Water Resources in Rockland - Planning in a Changing World

Desalination in a Global Context

earth from spaceAs of March 08 there were 15,000 desalination plants globally with 40,000,000 m3/day total production (Schiermeier, 2008). The biggest plant operating is 300,000 m3/day (Ashkelon in Israel), most have a capacity under 5000 cubic meters(m3)/day. At present 75 major plants are at various stages of development globally, and Australia, Britain, the US and China all have projects in the works. While desalination does provide a substantial part of water supply in certain oil rich nations, desalination capacity globally is .3% of total world freshwater use.

Looking at desalination plants that were planned, in construction or operating as of January 2005, 9 of the top 10 were in the Middle East and 68 of the top 100 were in the Middle East and North Africa. 18 were in the US, 4 in India, Pakistan, Iran, 3 in Spain and 3 in Singapore and 1 each in Australia, China, Mexico and Trinidad and Tobago (Gleick, et al, 2006).

 
 

As of Jan 2005 the US had over 2000 plants with capacity greater than 100 m3/day installed or contracted, with a total installed capacity of ~ 6,000,000 m3/day, about .3% of US use. While earlier in the decade almost all US desalination facilities were small systems for high value commercial and industrial needs, but this has been changing as demand increases, technology improves and prices drop. Four states have most of the US installed capacity - Florida, California, Arizona and Texas, though large plants included in the estimate are not operating (e.g. Yuma Arizona, Santa Barbara, CA). California has proposals for at least 20 new large desal facilities, which together could ultimately supply 6% of state's urban water demand. All plants proposed use reverse osmosis, with a total capacity between 1.9-2.3 million m3/day.

 

desal plant

While thermal evaporation and distillation is very common in the Middle East, reverse osmosis is the most common technology globally and accounts for 70% of capacity in the US. Multi-stage flash distillation is the second most common technology used in the world but it's rare in the US(1%). Nanofiltration which is rare globally accounts for 15% of US capacity. This is 70% of nanofiltration globally. Globally, 56% of desalination plants are designed for seawater, 24% brackish, and 9% for river water. In contrast, in the US, half of the capacity is designed for brackish water and less than 10% of capacity is designed for seawater ("With a Grain of Salt: An Update on Seawater Desalination", Gleick et al).

Costs for desalination have come steadily down. Ashkelon in Israel, once the world's largest at >300,000 m3 daily for ~ 50 cents per m3. But on average water produced by desalination is 3.5x more expensive than water from other freshwater sources, and energy will always remain the crucial constraint accounting for at least 1/3 to 1/2 the cost of the produced water. While twenty years ago it took 5-10 kwh of electricity to produce one m3 of desalinated water, plants like Ashkelon use about 2kwh but given that high flux rates require extra energy Ashkelon may be close to the limit of what's achievable in reducing energy use. Experts anticipate a delivered (vs. produced) price of water from desalination to stay in the range of $1/m3 - $3/m3, with the possibility for larger plants of getting down to the .80-.90 range (Gleick, 2006). The mitigation of environmental impacts can add to the cost as well. The principal impacts arise from the water intakes which can sometimes be mitigated by the use of beach wells and the discharge of the brine.

 

National Research Council on Desalination

The Committee on Advancing Desalination Technology, formed by the Water Science and Technology Board of the National Research Council published in 2008 the results of their analysis of current desalination technologies and barriers to broader implementation. The study was motivated by the observation that while concerns are growing in the US about options for meeting future water needs the US lags behind many other countries in the adoption of desalination technologies. The study produced a framework for a national research agenda, considering environmental an implementation issues as well as technology and costs. Read an excerpt from their conclusions.

Rockland County

Haverstraw Water Supply Project

United Water New York is proposing a desalination plant, the Haverstraw Water Supply Project, as the second phase of meeting Rockland County's water needs over the next twenty years. A series of short-term actions make up the first phase. The proposed Haverstraw project would provide up to an additional 7.5 mgd and it is proposed to be built in 3 separate 2.5 mgd phases. At full build-out the desalination plant will take in 10mgd in order to produce 7.5mgd. For comparison, average sustainable water supply in Rockland County in 2006 was 32-34mgd.

 

Environmental Impacts

Environmental impacts of desalination plant operations for potable water production

The National Research Council (NRC) 2008 analysis identified three major categories of environmental impacts associated with operation of a desalination plant that are relevant to the Haverstraw Bay, NY project proposed by United Water New York: (1) Impacts associated with the water intake; (2) Impacts associated with disposal of concentrate liquids; and (3) Greenhouse gas emissions associated with increased fossil fuel emissions derived from electricity generation.

There would be environmental impacts of construction of the Haverstraw project as well as those associated with plant operations but they are not addressed here. The material provided on this web site on the environmental impacts of desalination plant operation is not intended to be comprehensive.

Environmental impacts associated with brackish water withdrawals for the Haverstraw Project

Two main issues - impingement (fish being trapped and killed on the screens) and entrainment (fish eggs, larvae and fish being killed by being transported into a plant with the water) - have been extensively studied with respect to electricity generation plants. A number of technologies and practices have been identified that can reduce impingement and entrainment impacts. These practices and technologies include intake location and intake placement, screen design, intake volume reductions and intake timing.

Amount of Hudson River water withdrawals from the proposed desalination plant and nearby power plants

Most desalination plants withdraw at least an order of magnitude less water than that required for water used for once-through cooling of a medium size power plant (NRC 2008). The 9/26/08 Haverstraw Project Pre Draft Environmental Impact Statement (pre-DEIS) prepared by United Water New York compared the proposed Haverstraw desalination plant intake to that of the two large power plants in the area. At full build-out the desalination plant would be intended to withdraw 10 mgd continuously or 20 mgd during the late ebb/early flood portion of the tidal cycle (to minimize the salinity of the water withdrawn for reverse osmosis treatment). The Bowline Fossil Fuel Generation Plant just south of the proposed project area takes in an average of 783 mgd, and the Indian Point Nuclear Generation Units #2 and #3 north of the project area together withdraw an average of 1869 mgd. The Bowline plant has a design capacity of 1125 megawatts. The Indian Point units 2 and 3 together have a combined generating capacity of 1910 megawatts.

Two other power plants along the lower Hudson (Roseton and Danskammer north of Newburgh, with a combined generating capacity of 1700 megawatts) make significant daily cooling water withdrawals and are included in the case argued before the Supreme Court in December 2008 seeking EPA to require “best available technology” for cooling water which in this case would mean closed-cycle cooling rather than once-through cooling. With closed-cycle cooling the combined Hudson River water withdrawals from Indian Point and Bowline would drop from approximately 2650 mgd to 53 mgd.

 
Impingement and entrainment

In the UWNY pre-Draft Environmental Impact Statement, chapter 2 and chapter 9. United Water of New York identified approaches it currently plans to minimize adverse impacts associated with water intake and reports some analyses to assess resulting impacts.

United Water New York proposes to withdraw brackish water from the Hudson River using EPA’s best technology available, specifically using a wedge-wire screen incorporating design parameters that it states have been found to virtually eliminate impingement and reduce entrainment, i.e. , wedge-wire screens with through-screen velocities of 0.5 feet/second, an approach velocity of less than 0.25 fps and 2 mm spacing of wire mesh screening.

United Water NY analyzed the impacts of entrainment in three ways.

  1. Equivalent losses: information on life stage specific natural mortality rates and durations was used to convert the entrainment losses of early life stages into equivalent losses of one-year-old fish for each target species. (Appendix 9.1)
  2. Conditional mortality rate: potential losses due to entrainment are expressed as a fraction of the species population in the entire reach of the tidal Hudson in the fall of the first year of life. (Appendix 9.1)
  3. An alternative methodology was also used to compare with the first two: actual entrainment estimates from the Bowline Generating Plant (1981-1987) were scaled down from ~800 mgd to a 10 mgd withdrawal rate. (Appendix 9.2)
 

Based on their analyses, United Water NY asserts that water intake from the operation of a desalination plan would not have significant adverse impacts on brackish phytoplankton species and zooplankton populations in Haverstraw Bay, but notes that plant operation do have the potential to result in long-term adverse impacts to fish and macroinvertebrates in the Hudson River.

Environmental groups in the region who have expressed concerns about the impact of a desalination plant on aquatic life in the Hudson River point to an April 2008 Pisces Conservation Ltd assessment of status and trends in Hudson River fish populations based on the 2005 class year report. Their conclusion was that “All the evidence points to the Hudson estuary ecosystem presently being in a state of change, with declining stability. Neither the ecosystem as a whole, nor many of the individual constituent species’ populations is in a healthy state. ”

These discussions about potential biological impacts on fisheries and other ecosystem parameters were derived from gross simplifications about productivity averaged over the entire surface area and volume of the tidal Hudson River, which can be defined by the distance between mile point zero at the southern tip of Manhattan (Battery) inland to the dam at Troy, NY, a total distance of more than 150 miles.

 

However, fisheries biology and other ecosystem elements can be highly dependent upon very local features of the natural and human environment. Since none of this complexity has been incorporated in the analyses to date the possible impacts of a particular perturbation such as a desalination plant on an estuarine system could be much greater than or less than a simplistic “average” calculation might imply.

The area of Haverstraw Bay in which a desalination plant would be constructed is within a relatively short distance of several very large electricity generation stations, both nuclear and fossil fuel in terms of fuel sources. An additional perturbation from a potable water desalination plant might be relatively minor in the face of very large withdrawals of cooling water already occurring. However, the impact of additional water withdrawals might also be quite non linear with respect to average water withdrawal rates and result in significant ecological impacts, well in excess of what scaling based on water withdrawal rates might indicate. Research on salt movements in the lower Hudson provides some details on the complexity and dynamics of Haverstraw Bay.

 

 
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The reach of the Hudson estuary from the northern portion of Haverstraw Bay into the Hudson Highlands includes one of the most dramatic changes in estuarine topography in the entire tidal Hudson River. The relatively shallow, wide reach of Haverstraw Bay changes to the narrow, very deep reach of the Hudson Highlands. When saline water re-intrudes the Hudson following an episode of very high freshwater discharge, such as typical spring snow-melt runoff or the period of a few weeks to months following a major rainfall episode, such as that associated with a hurricane or tropical storm, the pattern of salinity intrusion is quite distinctive and potentially important to the ecosystem of the Hudson (Simpson et al, 1974). Salt-wedge high salinity water moves rapidly upstream in the navigation channel, overlain by fresh or very low salinity water. When the saline water in the shallow expanse of Haverstraw Bay encounters the very deep channel of the Hudson Highlands, the salt wedge inland transport is strongly inhibited by mixing of saline water upward through the water column. The net effect is to cause the rapid re-intrusion of saline water to be greatly slowed down as the result of the loss of strong density-driven circulation. The zone over which this change occurs represents a relatively small fraction of the entire reach of the tidal Hudson. This region at the boundary of the shallow expanse of Haverstraw Bay and the Tappan Zee adjacent to the Hudson Highlands could play a critical ecological role for some of the Hudson biota, including fisheries. The “accident” of the coincidence of this zone with some of the largest withdrawals of water from the Hudson estuary for power plant cooling water has not been carefully examined, despite the long history of public conflict over the best approach to reconciling the need for electric power generation with ecological considerations for this important estuary. (Simpson, et al 1974)

 
 
Environmental impacts associated with disposal of concentrate remaining after reverse osmosis is used for desalination

Briefly, the process of reverse osmosis would take the brackish (mixture of sea water and fresh water) Hudson River water and remove the salt, producing fresh water for drinking. The waste product of this process is known as concentrate: a relatively small volume of water, which is much saltier than the original brackish water.

In its 9/08 pre- DEIS United Water NY reports that the reverse osmosis (RO) concentrate produced would have concentrations of total dissolved solids (TDS) and chloride (Cl) that are 6-7x higher than the Hudson River water withdrawn. To minimize the potential for adverse impacts to the aquatic resources United Water plans to discharge the concentrate into the effluent of the Rockland County Sewage Treatment Plant (STP) with the combined effluent and RO concentrate discharged together into the Hudson River under the STP’s SPDES permit. The desalination plant is projected to produce 1.32 mgd of RO concentrate with a salinity of 20 ppt at full build-out By 2015 United Water New York expects the STP to generate 8 mgd of effluent according. The salinity of the effluent concentrate combination at that point is anticipated to be about 3.4 ppt. While United Water states that this is well within the range of salinities in Haverstraw Bay, they note that under some conditions it would exceed salinity in the Hudson River at the point of discharge from the diffuser.

 
Greenhouse gas emissions associated with electrical energy consumption

The pre-DEIS estimates total emissions over the 50-year lifetime of the project. 89% of the total lifetime emissions associated with the project would come from generation of the electricity consumed during plant operation.

Desalination is an energy-intensive process. A comparison of the energy requirements for different water sources in California indicates seawater reverse osmosis as using approximately 10x as much energy as surface water treatment (Cohen et al, 2004, cited in NRC, 2008). The proposed Haverstraw RO desalination plant would use less energy than a seawater RO plant because the water withdrawn is less saline than seawater

The United Water NY Draft EIS includes a chapter on global climate change which details projected electricity use for the desal plant and figures from other US water utilities for comparison. The proposed Haverstraw Project is anticipated to use 4,427 kWh/million gallons of potable water produced at the full build-out (7.5 mgd) and 6,520 kWh/million gallons of potable water produced in Phase 1 (2.5 mgd). For comparison United Water NY offers a study of 137 water utilities across the US ranked by energy usage. The 10 highest scoring utilities (high energy efficiency = low energy consumption per unit of potable water volume) averaged 324 kWh/million gallons and the 10 lowest scoring utilities averaged 2,360 kWh/million gallons.

 
 
 
 

Desalination: Swansea, MA

Swansea, Massachusetts (located ~12 miles from Providence, RI) broke ground for a desalination plant in November 2007. This plant will take water from the saline Palmer River (just as any proposed plant in Rockland County would take water from the saline Hudson River). This project is in response to the drought of 1999-2000. Mandatory water use restrictions were imposed in 1995 and a water main extension moratorium enacted in 1999.

Massachusetts set a residential water use standard of 65 gallons per capita per day for river basins that experience medium to high levels of stress. As a result of demand management and conservation measures instituted, Swansea's residential water use was 46.5 gpcd during the period the desalination option was selected.

Desalination was determined to be the most cost-effective technology for this municipality. The plant will process up to 2 million gallons per day (MGD) by reverse osmosis, a technology that removes salts and other dissolved components and 1 MGD of groundwater will be treated with very fine filtration. This desalination facility is anticipated to activate in 2010 and will expand water production to the Swansea Water District's customers when demand is the highest (summer).

Click here for additional discussion of alternatives considered.
 
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At the outset of the evaluation process groundwater supplies depleted and no other freshwater sources were identified. The choice among brackish and saline sources was based on maintenance requirements and fisheries impacts. The site chosen was the only coastal source with no future maintenance dredging requirements and no significant winter flounder spawning grounds.  In addition, other sites had higher ambient salinity levels and shallower water depth that would expose more marine life to impingement and entrainment.

In evaluating its options, the Swansea Water District evaluated the purchase of water from Fall River and a hookup to Aquaria Water LLC's 5 MGD Taunton River desalination facility in Dighton, Massachusetts.  The Taunton River facility is to contractually furnish desalinated water to the City of Brockton, Massachusetts with minimum amounts purchased by Brockton over a twenty-year term. Between the necessity of successfully arranging inter-municipal transmission through adjoining water distribution networks, and Brockton's contractual pricing with Aquaria, Swansea's overall estimated desalination production cost of $3.35/1000 gal was determined to be more cost-effective.


The Palmer River, the raw water source for the Swansea desalination facility, is a brackish to saline estuary with total dissolved solids (TDS) ranging from 1,000 mg/l to 22,000 mg/l and with shad spawning serving as an important local fishery source. Producing 1.3 MGD of desalinated water is based on treating the upper end of this range of TDS; when the Palmer River's TDS is 14,000 mg/l, the plant can recover 50% of the raw water intake to produce 2 MGD of potable water. In comparison, Hudson River, TDS levels approach 6,000 mg/l near the proposed Haverstraw intake site during the summer months of lower flows.

Construction of the Swansea desalination facility was estimated to cost $15.097 million with actual costs coming in at $14.5 million. The cost to produce potable water from the Swansea desalination facility is estimated to be $3.25/1,000 gal (in 2005$). In its January 15, 2007 submission to the New York Public Service Commission, United Water estimated capital costs for full buildout of the proposed 7.5 MGD desalination facility, including improvements to its distribution system, to be $98 million (in 2006 dollars). In the same submission to New York's PSC, United Water estimated operating costs to be $1.79/1,000 gal (in 2006 dollars).

Both the Swansea and proposed Haverstraw desalination facilities draw raw water from estuaries and use the microfiltration/reverse osmosis (MF/RO) process to obtain, produce and deliver finished water to their customers. The Swansea facility is currently in pilot stage with 4-season life cycle testing underway. The proposed Haverstraw desalination plant will include a pilot study of its proposed treatment process.

For details of the desalination plant process at Swansea compared to the proposed Haverstraw plant, click here.

The environmental impact analysis identified the impact on fisheries as the most serious issue with respect to the Swansea plant. Intake velocities and intake screen design were selected to mitigate fisheries impacts and concentrate salinity vs. intake volume tradeoffs were analyzed and the least impact alternative chosen.

 

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