LARGE-SCALE DESALINATION

Water & Wastes Digest | By Gregory Shtelman | May 08, 2017


Israel desalination plant provides potable water to 1.5 million people

Designing successful mega-sized seawater desalination plants is an art that is constantly changing according to new needs and requirements. Companies cannot remain entrenched in old technology; they must constantly be innovative, refining and improving the technology. At the same time, low water costs, high plant availability and reliability, and optimal operation and maintenance processes are of paramount importance.

One example of setting new industry benchmarks in desalination technology, capacity and water cost is the Sorek 150-million-cu-meter-per-year seawater desalination plant, located 2.2 km from the Mediterranean Sea, approximately 15 km south of Tel Aviv, Israel. It is a joint venture of IDE Technologies Ltd. (51%) and Hutchison Water International Holdings Pte. Ltd. (49%). The joint venture, known as Sorek Desalination Ltd. (SDL), constructed the project under a build, operate, transfer model, following SDL’s selection by the government of Israel. The plant went online in mid 2013 and today provides clean, potable water for more than 1.5 million people.

Technology Talks

The Sorek plant has two 75-million-cu-meter-per-year plants able to operate separately from, and independently of, one another. Most subsystems are double (one for each 75-million-cu-meter-per-year plant), with the exception of the intake system and the independent power plant. These systems are common for the entire 150-million-cu-meter-per-year plant, but are designed with the required redundancy to serve each plant separately.

The technological innovations that have the greatest impact on plant capacity and operating costs include:

Large-diameter membrane elements. Emphasis has been placed on the optimization of the reverse osmosis (RO) banks’ configuration by utilizing a patented design that includes a minimal number of independent trains fed by both feed pumping centers. The design uses 16-in. membrane elements installed in vertical pressure vessels. The behavior of the 16-in. membrane element was confirmed by the installation and successful continuous operation of vertical 16-in. pressure vessels in pilot installations, as identical to that of the 8-in. membrane, resulting in identical salt rejection performance and a correspondingly four-times-larger flow rate at the same feed pressure and operation conditions.

This approach allows a significant reduction in plant footprint (an important consideration in a small country such as Israel), shorter high-pressure pipe headers, an improved membrane loading method, and a significant reduction in membrane handling for maintenance purposes. In addition, due to the larger volumes of feedwater, there is a lower tendency for membrane fouling and polarization in the second stage. If required, the same configuration can produce larger quantities of permeate by operating at the high-production regime for longer periods and increasing the flux through the membrane elements within the limits of manufacturer recommendations.

Pipe jacking methodology. With ever-stricter environmental requirements to be met, emphasis was placed on minimizing the plant environmental impact (air, land and marine). To this end, the feed and brine pipelines were installed using the pipe jacking method, an environmentally conscious tunneling method for the installation of underground pipelines. This method was used for in total for 7,500 meters from the shore, and for the all-onshore pipelines, while use of the less environmentally desirable cut-and-cover method was limited to applications required to complete the installation. The advantages and benefits of laying pipe using the pipe jacking method include minimal surface (seabed) disruption, lower emissions, minimal impact on existing infrastructures, no impact on navigation, and longer overall lifetime of the pipeline.

Self-generating energy supply system. With the goal of minimizing the cost of the project’s electrical power while assuring its reliability, Sorek deployed two redundant energy sources: a self-generating energy supply system (IPP) is built on site to serve as the plant’s primary source of energy. In addition, a 161-KV overhead line from the Israel Electric Co. grid operates mainly during off-peak times. The self-generating energy supply system is fueled by natural gas, resulting in lower electricity costs that contribute to a lower overall water price.

Pressure center concept. The pressure center concept was adopted for Sorek due to its economy of scale, simplification of erection, and operational flexibility, reliability and availability.

The pressure center includes the RO membrane segment and the feed pumping center. Water to be desalinated is supplied to the RO section by the feed pumping center, which comprises both the high-pressure (HP) pumps and the advanced energy recovery systems (ERS). The HP pumping and energy recovery centers, which supply the HP feed to the RO banks via common feed lines, can be optimized by the selection of a minimal number of large HP pumps and ERS units working at the highest efficiency rates and best operational conditions.

An advantage of the pressure center design is that it allows the plant to vary production rates during the day, resulting in the high level of flexibility needed to maximize efficiency. Since it allows for increasing and decreasing the feed pressure to the RO trains, all RO trains remain operational during stages of decreased production, thereby decreasing system recovery without increasing the total feed to the plant. This feature reflects the pressure center concept’s ability to produce at low recovery yields, resulting in lower osmotic pressures and furthermore producing at lower permeate fluxes through the entire available membrane area.

Operational Success

Successful commissioning, operation and maintenance also are central to the plant’s success.

Despite the project’s magnitude, Sorek’s commissioning was completed in six months. Among the challenges was commissioning the 16-in. vertical membrane arrangement, which was a world first for a mega-plant. Technical setbacks were quickly resolved, and the plant was commissioned on time at a low chemical consumption and energy cost.

The Sorek plant operates according to a variable electricity tariff—production varies by the time of day and year, according to the cost of electricity. Production under many constraints is not a simple task, taking the varying costs of electricity and the annual water demand of the Israeli Water Desalination Authority into account.

Energy consumption is one of the most important indicators of an efficient plant operation. In 2014, the plant performance was impressive in this aspect, achieving an average electric consumption of less than the contracted 3.414-KWh-per-sq-meter value. These energy consumption savings are the direct result of optimized plant design and proper operation. It is worth noting that these results were achieved with a relatively low rate of failures and fewer issues at this stage of the project than other plants of a similar size.

Plant design cannot remain stagnant, it must adapt as needs expand. The Sorek plant is a notable example of this innovation, as it utilizes a number of advanced technologies to decrease energy requirements and increase overall efficiency. These have enabled the world’s largest seawater RO desalination plant to achieve low costs for high-quality desalinated water.

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