Three Pressure Retarded Osmosis (Pro) Processes

THREE PRESSURE RETARDED OSMOSIS (PRO) PROCESSES

Authors: Boris Liberman, Gal Greenberg, Vitaly Levitin, Tal Oz-Ari, Udi Tirosh

Presenter: Dr. Boris Liberman CTO, VP Membrane Technology – IDE Technologies Ltd. – Israel [email protected]

Abstract Pressure retarded osmosis (PRO) can be implemented on a number of water types, using different technologies and achieving various power outcomes. This paper presents the three most practical options:

 Option 1 - Seawater with river water, driving force 25 bar, power output 5-10 watt/m2

 Option 2 - SWRO brine with wastewater, driving force 50 bar, power output 10-20 watt/m2

 Option 3 - Dead Sea or salt lake with river water, driving force 250 bar, power output 50-100 watt/m2 Each of the above options requires a different PRO technology.

 Option 1 necessitates movement of huge water volumes with extremely low power losses. All water movement must be at seawater level. The pressure exchanger consumes 15 times less power than it does in RO technology. Pretreatment CAPEX and OPEX expenses are considerably less than those currently implemented by RO technology, while water quality has to be as good as that required from RO.

 Option 2 is the most economical and ready to use. This option uses already filtrated and pressurized brine from an SWRO plant. The main obstacle to the implementation of this option is finding a wastewater source with no cost.

 Option 3 implements natural exotic resources such as Dead Sea water with extremely high osmotic pressures. The PRO technology in this option requires membranes and pressure exchangers that are able to operate at extremely high pressures.

The paper describes these three options in detail.

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I. INTRODUCTION

Saline water has vast potential energy in the form of osmotic pressure [1]. The challenge in the coming years is to determine the best technique for recovering the osmotic pressure as mechanical or electrical power [2]. Osmotic pressure is unique because it can only be achieved in contact with lower osmotic pressure water, using semi-permeable membranes dividing the water with different salinities.

Implementing all the well-known processes in osmotic power generation will possibly be less cost effective than other methods of green power generation, which must implement green methods. It is illogical to generate green power and discharge the harsh chemicals in the environment. The technology associated with osmotic power generation has to be more efficient and less expensive than conventional RO, both challenging requirements.

Osmotic power can be recovered from a large range of saline water that exists in nature or that is a result of industrial processes. The types of saline water can be divided into three groups by the type of technology used to achieve their potential energy:

 Seawater with river water, driving force 25 bar, power output 5-10 watt/m2

 SWRO brine with wastewater, driving force 50 bar, power output 10-20 watt/m2

 Dead Sea or Salt Lake with river water, driving force 250 bar, power output 50-100 watt/m2

II. OPTIONS IN DETAIL

2.1 Seawater with River Water

The most practical process for recovering osmotic power is seawater with river water [3]. While seawater is the most common type of saline water, the limiting factor is river water. The osmotic pressure of seawater in regions where river water is present is usually approximately 26 bar. Not all of the 260 m head potential can be converted to useful power. Each water molecule that enters the saline water diminishes the osmotic pressure, and if the osmotic pressure recovery is not implemented correctly, power losses can be significant enough to make this process cost-prohibitive.

Osmotic power generation uses the same conventional sub processes and equipment as the RO desalination industry: intake, pretreatment, energy recovery devices and semi-permeable membranes.

Implementing these well-known processes in osmotic power generation (as they currently are) will result in lower cost effectiveness in comparison with other methods of green power generation. Osmotic power is green energy that must use green methods, and it is counter-intuitive to generate green power and then discharge the harsh chemicals into the environment.

Osmotic power generation faces two challenging requirements – it has to be more efficient and less expensive than conventional RO.

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Seawater-river water osmotic power generation technology is associated with movement of significant water volumes. It requires a minimal pressure drop in all pipes and valves and prevents lifting water above seawater level.

The PRO pretreatment must be as efficient as the conventional RO pretreatment process. It is sometimes thought that seawater pretreatment for the PRO process can be less demanding because seawater passes by, but does not penetrate, the membranes. Hydraulically, this is correct, but biologically it is not. Seawater is a highly populated medium; each cubic centimeter may contain thousands of algae, bacteria and viruses. The seawater ecological system is in a state of starvation (a deficit of nutrients), but oxygen is present in super saturated quantities.

As buds in the spring, the seawater is ready to burst into bacteria that develop in just a few hours when nutrients become available. Bacteria do not consider live algae as nutrients. They are not able to kill healthy algae and consume their nutrients, but can consume nutrients from mechanically damaged algae. Any straining activity, seriously damages algae and releases huge amounts of nutrients into the water, releasing fast bio-fouling mechanisms.

Media filtration is significantly gentler and does not damage the algae. It increases the starvation state of seawater by consuming available nutrients that are less ready to consume, as well as oxygen, which is the second largest risk factor for bio-fouling after nutrients.

Standard media filtration with chemical coagulation and flocculation is a less suitable option for several reasons. First, bacteria consume nutrients less effectively when coagulation and flocculation are aided by chemicals. Second, chemical coagulation and flocculation require additional processing to enable discharge of the media backwash water to the sea. Solid waste handling facilities are expensive and require human technical intervention. IDE has developed the chemical-free IDE PROGREEN™ technology, which has the most suitable pretreatment solution for seawater-river water pretreatment for osmotic power generation.

The seawater-river water osmotic power generation process requires a large energy recovery system (ERS) suitable for high flow rates. Current energy recovery systems, including ERI, DWEER and other ERS, are able to provide constant high pressure flow rates from each energy recovery subsystem. Seawater-river water osmotic power generation does not require this uninterrupted flow, which consumes unnecessary power. IDE’s pressure center approach has several ERS units working together, with the pressure center as a whole, providing the uninterrupted high pressure flow. For the PRO process, it is not necessary for each subsystem to provide uninterrupted flow rates, enabling reduction of the ERS pressure drop by a factor of more than 15.

Seawater-river water osmotic power generation should be designed with a pressure center. It is not economical to implement a single train approach where each bank of PRO membranes has its own ERS and turbine with generator. The single PRO train approach is prohibitively costly for both CAPEX and OPEX.

The main obstacle to a cost effective seawater-river water osmotic power generation plants is the PRO membrane. The main difference between RO and PRO membranes is the direction of permeate “A” flow and salt “B” flow. In the RO process, permeate “A” and salt “B” flows are concurrent; they move

The International Desalination Association (IDA) World Congress on Desalination and Water Reuse REF: IDAWC/TIAN13-422 -3- in the same direction, allowing RO permeate to have constant salinity and a single outlet. Permeate and salt move together to one common, central permeate collection tube. The RO spiral membrane industry is based on this approach.

The permeate spacer, usually TRICO, is constructed so that permeate flow with relatively low-pressure losses is able to reach the central permeate tube from any location in the membrane. Permeate spacer allows equal permeate flow distribution on the all membrane area under significant pressure from the seawater side.

In addition to semi-permeability, the membrane has to be strong enough to tolerate the difference in gauge pressures between the sea and river water sides. A support layer increases the durability.

In the PRO process, the river water “A” flow from the membrane’s river water side to the seawater side and salt “B” flow from the seawater side to river side are countercurrent, meaning that salt is concentrated on the membrane’s river water side. Without bleeding out part of river water, the osmotic pressure will equalize on both sides of the membrane after the PRO process runs for a few minutes. The river “A” flow into the seawater side will be stopped.

Part of the river water flow must be discharged (bleed out) to allow uninterrupted discharge of the salt “B” flow to avoid salt concentration on the river flow side. The continuous discharge requires:

 The PRO membrane must have inlet and outlet on the river water side. The standard spiral membrane has single outlet.

 The permeate spacer has to be able to allow low pressure drop movement not only for river water inlet flow as well as bleeding outlet flow.

 The structure of the membrane support layer (S value) has to be sufficient to diminish concentration polarization and allow easy salt evacuation by bleeding river flow.

These requirements are quite challenging for membrane manufacturers [3]. IDE has already taken steps to solve this problem and has developed PRO membrane elements (patent pending) with central division of permeate tube that allows river water distribution between several PRO elements in PRO pressure vessels, and collection of bleeding from all elements.

The membrane element shown in Figure 1 allows parallel river water entrance in several spiral membranes located in a pressure vessel, as well as consecutive seawater movement throw all membrane elements.

A partition wall located along the length of the central tube divides it into separate inlet and outlet channels. Inlet holes are located on the left half of the central tube and are connected to the inlet channel. Outlet holes are located on the right half of the central tube and are connected to the outlet channel.

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The river water spacers are located around the central tube and separate the river water inlet and outlet.

Figure 1: PRO Membrane Element

2.2 SWRO Brine with Wastewater

The second type of osmotic power generation is SWRO brine with wastewater. In addition to being more cost effective, this has several benefits:

 Power density is expected to be approximately 10-12 W/m2.

 SWRO brine does not require pretreatment.

Figure 2 and Figure 3 show the flow scheme of a standard RO plant and an RO plant equipped with PRO block, respectively. PG 62 is pressure gauge 62 bar, PO 26 is pressure osmotic 26 bar, FL 1000 means flow of 1000 m3/hr.

Wastewater at a rate of 550 m3/hr is pumped to the PRO blocks at the inlet and outlet of the SWRO brine at 20 and 40 bar respectively. Flow at a rate of 50 m3/hr bleeds out from the PRO block.

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PG 59 RO PO 26 FL 500 PERM

HP PUMP PG 57 PO 53 FL 500 PG 59 FL 500 PO 26 C-PUMP

ERS 1

PG 2 PO 30 FL 1000 BR

Figure 2: Standard RO Process

PG 59 RO PO 26 FL 5 PERM

HP PUMP PG 57 PG 20 PO 53 PO 0.1 FL 500 FL 275 PG 59 WW1 PO 26 FL 995 C-PUMP PRO

PG 56 ERS 2 PO 26 WW2 FL 1000 PG 40 PO 0.1 FL 275

PG 2 PO 26 PO 1 FL 1000 FL 50 PO 26 BR FL 1000

Figure 3: RO PRO Process

Figure 4 and Figure 5 show the balance of forces applied at the beginning and end of the PRO process respectively. The net driving force (NDF) at the beginning of the PRO process is 16 bar and 10 bar at the end of the PRO process.

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Figure 4: Balance of Forces - Beginning of PRO

Figure 5: Balance of Forces - End of PRO With almost the entire volume of high pressure seawater coming from ERS 2, it is possible to reduce the flow from the high pressure pump one hundredfold. The high pressure pump duty compensates for losses of pressurized brine in the ERS and at the start of the RO system. Wastewater is pumped into the PRO block. Converting osmotic pressure to useful power is very efficient, because it is used as pumping pressure for the incoming seawater. The conversion of osmotic power by the ERS has a recovery of 97%. Standard energy recovery systems can be used to convert the power. Standard RO power consumption is higher than RO and PRO power consumption (see Table 1 and Table 2). If a standard RO process has 3.55% TDS, 30c, 50% recovery and 15.3 lmh flux, the power consumption will be 2.17 kWh/m3. An RO plant equipped with a similarly-sized PRO block will require 1.5 kWh/m3, providing savings of 0.67 kWh/m3. Additional saving will come from a smaller high pressure pump, MCC, cables, transformers, and control equipment. Additional expenses associated with wastewater pumping will be 20 bar for RO and 40 bar for RO and PRO. Table 1: Standard RO

Pump Motor Flow Section Pressure [bar] Efficiency Efficiency kW [m3/hour] [%] [%] Intake 1000 2 85 95 69 HP 500 57 85 95 980

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Pump Motor Flow Section Pressure [bar] Efficiency Efficiency kW [m3/hour] [%] [%] Circulation 500 2 85 95 34 Power Consumption 2.17 kWh/m3 1,084

Table 2: RO and PRO

Pump Motor Flow Section Pressure [bar] Efficiency Efficiency kW [m3/hour] [%] [%] Intake 1000 2 85 95 69 HP 5 57 85 95 10 Circulation 995 3 85 95 103 WW1 275 20 85 95 189 WW2 275 40 85 95 378 Power Consumption 1.50 kWh/m3 749

Practical implementation of osmotic power recovery can be different in new and existing RO plants. Existing SWRO plants can be refurbished for osmotic power recovery fully, or partially.

Full-refurbishment means that all SWRO brine flow will pass through the PRO membrane block and an additional ERS subsystem will be added to the existing ERS. An additional circulation pump will be required.

The benefit of osmotic power generation will be expressed as increased permeate production without increasing the high-pressure pump capacity or power consumption. Additional membrane elements have to be installed and/or replaced for high permeate production.

Partial-refurbishment can be achieved by adding PRO membranes to part of the SWRO brine stream.

Filtered water can be used as an alternative to wastewater. The pressure difference between the SWRO brine and the seawater can be about 30 bar. This can be more attractive if the SWRO plant is adjacent to a power station, in which case the power station cooling water can be directed for osmotic power recovery.

The commercialization of SWRO brine with seawater will mature at later stage when a lower pressure will be required to mechanically pass the membrane skin. It has already been announced that this type of semipermeable carbon membranes are in the development stage.

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2.3 Dead Sea or Salt Lake Water with River Water

The third type of PRO power generation plant is based on Dead Sea or Salt Lake water with river water. This approach uses exotic sources of saline water. Sometimes industrial wastewater has extremely high osmotic pressure and has to be diluted before it can be discharged environmentally to a river or the sea. In this case, both streams are present and must be combined.

The difference in osmotic pressure between the Dead Sea and river water is extremely high, about 300 bar. The power output is expected to be 50-100 W/m2 of membrane area, if extremely high pressure membranes and ERS are used. The development of this type of equipment will take time. Presently, power can be generated at a lower pressure of about 82 bar, with a river water inlet at the continuation of a Dead Sea stream, as shown in Figure 6.

RWin RWout RWin RWout RWin RWout

PRO-1 PRO-2 PRO-3 T

C-PUMP

ERS

DSin DSout

Figure 6: High Osmotic Pressure PRO Process

Figure 6 shows a scheme where the usual pressure for a seawater RO application is about 82 bar (1200 psi). River water enters the PRO blocks at three stages, until all osmotic power is recovered.

III. CLOSING REMARKS

The semipermeable membrane technology is nearing the stage of development when, in addition to widely implemented RO technology and first attempts at the commercial implementation of Forward Osmosis, Pressure Retarded Osmosis is coming of age and will shortly be implemented in commercial plants.

It can be expected that the options of seawater with river water, and SWRO brine with wastewater, will be in commercial use within the next two to three years.

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IV. REFERENCES

1. Pressure Retarded Osmosis: From the vision of Sidney Loeb to the first experimental installations. Andrea Achilli, Amy E. Childress. Desalination (2010) doi:10.1016

2. Global Challenges in Energy and Water Supply: The Promise of Engineered Osmosis. Robert L. McGinnis, Menachem Elimelech. Vol. 42 No. 23, 2008 Environmental Science & Technology.

3. Forward Osmosis: Principles, Applications, and recent developments. Tzahi Y. Cath, Amy Childress, Menachem Elimelech. Science Direct Journal of Membrane Science 281 (2006) 70-87.

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