System for pre-treating seawater by ultrafiltration

Abstract

The present invention relates to a system for pre-treating seawater by ultrafiltration, comprising ultrafiltration membranes (1) submerged in a tank (2) at atmospheric pressure which is provided with two inlets (21, 22), an upper and lower inlet, for a supply flow (Qa) of seawater propelled by a supply pump (3) and two outlets (23, 24), an upper and lower outlet, for an entrainment flow (Qar) of seawater circulating between the inlets (21, 22) and the outlets (23, 24). The system also comprises a suction pump (4) for a suction flow (Quf) of seawater ultrafiltrated in the ultrafiltration membranes (1). The supply flow (Qa) of seawater is greater than the suction flow (Quf) of ultrafiltrated water and the difference between said flow rates is equal to the entrainment flow (Qar) that forms a cleaning means for the tank (2).

Description

[0001] DESCRIPTION

[0002] Seawater pretreatment system by ultrafiltration.

[0003] Technical sector.

[0004] The present invention relates to a seawater pretreatment system by ultrafiltration, applicable in desalination plants, preferably with an open intake, and comprising ultrafiltration (UF) membranes housed and submerged inside an atmospheric pressure tank fed with seawater.

[0005] Prior art

[0006] The main difficulty faced by seawater desalination plants, using a reverse osmosis membrane process, is to achieve a pretreatment that removes, if not all, most of the elements and components of seawater, other than its saline composition, such as: organic matter, inert inorganic matter (sands and clays), microbiological matter (bacteria and viruses), zooplankton and phytoplankton, and that is able to adapt to any changes in composition during the operation of the desalination plant.

[0007] Currently there are various combinations of physical and chemical processes used for this purpose, with ultrafiltration (UF) using polymeric membranes being one of the most recent and effective pretreatment processes.

[0008] There are two types of ultrafiltration systems: pressurized membrane systems and submerged membrane systems. In both systems, during the filtration period, the inlet flow rate to the UF modules is the same as the outlet flow rate, and the UF membranes retain on their surface components larger than the membrane pore size. Neither system is well-suited to the changing conditions of seawater quality.

[0009] Once the membrane surface reaches a certain degree of clogging, the system is stopped and a backwash (BW) is performed by injecting under pressure a countercurrent flow of brine generated by reverse osmosis or ultrafiltered seawater, to dislodge the particles retained by the membranes.

[0010] In the system closest to the present invention, the UF membranes are housed and submerged inside an atmospheric pressure tank fed with seawater. Filtration is carried out by suction pumps located at the top of the tank, so the membranes operate under negative pressure.

[0011] In the aforementioned backwashing process, the particles detached from the surface of the membranes remain in the tank, making it necessary to subsequently empty the tank and remove its contents.

[0012] To begin a new filtration process, the tank must be refilled with seawater, and once full, the suction pumps must be started. After a certain number of backwashes, a maintenance chemical wash (called CEB or MW) is scheduled to remove scale and disinfect the membranes. This longer maintenance chemical wash is performed under pressure in a countercurrent flow by injecting chemicals into the ultrafiltered water using the chemical cleaning pumps. Finally, after a certain number of maintenance chemical washes, an intensive chemical cleaning (CIP) of all the UF membrane modules is performed.

[0013] Both the backwashing phase and the maintenance chemical cleanings and the intensive chemical cleanings involve a high number of cycles with the following associated drawbacks with respect to the invention:

[0014] - Higher energy consumption due to the frequency of cycles and therefore the accumulated operating time of the backwash (BW) pumps, the chemical cleaning pumps (CEB or MW and CIP) and a higher consumption of chemicals used in the chemical cleaning phases.

[0015] - Increased seawater consumption due to the increased emptying and filling of UF tanks and therefore increased consumption of seawater pumps that feed these tanks.

[0016] - Increased deterioration of motor pump equipment due to the number of starts / stops.

[0017] Furthermore, if ultrafiltered water is used for backwashing (a procedure recommended by most UF membrane manufacturers), the overall ultrafiltration conversion rate (the ratio of ultrafiltered water produced to feed the reverse osmosis systems to seawater fed to the UF membranes) is reduced because more extra ultrafiltered water must be produced that is not used in reverse osmosis. This reduction in conversion, or in other words, this extra production of ultrafiltered water, results in higher energy consumption by the intake pumps that supply seawater to the UF tanks.

[0018] - All these problems are exacerbated by the worsening of seawater quality and particularly in episodes commonly known as red tides (HAB).

[0019] - UF system manufacturers require the installation of pre-UF filters with cutoff capacities between 100 and 200pm, which increases the CAPEX of pretreatment.

[0020] US2004007527 describes a process for filtering water containing solids using filtration membranes in an open tank, removing the permeate produced in the membranes from the tank; replacing the permeate removed from the tank with feed water and reducing the concentration of solids in the tank water from time to time by deconcentrations.

[0021] Document CN 109126218 A provides a wastewater sedimentation tank, comprising a sedimentation tank body and a sludge removal device. Said tank comprises inclined sedimentation plates rotatably connected to the sedimentation tank body via spindles.

[0022] Document US2022274855 A1 describes a submerged membrane system, the influent flows into an open membrane tank and one or more conduits are provided in the tank to direct the influent flow towards the submerged membrane units.

[0023] Explanation of the invention.

[0024] The seawater pretreatment system by ultrafiltration that is the subject of the invention is of the type mentioned above and comprises ultrafiltration (UF) membranes submerged in a tank at atmospheric pressure, fed with a flow of seawater driven by a feed pump, and a suction pump of a suction flow (Quf) of ultrafiltered water in the UF membranes.

[0025] A primary objective of the invention is the development of a seawater pretreatment system that provides water circulation that prevents the deposition of organic matter and inorganic inert matter in the tank in which the UF membranes are submerged, and therefore a flexible pretreatment capable of adapting to changing seawater quality conditions

[0026] To this end, and in accordance with the invention, the tank containing the UF ultrafiltration membranes comprises two seawater feed inlets (Qa) arranged at different heights in the tank and two seawater drag outlets (Qar) arranged in an upper and lower part of the tank, wherein the seawater feed inlet (Qa) is greater than the ultrafiltered water suction inlet (Quf), the difference between the two inlets providing the seawater drag inlet (Qar) that circulates between the inlets and outlets of the tank during ultrafiltration and constitutes a means of cleaning the tank.

[0027] With the indicated characteristics, this system prevents the accumulation of organic matter inside the tank, adapts to the quality of the seawater feed, and provides a technical advantage consisting of a decrease in the frequency of backwash (BW) cycles, maintenance chemical cleanings (CEB or MW) and intensive cleanings (CIP), and therefore an increase in the filtration period.

[0028] The arrangement in the tank of two outlets for the drag flow, one upper and one lower, and the introduction into the tank of a feed flow (Qa), greater than the suction flow (Quf) of ultrafiltered water produced by the membranes, allows for a drag flow (Qar) that removes floating particles from the tank through the upper outlet and settleable particles through the lower outlet, reducing the accumulation of dirt inside the tank.

[0029] Another feature of the invention is that at least the lower outlet of the tank has a valve for controlling and regulating the carryover flow through each of the outlets, upper and lower. The seawater feed pump to the tank includes a frequency converter designed to regulate the feed flow rate and vary the ratio between the carryover flow rate and the nominal operating flow rate of the ultrafiltration system.

[0030] The choice of the percentage of carryover flow (Qar) with respect to the feed flow (Qa) will depend on the quality of the seawater to be treated.

[0031] According to the invention, the upper and lower inlets of the tank are provided with separate valves for controlling and distributing the feed flow through each of said inlets.

[0032] Advantageously, the tank has additional features that substantially improve the pretreatment of seawater and the cleaning of the tank, by directing the particle carryover flow towards the upper and lower outlets of the tank.

[0033] Through the development of fluid dynamic models in different operating scenarios, a UF tank has been designed that optimizes the operation of the UF membranes and allows water circulation under all conditions, including those with high organic matter load.

[0034] Using computational fluid dynamics techniques, a homogeneous distribution of flow velocities on the membranes has been determined, in order to achieve adequate efficiency in the removal of particles attached to the external surface of the UF membranes.

[0035] Specifically, and in accordance with the development carried out, the lower inlet of the tank is located in a high position and specifically at an equidistant distance from the bottom of the tank and the upper inlet of the same.

[0036] The lower inlet of the tank includes a flow distribution wedge for the feed flow rate (Qa) and for reducing the recirculation of the carryover flow to the inlets. This wedge has a pointed end facing the lower inlet. Another feature of the pretreatment system is that the tank internally contains two baffles, lower and upper, located near the lower and upper inlets, respectively, and each consisting of a flat plate. The upper baffle is positioned horizontally above the upper inlet of the tank, and the lower baffle is positioned below the lower inlet and inclined downwards, forming an angle (a) of between 25 and 35 degrees with the horizontal.

[0037] As mentioned previously, the proposed system reduces the frequency of backwash and chemical cleaning cycles, and the advantages that result directly from this reduction are as follows:

[0038] Reduction in the overall energy consumption of the pretreatment system. Although the carryover flow implies a slight increase in the energy consumption of the intake pumps, the overall balance results in a net decrease in energy consumption as a consequence of: o Less accumulated operating time of the backwash pumps (BW) and the chemical cleaning pumps (CEB or MW and CIP).Reduced seawater consumption due to fewer emptying and filling cycles of the UF tanks, and therefore lower energy consumption by the seawater pumps that supply the UF system. If ultrafiltered water is used for backwashing (a procedure recommended by most UF membrane manufacturers), the overall ultrafiltration conversion rate (the ratio of ultrafiltered water produced to feed the reverse osmosis trains to seawater fed to the UF membranes) increases, as less ultrafiltered water needs to be produced for chemical cleaning cycles. This increased conversion rate, or in other words, this reduction in the extra production of ultrafiltered water, results in lower energy consumption by the intake pumps that supply seawater to the UF tanks.

[0039] Reduction of the cost of the pretreatment system by eliminating the pre-filtration system required by the manufacturers of UF systems.

[0040] Reduction in maintenance activities for motor pump equipment, by decreasing the frequency of start-stop cycles. Lower consumption of chemicals used in chemical cleaning phases.

[0041] Brief description of the content of the drawings.

[0042] To complement the description being made and in order to facilitate the understanding of the characteristics of the invention, a set of drawings is included with this descriptive report in which, for illustrative and non-limiting purposes, the following has been represented:

[0043] Figure 1 shows a schematic view of an example of the implementation of the seawater filtration pretreatment system by ultrafiltration, connected to the reverse osmosis trains of a desalination plant.

[0044] Figure 2 shows a schematic elevation view of an example of the ultrafiltration membrane tank, equipped with baffles at the seawater feed inlets and a flow distribution wedge facing the lower feed inlet. The UF membranes and the flow directions of the feed from the upper and lower tank inlets are represented by dashed lines in this figure.

[0045] Detailed exposition of modes of implementation of the invention.

[0046] In the embodiment shown in Figure 1, the seawater treatment system by ultrafiltration comprises ultrafiltration membranes (1) housed and submerged in a tank (2) at atmospheric pressure comprising: two inlets (21, 22), lower and upper, for a feed flow (Qa) of seawater supplied by a feed pump (3), arranged on one side of the tank (2) and outlets (23, 24) for a carryover flow (Qar) of seawater arranged on the opposite side of the tank (2).

[0047] For their part, the ultrafiltration membranes (1) are connected to a suction pump (4) that extracts a suction flow (Quf) of ultrafiltered water in said membranes, and which is sent to the reverse osmosis trains (6) of a desalination plant, represented schematically by an osmosis membrane (61), a high pressure pump (62) and a recirculation circuit (63) for energy recovery from the brine.

[0048] The feed flow rate (Qa) is supplied to the tank (2) by at least one feed pump (3) provided with a frequency converter for regulating the feed flow rate (Qa) introduced into the tank through the two upper and lower inlets (21, 22).

[0049] This seawater feed flow rate (Qa) is greater than the suction flow rate (Quf), the difference between both flow rates being what provides the seawater drag flow rate (Qar) that circulates between the inlets (21, 22) and the outlets (23, 24) of the tank (2).

[0050] The purpose of this drag flow (Qar) is to extract from the tank small sands, microalgae and other particles contained in the seawater introduced by the feed pump (3) into the tank (2), adapting to the quality of the seawater captured and fed to the system

[0051] The outlets (23, 24) are arranged at the bottom and top of the tank to facilitate the extraction, by the action of the entrainment flow, of the particles settled through the lower outlet and of the floating particles through the upper outlet.

[0052] The inlets (21, 22) and outlets (23, 24) of the tank (2) are provided with valves (51, 52) and (53, 54) that allow the distribution of the feed flow rate (Qa) of seawater that enters through each of the inlets (21, 22) and the distribution of the carryover flow rate (Qar) between the outlets (23, 24), optimizing the operation of the pretreatment system according to the amount of floating and settleable material contained in the seawater, which depends on the changing conditions of the sea.

[0053] To increase turbulence and ensure that the seawater flow is uniformly distributed throughout the tank (2), different alternatives have been studied, including the analysis of the pretreatment system with three different entrainment flow rates to guarantee proper operation under any entrainment flow rate condition. The analysis and selection criteria for the optimal system were based on achieving the best flow distribution, thus avoiding dead zones and ensuring homogeneous system performance in the ultrafiltration membrane zone (1).

[0054] Thus, and based on the simulations carried out, a tank configuration (2) with the specific characteristics shown in figure 2 has been chosen.

[0055] In this configuration the lower inlet (21) is in a high position, at a distance (d) equidistant from the bottom of the tank (2) and the upper inlet (22).

[0056] The tank (2) comprises at the lower inlet (21) a wedge (25) that distributes the flow of the feed flow (Qa) inside the tank (2) and reduces the recirculation of the feed flow towards the inlets.

[0057] Said wedge (25) comprises a pointed end facing said lower inlet (21) and forming an angle between 65 and 75 degrees; specifically 70 degrees in the embodiment shown in figure 2.

[0058] The tank (2) internally comprises two lower and upper deflectors (26, 27), located close to the lower and upper inlets (21, 22) respectively, and which are made up of two flat plates.

[0059] The upper deflector (27) is arranged horizontally, above the upper inlet (22), and the lower deflector (26) is arranged below the lower inlet (21) and inclined downwards, forming an angle (a) between 25 and 35 degrees with the horizontal.

[0060] Once the nature of the invention has been sufficiently described, as well as an example of a preferred embodiment, it is noted for the appropriate purposes that the materials, shape, size and arrangement of the elements described may be modified, provided that this does not imply an alteration of the essential characteristics of the invention claimed below.

Claims

CLAIMS 1. A seawater pretreatment system by ultrafiltration, comprising ultrafiltration membranes (1) submerged in a tank (2) at atmospheric pressure, fed with a flow of seawater driven by a feed pump (3) and a suction pump (4) of a suction flow (Quf) of ultrafiltered water in the ultrafiltration membranes (1), characterized in that the tank (2) containing the ultrafiltration membranes (1) comprises: two inlets (21, 22), lower and upper, for a feed flow (Qa) of seawater, arranged on one side of the tank (2), connected to the feed pump (3), and comprising respective valves (51, 52) for control and distribution through said inlets (21, 22) of a feed flow (Qa) of seawater greater than the suction flow (Quf) of ultrafiltered water in the ultrafiltration membranes;and two outlets (23, 24) lower and upper, for a seawater entrainment flow rate (Qar), arranged on the opposite side of the tank (2); providing at least one of the outlets (23, 24) with a valve (53, 54) for controlling and regulating an entrainment flow rate (Qar) flowing through said outlets (23, 24); wherein the feed flow rate (Qa) of seawater, controlled by valves (51, 52), is greater than the suction flow rate (Quf) of ultrafiltered water, and the difference between said feed flow rates (Qa) and suction flow rates (Quf) is equal to the carryover flow rate (Qar) controlled by valve (53, 54) of at least one of the outlets (23, 24) and which circulates during ultrafiltration between the inlets (21, 22) and outlets (23, 24) of the tank, forming a means of cleaning the tank (2).; 2. The pretreatment system according to claim 1, wherein the seawater feed pump (3) to the tank (2) comprises a frequency converter intended to regulate the feed flow rate (Qa) and vary the ratio between the carryover flow rate (Qar) and the nominal operating flow rate of the ultrafiltration system depending on the quality of the captured seawater.

3. The pretreatment system, according to any preceding claim, wherein the lower inlet (21) is in a high position relative to the bottom of the tank (2), at a distance (d) equidistant from the bottom of the tank (2) and from the upper inlet (22).

4. The pretreatment system, according to any preceding claim, wherein the lower inlet (21) of the tank (2) comprises a wedge (25) distributing the flow of the feed flow rate (Qa) and reducing the recirculation of the flow rate feed to the tank inlets.

5. The pretreatment system according to claim 4, wherein the wedge (25) comprises a pointed end facing said lower inlet (21) and forming an angle between 65 and 75 degrees.

6. The pretreatment system, according to any previous claim, wherein the tank (2) internally comprises two lower and upper deflectors (26, 27), located close respectively to the lower and upper inlets (21, 22), and consisting of two flat plates.

7. The pretreatment system according to claim 6, wherein the upper deflector (27) is arranged horizontally, above the upper inlet (22).

8. The pretreatment system according to claim 6, wherein the lower deflector (26) is arranged below the lower inlet (21) and inclined downwards, forming an angle (a) between 25 and 35 degrees with the horizontal.

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