A method of removing organics and particulate plastics from water

Abstract

A method of treatment of organic-, especially microplastics-containing aqueous medium is disclosed. The method comprises the steps of contacting the medium with a solid adsorption agent, introducing ozone into the medium, lowering the pressure of the medium at a rate sufficient to cause cavitation in the medium, continuing mixing for a predetermined time.

Description

[0001] A Method of Removing Organics and Particulate Plastics from Water

[0002] Field of the Invention

[0003] The present invention relates to a method of removing organic materials and especially particulate plastics materials from a body of water. This is achieved through the chemical oxidative degradation of the materials. Also disclosed is an apparatus in which said removal is carried out. to the Invention

[0004] Organics and particulate plastics are recognised as pollutants in the potable water supply and increasingly, certainly in the case of microplastics, as a pollutant of natural water courses and the oceans. A reasonable estimate of the scale of problem states that around 500,000 tonnes of microplastics are discharged to the environment each year. The problem is particular acute when dealing with polyfluoroalkyl substances (PFAS) and also the related perfluorooctanesulphonic acid (PFOS). These chemicals, often referred to as 'forever' chemicals, are particular difficult to degrade due to their chemical stability.

[0005] These materials arise from many different sources. For example, microplastics can arise from many modern articles of manufacture from clothes to vehicle tyres. In general, microplastics are characterised as particulate materials of size less than 5mm, but can go down to sub- microscopic sizes on the nanometre scale. Often from land, the particles make their way into watercourses and then into the bodies of aquatic animals. Typical solutions attempt to filter out the materials using microfilters. This can be difficult to achieve however especially for the nanomaterials, where filtration can be a slow process. Moreover, there then remains the problem of dealing with the filtered microplastics along with either regeneration or disposal of the filter material. Other methodologies attempt to bind microparticles together into a clump using a flocculant which can then be removed, often without the use of filters. Again, disposal of the microplastics still needs to be carried out, along with the flocculant, in a safe and environmentally friendly manner.

[0006] The present invention seeks an alternative methodology to eliminate organic materials and microplastics materials through degradation of the materials into relatively harmless degradation products such as CO2.

[0007] Summary of the Invention

[0008] According to a first aspect of the invention there is provided a method of treatment of a microplastics-containing aqueous medium, the method comprising the steps of mixing the medium with a solid adsorption agent, the solid adsorption agent having a particle size of from lpm - 1,000 pm, introducing ozone into the medium, lowering the pressure of the medium at a rate sufficient to cause cavitation in the medium, continuing mixing for a predetermined time, forwarding the medium to a filtration vessel, the filtration vessel including a filtration membrane to remove adsorption agent from the aqueous medium.

[0009] The organic materials and microplastics are thereby degraded to carbon dioxide and water and the aqueous medium can be released to drain, without further treatment or handling of the organic materials and microplastics.

[0010] Preferably the aqueous medium is pressurised above atmospheric pressure prior to addition of the ozone. Further preferably, the aqueous medium is pressurised to around 2.5 barg. Preferably the ozone introduced is at a concentration of from 100g / Nm3to 300g / Nm3, further preferably 240g / Nm3. In the aqueous medium the concentration of dissolved ozone is initially at least 3mg / l to provide a reasonable reaction rate.

[0011] Preferably, the particle size of the solid adsorption agent is 5pm.

[0012] Preferably the pressure lowering is greater than or equal to 0.5 bar, and further preferably greater than or equal to 1.0 bar to produce the required cavitation. Alternatively preferably, the pressure lowering is from 0.5 - 0.8 bar.

[0013] Optionally, once the predetermined time has expired, the aqueous medium is forwarded to a filtration vessel. The filtration vessel is further preferably formed of a ceramic material to provide resistance to chemical degradation of the filtration vessel.

[0014] The filtration membrane is preferably formed of silicon carbide. The filtration membrane is yet further preferably has a pore size of 0.1pm - 1.0 pm, and preferably of 0.1pm, enabling good filtration, without reducing water flow through the membrane too much.

[0015] According to a second aspect of the invention, there is provided an apparatus for the treatment of a microplastics-containing aqueous medium, the apparatus comprising a feed tank for water to be treated, the feed tank being fluidly linked to a mixing vessel, , an adsorption agent feed tank, fluidly linked to the mixing vessel, said mixing vessel including a mixing means to mix the adsorption agent and the water to form a reaction mixture, the apparatus further comprising a cavitation device having an ozone tank fluidly linked thereto to feed ozone into the cavitation device, the cavitation device including means to induce a pressure drop within the cavitation device, the mixing vessel and the cavitation device being fluidly linked to allow the reaction mixture to flow into the cavitation device, a reaction tank, fluidly linked to receive able to retain the reaction mixture at greater than atmospheric pressure, a filtration tank, fluidly linked to receive reaction mixture from the reaction tank and including filtration means to retain adsorption agent from the reaction mixture.

[0016] Brief Description of the Drawings

[0017] The invention is now described with reference to the accompanying drawing which shows by way of example only, an embodiment of an apparatus. In the drawing:

[0018] Figure 1 shows a layout of an apparatus suitable for removing organic and plastics materials from a body of water.

[0019] Detailed Description of the Invention

[0020] Conventional methodologies which seek first to collect microplastics materials from bodies of water are still left with the problem of safely disposing of the materials used in the collection and also the collected microplastics. Thus at least a 2-stage process is required with the concomitant higher costs and equipment requirement. Moreover, certain classes of chemicals (e.g. PFAS and PFOS) have the additional problem of being highly resistant to simple chemical degradation.

[0021] The present invention utilises a single pass, semi-continuous process to remove the target materials. In the following, the apparatus and method are described with respect to pollutant organic microplastic materials, but the skilled person will appreciate that the same can also be used to remove other organic contaminants.

[0022] An embodiment of an apparatus suitable for carrying out the method of the current invention is shown in Figure 1. In Figure 1, arrows indicate the direction of flow of materials within the apparatus. In overview, the water to be treated is fed in through the pipe 10 into a feed tank 12. Processing of the water then takes place, with cleaned water being discharged through the outlet line 48. During the process, microplastic materials are degraded into smaller, tractable chemical components which can be relatively easily dealt with.

[0023] In more detail, the water fed into the feed tank 12 can be pre-treated to remove biological materials and any secondary effluent, such as solid biological materials. Once sufficient water has accumulated in the feed tank 12, the water is pumped out via the pipe 14 by means of the first centrifugal pump 16 into the adsorption agent mix tank 18. In the adsorption agent mix tank 18, the water is mixed with active ingredients which are involved in the breakdown of the microplastic particles. An adsorption agent material, in powdered form, stored in an adsorption agent storage tank 20 passes via a feed pipe 22 through an automatic feeder 24 into the adsorption agent mix tank 18. The adsorption agent and the water are mixed by means of the stirrer 26. The adsorption agent is preferably an activated carbon whose eventual breakdown products are carbon dioxide, which can dissolve safely in the water.

[0024] Following a pre-determined residence time in the adsorption agent mix tank 18, the mixture is pumped therefrom by means of the second centrifugal pump 28 and into a cavitation device 30. The second centrifugal pump 28 increases the pressure of the solution to around 2.5 barg which increases the solubility of ozone in the water in accordance with Henry's Law, thus increasing the ozone concentration. The residence time is calculated as a function of the adsorption agent mix tank volume and the water flow design, but ideally should be greater than 2 minutes. The cavitation device 30 is equipped with means to produce a pressure drop within the mixture held by the cavitation device 30. A typical pressure drop produced is greater than 0.5 bar and especially over 1.0 bar. A pressure drops of 0.5 bar - 0.8 bar is preferred.

[0025] As an example of how the pressure drop is achieved, then high pressure water is brought to the cavitation device 30 where reduced flow constriction increases velocity for a while up to 10-15m / s. This acts to reduce pressure to very low levels, approaching vacuum, before going back to high water pressure after the cavitation device 30. The pressure difference is measured before the cavitation device 30 and after the cavitation device 30 and that drop can be equated to the energy which was spent on cavitation and vacuum generation required for gas suction. Gas enters the water in the vacuum region and is then compressed to the cavitation device's 30 outlet pressure which improves the gas transfer and gas hold up in the water.

[0026] In the exemplified embodiment, an ozone generator 32 produces the ozone which is passed into the cavitation device 30 via the ozone gas pipe 34. The cavitation device 30 acts to dissolve the ozone in the mixture and to produce cavities in the mixture. A typical concentration of ozone in the mixture initially within the cavitation device 30 is around 3mg / l, and preferably at least 3mg / l, although this value can be selected dependent on the concentration of organic material in the mixture. Typical concentration ranges of ozone concentration in the ozone supplied is from 100g / Nm3to 300g / Nm3, preferably 240g / Nm3. Within the cavitation device 30, the ozone reacts with the adsorption agent and is converted into the reactive hydroxyl radical, which is a stronger oxidising agent than ozone itself. The hydroxyl radical so generated oxidises the microplastic particles, along with any long-chain organic materials, in the mixture, breaking down the polymeric chains and destroying the physical integrity of the microparticles. The destruction of the microparticles is aided by the cavitation process. As such therefore, the method disclosed herein is suitable for breaking down any type of organic polymer, although is unsuitable for flocculants and surfactants.

[0027] Once the required amount of ozone is delivered into the mixture and the cavitation process is complete, the solution is delivered via the pipeline 36 into a pressurised reaction vessel 38, having a pressure of 1.5 - 2.0 barg, in which reaction vessel 38 the oxidation process continues. The mixture is then delivered into a silicon carbide ceramic filtration tank 42 via the feed pipe 40. In the ceramic filtration tank 42, silicon carbide membranes having a 0.1 pm to 1.0 pm porosity, preferably 0.1pm, act to filter the adsorption agent material and also particulate pollutant material from the solution. The pollutant material and the adsorption agent are thus brought into close spatial proximity to each other. This increases the frequency of hydroxyl, and other radicals - generated from reaction between the ozone and the adsorption agent - encountering the pollutant material and thus being able to chemically break down the pollutant material. Once the pollutant material has been degraded, the breakdown products pass into solution, passing through the membrane, allowing new pollutant material particles to be brought onto the membrane surface, adjacent the adsorption agent.

[0028] The clean water produced passes through the pores of the membrane. The water at this stage is clear and the Chemical Oxygen Demand (COD) and the Biochemical Oxygen Demand (BOD) are ideally below detection thresholds and so within acceptable limits. The cleaned water is sucked through the pores by means of the centrifugal and self-priming pump 44 which pumps the cleaned water via the pipeline 46 to the outlet line 48. The concentrated aqueous solution and adsorption agent material is discharged from the bottom of the ceramic filtration tank 42. From there, a centrifugal pump 50 pumps the discharged water and adsorption agent back into the adsorption agent mix tank 18 via the drain line 52.

[0029] The adsorption agent material can then be reutilised. This is enabled because during the cavitation / ozonation step, materials are adsorbed onto the surface, or within shallow pores of the adsorption agent material in order to enable the adsorption agent to function. Because the adsorption agent material is in powdered form, having a particle size of from 1pm - 1,000pm, and preferably 5pm, the adsorbed material does not lie deep within a pore of the adsorption agent material. This enables easier recycling of the adsorption agent material.

[0030] The oxidation potential of the adsorbed ozone towards oxidising organic materials due to the ozone being near the catalytic surface is increased, enabling the ozone to form hydroxyl radicals more easily. The high level of hydroxyl radicals generated results in greater breakdown of organic materials than is known from the prior art. There is therefore no need to remove adsorbed material which is often responsible for lowering the catalytic properties of many adsorption agents as the destruction of the organic material into smaller, more soluble molecules is greater. For prior art systems, once the active sites on the adsorbent are occupied, the active material, along with the target molecules or particles need to be removed and safely disposed of. The drain line 52 from the ceramic filtration tank 42 is equipped with 2 valves. The first valve 54 is responsible for the operation of the recirculation loop for the adsorption agent material. The second valve 56 is only opened when water needs to be discharged from the system via the drain line 58. In this manner a closed loop is maintained for the circulation of adsorption agent material.

[0031] In a procedure carried out, 10001 of river water was treated was treated in accordance with the method described above. Using the method, complete removal of PFAS & PFOS group micropollutants, with constant flux rate equal to 135 litres per square meter per hour (Imh), maintaining very low Transmembrane Pressure at -0.15 bar. Samples of a feed water and treated water were analysed for more than 40 parameters of PFAS and PFOS based pollutants detected and measured in the initial sample. None of the measurements were such that the water remained above acceptable and regulated values. Some of the detected micropollutants level, before and after treatment are presented below:

Claims

Claims1. A method of treatment of a microplastics-containing aqueous medium, the method comprising the steps of mixing the medium with a solid adsorption agent, introducing ozone into the medium, lowering the pressure of the medium at a rate sufficient to cause cavitation in the medium, continuing mixing for a predetermined time, forwarding the medium to a filtration vessel, the filtration vessel including a filtration membrane to retain adsorption agent from the aqueous medium.

2. A method according to Claim 1, wherein the aqueous medium is pressurised above atmospheric pressure prior to addition of the ozone.

3. A method according to Claim 2, wherein the aqueous medium is pressurised to around 2.5 bar.

4. A method according to any preceding Claim, wherein the concentration of ozone introduced in the aqueous medium is at a concentration of from 100g / Nm3to 300g / Nm3preferably 240g / Nm3.

5. A method according to any preceding claim, wherein the initial concentration of ozone in the aqueous medium is at least 3mg / l.

6. A method according to any preceding Claim, wherein the pressure lowering is 0.5 bar or above.

7. A method according to Claims 1 - 6, wherein the pressure lowering is 1.0 bar or above.

8. A method according to Claims 1 - 6, wherein the pressure lowering is from 0.5 - 0.8 bar.

9. A method according to any preceding Claim , wherein the filtration vessel is formed of a ceramic material.

10. A method according to any preceding Claim, wherein the filtration membrane is formed of silicon carbide.

11. A method according to Claim 10, wherein the filtration membrane is yet further preferably has a pore size of , of 0.1pm.

12. An apparatus for the treatment of a microplastics-containing aqueous medium, the apparatus comprising a feed tank for water to be treated, the feed tank being fluidly linked to a mixing vessel, an adsorption agent feed tank, fluidly linked to the mixing vessel, said mixing vessel including a mixing means to mix the adsorption agent and the water to form a reaction mixture, the apparatus further comprising a cavitation device having an ozone tank fluidly linked thereto to feed ozone into the cavitation device, the cavitation device including means to induce a pressure drop within the cavitation device, the mixing vessel and the cavitation device being fluidly linked to allow the reaction mixture to flow into the cavitation device, a reaction tank, fluidly linked to receive able to retain the reaction mixture at greater than atmospheric pressure, a filtration tank, fluidly linked to receive reaction mixture from the reaction tank and including filtration means to retain adsorption agent from the reaction mixture.

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