Abstract:
Devices and methods for sensing the charge condition of a mixed stream of aggregant and aqueous dispersion of charged particles are based on sensing changes in the potential of an electrode contacting the stream. Sensing the changes in potential permits better control of aggregation in raw water and effluent treatments.

Description:
This invention is concerned with a method of following the influence of a coagulant and/or flocculant (hereafter referred to as an &#34;aggregant&#34;) on a stream of an aqueous dispersion of charged solid or liquid particles and a device suitable as one component of a control loop process governing dosing of the aggregant. 
     THE FIELD OF THE INVENTION 
     The process of aggregating aqueous dispersions of charged particles is widespread in many industries, including, for example, the treatment of raw water in making drinking water, effluent and sludge treatment and paper-making. 
     The stability of an aqueous dispersion depends on the electrical double layers which exist at the surfaces of the particles and extend into the liquid phase. Coagulation is caused by changes in the structure of the double layers. 
     The most widely used method of coagulating a disperse system, is to add colloids having an effective change opposite in sign to that carried at the surface of the disperse phase: thus &#34;solutions&#34; of alum or ferric salts (in fact dispersions of colloidal cationic hydroxylic polymers of aluminum or of ferric iron) are effective coagulating agents for natural dispersions such as raw water, sludge and effluent and some man-made dispersions such as pulp dispersions used in paper-making, all of which are anionic. 
     Maintaining ideal coagulation conditions in a stream has never been easy. The jaw test which is in widespread use is at best an uncertain guide; the process is so lengthy that by the time the result is available, the condition of the stream may have changed so significantly that the test is no longer appropriate to determine the action to be taken. 
     Flocculation involves the interaction of a colloidal solution of a flocculant with the disperse phase. 
     DISCUSSION OF THE PRIOR ART 
     The first, and hitherto the only, step towards automating the maintenance of a chosen aggregation condition in a mixed stream of an aqueous dispersion and an aggregant, was to use devices utilising the electrokinetic phenomenon of streaming current. An account of the competence of these devices generally known as Streaming Current Detectors, or SCDs, is contained in the specification of European Patent Application 88901621.8 SCDs have a shared essential characteristic, namely that samples of the stream are caused to flow back and fore through a capillary, limiting the in-line application of SCDs to monitoring the treatment of very dilute dispersions of colloids of small particle size, like some raw waters in making drinking water. The range of applications of SCDs was increased by the discovery that the charged condition of the filtered mixed stream was identical with that of the unfiltered mixed stream, but that introduced the addition step of filtration and a delay in the analysis. 
     The addition of alum to a pulp dispersion was thought to be controllable indirectly by the acidity of the mixed stream using a pH meter with a hydrogen-selective electrode, but the method took insufficient account of the accumulation of the sulphuric acid in the mill&#39;s recycled water and has been displaced by SCDs used off-line. 
     There is a need for an aggregation control method able to operate in-line on such aqueous dispersions as sewage, effluent, pulp dispersions and raw waters of all kinds. 
     One object of the invention is to provide such an in-line method and a device for employment in the method. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided a potentiometric method for sensing the charge of a mixed stream of an aggregant and an aqueous dispersion, suitable for use in an aggregation control loop process, which comprises flowing the mixed stream in contact with a metallic indicator electrode, measuring the potential difference between the indicator electrode and a second electrode of substantially stable potential different from that of the indicator electrode, whilst preventing any substantial electrical current from flowing between the electrodes. 
     For the purposes of this specification, aggregation means the process of coagulation, or that of flocculation. 
     Desirably the potential difference is measured with a device for measuring potential which allows only negligible electric current (e.g. less than 10 -10  amps) to flow between the electrodes. 
     The second electrode may be a reference electrode, or (and preferably, when the mixed stream is earthed) an earthed electrode or an earthed connection to the reference terminal of the device for measuring potential. 
     It is common practice to classify electrodes as either metallic electrodes or membrane electrodes; a carbon electrode is classed as a metallic electrode. 
     The method may be carried out using a direct-reading pH meter or potentiometer equipped with the metallic indicator electrode. Such meters, hitherto, have been considered to respond solely to solute ions, their reactions and concentrations and are generally equipped with ion-specific membrane electrodes. No such electrochemical cell has been proposed for use in directly discriminating the reaction of an insoluble colloid with another oppositely charged colloid or colloidal solution. Earthing one electrode of a pH meter has never been proposed but surprisingly if such earthing is effected and an appropriate choice of indicator electrode is made, highly effective aggregant addition control of an earthed mixed stream can be achieved using such a pH meter. 
     There is no accepted theory which explains the present method. The inventors would not wish to be held to either of the following possible explanations, but is appears to them that the negatively charged surface of a cathode is a bite exerting electrostatic attraction on the free colloidal cationic aggregant which appears when the interaction of the colloids is substantially complete: there may follow a close association of the aggregant and the electrode surface, or a compression of the Helmholtz layer. Alternatively, an anionic aggregant (for example the copolyacrylic acids used in paper-making) may act as a ligand for metal ions released at the anode. Any such process would affect the potential of the electrode and the potential difference reading. 
     The interaction of an electrode surface and the aggregant will depend on the nature of the aggregant, the relative proportions of disperse phase and aggregant in the mixed stream, as well as on the electrode potential, electrode material, pH, temperature and flow rate. The latter three variables may be compensated, or so reduced in consequence that no compensation is required, for example when the aqueous dispersion is subject to pH, temperature or flow control. 
     Thus the method is directly sensitive to the charge condition of the mixed stream. No sensor, apart from SCDs, has been suggested for this use and none, including the SCDs, has been capable of in-line use in high solid streams such as sewage, effluent and pulp dispersions. 
     The indicator electrode should have no particular ion-specificity and a stable surface substantially chemically inert to the electrolytes in the mixed stream. Suitable metallic electrodes include stainless steel, gun metal bronze, carbon, gold, lead, platinum and silver, of which the first four have proven to be the most widely useful in the various mixed streams to which the method and device may be applied: the others are selectively useful, depending on the chemical nature of the stream. For example, lead and silver are changed at their surfaces by streams which contain hydrogen sulphide (sewage effluents) and platinum, from its sensitivity to hydrogen ion, is not preferred in streams with variable acidity. 
     Specifically excluded as indicator electrodes are membrane and ion-selective electrodes such as hydrogen electrodes used in pH measurement, where the response to a particular ion is dominant and obscures whatever response such as electrode may have to the charge condition of the stream. 
     According to a further aspect of the invention a device for sensing the course of the aggregation of an aqueous dispersion of charged particles in an aggregation control loop process, comprises a galvanic cell having a metallic indicator electrode without any particular ion-specificity and a second electrode, the electrolyte of the cell being a mixed stream of the dispersion and an aggregant for the charged particles and being in contact with the indicator electrode, and a means for sensing the potential difference appearing between the electrodes. 
     The potential difference of the galvanic cell may arise spontaneously from the difference in the potentials of the two electrodes, or the potential difference may be impressed on the cell by an external source of stable e.m.f. bridging the electrodes. In the latter case the electrodes may be of similar composition. 
     In a cell with an impressed potential difference, preferably both electrodes are metallic. 
     In a cell in which the potential difference arises spontaneously, the second electrode acts as a reference electrode developing a substantially stable potential so that the changes in the potential of the indicator electrode may be recognised by changes in the cell e.m.f.. The known reference electrodes comprising half cells of a metallic electrode in a standard solution, for example the silver/silver chloride(solid)/KCal (solution) composite are useful, as are a silver/silver chloride solid electrode with no surrounding standard solution, and, particularly for use in the aggregation of particles in effluent and sewage, a silver/silver sulphide electrode. 
     Whilst the indicator electrode must contact the mixed stream, it is not always necessary that the second electrode should do so. If there is no extraneous, significant, variable electrical field, such as would form a ground loop affecting the potential difference, and the mixed stream is earthed, it is sufficient that the second electrode is an earthed connection from the voltmeter directly to earth or to a metal conduit carrying the stream. Preferably such earthed connection is the anode. 
     The indicator electrode or both electrodes may be housed in a conduit which determines the path of the mixed stream, or they may be mounted externally on a body for immersion in the mixed stream. 
     The potential difference is desirably measured by a high impedance voltmeter drawing insufficient electric current to affect the potential difference between the electrodes--available electronic voltmeters operating with a current of from 10 -12  to 10 -14  amperes are ideal, preferably the voltmeter incorporates electronic amplification to produce a signal which can be displayed in a digital and/or analog form. The conductivity of the mixed stream is of no consequence to the operation of a sensor cell comprising such a voltmeter. 
     There is, in the vicinity of aggregation, for each substantial change in the electrical double layers of the particles, a corresponding and substantially unique potential difference which is sensed by the method and device of this invention. If there is efficient aggregation of the particles, the corresponding measured potential difference can be used as a datum signal for the preservation of that condition despite changes in the concentration of particles and/or the charges they carry. 
     Furthermore, the indicator electrode may be situated no more than 30 seconds in real time downstream of an aggregant dosing point: this short response time is valuable to close control and compares with 40 minutes real time delay for a jar test and 5 to 8 minutes delay for an SCD operating on a filtrate derived from the stream. 
     It should be understood that a control loop method based on the method of the invention does not determine the best coagulation conditions, but will preserve a selected coagulation condition established by other means by preserving the datum signal generated by the selected condition. 
     The method of the invention may discriminate the effect of a trace, say less than 1 part per million, of coagulants such as hydroxylic polymers of aluminium and of ferric iron in the vicinity of efficient coagulation, and flocculants such as synthetic polycations. The method may also, in similar circumstances, discriminate traces of polyanionic substances, for example copolyacrylic acids and polyphosphates, used in paper-making. 
     When it is intended that the indicator electrode and a reference electrode both contact the mixed stream, the electrodes may be disposed relatively to each other transversely or longitudinally of the path of the stream. 
     A range of useful datum potential differences is available from the choices of cell conformations, electrode materials and/or externally applied e.m.f.. 
     Monitoring aqueous streams in conduits from capillaries to pipes of several hundred millimeters diameter would encompass most practical applications, but no limit has been found on the size of the conduit or the rate of flow of the liquid. 
     In dealing with some mixed streams, it is seldom possible to use a sampling pump to abstract a side stream from the very large works&#39; pipes (frequently 700 mm diameter) to feed to the sensor cell, firstly because such mixed streams are prone to block the pump and secondly such fast acting pumps wear quickly from contact with the abrasive material such mixed streams frequently contain. In these circumstances and the absence of interfering ground loops, the indicator electrode may be let into the pipe (electrically insulated from the pipe if it is metallic) and the second electrode may be a connection between the voltmeter and earth or to the pipe as appropriate. 
     The surface of an electrode in contact with some mixed streams, will be kept in working condition by abrasion from components in the mixed stream. In other cases the surface may become fouled and then must be cleaned by mechanical abrasion, electrolytically, or ultrasonically. The or each electrode may be placed or shaped to reduce the incidence of fouling--flush fitting in the inner wall of the conduit, cambering, or streamlining. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: 
     FIGS. 1 to 5 are schematic cross-sectional representations of a series of devices according to the invention showing variations in a measuring cell. 
     FIG. 6 is a cross-sectional representation of a cell with external electrodes. 
     FIG. 7 is a reproduction of a chart recording of an inefficiently coagulated effluent stream monitored by a device according to the invention. 
     FIG. 8 is a reproduction of a chart recording of an effluent stream dosed in accordance with the output of a device according to the invention. 
     FIGS. 9 and 10 show alternative arrangements of electrodes in sensing devices according to the invention, and 
     FIG. 11 shows a portable cell for use in the method of the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a length of horizontal works&#39; pipe 1 (through which flows the grounded works&#39; effluent) shown sectioned on a vertical diameter, has a port 2 closed by an insulating rubber collar 3 surrounding a silver indicator electrode 4. An earthed connection 5 serves as a reference electrode. The electrodes 4 and 5 are bridged by a high impedance voltameter 6 via screened cables 7a and 7b respectively. The output of the voltameter is connected to a signal processor 8 from which a control signal is relayed to a chemical dosing pump 9 via a control line 10. 
     Apparatus according to FIG. 1 was installed at an effluent works using a commercial polyquaternaryammonium salt as flocculant. The works&#39; chemist established the dose of flocculant to give satisfactory flocculation and the datum reading of the voltmeter 6 was established as 97. The control loop was put in control of the flocculant dosing pump 9 to preserve a datum reading of 97 and through three weeks of continuous operation, the quality of flocculation remained constant without need of recalibration of the datum reading. During a period of about two hours, the dispersion was of low solids content and the dosing pump stalled under the low revolutions demanded by the automatic control. Manual control was instituted to maintain the datum reading. The quality of the flocculation continued satisfactory and was later returned to automatic control. Over the three week period a 25% saving of flocculant was logged and the plant had not previously enjoyed such a sustained period of acceptable flocculation. 
     Referring to FIG. 2, a polypropylene pipe 11 shown in longitudinal vertical cross-section, conducts an ungrounded mixed stream. A port 12 in the pipe is sealed by a collar 13 of synthetic rubber surrounding a stainless steel indicator electrode 14. A reference electrode 15 of silver/silver sulphide is housed within an insulating collar 13a sealing port 12a. The electrodes 14 and 15 are bridged by a high impedance voltameter 16 via screened cables 17a and 17b and the output of the voltmeter is connected to a signal processor 18. The processor 18 is used to control the addition of aggregant to the stream flowing in pipe 11 upstream of the region shown. 
     Referring to FIG. 3, a right-sectioned cylindrical conduit 21, of polyvinyl chloride, shown end-on, has a screwed-in indicator electrode 24 and a reference electrode 25, both of stainless steel. The electrodes are bridged firstly by a high impedance voltmeter 26 through cables 27a and 27b, and secondly by a battery B having an e.m.f. of 2 volts connected in series with a resistor R of 0.4 megohms. The voltmeter 26 is connected to a signal processor 28. The processor 28 is used to control the addition of aggregant to the stream flowing in pipe 21. 
     FIG. 4 shows an alternative arrangement to FIG. 2, the electrodes 14 and 15 being opposite each other and differently mounted in the tube. 
     Referring to FIG. 5, the numbered items 11 to 18 have the meaning given to items 11 to 18 in FIG. 4, but an additional electrode 19 is set upstream of the other electrodes and a screened cable 20 connects electrodes 14 and 19. The third electrode 19 increased the potential difference available from the cell and ay make amplifying of the output signal unnecessary. 
     In a trial at a slaughterhouse, unfiltered effluent from the slaughterhouse was flowed through the conduit 11 of FIG. 5 (diameter 5 centimeters) and the characteristic reading was exhibited on a digital display 18A. FIG. 7 shows the record of the display over a twelve hour period with dosing of a suitable coagulant determined under manual control guided by jar tests. During this period the coagulation was more often poor than good. FIG. 8 shows the record of continuous manual adjustment of the coagulant dose guided by the digital display 18A. Good coagulation was known to occur at a reading of 450 and the dosing adjustments were undertaken to seek to maintain this figure. The coagulation was of consistently good quality throughout this period and it was found that there was a display band width of from 443 to 455 in which this quality was assured. This performance would be guaranteed if the output signal of the processor 18 was used to control coagulant dosing in a coagulant dose control loop. 
     Referring to FIG. 6, a portable cell for use in the method of the present invention comprises a T-piece 31 of polyvinyl chloride which carries external electrodes 32, 33 and 34 of M316 stainless steel. Electrodes 32 and 33 are connected by screened cable 35 and electrode 34 is connected to a cable 36. Cables 35 and 36 issue from the T-piece 31 for connection of a high impedance amplifier and signal processor (not shown). This form of device is useful for occasional or permanent examinations of industrial effluent streams running in otherwise inaccessible gullies. The device of FIG. 6 has been tested successfully in a gully 3.7 meters deep in which ran effluent from a paper mill. This effluent had routinely blocked a sampling pump meant to lift a continuous sample to the filter of a filter/SCD sensing device used for monitoring a parameter of the effluent. The cell shown in FIG. 6 enabled effective parameter monitoring to be achieved without blockage problems. 
     Referring to FIG. 9, a conduit 41 of partial frusto-conical shape is shown, housing a larger annular electrode 42 having an internal diameter of 12.5 millimeters, at the wide end of the frusto-conical part and a smaller annular electrode 43 having an internal diameter of 6.25 millimeters at the narrow end; each of the electrodes 42 and 43 is equipped with an extension 45 or 46 respectively, which penetrates the wall of the conduit for electrical connection to a voltmeter (not shown). 
     An advantage of a venturi-shaped zone with the electrodes situated at the narrowest bore is that there is a self-cleaning effect achieved with this set-up caused by the acceleration of flow of the liquid as it streams through the constricted bore. 
     Referring to FIG. 10, a conduit 61 of polypropylene has a waisted bore 67 and houses stainless steel electrodes 62 and 63 with non-conducting spacers 68 and 69 all located at the waist, flush with the boundary of the bore. Stainless steel lugs 65 and 66 are secured to the electrodes 63 and 62 respectively for connection to other elements of the cell. 
     FIG. 11 shows a modified form of portable test device for use in the method of the invention. Hollow tube 71 includes an electrode pair (e.g. such as shown in FIG. 10) which can be dipped into a flowing stream using handle 72 and hollow support shaft 73. The potential created across the electrode pair is sensed by a millivoltmeter 74 via wires passing through the hollow shaft 73. An earthing spike 75 can be clipped to the shaft 73 when not required, but is used to ground one side of the voltmeter if a grounded effluent stream is being monitored, in which case only the indicator electrode (62) need be connected to the other side of the voltmeter.