Increasing demand for allocated freshwater resources, declining freshwater quality, drought, and the need for a diverse water supply portfolio are among the many reasons that people across the US and the world are looking to the sea as a potential supply. However, in the US, the high cost of desalination has historically hindered interest in seawater as a possible fresh water supply.
Sensitive to the issue of cost as a limitation to realising large-scale implementation of seawater desalination, engineers, scientists, and the manufacturing industry have worked over the last two decades to reduce both the capital and operating cost associated with desalinated water. Little attention was given to energy consumption when seawater desalination was commercialised in the 1970s.
As indicated in Figure 1, energy consumption for the desalination process was approximately 12 kW-hr/m3, or 50% of the total costs for a seawater desalination plant. By 2000, the power consumption rate decreased to approximately 3.7 kW-hr/m3. This was in large part due to several advances in technology that occurred during the 1990s, which include:
- New low-energy reverse osmosis (RO) membranes with improved salt rejection
- High efficiency pumps and motors
- More efficient energy-recovery devices (ERDs)
While these advances continue to occur, the industry’s perception of seawater desalination energy consumption has not changed significantly since 2000. Many experts in the industry still believe that the seawater desalination process requires between 2.6 to 3.7 kW-hr/m3.
As indicated in Figure 2, using these energy requirements, the power required for seawater desalination is significantly higher than other water supply options in Southern California, which is, in part, why large-scale seawater desalination has not become a reality. However, as presented in Figures 2 and 3, based upon the work conducted during this project, using commercially available technologies applied in a manner where design emphasis is placed on energy efficiency and responsibly reducing the overall total water costs, a new paradigm for the costs of seawater desalination is now available. Seawater desalination can now be considered cost competitive with other new water supply options in Southern California.
The Affordable Desalination Collaboration (ADC) is a California non-profit organisation composed of a group of leading companies and agencies in the desalination industry that have agreed to pool their resources and share their expertise in the mission to realise the affordable desalination of seawater.
Using a combination of proven technologies, the ADC has demonstrated that seawater reverse osmosis (SWRO) can be used to produce water at an affordable cost comparable to other supply alternatives. As a result, the ADC is pleased to announce their mission is a success. Desalination is affordable and can provide another cost effective tool to water agencies seeking a diverse water supply portfolio.
The ADC’s demonstration scale SWRO plant completed over six months of testing at the US Navy’s Seawater Desalination Test Facility in Port Hueneme, California, in March of 2006. Three membranes were tested, while varying flux and recovery, to estimate the most affordable operating point. The most affordable operating point was estimated by calculating the net present value for each tested condition, accounting for both capital and operating costs.
Details of the consortium, test rig, test results and other data on the project can be found at the ADC website: www.affordabledesal.com.
Outcome
Conclusions based upon the work conducted by the ADC include:
- The ADC’s results must be taken within the context of the raw water quality conditions tested. These conditions include a lower feed temperature than would typically be seen at a SWRO plant fed warm water from a once through cooling power plant. Therefore, at higher temperature, the membranes, at a flux of 6 gfd, will produce water with higher permeate total dissolved solids (TDS), but with about lower specific energy. Further testing and evaluation is required to determine the impact of temperature.
- Increasing flux (at constant recovery) on the SWRO membranes results in lower concentrations of TDS and boron in the permeate.
- Increasing recovery (at constant flux) results in higher concentrations of TDS and boron in the SWRO permeate.
- Direct contact of brine to SWRO feed water in the PX device resulted in approximately 4-6% increase to the SWRO system feed water TDS. This increase in feed water TDS resulted in approximately 30 psig higher feed pressure (ie. at 50% recovery) to produce the same permeate flow.
- Specific power consumption using the ADC’s SWRO process design was demonstrated to range from 1.80-2.00kW-hr/m3 at the most affordable operating point (i.e., 9 GFD, 50% recovery for the SW30HR-380 and SW30XLE-400i membranes, and 6 GFD, 50% recovery for the SW30HR LE-400i). The lowest SWRO process energy consumption, 1.58kW-hr/m3, was demonstrated using the SW30XLE-400i membrane at 6 GFD, 42.5% recovery.
- The ADC’s design has demonstrated the ability to reduce power consumption by 38 to 40% over industry experts’ perception of power required for SWRO system designs.
- As train size gets larger, the ADC’s power consumption may be difficult to replicate. Careful consideration of pump type, size and energy recovery system pressure centres should be considered to minimise power consumption.
- Data indicates that there is an optimal recovery point with regards to energy consumption for a given membrane array and site conditions.
- Data indicates that flux versus energy consumption is not linear.
- While high recovery consistently resulted in the lowest treatment costs, the impact of flux rate was questionable in some cases.
- A recovery rate of 50% consistently demonstrated the lowest estimated total water costs.
- Based upon the ADC’s cost model, as presented in Figure 2, the cost for seawater desalination in California has been shown to be competitive with other new supply options, with costs ranging from US$772 to US$913/AF (US$0.63 to $0.74/m3).
Recommendations
The data gathered during this study has led to some very promising results. To further validate and improve upon the findings of this study, the authors recommend the following:
- Additional testing at warmer temperatures is recommended to help draw conclusions with regard to the acceptability of each membrane to meet permeate quality standards and the feed pressure (ie. energy) required.
- Pretreatment is a critical aspect of a successful seawater RO process. While media filtration is very capable of meeting the SDI and turbidity standards required for RO, a ‘red tide’ event that occurred early during the study resulted in excessive backwashing frequencies and ultimately placing the study on standby. While the persistence of this event was an apparent anomaly in California, and even those seawater systems treating the Pacific Ocean using membrane pretreatment were challenged to produce enough water, the membrane pretreatment provided a consistent and reliable quality of water, which the ADC’s media filter design could not. As a result, the authors recommend a further study to compare other types of media and advanced filtration designs.
- SWRO system designers should consider public values to issues such as water quality and cost when selecting design conditions such as flux, recovery and membrane type. The community values may require the use of a membrane that rejects more TDS and boron, but requires more energy to produce water. Factors of safety in permeate quality may also be considered. The data presented in this paper indicated that the SW30XLE-400i membrane barely met the California standard for boron at a flux of 6 gfd. A higher flux or use of a different membrane may make sense for some communities.
- The ADC’s test results represent conclusions based upon the performance of new membranes. The concept of the Cumulative Annual Replace-ment Rate (CARR) was used to adjust costs and normalise performance with respect to permeate quality and energy consumption. Long-term testing is required to validate the flux and recovery at the most affordable operating point. In addition, long term testing required to determine how specific power will vary with time and cleaning cycles. Furthermore, industry experience indicates that high flux and high recovery operation results in more frequent chemical cleaning and shorter membrane life. However, when balanced with capital costs on a life cycle basis, incurring these incidental operating costs often proves to be more economical, but more labor intensive to maintain. A longer study is required to help quantify the differences that could not be derived from the ADC’s data due to the short testing duration.
- Additional configurations for the SWRO system should be tested to compare alternate membrane types, energy recovery devices and pumping technologies. Many manufacturers have comparable technologies that are worthy of testing.
- Cost estimates should consider the possible economy of large diameter pressure vessels and membrane elements, which may reduce capital costs by approximately 20%.
- Seek out, test and demonstrate system designs and technologies that can increase the achievable recoveries of SWRO systems.