Abstract:
A system for electric pH control of saltwater swimming pools, including a pump-assisted circuit for circulating saltwater to and from a swimming pool, means for determining the pH of the saltwater, a pH control cell having at least one pair of electrodes arranges: for electrolytically creating an alkaline and an acidic chemical, the cell including a water flow-through compartment and a species separation compartment, the compartments being separated by a separator structure, a drainage structure, and a controller functionally operative to compare the pH determined or sensed with a desired pH value, apply an electric potential across the electrodes of the cell and control one or both of the potential and electric current supplied to the electrodes as a function of the pH comparison and regulate drainage of an alkaline or acidic species which has been electrolytically generated.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application the National Stage of International Application No. PCT/AU2015/050285 having an International Filing date of 27 May 2015, which designated the United States of America, and which. International Application was published under PCT Article 21(2) as WO Publication No. 2015/179919 A1, and which claims priority from, and the benefit of, Australian Application No. 2014902004, filed on 27 May 2014, the disclosures of which are incorporated herein by reference in their entireties. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The presently closed embodiment relates primarily to the control of in swimming pools, and also to a means of saltwater chlorination of swimming pools. 
         [0004]    2. Brief Description of Related Developments 
         [0005]    Swimming pools are mostly sanitized by the use of chlorine. The chlorine may be added to the pool in many ways, including chlorine bearing compounds in solid form, liquids (usually as sodium hypochlorite or bleach, in solution), or as a gas (typical as chlorine or chlorine dioxide); in over 85% of Australian residential swimming pools, chlorination is effected by electrolysis of pool water to which a salt has been added, i.e. salt water chlorination. 
         [0006]    Saltwater chlorination is a process that uses a salt, usually NaCl but could be other chloride or bromide salts, dissolved in pool water at typically 2,500 to 6,000 parts per million (ppm), as a source of chlorine (or bromine) in generating sanitizing chlorine (or bromine) compounds, in particular the preferred hypochlorous acid (HClO). 
         [0007]    The term ‘saltwater’ is used in the present document to denote pool water with a typical load of salt (which need not be but preferably in its bulk amount is NaCl) in the range of 3,000-6,000 ppm, but could range from 500-1,000 ppm to seawater salt concentrations in practice, as source of the disinfectant halide entity (usually Cl, Br). Further, the aspects of the presently disclosed embodiment will be described in the context of use of chlorine as the halide, but it will be understood that other compounds are and may be used. 
         [0008]    To effect saltwater chlorination, ‘salted’ pool water is pumped through an electrolytic chlorine generator (or cell) comprising at least one anode-cathode plate set. Usually, titanium is used for the electrodes, at times the plates are coated with a metal oxide such as that of ruthenium, or iridium. Other plate materials (such as carbon, graphite or platinum) and other coatings and/or doped materials are also used. Perforated plates (and coaxial mesh cylinders) may also be used rather than parallel solid plates. 
         [0009]    Irrespective of the precise details of plate materials, number of plates and geometry, when a voltage (i.e. a potential difference), usually in the 2 volt to 8 volt range, is applied between the electrodes, electrolysis of salt water will lead to water being dissociated generating hydrogen gas at one electrode (cathode), and chlorine at the other electrode (anode). The otherwise flammable and potentially explosive hydrogen gas generated at the cathode is safely flushed from the cell in the water stream passing through it, noting that conventional salt water chlorine generators are usually installed in line after the swimming pool filter as the last item in the recirculation line towards the swimming pool. 
         [0010]    Chlorine is produced at the anode of the chlorinator according to the reaction 
         [0000]      2Cl − →Cl 2 +2 e −   (1)
 
         [0011]    The chlorine reacts rapidly with water according the reaction 
         [0000]      Cl 2 +H 2 O→HCl+HClO   (2)
 
         [0012]    The hydrochloric acid is fully dissociated. The hypochlorous acid is in equilibrium with its conjugate base, the hypochlorite ion, according the equation 
         [0000]      HClO⇄H + +OCl −   (3)
 
         [0013]    the cathode the main reaction 
         [0000]      2H 2 O+2 e − →H 2 +2OH −   (4)
 
         [0014]    For the purposes of sanitation, these re the main reactions occurring in saltwater chlorination, though many other reactions also occur depending on the chemical composition of the pool water, potential difference, configuration of the chlorinator plates, and other variables. 
         [0015]    Hypochlorous acid is a much stronger disinfectant than the hypochlorite ion, and is the principal and preferred disinfecting agent. 
         [0016]    Relevantly, the concentrations of hypochlorous acid and hypochlorite ion, which are the chlorine hydrolysis compounds, are controlled by pH according to the above equilibrium (equation3), and so the sanitizing effectiveness of chlorination varies considerably with the pH of the water, which also affects comfort of users of the swimming pool. 
         [0017]    Put in another way, disinfection of pool water is much more effective at lower pH values because the chlorine hydrolysis products are mostly present in the form of highly disinfectant hypochlorous acid rather than the mild disinfectant hypochlorite ion. The desired pH range for swimming pools, considering these and other factors such as the longevity and the appearance of the finish on the poor&#39;s structural surface, ought thus typically to be set at 6.9 to 7.8, but in Australia more commonly at 7.2 to 7.6. 
         [0018]    It can be seen further from the equations above that for every two moles of strongly basic (alkaline) hydroxide ion produced at the cathode, the anode produces one mole of a strong acid (HCl) and one mole of weak acid (HClO). When mixed together as in the output stream of conventional saltwater chlorinators, the acid compounds (herein also referred to as acidic chemical species) may completely or incompletely neutralize the alkaline compounds (herein also referred to as alkali chemical species), depending on the pH and the degree of dissociation of the hypochlorous acid. So, the overall chlorination process either does not change the pH or it increases the pH; it does not decrease the pH of the chlorinated pool water. Optimising the pH setting must therefore be done using other techniques, as noted below. 
         [0019]    Another factor to consider is that most in ground pool shells are made of concrete or have surface finishes that incorporate cement, both of which are alkaline and which tend to leach alkali into the pool water. 
         [0020]    Consequently, the combination of such leaching and the electrolytic chlorination process tend to drive the pool alkaline with time, that is, to higher pH values. Even pools made of more neutral materials such as fibreglass may naturally drift to higher pH values due to the effects of the electrolytic process which, overall, tends to be alkaline. 
         [0021]    This tendency to high pH in swimming pools is typically countered in conventional pool set-ups by adding hydrochloric acid (in addition to that created in the chlorinator), in amounts as required to maintain the pH within the desired range. Other acids can also be used and bubbling carbon dioxide into pool water to form carbonic acid is one such other method. Concentrated hydrochloric acid, also known as muriatic acid, is most widely used. 
         [0022]    The use of concentrated hydrochloric acid usually means the storage of significant quantities of this dangerous substance in domestic situations, often without the precautions and due care that are appropriate. Dispensing is done either by manual methods, which require careful measurement and handling, or in automated acid dispensing systems; or by pumps or other mechanical dispensing methods to deliver metered amounts of acid. Such mechanised dispensing methods can be automated using sensors, or semi-automated (pre-set to a daily quantity dispensed). 
         [0023]    Both methods have significant problems. Manual methods are notoriously inaccurate and unreliable and, in typical domestic situations, are seldom performed regularly and very rarely performed often enough to achieve effective control of pH. Weeks and sometimes months pass between treatments when, in reality daily or every second-day treatment is required in some pools to ensure good or even acceptable sanitizer performance. In addition, manual handling of acid is undesirable for safety reasons. 
         [0024]    In automatic systems and semi-automatic systems, peristaltic pumps are usually used, which are notoriously unreliable and break down, resulting in ineffective pH control and sanitation in the pool and drums of unused acid left deteriorating on-site for long periods. In short, residential sites are seldom managed correctly for a variety of reasons. 
         [0025]    As noted above, overall, most in-ground pools tend to drift to being alkaline over time, and the most common pool, being the concrete one with a cement-based finish, strongly so to the extent that many litres of acid a year may be required to balance the water. This is especially true of new pools where leaching rates and alkalinity are much higher a old pools that are more chemically stable. 
         [0026]    On the other hand, although more rare, some pools can become acidic. In this case, an alkali needs to be added to restore pH to the desirable range. This is often sodium bicarbonate. This chemical is also very commonly dissolved in the pool water as a buffer to stop the pool going acid and reduce the rate of change of pH. 
         [0027]    Electrolytic systems for the automatic control of chlorine content and pH in swimming pools have been proposed, such as in U.S. Pat. No. 4,767,511 (Aragon). The system described by Aragon uses a dual-compartment electrolytic cell for generation of chlorine and caustic soda (NaOH) from a sodium chloride solution (brine) and water, as well as an acid supply system for adding hydrochloric acid directly to the pool water as required for pH control. Generation of chlorine and addition of HCl are controlled automatically in response to sensed oxidation-reduction potential (ORP) and pH in the swimming pool water. The dual-compartment electrolytic cell has a porous diaphragm (or separator) dividing the cell into anolyte and catholyte compartments. Chlorine gas generated in the analyte compartment of the cell is separated from spent brine which is recirculated back into the NaCl+H2O (brine) supply tank where is re-saturated, whereas caustic soda, H2 gas and water are supplied from the catholyte compartment of the cell into the pool water return line of the cell. 
         [0028]    The system of Aragon requires a dedicated brine supply tank (storage) and recirculation circuit between tank and electrolytic cell, as well as a separate HCl storage facility and supply line to pool, to effect both the pH and chlorination control. 
         [0029]    The presently disclosed embodiment seeks to provide an electric pH control system, using electrolysis of saltwater, which is preferably automated, and without the need for bulk acid addition to the swimming pool water. 
         [0030]    It would be beneficial too to define an electric control cell which could be used simultaneously as a chlorinator cell. 
         [0031]    It would be beneficial also for the system to enable a reduction of regular bulk material inputs into the pool water, i.e. consumables, in particular acids such as HCl. 
         [0032]    It is also desired to simplify the make up of a pH and chlorination control system which is effective in maintaining effective pool water sanitation levels. 
         [0033]    It would also be desirable to devise an electrolytic pH control cell in which build-up of scale on the electrodes (and other metallic components of the cell) can be minimised or cleaned-up in operation of the cell. 
       SUMMARY 
       [0034]    In the different aspects of the presently disclosed embodiment, swimming pool saltwater is subjected to hydrolysis, whereby chlorine is generated in a fairly conventional manner. Relevantly, however, the inventive lay out of the electrolytic cell is such that the pH of the saltwater exiting the cell is controlled by selective removal of chemical alkaline species, created in the electrolysis process, from the stream of water flowing through the cell. This process renders the saltwater more acidic prior to being delivered from the cell into the swimming pool, reducing the pH. Furthermore, the below described inventive cell can also be operated in a manner to selectively remove chemical acidic species, thereby increasing the pool pH where such is necessary, preferably by temporary inversion of the polarity applied to the cell&#39;s electrodes. 
         [0035]    In a first aspect of the presently disclosed embodiment, there is provided a system for electric pH control of saltwater swimming pools, comprising: (a) a pump-assisted circuit for circulating saltwater to and from a swimming pool; (b) means for determining the pH of the saltwater, preferably a pH sensor; (c) an electrolytic pH control cell with an inlet and outlet connected to the pump-assisted circuit for receiving and discharging saltwater from/to the pool, respectively, the pH control cell having at least one pair of electrodes arranged for creating an alkaline and an acidic chemical species from saltwater flowing through the cell, the cell comprising a water flow-through compartment in which one of the electrodes is located and a species separation compartment in which the other of the electrodes is located, the compartments being separated by a separator structure which is permeable to cation and anion transfer and restrictive to electrolyte flow between both compartments; (d) a drainage structure arranged for selectively draining liquid from the species separation compartment in controlled manner to waste; and (e) a controller functionally operative to (i) compare the pH determined or sensed with a desired pH value, (ii) apply an electric potential across the, electrodes of the cell and control one or bath of the potential and electric current supplied to the electrodes as a function of the pH comparison, and (iii) regulate drainage of an alkaline or acidic species which has been electrolytically generated within the separation compartment from pool water flowing through the flow-through compartment as a function of positive or negative potential being applied to the electrode in the species separation compartment. 
         [0036]    In operation of the pH control cell, with saltwater being pumped at a selected rate through the flow-through compartment, and saltwater being present in the species separation compartment of the cell, the electrochemical reaction at the negative electrode (cathode) changes the chemistry of the saltwater in contact with it in accordance with the reactions described above. This process generates a liquid that can be referred toy as a catholyte. In essence, it is still saltwater, but with alkaline chemical species added to the initial liquid charge, with the degree of alkalinity depending upon various adjustable parameter&#39;s of the system, including one or both of electric potential difference across the electrodes and current flow between the electrodes of the cell. Hydrogen gas also produced at the cathode is preferably collected at a gas head space within the cell and ultimately dispersed safely. At the other electrode, the positively charged anode, an anolyte is produced by the chemical reactions described above, which ultimate leads to acidic chemical species being added to the saltwater as it flows through the cell. The anolyte also carries chlorine produced at the anode and oxidants mixed into the water, being oxidizing agents containing at least oxygen and/or chlorine in various chemical forms. 
         [0037]    Noting that in most cases saltwater pools tend towards alkaline pH over time, it is particularly preferred to devise the system controller to be operative to apply a negative potential to the electrode within the species separation compartment sufficient to drive hydroxide ion (OH − ) production from saltwater and produce an alkaline catholyte, and H 2 , in the species separation compartment wherein catholyte can then be drained in a controlled and selective manner to waste (or storage for alternative uses) and H 2  gas accumulating at a gas head space of the compartment vented preferably into the saltwater stream of the flow-through compartment. A positive potential will be present at the electrode in the flow-through compartment sufficient for producing an acidic anolyte from saltwater flowing in the flow-through compartment of the cell. The net output of liquid from the cell towards the pool water return line will thus be acidic, lowing pH in the pool. 
         [0038]    In contrast to a conventional in-line chlorinator, in which the anolytes and catholytes created during electrolysis within the cell are mixed downstream of the electrodes and returned to the pool, thus creating a chlorinated and potentially basified (alkaline) stream of salt water, carrying hydrogen gas as well as some mixed oxidants and other electrochemically generated species, the presently disclosed embodiment requires the electrolysis output streams to be kept separate in compartments that are chosen large enough in volume to allow effective separation of alkaline and acidic electrolyte. The pH of the net output fluid from the pH control cell to pool can then be controlled by discharging in controlled manner part or all of either the alkaline catholyte or the acidic anolyte to waste without mixing it into the output liquid stream which recirculate back to the pool. 
         [0039]    The output stream can be chosen to be alkaline by discharging some of the acid anolyte, or acidic by discharging of some of the alkaline catholyte, or neutral. If anolyte is dispensed to waste for pH control reasons, then the chlorine generated in the pH control cell will be simultaneously lost as it is dissolved in the anolyte. In pools requiring this, that is, in the small minority of pools that tend to go acidic, supplemental chlorination will be required over time. 
         [0040]    The drainage structure will at include a variable flow valve so that the drainage rate of liquid from the separation compartment can be set to a predetermined value. In its simplest form it can be a crimp valve. A peristaltic pump could also be used, this providing the added functionality of allowing pump assisted, more precisely metered draining (rather than purely gravitational purging) of the compartment). Drainage rates are very slow compared to flow rates of pool water through the flow-through compartment of the cell. Drainage rates can be set at between 0.1 to 1.0 ml per second (0.36-3.60 1 per hour), noting that the pH cell will not be operated on a continuous basis but intermittently, thus avoiding unnecessary loss of saltwater volume from the pool. Ultimately, drainage rate is a function of separation compartment volume, saltwater flow-through rate through the cell, leakage rate between flow-through and separation compartments across the separator structure between the compartments, hydroxide or migration rate through the separation structure, and needs to be fast enough to exchange the electrolyte contents within the separation compartment in a time that is short compared to the duty cycle of the cell when running as an acid generator (see below). 
         [0041]    The cell will preferentially be operated such that the concentration of chemical species the discharge is high, so that the pH of discharged liquid is quite alkaline (greater than 11), or quite acidic (less than 3), depending on the polarity applied to the electrode in the separation compartment of the cell. The result is that the of the pool will be shifted by removing a small volume of liquid at an extreme pH at the cell. When neither catholyte nor anolyte are dumped to waste, then the net output stream from the pH control cell is either unchanged or slightly more alkaline than the incoming saltwater. 
         [0042]    The removal of catholyte (or anolyte where the potential to the electrodes has been temporarily reversed) from the pH cell&#39;s separation compartment may be assisted by pumps, venturis, other mechanical devices or gravity, depending upon the hydraulic set-up of the pool in any one situation. 
         [0043]    It is possible to measure the pH aria ORP levels of the output of the pH control cell directly at the cell, but typically this is not necessary. If electronic sensors are located after the pool filter and before the pH control cell, then the resultant pH and ORP of the pool can be sensed and used to appropriately control the liquid output of the cell. Optimally, a discharge rate to waste is chosen whereby the chlorine and pH are simultaneously optimised. 
         [0044]    Under alkaline conditions, some dissolved salts may precipitate as solids, usually hydroxides or carbonate compounds, at the cell. For instance, dissolved calcium may precipitate as “lime scale”, which is principally a complex mix of hydroxide and carbonate salts of calcium. These residues may foul the separation structure of the cell which allows ion transfer between the compartments of the cell, valves and other cell structures, which if unchecked, may cause device failure or reduced lifetime of components. As these residues are usually redissolved by acid, the pH control cell can be ‘switched’ (through its controller) to clean itself. For instance, after a period of operation in one polarity, in which some residue forms in the alkaline compartment, the polarity can be briefly reversed while drainage from the separation compartment to waste is stopped, to produce an acid environment to dissolve the residue. After a period of time, the separation compartment is flushed by draining to waste, and normal operation is then resumed. 
         [0045]    In one preferred aspect the above system, the pH control cell works together with a separate, in-line salt water chlorinator located downstream in the pool water recirculation circuit such as to allow for the pH control cell to work at an operating point optimised for pH control (vs chlorine generation) and allowing the dedicated saltwater chlorinator cell to be a primary chlorine source for sanitation. Such arrangement provides improved efficiency and improved cell electrode (plate) maintenance at both the pH control and chlorinator cells. 
         [0046]    Normally, conventional in-line chlorinator cells in a properly managed saltwater pool are fed with pool water at a pH from 7.2 to 7.8, which produces chlorine as well as some oxidants, being a mixture of oxygen, hydrogen and chlorine compounds, some of which are useful as a sanitiser, for example, hydrogen peroxide. If, in accordance with the presently disclosed embodiment, the pH control cell is set to feed the conventional in-line salt chlorinator with a stream of saltwater at below pH 7.0, then the mixture of compounds produces changes and can include, for example, chlorine dioxide, which is an excellent sanitiser. The chlorinator also tends to operate more efficiently and at higher electrical currents for the same salt concentrations at lower pHs. 
         [0047]    In normal operation with both the pH control cell operating to deliver an acidic saltwater stream and the chlorinator cell operating to deliver chlorine, feeding an acidic saltwater stream to the chlorinator cell also reduces the deposition of Calcium Carbonate on the anode of the chlorinator cell. This is normally a significant problem in in-line pool chlorinators. Calcium deposition typically needs to be removed by either regular removal of the electrodes and acid washing, or by reverse polarity operation. Reverse polarity operation appreciably decreases the allowed current density in the electrode plates by a factor of at least 3 and often 5, depending on the coating on the plate. It necessitates larger electrode plates by said factor and also means that both anode and cathode of the chlorinator cell must be coated with an expensive material such as Ruthenium or Iridium oxide or Platinum, depending on the technology being used. Reverse polarity operation also reduces coating life by a significant margin. 
         [0048]    Furthermore, not only will the acidic saltwater output stream from the pH control cell be beneficial in normal operation, but it can also be used to clean the in-line chlorinator plates. This is done by setting the controller of the pH control cell to ‘minimum pH setting’, by increasing the electric current to the electrodes and/or reducing flow of saltwater by the pool pump (or dedicated cell pump), switching off the chlorinator cell and reducing filter speed to very slow so as to push a stronger acidic saltwater stream than normal into the chlorinator cell. This can also be done by stopping and starting the filter pump or in other ways but the essence is that either a stronger acidic saltwater stream is caused to flow continuous into the in-line chlorinator or it is pushed in in batches and allowed to reside for a period appropriate for the acid scrubbing alkaline deposits and thus cleaning the plates, before being refreshed or terminated as the case might be. This avoids the need for manual or other acid cleaning, or, reverse polarity operation in the in-line chlorinator. 
         [0049]    As hinted previously, the system can furthermore be devised/controlled such that the controller of the pH cell is operative to apply a positive potential to the electrode within the flow-through compartment sufficient to produce an effective amount of chlorine from saltwater within the flow-through compartment of the cell to enable the pH control cell to simultaneously serve as a sole chlorination source for the swimming pool. 
         [0050]    In a second aspect of the presently disclosed embodiment there is provided method for electric pH control of saltwater swimming pools, comprising: (a) determining the pH of saltwater in a swimming pool or flowing through a swimming pool water recirculation circuit; (b) circulating saltwater to and from the swimming pool past a saltwater electrolysis cell, the cell arranged for generating alkaline and acidic chemical species from saltwater using at least one pair of cell electrodes, the cell comprising a flow-through compartment in communication with the pool water recirculation circuit and in which one of the electrodes is located and a species separation compartment in which the other of the electrodes is located, the cell compartments being separated by a separator structure which is permeable to cation and anion transfer and restrictive to—yet preferably not fully blocking of—electrolyte flow between both compartments; (c) selectively applying an electric potential difference across the electrodes as a function of the pH determined and a desired pH of the pool water to produce alkaline or acidic chemical species from the saltwater at the electrode in the species separation compartment while maintaining pool water flow in the flow-through compartment; and (d) selectively draining liquid containing the alkaline or acidic chemical species from the species separation compartment away from the pool water. 
         [0051]    In a third aspect, the presently disclosed embodiment provides a method for electrolytic pH control and chlorination levels of saltwater swimming pools, comprising: a) determining the pH and ORP (or chlorine) levels of saltwater in a swimming pool; (b) circulating saltwater to and from the swimming pool past a saltwater electrolysis cell, the cell arranged for generating chlorine and alkaline and acidic chemical species from saltwater using at least one pair of cell electrodes, the cell comprising a flow-through compartment in communication with the pool water recirculation circuit and in which one the electrodes is located, and a species separation compartment in which the other of the electrodes is located, the cell compartments being separated by a separator structure which is permeable to cation and anion transfer and can be either fully blocking of or strongly restrictive to electrolyte flow between both compartments; (c) selectively applying an electric potential difference across the electrodes as a function of the determined pH and chlorine level and a desired pH and desired chlorine level in the pool water, whereby the electrode in the species separation compartment is negative relative to the electrode in the flow-through compartment so that chlorine and acidic chemical species are produced from the saltwater at the positive electrode and hydroxide is produced at the negative electrode in the species separation compartment; and (d) maintaining pool water flow in the flow-through compartment for delivering the chlorine and acidic chemical species produced during electrolysis into the pool water circulation circuit and selectively draining liquid containing the alkaline chemical species from the species separation compartment in controlled manner away from the pool water. 
         [0052]    Control of the chlorine and pH levels at chosen set-points in the pool can be advantageously achieved using closed loop control of the components of the net output liquid stream of the electrolytic pH control cell (these being liquid passing through the flow-through compartment and liquid contained and selectively drained to waste from the species separation compartment of the cell) using, for example, electronic ORP and pH sensors which would usually be located upstream of the cell in the recirculation/filtration line for swimming pool water. As noted, the other operating variables of the pH control cell that can be controlled and set are the potential difference applied across the electrodes and the electric current supplied to these. 
         [0053]    One of the advantages provided by the different aspects of the presently disclosed embodiment can be seen in the elimination (or at least substantive reduction) of a need to store acid and/or alkali on-site the swimming pool location in order to effect pH control and also eliminating the need to dispense stored acid or alkali manually or via some metering system, given that such control is effected by ‘manipulating’ the saltwater of the pool itself. 
         [0054]    Another benefit that flows from implementing the inventive aspects is a reduction or complete removal of the need for a dissolved buffer solution in the pool, such as sodium bicarbonate. Sodium bicarbonate is no longer required as the presently disclosed embodiment provides both acid and alkali control; buffer can optional be used in conjunction with the presently disclosed embodiment where there is a natural tendency for pools to drift to be acidic. 
         [0055]    In a further aspect, the presently disclosed embodiment provides a swimming pool pH control cell, comprising: (a) a water flow-through compartment within a housing and which can be coupled into a pump-assisted circuit for circulating saltwater between a swimming pool and the cell; (b) a species separation compartment at or within the housing, arranged to receive saltwater from the swimming pool, preferably via the flow-through compartment, and having a drainage arranged for selectively draining liquid from the species separation compartment in controlled manner to waste, preferably through controlled valve; (c) a separator structure between the compartments which is permeable to cation and anion transfer and which is blocking of or restrictive to electrolyte flow between both compartments; (d) at least one pair of electrodes arranged for creating an alkaline and an acidic chemical species from saltwater flowing through the cell, one of the electrodes located in the water flow-through compartment and the other located in the species separation compartment, the electrodes being connectable to a DC electricity source for effecting saltwater electrolysis; and (e) a controller operative on the electrode pair and having a controller functionally devised for (i) comparing a sensed pH of pool saltwater with a desired pH value, (ii) controlling one or both of electric potential across the electrodes of the cell and electric current supplied to the electrodes as a function of the pH comparison and (iii) regulating drainage of an alkaline or acidic species produced by electrolysis from saltwater within the separation compartment from pool water flowing through the flow-through compartment as a function of positive or negative potential being applied to the electrode, in the species separation compartment. 
         [0056]    In its simplest form, one of the control functionalities in the different aspects of the presently discloses embodiment could be performed in a semi-automated manner, wherein the pH is determined manually and compared with a desired/optimal pH level for a given swimming pool based on sanitation/chlorine settings, and based on a look up table (stored in controller memory) the electrolytic cell is then activated automatically and run (e.g. timer controlled) for a time sufficient to achieve the desired pH change, with draining of the species separation chamber being performed manually as well. 
         [0057]    However, it will be immediately appreciated that the different aspects are best performed in a fully automated implementation. A micro-processor controller can be suitably programmed and appropriate sensors and actuators can be provided at the cell/pool water recirculation circuit, and linked to the controller, to effect pH control in automated fashion in a closed (or open) control loop. 
         [0058]    Advantageously, the species separation compartment is located within the housing in the flow through compartment or at the housing besides the flow-through compartment, separated from the latter by the anion and cation separator structure. The species separation compartment can hereby be devised to receive saltwater via the flow-through compartment via a suitable lock structure or mechanism, as described below, or through a separate line with flow regulation valve, from the pool recirculation circuit. 
         [0059]    The species separation compartment will advantageously be provided with facilities for one or more of, but preferably all of (i) venting of as generated during electrolysis of salt water, (ii) for maintaining a gas lock between the flow-through compartment and the species separation compartment to keep the liquids in the respective compartments separate from one another during electrolysis of saltwater, (iii) for allowing liquid ingress from the flow-through compartment into the species separation compartment when the latter is being drained, and (iv) for liquid fill (or level) control of the species separation compartment to ensure that the electrode located therein remains fully submerged during the electrolysis process. 
         [0060]    These facilities may be provided dedicated mechanism/devices/structures such as valves, pumps and sensors which may be actively controlled, or passive structures that utilise hydraulic principles in achieving such functionality. Such structures are described below and identified in the claims at the end of this specification. 
         [0061]    The separator structure between the flow-through and species separation compartments can be described as a ‘porous’ separator in that while it aims to substantially restrict passage of liquid through it, it has a degree of permeability to liquid passage at extremely low rates, the porosity being chosen to substantially prevent bulk liquid flow between the compartments while ensuring adequate exchange of ions between the compartments. Material selection of the separator structure is also predicated to allow electrical current flow between electrodes to effect electrolysis. 
         [0062]    In a preferred aspect, the porous separator structure can include a polymer membrane having a thickness in the micrometer range, covering a window in a liquid-impervious wall separating the flow-through and species separation compartments. Such membrane will preferably be inert (i.e. not having inherent polarity preferences), the many fine pores being sized to allow water containing dissolved salts to provide the path for electric flow across the membrane between the electrodes, with low electrical resistance. The porosity will be chosen to restrict the flow of bulk liquid through the partition membrane to a flow rate that is at least an order of magnitude smaller than the drainage rate at which liquid is drained in controlled fashion from the species separation compartment and orders of magnitude smaller than the flow rate of saltwater from the pool through the flow-through compartment defined within the housing of the cell. In a specific example of such membrane, applicants have selected a microporous hydrophilic PTFE membrane laminated on a non-woven polypropylene substrate, “JMTL-100” from Anow Microfiltration Company, PR China. Such composite membrane is about 120 microns thick, the PTFE layer being about 20 micron, with pore size 1 micron. The PTFE membrane is believed to be furthermore quite resistant to the chemical environment in the cell. 
         [0063]    Relevantly, the membrane base material should be selected also to take account of the relatively chemically aggressive environment of the anolyte or catholyte in the species separation compartment in particular to achieve acceptable ‘wear’ properties. In that regard also, the housing of the cell will advantageously be constructed to allow access to and replacement of the separator membrane (or other structure), which is mounted within the housing between the flow-through and species separator compartments, when and if required. 
         [0064]    The electrodes used in the cell are preferably plate-like in design so as to extend parallel and closely spaced on either side of the planar separator structure, only a few millimetres apart. While the plates could simply be flat and rectangular, they could also be concentric cylinders or have other shapes. 
         [0065]    All structures used in the manufacture of the cell are made from materials that are chemical resistant to acidic, alkaline and oxidising environments, including chlorine hypochlorous acid and hypochlorite. 
         [0066]    Preferred aspects of tie presently disclosed embodiment, and optional features thereof, will be described herein below with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0067]      FIG. 1  shows a schematic and simplified recirculation and filtration circuit for a saltwater swimming pool, into which an inventive electric pH control cell has been plumbed in-line downstream the pool filter, in a first aspect of a system for electric pH control of saltwater swimming pools in accordance with one aspect of the presently disclosed embodiment; 
           [0068]      FIG. 2  shows a schematic and simplified recirculation and filtration circuit for a saltwater swimming pool, into which an inventive electric pH control cell has been plumbed in parallel flow, by-passing the pool filter, according to a second aspect of a system for electric pH control of saltwater swimming pools in accordance with one aspect of the presently disclosed embodiment; 
           [0069]      FIG. 3  is a schematic, vertical section of an embodiment of an electrolytic cell in accordance with another aspect of the presently disclosed embodiment, for use as the pH control cell in the systems of  FIG. 1 or 2 ; 
           [0070]      FIG. 4  is an enlarged detail view of the upper portion of the separation compartment located within and forming part of the cell illustrated in  FIG. 3 ; 
           [0071]      FIG. 5  is a plotted pH—time graph illustrating results of a pH control experiment conducted on a small volume of NaCl-salted water using an experimental pH cell such as schematically illustrated in  FIG. 3 ; 
           [0072]      FIG. 6  shows a graph with pH and ORP curves over a 21 day period, of water in a 45,000 litre salt water pool, whose pH was controlled using the experimental pH cell schematically illustrated in  FIG. 3  in accordance with the aspect of  FIG. 2 ; and 
           [0073]      FIG. 7  shows a second aspect of an electrolytic cell, schematically, in accordance with the presently disclosed embodiment, whereby same reference numbers as appear in  FIGS. 3 and 4  have been used to denote functionally equivalent cell components. 
       
    
    
     DETAILED DESCRIPTION 
       [0074]      FIG. 1  schematically illustrates a saltwater swimming pool  10  with a conventional water filtration and recirculation circuit  12 . Circuit  12  draws saltwater from pool via suction line  13  using pool pump  14 . Saltwater is circulated into rapid sand filter  16  for particulate matter scrubbing, and directed into an inline chlorinator in form of a conventional electrolytic cell la for adding of chlorine. The scrubbed and chlorinated water is returned via return line  20  to pool  10 . Box  22  denotes summarily a suite of pool water quality sensors, including in particular sensors for determining pH and oxidation reduction potential (ORP) of water passing through the pipe work from/to pool  10 . Water salinity can be set to between 2,500 to 6000 ppm sodium chloride by dissolving solid salt into the pool water as practiced conventionally. Salt need only be replaced when water levels in the pool are topped-up, due to, backwashing water losses or draining of water in the process of pool cleaning or after heavy rain, as normal evaporation of pool water leads to concentration of salt level. 
         [0075]    The pool water recirculation circuit components are conventional in nature and well known to the skilled pool operator. Circuit components such as valves, power supply circuitry for the pump and chlorinator cell, optional pool water heating recirculation equipment and infrastructure, and pool equipment control circuitry, which in its simplest form would include a timer for setting operating times of the pump and chlorinator, have been omitted for clarity purposes. 
         [0076]    In accordance with a first aspect of the circuit lay-out, an electrolytic pH control cell  25  (also referred to as a pH controller) in accordance with one aspect of the presently disclosed embodiment is mounted (plumbed) in-line downstream the sensor suit  22  and upstream the chlorinator cell  18  in the water recirculation circuit  12  to deliver saltwater passing through cell  25  into cell  18  via line  21 . Relevantly, pH controller  25  is connected also to a liquid discharge pipe or line  26  for reasons which will be described in detail below with reference to  FIG. 3 , which drains part of the liquid received in cell  25  towards waste (e.g., sewerage). 
         [0077]    In the circuit of  FIG. 1 , noting that the water flow rate and pressure will be dictated by the pool pump  14  and hydraulic parameters of the filter  16 , water pipes/lines and valves in the circuit  12 , in order to ensure adequate operation of pH controller  25 , controller  25  may be partially by-passed by an appropriately sized or valve-controlled pipe (not shown) chosen to bypass a set (or otherwise controllable) amount of pool water towards chlorinator cell  10 . Equally, care must be taken that the water-flow through the line downstream controller  25  has sufficient pressure to clear any accumulation of air or gas in that section of pipe and from the pH controller back into the pool for release to the atmosphere, as will become clear later on. 
         [0078]    In accordance with a second circuit lay-out, as shown in  FIG. 2 , the controller  25  may instead be located within a dedicated pH control line  28  Which draws pool water from pool  10  via a suitably sized suction pipe  29  through a separately controlled controller pump  27 , thus by-passing pool pump  14  and filter  16 . Pool water can thus be pumped through pH controller  25  at a separately controlled rate independent from the flow rate of the filtration circuit  12 , from where it is supplied into the recirculation circuit  12  upstream of chlorinator  18  through appropriate plumbing  21   a.    
         [0079]    Turning then to  FIGS. 3 and 4  which illustrate schematically the make-up of an experimental electrolytic pH control cell  25  as manufactured by the applicant, reference number  30  denotes the cell&#39;s primary housing, a clear PVC pipe section with an outside diameter of 90 mm, inside diameter of 80 mm and length of 700 mm. In operating cell  25 , housing  30  will be mounted oriented vertically. The lower end of cell body  30  is inserted in sealing engagement into an upper arm of a T-piece pipe fitting  32 . The lower vertically oriented port of the T-piece  32  is devised for coupling with a pool water inlet hose or pipe via suitable pipe fittings (schematically alluded to at  33 ), so that pool water can be pumped from the pool  10  into the lower end of the hollow cell body (housing  30 ). The horizontally oriented port of the T-piece  32  is sealed with a PVC cap  34  which contains a central port  35   a  to pass through the above mentioned cell drain line or pipe  26  in sealing manner, and separate side ports  35   b  for electrical cables  36   a  and  36   b  of the cell&#39;s two electrodes  38 ,  40  without leakage. The upper end of cell body  30  is in turn coupled via a suitable pipe fitting (shown schematically only at  41 ) to a hose or pipe which feeds into chlorinator cell  10  as per  FIG. 1 or 2 . Consequently, pool water will enter cell  25  via T-piece  32  and pass through flow channel or compartment  42  defined within hollow pipe section  30  for discharge via pipe fitting  4 l for return to pool. 
         [0080]    A liquid separation compartment  44  is present inside the cell&#39;s main body (pipe)  30 , preferably with sufficient spacing from the tubular wall of pipe section  30  to minimise flow constriction for pool water passage within flow compartment  42 . Separation compartment  44  is a box-like hollow structure fabricated from 3 mm thick acrylic sheet wall sections bonded with silicone elastomer, defined an inner enclosure or chamber  45 , and is substantial rectangular prismatic in shape, with height of 550 mm, width of 66 mm and depth of 26 mm. A rectangular window 420 mm high and 40 mm wide is cut in the acrylic sheet providing one of the walls  46  of the liquid separation compartment  44 . A liquid separation membrane  48  mounted over this window using silicone elastomer adhesive to form a leak-proof seal  43  around the window&#39;s perimeter. Membrane  46  thus separates the flow compartment  42  defined within cell body  30  from the chamber  45  defined inside of separation compartment  44 . Membrane  46  is preferably a microporous polypropylene foil with PTFE coating, 25 to 125 micron thick with 55% pore volume fraction, and an average pore diameter of 64 nanometres to 1 micron, but could be made from other materials capable of operating in salt water concentrations typically encountered in domestic swimming pools without fouling. A relevant selection criterion for the membrane, which could be thicker than foil material, is its capability for adequate ion transfer in the process of electrolysis of salt water, as will become clear later on. 
         [0081]    It will be noted from  FIG. 3  that drainage line  26  connects sealing fashion into a port formed at or near the lower end of vertical wall  47  of separation compartment  44  so as to communicate with chamber  45 , opposite the membrane-carrying wall  46 . A manually,but preferably otherwise operated valve  49  (e.g. pneumatically, electrically, hydraulically) is present in discharge line  26  to control the rate of flow of liquid that may pass through drain line  26  from chamber  45  of separation compartment  44 , towards waste as is explained below. 
         [0082]    The two electrodes  38 ,  40  of electrolytic pH control cell  25  are fabricated from 0.5 mm thick titanium plates coated on each side with a catalytic coating of rare earth metal oxides, primarily ruthenium oxide and iridium oxide. The electrodes  38 ,  40  are 430 mm high and 50 mm wide plates, secured within cell  25  by way of small acrylic bracket structures (not shown) affixed to the wall  46  featuring the window, either side of and parallel with membrane  48  so that one electrode  40  is located in the chamber  45  inside the liquid separation compartment  44  and the other electrode  38  is outside thereof in the flow channel  42  defined the cell&#39;s main body (tube section  30 ). Electrode separation is approximately 9 mm, and a small hole is drilled in each plate so that electrical connection to each plate is made with insulated wires  36   a  and  36   b  whose exposed ends are received in the holes and encapsulated using an epoxy putty to prevent contact with pool water and other liquids. The electric wire  36   a  connected to the inner electrode  40  is passed through a small port in wall  47  of separation compartment  44  opposite the membrane covered window, and appropriately sealed off to prevent leaks. As is known from conventional electrolytic cells, the electrodes  30 ,  40  will be connected to a switchable DC power supply (not shown) in known fashion. 
         [0083]    The box-like structure of separation compartment  44  is provided with fixtures to (i) enable liquid level control within cavity  45  of compartment  44 , (ii) permit venting of gas generated as a by-product of salt water electrolysis within cavity  45  of separation compartment  44 , (iii) allow liquid re-filling to replace liquid selectively drained through drainage line  26  from cavity  45  of compartment  44  and (iv) provide a gas lock (as in an air lock) to ensure that liquid contained within the separation compartment cavity  45  is discontinuous from the pool water flowing outside the separation compartment  44  in the flow-through compartment  42  defined within cell body  30 . 
         [0084]    Rather than having actively controlled valves and similar fixtures with moving parts to effect the above mentioned functions, the inventive pH controller  25  is devised with a set of what will be termed passive, constructional elements at an upper region of the separation compartment  44  to provide the required functionality These constructional elements are schematically shown in  FIG. 4 . Essentially, the stated functionality can be achieved using a number of weirs and inverted weirs, identified at  50 ,  58  and  54 ,  62 ,  66 , respectively, in  FIG. 4 . A weir (such as at  50  and  58 ) is a structure which confines a body of liquid until a rise in liquid level allows the liquid to spill over it. Analogously, an inverted weir (such as at  54 ,  52  and  66 ) is a structure which confines a submerged body of gas until a drop in liquid level allows the gas to bubble out from under it. 
         [0085]    The weirs  50 ,  58  and inverted weirs  54 ,  62  and  66  which achieve the required functions at the liquid separation compartment  44  are created by providing rectangular windows or slots  51 ,  56  in the wall  46  above the membrane  48 , and using sections of the same acrylic sheet material which make up the walls of box-like separation compartment  44 . Slots operate more reliably as they are less prone to blockages or vapour locks than circular or low aspect ratio holes. 
         [0086]    There is provided one upper set of weir and inverted weir  50 ,  54  about a rectangular cut out (slot)  51  in the terminal upper edge of wall  46 , and one lower set of a weir  58  and two inverted weirs  62 ,  66  about a lower rectangular window  56  in wall  46  above the membrane covered window of compartment  44 . It should be noted though that the upper weir and inverted weir set  50 ,  54  need not necessarily be present in the same wall as the lower weir and inverted weir set  58 ,  62 ,  66 . 
         [0087]    Clearances between the acrylic sheet pieces comprising these structures, and the height overlaps of the weirs and inverted weirs should exceed the capillary length, which is the length scale over which gravitational forces on a liquid are larger than capillary forces. This ensures the behaviour of quid interfaces in the complex structures is reliable and predictable and not confounded by capillary rise and meniscus curvature of liquid The capillary length λ is given by the formula 
         [0000]      λ=√(γ/ρ g )
 
         [0088]    where γ is surface tension, ρ is density, and g is gravitational acceleration. The capillary length of clean water is about 3 mm. Consequently the weir and inverted weir structures  50 ,  54 ,  50 , and  66  within the upper part of the inner compartment  44  have clearances and defined level differences of preferably about 5 mm (but could be greater if desired). 
         [0089]    It will be noted that the otherwise open upper end of compartment  44  is capped off in sealing manner by a top plate  52  which is 26 mm wide and protrudes beyond vertically extending wall  46  to cooperate with a vertically extending face plate  53  to define the upper inverted weir  54  outside the cavity  45  of compartment  44 . A horizontally extending shelf plate  55 , which is 18 mm wide, is inserted into the lower rectangular slit  56  formed in wall  46  and secured (bonded) to the upper edge of slit  56  at wall  46  to protrude into the cavity  45  defined within compartment  44  and cantilever to similar extent than top plate  52  on the outside of compartment wall  46 . An outer face plate  57  is secured to depend vertically from the outside terminal edge of shelf plate  55  to define the externally located lower inverted weir  66 , whereas an inner face plate  59  is bonded to the inner terminal edge of shelf plate  55  to depend vertical therefrom. The upper weir  50  has a clearance of 10 mm height, and the three inverted weirs  54 ,  62  and  56  have a clearance height of 13 mm. The lower terminal edge of face plate  53  of upper inverted weir  54  is 5 mm lower than the top edge of the upper weir  50 , and the lower terminal edge of external inverted weir face plate  57  is 5 mm lower than the top edge of the lower (normal) weir  58 . 
         [0090]    While the upper set of weir and inverted weir  50  and  54  provide the above mentioned gas venting facility to allow gas generated during saltwater electrolysis, which is ‘trapped’ in the head space  64  defined between the lower and upper weir sets within the separation compartment  44 , to escape into water streaming past outside of separation compartment  44 , the set of lower external inverted weir  66  and normal weir  58  provide the liquid refilling functionality noted above and whose function is described in more detail below. 
         [0091]    The location of the terminal lower edge of face plate  59  of the inner inverted weir  62  sets the lower liquid-fill control level of chamber  45  within separation compartment  44 . In the experimental cell  25  described herein and manufactured by the applicant, this edge is situated 85 mm above, the upper edge, of the inner electrode  40 , thereby ensuring electrode  40  is always submerged during operation of the pH control cell  25 , as explained below. 
         [0092]    Before turning to describing the operation mode of the electric pH control cell, attention is drawn to  FIG. 7  which shows a highly schematised and simplified further aspect of such cell, whereby it is very similar to the one described with reference to  FIGS. 3 and 4 , and thus uses the same reference numbers (but with an increment of 100) to denote similar components, but for the differences noted in the following. 
         [0093]    Housing  130  is not tubular but box like in configuration, with an internal separation wall  146  subdividing the hollow space into unequally sized chambers such that the flow-through compartment  142  is arranged parallel with and to one side of the liquid separation compartment  144 . Pool water supply line  133  and ‘treated’ (pH adjusted) pool water return line  141  connect in a manner previously described via suitable pipe fittings to the flow-through compartment  142  of upright installed cell  125  at its lower and upper end, respectively. 
         [0094]    Separation all  146  has inverted upper and lower weir structures  150  and  158  substantially as previously described. Equally, separation wall  146  has a rectangular window which is covered by micro porous membrane  148  as described above, with anode and cathode electrodes  138 ,  140  being mounted in flow-through and species separation compartments  142 ,  145  respectively, and connected to an electric voltage source. A drainage arrangement comprising simple crimp valve  149  and pipe  126  allow drainage of species separation compartment (chamber)  145  as previously described. 
         [0095]    The box-like housing configuration with inner separation wall  146  facilitates manufacture of the cell  125  either from injection moulded, chemically resistant polymer housing parts, suitably welded together or otherwise sealingly secured to one another to allow access to the exchangeable separation membrane  148 ; assembly from discrete poly carbonate sheet sections welded to one another is an alternative manufacturing option, as are 3-D printing techniques. 
         [0096]    In the following, operation of the pH control cell  25 , and in particular the weir structures, will be described with reference to  FIG. 4 ; an analogous mode applies to the cell aspect  125  of  FIG. 7 . 
         [0097]    Initial filling of the species separation compartment  44  (i.e. its inside cavity  45 ) with electrolyte (i.e. saltwater) takes place in the process of bringing cell  25  on line when pool water is pumped through the recirculation circuit  12 , as per the circuit lay out in  FIG. 1 , or when dedicated controller pump  27  in the pH controller line  23  of the circuit lay-out of  FIG. 2  is turned on, as part of the pH control process. Pool water is pumped in to the bottom of cell  25 , and fills flow-through compartment  42 , and as water level rises above the top of the lower weir  58 , it spills over the edge of the lower weir&#39;s vertical wall into the separation compartment&#39;s cavity (or chamber)  45 , displacing air out past the upper inverted weir  54 . Pool water can rise inside the inner (i.e. separation) compartment  44 , but this will not completely fill cavity  45  because a gas headspace will be trapped at  56  below shelf plate  55  between the face plates  59  and  57  of inner and outer lower inverted weirs  62  and  66 , and another gas headspace will be trapped at  51  between the face plate  53  of upper inverted weir  54  and the back wall  47 . These two headspaces, and the membrane  48 , separate saltwater received within the electric pH controller  25  into two discontinuous bulk bodies of liquid, one body within the cavity  45  of separation compartment  44 , and one body surrounding compartment  44  within the flow-through compartment  42  formed within housing  30 . 
         [0098]    There is no means for free bulk (i.e. substantial) exchange of liquid volume between the two compartments once the inner (separation) compartment  44  has been filled and gas head spaces formed. There may be minor exchange of volume through the porous membrane  48 , depending on its porosity and pressure gradients between inner compartments  44  and outer compartment  42 , or by fillets of fluid retained in corners of the structure by capillary action. Relevantly, any such exchange does not compromise the functionality of the cell  25 , as such fluid exchange is at least an order of magnitude slower compared to the electrolysis and pH adjustment processes of interest. 
         [0099]    The purpose of separating the two bodies of liquid is to ensure that chemical alkaline species created in the saltwater contained within cavity  45  of compartment  44  during ‘normal’ operation of cell  25 , in which inner electrode  40  is switched to a negative potential (thus becoming the cell&#39;s cathode) compared to the outer electrode  38  (which is thus the cell&#39;s anode), does not mix back into the main flow of saltwater flushing through flow-through compartment  42  of cell  25 . When a sufficient voltage is applied and current supplied to electrode  40  within separation compartment  44 , H 2  gas is liberated on the electrode surface. The H 2  gas rises through the saltwater in cavity  45  from the inner electrode  40  and bubbles into either of the two internal headspaces  56  and  64 . The volume of the headspaces increases, until the gas escapes as bubbles from the inner compartment  44  by spilling over either the lower or upper outer inverted weirs  66 ,  54 . In this process each headspace is maintained, and liquid segregation is also maintained while the gas can freely vent. 
         [0100]    The liquid level in the cavity  45  of separation (inner) compartment  44  must not be allow to drop to expose the inner electrode  40 , otherwise a hazardous condition may result from overheating of the electrode. By the same token, the cavity  45  of separation compartment.  44  must be slowly drained, at the same time as gas is being evolved within it. Under some conditions, liquid may also be lost by foaming action carrying some entrained liquid out past the inverted weirs. Therefore, the inner liquid level must be controlled such that the cell refills if the liquid drops below a lower control level. 
         [0101]    The bottom edge of inner inverted weir  62  sets the lower control level. If the liquid in cavity  45  of the inner compartment  44  drops below the free edge of inner inverted weir  62 , saltwater from the outer, ie flow-through compartment  42  (see  FIG. 3 ) can spill over the lower weir  58  into cavity  45 , while gas is displaced past the upper inverted weir  54  from separation compartment  44  to the flow-through (or ‘outer’) compartment  42  of cell  25 . The liquid in the inner (separation) compartment  44  will rise until it reaches the lower control level at  62 . 
         [0102]    This requirement is the reason for the double inverted weir structure, rather than a simpler single inverted weir, such as a design in which the inner inverted weir  62  were absent Such a design would still separate the liquids within separation compartment  44  and flow-through compartment  42  into two bodies, allow gas venting, or allow refilling when being drained, but it would fail to maintain a lower control level when separation compartment  44  is simultaneously drained while the separation compartment&#39;s electrode  40  is producing gas. 
         [0103]    The above described cell  25  has been tested in two environments. In a first experiment, cell  25  was used to control in a small amount of liquid, and to confirm operation of the liquid control level functionality provided by inner inverted weir  62  of the separation compartment  44  of cell  25 , whereas in a second experiment, a large saltwater poll was subjected to pH control over an extended period of time. 
         [0104]    In the first experiment, pH controller was installed next to a tank containing 500 litres of 6000 ppm NaCl water solution. A small pump circulated water from the tank to the bottom of the pH controller, through the cell  25  and back to the tank via a hose. An electric potential was applied to the electrodes, such that the (inner) electrode  40  within separation compartment  44  acted as the cell&#39;s cathode. 
         [0105]    A small manual valve (as per  49  in  FIG. 3 ) was set to drain the cavity  45  of separation compartment  44  at a constant slow rate. The pH and oxidation reduction potential (ORP) in the NaCl water solution was monitored using sensors attached to the tank. 
         [0106]    The rate of flow of pool water through the pH controller was set to 6 litres per minute, whereas the rate of drainage of the separation compartment  44  was set to approximately 1 ml per second (60 ml per minute). A potential of 13.3 V was applied to inner and outer electrodes  40  and  38 , which produced a current of 7.9 amps. 
         [0107]    The change in pH with time through the experiment is shown on the graph of figure in which the vertical axis is pH multiplied by 100, and the horizontal axis is time in hours and minutes. The initial pH of the tank was 8.1. The pH dropped by a full pH unit to 7.1 in approximately 3.5 hours. The pH of the drained stream from the species separation compartment  44  was significantly alkaline, at approximately 12.3. 
         [0108]    Hydrogen evolved in the separation compartment  44  was vented into the main flow (flow-compartment  42  of cell  25 ) and returned to the tank. Despite constant drainage and gas evolution, the liquid level within the chemical species separation compartment  44  was always maintained not lower than the lower liquid control level (inner inverted weir  62 ), and the inner electrode  40  always remained submerged. 
         [0109]    In the second experiment, cell  25  was used in the control of pH in a large, outdoor saltwater swimming pool. The electric pH controller  25  was installed poolside, above the water level of an outdoor domestic pool of approximately 45,000 litre capacity, with a pump, filter and conventional saltwater chlorination unit installed in a conventional manner, as per  FIG. 2 . The pool surface was comprised of tiles and grout, which when unmanaged buffers the pool to a high pH of around 8.2. As noted, the pH controller  25  was not incorporated in the main pumped pool loop, but operated in a standalone mode with its own small pump, similar to the lay-out in  FIG. 2 . Water was pumped from the pool, up through the pH controller, and returned to the pool via a hose. An electric potential was applied to the electrodes, such that the electrode  40  within species separation compartment  44  functioned as the cathode of the cell  25 . The separation compartment  44  was connected to a small manual valve in order to effect draining at a constant slow rate. Thee pH and oxidation reduction potential (ORP) was monitored using sensors installed in the pool loop in conventional manner. The ORP is a direct measurement of the disinfection action in the pool, and is a function primarily of the concentration of hypochlorous acid, hypochlorite ion, and pH in the pool. A conventional saltwater chlorination system operated on a timed cycle through part of the experiment. 
         [0110]    The rate flow of pool water through the pH control cell  75  was set to 18 litres per minute. The rate of drainage of the separation compartment  44  was set to 0.18 ml per second (10.8 ml per minute). A potential of 14 V was applied to the electrodes, which produced a current of 8.0 amps. 
         [0111]    The electrodes were first ‘turned on’ at 11.00 am on the 30 th  of April 2014 and then turned off at 11.30 pm on the 3 rd  of May 2014. The pH control cell thus ran continuously at 8 amps for 3.5 days (84.5 hours). 
         [0112]    The pool chlorinator cell ran on a schedule from 10:15 pm to 7:45 am overnight and from 12:15 pm to 1:45 pm during the day, each day. This schedule was in operation when the pH controller was turned on. The chlorinator was turned off at 11:00 am on the 2 nd  of May and did not run thereafter. 
         [0113]      FIG. 4  shows the pH and ORP of the pool from the of April to the 10 th  of May 2014, ie during a period prior to, during and after operation of the control cell. The vertical axis is the pH multiplied by and the ORP value is in millivolts. 
         [0114]    Prior to turning on the pH control cell  25 , switched to act as an acid species generator, the pool pH oscillated between constant bounds of about 8.2 and 7.8. This oscillation is due to the daily cycle of production of hypochlorous acid overnight by the chlorinator, which drives the pH up, and the destruction of hypochlorous acid during the day by sunlight, which drives the pH down. The cycling in the ORP trace is also due to this effect. The low spikes, in the pH curve are an artefact of the main pool pump cycling off, leaving stagnant pool water in contact with the sensors. The sensors do not truly represent the state of the pool at these times. 
         [0115]    On April 25 th , 500 ml of concentrated hydrochloric acid was added manually to the pool, which led to a drop of the pH to about 7.4. The pool then recovered over the next for days to its natural value. This pool therefore required addition of approximately 500 ml per four days to maintain pH in a range suitable for adequate disinfection, in the absence of other means of pH control. 
         [0116]    The electric pH controller was turned on at 11:00 am on the 30 th  of April. The pH in the pool immediately began to drop. The pH dropped from a high of about 8.2 to a low of about 7.2 over the course of 3.5 days. The pH controller was turned off on May 3, and the pH began to recover, ie drift towards the ‘natural’ more basic side present in pools of the type controlled the experiment. This demonstrates effective control of the pool by the electric pH control cell in accordance with one of the aspects of the presently disclosed embodiment. 
         [0117]    The ORP increased to very high levels after the pH controller was turned on. This was due in part to additional production of chlorine by the pH cell (which was acting as an acid generator), but in the main due to reduced pH. As the pH drops, pool chlorine present as hypochlorite ion converts to hypochlorous acid, which increases the ORP, and the disinfection action within the pool. 
         [0118]    The rate of increase of pH after turning off the pH controller is slower than after the manual addition of acid, because of the high loading of chlorine in the pool. As hypochlorous acid and hypochlorite ion are destroyed by sunlight or reaction with organic molecules, they constitute a source of H +  ions. The residual chlorine therefore provides some pH buffering to the pool system. This also stabilizes the ORP level for some days, as the effect on the ORP of the loss of active chlorine is compensated for by the concomitant production of H +  ions. The use of the pH controller is there fore particularly efficacious in setting up a pool condition that can hold the ORP at a level sufficient for adequate disinfection over an extended time without any interaction with the pool, whether by manual addition of chemicals such as acid or chlorine compounds, or electrical chlorination, or electrical pH control. It will be appreciated that the different aspects of the presently disclosed embodiment, in particular the specific lay out of the pH control cell  25  may be varied, as long as the above mentioned functionality is implemented, i.e. temporarily separating two volumes of saltwater which enter the cell, during the electrolysis process, and removing a concentrated catholyte (base chemical species) for lowering pH or removal of concentrated anolyte (acidic chemical species) for increasing pH, from the stream of water being returned from the cell to the pool.