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. The International Desalination Association World Congress on Desalination and Water Reuse 2013 / Tianjin, China REF: IDAWC/TIAN13-422 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. The International Desalination Association (IDA) World Congress on Desalination and Water Reuse REF: IDAWC/TIAN13-422 -2- 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].