Sanctuary’s regulations relate directly to desalination. The first involves a prohibition on discharging or depositing any material within Sanctuary boundaries. Since the brine effluent, and in some cases other materials, are usually disposed of in ocean waters, this activity requires Sanctuary authorization of Regional Water Quality Control Board (RWQCB) permits. The second Sanctuary regulation pertains to discharging materials outside of the boundaries, which subsequently enter Sanctuary waters and negatively impact MBNMS resources. As with the previous regulation, Sanctuary approval via authorization of the RWQCB permit is required. The third relevant regulation involves a prohibition on activities that cause alteration of the seabed. Thus installation of certain desalination facility structures such as an intake/outfall pipeline on or beneath the ocean floor will also require Sanctuary authorization.

Three small desalination plants currently operate in the Sanctuary:

Duke Power Plant in Moss Landing contains a seawater distillation plant that produces a little less than 0.5 million gallons per day (MGD) for use in its boiler tubes for the power production process. This facility uses power plant cooling water as the source for the desalination feed water and brine effluent discharge. Due to the large volume of cooling water being discharged by the plant, the brine effluent is diluted and impacts from the salinity are eliminated.

Marina Coast Water District in the City of Marina operate a small plant with the capacity of 0.45 MGD, currently supplies about 13% of the city’s annual municipal water consumption. This plant uses a beach well for intake water, and an injection well for discharging brine effluent. This facility was originally built in 1996, and will be renovated in the near future, with new technologies that will greatly increase its efficiency.

The Monterey Bay Aquarium operates a very small facility that provides about 0.040 MGD for maintenance purposes such as flushing the toilets. The saline brine discharge is blended with, and effectively diluted by the exhibit water outfall.

Although there are currently only three facilities in operation, there has recently been an increase in proposals for both private and public desalination plants. There are approximately ten additional facilities in the Sanctuary region that are in some stage of initial consideration or planning. These range from small, less than 50,000 GPD private facilities such as the proposed RO plant for the Ocean View Plaza to be built on Cannery Row in Monterey, to larger multi-city regional projects like the ones the City of Santa Cruz and Monterey Peninsula Water Management District are currently investigating. There are also several proposals for small to medium size projects to serve a single city, such as the proposed plants in Cambria or Sand City. Due to population growth in the area, continuing shortages and degradation of conventional water supplies, and advances in desalination technology, the trend will likely continue.

OTHER RESOURCES:

California Coastal Commission. "Seawater Desalination and the California Coastal Act". March 2004
http://www.coastal.ca.gov/energy/14a-3-2004-desalination.pdf

California Department of Water Resources
http://www.owue.water.ca.gov/recycle

National Water Research Institute
http://www.nwri-usa.org

Sanctuary Integrated Monitoring Network (SIMoN)
http://mbnms-simon.org

State of California Water Desalination Task Force. "Findings and Reccomendations" Report
http://www.owue.water.ca.gov/recycle/desal/Docs/FinalReport.htm

The ABC's of Desalting
http://www.idadesal.org/PDFS/Publications/ABCs.pdf

PHOTOGRAPHS: All photos courtesy of Brad Damitz (MBNMS)