WO2011150473A1

WO2011150473A1 - Controlling activity of microorganisms in wastewater systems

Info

Publication number: WO2011150473A1
Application number: PCT/AU2011/000703
Authority: WO
Inventors: Ilje Pikaar; Korneel Pieter Herman Leo Ann Rabaey; Rene Alexander Rozendal
Original assignee: The University Of Queensland
Priority date: 2010-06-03
Filing date: 2011-06-03
Publication date: 2011-12-08

Abstract

A process for controlling microbial growth in aqueous streams or environments comprising providing an electrochemical system having an anode and a cathode separated by a separator, feeding an aqueous stream to the anode, producing an alkaline stream and/or a hydrogen peroxide solution at the cathode and supplying the alkaline stream and/or the hydrogen peroxide solution to the aqueous stream or environment. In other embodiments, an acidic stream or oxygen is produced and added to the aqueous stream or environment.

Description

CONTROLLING ACTIVITY OF MICROORGANISMS IN WASTEWATER SYSTEMS

FIELD OF THE INVENTION

The present invention relates to a method for controlling the activity of microorganisms, such as the activity of sulfate reducing bacteria and/or methanogens (methanogenic Archaea) (or both) in environments containing such organisms. In some aspects, the present invention relates to a method for controlling the activity of sulfate reducing bacteria and methanogenic archaea (or both) in aqueous streams, sewers or wastewater treatment systems. The present invention also relates to a method for treating or controlling biofilm in sewers or other wastewater treatment systems.

BACKGROUND TO THE INVENTION

Sulfate reducing bacteria and methanogenic archaea (also referred to as methanogens) are groups of microorganisms present in a wide range of environments including marine sediments, hot springs, oil reservoirs, UASB reactors, sewers and wastewater treatment systems. Their presence in sewer networks and other wastewater treatment systems is considered unfavourable due to their capacity to produce hydrogen sulfide and methane under anaerobic conditions. Emission of hydrogen sulfide to the gas phase leads to a number of deleterious effects including corrosion of sewer infrastructure, generation of noxious odours and health problems. Methane is an explosive gas at concentrations of 5- 15%, and is also a potent greenhouse gas.

Sulfide is generated in sewers by sulfate-reducing bacteria (SRB) present in sewer biofilms under anaerobic conditions (USEPA, 1974; Bowker et al., 1 89). When sulfides build up in the aqueous phase they can be emitted to the sewer atmosphere as H₂S gas, which induces damage to sewer concrete structures and creates occupational hazards and odour problems (Thistlethwayte, 1972; Bowker et al., 1989; Hvitved-Jacobsen, 2002). A number of sulfide control strategies and technologies are being used by the wastewater industry. These methods can be roughly divided into three categories, namely the inhibition of bacterial activities of sewer biofilms thus reducing the production of sulfide and other odorous compounds, the chemical and/or biological oxidation of sulfide formed, and the reduction of H₂S transfer from liquid phase to gas phase.

Sulfide removal by chemical oxidation has been achieved through the injection of ozone, hydrogen peroxide, chlorine or potassium permanganate (Tomar and Abdullah, 1994; Boon, 1995; Charron et al., 2004). Biological sulfide oxidation has been achieved with the addition of oxygen, nitrate, and nitrite, while oxygen injection induces both chemical and biological oxidation of sulfide (Gutierrez et al., 2008). The addition of nitrate and nitrite salts stimulates the development of nitrate-reducing, sulfide-oxidising bacteria, thus achieving sulfide oxidation with nitrate or nitrite as the electron acceptor (Bentzen et al., 1995; Nemati et al., 2001 ; Yang et al., 2005; Mohanakrishnan et al., 2009).· These strategies for controlling sulfide removal will require the continuous addition of oxidants, which incurs substantial operating costs. The reduction of H₂S transfer from water phase to gas phase can be achieved by pH elevation (Thistlethwayte, 1972; Gutierrez et al., 2009) or addition of metal salts (Bowker et al., 1989). Molecular H₂S is the form of sulfide released from water to air. In water, dissolved H₂S forms chemical equilibrium with HS^* with ratios between the species determined by pH and temperature, among other factors. The proportion of H₂S is reduced when pH is increased. pH elevation through addition of e.g. Mg(OH)₂ is commonly used for reducing H₂S transfer. The reduction of molecular H₂S can also be achieved through precipitation of HS^* and/or S^2" with metal salts. The precipitation of HS^" and S^2" results in lowered total dissolved sulfide concentration and hence lowered dissolved H₂S concentration. Iron salts, either in the form of ferrous or ferric ions, have been widely used for the abatement of sulfide induced problems in sewer networks (USEPA, 1974; Jameel, ^' 1989; Hvitved-Jacobsen, 2002). These strategies also require continuous addition of chemicals, incurring substantial operating costs.

Addition of a strong base to elevate pH in wastewater to 10.5 to 13 (pH shock) has been used to deactivate bacteria in sewer biofilms (MMBW, 1989). Similarly, the addition of inhibitors such as biocides and molybdate has also been proposed to inhibit the production of H₂S (Nemati et al., 2001). Inhibition of sulfide production by addition of alternative electron acceptors such as oxygen, nitrate and nitrite has also been reported (Bentzen et al., 1995; Hobson and Yang, 2000). Therefore, oxygen injection may also be used to prevent anaerobic conditions, thereby inhibiting the activity of SRB and thus prevent or minimise sulfide generation. However, recent studies have shown that oxygen and nitrate have no long-lasting inhibitory/toxic effects on SRB in sewer biofilms (Gutierrez et al., 2008; Mohanakrishnan et al., 2008). In comparison to the previous two categories of control strategies, this category of control strategy does not require permanent or continuous dosage of chemicals. Intermittent addition of the chemicals is expected to be adequate. The "pH shock" technology has been demonstrated to be effective in reducing the activity of sulfate reducing bacteria (SRB). However, the activity of the sulfate reducing bacteria resumes quickly in 1-2 weeks. Therefore the dosage of strong base has to be applied frequently (e.g. weekly), incurring considerable costs. The limited use of this technology by the wastewater industry could imply that the dosing, storage and transport of base is presently posing cost and safety concerns

It may also be possible to add hydrogen peroxide solution to obtain elevated pH levels. However, the amount of hydrogen peroxide required to be added is likely to result in prohibitive costs for widespread adoption of this technique.

Bacterial growth in sewer pipes also results in the formation of a biofilm lining, the inner wall of the pipes. The biofilm in sewer pipes can attain significant thickness, for example, of the order of millimetres to tens of millimetres. The presence of the biofilm in sewer pipes has at least three undesirable side-effects, these being (1) microorganisms in the biofilm are somewhat protected from the main flow of liquid through the sewer; (2) flow area in the pipe is decreased, and (3) the friction between water flow and pipe walls increases and hence the energy consumption increases. Therefore, it becomes difficult to treat microorganisms in the biofilm by adding treatment agents to the flow in the sewer, as the biofilm acts to separate the treatment agents from the microorganisms. In this regard, the treatment agents will typically have to diffuse into the biofilm, thereby requiring significantly higher concentrations of treatment agents and longer addition of treatment agents to the sewer in order to adequately treat the biofilm.

There exists a need to develop a method for controlling the activity of sulfate reducing bacteria and/or methanogenic archaea (or both) in environments containing such organisms, which overcomes or at least ameliorates one or more of the above disadvantages, or provides a commercial alternative. BRIEF DESCRIPTION OF THE INVENTION

In a first aspect, the present invention provides a process for controlling microbial growth in aqueous streams or environments comprising providing an electrochemical system having an anode and a cathode separated by a separator, feeding an aqueous stream to the anode, producing an alkaline stream and/or a hydrogen peroxide solution at the cathode and supplying the alkaline stream and/or the hydrogen peroxide solution to the aqueous stream or environment. In some embodiments, the process further comprises generating oxygen at the anode and feeding the oxygen to the aqueous stream or environment.

In one embodiment, the aqueous stream or environment comprises a wastewater system (including a wastewater collection system) or an aqueous stream used in an industrial process or a food processing system or a beverage processing system. The present invention encompasses the control or inhibition of microbial growth in any aqueous stream or environment.

In one embodiment, the electrochemical system is operated such that oxygen is generated at the anode:

OR:

The oxygen generated at the anode is optionally supplied to the wastewater system. In this embodiment, anaerobic conditions in the wastewater system are minimised or avoided, thereby minimising or preventing sulfate reduction which, in turn, minimises or prevents sulfide production. In addition, the oxygen can oxidize reduced sulfur species, such as sulfide, to oxidized sulfur species, such as elemental sulfur, thiosulfate, sulfite, and sulfate. In another embodiment of the invention, the electrochemical system is operated such that reduced sulfur species, such as sulfide, are oxidized at the anode to form oxidized sulfur species, such as elemental sulfur, thiosulfate, sulfite, and sulfate. In yet another embodiment of the invention, the electrochemical system is operated such that mediators such as oxygen, chlorine and OH^* radicals are generated at the anode, while simultaneously reduced sulfur species are also oxidized at the anode to form oxidized sulfur species, such as elemental sulfur, thiosulfate, sulfite, and sulfate. In some embodiments of the invention, the electrochemical system is operated such that oxygen is reduced at the cathode, leading to the formation of water. The oxygen can be obtained from the air, oxygen supply, and/or from the anode. As the reduction of oxygen to water consumes protons, the cathode fluid typically becomes alkaline. In some embodiments of the invention, the electrochemical system is operated such that hydrogen gas is generated at the cathode:

OR:

The cathode may be catalyzed chemically and consumes electrons. Since the cathode reaction (equation 2) either consumes protons or produces hydroxyl ions, the pH will typically increase in the cathode chamber. This means that simultaneously with the hydrogen gas, an alkaline stream can also be produced.

In other embodiments, a hydrogen peroxide containing solution is formed at the cathode. The electrons (e^") that are generated in the oxidation reaction are transferred to the anode and transported from the anode to the cathode via an electrical circuit. The cathode may be catalyzed chemically and consumes electrons for the reduction of oxygen to hydrogen peroxide. The cathode reactions are as follows:

or:

Depending on the pH the cathodic production of hydrogen peroxide production exhibits an electrode potential of about -0.065 (pH 14) to 0.67 V (pH 0), relative to a standard hydrogen electrode. Methods for the production of peroxide at cathodes can for example be found in International patent application publication number WO2010042986, the entire contents of which are herein incorporated by cross reference.

The anode can be separated from the cathode by a cation exchange membrane. Cation exchange membranes are known to the person skilled in the art and include membranes such as CMI-7000 (Membranes International), Neosepta CMX (ASTOM Corporation), furnasep® FKB (Fumatech), and Nafion (DuPont). In cases where a cation exchange membrane is used as the membrane in the electrochemical system, cations are transported from the anode to the cathode to compensate for the negative charge of the electrons flowing from anode to cathode through the electrical circuit. Since aqueous waste streams, especially wastewaters, are typically about pH neutral, the cations that are transported through the cation exchange membrane are typically not protons, but comprise other cations present in the aqueous waste streams, such as sodium and potassium. At the cathode these cations combine with the hydroxyl ions that are produced in the cathode reaction (e.g. equation 3).

If the level of multivalent ions (e.g, calcium) is high in the aqueous waste streams (e.g., wastewater) that is fed to the anode chamber, there exists a risk of scaling of the cation exchange membrane due to precipitation of calcium salts (e.g., calcium hydroxide) on the cathode side of the membrane. This can irreversibly damage the membrane. This risk is especially high if the ion permeable membrane allows multivalent ions to pass therethrough. To prevent scaling damage to the membrane the cation exchange membrane may be a special type of cation exchange membrane, namely a monovalent ion selective cation exchange membrane (Balster et al., J. Membr. Sci., 2005, 263, 137-145). Monovalent ion selective cation exchange membranes are known to the person skilled in the art and include Neosepta CIMS (ASTOM Corporation). Monovalent ion selective cation exchange membranes selectively transport monovalent cations (e.g., sodium, potassium) and prevent multivalent cations (e.g, calcium) being transported therethrough. Therefore, the amount of multivalent ions reaching the cathode side of the membrane is significantly reduced and the scaling risk diminishes. An additional advantage gained by using monovalent ion selective cation exchange membranes is that traces of iron ions, which might be present in the aqueous waste stream, are blocked by the membrane too. In another embodiment issues arising from scaling at the cathode can be reduced through the addition of anti-scaling agents to the cathode fluid.

In another embodiment of the invention the ion permeable membrane that separates the anode and the cathode chamber comprises an anion exchange membrane. Anion exchange membranes are known to the person skilled in the art and include membranes such as AMI-7001 (Membranes International), Neosepta AMX (ASTOM Corporation), and fumasep FAA® (fumatech). In cases where an anion exchange membrane is used as the membrane in the electrochemical system, anions are transported from the cathode to the anode to compensate for the negative charge of the electrons flowing from anode to cathode through the electrical circuit. As cations are blocked completely by the anion exchange membrane, multivalent cations cannot be transported from anode to cathode and scaling issues are prevented. Moreover, also iron ions are blocked so if iron is present in the aqueous waste stream decomposition of the hydrogen peroxide is prevented.

In another embodiment of the invention, the separator between the anode and the cathode is a porous membrane or separator. Porous membranes are known to the person skilled in the art and include microfiltration membranes, ultrafiltration membranes, and nanofiltration membranes. Porous separators include sintered PVC, glass fibers and other permeable materials as known to a person skilled in the art. In some embodiments the anode reaction may be chemically catalysed. Metals or metal alloys may also be used as the anode material. A mixed metal oxide coated titanium electrode may also be used as the anode. These electrodes are known to the person skilled in the art and include stainless steel, nickel, iridium oxide coated titanium electrodes, platinum/iridium coated titanium electrodes, mthenium-iridium oxide coated titanium electrodes, tin oxide coated titanium electrodes, lead oxide coated titanium electrodes, boron doped diamond electrodes, etc.

In some embodiments the cathode reaction may be chemically catalyzed. Metals or metal alloys may also be used as the cathode material. A mixed metal oxide coated titanium electrode may also be used as the cathode. These electrodes are known to the person skilled in the art and include stainless steel, nickel, iridium oxide coated titanium electrodes, platinum/iridium coated titanium electrodes, ruthenium-indium oxide coated titanium electrodes, tin oxide coated titanium electrodes, lead oxide coated titanium electrodes, boron doped diamond electrodes, etc.

In some embodiments, the present invention involves anodic oxygen generation at mixed metal oxide electrodes coupled with cathodic generation of caustic. In some embodiments of the present invention, the anode is positioned in an anode chamber and the anode chamber receives wastewater from the wastewater system. For example, the anode chamber may be positioned in the wastewater system, such as the anode chamber being positioned within a sewer pipe, in a wet Well or in a pumping well. Alternatively, the anode chamber may be associated with the wastewater system by way of connection with a bypass pipe or a feed line. Wastewater may be provided from the wastewater system to the anode chamber and wastewater leaving the anode chamber may be returned to the wastewater system.

In an embodiment, the anode and the cathode may be periodically electrically switched over and alternatively used as the cathode and the anode, respectively. This may prevent scaling, particularly on the cathode. This implies that periodically alkaline solutions can be harvested from the cathode, while wastewater is supplied to the anode.

In one embodiment, the present invention inhibits the activity of sulfate reducing bacteria. In another embodiment, the present invention inhibits the activity of methanogenic archaea. In a further embodiment, the present invention treats or disrupts a biofilm present in the aqueous stream or environment. In one embodiment, the aqueous stream or environment comprises a wastewater system. The wastewater system may comprise a wastewater collection system, such as a sewer or a wet well of a sewer system or a rising main of a sewer system. In some embodiments, the alkaline stream and/or the hydrogen peroxide solution are periodically supplied or intermittently supplied to the aqueous stream or environment, such as a wastewater system.

In some embodiments, the alkaline stream and/or the hydrogen peroxide solution are supplied to the aqueous steam or environment during a pumping event. The pumping event may be timed to achieve a desirable residence time of the alkaline and/or hydrogen peroxide solution in the wastewater system.

In embodiments where an alkaline stream and/or hydrogen peroxide solution is generated in the cathode, the alkaline stream and/or hydrogen peroxide solution is injected into the aqueous stream or environment such that a pH of at least 8.3, more preferably at least 10, more preferably at least 10.5, more preferably at least 1 1, more preferably of at least 12, is obtained in the wastewater system after injection. In some embodiments of the present invention, the anode comprises a biocatalysed anode. As will be understood by persons .skilled in the art, the bioelectrochemical system used in this embodiment will include electrpchemically active microorganisms associated with the anode. For example, the anode may be present in an anode chamber and the electrochemically active microorganisms may also be present in the anode compartment or chamber (throughout this specification, the terms "compartment" and "chamber" are used interchangeably). In this embodiment, the electrochemical system will be a bioelectrochemical system.

In some embodiments of the present invention the electrochemical system comprises an anode chamber and a cathode chamber separated by an ion permeable membrane, as known to the person skilled in the art (in this embodiment, the separator comprises the ion permeable membrane). Ion permeable membranes suitable for use in the present invention include any ion permeable membranes that may be used in. electrochemical systems (Kim et al., Environ. Sci. Technol., 2007, 41, 1004-1009; Rozendal et al., Water Sci. Technol., 2008, 57, 1757-1762). Such ion permeable membranes may include ion exchange membranes, such as cation exchange membranes and anion exchange membranes. Porous membranes, such as microfiltration membranes, ultrafiltration membranes, and nanofiltration membranes, may also be used in the electrochemical system used in the present invention. The ion permeable membrane facilitates the transport of positively and/or negatively charged ions through the membrane, which compensates for the flow of the negatively charged electrons from anode to cathode and thus maintains electroneutrality in the system. Pervaporatipn membranes and membranes as used for membrane distillation may also be used.

The anode and the cathode will be connected to each other by an electrical circuit. In one embodiment, the electrical circuit may comprise a conductor having very low resistance such that in some cases the conductor acts as an electrical short circuit between the anode and the cathode. In another embodiment, a power supply may be included in the electrical circuit. This power supply can be used to apply a voltage on the system, which increases the rate of the electrochemical reactions taking place. The voltage applied with a power supply between the anode and the cathode may be between 0 and 100 V, preferably between 0 and 10 V, more preferably between 0 and 5 V. In some embodiments, a wastewater stream may be fed to the anode compartment and water or an aqueous stream may be fed to the cathode compartment.

In one embodiment, an alkaline stream is produced at the cathode. The alkaline stream that is produced at the cathode may contain caustic soda (NaOH) or potassium hydroxide (KOH), or indeed any other hydroxide containing solution that may be used for other purposes. Desirably, the alkaline stream that is produced on the cathode contains a dissolved hydroxide salt. This may be achieved by providing an electrochemical system that has an ion permeable membrane separating the anode and cathode, which ion permeable membrane selectively allows cations to pass therethrough.

In one particular embodiment, a wastewater stream is passed to the anode and an aqueous stream or water is passed to the cathode. The wastewater stream may contain dissolved sodium and/or potassium^', and/or other cations, which pass through the ion selective membrane between the anode and the cathode to thereby form sodium hydroxide and/or potassium hydroxide in the aqueous solution on the cathode side of the electrochemical system.

In this embodiment, the process may be operated such that the pH of the alkaline stream leaving the cathode is in excess of 10, more preferably greater than 1 1 or greater than 12, even more preferably greater than 13. The alkaline stream may be recovered for storage for subsequent continuous or periodic injection into the wastewater system.

Dosage of the alkaline solution and/or the hydrogen peroxide solution to the wastewater system may be conducted on a periodic or intermittent basis. In some embodiments, the electrochemical system is operated on a generally continuous basis and the alkaline solution and/or the hydrogen peroxide solution generated in the cathode is stored for intermittent or periodic dosage to the wastewater system. Alternatively, the electrochemical system may also be operated on an intermittent or periodic basis to thereby generate the alkaline solution and/or the . hydrogen peroxide solution on an intermittent or periodic basis. Appropriate valving to control feed of the aqueous streams to the anode in the cathode may be provided in this embodiment, as may appropriate electrical control systems. In one embodiment, a concentrated caustic solution is produced at the cathode and it is dosed to the wastewater system, such as a sewer system, to cause the pH in the wastewater system to be greater than or equal to 8.3 , more preferably greater than or equal to 10, more preferably greater than 1 1 or greater than 12, even more preferably greater than 13 in order to inactivate microorganisms in the wastewater system, or to prevent release of sulfide as H₂S. It is expected that dosing the wastewater system to obtain pH of greater than or equal to 10-12 or 11-12 will result in inactivation or inhibition of microorganisms for a period of time after the dosing of the alkaline solution has been completed. The person skilled in the art will appreciate that it will be a simple matter to conduct tests to determine an optimum residence time for each dosage of alkaline material and the duration between doses (or spacing or frequency of doses).

Similar considerations apply with regard to dosing with a hydrogen peroxide solution (dosage and the duration between doses will depend upon on the concentration of hydrogen peroxide that should be obtained in the sewer, and this also depends upon the concentration of the hydrogen peroxide solution added to the sewer and the amount of that solution is added in each dose).

In some embodiments, dosing of the wastewater system , with the alkaline solution and/or hydrogen peroxide solution in order to obtain a pH of greater than or equal to 12 is achieved by dosing every 1 to 7 days, more suitably every 3 to 5 days. It is anticipated that this dosage regime will result in the inhibition of sulfate reducing bacteria to prevent sulfide generation. The present invention also encompasses an apparatus suitable for use in the method of the present invention. Accordingly, in a second aspect, the present invention provides an apparatus for controlling microbial growth in an aqueous stream or environment comprising an electrochemical system comprising an anode and a cathode separated from the anode by a separator, anode feed means for feeding an aqueous stream from the aqueous stream or environment to the anode electrode, cathode feed means for feeding an aqueous liquid to the cathode, cathode effluent removal means for removing an alkaline stream and/or a hydrogen peroxide solution from the cathode and dosage means for dosing the alkaline stream and/or the hydrogen peroxide solution to the aqueous stream or environment.

The dosage means may comprise a pipe, tube, conduit or channel for feeding the alkaline stream and/or the hydrogen peroxide solution to the aqueous stream or environment. The dosage means may transfer the alkaline stream and/or the hydrogen peroxide solution directly from the cathode to the aqueous stream or environment (in which case, the cathode effluent removal means in the dosage means may comprise a single pipe, tube, conduit or channel). Alternatively, the cathode effluent removal means may remove the alkaline stream and/or the hydrogen peroxide solution from the cathode and direct it to a storage vessel or receptacle and the alkaline stream and/or the hydrogen peroxide solution may subsequently be transferred from the storage vessel or receptacle to the aqueous stream or environment via the dosage means.

The person skilled in the art will understand that the anode feed means, the cathode feed means and the cathode effluent removal means may comprise one or more pipes, tubes, conduits or channels, or indeed any other liquid flow or liquid transfer arrangements. In some embodiments, the anode is located within an anode chamber and the cathode electrode is located within a cathode chamber.

In some embodiments, the anode chamber and the cathode chamber will periodically be switched to prevent scale build-up in the cathode. This can be achieved by switching the polarity of the power supply. This means that the anode chamber becomes cathode chamber and the cathode chamber becomes the anode chamber. Similarly, the anode electrode becomes cathode electrode and the cathode electrode becomes the anode electrode. In this embodiment, the electrochemical apparatus will suitably be provided with appropriate feed pipes and effluent pipes (and valving) to enable the appropriate liquid streams to be fed to the appropriate compartments when a switch has occurred.

In some embodiments, an acidic stream is passed to the cathode in order to remove or reduce scale in the cathode. The acidic stream may be provided from an external source of acid. Alternatively, the acidic stream may be generated at the anode and subsequently passed to the cathode or to a storage vessel and then from the storage vessel to the cathode. In some embodiments, the wastewater flow is sent through the anode, and after a certain period the wastewater flow is stopped and an acidic stream is generated in the anode. This acidic stream may be supplied from an external vessel or be generated solely in the anode. Subsequently, the acidic stream can be supplied to the cathode, where calcium deposits or other scaling as known to a person skilled in the art are removed.

In some embodiments, the system is operated to produce an acidic stream at the anode. This may involve sending a wastewater stream through the cathode, while an aqueous solution is provided to the anode. At the anode, the acidic solution may be formed through consumption of protons during the oxidation of water with the formation of oxygen, or other anodic reactions as known to a person skilled in the art. The acidic stream may be dosed continuously or intermittently to the wastewater system. Similar technical embodiments as described for the production of an alkaline or hydrogen peroxide stream can be considered, as described earlier in this document.

In some embodiments, the anodic process may also lead to the production of chlorine or reactive oxygen species, as known to a person skilled in the art. The chlorine or reactive oxygen species containing stream may intermittently be fed to the wastewater system, in a manner similar to the earlier described embodiments.

Accordingly, in another aspect, the present invention provides a process for controlling microbial growth in aqueous streams or environments comprising providing an electrochemical system having an anode and a cathode separated by a separator, feeding an aqueous stream to the anode, producing an acidic solution at the anode and supplying the acidic solution to the aqueous stream or environment. An aqueous stream may also be fed to the cathode. The aqueous stream fed to the cathode may be a wastewater stream. The electrodes and separators described above may also be used in this aspect of the present invention.

In another aspect, the present invention provides a process for controlling microbial growth in an aqueous stream or environment comprising providing an electrochemical system having an anode and a cathode separated by a separator, feeding wastewater to one electrode, producing an alkaline or acidic stream at the other electrode and supplying the alkaline or acidic stream to the wastewater system. In yet another aspect, the present invention provides a process for controlling microbial growth in aqueous streams or environments comprising providing an electrochemical system having an anode and a cathode separated by a separator, feeding wastewater to the anode, producing an alkaline stream at the cathode and supplying the alkaline stream to the wastewater system.

In a further aspect, the present invention provides a process for controlling microbial growth in aqueous streams or environments comprising providing an electrochemical system having an anode and a cathode separated by a separator, feeding an aqueous! stream to the anode, producing an acidic solution at the anode and supplying the acidic solution to the aqueous stream or environment.

The method and apparatus of the present invention has the potential to effectively and efficiently prevent the formation of sulfide in sewer systems and in other wastewater · systems. Production of methane in sewer systems and in other wastewater systems may also be inhibited by the present invention. Biofilms may also be disrupted. The method and apparatus of the present invention can prevent or lower sulfide generation in wastewater systems in a very cost-effective way. The method of the present invention may also avoid any further risk for downstream sulfide production as the method may prevent or inhibit the presence of a biofilm containing sulfate reducing bacteria on the walls of the wastewater system. The process could also result in a decrease in the methane production from sewer systems. This is desirable as methane is a very strong greenhouse gas. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic side view, in cross-section, of an electrochemical apparatus suitable for use in one embodiment of the present invention; Figure 2 shows a schematic side view, in cross-section, of an electrochemical apparatus suitable for use in another embodiment of the present invention- Figure 3 shows a schematic side view, in cross-section, of an electrochemical apparatus suitable for use in a further embodiment of the present invention;

Figure 4 shows a schematic side view, in cross-section, of an electrochemical apparatus suitable for use in another embodiment of the present invention; and

Figure 5 shows a typical profile of the (a) anode potential, (b) cathode potential and (c) cell voltage: (Figure 5A) without spacers (n=10) and (Figure 5B) with spacers (n=3).

DETAILED DESCRIPTION OF THE DRAWINGS

It will be understood that the drawings have been provided for the purposes of illustrating preferred embodiments of the present invention. Therefore, it will be appreciated that the present invention should not be considered to be limited to the features as shown in the drawings. Figure 1 shows a schematic side view, in cross-section, of an electrochemical apparatus suitable for use in an embodiment of the present invention. The embodiment shown in figure 1 is suitable for controlling the activity of microorganisms in wastewater flowing through a sewer. The embodiment shown in figure 1 includes an electrochemical apparatus that is provided in submerged configuration within the sewer. In particular, the anode electrode is located such that it directly contacts an aqueous stream flowing through the sewer.

The sewer 10 shown in figure 1 is typically made from a pipe having a side wall 12. An anode electrode 14, which is shown in dashed outline, is positioned in the sewer 10 such that the aqueous stream flowing through the sewer can directly contact the anode electrode. As shown in figure 1, the anode electrode 14 is located close to an upper part 13 of the side wall 12 of the sewer pipe. In order to form the electrochemical apparatus in submerged configuration, either a part of the side wall 12 of the sewer pipe is removed and a housing 16 is positioned over the opening thus formed, or a special sewer pipe having the housing 16 integrally formed therewith is provided. A cathode electrode 18, which is shown in dashed outline, is positioned in a cathode compartment 17. A separator 20, which is shown in dotted outline in figure 1, is positioned between the anode 14 and the cathode 18. The separator 20 separates the cathode compartment 17 from the sewer line 10.

Metals or metal alloys may also be used as the anode electrode material. A mixed metal oxide coated titanium electrode may also be used as the anode. These electrodes are known to the person skilled in the art and include stainless steel, nickel, iridium oxide, coated titanium electrodes, platinum/iridium coated titanium electrodes, ruthenium- iridium oxide coated titanium electrodes, tin oxide coated titanium electrodes; lead oxide coated titanium electrodes, boron doped diamond electrodes, etc. Metals or metal alloys may also be used as the cathode electrode material. A mixed metal oxide coated titanium electrode may also be used as the cathode. These electrodes are known to the person skilled in the art and include stainless steel, nickel, iridium oxide coated titanium electrodes, platinum/iridium coated titanium electrodes, ruthenium- iridium oxide coated titanium electrodes, tin oxide coated titanium .electrodes, lead oxide coated titanium electrodes, boron doped diamond electrodes, etc.

The separator 20 may comprise an ion permeable membrane, as well known to the person skilled in the art.

The apparatus shown in figure 1 also include a cathode inlet 22 for feeding water or an aqueous stream to the cathode compartment 17. A PLC (programmable logic controller) 24 can be used to control, the overall process. The PLC 24 may be used to control the voltage difference between the cathode 18 and the anode 14. In this regard, the PLC may include an electrical circuit and voltage controller to complete the electrical circuit between the anode 14 and the cathode 18. Alternatively, a separate electrical circuit comprising an electrical conductor connected to anode 14, a source of potential difference and an electrical connector connected to cathode 18 may be provided, with the PLC controlling the voltage applied to the electrical circuit by the source of potential difference. As a further alternative, the PLC may be replaced by a power source or another controller. The person skilled in the art will readily understand how a potential difference is applied to the electrochemical system shown in figure 1 and therefore further description need not be provided.

In the cathode compartment 17, the cathode reactions result in the formation of a caustic stream or an aqueous stream containing hydrogen peroxide. This stream is removed from the cathode compartment 17 via cathode effluent line 26. A valve 28 can be selectively opened and closed to control the flow of effluent from the cathode compartment 17. When the aqueous medium in the cathode compartment 17 has been present for the desired residence time necessary to obtain the desired concentration of caustic/alkalinity or hydrogen peroxide, the valve .28 is opened' and pump 30 pumps the caustic stream or the hydrogen peroxide containing stream to a storage tank 32. Storage tank 32 has an outlet port connected to outlet pipe or line 34. When it is desired to dose the caustic stream or the hydrogen peroxide containing stream to the sewer ,10, the valve 36 is opened and the caustic stream or the hydrogen peroxide containing stream travels through line 34 and into sewer 10. This dosing of the sewer 10 with the caustic stream or the hydrogen peroxide containing stream may take place on an intermittent basis or on a periodic basis.

Figure 2 shows an alternative arrangement of an electrochemical apparatus suitable for use in treating an aqueous environment, in this case wastewater flowing through a sewer. In figure 2, the sewer 1 10 is provided with a first sidestream pipe or line 1 12 and a second sidestream pipe or line 114. Sidestream pipe or line 1 12 provides wastewater from the sewer 110 to the anode compartment 116 of an electrochemical apparatus 115. Anode compartment 1 16 includes an anode 114. A pump 113 may assist the transfer of wastewater from the sewer to the anode compartment 1 16. Alternatively, if the pressure in the sewer 110 is sufficiently high, the wastewater may simply flow under its own pressure into the anode compartment 1 16. Sidestream pipe or line 1 14 removes a stream of wastewater from the anode compartment 116 and returns it to the sewer 110. The electrochemical apparatus 115 also comprises a cathode compartment 1 17. Cathode compartment 1 17 includes a cathode electrode 1 18. A separator 120, which will typically be an ion permeable membrane, separates the anode compartment 1 1 from the cathode compartment 117. The cathode compartment 117 includes an inlet line 122 for feeding an aqueous stream or a water stream to the cathode compartment 1 17. A PLC 124 is used to control the electrochemical process and to automate any electromechanical processes that may need to occur. The cathode compartment 117 includes a cathode outlet line 126. A valve 128 may be selectively opened and closed to control the flow of effluent from the cathode compartment 1 17 into the cathode compartment effluent line 126. A pump 130 is used to pump a cathode effluent to a storage tank 132. The storage tank 132 has an outer line 134. A pump Γ_.38 controls the flow of cathode effluent from the storage tank 132 to the sewer 1 10.

Operation of the apparatus shown in figure 2 is generally similar to that as shown in figure 1. In particular, the cathode compartment is operated such that a caustic stream or hydrogen peroxide containing stream is generated. When the aqueous medium in the cathode compartment has attained the required concentration of caustic or hydrogen peroxide, valve 128 is opened and the caustic stream or hydrogen peroxide containing stream is pumped to the storage tank 132. When it is desired to dose the sewer 1 10, pump 138 is operated to pump the caustic containing stream or the hydrogen peroxide containing stream from the storage tank 132 to the sewer 1 10. It will be appreciated that line 134 may also be provided with a valve 136.

Figure 3 shows an embodiment that is generally similar to figure 2. For convenience and brevity of discussion, the features in figure 3 that are common with the features in figure 2 will be denoted by the same reference numeral but with a ' added. These features need not be described further.

In the embodiment shown in figure 3, the sewer 110' receives wastewater from a wet well 150. Wet well 150 includes a submerged pump 152 that can be used to pump wastewater from the wet well 150 to the sewer 110'. The wet well 150, in turn, receives wastewater from a wastewater collection pipe 154.

Figure 4 shows one arrangement for achieving switching between the cathode and the anode. In the apparatus 300 shown in figure 4, electrode chamber 302 is provided. Electrode chamber 302 houses a first electrode 304 and a second electrode 306. A separator 308 separates the electrodes 304, 306. Appropriate electrical wiring 310, 312 is provided from each electrode in order to enable an external circuit to be completed between the electrodes. The separator 308 also divides the electrode chamber 302 into a first chamber 305 and a second chamber 307. Chamber 305 has an outlet 314. Chamber 307 has an outlet 315.

A wastewater stream 316 and an aqueous solution 318 are alternately supplied to the first chamber 305 and the second chamber 307. In this regard, the wastewater stream 316 is provided to the first chamber 305 via line 320. Line 320 includes a valve 322. Similarly, aqueous solution 318 is provided to chamber 307 via line 324. Line 324 includes a valve 326. As can be seen from figure 4, line 316 also has line 328 extending therefrom. Line 328 includes a valve 330. Similarly, line 318 has a line 332 extending therefrom. Line 332 has a valve 334. When it is desired to supply wastewater to compartment 305, valve 330 is closed and valve 322 is opened. Pump 317 is operated and wastewater is provided to the first compartment 305. Similarly, valve 326 is opened and valve 334 is closed so that operation of pump 319 supplies aqueous solution to the chamber 307. In this embodiment, chamber 305 acts as an anode and chamber 307 acts as a cathode.

When it is designed to swap over the anode and the cathode, valve 322 is closed and valve 330 is opened so that wastewater from line 316 can pass to the chamber 307. Similarly, valve 326 is closed and valve 334 is opened so that aqueous solution can pass to the chamber 305. In this arrangement, chamber 305 acts as the cathode and chamber 307 acts as the anode. Appropriate electrical switching also takes place to swap the anode to the cathode and vice versa.

The method in accordance with some embodiments of the present invention can effectively prevent sulfide generation by producing a concentrated caustic solution or hydrogen peroxide solution at the cathode at a cost of AUD$ 1.15-2.0 kg^"1 S removed (based on electricity cost, 0.10$/kWh, applied cell voltage 10 V, assuming 95% current efficiency and a required 'shock-loading' of 1/3 of the sewer pipe at pH=12 every three days), which is far below the chemical costs of sulfide removal in sewer systems ($ 2.7 - 10.2 kg^"1 S removed) as reported in literature. In some embodiments, the process also involves generating oxygen at the anode, with the oxygen being returned to the aqueous environment to prevent or minimise the formation of anaerobic conditions. This also has the potential to reduce or control the activity of sulfide generating bacteria and methanogens. In other embodiments of the present invention, oxygen is generated at the anode and the oxygen is supplied to the wastewater system. In this embodiment, anaerobic conditions in the wastewater system are minimised or avoided, thereby minimising or preventing sulfate reduction which, in turn, minimises or prevents sulfide production. In other embodiments, an acidic stream is generated, typically at the anode, and it is dosed to the wastewater system to control or inhibit microorganism activity or growth.

In other embodiments, an alkaline stream is generated, typically at the cathode, and it is dosed the wastewater system to control or inhibit microorganism activity or growth.

Example

Sewer corrosion caused by hydrogen sulfide generation represents a major issue in sewer management. A recent industry survey revealed that the addition of caustic to reach toxic levels (i.e. pH>10.5 to kill or inactivate the sulfate reducing bacteria, SRBs) is commonly used by the Australian water industry for sulfide control in sewers. However, despite its strong benefits, this method also has some important disadvantages, including the frequent transport, handling and storage of concentrated caustic, all of which constitute serious occupational health and safety hazards.

Caustic is normally produced via the chlor-alkali process, in which chlorine is produced from a concentrated brine solution at the anode while caustic and hydrogen are generated at the cathode. Here, we investigated the feasibility of in-situ electrochemical caustic production from sewage in the context of sewer corrosion abatement. In contrast to the pure sodium chloride solution used in the chlor-alkali process, sewage contains a host of multivalent cations, which can also cross the cell membrane, potentially entailing scaling issues. We therefore also studied the feasibility of avoiding the scaling issue by periodic polarity switching of the electrodes.

Experimental

Electrochemical cell and operation

The two-chambered electrochemical cell consisted of two parallel Perspex frames separated by a cation exchange membrane (Ultrex CM 17000, Membranes International Inc., USA). The internal dimensions of the anode and cathode compartment were 20x5.0*0.9 cm creating an anode and cathode volume of 90 mL, respectively. Pt/Ir (Pt0₂/Ir0₂: 0.70/0.30) coated titanium electrodes (diameter: 240 mm; thickness: 1 mm; specific surface area: 1.0 cm²/cm²) with a projected surface area of 24 cm² were used as the anode and cathode material (Magneto Special Anodes BV, The Netherlands). In all experiments, an Ag/AgCl reference electrode (+197 mV versus NHE) was used. Sewage was fed to the anode chamber at a flow rate of 9.1 L/h using a peristaltic pump (Watson Marlow, UK). At this flow rate, the applied current density corresponded to an anode-to- cathode-flux of 15-25% of the all the sodium present in the sewage. A recirculation flow of 60 L/h was applied to obtain a good mixing rate in the anode chamber by using a peristaltic pump (Watson Marlow, UK). The sewage, used as feedstock for the anode, was stored at 4°C and sent through a water bath to achieve ambient temperatures (i.e. 25.1±1.0 °C) before entering the reactor. 120 mL of a 10 g/L NaCl solution was fed to the cathode chamber using a peristaltic pump (Midstreams Instruments, Australia). A water-lock (100 mL) containing 0.05M HC1 was used to trap any ammonia (NH₃) that may strip from the cathode solution as ammonium (NH₄-N).

The experiments were controlled galvanostatically using a Wenking potentiostat/galvanostat (KP07, Bank Elektronik, Germany). Two series of experimental runs were performed. The first serie (n=10) was performed to determine the coulombic efficiency of caustic generation at a current density of 10 mA/cm², the required energy input (i.e. the obtained cell voltage) per unit caustic produced, and the feasibility of polarity switching to avoid scaling. After each experimental run of 4 hours the polarity of the cell was switched, i.e. the anode of the previous experimental run became the cathode of the subsequent experimental run and vice versa. In the second series of experiments (n=3), spacers were used to investigate their impact on (a) the coulombic efficiency for caustic generation and (b) the cell voltage. Also here, polarity switching every 4 hours was used.

The anode and the cathode potentials were recorded every minute using an Agilent 34970A data acquisition unit. The coulombic efficiency was calculated as the ratio between the amount of charge transfer used for the production of caustic (based on the one electron reduction of water to hydroxide) and the total charge added to the system. All electrode potentials are reported versus NHE. Chemical analyses

The pH of the cathode compartment was determined by alkalinity titration using a 1 M hydrochloric acid solution at the end of every batch. The conductivity and temperature were measured by using a handheld conductivity meter (Cyberscan PC 300, Eutech Instruments). The ammonium concentrations were analysed using a Lachat QuikChem8000 (Lachat Instruments, USA) flow injection analyser (FIA). Elements (e.g. Sodium and Calcium) were measured with inductively coupled plasma optical emission spectrometry (Perkin Elmer ICP-OES Optima 7300DV, Perkin Elmer, USA). Chloride concentrations were measured with Ion Chromatography (IC) using a Dionex 2010i system.

Results and discussion Caustic generation from sewage

During 4-hour experiments, caustic at a concentration of 0.61±0.1 wt % was generated from sewage using Pt/Ir coated titanium electrodes at a current density of 10 mA/cm . The average coulombic efficiency obtained was 53±8% (without the use of spacers). A coulombic efficiency lower than 100% implies that either (i) protons and/or ammonium migrated from anode to cathode, or (ii) hydroxyl ions diffused back from cathode to anode. Analysis of the ammonium concentrations at the cathode revealed that there was insignificant transport of ammonium (Table 1). Hence, the transport of protons and/or back-diffusion of hydroxyl were the most likely reason for the decrease in coulombic efficiency.

Protons are generated in the anodic reaction. At high current densities, this can result in high proton concentrations in close vicinity of the anode surface. As the anode is located directly at the membrane surface, these high proton concentrations could cause a significant migrational protons transport from anode to cathode, even though the average anodic concentration of protons compared to other cations (e.g. Na ^") is low. Thus, increasing the distance between anode and membrane, and thereby locally improving mixing, conditions, may lead to reduced migrational proton transport and hence a higher caustic recovery. Therefore, experiments were performed with the use of spacers between the electrode surface and the membrane in both the anode and cathode chambers. The average obtained caustic recovery during the experiments (n=3) was 46±7%. Hence, this suggests that either the use of spacers did not improve the mixing conditions and/or that the back-diffusion of hydroxyl ions from the cathode to the anode compartment also played an important role.

Chemical analysis of the elements (e.g. Na⁺ and Ca²⁺) and ammonium revealed that the transport of cations was not sufficient to maintain electroneutrality of the solution. Chloride analysis (n=3) revealed that there was significant back-diffusion of chloride towards the anode compartment (see Table 1). As the hydroxyl concentration (i.e. based on pH) is in the same range as the chloride concentration (see Table 1 ) it can be expected that the hydroxyl back-diffusion is of the same order of magnitude as the chloride back- diffusion and thus significantly affects the caustic recovery. The distribution of the average transport of the cation and back-diffusion of anion species is presented in Table 2.

Figure 5 A and 5B show the anode, cathode and overall cell potentials during the experiments without and with the use of spacers, respectively. In all experimental runs the anode and cathode potentials remained constant over the course of the experiments and were 1.8±0.1 and -0.8±0.02 Volt, respectively. However, the overall cell potential increased over the course of all experimental runs. In the experimental runs without spacers (Figure 5A), the average cell potential at the start of a cycle was 4.5±0.2 Volt and increased to 5.9±0.5 Volt by the end of the cycle, whereas in the experimental runs with spacers (Figure 5B), the average cell potential at the start of a cycle was 5.4±0.4 Volt and increased to 8.3±0.6 Volt by the end of the cycle. Hence, the use of spacer resulted in an increase in the average cell voltage from 5.2±0.7 (no spacers; Figure 5A) to 6.8±0.2 Volt (with spacers; Figure 5B). This increase can be explained from the low conductivity of domestic wastewater (~1 mS/cm), which theoretically results in an increase in cell potential of ~1V per mm distance between the anode and cathode at a current density of lOmA/cm².

Since the anode and cathode potentials remained constant, the increase in overall cell potentials over time were likely caused by an increasing ohmic resistance. This increase was most likely caused by the transport of bivalent cations like calcium to the cathode causing precipitation of inorganics such as calcium hydroxide or calcium carbonate. This hypothesis was supported by the ICP analysis, which showed transport of calcium and magnesium to the cathode compartment (Table 2). In addition, scaling was visually observed on the membrane surface by the end of each cycle. The experiments showed that switching the polarity of the electrodes (i.e. anode becomes cathode and cathode becomes anode) every 4 hours returned the cell potentials to their original values (see Figure 5A). Moreover, the observed scaling disappeared. Hence, it seems that periodic switching of the polarity is successful to prevent scaling, and thus avoids the necessity for frequent chemical cleaning. Further research is warranted to elucidate the impact of the switching and scaling on the electrode and membrane lifetime.

The results showed that it is feasible to produce caustic from sewage without significant pH changes at the anode. To the inventor's best knowledge, this is the first time that caustic is directly generated from sewage, potentially leading to a reagent free method for sulfide abatement in sewer systems.

In this study, we investigated the feasibility of caustic generation from sewage by demonstrating its working principle. According to our calculations, the cost would be $0.83-1.17 (kg S^*1) compared to $1.7-7.2 (kg S^"1) for conventional sulfide abatement strategies. The costs are based on the obtained results (i.e. coulombic efficiency and cell voltage) and a required caustic dosing frequency of once per week to pH 11. The costs mainly depend on the length and diameter of the sewer pipe and the required frequency of dosing. The results indicate that there was significant cross-over of protons as well as back- diffusion of anions. Further optimization of the process can occur by selecting ion exchange membranes less prone to back-diffusion, such as the ion exchange membranes that are used in the chlor-alkali industry. In this study, Pt/Ir coated titanium electrodes were used.

Continuous experiments to determine the long-term performance at larger scale are required to accurately assess the potential of this method. The coulombic efficiency, obtained cell voltage, attainable caustic concentration (wt % NaOH), the electrode life time and engineering aspects of the system will be critical parameters. Conclusions

Caustic can be produced from sewage using Pt/Ir coated titanium electrodes at a current density of 10 mA/cm², with a coulombic efficiency of 53±8% at an average cell voltage of 5.2±0.7 Volt. Periodically switching the polarity of the electrodes was successful to overcome problems with scaling. The use of spacers did not increase the caustic recovery, but higher cell voltages were obtained. This technology constitutes a promising reagent free method for sulfide abatement in sewer systems.

Those skilled in the art will appreciate that the present invention may be susceptible to variations and modifications other than those specifically described. It will be understood that the present invention and compasses all such variations and modifications that fall within its spirit scope.

Throughout the specification, the term "comprising" and its grammatical equivalence shall be taken to have an inclusive meaning unless the context of use indicates otherwise.

Claims

CLAIMS 1. A process for controlling microbial growth in aqueous streams or environments comprising providing an electrochemical system having an anode and a cathode separated by a separator, feeding an aqueous stream to the anode, producing an alkaline stream and/or a hydrogen peroxide solution at the cathode and supplying the alkaline stream and/or the hydrogen peroxide solution to the aqueous stream or environment.

2. A process as claimed in claim 1 wherein an alkaline stream is produced at the cathode.

3. A process as claimed in claim 1 or claim 2 wherein the process further comprises generating oxygen at the anode and feeding the oxygen to the aqueous stream or environment.

4. A process as claimed in any one of claims 1 to 3 wherein the aqueous stream or environment comprises a wastewater system (including a wastewater collection system) or an aqueous stream used in an industrial process or a food processing system or a beverage processing system.

5. A process as claimed in any one of the preceding claims wherein the electrochemical system is operated such that oxygen is generated at the anode and the oxygen generated at the anode is supplied to the wastewater system.

6. A process as claimed in any one of the preceding claims wherein the electrochemical system is operated such that reduced sulfur species are oxidized at the anode to form oxidized sulfur species, such as elemental sulfur, thiosulfate, sulfite, and sulfate.

7. A process as claimed in any one of the preceding claims wherein the electrochemical system is operated such that mediators such as oxygen, chlorine and

radicals are generated at the anode, while simultaneously reduced sulfur species are also oxidized at the anode to form oxidized sulfur species, such as elemental sulfur, thiosulfate, sulfite, and sulfate.

8. A process as claimed in any one of the preceding claims wherein the electrochemical system is operated such that oxygen is reduced at the cathode, leading to the formation of water. -

9. A process as claimed in any one of the preceding claims wherein the electrochemical system is operated such that hydrogen gas is generated at the cathode and the cathode reaction either consumes protons or produces hydroxyl ions to cause pH in the cathode chamber to increase and simultaneously produce the hydrogen gas and an alkaline stream.

10. A process as claimed in any one of the claims 1 to 8 wherein a hydrogen peroxide containing solution is formed at the cathode.

1 1. A process as claimed in any one . of the preceding claims wherein the anode is separated from the cathode by a cation exchange membrane.

12. A process as claimed in claim 1 1 wherein the cation exchange membrane comprises a a monovalent ion selective cation exchange membrane.

13. A process as claimed in any one of the preceding claims wherein one or more anti_^ scaling agents are added to the cathode.

14. A process as claimed in any one of claims 1 to 10 wherein the anode is separated from the cathode by an anion exchange membrane

A process as claimed in any one of claims 1 to 10 wherein the separator between the anode and the cathode comprises a porous membrane or separator.

16. A process as claimed in any one of the preceding claims wherein the anode reaction is chemically catalysed.

17. A process as claimed in any one of the preceding claims wherein the cathode reaction is chemically catalyzed.

18. A process as claimed in any one of the preceding claims wherein the processcomprises anodic oxygen generation at mixed metal oxide electrodes coupled with cathodic generation of caustic.

19. A process as claimed in any one of the preceding claims wherein the anode is positioned in an anode chamber and the anode chamber receives wastewater from the wastewater system.

20. A process as claimed in claim 19 wherein the anode chamber is positioned in the wastewater system such as the anode chamber is positioned within a sewer pipe, in a wet well or in a pumping well, or the anode chamber is associated with the wastewater system by way of connection with a bypass pipe or a feed line with wastewater being provided from the wastewater system to the anode chamber and wastewater leaving the anode chamber being returned to the wastewater system.

21. A process as claimed in any one of the preceding claims wherein the anode and the cathode may be periodically electrically switched over and alternatively used as the cathode and the anode, respectively.

22. A process as claimed in any one of the preceding claims wherein the process inhibits the activity of sulfate reducing bacteria or the process inhibits the activity of methanogenic archaea otthe process treats or disrupts a biofilm present in the aqueous stream or environment.

23. A process as claimed in any one of the preceding claims wherein the aqueous stream or environment comprises a wastewater system selected from a wastewater collection system, such as a sewer or a wet well of a sewer system or a rising main of a sewer system.

24. A process as claimed in any one of the preceding claims wherein the alkaline stream and or the hydrogen peroxide solution are periodically supplied or intermittently supplied to the aqueous stream or environment.

25. A process as claimed in any one of the preceding claims wherein the alkaline stream and/or the hydrogen peroxide solution are supplied to the aqueous steam or environment during a pumping event.

26. A process as claimed in any one of the preceding claims wherein an alkaline stream and/or hydrogen peroxide solution is generated in the cathode, and the alkaline stream and/or hydrogen peroxide solution is injected into the aqueous stream or environment such that a pH of at least 8.3, more preferably at least 10, more preferably at least 11, more preferably of at least 12, is obtained in the wastewater system after injection.

27. A process as claimed in any one of the preceding claims wherein the anode comprises a biocatalysed anode.

28. A process as claimed in any one of the preceding claims wherein the anode and the cathode are connected to each other by an electrical circuit.

29. A process as claimed in claim 28 wherein a power supply is included in the electrical circuit.

30. A process as claimed in any one of the preceding claims wherein a wastewater stream is fed to the anode compartment and water or an aqueous stream is fed to the cathode compartment.

31. A process as claimed in any one of the preceding claims wherein an alkaline stream is generated at the cathode and the alkaline stream is recovered for storage for subsequent continuous or periodic injection into the wastewater system.

32. A process as claimed in any one of the preceding claims wherein the electrochemical system is operated on a generally continuous basis and the alkaline solution and/or the hydrogen peroxide solution generated in the cathode is stored for intermittent or periodic dosage to the wastewater system, or the electrochemical system is be operated on an intermittent or periodic basis to thereby generate the alkaline solution and/or the hydrogen peroxide solution on an intermittent or periodic basis^'.

33. A process as claimed in any one of the preceding claims wherein a concentrated caustic solution is produced at the cathode and it is dosed to the wastewater system to cause the pH in the wastewater system to be greater than or equal to 8.3, more preferably greater than or equal to 10, more preferably greater than 1 1 or greater than 12, even more preferably greater than 13 in order to inactivate microorganisms in the wastewater system, or to prevent release of sulfide as H₂S.

34. A process as claimed in claim 33 wherein the alkaline stream is added to the wastewater stream to obtain pH of greater than or equal to 10-12 or 1 1-12 to cause inactivation or inhibition of microorganisms for a period of time after the dosing of the alkaline solution has been completed.

35. A process as claimed in any one of the preceding claims wherein the wastewater stream is dosed with the alkaline solution and/or hydrogen peroxide solution in order to obtain a pH of greater than or equal to 12 by dosing every 1 to 7 days, more suitably every 3 to 5 days.

36. An apparatus for controlling microbial growth in an aqueous stream or environment comprising an electrochemical system comprising an anode and a cathode separated from the anode by a separator, anode feed means for feeding an aqueous stream from the aqueous stream or environment to the anode electrode, cathode feed means for feeding an aqueous liquid to the cathode, cathode effluent removal means for removing an alkaline stream and/or a hydrogen peroxide solution from the cathode and dosage means ^' for dosing the alkaline stream and/or the hydrogen peroxide solution to the aqueous stream or environment.

37. An apparatus as claimed in claim 36 wherein the dosage means comprises a pipe, tube, conduit or channel for feeding the alkaline stream and/or the hydrogen peroxide solution to the aqueous stream or environment.

38. An apparatus as claimed in claim 36 or claim 37 wherein the dosage means transfers the alkaline stream and/or the hydrogen peroxide solution directly from the cathode to the aqueous stream or environment, or the cathode effluent removal means removes the alkaline stream and/or the hydrogen peroxide solution from the cathode and directs it to a storage vessel or receptacle and the alkaline stream and/or the hydrogen peroxide solution is subsequently be transferred from the storage vessel or receptacle to the aqueous stream or environment via the dosage means.

39. An apparatus as claimed in any one of claims 36 to 38 wherein the anode chamber and the cathode chamber are periodically switched to prevent scale build-up in the cathode and the apparatus further comprises feed pipes and effluent pipes and valving to enable appropriate liquid streams to be fed to the respective compartments when a switch has occurred.

40. A process for controlling microbial growth in aqueous streams or environments comprising providing an electrochemical system having an anode and a cathode separated by a separator, feeding an aqueous stream to the anode, producing an acidic solution at the anode and supplying the acidic solution to the aqueous stream or environment.

41. A process for controlling microbial growth in an aqueous stream or environment comprising providing an electrochemical system having an anode and a cathode separated by a separator, feeding wastewater to one electrode, producing an alkaline or acidic stream at the other electrode and supplying the alkaline or acidic stream to the wastewater system.

42. A process for controlling microbial growth in aqueous streams or environments comprising providing an electrochemical system having an anode and a cathode separated by a separator, feeding wastewater to the anode, producing an alkaline stream at the cathode and supplying the alkaline stream to the wastewater system.

43. A process as in any one of claims 40 to 42 wherein hydrogen peroxide is formed at the cathode and/or chlorine is formed at the anode.

44. A process as claimed in any one of claims 1 to 35 or 40 to 43 wherein the anodic process also leads to the production of chlorine or reactive oxygen species.

45. A process as claimed in claim 44 wherein the chlorine or reactive oxygen species containing stream are intermittently fed to the wastewater system.

46. A process as claimed in any one of claims 1 to 35 or 40 to 45 wherein an acidic stream is passed to the cathode in order to remove or reduce scale in the cathode.

47. A process as claimed in claim 46 wherein the acidic stream is provided from an external source of acid or the acidic stream is generated at the anode and subsequently passed to the cathode or to a storage vessel and then from the storage vessel to the cathode.

48. A process as claimed in claim 47 wherein wastewater flow is sent through the anode, and after a period the wastewater flow is stopped and an acidic stream is generated in the anode, and the acidic stream is supplied to the cathode, where calcium deposits or other scaling deposits are removed.