Keeping the RO Membranes of the Future Continuously Clean. Author: Boris Liberman, Ph.D. VP CTO IDE Technologies Ltd

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1 1 Keeping the RO Membranes of the Future Continuously Clean Author: Boris Liberman, Ph.D. VP CTO IDE Technologies Ltd INTRODUCTION 1 2 The current state of the art RO membrane technology, polyamide TFC, is now 37 years old (having replaced the first RO membrane technology, cellulose acetate, which was used for the previous 19 years). There is broad agreement that a third generation desalination membrane is due to replace polyamide. Currently in development are carbon nanotubes, graphene, aquaporin, and others. The common feature of all these membranes is high flux operation, dozens of times higher than that of the present membranes. This dramatic increase in flux promises a list of technological and commercial benefits. The drawback of the new RO membrane is related to a misbalance between ultrafiltration (UF) pretreatment, which will be similar to that currently in use, and the high flux operation of the new RO membranes. The UF pretreatment membrane allows the passage of small organic molecules that will plug the high flux membranes in a short time. This, in turn, will increase the required feed pressure and negatively affect the expected future desalination technology. The standard clean-in-place (CIP) method is not applicable for this new, high flux RO technology. Currently, the period between CIP procedures is days. This will be reduced by approximately 0 times, proportionally to a 0-time increase in flux. Each CIP procedure requires the stoppage of an RO train, connection to the CIP system, circulating the cleaning solutions a few times, soaking, flushing between solutions, reconnecting to the HP pumping system, and startup. Normally the CIP process requires 6-8 hours of RO train stoppage, i.e. every two days of high flux operation will require a 6-8 hour stoppage for membrane cleaning. This operation regime makes the new high flux membranes not effective. The polyamide TFC membrane, currently on the market, can perform significantly better if the current (standard) RO desalination technology, based on continuous and stable flow, is replaced by the new PFRO desalination technology. The current membranes can operate with a significantly higher recovery, approximately % energy saving, with fewer membranes in operation, and allowing for lower raw water quality. CURRENT BRACKISH AND WASTEWATER RO DESALINATION TECHNOLOGY 30 Although this paper presents an implementation of PFRO in brackish water plants, the technology can also be fully implemented in seawater applications. The staging design of the standard brackish and wastewater RO desalination technology is shown in Figure 1.

2 2 Figure 1: Staging Design of BWRO Train In this staged design, the brine flow continuously diminishes from stage to stage. The pressure drop in each stage is usually approximately 1-2 bar, which results in a total pressure drop per RO train of 4- bar. To overcome these pressure losses and compensate for the increase in osmotic pressure in subsequent stages, a booster pump may be installed, which increases the complexity of the RO train construction and operation. The standard RO design provides the best possible conditions for scaling and fouling formation. The technological and operational limitations of the standard RO design are: Limited recovery Low fluxes in second and third stages More membranes and pressure vessels, required by the low fluxes Increased permeate TDS, caused by the low fluxes of the second and third stages Fouling and scaling caused by the low reject flow velocity at the end of the RO train Sensitivity to raw water quality 1 High differential pressure in each stage Power losses with reject flow Shortcomings of the current energy recovery systems for BWRO, which are not energetically efficient, are costly, and not flexible if a change in recovery is needed Necessity of an inter-stage booster Complicated piping THE NEW PULSE FLOW RO DESALINATION TECHNOLOGY The technology presented in this paper and GB Patent and PCT/IB1/066 allows: Significantly increased concentration of reject brine 2 Diminished power consumption by approximately % Lower requirements for the quality of raw feed water Simplified RO train structure Avoiding the necessity of inter-stage boosters and energy recovery systems Diminished number of membrane and pressure vessels 30 Improved permeate TDS

3 3 The PFRO train has a simple one-stage structure, as shown in Figure 2, where all the pressure vessels operate in parallel. Figure 2: PFRO One-stage Structure The PFRO operates somewhat similarly to membrane ultrafiltration. The PFRO operation regime may include one or several sub-regimes: 1. Pulse flow, dead-end, no residual brine discharge permeate production, followed by a short and intensive brine discharge. 2. Periodic, variable frequency, pulse flow cleaning, conducted synchronously with micropressure strokes. 3. Pulse flow fouling evacuation by high shearing velocity reject flush. 4. Periodically, forward osmotic membrane backwash may be performed. The PFRO desalination technology is based on a pulse flow generator that operates a synchronized sequence for several permeate and reject flow valves. 1 During the dead-end 0% recovery permeate production regime, the stage pressure drop is low. The gauge pressure is practically equal in all the membranes of the pressure vessel. This allows obtaining an almost equal flux from all the membranes of the PFRO train, taking in account the changeable osmotic pressure along the pressure vessel. A smaller inequality of flux in the single stage between the first and the last membrane allows diminishing the total number of membranes in the RO train for the same production rate. In some applications, the PFRO regime may include partial instead of full stoppages of the reject bine flow. HIGH RECOVERY OPERATION 2 In the standard RO train (see Figure 4, left), the brine concentration does not change for a long time. These are perfect conditions for scaling formation. In PFRO operation (see Figure 4, right), the brine concentration changes very fast and reaches very high values for extremely short periods.

4 4 Standard Production Regime PFRO Production Regime Figure 3: Standard vs. PFRO Production Regime Typical Crystallization Process PFRO Production Regime Figure 4: Typical Crystallization Process; Standard RO and PFRO Source: 1 Figure, left, presents the three standard stages of crystallization. Figure, right, shows a graph of the PFRO concentration cycle (grey line). The green line represents the conventional RO stable concentration. The critical super-saturation is the level of supersaturation above which nucleation will occur very fast. The gap between solubility level and critical super-saturation may be significant. The crystallization process consists of two major events, nucleation and crystal growth, which are driven by thermodynamic properties as well as chemical properties. In nucleation, the solute molecules or atoms dispersed in the solvent start to gather into clusters, the microscopic scale (elevating solute concentration in a small region), the clusters need to reach a critical size in order to become stable nuclei, able to attach to the present protocrystal of form a new proto-crystal. Reaching critical size requires a period of time dictated by the dynamics of nucleation. The feature of the PFRO process is providing a brine concentration cycle shorter than the nucleation time. Staying in critical super-saturation concentration for a shorter time than it takes the nucleation dynamic to form proto-crystals, allows reaching an extremely high brine concentration without the risk of scaling and operat the system at extremely high recovery rates.

5 1 One of PFRO benefits is the ability to concentrate the reject stream ( A point) significantly above the standard RO process ( B green line). This is possible because in the PFRO cycle, the concentration rapidly increases during the dead end production step and sharply decreases during the short and intensive brine-flush. The concentration polarization part of the brine stream will never reach the critical super-saturation limit. This short few-second stay in the A point concentration allows reaching an extremely high recovery in the PFRO process, significantly higher than B standard RO may reach. If some fouling particles may deposit on the membrane, the pulse flow membrane cleaning regime will remove them. The PF membrane cleaning is conducted by micro-pressure strokes, possibly followed by high shearing velocity reject brine fouling evacuation flow. This pulse flow membrane cleaning regime may allow using a more fouled feed into the RO membrane than the standard RO allows. Online forward osmosis backwash may be periodically applied in two different forms: draw solution driven, or by equalizing the permeate gauge pressure with the feed gauge pressure. The draw solution driven FO process comprises injecting for a short time a high-osmotic-pressure solution in the suction site of the high pressure pump. The wave of the draw solution moves along the membranes in the pressure vessel, changing the process from RO to FO and back to RO, backwashing membranes and dehydrating bacteria. In the second option the FO process is driven by equalizing the permeate gauge pressure with the feed gauge pressure for a short time, and using the osmotic pressure of the concentration polarization as draw solution for performing the FO process. This second type of FO cleaning takes place immediately on all the membranes in the pressure vessel. It is a short process, and may be performed frequently. POWER SAVING 2 Power consumption in the PFRO process can be understood from Figure 6, which shows the PFRO pumping duty points (1) and (2) versus the standard RO operation duty point (3). The PFRO system does not use any energy recovery system because power is not consumed for reject flow pressurization in dead end 0% recovery production, see duty point (1). 30 Power is consumed for reject brine flow movement, duty point (2). In duty point (2) the pressure is lower, the flow bigger, staying in duty point (2) is about times short than staying in point (1).

6 6 Figure : Pump Operation in PFRO and Standard RO Regimes The standard RO requires bigger pumps than PFRO, because in standard RO permeate and reject flow are simultaneously pumped at the same pressure. Afterwards, the power in the reject flow may be wasted or recovered by an energy recovery system. In many cases, the standard RO process reject flow pressure is so diminished by the differential pressure per stage, that the recovery of this pressure in the reject brine stream is not cost-effective. From the analysis of these duty points we can see that the PFRO production regime consumes less energy and requires smaller pumps than the standard RO. This PFRO process, in operation for a few months in two pilots, shows stable performance.