Brackish Water Desalination: Energy, Costs and Energy Recovery Devices

By Satish Shaligram

Energy recovery devices are in nearly all seawater reverse osmosis (SWRO) desalination plants to recover pressure from the membrane reject stream and return it to the process. Because of the high pressures and low membrane permeate recovery rates in these systems, the membrane reject stream contains a considerable amount of energy, thus, energy recovery devices in seawater RO are readily justified on the basis of operating cost savings.

Figure 1. Typical Two-stage System with Turbocharger for Interstage Boost

Typically, the application of energy recovery is much less common in brackish water RO systems, primarily because of the relatively low feed pressure and low flow rate of the membrane reject stream. The fear is energy recovery devices can also potentially limit the flexibility of a brackish RO process because of efficiency losses or flow-rate constraints encountered during off-peak operation.

Recently, low-cost isobaric energy recovery devices have moved to the forefront for brackish RO applications. These devices provide greater energy-savings payback and greater operational flexibility than was previously achievable. They also have the potential to reduce the overall capital costs of an installation since they can be less expensive than the high-pressure pump capacity necessary in their absence.

Energy Recovery Technology

Energy Recovery Devices (ERDs) can be installed on or adjacent to an existing RO rack, and will use almost the same amount of space as an Interstage Booster Pump.

The different type of ERDs for brackish applications are mainly centrifugal devices, such as turbochargers, as well as isobaric devices such as the Brackish Water PX™ Pressure Exchangers.

A turbocharger transfers pressure energy from one liquid stream to a second liquid stream. It consists of a pump impeller section and a turbine rotor section. Both pump and turbine sections contain a single stage impeller or rotor. The turbine rotor extracts hydraulic energy from the brine stream and converts it to mechanical energy. The pump impeller converts the mechanical energy produced by the turbine rotor back to pressure energy in the feed stream. Thus, the turbocharger is entirely energized by the brine stream. It has no electrical, external lubrication or pneumatic requirements.

The turbine rotor depressurizes the brine while extracting the energy in the form of high speed rotational torque. The brine, depressurized up to 5 psi (brine exhaust can be any value, even hundreds of psi) is exhausted to the discharge piping.

The turbocharger eliminates the need for a brine control valve, which is a major expense in an RO plant.

The interstage boost application shown in Figure 1 is a particularly advantageous application of a turbocharger. In addition to saving energy, the turbocharger acts to balance flux between the stages. The closer match between the interstage and second stage concentrate flows, as compared to that of the first stage feed and concentrate, which means that the turbine and impeller will operate at a higher overall efficiency. Additionally, the capital cost will be lower because the size of the pump stage is proportionally smaller.

Ideally, a turbocharger for BWRO applications will be custom designed for the specific application. This design will include machining the components to optimize hydraulic performance for the flow and pressure conditions of the application, as well as integrating an auxiliary nozzle to maintain high efficiency during process variations.

The BWRO plant designer would normally design the system considering the warnings that the Membrane Projection Program issues.

Even in cases without any warnings, it is necessary to equilibrate permeate flow between stages i.e. decrease permeate flow from the first stage and increase permeate flow from the last stage. This can be accomplished in one of two design configurations.

One solution is to install a valve on the permeate line from the first stage. By throttling this valve, permeate back pressure will increase, reducing net driving pressure and reducing permeate flux from the first stage. The differential permeate flux is produced from the second stage by operating the RO unit at a higher feed pressure. The other solution is to install a booster pump on the concentrate line between the first and the second stage. The booster pump will increase feed pressure to the second stage resulting in the ability to balance flux between the two stages.

Figure 2. Typical Two-Stage RO System with Interstage Boost and Isobaric ERD

The interstage boost solution also provides an option to use isobaric devices such as the pressure exchanger that will recover energy from the concentrate and reduce the overall power consumption.

The advantage of the permeate throttling design is simplicity of the RO unit and low capital cost. However, this design results in additional power consumption due to hydraulic losses associated with permeate throttling. The interstage pump design requires modification of the interstage manifold and an additional pumping unit. The investment cost is higher than in the first design, but the power consumption is lower.

Conclusion

Energy recovery devices would be economically beneficial in most brackish systems. In addition to retrofit applications, isobaric ERDs can enable a plant to be expanded by the reject ratio without replacing the high pressure feed pump or significantly increasing energy consumption.

In contrast to seawater RO applications, the successful application of ERDs to brackish applications requires detailed analysis of the entire RO system. The energy cost savings and operating improvements will be even greater if all parts of the BWRO system, including membrane and pump designs, are considered in conjunction with ERD selection. In most cases, such effort is rewarded with a good return on investment (ROI) as well as an overall carbon footprint reduction.