WO2013030591A1

WO2013030591A1 - An electrocoagulation apparatus

Info

Publication number: WO2013030591A1
Application number: PCT/GB2012/052143
Authority: WO
Inventors: Carlos Filipe Duarte; Duarte Tito; David Jones; Oliver PUCKERING; Nicola RANDLES
Original assignee: Aguacure Limited
Priority date: 2011-08-31
Filing date: 2012-08-31
Publication date: 2013-03-07
Also published as: GB2494299B; GB201215567D0; GB2494299A

Abstract

An electrocoagulation method comprising : providing an electrochemical cell comprising at least one cathode and at least one anode having a consumable surface, introducing a first liquid into the electrochemical cell, passing a current between the anode and cathode, such that the consumable surface of the anode dissolves in the first liquid, providing conditions that allow a floe to form in the first liquid to provide a floc-containing-liquid, holding the floc-containing-liquid for a residence time to allow floe growth to occur, and contacting the floc-containing-liquid with a second liquid.

Description

AN ELECTROCOAGULATION APPARATUS

The present invention relates to an electrocoagulation method and, in particular, an electrocoagulation method for removing contaminants from a liquid.

It is often necessary to remove contaminants from a liquid, especially water, to render the liquid suitable for use or discharge into the environment. Contaminants include metal ions, such as arsenic, chromium, copper, cadmium, nickel, lead and zinc; suspended solids, such as silt and clay; organic compounds, such as

hydrocarbons; and salts, such as phosphates.

Various methods of removing contaminants from liquids are known. For example, a contaminated liquid may be treated with a solution of a coagulating agent, which reacts with the contaminants to form insoluble compounds that aggregate or flocculate to form larger particles. These larger particles can then be separated by physico-chemical methods, such as by settling, filtration and/or flotation .

As an alternative to chemical coagulation, contaminants can also be removed from liquids by electrocoagulation.

Electrocoagulation is similar to chemical coagulation in that it relies on the reaction between a coagulating agent and the

contaminants to form insoluble compounds, which aggregate or flocculate to form larger particles. With electrocoagulation, however, a current is passed between an anode and a cathode to induce the corrosion of the anode in the liquid under treatment, a result, the anode dissolves in the liquid, thereby forming a coagulant solution which is subsequently treated to form a floe, conventional electrocoagulation processes, the electrodes are in physical contact with the liquid or effluent under treatment, and the coagulating agent is delivered into the liquid to be treated directly. The rate of corrosion and, hence, the rate of dosing, can be controlled by varying the current through the electrodes.

Although such prior art processes may be used to treat certain liquids, they cannot be used to treat a wide variety of liquids efficiently and cost-effectively. For example, some liquids may require expensive pre—treatment steps (e.g. to remove suspended solids, which may obstruct flow through the electrocoagulation unit) Furthermore, some liquids can cause deactivation or passivation of the electrodes, reducing the efficiency and efficacy of the

electrocoagulation process.

It is among the objects of embodiments of the present

invention to provide a more efficient electrocoagulation process . It is also among the objects of embodiments of the present invention to provide an electrocoagulation process that can be used to treat a wide variety of liquids or effluents .

According to a first aspect of the present invention, there is provided an electrocoagulation method comprising:

providing an electrochemical cell comprising at least one cathode and at least one anode having a consumable surface,

introducing a first liquid into the electrochemical cell, passing a current between the anode and cathode, such that the consumable surface of the anode dissolves in the first liquid,

providing conditions that allow a floe to form in the first liquid to provide a floc-containing-liquid,

holding the floc-containing-liquid for a residence time to allow floe growth to occur, and

contacting the floc-containing-liquid with a second liquid.

According to a second aspect of the present invention, there is provided an electrocoagulation apparatus or system comprising

an electrochemical cell comprising at least one cathode and at least one anode having a consumable surface, whereby, upon passing an electrical current between the anode and the cathode, the consumable surface of the anode dissolves in liquid present in the electrochemical cell,

means for providing conditions that allow a floe to form in the liquid to provide a floc-containing-liquid,

means for retaining the floc-containing-liquid for a residence time to allow floe growth to occur,

means for contacting the floc-containing-liquid with a second liquid,

optionally, a treatment zone for facilitating further floe growth in a mixture comprising the floc-containing-liquid and the second liquid,

and, optionally,

a separator for separating the floe formed in the treatment zone .

When current is passed between the anode and cathode, the consumable surface of the anode dissolves in the first liquid. In the present invention, the process is controlled to provide

conditions that allow a floc-containing-liquid to form. In

particular, the pH and/or the residence time are controlled to allow the floc-containing-liquid to form and/or grow in a desirable manner. It is also desirable to ensure that the metal concentration in the floc-containing-liquid is sufficiently high, as, as discussed in further detail below, this improves both the quality of the floe produced and the efficacy of the coagulant.

In a preferred embodiment, the process of the present

invention involves controlling at least one of i) pH, ii) the residence time and iii) the metal concentration in the floc- containing-liquid to allow the floc-containing-liquid to form and/or grow. Preferably, all three of i) , ii), and iii) above are

controlled to allow the floc-containing-liquid to form and/or grow in an optimised manner. It may also be desirable to control the level of oxygen in the liquid to facilitate floe growth. The target oxygen concentration may vary depending on, for example, the

coagulant type and nature of the first liquid. The process may be controlled such that floe begins to form in the electrochemica 1 cell. Alternatively, the anode may dissolve in the first liquid t 0 form a coagulant solution in the electrochemical cell, which is then subjected to conditions to allow the floc to form. For example , the pH of the coagulant solution may be adjusted (e.g. after remova 1 from the electrochemical cell) to initiate floe formation. Thereafter, the resulting floc-containing-liquid may be retained for a residence time to allow floe growth to occur, As explained in greater detail below, however, contacting a floc- containing-liquid with the second liquid provides advantages

(improved removal of contaminants and lower residual coagulant levels in the treated second liquid) over contacting a coagulant solution with the second liquid.

Preferably, the floc-containing-liquid is held or retained in a storage vessel or flow cell for a residence time to allow floe growth to occur. The storage vessel or flow cell may be coupled to the electrochemical cell via fluid communication means, such as a pipe. Fluid communication means may also be provided to fluidly couple the storage vessel or flow cell to the volume of second liquid. Accordingly, the process or system of the present invention may be operated continuously or semi-continuously, if desired.

In the process of the present invention, a first liquid is introduced into the electrochemical cell. When an electrical current is passed between the anode and cathode of the cell, the consumable surface of the anode dissolves in the first liquid. The conditions of the first liquid are preferably controlled such that seed particles instantaneously or quickly form to develop a floc- containing-liquid . Accordingly, when the floc-containing-liquid is mixed with the second liquid, the floes react with contaminants in the second liquid to form large floes. The floe of coagulated contaminants can then be separated from the second liquid by, for example, settling, filtration and/or flotation, allowing the second liquid to be purified. One or more electrochemical cells may be provided.

Preferably, the coagulant formed by the dissolution of the anode in the first liquid forms a floc-containing-liquid . This occurs when the seed particles grow into floe as a result of aggregation and/or flocculation with themselves and/or with

suspended particles and/or contaminants in the first liquid. This may be called the first-floc-containing-liquid to distinguish it from the floe formed in the second liquid.

Preferably, once the consumable surface of the anode has dissolved in the first liquid to form a floc-containing-liquid, the floc-containing-liquid is then retained in a storage vessel or flow cell for a residence time after leaving the electrochemical cell, and is then passed from the electrochemical cell into the second liquid via fluid communication means .

Preferably, once the consumable surface of the anode has dissolved in the first liquid to form a floc-containing-liquid, the floc-containing-liquid is retained for less than 20 min, less than 10 min, more preferably less than 5 mins . In one embodiment, the floc-containing-liquid is held for more than 2 seconds but less than 3 minutes, preferably, less than 2 minutes, and more preferably, less than 1.5 minutes. For example, the floc-containing-liquid may be held for less than 1 min, or most preferably between 10 and 40 seconds before being introduced into the second liquid. As

discussed above, the residence time may desirably be controlled. Other factors that may be controlled include pH, metal coagulant concentration and/or oxygen levels, oxidation-reduction potential (ORP) , to optimise floe formation and/or growth. Typically the first floc-containing-liquid is transferred directly to the second liquid, however, it may undergo intermediate treatment steps prior to being introduced to the second liquid.

In one embodiment of the invention, the floc-containing-liquid that is contacted with the second liquid has a zeta potential of 0 to +30mV. Accordingly, floe formation and growth may be optimised to achieve a zeta potential within this range prior to contact with the second solution. This optimisation may be achieved by

controlling the residence time. Optimisation may also be achieved by controlling other factors, such as pH, metal coagulant

concentration and/or oxygen levels, as mentioned above.

Alternatively or additionally, the floc-containing-liquid that is contacted with the second liquid desirably has an average particle size of less than 3.5 mm. The floe in the floc-containing- liquid should be visible to the naked eye and/or be capable of settling prior to contact with the second liquid. Accordingly, floe formation and growth should be optimised to achieve the desired floe size prior to contact with the second solution. This optimisation may be achieved by controlling the parameters mentioned above.

The present inventors have found that several advantages are associated with producing a floc-containing-liquid from a first liquid and then subsequently passing the floc-containing-liquid into a second liquid. (Examples of such a method are when the first liquid is a side stream of the second liquid, when the first liquid is a pure electrolyte, or when the first liquid is a sidestream of the treated second liquid, and wherein the conditions of the first liquid such as pH, residual oxygen, total suspended solids and other parameters are controlled prior to introduction into the

electrochemical cell, and the first liquid is then introduced into the electrochemical cell where a floc-containing liquid is formed, which is then mixed with the second liquid) . Using the method described herein allows the process to be carried out such that direct contact between the electrodes and contaminants in the second liquid can be reduced or avoided. This means that passivation of electrodes may be reduced or avoided. Such a method also means that blockage of the electrochemical cell and/or deposition due to contaminants (for example oil and fat, suspended solutions) which are present in the second liquid may be avoided or reduced. The present method also avoids or reduces undesirable competitive processes occurring, for example flotation. The possibility of using a sidestream of the second liquid as the first liquid means that pre-treatment of the second liquid is limited to a small volume or flowrate . The possibility of using a sidestream of the treated second liquid or a different liquid such as a clean electrolyte solution means that pre-treatment of the second liquid can be completely avoided and also presents the benefit that flow through and current applied to the electrochemical cell can be

intermittently stopped, allowing for any passivation formed on the electrodes to be removed through dissolution into the first liquid.

The process described herein allows greater versatility and/or a broader range of treatment. For example, the present process enables a second liquid (for example an effluent) having a high solids loading to be treated. The process also enables a second liquid (for an example effluent) with passivating chemistry (with regards to electrocoagulation) to be successfully treated, (for example a nickel plating bath, or wastewaters containing fats, oils or greases ) .

Moreover, the present inventors have found that several advantages are associated with forming a floc-containing- liquid which is subsequently passed from the electrochemical cell into a second liquid, over forming a coagulant solution which is

subsequently passed from the electrochemical cell into a second liquid. These advantages include one or more of (i) increases in the rate of floe formation in the second liquid; (ii) improvement in the strength of the floe formed in the second liquid; (iii) larger and/or more stable floe formation; (iv) lower residual coagulant levels; (v) lower or substantially no colloidal coagulant formation (vi) reduced coagulant requirement and (vii) faster settlement rates of the floe formed in the second liquid. Typically all these advantages are observed when a floc-containing-liquid compared to coagulant solution is added to a second liquid. This results in the process being more efficient and in improved contaminant removal rates. For example, by using a floc-containing-liquid as opposed to a coagulant solution, the turbidity, suspended solids content and/or residual coagulant content may be reduced. Without wishing to be bound by any particular theory it is thought that increases in the size of floe formed in the second liquid are observed when a floe is introduced into the second liquid, rather than when a coagulant solution is introduced because floe is allowed to pre-form at higher coagulant concentrations (in the first liquid) irrespective of the final dosing rate. The size of floe formation may be monitored visually during growth. Another method of monitoring floe size formation, which is not typically carried out during the process, but may be carried out in order to optimise the process is to measure the coagulant (for example Fe) concentration in solution after a period of settlement. The larger the floe size, the lower is the residual concentration of coagulant (for example Fe) post settlement.

Lower residual coagulant levels in the second liquid are advantageous . As used herein the term "residual levels" means the amount of coagulant remaining in the second liquid after treatment. The amount of coagulant (for example Fe or Al) in the second liquid after treatment may be measured and its discharge level may be regulated. Also, since the coagulant may be bound to contaminants, residual concentrations of coagulant also carry residual levels of contaminant. Residual coagulant in the second liquid after treatment means reduced efficacy of the coagulant and increasing operating costs. Thus, lower residual coagulant levels in the second liquid are desirable.

Faster settlement times may reduce the required size of

downstream process equipment and residency time for treatment (i.e. contaminant removal) .

Preferably, the floc-containing-liquid formed from the first liquid has a pH of from 5 to 9, more preferably around 7. In one embodiment, the process involves controlling or tailoring the pH to allow the floc-containing-liquid to form in an optimised manner.

The pH may be controlled within, for example, 6 to 9 or 7 to 8. The precise value will depend on a number of factors, including, for example, the nature of the metal of the anode. One advantage of this is that the pH of the first liquid (for example liquid in the side stream) may not need to be subjected to exactly the same pH constraints as the second liquid.

To the inventors' knowledge, it is not known to introduce a pre-formed floc-containing-liquid into a second liquid to be treated in order to provide improved coagulation in a second liquid in the manner described herein. This is in contrast to other methods wherein a chemical and/or metal ion solution may be prepared as a solution at low pH, which is then dosed into liquid to be treated. In this latter case, floc-formation only occurs after addition of the chemical and/or metal ion solution (coagulant solution) to the water/liquid to be treated.

The present inventors have found that further advantages are associated with the present invention, which include the avoidance of chemical handling and/or the avoidance or reduction of coagulant solution and/or chemical storing. This is because the floc- containing-liquid may be formed in-situ in the first liquid and be transferred to the second liquid by fluid communication means.

Further advantages are associated with directly treating by electrocoagulation (i.e. contacting the electrodes) with a smaller volume of liquid (i.e. when the volume of the first liquid is smaller than the volume of the second liquid) than the resulting amount of liquid to be treated. These include reduced system footprint and greater ease of retro-fitting.

Typically as the flow rate of the first liquid is typically in the order of ten to one hundred times less than the flow rate of the second liquid, the amount of hardware (e.g. pumps, pipework, valves etc) required to handle this smaller flowrate is much smaller

(smaller footprint) and cheaper than would be required to directly treat the second liquid.

Moreover, a smaller system is generally easier to retro-fit into an existing infra-structure due to fewer space constraints . In the process of the present invention, it is not necessary for the liquid or effluent requiring treatment to be treated in the electrochemical cell (although a portion of the effluent may be so treated, if desired) . In contrast, a first liquid may be selected for treatment in the electrochemical cell to produce a floc- containing-liquid for dosing into the liquid or effluent requiring treatment (second liquid) . This opens up the possibility of treating a wide range of liquids by electrocoagulation. Moreover, the concentration of the floc-containing-liquid produced may be varied by varying the current passed between the anode and cathode of the cell. In this way, the concentration of the floc-containing- liquid may be tailored according to the characteristics of the second liquid. This allows a concentrated floc-containing-liquid to be produced for dosing into the second liquid. Such a concentrated floc-containing-liquid can be produced with a relatively low volume of first liquid. As a result, the size of the electrochemical cell required to treat a given volume of second liquid is reduced compared to the size of a conventional electrocoagulation cell used to treat liquid directly. This allows the process to be carried out in locations where space is limited, for example, on board a ship. Preferably, for every 100 m³ of second liquid to be treated, 1 to 50 m³, more preferably 1 to 10 m³, even more preferably 1 to 5 m³ of first liquid is introduced into the electrochemical cell. This compares with 100 m³ of liquid that would need to be fed into the electrochemical cell of a conventional electrocoagulation unit.

The process of the present invention may be operated as a batch process. However, in a preferred embodiment of present invention, the floc-containing-liquid, once retained for an

appropriate residence time, is passed from the electrochemical cell into the second liquid via fluid communication means fluidly linking the electrochemical cell to the second liquid. Preferably, the floc-containing-liquid is passed continuously or substantially continuously into the second liquid, for example, at a desired flow/drip rate. The rate will depend on a range of factors, including, for example, the concentration of the floc-containing liquid and the characteristics of the second liquid to be treated. However, by way of illustration, for a single cell with an internal square section with a width of 0.057 m and a length of 0.059 m and variable height, example velocities range from 0.005 to 0.070 m/s, more preferably 0.010 to 0.035 m/s and most preferably 0.020 to 0.025 m/s. Velocities as high as 3 m/s may also be suitable.

The ratio of the volume of the floc-containing-liquid

delivered to the second liquid to the volume of second liquid to be treated is desirably 1:5 - 1:100, preferably 1:10 - 1:100, more preferably 1:50 - 1:100.

Preferably, the floc-containing-liquid is not stored in a static (i.e. non-flowing) reservoir prior to being introduced into the second liquid. This avoids the need for storage containers, reducing the hardware requirements of the overall process. Moreover, by avoiding such a storage step, degradation of the efficacy of the floc-containing liquid is reduced or eliminated. This opens up the possibility of using a wider range of liquids in the electrochemical cell. In this connection, the floc-containing-liquid produced in the electrochemical cell may be less pure than solutions of

coagulating agents typically used in conventional chemical

coagulation techniques. For example, in one embodiment of the invention, impure liquid streams, such as seawater and waste water (e.g. grey water or black water) may be used as the first liquid introduced into the electrochemical cell. Although coagulant solutions and/or floc-containing-liquid produced from such liquids may degrade with extensive storage, this may not be a significant issue with the process of the present invention, since the coagulant solution and/or floc-containing-liquid may be dosed into the second liquid as or shortly after it is formed. Preferably, the floc- containing-liquid is dosed into the second liquid within two hours, preferably within one hour from the time when the consumable surface of the anode dissolves into the liquid.

As discussed above, a current is passed between the anode and cathode, such that the consumable surface of the anode dissolves in the first liquid to form a floc-containing-liquid . The rate of corrosion is controlled by varying the current through the

electrodes. The current, therefore, can be used to vary the concentration of floc-containing-liquid produced or, when adjusted together with the flow rate of the first liquid through the

electrochemical cell, the volume of the floc-containing-liquid produced. In this way, the process can be controlled by tailoring the current through the electrochemical cell to the characteristics of the second liquid to be treated. For example, the current through the electrochemical cell may be tailored according to the conductivity, volume or flow rate of the second liquid.

Alternatively or additionally, the current may be tailored according to the impurity content of the second liquid. Examples of such impurities include the liquid's phosphorus (e.g. as phosphate) content, heavy metal content and/or suspended solids. It may also be possible to tailor the current to the colour, COD and/or BOD of the second liquid. Since there is preferably fluid communication between the electrochemical cell, means for retaining the floc- containing-liquid and the second liquid to be treated, the current through the electrochemical cell is desirably a function of the volume or flow rate of the second liquid. Additionally and/or alternatively, the flowrate through the electrochemical cell may be controlled as a function of the flowrate and composition of the second liquid.

Any suitable liquid may be introduced into the electrochemical cell as the first liquid. Preferably, the first liquid comprises water. Water from a range of sources may be employed, including, for example, seawater, river water, brackish water or waste water. Examples of waste water streams include grey water or black water. The waste water may be from a domestic, agricultural or industrial process. If desired, a clean water stream may also be used.

Although the first liquid need not be the same as the second liquid under treatment, it may be possible to use a portion of the second liquid as or as part of the first liquid. Effectively, therefore, the first liquid may be or include a side-stream from the second liquid requiring treatment. This side-stream may form 1 to 100 volume %, preferably 50 to 100 volume %, from 80 to 100 volume %, or from 60 to 90 volume % of the volume of second liquid that is mixed with the floc-containing-liquid produced in the electrochemical cell,

The first liquid may be pre-treated before it is introduced into the electrochemical cell. Suitable pre-treatment steps include filtration, pH adjustment and oxygenation/aeration . A filtration step may be required to ensure that the solids content of the first liquid can be tolerated in the electrochemical cell. As an

alternative or in addition to the filtration step, a liquid having a reduced solids content may be included in the first liquid to ensure that the liquid introduced into the electrochemical cell has an appropriate solids content. A pH adjustment step may be required to ensure that the first liquid is of a suitable pH for

electrocoagulation to occur. An oxygenation or aeration step may be required to ensure that the levels of dissolved oxygen in the first liquid are sufficient, for example, to facilitate the desired oxidation of soluble to non-soluble particles. This oxygenation or aeration step may be carried out by bubbling oxygen or air into the first liquid, for example, prior to treatment in the electrochemical cell. It may also be possible for air to be introduced directly into the electrochemical cell.

The first liquid, with optional pre-treatment, is then

introduced into the electrochemical cell. An electrical current (DC or AC) is then passed between the anode and cathode, such that the consumable surface of the anode dissolves in the first liquid to form a floc-containing liquid. During this electrochemical step, gases, such as hydrogen, oxygen and chlorine, may be produced.

Preferably, therefore, the process of the present invention further includes a step of gas removal. For example, gas removal may be carried out before or after the floc-containing-liquid is mixed with the second liquid. It may also be desirable to monitor the

concentration of gases, such as chlorine and hydrogen, in the plant e.g. for safety reasons. Any suitable liquid may be used as the first liquid. Examples include impure water streams, such as seawater, river water, brackish water or waste water. Examples of waste water streams include grey water or black water. The waste water may be waste water from a domestic, agricultural or industrial process. One or more of the contaminants which may be present in the first liquid may be desirable products.

The conductivity of the first liquid may be adjusted prior to entry into the electrochemical cell in order to ensure good

electrical efficiency in the cell and to aid the speed of conversion of metal ions to floe. In conventional electrocoagulation wherein the entire water/ wastewater (second liquid) is passed through the electrochemical cell, adjustment of conductivity may not be

practically possible as this would cause difficulties with further processing/ discharge of the treated water/ wastewater. However, passing only a small volume of first liquid through the

electrochemical cell and mixing first liquid with the water/ wastewater to be treated (second liquid) allows the conductivity of first liquid to be adjusted without significantly altering the conductivity of second liquid. By way of example, the conductivity of first liquid may preferably be at least 500 uS/cm.

The cathode (s) and anode (s) of the electrochemical cell may be formed of any suitable material. For example, the anode (s) and/or the cathode (s) may comprise of at least one of the following:

aluminium, iron, copper, graphite, reticulated vitreous carbon and a dimensionally stable electrode (e.g. alloy) . Where the anode (s) and/or cathode (s) comprise iron, they may be formed of steel or stainless steel. The cathode (s) and anode (s) may be formed of the same material. For example, both the cathode (s) and anode (s) may be formed of aluminium, steel and/or iron. Alternatively, the

cathode (s) and anode (s) may be formed of different materials. In one embodiment, the cathode (s) comprises iron (e.g. steel) which in this instance serves as a pseudo-dimensionally stable material, while the anode (s) from which the floe is generated comprises aluminium. Other suitable cathode/anode combinations include

aluminium/aluminium, iron/dimensionally stable electrode and

aluminium/dimensionally stable electrode.

Preferably, the electrochemical cell is configured to dose aluminium or iron ions into the first liquid to produce the floc- containing liquid. The concentration of metal coagulant in the floc-containing liquid may range from 50 to 5000ppm, preferably 100 to 3000ppm, more preferably 200 to 2000ppm, most preferably from 500-1000ppm. The metal coagulant concentration may be controlled to optimise floe formation and/or growth. For the avoidance of doubt, the metal coagulant concentration refers to the concentration of metal present in any form, including ionic, colloidal, complexes or floe, with the objective to ensure that particulate forms

predominate. This concentration can be readily determined by well- known techniques. For example, the sample can be collected,

digested in nitric acid (to convert the metal present in the sample into ionic form) and then analysed by atomic absorption spectroscopy.

The metal coagulant concentration in the floc-containing- liquid may desirably be at least 1 mmol/litre, more preferably greater than 3.5 mmol/litre, most preferably greater than 9

mmol/litre .

In one embodiment, the electrochemical cell comprises a

plurality of anodes and a plurality of cathodes. The anode (s) and/or cathode (s) may take the form of plates. Such plates should be suitable for immersion in any first liquid contained in the cell.

Preferably, flow through the electrochemical cell may be periodically reversed. For example, the electrochemical cell may be configured to operate "upside-down" or "back to front" to allow flow through the cell to be reversed. This reduces the risk of fouling in the electrochemical cell.

As discussed above, the floc-containing-liquid produced in the electrochemical cell is introduced into the second liquid via the fluid communication means connecting the electrochemical cell to the volume of second liquid. The fluid communication means may be any conduit that is capable of being used to deliver a flow (e.g. a fluid stream or series of drops) of floc-containing-liquid from the electrochemical cell to the volume of second liquid. The fluid communication means may be or comprise one or more pipes. If desired, the floc-containing-liquid may be treated by intermediate processing steps as it passes from the electrochemical cell to the volume of second liquid. For example, the pH of the floc- containing-liquid may be adjusted prior to being introduced into the second liquid. With the latter, a conduit, such as a pipe or series of pipes, may be used to transport the floc-containing-liquid through a pH adjustment unit before it is introduced into the second liquid .

If desired, the floc-containing-liquid may be treated by agitation and/or degassing as, or before, it passes from the electrochemical cell to the volume of second liquid. For example, the floc-containing-liquid may be mixed or agitated prior to being introduced into the second liquid. This may promote floe formation.

The second liquid may be provided as a static volume. For example, a static container of the second liquid may be treated. Once treated, the liquid may be removed and, for example, subjected to conditions to promote floe growth. Alternatively, floe growth may occur within the container and the liquid subsequently removed for separation of the flocculated contaminants. Once removed, the container may be replenished with further second liquid.

Preferably, however, the second liquid is provided as a moving body of liquid, such as a stream or series of drops . Accordingly, the floc-containing-liquid is preferably introduced into a moving body or volume of second liquid. Preferably, the floc-containing- liquid is introduced in a turbulent flow, and preferably grows in a laminar flow. The second liquid may be any liquid that contains contaminants that can be removed by coagulation. Examples include impure water streams, such as seawater, river water, brackish water or waste water. Examples of waste water streams include grey water or black water. The waste water may be waste water from a domestic,

agricultural or industrial process.

One or more of the contaminants which may be present in the second liquid, and which can be removed by coagulation in the process and/or apparatus of the present invention, may be desirable products .

It may be an object of the process and/or apparatus as described herein to remove contaminants from a liquid and/or to purify a liquid. In this case, the decontaminated and/or purified liquid may be the primary desired product of the process and/or apparatus .

Alternatively, it may be an object of the process and/or apparatus as described herein to isolate and/or separate

contaminants from a liquid. In this case the isolated and/or separated contaminants obtained from the liquid may be the primary desired product of the process and/or apparatus.

It may be that

(i) the separated and/or isolated contaminants which were originally present in a liquid; and

(ii) the liquid having had the contaminants at least partially removed

are both the desired products of the process and/or apparatus as described herein.

Once introduced into the second liquid, the resulting mixture is preferably treated to allow floe formation and growth to occur. In a preferred embodiment, the apparatus of the present invention includes a treatment zone located downstream of the point at which the floc-containing-liquid is introduced into the second liquid. I this treatment zone, the liquids may be first mixed or agitated at a relatively fast rate to promote contact between the floc-containing- liquid the second liquid. The resulting mixture may then be allowed to contact, optionally, with gentle mixing, to promote growth (or further growth) of the floe particles. In one embodiment, the liquids may be treated in two continuously stirred tanks operated at the required stirring rates. Alternatively, the floc-containing- liquid may be injected or mixed at high speed into the second liquid and the resulting mixture then introduced into a reactor, such as a tubular reactor, to allow the floe to grow. Optionally, air may be introduced into the liquid to facilitate floe formation.

The floe (sludge) of coagulants formed in the liquid may then be separated using conventional separation techniques. Examples include floatation, settling, filtration or a combination of two or more of these. The apparatus of the present invention may include a separator e.g. downstream of the treatment zone where floe growth occurs. Suitable separators include flotation units, settling units and filtration units . Additional flocculating agent may be used to aid the separation step, if desired. Once separated, the sludge can be processed, for example, by drying in a filter press. The separated or treated liquid has a reduced contaminant content and may be used or purified further, as desired. Optionally, a portion of the treated liquid may be recycled for use as the or part of the first liquid introduced into the electrochemical cell. In one embodiment, a portion of the treated liquid is mixed with a side stream from the untreated second liquid stream and, optionally, liquid from another source. The resulting mixture is then

introduced to the electrochemical cell as the first liquid. The use of the treated liquid in this manner may reduce the solids content of the overall stream making it more suitable for use in the electrochemical cell.

As discussed above, the second aspect of the invention relates to an electrocoagulation apparatus comprising an electrochemical cell comprising at least one cathode and at least one anode having a consumable surface. When the electrochemical cell is not in use and no current is passed between the anode and cathode, it may be desirable to protect the electrodes from undesired corrosion. A suitable system for protecting the electrodes is described in UK Patent Application No. 1017725.1. To implement this system, the apparatus of the present invention is further provided with a protective electrode that is connectable to the cathode and/or anode of the electrochemical cell via a source of direct current, such that electron flow takes place from the protective electrode to the cathode and/or anode in the electrochemical cell, said protective electrode being formed from a different material to the cathode or anode in the electrochemical cell.

With no current flowing between the anode and cathode of the cell, a direct current can be passed from the protective electrode to the cathode and/or anode of the electrochemical cell. This causes an impressed flow of electrons to take place from the protective electrode to the cathode and/or anode of the

electrochemical cell. Accordingly, in "protection mode", the protective electrode acts as the positive electrode, while the cathode and anode of the electrochemical cell acts as the negative electrode of the "protection" cell. This negative electrode is protected from corrosion by the impressed current, which reduces the negative electrode's susceptibility to oxidation. The effect is known as cathodic protection and is particularly important when the electrochemical apparatus is out of use, and no potential difference is applied across the anode and cathode. Under these conditions, the anode and cathode of the electrochemical cell may be susceptible to natural corrosion, particularly if the liquid in the chamber contains contaminants that aid the corrosive process. This is undesirable, as it can lead to degradation of the electrode (s) without the benefit of e.g. effective electrocoagulation. The protective electrode, therefore, can be used to reduce the risk of corrosion of the anode and cathode particularly during such out-of- use periods, thereby increasing the longevity of the electrode (s) and the cost-effectiveness of e.g. the overall electrocoagulation process . The protective electrode is preferably formed from a material that is different to that of the anode and/or cathode of the electrochemical cell. More preferably, the protective electrode comprises a noble metal and/or an alloy.

A third aspect of the present invention provides an

electrocoagulation method comprising:

introducing a first liquid into the cell,

passing a current between the anode and cathode, such that the consumable surface of the anode dissolves in the first liquid to form a coagulant solution and/or a floc-containing-liquid,

providing fluid communication means for fluidly coupling the electrochemical cell to a volume of second liquid, and

passing the coagulant solution and/or a floc-containing- liquid from the electrochemical cell into the second liquid via said fluid communication means.

A fourth aspect of the invention provides an

electrocoagulation apparatus comprising

an electrochemical cell comprising at least one cathode and at least one anode having a consumable surface, whereby, upon passing an electrical current between the anode and the cathode, the consumable surface of the anode dissolves in liquid present in the electrochemical cell to form a coagulant solution and/or the floc- containing- liquid,

means for introducing the coagulant solution and/or the floc- containing- liquid formed in the electrochemical cell into a second liquid,

a treatment zone for facilitating floe growth or further floe growth in the mixture of coagulant solution and/or the floc- containing-liquid and second liquid,

and, optionally,

a separator for separating the floe formed in the treatment zone . Where appropriate or desirable, the preferred and optional features of the first and second aspects of the present invention described above and herein may be applied to the third and fourth aspects of the present invention.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

These and other aspects of the present invention are now described with reference to Figures 1 to 3.

Figure 1 is a schematic diagram of an apparatus according to the present invention.

Figure 2 is a schematic diagram of an electrochemical chamber for use in the present invention.

Figure 3 is a schematic diagram of an electrochemical chamber mounted on a bracket for use in the present invention.

Figure 1 depicts an apparatus 10 comprising an electrochemical cell 12, a tubular reaction zone 14, a degasser 16 and a

solid/liquid separation unit 18. The apparatus 10 also includes a filtration unit 20 and a pH adjustment zone 22. The electrochemical cell 12 comprises a series of cathodes and a series of anodes, each of the anodes having a consumable surface.

The apparatus 10 may be used to treat a waste effluent stream 24 by electrocoagulation. To operate the apparatus 10, a first liquid 26 is passed through the electrochemical cell 12. A current is then passed from the anodes to the cathodes of the

electrochemical cell, such that the consumable surfaces of the anodes dissolve in the first liquid 26 to form a floc-containing- liquid. The floe -containing-liquid contains seed particles that, when mixed with the waste liquid stream, can form a floe.

Alternatively, or additionally, the floc-containing-liquid contains floe formed from the seed particles in the first liquid.

Prior to introduction into the waste effluent stream 24 at mixing point 28, the floc-containing-liquid is retained within a residence time chamber 33 connecting cell 12 to mixing point 28 to allow floe development and enhance floe efficacy. The floc- containing-liquid is mixed with waste effluent stream 24 under low shear conditions, facilitating contact between the seed particles and the contaminants in the waste effluent stream 24. The mixture is then introduced into a reactor zone 14 where floe growth

continues .

Gas 17 is vented from the mixture in the degasser 16 before the mixture is passed to the solid/liquid separation unit 18. The solid/liquid separation unit separates the floe of coagulated contaminants from the mixture as sludge 30. The treated water 32 emerging from the solid/liquid separation unit 18 has a reduced contaminants content and may be used or disposed of as desired.

The first liquid 26 introduced into the electrochemical cell 12 is formed from a side-stream 34 of the waste effluent stream 24. This side-stream is optionally filtered in the filtration unit 20 to reduce its solids content. It is then mixed with a separate source of water, such as seawater 36, and a portion of the treated water 32 from the solid/liquid separation unit 18 before being used as the first liquid 26 fed into the electrochemical cell 12. Optionally, the pH of the first liquid 26 is adjusted in pH adjustment zone 22 before it is introduced into the cell 12.

One of the advantages of the apparatus 10 is that only a portion of the waste effluent stream 24 is treated in the

electrochemical cell 12. With conventional electrocoagulation, the entire waste effluent stream 24 would need to be passed through the electrochemical cell. Depending on the nature of the waste products in the stream, this may also require the entire waste effluent stream to be pre-treated. This adds considerably to the cost and hardware requirements of the process.

Figure 2 provides a more detailed view of an electrochemical chamber for use in the present invention. The figure shows the cable gland (cable entry point) 40, the cable gland (cable entry point) 42, the electrode plate 44 within the electo-coagulation (EC) pack and the external view of the electrochemical chamber 48.

Figure 3 shows an electrochemical chamber held in a mounting bracket 56. The electrochemical chamber 58 has a inlet 60 (when there is an upflow through the chamber) and an outlet 52 (when there is an upflow through the chamber) . Also shown in the figure is cable glands 54 and 50.

Examples

Example 1

The electrocoagulation method and apparatus will now be further described by reference to the following non-limiting examples .

The electrocoagulation method discussed herein was used to treat a domestic effluent stream containing 1000 ppm Total Suspended Solids (TSS) with a conductivity of 3.2 S/m at a flowrate of 0.9 m³/h. A concentrated primary coagulant stream (PCS) of 1000 ppm was generated through electrochemical dosing of iron (Fe) into a seawater stream (3.5% NaCl (volume/volume), 6.7 mS/cm), at a flowrate of 0.1 m³/h, through the controlled corrosion of mild steel plates enclosed in a closed flow-through reactor (electrochemical cell) . The PCS was held for a residence time of 30 seconds. Both streams were fed into a continuously-stirred (25 rpm) mixing tank

(0.16 m³ volume) resulting in a final dose of coagulant of 100 ppm at a total flow of 1.0 m³/h, with a PCS dilution of 1:10. The blended stream from the mixing tank then entered a 0.5 m³ separation

(settlement) stage with a residence time of 30 minutes, resulting in a separation efficiency of 98%, with a final TSS concentration at discharge of 21 ppm.

Example 2

This example shows the benefits of treating a first liquid to form a floc-containing liquid and introducing this into a second liquid to be treated, rather than to simply directly treat a second liquid using electrocoagulation means .

For evaluation of the Side Stream methodology in the removal of solids from marine effluents, treatment of synthetic wastewater containing lOOOmg/1 solids (macerated dog food) and high salinity (1.5% NaCl, approx. 32mS/cm) was carried out.

Solids removal was evaluated through iron (Fe) dosing in SS mode (the method described herein) where an iron (Fe) primary coagulant stream (PCS) of lOOOmg/1 was generated by

electrochemically dosing iron into a seawater stream through the controlled corrosion of mild steel plates enclosed in a closed, flow-through reactor. The PCS was held for a residence time of 30 seconds .

The seawater (approx.3.5% NaCl, approx 67mS/cm) used to generate PCS did not undergo any pre-treatment and was fed into a mixing tank (80L, 25 rpm stirring) via the EC reactor at a constant flow rate (0.81/min). Synthetic effluent was also fed into the mixing tank (7.2L/min) diluting the PCS 10 x (final dosing rate lOOppm) where flocculation of suspended solids occurred. The combined flow was allowed to run into a separation stage were sedimentation occurred.

Residual suspended solids in the treated stream was measured after a settlement period of 30 minutes. Reductions of 98% (avg. residual TSS = 21mg/l) were recorded.

For comparison, direct electrochemical dosing of Fe into a synthetic effluent stream was carried out over a range of dosing rates (50 - 400mg/l) where similar residual levels (residual TSS = 13mg/l) were achieved at 400mg/l of Fe . At lOOmg/1 Fe dosed, residual TSS levels of 190 mg/1 were

measured .

Example 3

In this Example, the effect of residence time on the efficacy of the process is demonstrated.

The electrocoagulation method discussed herein was used at a laboratory scale with a varying retention time of the floc- containing-liquid between the electrocoagulation cell and the mixing point .

A synthetic effluent solution of 1.5% saline containing asolids loading content of 1,000 ppm and a pH of 7.1 was produced.

A 1.5% saline electrolyte solution (pH 7.2, conductivity 34 mS/cm) was produced to be used as a stock for the production of floc-containing-liquid.

The electrolyte solution was fed to the cell at a constant flow rate of 15 L/hr to produce a primary coagulant stream (PCS) of 500ppm/Fe . Raw effluent was mixed with floc-containing-liquid at a constant flow rate of 285L/hr, thus achieving a dilution ratio of 1:20 PCS to raw effluent, giving a final metal coagulant dose of 25ppm.

Residual suspended solids, turbidity and metals analysis

Cr, Ni) were carried out and averaged for each retention time investigated .

The table below summarises the residual suspended solids concentrations and turbidity recorded at various PCS retention times for Stainless Steel (SS) and Mild Steel (MS) electrode packs. These results are further illustrated in figures 4 and 5.

Table for residual TSS and turbidity for both MS and SS :

Example 4

In this Example, the effect of metal coagulant concentration in the floc-containing liquid on the efficacy of the process is demonstrated .

The electrocoagulation method discussed herein was used at a laboratory scale with varying coagulant concentrations in the floc- containing liquid. Raw municipal effluent containing 5 ppm (0.16 mmol/L) of total phosphorus and a conductivity of 0.78 mS/cm at pH 6.5 was fed to the cell at a constant flow rate of 200 mL/min.

Current input was altered to produce iron coagulant concentrations of 40 - 200 ppm/Fe (0.72 - 3.58 mmol/L) . A second volume of raw municipal effluent (of identical characteristics) was mixed with the floc-containing liquid at the various coagulant concentrations such that the final coagulant concentration in 1 L samples collected was at a constant 13 ppm/Fe (0.23 mmol/L) . Residual iron analysis was carried out in the settled supernatant volumes of collected samples . The incoming wastewater (second liquid) contained 0.2ppm Fe . A low residual Fe after treatment indicates an effectively use of Fe coagulant. As can be seen from the table below and Figure 6, as the concentration of Fe coagulant in the floc-containing liquid (PCS) increases, the residual Fe reduces, showing that the coagulant Fe is being more effectively used in coagulation.

PCS Fe Residual Fe in

concentration treated wastewater

0 (incoming 0.2

wastewater )

40 3.2

60 2

80 1.3

100 1.2

150 0.8

200 0.7

The foregoing detailed description has been provided by way of explanation and illustration and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents .

Claims

1. An electrocoagulation method comprising:

holding the floc-containing-liquid for a residence time to allow floe growth to occur,

and

contacting the floc-containing-liquid with a second liquid.

2. A method as claimed in claim 1, wherein the pH is controlled to allow a floe to form in the first liquid to provide a floc- containing-liquid .

3. A method as claimed in claim 2, wherein the pH is controlled at 6 to 9, so as to allow a floe to form to provide a floc- containing liquid.

4. A method as claimed in claim 1 or 2 , wherein the floc- containing-liquid is formed having a metal coagulant concentration of at least 1 mmol/L.

5. A method as claimed in any one of the preceding claims, wherein the second liquid is provided as a flowing volume of liquid.

6. A method as claimed in any one of the preceding claims, which further comprises treating the mixture of floc-containing-liquid and the second solution to allow floe growth and/or further floe growth to take place.

7. A method as claimed in any one of the preceding claims, which further comprises the step of separating any floe formed in the second liquid following treatment with the floc-containing-liquid .

8. A method as claimed in claim 7, wherein a portion of the treated liquid from the separation step is recycled for use as or as part of the first liquid.

9. A method as claimed in any one of the preceding claims,

wherein a portion of the second liquid is used as or as part of the first liquid.

10. A method as claimed in any one of the preceding claims,

wherein the first liquid and/or the second liquid comprises seawater, brackish water, river water or waste water.

11. A method as claimed in any one of the preceding claims,

wherein conductivity of the first liquid is adjusted prior to entry into the electrochemical cell.

12. A method as claimed in any one of the preceding claims,

wherein pH of the floc-containing-liquid is adjusted prior to contact with the second liquid.

13. A method as claimed in any one of the preceding claims,

wherein the pH of the first liquid is adjusted before it is

introduced into the electrochemical cell.

14. A method as claimed in any one of the preceding claims, which further comprises the step of degassing the second liquid.

15. A method as claimed in any one of the preceding claims, which further includes an aeration step.

16. A method as claimed in claim 15, wherein the aeration step takes place in the electrochemical cell or after the floc- containing-liquid is introduced into the second liquid.

17. A method as claimed in any one of the preceding claims,

wherein the anode and/or cathode comprises iron, stainless steel and/or aluminium.

18. A method as claimed in any one of the preceding claims,

wherein the floc-containing- liquid is continuously flowed or dripped into the second liquid via the fluid communication means.

19. A method as claimed in any one of the preceding claims,

wherein the direction of flow through the electrochemical cell is periodically reversed.

20. A method as claimed in any one of the preceding claims, wherein the flow through the electrochemical cell is periodically stopped to allow for electrode recovery from passivation.

21. A method as claimed in any one of the preceding claims,

wherein the current input to the electrochemical cell is controlled by the flow and/or characteristics of the second liquid.

22. A method as claimed in any one of the preceding claims,

wherein the flow of first liquid to the electrochemical cell is controlled by the flow and/or characteristics of the second liquid.

23. An electrocoagulation apparatus or system comprising

an electrochemical cell comprising at least one cathode and at least one anode having a consumable surface, whereby, upon passing an electrical current between the anode and the cathode, the consumable surface of the anode dissolves in liquid present in the

electrochemical cell,

means for contacting the floc-containing-liquid with a second liquid, optionally, a treatment zone for facilitating further floe growth in a mixture comprising the floc-containing-liquid and the second liquid,

and, optionally,

a separator for separating the floe formed in the treatment zone.

24. An apparatus or system as claimed in claim 23, wherein means for introducing the floc-containing- liquid formed in the

electrochemical cell into the second liquid is a fluid communication means fluidly coupling the electrochemical cell to the second liquid.

25. An apparatus or system as claimed in claim 23 or 24, which further comprises a protective electrode, said protective electrode being connectable to the cathode and/or anode of the electrochemical cell via a source of direct current, such that electron flow takes place from the protective electrode to the cathode and/or anode in the electrochemical cell, said protective electrode being formed from a different material to the cathode or anode in the

electrochemical cell.

26. An apparatus or system as claimed in any one of claims 23 to 25, wherein the electrochemical cell is configured to allow the direction of flow through the cell to be reversed at periodic intervals .