Integrated ultrafiltration and reverse osmosis desalination systems

Abstract

An open architecture desalination system having a field of water desalination using porous micro filtration or ultrafiltration (MF or UF) membranes followed by high pressure reverse osmosis (RO) membranes for salt removal. A novel integrated system with a unique process flow allowing use of multiple UF and MF membrane configurations on same platform is also disclosed. Additionally, the systemutilizes a noble process flow to enable high efficiency operation of the MF and UF membranes thus reducing footprint, longer life of the membranes and reduced energy.

Claims

1. A novel desalination system comprising: i. a first-stage pre-filtration segment and a second-stage microfiltration or ultrafiltration (MF/UF) segment, the first stage pre-filtration segment hydraulically coupled to the second-stage MF/UF segment, a first built-in buffer tank and a third-stage reverse osmosis (RO) membrane segment following the second-stage MF/UF segment, the first built-in buffer tank hydraulically coupled to the third-stage RO segment, a second built-in buffer tank following the third-stage RO segment, the second built-in buffer tank hydraulically coupled to the second-stage MF/UF segment and to the third-stage RO segment, and a programmable logic controller; ii. the second-stage MF/UF segment comprises a membrane bank, wherein the membrane bank is an inside-out membrane bank, an outside-in membrane bank, or a cross-flow membrane bank, wherein the inside-out membrane bank, the outside-in membrane bank, and the cross-flow membrane bank are interchangeable within the second-stage MF/UF segment; iii. the first built-in buffer tank stores MF/UF filtered water and the second built-in buffer tank stores RO concentrate, wherein the first built-in buffer tank has a volume capacity to feed the third-stage RO segment without interruptions when the second-stage MF/UF segment goes through a backflush process, wherein the second built-in buffer tank has a volume capacity to backflush the second-stage MF/UF segment; iv. the first built-in buffer tank is hydraulically coupled to the second built-in buffer tank; v. the first-stage pre-filtration segment comprises a sequential combination of a centrifugal separator followed by a screen filter component in order to remove settleable and suspended particles; vi. a single pump drives the first-stage pre-filtration segment and the second-stage MF/UF segment with intra-stage differential pressure measurement across the centrifugal separator, the screen filter component and the second-stage MF/UF segment; vii. the screen filter component in part (v) has two parallel screen filters with 3-way isolation valves to replace one of the two parallel screen filters without interrupting system operation.

2. The novel desalination system of claim 1, further comprising a plurality of two-way valves, wherein, the programmable logic controller controls the plurality of two-way valves for process control of the inside-out membrane bank, the outside-in membrane bank, or the cross-flow membrane bank within the second-stage MF/UF segment.

3. The novel desalination system of claim 2, further comprising an operating interface in communication with the programmable logic controller for selecting a membrane specific process flow through the plurality of two-way valves for the second-stage MF/UF segment.

4. The novel desalination system of claim 1, wherein, a pump drives backflush, clean-in-place of the second-stage MF/UF segment, wherein the pump further drives flush and clean-in-place of the third-stage RO segment.

5. The novel desalination system of claim 1, wherein the second built-in buffer tank is used for backflush and clean-in place of the second-stage MF/UF segment, wherein the second built-in buffer tank is further used for flush and clean-in-place of the third-stage RO segment.

6. The novel desalination system of claim 1, wherein backflush of the second-stage MF/UF segment with RO concentrate produces a high salinity surge in the third-stage RO segment causing disruption of bacterial growth and reduction in biological fouling.

7. The novel desalination system of claim 6, wherein the high salinity surge in the third-stage RO segment produces a reverse permeate flow to effect removal of scalants and foulants from a membrane surface of the third-stage RO segment and to effect reduced long-term fouling.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawling(s) will be provided by the Office upon request and payment of the necessary fee. A more complete and thorough understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings wherein:

(2) FIG. 1: A configuration table in the process control software interface allowing selection of either inside-out, outside-in or cross-flow configuration for installed membranes. Each configuration type has its dependent/common operating parameters as shown in table. Once a configuration has been selected, a pre-programmed process moves water in a manner consistent to the requirements of a given membrane configuration. This table also defines a control parameter (BF1/BF2 backflush) which controls which type of backflush operation will be performed for certain type of membranes and it is configurable. In one case, membrane can be back flushed from top end (BF1) for 5 cycles and from bottom end (BF2) for 1 cycle. In other case it can be reversed and in a third case it can be an alternate cycle and other combination thereof, thus allowing efficient operation of each membrane in an open architecture equipment design without complicated programming requirements.

(3) FIG. 2: A process flow layout of the open-architecture system with brine based back-flush of the UF/MF membranes and operation of various configuration options for UF/MF membranes. A feed pump (P1) sends water to UF/MF membranes from bottom end and the permeate from UF/MF is sent to a RO feed tank. This UF/MF treated permeate is boosted through a booster pump (P3) and further pressurized by pump (P4) for reverse osmosis process. The concentrate from RO is send to a temporary holding tank (Backwash and CIP tank) that is used for backwashing of the MF/UF section of the filtration. The RO feed tank capacity is designed to keep it running without interruptions when MF/UF section goes through periodic backflush process. Combination of 5 valves as shown in the FIG. allows for various combination necessary to do filtration, backflush, drain, forward-flush, clean-in-place for both MF/UF membranes as well as flush and clean-in-place for RO membranes.

(4) FIG. 3: Two pictures of the same desalination system with open architecture that allows for a quick swap of inside-out UF/MF to outside-in UF/MF membranes with minimal efforts. The system also allows operation of the integrated UF/RO process with built-in buffer tanks as shown in FIG. 2 with a common PLC based control system. This approach allows significant reduction in foot-print for same capacity and eliminates dependence on one type of membrane use, a key component of the invention.

DESCRIPTION OF THE INVENTION

(5) While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of distinct ways to make and use the invention and do not delimit the scope of the invention.

(6) To facilitate the understanding of this invention, a number of terms are defined below. Terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present invention. Terms such as a, an and the are not intended to refer to only a singular entity, but include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments of the invention, but their usage does not delimit the invention, except as outlined in the claims.

(7) A membrane is a permeable, often porous material in the form of a film, a tube, a powder or a block and capable of filtering certain material while blocking others. The pores or thin layer of the membrane defines its unique characteristics for serving as a selective barrier. Membranes are widely used in purification of gases and liquids. They are highly energy efficient; however they generally require a pressure differential to work. Advanced purification of water today mostly uses membranes. Based on their pore size, membranes are classified as reverse osmosis (RO), nano-filtration (NF), ultra-filtration (UF) and micro-filtration (MF). While all of these membranes are permeable to water, they reject certain size impurities while allowing water to go through them.

(8) Generally, for filtration through MF and UF membranes, the influent water must be clarified to remove particulates larger than 150 um in size and high density particulates should also be removed to minimize the abrasion damage of the active separation layers. In one embodiment of the invention, we disclose a combo, sequential implementation of a centrifugal separator followed by a screen filtration providing the desired water quality prior to MF and UF stage. This sequential combination step significantly reduces the footprint utilized by other methods such as disk-filtration or use of single step screen or sock filtration.

(9) The screen and sock filtration generally require frequent replacement of screen/sock for cleaning. In one embodiment of the invention, we disclose a parallel screen filter with isolation valves to remove and replace filter while system continues to operate.

(10) In one embodiment of the invention, we disclose use of a single pump to drive filtration stages for centrifugal separator, screen filtration followed by ultra-filters while measuring the pressure differential across each stages to monitor pressure loss across each stages. This invention simplifies the filtration pump hardware requirement with properly design pump capacity.

(11) As discussed above, the ultrafiltration and microfiltration membranes come in two types of geometries, inside-out and outside-in filtration. In addition to two geometries, several membrane material compositions such as PVDF, PAN and PES and require very different operation parameters, such as flux or flow rates, trans-membrane pressures (TMPs), filtration direction (inside lumen or outside lumen), back flush directions, the configurations can vary significantly and can be very dependent on system design. In one embodiment of the invention, we disclose a process flow that is able to incorporate various flow-directions, varying back-flush requirements and membrane specific operating parameters.

(12) In another embodiment of the invention, we disclose the user selection driven operation of the various configurations using a common programmable logic controller and an operating interface.

(13) An RO membrane is designed to rejects material as small as ions like Na.sup.+ and Cl.sup., thus enabling desalination of water. Typically, they are capable of rejecting more than 99% of the monovalent and divalent ions such as Na.sup.+, Cl.sup., Ca.sup.++, Mg.sup.++ and SO.sub.4.sup.. Most RO membranes require pressure to filter water through them. This pressure requirement is directly related to the amount of salt concentrations (or total dissolved solids, TDS) for the water processed. Higher TDS requires higher feed pressure to overcome the osmotic pressure. Higher pressure means increased energy use by pumps to permeate water through RO membranes. Today, for seawater desalination, energy is a significant cost component and can be as high as 30% of the total cost to desalinate water. Seawater usually has TDS in the range of 3-4%, brackish water (underground) TDS can vary in the range 0.5-2% while surface water (rivers and lakes) TDS can be lower, in the range of 0.1-1%. For drinking purposes, TDS of less than 0.05% (500 ppm) is required and lower is always better. For desalination, it is necessary to optimize applied pressure for maximum permeate efficiency (ratio of permeate and feed). For seawater desalination, typical feed pressures can range from 600 to 1000 psi while achieving permeate efficiency in the range of 25-40%. For brackish water, the feed pressures typically range from 200 to 400 psi while achieving permeate efficiency in the range of 50-70%. Surface water desalination typically requires feed pressures in range of 100-200 psi and is able to achieve 60-80% efficiency. For most of these desalination scenarios, concentrate brine (the reject) can have salt concentration in the range of 4-5%, if processed already through a UF or MF pretreatment stage is quite clean of suspended particulates and mostly contains dissolved solids. In one embodiment of the invention, we disclose a temporary storage of the brine for use as backflush of the UF/MF membranes, thus increasing the overall efficiency of the MF/UF segment.

(14) In another embodiment of the invention, we disclose the impact of such high salinity backflush on disruption of any bio-growth of bacteria due to increased salinity.

(15) A system process flow diagram is included below for reference covering the various inventive steps and processes discussed above. A combination of several 2-way (open/close state) valves (FIG. 2) provides the required control of the process for various UF/MF membranes. Additional valves provide use of intermediate holding tanks for various processes involved for maintenance of the UF/MF filters. In one embodiment of the invention, a common pump is used for multiple purposes such as MF/UF backflush, MF/UF clean-in-place, RO-flush, and RO clean-in-place.