Energy Recovery Devices in Seawater Reverse Osmosis Desalination Plants with Emphasis on Efficiency and Economical Analysis of Isobaric versus Centrifugal Devices

With huge demands for potable water in regions lacking fresh water sources such as surface or ground water, various potential technologies have been explored for eliminating water shortage. Seawater emerged as a potential source and a major lifeline for such water-deprived areas. The development of seawater reverse osmosis (SWRO) technology proved to be a groundbreaking innovation, making it easier to extract pure water from seawater. Ever since its inception, SWRO technology has taken many leaps towards the development of energy efficient and high yielding systems. The reduction in energy consumption of desalination plants that were based on the SWRO technology emerged as a major driver of the technology revolution in this field. The improvement of membrane life and salt rejection, increase in recovery, and decrease in energy consumption has been the primary criteria for sifting through available technologies for incorporation in desalination plants. Many developments have, ever since, occurred in this direction. The membrane life has multiplied and the Total Dissolved Solids in the product are now as low as 100 mg/L. In addition, recoveries of 40-50% have been achieved. By recycling energy, many SWRO desalination plants have significantly lowered their total energy consumption. With the help of energy recovery devices (ERDs), it is now possible to decrease power consumption and increase efficiency of the seawater reverse osmosis desalination plant. The first large-scale municipal SWRO plant was installed in 1980 in Jeddah, Saudi Arabia. This plant consumed 8 kilowatt-hours energy per cubic meter of water produced. This consumed energy was less than half of what was usually consumed by other conventional distillation processes. However, the SWRO desalination technology has one disadvantage. The seawater, which is to be desalinated, is pressurized with the help of high-pressure pumps. A large amount of energy is consumed during this process. Once the desalination is complete, the remaining reject water has to be eliminated as waste. Since the brine reject produced in this process has a high pressure, simply dumping it back into the sea is a waste of energy. This pressure can be reused and thus, the energy could be recycled. This idea led to the innovation of energy recovery devices (ERDs) that prevent the wastage of energy in the SWRO process. The hydraulic energy in the highly pressurized reject brine can be re-used with the help of ERDs, and energy consumption can thus be reduced by significant high amounts. The development of ERDs helped in the set-up and operation of large-scale SWRO plants, and facilitated the economic viability of the desalination process. The energy requirements of conventional SWRO plants are presently as low as 1.6 kWh/m3, making the process more cost effective and energy efficient than other technologies. About 80% of the total cost of desalinated water is due to energy consumption and capital amortization. The remaining costs are associated with other maintenance operations such as replacement of membranes and other components, labor associated costs etc. Since energy consumption is the main determinant of final costs of the product, increasing energy efficiency of the plants is of primary concern. This paper deals with various energy recovery devices such as the Francis turbine, Pelton wheel, turbocharger, Recuperator, DWEER and Pressure Exchanger, used in SWRO desalination plants along with case studies associated with each of these. Special focus is given to the energy efficiency and costs associated with these devices. A brief discussion of the devices that are currently under investigation is also provided in the conclusion. An analysis of isobaric versus centrifugal devices is also conducted in this work. A comparison between the energy recovery turbine (ERT) manufactured by Pump Engineering Inc. (PEI) and the pressure exchanger (PX) manufactured by Energy Recovery Inc. (ERI) energy recovery systems is performed using collected data from provided water analyses and respective manufacturers\u27 device specifications. The different configurations used for this comparison were applied to the Jeddah SWRO desalination plant for a total productivity of 240,000 m³/day. As a result of this analysis, the specific energy consumption of the ERT and PX configurations were 2.66 kWh/m3 and 2.50 kWh/m3 respectively. Analysis shows however that although the PX configuration achieved the best specific energy consumption, the ERT was favored over it due to its lower capital and maintenance costs. Therefore, the final conclusion of this work, in this special case, is that the ERT configuration is more economical than the PX configuration