FILTRATION MEMBRANE CLEANING

The subject of the invention is a water treatment system and a method of filtration membranes cleaning.

Invention belongs to the field of water treatment systems and installations. In water treatment systems, solid and colloidal impurities from liquids are removed by using membrane type filtration baffles, usually with holes smaller than 1 micrometer. The separated suspension with positive pressure flows vertically or tangentially to the surface of the membrane. The liquid flows through the surface of the membrane and on its surface and in a porous membrane structure the sediment builds causing an increase in flow resistance defined as trans-membrane pressure. An increase in the trans-membrane pressure means a decrease in the value of the liquid stream. When the trans-membrane pressure reaches a certain high value and the filtration efficiency from the surface unit decreases, the filtration process is stopped, and the membrane is subjected to washing to regain its permeability. Depending on the type of membrane, it is subjected to washing by means of pumping a part of the clean liquid or washing it tangentially to the surface of the membrane with a stream of liquid while the outflow of liquid behind the membrane is closed. Periodically, the membranes are additionally subjected to chemical washing in order to dissolve and remove the impurities accumulated in the porous structure of the membrane, blocking the filtration process. One of the commonly used methods of counteracting impurities settling on the membrane is the bubbling of gas bubbles along the vertical surface of the membrane. Large diameter air bubbles, with the diameter of few mm, are used. As a result of the mixing accompanying bubbling, part of the sludge is removed from the membrane surface mechanically, similarly to the liquid flow along the membrane during "cross-flow" filtration. Solid and colloidal particles are removed from the surface of the membrane and are carried to the surface of the liquid. The larger the particle size, the easier it is to remove them. The most difficult to remove particles of the smallest size create deposits with very low porosity and with the greatest filtration resistance. The rate of membrane blocking depends on the concentration of solid and colloidal contaminants in the suspension. The membrane is faster to overgrow with impurities the more there is a deposit in the liquid supplied to the surface of the membrane and the smaller the particles of sediment in the suspension. It is natural that in order to filter out solid particles of a given size, one-stage filtration is not carried out and the suspensions are not filtered through a membrane with a hole size equal to the size of the partition grain. A better solution is to gradually filter out the smaller and smaller solid phase grains from the liquid on subsequent filtration baffles with smaller and smaller holes. Another effective solution is to remove larger particles by chemically coagulating suspensions, followed by sedimentation or flotation methods. Sedimentation as a process based on the use of gravity is slow and its effectiveness depends on the correctness of selected chemicals. Large reservoirs, so-called clarifiers, are needed for sedimentation. Flotation is a process faster than sedimentation and requires smaller tanks, but its implementation requires continuous feeding to the flotation chamber a large amount of gas in the form of small bubbles that attach to solid particles and put them on the liquid surface in the form of an easily mechanically removable coat. The efficiency of suspensions removal using flotation depends on the size of gas bubbles introduced into the flotator. Commonly used flotators are equipped with small bubble generators with sizes from 300 to 1000 micrometers. Such bubbles relatively quickly float in the liquid, the probability of their contact with solid particles is often lower than 50%, and the efficiency of separation rarely reaches 98%. Smaller bubbles with sizes of 70-300 micrometers give better results, but quickly combine to larger ones and float to the surface of the liquid, often without contact with solid particles.

Such a solution including flotation prior to the ceramic membranes is described in application EP 2867174. Air bubbles of approximately 70 micrometers are introduced into the liquid at the inlet to the flotation chamber and remove (flotate) suspensions, colloids and fats. The efficiency of separation by such a flotation method reaches 99.5%. The liquid with a small amount of suspensions is filtered through ceramic membranes, immersed in the flotation chamber. The filtrate is drawn from the inside of the membranes by a vacuum produced by a vacuum pump. Air bubbles from fine-mesh diffusers flow along the surface of the membranes. On the surface of the membranes very fine suspensions are deposited, which create filtration sediments (cake) with a high filtration resistance even at low thickness. The sludge is periodically removed by the counter flow wash method when the trans-membrane pressure rises to the critical value. As a result of counter-flow washing, the rinsed sludge is carried to the surface of the liquid to the layer of the float removed mechanically. Such a combination of flotation and membrane filtration using ceramic membranes gives the possibility of obtaining both higher filtration efficiency and better filtration effectiveness. The disadvantage of the described method is the necessity of using a relatively large amount of chemicals and longer breaks in the operation of the installation, as well as losses of energy and filtrate on frequent membranes washing. The method described in the European application allows reducing the frequency of membrane washing by reducing sediment concentration in the suspension but does not reduce the frequency of chemical membrane washing as it does not improve the cleaning of the porous internal membrane structure. Cleaning the membrane surface with small bubble aeration is too ineffective and does not significantly improve the filtration conditions.

In addition, in traditionally used water treatment systems, the filtration membranes are multi-layered porous structures where the thinnest layer with the smallest apertures is located on the surface, and the subsequent layers are a mechanical undercoat and drainage for draining liquid that has passed through this thinnest layer. During the membrane filtration, sediment is deposited both on their surface and in the porous structure of the membrane. During the filtration, the thickness of the sludge layer on the membrane surface gradually increases, the trans-membrane pressure increases, and the filtration efficiency decreases from the surface unit. After reaching the preset trans-membrane pressure limit, it is necessary to clean the membrane in order to restore its initial permeability.

In one of the well-known solutions, the membranes are cleaned by closing the liquid flow through the surface of the membrane in order to wash away the deposit accumulated on the surface of the membrane. The sludge removal takes place as a result of the tangential flow of the washing agent tangentially to the membrane surface. The washing agent is usually a filtered suspension which tangential flow separates the sludge mechanically from the membrane surface. In many solutions, the additional cleaning agent is macro-gas-bubbles, which detach the sediment from the membrane during a fast, pulsating, flow to the surface of the liquid. This technical solution is also used in the case of plastic membranes.

Another, partly above-mentioned method of cleaning the filtration membranes is to generate a filtrate flow in the direction opposite to the direction of filtration - the counter flow washing method. With sufficient mechanical resistance of the membrane, washing liquid under pressure is pumped and the sludge is mechanically removed from the membrane surface and partly from the inside of the pores of the drainage layers. The removed sludge is removed to the filtered suspension. If the membrane is characterized by high mechanical resistance, the so-called "hydraulic hammer" method, consisting of a rapid impact of a hydraulic portion of the filtrate in the direction opposite to the direction of filtration, is used. Even 4-5bar pressure is used for such a membrane cleaning method. The main advantage of this cleaning method is a very short cleaning time. The disadvantage of this solution is the danger of accumulation and sludge compression in the inner porous structure of the membrane, a decrease in the volume porosity and a gradual increase in the hydraulic resistance of the membrane. In practice, this method is used to clean membranes from materials with very high mechanical strength, e.g. silicon carbide SiC. It should also be noted that any such washing of the membrane causes a decrease in the efficiency of the device and the creation of additional wastewater from washing of the membranes .

The filtration membranes are also cleaned by chemical and biochemical removal of accumulated deposits - dissolution. After repeated mechanical cleaning of the outer surface of the membrane and the internal structure of the membrane, it is necessary to clean the membranes chemically or biochemically. The cleaning operation consists in passing through the membrane successive washing solutions (acids, alkalis , enzymatic substances), which dissolve impurities accumulated in the internal structure of the membrane. Each time, chemical and biochemical washing creates a sewage from the washing, which must be discharged separately and recycled. The washing time is equivalent to the stoppage of the membrane operation. The costs of chemicals and their consumption significantly affect the operating costs of filtration.

From the prior art flotators equipped with 1-40 micrometers diameter micro-nano-bubbles generators are known. Such small bubbles do not combine into larger ones but disintegrate into smaller ones and practically do not float to the liquid surface. The surface of micro-bubbles is negatively charged, which makes it easy to wrap positively charged solid and colloidal particles, cause self-coagulation and allow to remove very fine impurities from the suspensions, especially organic ones. Replacing standard bubbles with micro-bubbles significantly increases the separation efficiency, and at the same time reduces operating costs and reduces the necessary working volume of flotators. Cracking micro-bubbles generate ultrasonic waves which clean all solid surfaces. As a result of the cracking of micro-bubbles hydroxyl OH* radicals are formed, which are very strong oxidants, stronger than ozone and atomic oxygen. Hydroxyl radicals break down both colloidal and dissolved organic substances that block the surface of the membrane and its porous interior. Both the solid particles in the separated slurry and the particles removed by the "counter flow" cleaning and the action of ultrasounds from the membrane surface are subjected to self-coagulation as a result of negative charging of the surface of solid and colloidal particles. If the flocolis produced in this way are deposited on the surface of a also negatively charged hydrophilic membrane, they will form a sludge (cake) with high porosity and significantly lower filtration resistance. Lower filtration resistance means a higher filtration rate from the filtration surface unit. Such deposits are easier to remove from the surface of the membrane by "counter flow" rinsing. If a sufficient number of micro-nano-bubbles that flow along the vertical surface of the membrane is provided, the effect of removing deposits from the membrane surface will be obtained and solid cleaning of the membrane and sludge floating to the surface of the liquid will be ensured. The above solution, however, does not provide a satisfactory level of water filtration efficiency due to the insufficient content of micro- nano-bubbles in post-flotation water.

The main objective of the solution according to the invention is to improve the efficiency of water treatment plants using filtration membranes, with particular emphasis on water filtration installations in reverse osmosis plants - water intended for filtration in reverse osmosis systems must be very carefully prepared, which generates e.g. costs related to the consumption of chemicals and electricity. The solutions known from the prior art use micro-nano-bubbles to catalyze the flotation process, making it more effective. Generated micro- nano-bubbles are added to the water before the flotation process and used to increase its efficiency. However, they do not get in enough quantity to the set of filtration membranes built into the flotation machine or installed externally. The solution according to the present invention allows saturation of post-flotation water, i.e. pre-purified by the flotation process with micro-nano-bubbles so that the pre-treated water again contains the maximum concentration of micro-nano-bubbles, enabling usage of their beneficial properties of preventing deposition on filtration membranes. The effectiveness of this solution is ensured by feeding water with micro-nano-bubbles directly to the filtration membranes. This makes the filtration process using membranes significantly more efficient and the water treated in this way can be again saturated with micro- nano-bubbles to improve the efficiency of the filtration process in reverse osmosis systems.

The most important cost in producing of a distillate using a reverse osmosis system is the cost of the electricity consumed. The production of the distillate in the system according to the invention consumes a total amount of electric energy at the level of 2.86 kWh/m ³ of distillate, whereas conventional reverse osmosis systems are able to achieve a minimum energy consumption of 3.7 kWh/m ³ of distillate. The above is achievable due to the fact that a flotator with a built-in micro-nano-bubble generator consumes 0.713kWh/m ³ , a 0.1kWh/m ³ is used by the membrane system, a micro-nano-bubble generator operating for the needs of reverse osmosis system consumes 0.563kWh/m ³ , and a reverse osmosis system as a result of the use of these devices achieves 2.5 times increase in efficiency because the micro- nano-bubble saturated brine effectively counteracts the build up of membranes by ultrasound waves breaking any accumulated impurities disturbing the flow through the membranes - waves are formed during micro-nano-bubble cracking. The above leads to the reduction of electricity consumption from 3.7kWh/m ³ to 1.48 kWh/m ³ in the area of the reverse osmosis system itself, giving a total of 2,86kWh/m ³ within the installation of water preparation. Assuming electricity costs in Saudi Arabia at 0.32SAR per kWh, the proposed solution reduces the cost of producing lm ³ distillate from 1.184SAR to 0.92SAR, which is a 23% reduction in production costs which should additionally be increased by reducing the cost of chemicals to the process in quantities around 0.05 $ per m ³ .

The essence of the invention is a water treatement system comprising a source of water for filtration, a flotation chamber, a sludge scrubber, a micro-nano-bubble generator, a set of filtration membranes connected to a filtrate drainage pipeline equipped with a filtrate extraction and a membrane washing pump characterized in that the flotation chamber is connected via a pipeline with a micro-nano-bubble generator to which a pipeline is connected whose outlet is directed at a set of filtration membranes and at least one of the pipelines connecting the micro-nano-bubble generator with the flotation chamber or filtration membranes chamber is equipped with a pump.

Preferably, the set of filtration membranes is outside the flotation chamber in the filtration membrane chamber.

Preferably, the flotation chamber is additionally connected to the filtration membranes chamber.

Preferably, the micro-nano-bubble generator is connected to a water source pipeline via a pipeline with a pump.

Preferably, the system according to the invention further comprises a filtrate tank connected to the filtration membrane set by a pipeline which consists of a filtrate drainage pipeline and a return pipeline, where the filtrate drainage pipeline is equipped with a pump.

Preferably, the system according to the invention comprises a micro-nano-bubble generator connected by a pipeline equipped with a pump to the filtrate tank and an additional micro-nano- bubble generator is connected via a pipeline equipped with a pump to a reverse osmosis desalination system.

Preferably, the set of filtration membranes consists of one or more filtration membranes.

Preferably, the pipeline leaving the micro-nano-bubble generator is connected to nozzles whose outlets are targeted directly at the filter membrane set.

The method of cleaning the filtration membrane set characterized in that water through the pipeline being the water source is pumped into the flotation chamber and then through the pipeline, it is pumped to a micro-nano-bubble generator, where it is saturated with micro-nano-bubbles with a diameter of 1 to 50 micrometers and then pumped through a pipeline to a set of filtration membranes.

Preferably, when the micro-nano-bubbles saturated water from the micro-nano-bubble generator is pumped through the pipeline to a set of filtration membranes through nozzles whose outlet is targeted directly at the filter membrane set.

Preferably, micro-nano-bubbles saturated water is pumped into the flotation chamber via a pipeline.

The subject of the invention in a preferred embodiment is shown in the drawing showing the water treatment system.

The invention has been realized by creating a water treatment system, where in a multi-stage manner sea water is prepared by reducing the SDI Salt Density Index, informing about the percentage decrease in the flow rate during one minute, which increases with increasing solids content in the flowing medium - the lower the coefficient, the lower the flow resistance during the membrane filtration process, which is very advantageous to the value of 1 or lower and the saturation of the micro-nano-bubble just before the inlet to the reverse osmosis system.

The water treatment system according to the invention in a preferred embodiment comprises a water source 3 for filtration connected via a plastic pipeline with a flotation 1 chamber comprising a sludge scraper 8 and a sludge tank 9. The micro- nano-bubble generator 4 is connected by a plastic pipeline 10 to the flotator 1, a plastic pipeline 22 equipped with a centrifugal pump 23 with a water source 3, a plastic pipeline 11 equipped with a centrifugal pump 12 with a set of nozzles 13. The set of nozzles 13 is located in the membrane chamber 19 also comprising a set of filtering membranes 2 connected to the steel filtrate tank 6 by means of a plastic pipeline 25 and a plastic pipeline 24 comprising a centrifugal pump 5. The steel filtrate tank 6 is connected to the micro-nano-bubble generator 14 by a plastic pipeline 15 comprising a vacuum pump 7. The micro-nano-bubble generator 14 is connected to the reverse osmosis system 18 by means of a plastic pipeline 16 equipped with a water pump 17.

The method according to the invention was implemented by supplying seawater with a salt concentration of 42 g/1 into the flotation chamber 1 by a pipeline being a source of water for filtration 3 made of fiberglass. The water was supplied by means of a centrifugal pump 21. In parallel to the above-described process, using a pipeline 10 made of PVC by means of centrifugal pump 23, the water is supplied to the micro-nano-bubbles generator 4 and saturated with ozone gas micro-nano-bubbles. In the discussed process, the VNN BN 200 type micro-nano-bubble generator was used. Then, water with micro-nano-bubbles with a diameter of less than 45 micrometers is fed via pipeline 22 to the pipeline which is the source of water 3 for filtration, where it mixes with seawater and goes to chamber 1 of the flotator. The saturation of seawater with micro-nano-bubbles causes the sediments contained in seawater to be led into the upper section of the flotator, from which they are collected via a chain-type scraper 8 and transported to the sludge discharge chamber 9 made of 316L steel. In this way pre-treated sea water is deprived of part of the sediment and microorganisms. At the same time, pre-treated water is saturated with micro-nano-bubbles in the micro-nano-bubble generator 4 and through the pipeline 11 by means of the pump 12 is fed to the section where the ceramic membranes 2, with a length of 800mm each, are arranged vertically and packed with a PVC collector. Water with micro-nano-bubbles goes to nozzles 13, of which under increased pressure (which results from the difference in diameters of nozzle outlets 13 in relation to the diameter of the pipeline 11) is fed directly to the set of filtration membranes 2. In the filtration membranes section, another stage of seawater treatement occurs with the use of ceramic filtration membranes 2, whose efficiency is significantly increased by saturating water purified on filtration membranes 2 with micro-nano-bubbles distributed with nozzles 13 on the outer surfaces of the membranes. The use of the above described method allows to reduce the frequency of the backwash process (consisting in generating a hydraulic impact, mechanically cleaning the surface of membranes from sediments accumulated in the process of purification and filtration of seawater) five times.

In addition, the advantage of this solution is a significant increase in the recovery rate of water in relation to the amount of seawater brought to the desalination plant from conventional 20-30%, up to 80%. This gives an advantage in the form of the amount of water supplied to the desalination process - in a system producing 1000 t/h nominally, we should provide 3400 t/h of seawater, while using the proposed solution to produce 1000 t/h of distillate should be delivered to the system about 1250 t/h of water sea. Another advantage is the limitation of the installation footprint by completely eliminating the sand filters. The costs of chemical agents added as preparation of water for the process are considerably reduced, which leads to a direct benefit of $ 0.05 /m ³ - which gives an annual saving of $ 540, 000 per 30, 000 m ³ per day. The service life of membranes is more than doubled, which reduces two times the downtime associated with the need to replace reverse osmosis membranes.