NL2021763B1

NL2021763B1 - Liquid purification method and system

Info

Publication number: NL2021763B1
Application number: NL2021763A
Authority: NL
Inventors: Krasnoholovets Volodymyr; Zabulonov Yuriy; Gandzha Ivan; Puhach Oleksandr; Nikolenko Viktor; Stokolos Maksym
Original assignee: Indra Scient Sa
Priority date: 2018-10-04
Filing date: 2018-10-04
Publication date: 2020-05-11

Abstract

A method for purification of a liquid comprising a contaminant is disclosed. The method comprises letting the liquid flow through at least two serially arranged cavitator chambers, thereby generating in each following cavitator chamber bubbles in the liquid. The method further comprises applying, downstream of the at least two cavitator chambers, a discharging voltage between electrodes in contact with the liquid to cause an electrical discharge in the liquid thereby generating a plasma which interacts with the contaminant. Each cavitator chamber of the at least two serially arranged cavitator chambers may have a shape of a pseudosphere. The process may be enhanced by having a plurality of discharging paths in the reactor chamber.

Description

Title: Liquid purification method and system

The invention relates to a method and a system for purification of a liquid comprising one or more contaminants.

Liquids containing contaminants are a well-known by-product of many different parts of modern society. Industries such as chemical industries, pharmaceutical industries, and agricultural industries often produce liquids with one or more contaminants such as persistent organic molecules that may survive in the liquid for a very long time. Urban areas may produce wastewater that contains many contaminants, such as, make-up, plastics, pesticides, waste and/or other contaminants.

These contaminants may remain in the liquid for a very long time if they are not removed. This may lead to a large and/or negative impact of said contaminants on, for instance, the environment, which may be the case even if the rate at which the contaminants enter the environment is small. Other contaminants may not survive a long time but nevertheless have a large and/or negative impact if they are not removed from the liquid.

Therefore, it may be desirable to remove said contaminants, or a part thereof, from the liquid, preferably in a cost effective and efficient way.

Many different methods of purifying liquids by removing contaminants from liquids or by breaking contaminants into smaller, potentially less harmful, pieces are known in the art. These methods may be chemical or non-chemical in nature. Chemical methods may make use of a chemical interaction of the contaminant with another substance, such as a reagent, catalyst or other substance. Examples of such substances may include substances that are naturally present in the liquid, such as oxygen, some radicals or salt. Chemical methods may have the disadvantage of adding additional chemical substances in the liquid leading to an even less pure liquid. Such chemical substances would have to be removed in an additional purification process.

Non-chemical methods are methods that do not rely on chemical reactions. Non-chemical methods may be based on mechanical processes such as cavitation to create vapour bubbles in the liquid or on electrical and magnetic methods. Electrical and magnetic methods include treatment with light and applying electrical discharges to the liquid.

-2A system and a method for treating flowing water systems are known from US 2014/0326681 A1. The system and method use a high voltage discharge of 200 kV to generate plasma and uses the ozone by-product from the high voltage generation. A reactive gas may be added to the water stream and a cavitator system may be used to create micro-bubbles into a reaction zone within a reaction chamber. The plasma discharges may remove or control growth of microbiological species.

A method and apparatus for generating plasma in a fluid are known from US 2006/0060464 A1. The fluid is placed in a bath having a pair of mutually spaced apart electrodes forming a cathode and an anode. A stream of bubbles is introduced or generated within the fluid adjacent to the cathode. A potential difference across the cathode and anode forms a plasma of ionised gas in the fluid.

The present invention aims to provide an improved purification of a liquid from a contaminant.

The invention refers to a method for purification of a liquid comprising a contaminant, which method comprises the following steps:

- letting the liquid flow through at least two serially arranged cavitator chambers, thereby generating in each following cavitator chamber bubbles in the liquid, and

- applying, downstream of the at least two cavitator chambers, a discharging voltage between electrodes in contact with the liquid to cause an electrical discharge in the liquid thereby generating a plasma which interacts with the contaminant.

The method for purification of a liquid comprising one or more contaminants may for example be applied to the purification of wastewater from a chemical factory, or a pharmaceutical factory, or a dairy factory, or any type of factory that produces liquids comprising one or more contaminants. As another example, the method may be applied for the purification of liquids comprising one or more contaminants produced by urban areas such as sewage water. As a further example, the method may be used for the purification of a liquid comprising one or more contaminants which results from, for example, an environmental disaster. Many other applications where contaminants are to be removed from a fluid, are imaginable.

The liquid comprising one or more contaminants may be water, or any other suitable liquid. The one or more contaminants may be organic or non-organic molecules. For example, the contaminants may be dinitrotoluene, toluenediamine, anilines, nitrogen compounds, or molecules with aromatic rings. The contaminants may also be micro-biological in nature for instance bacteria or moulds.

-3The method comprises the step of letting the liquid flow through a series of serially arranged cavitator chambers. A cavitator chamber may be a channel through which liquid can flow with a narrow part and a wide part, whereby the liquid flows from the wide part ofthe cavitator chamber to the narrow part of the cavitator chamber. The terms wide part and narrow part are to be understood as parts of the cavitator chamber having a wide cross section and narrow cross section respectively, the cross sections seen in a plane perpendicular to a main direction of flow of the liquid in the series of cavitation chambers. As the liquid flows from the wide part to the narrow part of the cavitator chamber the liquid may start flowing faster causing a pressure in the liquid to drop. The liquid may have a higher pressure in the wide part of the channel where the liquid flows slowly, and the liquid may have a lower pressure in the narrow part ofthe channel where the liquid flows faster.

Cavitation occurs when a flow pipe through which the liquid flows is narrowed. The liquid will flow faster through the narrow part of the pipe than through the broader part and hence the liquid will have a lower pressure at the narrower part. A sufficiently low pressure, in relation to a local temperature ofthe liquid, may cause the liquid to locally start boiling, creating vapour bubbles in the liquid. Furthermore, the local pressure changes may result in associated local temperature changes ofthe liquid, which may have an influence on bubble formation.

As the pressure of the liquid may lower in the narrow part of the channel, locally the pressure may drop so low that the liquid may start boiling at temperatures well below the boiling temperature at one atmosphere causing the formation of vapour bubbles in the liquid. Said vapour bubbles, which may in the present document also be referred to as “bubbles”, may contain a gaseous form ofthe liquid. The bubbles may collapse after several milliseconds or minutes depending on external and internal factors such as the size ofthe bubble, the surface tension ofthe bubble, the internal pressure in the bubble, and the external pressure in the liquid.

The cavitation chambers are serially arranged, in that a downstream, narrow part of one cavitator chamber discharges into a wide part of a next, downstream cavitator chamber. Accordingly, each cavitator chamber is provided with an inlet at an upstream end thereof allowing liquid to enter the cavitator chamber and an outlet at a downstream end thereof allowing liquid to leave the cavitator chamber. As the cavitator chambers are serially arranged, the outlet of one cavitator chamber discharges into the inlet of a next, downstream caviator chamber. As the liquid flows through a channel which comprises the serially arranged cavitator chambers it flows from one cavitator chamber to the next undergoing pressure changes. It is possible to

-4have multiple channels with multiple serially arranged cavitator chamber series in parallel having the multiple channels meet downstream.

The dimensions of the cavitator chambers of the serially arranged cavitator chambers may be chosen depending on an application. The application may reflect a type of liquid which has to be purified, the contaminants comprised in the liquid, a total volume of the liquid, and other factors.

For example, for the purification of water a cavitator chamber in the cavitator chambers may have a radius in the wide part between 40 mm to 50 mm, a radius in the narrow part between 2 mm and 4 mm and a distance between the wide part and the narrow part between 40 mm to 60 mm. Bubbles created in cavitator chambers of these dimensions may for example have diameters between 10 micrometres and 500 micrometres.

The method comprises the step of applying, downstream of the series of cavitator chambers, a voltage difference over electrodes in contact with the liquid to cause an electrical discharge in the liquid. The potential applied to the liquid may result in generating a non-thermal plasma in the liquid which interacts with the one or more contaminants. The non-thermal plasma may comprise moving electrons and ions, and may be formed at the walls of the bubbles. The term non-thermal plasma is to be understood as plasma whereby the negatively charged electrons and the positively charged ions have different rates of movement. For example, the electrons may exhibit a higher movability as compared to the ions, thereby reflecting a higher temperature.

The electrical discharge may be applied downstream of the series of cavitator chambers, i.e. after the liquid has passed the serially arranged cavitator chambers. The electrical discharge may be performed by applying a discharging electrical voltage, such as discharging electrical pulses, to electrodes in contact with the liquid. Two or more electrodes may be used for the creation of a periodic voltage difference leading to the periodic electrical discharge in the liquid. A voltage difference of e.g. up to 30kV may be needed to generate a non-thermal plasma in the liquid depending on the distance of between the electrodes, the conductivity of the liquid, size and quantity of bubbles, and other factors.

The formed non-thermal plasma may interact with the contaminant, e.g. by breaking long molecular chains or circular forms of complex stable biological molecules to smaller, possibly relatively unstable, molecules or by ionising the contaminant. The ionised molecules may react with other charged particles in the liquid creating reaction products in the liquid. Contaminants

-5may be oxidised to inactive compounds more effectively after being treated by the plasma which may also create reaction products in the liquid.

The liquid may comprise reaction products of the contaminants after that are naturally present in the liquid or may be removed naturally by an ecosystem. The liquid may also comprise reaction products of the contaminants that are not naturally present or may not be removed by an ecosystem. The reaction products, preferably the ones that are not naturally present or removable by an ecosystem, may be removed after treatment using, for instance, a mechanical or a chemical process such as filtration or coagulation.

It may be advantageous to fill a large volume of liquid with plasma, so as to have an efficient purification method and to be able to process a largest amount of liquid possible. The method may be applied for the treatment of a volume per second of liquid of, for example, 300 ml/s.

The person skilled in the art assumed that bubbles created in a cavitator chamber would collapse when the liquid would undergo a rise in pressure. Hence, the person skilled in the art would not contemplate to subject the liquid, that has passed a cavitation chamber, to a following cavitation chamber, as the following cavitation chamber would result in a pressure rise, which would result in a collapse of bubbles as created in the cavitation chamber through which the liquid has passed.

The inventors have realized that this is only partially true. As the liquid flows from one cavitator chamber to the next in the serially arranged cavitator chambers, the liquid undergoes a rise in pressure. The inventors have realised that larger bubbles, for instance those with radii around 500 micrometres, may tend to collapse on average faster than smaller bubbles, for instance those with radii of 10 micrometres. This can be explained as follows: From balancing pressure inside the bubble with pressure outside the bubble it may be deducted that smaller bubbles have a higher internal pressure than larger bubbles. A smaller bubble, one having a higher internal pressure, will be more resilient to a rise in external pressure than a larger bubble, one having a lower internal pressure. When external pressure rises on average the bigger bubble will collapse first because the rising external pressure will overcome the internal pressure earlier.

As the liquid flows from one cavitator chamber to the next the external pressure will rise and some bubbles may survive the transition while other bubbles may collapse. On average the surviving bubbles will be smaller than the average of the bubbles generated in a cavitator chamber since the smaller bubbles will be more likely to survive.

-6The inventors have therefore realized, that, due to some on average smaller bubbles surviving, an overall concentration of bubbles in the liquid may increase with each cavitator chamber while an average size of the bubbles in the liquid may decrease with each cavitator chamber.

Hence, the inventors have realized that a sequence of serially arranged cavitator chambers may lead to a liquid with a high concentration of bubbles of relative small average size. The concentration of bubbles downstream of the serially arranged cavitator chambers may increase as the number of cavitator chambers increases and the average size of the bubbles may decrease as the number of cavitator chambers increases. Given the on average small size of the bubbles in the liquid, the bubbles may be relatively stable, hence may exhibit a low tendency to collapse over time. After flowing through a sufficient number of cavitator chambers, the liquid may be filled with a large number of relatively small and relatively stable bubbles, and may flow in this state to the reactor chamber where the liquid is subject to electrical discharging.

When an electrical discharge is propagating through the liquid, a current path of the electrons may tend to flow along an interface between liquid and gas (i.e. vapour bubble).

The gas in the bubbles may in each bubble undergo a phase change to the non-thermal plasma state. A ratio of surface area and volume of an object may relate to an object size: the smaller a size of the object, the larger the ratio of surface area and volume may tend to be. For example, in the case of relatively small bubbles, an effective overall surface of the interface between liquid and gas may be larger than for larger bubbles, thereby enabling a more effective creation of plasma, as the discharging electrons that may promote the creation of the plasma may flow along the interface of liquid and gas. Furthermore, given the relatively large interface between the bubbles and the liquid, the plasma may tend to penetrate the liquid in a more effective way, hence may tend to result in an enhanced interaction with the contaminant.

As a result of the relatively large interface surface between the liquid and the gas in the bubbles, due to relatively small bubbles, and as a result of the large density of vapour bubbles, a magnitude of an overall electrical current required to initiate the plasma generation may be relatively small. Thus, a relatively low electrical voltage differential may suffice to initiate the plasma generation process. Hence, a discharging voltage as may be applied to the electrodes may be relatively low.

-7The liquid to be purified may be any liquid, such as water. Thus, the method may be applied as a water purification method. Other applications are imaginable, such as purifying any other suitable liquid.

Radicals may be created in the liquid by cavitation. Radicals may be created as a product of bubble creation and/or collapse. Radicals my react with the contaminant in the liquid, before or after treatment. Hence, radicals may aid in the purification process.

Electrical discharges in the liquid may create a plasma in the liquid. Plasma is a partially or fully ionised gas and may consist of electrons, free radicals and ions. Plasma may be thermal or nonthermal depending on the relative electron and ion temperatures.

Plasma formation may lead to various physical and chemical effects, including formation of oxidizing species, shockwaves, ultraviolet light and cavitation. It may be used for the degradation of long and complex molecules such as pesticides, phenols, anilines, organic dyes, and other compounds.

As the at least two cavitator chambers provide for a high concentration of relatively small vapour bubbles in the liquid, plasma generation (and the associated generation of free radicals) may be promoted, enabling to promote interaction of the plasma with the contaminant.

In this document, the term purification of a liquid is to be understood as an (at least partial) removal of a contaminant from a liquid or an (at least partly) conversion of the contaminant into another, possibly less contaminating or possibly more easily removable, substance.

The wide part of a cavitator chamber of the serially arranged cavitator chambers may be connected upstream directly with the narrow part of the upstream cavitator chamber so that the liquid, as it flows through the serially arranged cavitator chamber is undergoing pressure changes. Accordingly, liquid discharged from the narrow part of a cavitator chamber will discharge into the wide part of a next, downstream cavitator chamber where the liquid, discharged at a relatively high velocity, will flow into liquid at a relatively high pressure in the wide part of the next cavitator chamber, which may cause the velocity of the discharged liquid to reduce and which may result in, for example, turbulence effects in the wide part. Furthermore, because of the direct connection, volumes of liquid in between the cavitator chambers, where

-8collapse of bubbles may occur, may be avoided to a large extent, thus avoiding to a large extent a reduction in bubbles in between successive cavitator chambers.

Alternatively, a connecting duct may be arranged between an outlet of a cavitator chamber and an inlet of a following, downstream cavitator chamber, The connecting duct may have a cross section or diameter e.g. comparable to the wide part of the downstream cavitator to which the duct connects or the narrow part of the upstream cavitator to which the duct connects.

In an embodiment, the cavitator chambers have a shape of a pseudosphere. The pseudosphere may be defined as a surface with constant negative Gaussian curvature. Locally, the cavitator chambers may have constant negative Gaussian curvature. The pseudosphere of the cavitator chambers may be truncated at the wide part and the narrow part of the cavitator chamber.

The pseudosphere may result in a relatively low flow resistance of the liquid that passes through the cavitator chamber.

As a result of the relatively low flow resistance which the liquid undergoes when passing the pseudosphere, the liquid flow is subjected to a low flow resistance enabling that a liquid pressure in following, downstream cavitator chambers may remain largely intact. Thus, even when making use of a relatively large number of cavitator chambers arranged in series connection, the cavitator effect (resulting in changes in pressure, temperature and speed of the liquid, and the associated generation of vapour bubbles) may remain present in the downstream cavitator chambers.

Furthermore, the pseudosphere may result in a relatively long passage where the liquid flows at a high speed and low pressure, hence providing a relatively large volume per cavitator chamber where the creating of vapour bubbles is promoted.

The serially arranged cavitator chambers may all have a same size or they may have different sizes. For example, a size of the downstream cavitator chambers may be smaller compared to a size of the upstream cavitator chambers, in particular the narrow part of the downstream cavitator chambers may be smaller than the narrow part of the upstream cavitator chambers, thus on the one hand at the downstream side reducing a bubble size even further, while on the upstream side keeping flow resistance to a more modest level given the larger size (e.g. diameter) of the narrow part of the cavitators. The wide part of a downstream cavitator chamber has a bigger radius than the narrow part of an upstream cavitator chamber.

-9The serially arranged cavitator chambers may be provided with any suitable plurality of cavitator chambers. For example, the inventors have found that increasing a number of serially arranged cavitator chambers in a range to between 5 and 20 may increase a number of bubbles while decreasing a bubble size. In experiments, it followed that between 10-15 cavitator chambers provided practicable results as described above, providing a high bubble concentration and small bubble sizes, promoting the plasma generation as described. The inventors have found that increasing the number of cavitator chambers increases the bubble concentration in the liquid. Each cavitator chamber may increase the concentration of bubbles by about 40%. The higher the concentration of bubbles the higher the effectiveness of the purification system. Increasing the number of cavitator chambers thus increases the effectiveness of the system.

The liquid may be supplied to the e.g. serially arranged cavitators by means of any suitable liquid supply means, such as a liquid pump or, in the case of a downwardly sloping liquid supply duct or downward liquid supply duct, by liquid pressure e.g. as a result of gravitational force leading the liquid downwards.

The discharging voltage may be generated by a discharging voltage generator. The discharging voltage generator may e.g. generate periodic high voltage peaks, e.g. by means of induction, e.g. making use of an inductance of a transformer and associated periodic switching and interruption from a primary voltage supply. Any other means to generate the high voltage may be applied. For example, a direct-current high voltage may be generated and periodically electrically connected to the electrodes by a corresponding switch.

In order to enable to substantially enhance the above process, in an embodiment, at least 3 electrodes are provided, the electrodes being spaced apart to provide at least two spaced apart discharging paths where the plasma is generated, wherein the discharging voltage is periodically applied to the electrodes at a discharging voltage repetition time, and wherein the discharging voltage repetition time is set below a lifetime of plasma gas bubbles generated along the discharging path.

Discharging paths are paths in the liquid along which the discharge travels between two electrodes.

Plasma bubbles are understood to bubbles in the liquid filled with plasma. After creation of plasma bubbles, they may collapse, slowly decreasing in size. A plasma bubble may experience a lifetime during which it may collapse. A collapse of a plasma bubble may take

- IQ20 milliseconds. A creation of a plasma bubble may be caused by the discharging voltage and may be nearly instantaneous.

The electrodes are arranged to be in contact with the liquid and spaced apart to provide at least two spaced apart discharging paths in the liquid. At least two electrodes are placed with a separation in a direction of liquid flow so as to create at least two discharging paths that are separated in a direction of liquid flow. Along the discharging paths, the discharges between the electrodes take place. The discharging voltage is applied periodically, i.e. repetitively, at a discharging voltage repetition time, thus causing periodic discharges. At a discharge, when no plasma was present along the discharging paths, plasma gas bubbles are created, and when plasma bubbles are present, but experiencing collapse, the plasma bubbles are forced to increase their size e.g. to their original size, e.g. effectively reverting the collapse and forcing the plasma bubble to restart with the collapse. In both cases plasma bubbles are said to be created.

At the creation of the plasma gas bubbles, an acoustic wave is generated which may propagate through the liquid and the plasma bubble. The acoustic wave may propagate through the liquid as an acoustic wave and through plasma as an ion wave. At the collapse of the plasma gas bubbles, again an acoustic wave may be created as a result of the shrinking size in relation to the collapse, which acoustic wave may likewise propagate through the liquid as an acoustic wave and through plasma as an ion wave. A shockwave created by a partial collapse, one that is reverted due to an electrical discharge, may be lower amplitude than a shockwave created by a full collapse.

A following discharging is applied to the electrodes during lifetime of the plasma gas bubbles,

i.e. while the plasma gas bubbles are collapsing. An interaction between the acoustic waves caused by the collapse of plasma gas bubbles and the acoustic waves caused by the creation of the plasma gas bubbles at a next discharge may be provided. In addition, an interaction between the acoustic waves caused by either the creation or collapse of two plasma gas bubbles created in separate discharging paths may be provided.

Acoustic ion waves traveling inside the plasma gas bubble may create zones of high pressure inside the plasma gas bubble.

The inventors have found that the interaction between the acoustic waves caused by the collapse of plasma gas bubbles and the acoustic waves caused by the creation of the plasma gas bubbles, provide for (e.g. locally) elevated conditions, such as an e.g. locally elevated

- 11 temperature and/or locally pressure, which may bring the liquid, e.g. locally where the acoustic waves interact, into or near supercritical fluid conditions, and or which may bring the fluid, preferably the plasma gas bubbles, into conditions where supercritical oxidation may occur. Supercritical oxidation is understood to be oxidation in highly favourable conditions such as favourably pressure and temperature.

Possibly, zones with supercritical oxidation conditions are found more favourably in plasma bubbles located on downstream discharging paths. The liquid that forms these plasma bubbles may already have been treated by the plasma along upstream discharging paths making the liquid more susceptible to plasma formation and formation of supercritical zones in the plasma. Hence, purification process may be more efficient in downstream plasma bubbles.

As a result of these conditions, a reaction speed at which the interaction with the contaminant takes place may be highly enhanced, which may provide for a substantial increase in reaction speed, involving e.g. the creation of free radicals, oxidation of the contaminant, decomposition of the contaminant, etc.

Thereby, on the one hand a reaction time may be substantially decreased, for example from minutes to seconds or even from tens of minutes to seconds. On the other hand, given the increased speed of the reaction, a substantially larger volume of the liquid may be purified from the contaminant. Still further, an energy consumption of the purification process may be substantially reduced, as, given the possible substantial reduction in reaction time, the time period during which the discharges are to be applied to the liquid, may be substantially reduced.

Furthermore, the inventors have observed that plasma oscillations may occur in the generated plasma gas bubbles. The oscillations may enhance an interaction between the plasma and the contaminant. Furthermore, the plasma oscillations may be observed electrically when measuring a voltage waveform during the lifetime of the plasma gas bubbles, in particular near an end of the lifetime where a stability of the plasma gas bubble may decrease. The plasma oscillations, e.g. formed by ion acoustic waves, may for example relate to the acoustic waves propagating in the fluid and/or to interaction of the plasma gas bubble with an electric circuit (the discharging voltage generator) that generates the discharging voltage.

- 12 The repetitive discharges may be supported by the high density of small (vapour and/or oxidizer) bubbles as may be provided in the liquid. Thus, the repetitive generation of plasma gas bubbles, and the associated interaction of the acoustic waves at the collapse of the plasma gas bubbles and the acoustic waves at the creation of the following plasma gas bubbles which may cause (e.g. local) supercritical fluid or near supercritical fluid, may be supported by the series arrangement of plural cavitation chambers.

In an embodiment, the discharging repetition time period is set to at least five times lower than the lifetime of the plasma gas bubbles, more preferably ten times lower.

Hence, as the plasma gas bubble collapses the collapse is reverted well below the lifetime of the plasma gas bubble. As a result the acoustic waves from collapse may be much smaller than the acoustic waves from creation. This may aid in the creation of zones of higher pressure since shockwaves of lower pressure (those due to collapse) are substantially smaller in amplitude.

As acoustic waves propagate to another one of the discharging paths, e.g. the adjacent discharging path the acoustic waves may interact with acoustic waves created in the another one of the discharging paths.

Two acoustic waves due to creation of plasma may interact to create a zone of high pressure in the fluid, preferably the plasma. Zones of high pressure are particularly favourable for oxidation reactions. In a zone where two acoustic waves due to creation of plasma gas bubbles interact supercritical oxidation may take place.

The inventors have devised that the plasma gas bubble lifetime may be in an order of magnitude of milliseconds, for instance 20 milliseconds. Accordingly, in line with such plasma gas bubble life times, the discharging repetition time period may be set in a range of 1 milliseconds -10 milliseconds, preferably 1 milliseconds - 5 milliseconds, more preferably 1 milliseconds - 3 milliseconds, thereby providing following discharges while the plasma gas bubble is still in existence.

Any suitable plurality of electrodes may be provided. In an embodiment, at least 5 electrodes are arranged along a direction of flow of the liquid, preferably at least 8 electrodes are arranged along a direction of flow of the liquid. The electrodes that are arranged along a direction of flow of the liquid may for example cooperate with a single common counter electrode, e.g. arranged at an opposite side of the reaction chamber, thus a discharging path

- 13crossing the reaction chamber is a direction perpendicular to a flow direction of the liquid in the reaction chamber. Alternatively, a counter electrode may be provided per electrode arranged along the direction of flow of the liquid or per subgroup of the electrodes arranged along the direction of flow of the liquid. Accordingly, plural discharging paths are generated. The electrodes arranged along the direction of flow are arranged downstream of each other.

For instance, for 5 electrodes arranged along the direction of flow of the liquid, the 5 electrodes cooperating with 5 counter electrodes, 5 discharging paths may be formed, namely one discharging path per electrode - counter electrode pair. The discharging paths downstream of each other. Alternatively, the 5 electrodes arranged along a direction of flow of the liquid may cooperate with a common counter electrode to form 5 discharging paths downstream of each other. In another example, with 8 electrodes arranged along the direction of flow of the liquid and 8 counter electrodes, i.e. where each electrode cooperates with a counter electrode, 8 discharging paths may be created, the discharging paths downstream of each other. It is noted that the 8 electrodes arranged along a direction of flow of the liquid, i.e. arranged downstream of each other, may alternatively cooperate with a common counter electrode or respective counter electrodes per group, e.g. per pair, of electrodes. Making use of at least 5, more preferably at least 8 electrodes downstream of each other, an enhancement of the above described effects of plasma generation, and in particular the reaching of supercritical or near supercritical conditions may be facilitated.

In an embodiment at least three electrodes are used to create the periodic electrical discharge. One (or more) of the electrodes may be placed further downstream from the other one of the electrodes. This may create a longer discharging path resulting in a larger area for the formation of the non-thermal plasma. The non-thermal plasma is advantageously created in a as large volume as possible so as to treat a large volume of liquid more efficiently and let little untreated liquid go through the system.

In an embodiment, at least one of the electrodes is displaceable in a direction towards another one of the electrodes. The electrode may for example be displaceable relative to the reactor housing along a lengthwise axis thereof. Accordingly, a distance between two electrodes in the reactor chamber may be set within a suitable range, e.g. between 1 mm and 35 mm. The setting of the electrode mutual distance may depend on the liquid to be treated, on the contaminants which are comprised in the liquid, on the voltage to be discharged, and on other factors. If electrodes are placed closer to each other the required voltage may be lower than when the electrodes are placed further apart. A distance between

- 141 mm and 35 mm may allow for a large volume flow of the liquid and a relatively low voltage across the electrodes.

In an embodiment the method further comprises injecting, by an injector, an oxidizer into the liquid, and mixing, downstream of the injector, the oxidizer with the liquid. The injector may be comprise any type of injector, such as an injector nozzle, an injector valve, etc.

An oxidizer may help in the formation of the non-thermal plasma, since by introducing the oxidizer gas in the liquid more bubbles are formed leading to the advantages discussed in this document. Furthermore, the oxidizer may directly react with the contaminants in the liquid, thereby possibly helping in the purification of the liquid. The oxidizer may interact more easily with the contaminants when the plasma is formed.

The mixing of the oxidizer with the liquid may be carried out by the flow of the liquid, the flow resulting from the liquid flowing through the serially arranged cavitation chambers. To achieve an effective mixing, in an embodiment the oxidizer may be mixed in the liquid using a further cavitator chamber. When an oxidizer is added to the liquid, the further cavitator chamber may mix the oxidizer more homogeneously with the liquid bubble mixture as a result of liquid flow that may be created in the cavitation chamber. Hence, other mixing operations, that may tend to promoting a collapse of bubbles, may be omitted. Furthermore, the further cavitator chamber may assist to increase a concentration of the oxidizer in the (vaporized liquid) bubbles, as a result of the bubble generation in the further cavitator chamber.

The further cavitator chamber may further aid in the creation of bubbles in the liquid thus helping in filling the liquid with bubbles. The bubbles after mixing may contain a mixture of oxidizer and gas (e.g. vaporized liquid). The further cavitator chamber may also aid in protecting the serially arranged cavitator chambers from pressure waves due to plasma formation by creating a pressure barrier or other acoustic barrier between the reactor chamber and the serially arranged cavitator chambers.

In an embodiment the oxidizer is injected downstream of the serially arranged cavitator chambers. Injecting the oxidizer in a liquid already containing bubbles may lead to a better mixing of the oxidizer with the liquid bubble mixture.

In an embodiment, the oxidizer is either air, oxygen, or ozone.

- 15In an embodiment the liquid is densely filled with bubbles after the series of cavitator chambers. The liquid is said to be densely filled with bubbles if the volumetric concentration of bubbles in the liquid is higher than 60%. A densely filled liquid can be obtained by serially arranging sufficient cavitator chambers. A densely filled liquid may be the most suited for creating a non-thermal plasma.

In an embodiment a magnetic field is applied along a discharging path between the electrodes, i.e. in the area where the non-thermal plasma is created. The magnetic field may have a number of advantages. As the magnetic field may interact with the charged particle flow in the discharging, the volume through which the charged particles (e.g. electrons) flow, may be increased, which may increase the area that is covered by the electrical discharge, thereby possibly increasing the volume of the created non-thermal plasma and the efficiency ofthe method. Furthermore, as the volume in the liquid through which the discharging takes place, increases, given the effectively wider discharging trajectory between the electrodes, an electrical conductivity of the discharging path may increase, which may also lower the threshold voltage over the electrodes for plasma creation. For example, the threshold voltage may drop to 15 kV.

In an embodiment a coagulant is added to the liquid to aid in the purification of the liquid by forming agglomerates of contaminants in the water. The coagulant may be added upstream ofthe serially arranged cavitator chambers, upstream ofthe reactor chamber but downstream of the serially arranged cavitator chambers, or downstream of the reactor chamber. The coagulants may react with the contaminants in the liquid creating agglomerates of the contaminants which may be removed from the liquid by a variety of processes such as cyclone, flotation, or filtration. Depending on the type of liquid and contaminant a different coagulant may be chosen.

In an embodiment the liquid is filtered using a filtration unit. The filtration unit may be placed upstream of the serially arranged cavitator chambers, upstream of the reactor chamber but downstream ofthe serially arranged cavitator chambers or downstream ofthe reactor chamber. The filtration unit may filter the liquid and remove any large contaminants which, for example, may have been formed by the addition of a coagulant or may already have been present in the liquid. The removal of such larger contaminants may be beneficial for the purification ofthe liquid using the non-thermal plasma. The filtration unit may also be used to remove contaminants from the liquid after the liquid has been treated by the non-thermal plasma.

- 16In an embodiment the liquid is flowing through the serially arranged cavitator chambers, the discharging voltage being applied periodically, and a following one of the discharges is applied when at least part of the liquid along the discharging path between the electrodes has been replaced as a result of the flowing of the liquid. When a discharge has been applied and a plasma has been formed in the liquid the liquid/plasma mixture may continue flowing making room for a new portion of liquid between the electrodes. The new portion of the liquid may be ready to receive an electrical discharge and to be treated by the non-thermal plasma. The frequency of the discharges may be such that the treated liquid has left the reactor chamber when a new discharge occurs.

In an embodiment the liquid is irradiated with Ultra Violet (UV)-light from a UV-light source. Some contaminants may react to UV-light and a liquid comprising some of these contaminants may be treated with UV-light. Depending on the liquid, contaminants and further setup, the liquid may be irradiated upstream of the serially arranged cavitator chambers, upstream of the reactor chamber but downstream of the serially arranged cavitator chambers, or downstream of the reactor chamber.

The invention also refers to a system for purification of a liquid from a contaminant, which system comprises:

- at least two serially arranged cavitator chambers, which serially arranged cavitator chambers are adapted to generate bubbles in the liquid,

- a reactor chamber, located downstream of the serially arranged cavitator chambers, which is in fluid communication with the serially arranged cavitator chambers, and which comprises at least two electrodes in fluid connection with the liquid,

- a discharging voltage generator connected to the electrodes and adapted to generate a discharging voltage over the electrodes in the reactor chamber to cause an electrical discharge to occur in the liquid in the reactor chamber, the electrical discharge cause a formation of a plasma in the liquid, which interacts with a contaminant.

The system of the invention may be used in the method of the invention.

The system comprises a discharging voltage generator which is connected to the electrodes and adapted to generate a discharging voltage over the electrodes in the reactor chamber to cause an electrical discharge to occur in the liquid in the reactor chamber. The generated electrical discharge may cause formation of a plasma in the liquid, which may interact with a contaminant.

- 17The discharging voltage generator may comprise a pulse generator and a high-voltage transformer. The pulse generator may generate pulses with frequencies up to 500 Hz. The working voltage of the high-voltage transformer may be 20-30 kV.

In an embodiment each cavitator chamber of the at least two serially arranged cavitator chambers of the system has a shape of a pseudosphere.

In an embodiment a narrow part of a cavitator chamber of the at least two serially arranged cavitator chambers is immediately connected with a wide part of a following, downstream cavitator chamber of the at least two cavitator chambers.

In an embodiment the system comprises between 5 and 20 serially arranged cavitator chambers, more preferably between 10 and 15 serially arranged cavitator chambers.

In an embodiment the system may further comprise an injector which is adapted to inject an oxidizer in the liquid and a mixing unit, located downstream of the injector, which is adapted to mix the oxidizer the bubbles in the liquid.

In an embodiment the mixing unit comprises a further cavitator chamber. Using a cavitator chamber as a mixing unit may be advantageous since it may simultaneously allow for a simple mixing mechanism and a bubble creation mechanism.

In an embodiment the injector is located downstream of the at least two serially arranged cavitator chambers. Adding an oxidizer to a liquid already containing bubbles may be advantageous over adding an oxidizer in a liquid before the bubbles are formed since it may allow better mixing between the liquid vapour and the oxidizer in the bubbles.

In an embodiment the oxidizer may be air, oxygen, or ozone.

In an embodiment the at least two serially arranged cavitator chambers are adapted to densely fill the liquid with bubbles.

In an embodiment, the system comprises a magnetic field generator adapted to generate a magnetic field along a discharging path in the reactor chamber. The magnetic field generator may be permanent magnets or electric magnets. The magnetic field may extend the area of the periodic electrical discharge increasing the volume where plasma is created and it may

- 18also lower the threshold voltage where non-thermal plasma is created, lowering the voltage needed for plasma creation.

In an embodiment at least three electrodes are located in the reactor chamber and one of the electrodes is located downstream from another one ofthe electrodes. Having electrodes separated along the stream of liquid may increase the volume of liquid that receives an electrical discharge thus increasing the volume of liquid that is treated by a non-thermal plasma created in said electrical discharge.

In an embodiment the system further comprises a filtration unit for filtering the liquid. Said filtration unit can be located anywhere.

In an embodiment at least one electrode is movable along a direction towards another electrode, a distance between the electrodes in the reactor chamber preferably being variable in a range from 1 mm to 35 mm. Placing electrodes closer together lowers the voltage threshold for discharge but may also lower the volume of plasma created. The distance may be varied depending on the use ofthe system.

In an embodiment the system further comprises a UV-light source for irradiating UV-light into the liquid. It is known that some contaminants react with UV-light. Irradiating the liquid with UV-light may improve the functionality of the system.

In an embodiment the cavitator chambers ofthe serially arranged cavitator chambers all have a same size. It may be advantageous to create a bubble population in the liquid with a very homogeneous distribution of sizes. This may be achieved by having the cavitator chambers all ofthe same size.

Furthermore, it is noted that the above described repetitive generation of plasma gas bubbles, and the associated interaction of the acoustic waves at the collapse of the plasma gas bubbles and the acoustic waves at the creation of the following plasma gas bubbles which may cause (e.g. local) supercritical fluid or near supercritical fluid, could be performed in other cavitator arrangements, i.e. other than the series arrangement of plural cavitatior chambers.

Accordingly, according to a further aspect, there is provided

A method for purification of a liquid comprising a contaminant, which method comprises the following steps:

- generating bubbles in the liquid, and

- applying, downstream of the bubble generation, a discharging voltage between electrodes in contact with the liquid to cause an electrical discharge in the liquid thereby generating a plasma which interacts with the contaminant, wherein at least 3 electrodes are provided, the electrodes being spaced apart to provide at least two spaced apart discharging paths where the plasma is generated, wherein the discharging voltage is periodically applied to the electrodes at a discharging voltage repetition time period, and wherein the discharging voltage repetition time period is set below a lifetime of plasma gas bubbles generated along the discharging path.

According to a yet further aspect, there is provided a system for purification of a liquid from a contaminant, which system comprises:

- a bubble generator which is adapted to generate bubbles in the liquid,

- a reactor chamber, located downstream of the bubble generator, which is in fluid communication with the bubble generator, and which comprises at least three electrodes in fluid connection with the liquid,

- a discharging voltage generator connected to the electrodes and adapted to generate a discharging voltage over the electrodes in the reactor chamber to cause an electrical discharge to occur in the liquid in the reactor chamber, the electrical discharge cause a formation of a plasma in the liquid, which interacts with contaminant, the electrodes being spaced apart to provide at least two spaced apart discharging paths for generation of the plasma, wherein the discharging voltage generator is arranged to periodically apply the discharging voltage to the electrodes at a discharging voltage repetition time period, and wherein the discharging voltage repetition time period is set below a lifetime of plasma gas bubbles generated along the discharging path.

The further features and embodiments as described with reference to the method and system according to the invention may apply, mutatis mutandis to the method according to the further aspect of the invention and the system according to the yet further aspect of the invention.

Further features, advantages and effects of the invention are explained in relation to the appended drawing, and the associated below description, wherein non-limiting embodiments of the invention are disclosed in which:

Fig. 1 shows a schematic overview of an embodiment of a liquid purification system;

Fig. 2 shows a cross sectional view of an embodiment of serially arranged cavitator chambers of the liquid purification system;

Fig. 3 shows a cross sectional view of an embodiment of a reactor chamber of the liquid purification system;

-20Fig. 4 shows a cross sectional view of an embodiment of serially arranged cavitator chambers placed in series with an injector and a mixing unit of the liquid purification system;

Fig. 5 shows a cross sectional view of an embodiment of a reactor chamber of the liquid purification system; and

Fig. 6 shows a graphical representation of a size of a plasma gas bubble in time in relation to a voltage discharge over time.

It is noted that, throughout the figures, the same reference numbers refer to the same or similar elements.

Fig. 1 shows a schematic overview of a liquid purification system. A section of a channel, labelled A, is shown in fig. 1 through which a liquid may flow comprising a contaminant. In embodiments, the liquid may be pumped through section A with a pressure between 0.5 bars and 3 bars. The pressure with which the liquid is pumped may depend on the type of liquid, the type of contaminant, a size of the cavitator chambers 2, and other factors.

Section A may be connected upstream with a liquid reservoir or a drain system that may supply the liquid purification system with liquid. In embodiments section A may comprise an injector and a mixing unit, a filtration unit, or a UV-light source. These may aid in the purification of the liquid by improving the efficiency of the plasma generating process, by removing contaminants from the liquid, by reacting with contaminants in the liquid, or in other ways.

Said liquid flows from left to right in fig. 1 hence the liquid flows from section A into at least two serially arranged cavitator chambers 1. Each one of the at least two serially arranged cavitator chambers 2 is wider at the upstream end 3 of the cavitator chamber 2 than on the downstream end 4 of the cavitator chamber 2. As the liquid flows from the wider part 3 to the narrower part 4 of each of the at least two serially arranged cavitator chambers 2, they may induce a local drop in pressure in the liquid, causing the liquid to locally start boiling, which may generate vapour bubbles in the liquid. Once the liquid flows out of the serially arranged cavitator chambers 1 the liquid comprises vapour bubbles.

For example, for the purification of water a cavitator chamber 2 in the serially arranged cavitator chambers 1 may have a radius in the wide part 3 between 40 mm to 50 mm, a radius in the narrow part 4 between 2 mm and 4 mm and a distance between the wide part 3 and the narrow 4 part between 40 mm to 60 mm. Bubbles created in cavitator chambers 2 of these dimensions may for example have diameters between 10 micrometres and 500 micrometres. For cavitator

-21 chambers 2 of this size the liquid may flow through a narrow part 4 of a cavitator chamber 2 with a speed of 0.5 m/s to 1 m/s.

Section B of the channel is in fluid connection with the serially arranged cavitator chambers 1 upstream and with a reactor chamber 5 downstream as shown in fig. 1. The liquid filled with vapour bubbles flows in section B between the serially arranged cavitator chambers 1 and the reactor chamber 7. In embodiments section B may comprise an injector and a mixing unit, a filtration unit, or a UV-light source which may aid in the purification of the liquid.

The liquid filled with vapour bubbles may flow through section B to the reactor chamber 5 where at least two electrodes 7 are in contact with the liquid. The electrodes 7 in fig. 1 are connected with a discharging voltage generator 13. The discharging voltage generator 13 is adapted to create a voltage difference over the electrodes 7 in the liquid. The voltage difference may induce an electrical discharge in the liquid which generates a non-thermal plasma in the liquid. For example, the voltage difference may be 20-30 kV before an electrical discharge is induced in the liquid.

The voltage generator 13 may comprise a pulse generator with a frequency up to 500 Hz and a high voltage transformer that is made of powder iron and does not enter the saturation regime at the operating current and voltage. The peak current during discharge may be 1500 A. The duration of a discharge may vary between 50 ps and 150 ps. The power consumption of the voltage generator may be 4 kWh per cubic meter of liquid treated.

In embodiments the electrodes 7 may be placed with a small off-set in the flow direction of the liquid to create a larger area that receives the electrical discharge. This may lead to a larger volume of liquid to be treated by the non-thermal plasma, which may be advantageous for purifying the liquid of contaminants.

In an embodiment, at least one of the electrodes 7 is displaceable in a direction towards another one of the electrodes 7. The electrode 7 may for example be displaceable relative to the reactor housing along a lengthwise axis thereof. Accordingly, a distance between two electrodes 7 in the reactor chamber 5 may be set within a suitable range, e.g. between 1 mm and 35 mm. The setting of the electrode 7 mutual distance may depend on the liquid to be treated, on contaminants which are comprised in the liquid, on the voltage to be discharged, and on other factors. If electrodes 7 are placed closer to each other the required voltage may be lower than when the electrodes 7 are placed further apart. A distance between 1 mm and

-2235 mm may allow for a large volume flow of the liquid and a relatively low voltage across the electrodes 7.

After having been treated by the non-thermal plasma the liquid may flow out of the reactor chamber 5 to section C of the channel. Section C of the channel may be connected downstream with a liquid reservoir or a drain system where the purified liquid may be stored or transported away from the purification system. In embodiments section C may comprise a filtration unit or a UV-light source to further treat the liquid.

The liquid flowing through section C may have been treated by the non-thermal plasma. The non-thermal plasma may still be present in the liquid in section C or the non-thermal plasma may have disappeared from the liquid. Some of the contaminants comprised in the liquid may have been broken down to smaller, preferably less contaminating, molecules or they may have reacted with molecules present in the liquid to form other molecules.

In fig. 2 a cross sectional view of serially arranged cavitator chambers 1 adapted for the method of the system of the invention is shown. The serially arranged cavitator chambers 1 of fig. 1 comprise nine cavitator chambers 2, each cavitator chamber 2 having the shape of a pseudosphere. The cavitator chambers 2 have a wide part 3, located at the upstream end of the cavitator chamber 2, and a narrow part 4, located at the downstream end of the cavitator chamber.

As can be seen, in the embodiment of fig. 2, the narrow part 4 of a first cavitator chamber 2 is immediately connected with the wide part 3 of a second chamber 2 located downstream of the first cavitator chamber 2. A liquid flowing through the serially arranged cavitator chambers 1 may undergo pressure changes. When the pressure drops, as the liquid flows to a narrower part 4 of a cavitator chamber 2, vapour bubbles may start forming in the liquid. As the pressure rises, for instance when the liquid flows to a wider part 3 of a cavitator chamber 2, some bubbles may collapse while others, on average the smaller bubbles, may not collapse.

Letting a liquid flow through the sequence of serially arranged cavitator chambers 1 may lead, downstream of the serially arranged cavitator chambers 1, to the liquid having a high concentration of bubbles of relative small average size. Having nine cavitator chambers 2 such as in fig. 2 may be sufficient to have the liquid densely filled with bubbles. This may aid in the formation of a non-thermal plasma and the subsequent treatment of the liquid by the plasma.

-23Cavitator chambers 2 may facilitate in the formation of radicals, such as HOH*, HO* HsO⁺. The radicals may be formed due to the energy released by the collapse of bubbles. Radicals may aid in the formation of plasma in the liquid by making the liquid more conductive and hence the discharge voltage lower.

Fig. 3 shows a cross sectional view of a reactor chamber 5 adapted for the method or the system of the invention. A liquid may flow through the reactor chamber 5 through the flow channel 6. The reactor chamber 5 shown in fig. 3 comprises three electrodes 7 that may induce an electrical discharge when a voltage difference is applied over them. A non-thermal plasma may be created in the flow channel 6 between the three electrodes 7 as a result of the electrical discharge. In an embodiment a distance between the electrodes 7 may be varied between 1 mm and 35 mm.

In the embodiment of the reactor chamber 5 as shown in fig. 3 two magnets 8 are placed around the reactor chamber 5. The magnets may maintain a magnetic field in the reactor chamber which may extend the area of the electrical discharge. This may increase the volume where plasma is created. In addition, the magnetic field may lower the threshold voltage where non-thermal plasma is created.

In the embodiment of the reactor chamber 5 as shown in fig. 3 a quartz tube 9 is placed in the reactor chamber. Said quartz tube 9 may protect a UV-light source which may be used to radiate the liquid and break contaminants into smaller molecules.

An aeration unit 10 or filtration unit 10 may be placed in the reactor chamber. An aeration unit 10 may aid in collecting agglomerates of contaminants created by adding coagulant to the liquid. A filtration unit 10 may aid in removing contaminants from the liquid.

Fig. 4 shows a cross sectional view of two serially arranged cavitator chambers 1a, 1b placed in series with an injector 11 and a mixing unit 12. The serially arranged cavitator chambers 1a, 1b, the injector 11, and the mixing unit 12 may be in fluid connection with the reactor chamber of fig. 3.

A liquid comprising contaminants may flow through the serially arranged cavitator chambers 1a, 1b and vapour bubbles may be created in serially arranged cavitator chambers 1a, 1b. Arranging multiple serially arranged cavitator chambers 1a, 1b to create bubbles in the liquid may increase the total volume of liquid where bubbles are created.

-24In this embodiment an injector 11 is placed downstream of the serially arranged cavitator chambers 1a, 1b. The liquid comprising contaminants and bubbles may flow from the serially arranged cavitator chambers 1a, 1b to the injector 11. An oxidizer may be added to the liquid by using the injector 11. The oxidizier may aid in the plasma formation process and the subsequent purification of the liquid.

A mixing unit 12 is located downstream of the injector unit 11 which may mix the oxidizer with the liquid. The oxidizer may be mixed with the liquid vapour in the bubbles or it may form oxidizer bubbles in the liquid. The mixing unit 12 comprises a cavitator chamber to aid in the creation of additional bubbles in the liquid. The cavitator chamber of the mixing unit 12 has the shape of a pseudosphere.

Fig. 5 shows a cross sectional view of an embodiment of a reactor chamber 5 of the liquid purification system where a discharging voltage is periodically applied. A liquid enters the reactor chamber 5 through a flow channel 6. The liquid flow direction is indicated by an arrow. Three pairs of electrodes 7 are provided giving three discharging paths. Along the discharging paths three plasma gas bubbles 13 are formed and sustained by applying a periodic discharging voltage between the electrodes 7. The discharging voltage repetition time period is set below a lifetime of plasma gas bubbles which may be 20 milliseconds. Shockwaves 14 may be created in the plasma gas bubbles 13 and in the liquid.

As liquid enters the reactor chamber 5 it encounters the first discharging path where a plasma gas bubble 13 is present. The bubbles in the liquid may become ionised forming part of the first plasma gas bubble 13. As a result of the periodic voltage discharges shockwaves may be formed in the plasma and near the plasma gas bubble creating zones of high pressure, preferably zones of supercritical density. In the zones of high pressure oxidation reactions may be favourable. Shockwaves of neighbouring plasma gas bubbles may also interact creating positive interference and hence zones of high pressure.

As the liquid leaves the first discharging path it has been treated by a first plasma gas bubble 13 and it may be more susceptible to the formation of a second plasma gas bubble 13 and/or the formation of shockwaves14 in the second plasma gas bubble 14. As the liquid enters the second discharging path a second plasma gas bubble 13 is formed.

As the liquid leaves the second discharging path it has been treated twice by plasma gas bubbles 13 and it may improve the treatment level and it may be even more susceptible to the formation of plasma gas bubbles. The liquid may be treated several times by several

-25discharging paths, each discharging path may improve the treatment level of the liquid and the effectiveness of the following plasma treatment.

Shockwaves may also aid directly in purification of the liquid by weakening the molecular bonds of the contaminants.

Fig. 6 shows a graphical representation of a size of a plasma gas bubble in time in relation to a voltage discharge over time. The horizontal axis has a time coordinate which increases to the right. The upper vertical axis has a size of a plasma gas bubble coordinate which increases upwards. The lower vertical axis has a discharging voltage coordinate which increase upwards.

As can be seen in the graph, after a discharging has occurred a size of the plasma gas bubble 13 may start to shrink. After some time period the size of the plasma gas bubble has decreased by a certain amount and a voltage discharge is induced, represented by a square peak in the lower graph. This forces the size of the plasma gas bubble to rapidly increase. After the discharge the size of the plasma gas bubble will slowly start to decrease again until another voltage discharge occurs. This process may repeat itself. In this way a plasma gas bubble of a certain minimal size may be maintained in a plasma discharging path.

As a result of the multiple discharges, the shockwaves and resonances, critical or near critical reaction conditions may occur, which may significantly shorten a reaction time, thus may significantly shorten a purification time of the liquid.

The following are examples wherein the system was tested. The system of the examples uses two parallel series of nine serially arranged cavitator chambers to create bubbles. Each cavitator chamber has a radius in the wide part of about 4 mm, a radius in the narrow part of 2 mm and a distance between the wide part and the narrow part of about 35 mm. The number of electrodes was different in the two experiments described below and a distance between the electrodes was 10-15 mm. The voltage over the electrodes was 25 kV and the voltage was discharged every 3.3 ms. Oxygen was added as an oxidized element.

-26In a first experiment wastewater comprising a chemical oxygen demand (COD) of 1350 mg/kg dinitrotoluene (DNT) was treated by the purification system. The number of electrodes used was

6. 10 L of the wastewater was cycled through the purification system for 25 minutes at a flow rate of 100 L/h. After treatment the original COD was reduced to 2 mg/kg, improving the purity of 5 the wastewater.

In a second experiment wastewater comprising a chemical oxygen demand of 2950 mg/kg toluenediamine (TDA) was treated by the purification system. The number of electrodes used was 10. 10 L of the wastewater was cycled through the purification system for 20 minutes at a 10 flow rate of 100 L/h. After treatment the original COD was reduced to 2 mg/kg, improving the purity of the wastewater.

Claims

A method of purifying a liquid comprising a contaminant, the method comprising the following steps:

- flowing the liquid through at least two cavitation chambers arranged in series, generating bubbles in the liquid in each subsequent cavitation chamber, and

applying downstream of the at least two serially arranged cavitation chambers a discharge voltage between electrodes in contact with the liquid to cause an electric discharge in the liquid generating a plasma interacting with the contaminant.

The method of claim 1, wherein each cavitation chamber of the at least two cavitation chambers arranged in series is in the form of a pseudosphere.

The method of claim 1 or 2, wherein a narrow portion of the cavitation chamber of the at least two cavitation chambers arranged in series is immediately connected to a wide portion of a subsequent downstream cavitation chamber of the at least two cavitation chambers.

A method according to any of the preceding claims, wherein the liquid flows through between 5 and 20 serially cavitation chambers, more preferably between 10 and 15 serially arranged cavitation chambers.

The method of any preceding claim, wherein at least 3 electrodes are provided, the electrodes being spaced apart to provide at least two spaced discharge paths where the plasma is generated, the discharge voltage being periodically applied to the electrodes having a discharge voltage repetition time period, and wherein the discharge voltage repetition time period is set lower than a lifetime of plasma gas bubbles generated along the discharge path.

The method of claim 5, wherein the discharge repetition time period is set at least five times less than the life of plasma gas bubbles generated along the discharge path, more preferably ten times shorter.

The method of claim 5 or 6, wherein the discharge repetition time period is set in a range of 1 millisecond - 10 milliseconds, preferably 1 millisecond - 5 milliseconds, more preferably 1 millisecond - 3 milliseconds.

A method according to any one of claims 5 to 7, wherein at least 5 electrodes are arranged along a direction of flow of the liquid, preferably wherein at least 8 electrodes are arranged along a direction of flow of the liquid.

A method according to any preceding claim, wherein at least one electrode is movable along one direction to another electrode.

The method according to any of the preceding claims, further comprising:

injecting, through an injector, an oxidizing agent into the liquid, and

- mixing, downstream of the injector, the oxidant with the bubbles in the liquid.

The method of claim 10, wherein the mixing of the oxidant with the bubbles in the liquid is performed through a further cavitation chamber.

The method of claim 10 or 11, wherein the oxidizing agent is injected downstream of the at least two series cavitation chambers into the liquid.

The method of any one of claims 10-12, wherein the oxidizing agent is air, oxygen or ozone.

A method according to any preceding claim, wherein the at least two series-arranged cavitation chambers fill the liquid with bubbles.

A method according to any preceding claim, wherein a magnetic field is applied between the electrodes along a discharge path.

A method according to any preceding claim, wherein a coagulant is added to the liquid.

A method according to any preceding claim, wherein the liquid is filtered through a filtration unit.

The method of any preceding claim, wherein the liquid flows through the serially-arranged cavitation chambers, the discharge voltage being applied periodically, and another of the discharges being performed when at least a portion of the liquid is along the discharge path between the electrodes due to the flow of the liquid has been replaced.

A method according to any preceding claim, wherein the method further comprises irradiating the liquid with a UV light source.

20. System for purification of a liquid from a contaminant, the system comprising:

- at least two serially arranged cavitation chambers, which serially arranged cavitation chambers are arranged to generate bubbles in the liquid,

- a reactor chamber located downstream of the serially arranged cavitation chambers, which is in fluid communication with the serially arranged cavitation chambers, and which comprises at least two electrodes in fluid communication with the liquid,

a discharge voltage generator connected to the electrodes and arranged to generate a discharge voltage across the electrodes in the reactor chamber so as to cause an electric discharge in the liquid in the reactor chamber, the electric discharge forming a plasma in the liquid causes it to interact with the contaminant.

The system of claim 20, wherein each cavitation chamber of the at least two cavitation chambers arranged in series has the shape of a pseudosphere.

The system of claim 20 or 21, wherein a narrow portion of a cavitation chamber of the at least two cavitation chambers arranged in series is immediately connected to a wide portion of a subsequent, downstream cavitation chamber of the at least two cavitation chambers.

System according to any one of claims 20-22, comprising between 5 and 20 serially arranged cavitation chambers, more preferably between 10 and 15 serially arranged cavitation chambers.

The system of any one of claims 20-23, comprising at least 3 electrodes, the electrodes being spaced apart to provide at least two spaced discharge paths for generation of the plasma, wherein the discharge voltage generator is arranged to periodically adjust the discharge voltage

-30 apply to the electrodes with a discharge voltage repetition time period, and wherein the discharge voltage repetition time period is set lower than a lifetime of plasma gas bubbles generated along the discharge path.

The system of claim 24, wherein the discharge voltage generator is arranged to set the discharge repetition time period at least five times less than the life of the plasma gas bubbles, more preferably at least ten times lower.

The system of any one of claims 24-25, wherein the discharge voltage generator is adapted to set the discharge repetition time period in a range of 1 millisecond - 10 milliseconds, preferably 1 millisecond - 5 milliseconds, more preferably 1 millisecond - 3 milliseconds.

System according to any one of claims 24 to 26, comprising at least 5 electrodes arranged along a direction of flow of the liquid, preferably at least 8 electrodes arranged along a direction of flow of the liquid.

The system of any one of claims 20 to 28, further comprising:

- an injector adapted to inject an oxidizing agent into the liquid,

a mixing unit located downstream of the injector adapted to mix the oxidant with the bubbles in the liquid.

The system of claim 28, wherein the mixing unit includes a further cavitation chamber.

The system of any one of claims 28 to 29, wherein the injector is located downstream of the serially arranged cavitation chambers.

The system of any one of claims 28 to 30, wherein the oxidizing agent is air, oxygen, or ozone.

The system of any of claims 20 to 31, wherein the at least two cavitation chambers arranged in series are arranged to fill the liquid with bubbles.

The system of any of claims 20-32, comprising a magnetic field generator adapted to generate a magnetic field along a discharge path in the reactor chamber.

The system of any one of claims 30 to 33, wherein the system further comprises a filtration unit.

The system of any one of claims 20 to 34, wherein at least one electrode is movable along one direction toward the other electrode, wherein a distance between the electrodes in the reactor chamber is preferably variable in a range from 1 mm to 35 mm is.

The system of any of claims 20 to 35, wherein the system further comprises a UV light source for radiating UV light into the liquid.

The system of any one of claims 20 to 36, wherein the cavitation chambers of the cavitation chambers arranged in series are all the same size.

38. A method of purifying a liquid comprising a contaminant, the method comprising the following steps:

generating bubbles in the liquid, and

- applying, downstream of the bubble generation, a discharge voltage between electrodes in contact with the liquid to cause an electric discharge in the liquid thereby generating a plasma that interacts with the contaminant, providing at least 3 electrodes, the electrodes are spaced to provide at least two spaced discharge paths where the plasma is generated, the discharge voltage is periodically applied to the electrodes having a discharge voltage repetition time period, and the discharge voltage repetition time period is set lower than a lifetime of plasma gas bubbles generated along the discharge path.

39. System for purifying a liquid from a contaminant, the system comprising:

- a bubble generator adapted to generate bubbles in the liquid,

- a reactor chamber, located downstream of the bubble generator, in fluid communication with the bubble generation, and comprising at least three electrodes in fluid communication with the liquid,

a discharge voltage generator connected to the electrodes and arranged to generate a discharge voltage across the electrodes in the reactor chamber to cause an electric discharge in the liquid in the reactor chamber, the electrical

32 discharge causes a formation of a plasma in the liquid that interacts with a contaminant, the electrodes being spaced apart to provide at least two spaced discharge paths for generation of the plasma,

5 wherein the discharge voltage generator is arranged to periodically apply the discharge voltage to the electrodes having a discharge voltage repetition time period, and wherein the discharge voltage repetition time period is set lower than a lifetime of plasma gas bubbles generated along the discharge path.