WATER QUALITY MONITORING METHOD AND APPARATUS

Abstract: The detection of unwanted metal cations in a process water is effected by monitoring the streaming electrical current generated when a sample of the water is made to flow forwards and backwards through a narrow passage (31) defined between confronting non-conducting solid surfaces (24, 26) whilst in contact with two electrodes (22, 23) spaced apart in the directions of the flows. The process water is either inherently free of particles above 10 microns or is clarified by removing (e.g. hydrocycloning and/or screening) particles above 10 microns.

Description: Water Quality Monitoring Method and Apparatus Technical Field This invention is concerned with monitoring various process waters which are required to be substantially devoid of polyvalent metal ions, for the incursion of such ions. For example softened water is required to be substantially devoid of calcium and magnesium ions and certain industrial effluents should be freed from cupric and mercuric ions before discharge into rivers or the sea: de-ionised water is reguired to be freed from all ions not derived from water: condensate from steam-powered turbine operations should be devoid of ions if it is to be recycled, but frequently becomes adulterated with metal cations. Another process water to which the invention relates is ground or river water, domestic waste water or sewage subjected to flocculation treatment. Discussion of Prior Art Some of these process waters are made by passing a raw water through a bed of cation-exchange material. The cation-exchange material retains the unwanted cations of the raw water and substitutes sodium ions (soft water; industrial effluent freed from heavy metal ions) or hydrogen ions (de-ionised water). Such processes are operated cyclically through a phase of cation exchange and a phase of regeneration of a partly exhausted cation-exchange material. The cation-exchange material, usually a synthetic resin, is regenerated after a set time interval or after a set volume of raw water has been passed through the bed and such intervals or volumes are set to ensure the quality of the process water by allowing only partial exhaustion of the cation-exchange material. This state of affairs is dictated by the lack of a reliable in-line monitor of the quality of the process water, able to detect the first signs of breakthrough of the unwanted cations from a fully exhausted bed. If such a monitor were available, the beds of cation-exchange materials could be run to exhaustion in each cycle leading to savings in the frequency of regenerations and the quantities of regenerant used. Another process water is potable water derived from turbid raw water or sewage by flocculation. The process utilises a flocculating agent, usually alum or a ferric salt, which reacts with negatively charged dispersed particles, forming uncharged floes which more easily agglomerate and are then in a form to sediment faster. Adding only the necessary quantity of flocculating agent is important to the process both economically and in terms of its effectiveness. Any excess flocculating agent would result in the presence of soluble aluminium or ferric ions or soluble cationic hydroxides of those metals. This problem was addressed by W.F. Gerdes in his paper "A New Instrument - The Streaming Current Detector" presented and published at the 12th National ISA Analysis Instrument Symposium, Houston, Texas, U.S.A. (The instrument and presently available commercial examples thereof dif ering effectively only in the introduction"of solid state electronics, and, occasionally an ultrasonic generator as hereafter disclosed, are referred to herein as "the SCD"). Gerdes explained the generation of the streaming current in the SCD as requiring the immobilisation of charged particles on the surfaces of the piston and the surrounding cylindrical pot of the SCD, whilst the mobile counter ions asssociated with the par¬ ticles are swept downstream through the annular gap between the piston and pot; this relative motion of opposite charges generates the electric current picked up by electrodes spaced apart in the gap. Central to Gerdes' theory is the existence of particulate matter in the liquid under test and Gerdes dismisses the influence of dissolved salts as causing "no problems and not requiring any correction of the SCD reading". This theory and the requirement that the total solids of the treated water must be present in the liquid submitted to the SCD is perpetuated in the literature of those companies currently offering SCD's for sale. The requirement has limited the application of the SCD because the solids tend to accumulate in the cylindrical pot disturbing the electrical signal to the extent that it cannot be relied on to control the rate of addition of the flocculant. This deficiency has, to an extent, been alleviated by vibrating the pot at ultrasonic frequencies as claimed in European Patent Application No. 83901975.9 owned by the Milton Roy Corporation, one of the companies currently offering for sale an SCD. The specification of that Application contains the passage "The present state of the art has advanced to the point of the above apparatus (the SCD), however nothing in the prior art has been practically operable due to the accumulated particles build-up from the sample stream on the walls of the receiving tube (cylindrical pot)": we have added the explanatory phrases in parentheses. . Even so, we have found that an SCD equipped with an ultrasonic generator can be overwhelmed by levels of turbidity in raw ground or river water which are not uncommon during a rainy season and the professed aim of the invention of that Application, namely that of eliminating the problem, has not been achieved. It is apparent that there has not been advanced any industrial application of the SCD to a clarified water. This invention is concerned with such applications. Summary of the Invention According to the present invention a method of monitoring a process water for the presence of unwanted metal cations, comprises abstracting samples sequentially from a stream of said process water, urging each sample to flow forwards and backwards through a passage defined between confronting, non-conducting solid surfaces, whilst in contact with two electrodes spaced apart in the directions of the flows and incorporated in an electric current measuring circuit, thereby generating a streaming electrical current characteristic of each sample and monitoring said streaming currents, the method being characterised in that the process water is substantially free from particles above 10 microns, and suitably above 2 microns. The process water which is substantially free from particles above 10 microns will be described in this specification as "clarified water". Thus "clarified water" means a process water which, flowed as a continuous stream through an operating SCD for a period of 48 hours, does not deposit accumulated particles within the SCD. A process water which inherently contains solid particles in excess of 10 microns, may be rendered into a clarified water by the removal of the oversized particles. Means to impose constant flow on the stream, may be used (e.g. a constant head device: a head of from 10 to 15 centimetres is adequate). The rate of flow of the stream is preferably between 0.5 and 3 litres per minute. Some of the process waters referred to herein are inherently clarified waters. This applies for example to softened water, condensate and de-ionised water. Industrial effluents, flocculated raw water and sewage require at least a reduction in the solids content to render them into clarified waters. According to a further aspect of the present invention there is proposed a method of monitoring a treated water for the presence of excess flocculating agent comprising abstracting samples sequentially from a stream of said treated water; urging each sample to flow forwards and backwards through a passage defined between confronting, non-conducting solid surfaces, whilst in contact with two electrodes spaced apart in the directions of the flows and incorporated in an electric current measuring circuit, thereby generating a streaming electrical current characteristic of each sample and monitoring said streaming currents, the method being characterised in that at least a substantial proportion of the floes created in the treated water by the said agent are removed prior to the samples being abstracted therefrom. We have found, surprisingly in view of Gerdes' theory, that rendering a flocculated raw water or sewage into a clarified water by removing the bulk of the dispersed solids and submitting the clarified water to streaming current measurement, signals are generated which duplicate the signals got from an SCD operating on the. water containing floes and other solids. Thus an SCD operating on the clarified water may be used to establish the set point characteristic of good quality flocculation and to control the addition of flocculant to maintain or recover the set point. We prefer that the confronting surfaces defining the passage should be low energy surfaces with the intention of avoiding the immobilisation of particles thereon. To this end we prefer to form at least the confronting surfaces of polytetrafluoroethylene or a polyurethane-co-polysiloxane block copolymer. The obvious methods of reducing the solids content of a process water - sedimentation and filtration through a bed of sand - are not best suited to making a clarified water when an SCD is used as an in-line controller of the flocculation process. Sedimentation is normally a slow process so that as much as 45 minutes may elapse before supernatant liquid qualifying as a clarified water would be available to pass to the SCD. This period is too long for close control purposes, although it matches the performance of the jar test common in the industry. Bed filtration of waters containing gelatinous floes, is also difficult: the filter tends to block quickly and requires frequent back-washing. Such discontinuities are not desirable. We prefer to reduce the solids content of a process water using a hydrocyclone. It is known that as the diameter of a hydrocyclone is reduced, successively smaller particles may be removed from a feed of water containing dispersed solids: the reduced solids content water exits via the vortex finder as the overflow whilst the reject stream entraining enhanced solids, exits as the underflow from the apex of the conical part of the hydrocyclone. The hydrocyclone has three significant advantages over- sedimentation and filtration: firstly the water of reduced solids content is quickly produced; secondly there is a sharp and selectable cut between the smaller particles retained in the water of reduced solids content and the larger ones in the reject stream; thirdly the hydrocyclone is self-cleaning and is therefore capable of prolonged continuous operation. We have found that a 10 mm diameter hydrocyclone (where the diameter is that of the cylindrical collar carrying the water inlet end surmounting the conical part of the hydrocyclone) receiving a flocculated raw water under a pressure of 5.6 Kg/cm 2 (80 p.s.i.), delivered three litres per minute of clarified water (more than adequate to supply an SCD) and one litre of underflow, the clarified water containing only 5 per cent of the original concentration of particles of 10 microns and no larger particles. The hydrocyclone may need to be protected from over-sized particles liable to block the hydrocyclone and in these circumstances we would pass the process water through a screen en route to the hydrocyclone. We prefer an angled screen allowing the retained solid to fall across the screen rendering the screen self-cleaning. We prefer to pump the water through the hydrocyclone. An alternative to a hydrocyclone is a fine meshed screen, for example a "Fourdrinier wire" - a sheet of woven metal or synthetic polymer filaments adapted to retain solids larger than about 10 microns. The screen can be set at an angle to become self-cleaning. We particularly prefer that such a sheet should be calendered to a smooth condition on the face adapted to receive the process water. The smooth face allows the retained solid to fall across the face more easily. Two or more such sheets of increasingly finer mesh may be arranged below each other to subject the process water to serial filtrations. Clarifying by means of an hydrocyclone or a screen, may be followed by filtration through a membrane filter if the process water requires additional clarification. We have found that the streaming current changes electropositively in sympathy with, but not in direct proportion to the concentration of soluble polyvalent cations. However the inclusion of a trace of a cation of higher valency than any already present in the process water, produces a greater change in the generated streaming current than when that trace amount is additional to an existing population of similarly charged cations in the water. That the response of an SCD and the method of this invention should be at its most sensitive when monitoring the incursion of foreign cations, is of considerable significance. Nevertheless, the sensitivity to increments in the concentration of already existing cations in the water, is not insignificant. It is possible to discriminate increments of one part per million of calcium ions in softened water over the range 0 to 30 parts per million and increments of two parts per million over the range 30 to 100 parts per million. We have found that 0.5 parts per million of calcium in a softened water may be used to generate a steaming current which is differentiable from the current generated with a completely softened water. This differentiation persists irrespective of the concentration of monovalent cations in the water, so long as at least 50 p.p.m. of such salts are present. Softened water made from waters of varying hardness, so that they had sodium salt contents of from 40 to 400 p.p.m. , have all proved susceptible to monitoring for the incursion of divalent cations by the method and apparatus of this invention. The streaming current signal may simply be recorded or the change in the current consequent on the incursion of the cation may be used to trigger an alarm circuit, or the regeneration sequence of a cation exchange material. Thus the invention includes the apparatus of this invention electrically connected to a regeneration sequence controller and adapted to relay a command signal when the output signal changes electropositively to a predetermined extent. Practitioners using SCD's to control flocculation, particularly on relatively soft raw ground, surface and river waters, have commented on the frequent need to recalibrate the set point characteristic of good quality flocculation. Such recalibrations are needed most during spate conditions and during drought and these are the conditions which produce substantial changes in the naturally occurring ionic contents of the water - hardness and detergent concentrations are greater during a drought than in spate water. This variation in the ionic content of a raw water can be followed by an SCD operating on a clarified raw water - increasing hardness in a relatively soft water produces a significant electropositive change in the signal. The electropositive increase due to hardness is without effect on the flocculation process (calcium and magnesium ions do not flocculate suspended solids) but should be taken into account in reckoning the set point for flocculation. A still further aspect of this invention, therefore, relates to an apparatus for monitoring flocculation of a raw water comprising a source of clarified raw water, a source of clarified treated water, an SCD adapted to receive flows of the clarified waters and switching means to alternate flows of the clarified waters to the SCD, the duration of each flow being sufficient to allow the SCD to acclimatise to the clarified water and produce a signal characteristic of the clarified water. The difference in the characteristic signals from the two may be calibrated for the adjustment of the set point. In its apparatus aspect, the present invention comprises a clarifier adapted to clarify a process water, means to impose substantially constant flow conditions on a stream of water from the clarifier and an SCD disposed to receive the stream of water. It is a further feature of this invention that additional analytical procedures may, with advantage, be carried out on the clarified water provided by the hydrocyclone. For example, the pH of the clarified water is an important parameter dependent solely on the dissolved ions of the water: if the pH electrodes are placed in a stream of the clarified water they will need to be cleaned less frequently. It will also be possible to estimate continuously and accurately the colour of the raw water and treated water due to dissolved organic materials and, if necessary, change the set point of the SCD or the pH of the flocculation to eliminate such chromophores. Brief Description of the Drawings The invention will now be further described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a flow diagram of an arrangement for using the device of Figure 2 in one embodiment of method according to this invention, Figure 2 is a schematic representation of a device for measuring streaming current. Figures 3 to 5 illustrate the relationship between streaming current signals and concentrations of metal cations in three water samples discussed in the following Example 1 and presented in table and graphic form, and Figures 6 and 7 show streaming current signals obtained in the following Examples 2 and 3. Description of Preferred Embodiments Referring to Figure 1, a pipe 1 through which is flowing a treated raw water containing floes has a branch 2 controlled by valve 3. The treated water falls from the branch onto a screen 4 set at angle to displace the retained solids downwardly across the screen. The partially clarified water passing through the screen collects in a container 5 equipped with an overflow 6. Water is withdrawn at the rate of 4 litres per minute from the container via pipe 7 by a pump 8 which feeds the water at 5.6 Kg/cm 2 (80 p.s.i.) via pipe 9 to a 10 millimetre hydrocyclone 10. Clarified water (i.e. containing no particles above 10 microns) flows from the hydrocyclone via the vortex finder and pipe 11 to a constant head tank 12 having an internal weir 13 (shown as a dotted line), an overflow pipe 14 and a supply pipe 15 delivering the clarified water to an SCD 16. The rate of delivery of the overflow from the hydrocyclone is 3 litres per minute and the ratio of the volumes of overflow to underflow (leaving the hydrocyclone at the apex 17 of the hydrocyclone) is 3:1. Figure 2 shows, in purely schematic form, (and not to scale) the SCD 16. Electrical connections 20 and 21 connect a sensor 34 and annular silver electrodes 22 and 23 respectively. The electrodes 22 and 23 are let into the inner wall 24 of a cylindrical dashpot 25 made of polytetrafluoroethylene and secured in a chamber 19 by a sleeve 29 having ports 30. A piston 26, also of polytetrafluoroethylene, is connected to an eccentric 27 mounted on the shaft of an electric motor 28. The piston 26 has a throw of 0.64 centimetre in the dashpot 25, the diameter of the piston 26 is 1.27 centimetre; the narrow annular passage 31 between the side wall of the piston and inner wall of the dashpot is 0.013 centimetre wide and the clearance 32 of the piston base from the inner wall of the base of the dashpot at closest approach, is 0.8 centimetre. In use, water from the flow to be monitored is fed through the supply pipe 15 so that clarified water devoid of solid matter, including adventitious dust particles, is discharged under constant head pressure to the inlet flooding the chamber 19 and submerging the dashpot 25 with a changing supply of water which overflows from the chamber 19 at the level 33. Within the chamber 19 the electric motor 28 causes the piston 26 to reciprocate within the dashpot 25 at, say, 230 cycles per second. The withdrawal of the piston 26 within the dashpot 25 draws water from the chamber 19 into, and the reverse stroke expels water from the dashpot 25 through the annular passage 31 between the piston wide wall and the inner wall of the dashpot. The displaced water travels through the gap at an average linear speed of about 1.2 metres per second whilst passing sequentially over the electrodes 22 and 23. The current generated by this process is sensed intermittently, once in each cycle, at similar instants of rapid flow of the displaced water. The current signal is sensed, amplified and displayed by the electrometer sensor 34 connected in series with electrodes 22 and 23 by wires 20 and 21. The constant head tank 12 may optionally be equipped with a fail safe low level sensor (not shown) adapted to switch off the electric motor driving the piston within the SCD, when the water level falls, as may happen if for any reason the supply of water to the constant head tank is interrupted. This preserves the piston from dry-running, an occasional cause of damage to SCD'ε. The signal generated by the SCD operating on the clarified water is characterised as the set point when the operator decides that the quality of the flocculated water is satisfactory. Minor movements of the signal in an electropositive or electronegative sense away from the set point may be used respectively, to decrease or to increase the rate of addition of the flocculating agent to the raw water. The invention is further exemplified by the followin Examples:- Example 1 The response of an SCD to changing concentrations of three salts dissolved in de-ionised water, together with determinations of pH and conductance are set forth in Figures 3, 4 and 5. The SCD readings are also presented graphically so that the substantial changes in those readings with minor changes in the salt concentrations may be more immediately appreciated. Example 2 A soft water containing 40 parts per million of sodium salts was fed through an SCD and a steady output current of -70 arbitrary units was established after 30 minutes of acclimatisation. A branch pipe from a hard water source was connected to the main, soft water delivery pipe; the branch pipe was equipped with a solenoid valve controlled by a timer which closed the valve for 30 minutes so that only soft water was supplied to the apparatus and then opened the valve so that for a further 30 minutes a mixture of hard and soft water was supplied to the SCD. It was established by chemical tests that the mixture of hard and soft water had a hardness of 6 parts per million of calcium. Thereafter the cycle was twice repeated. Meanwhile, the output signal from the SCD was fed to a chart-recorder and a reproduction of the trace appears in Figure 6. It will be seen that the steady signal of -70 arbitrary units, characteristic of the softened water, was changed electropositively to -60 units immediately the water with a hardness of 6 parts per million was introduced into the apparatus and that the characteristic signal for softened water was recovered as abruptly when the hardness was eliminated. These observations apply equally to the later cycles. Example 3 A water-softening, ion exchange bed was run continuously with the effluent softened water monitored by an SCD. For the better part of the run of eight hours the characteristic signal (set point) of -70 units for this particular softened water was recorded on a chart recorder. The trace from the recorder for the latter stages of the run is reproduced in Figure 7. The trace shows about one hour of production of fully softened water and then the effect of the incursion of calcium ions as the hardness broke through. ' As may be expected the breakthrough is a gradual rather than a sudden process and the level of breakthrough even after 120 minutes (e.g. after a further hour) would be a tolerable pollution for most uses of softened water. It will be appreciated that the backward and forward flow of water through the passage 31 in the apparatus of Figure 1 generates sinusoidal alternating streaming currents in the wires 20, 21 which enables AC signal pro¬ cessing to be performed in the sensor unit 34. This is a most valuable feature having regard to the small magnitude of the signals it is desired to monitor and the high level of disturbing background signals. Having a constant head in the tank 12 and thus a constant water level 33 in the chamber 19 imposes substan- tially constant flow conditions on the stream of clarified process water from which the samples pulsed through the passage 31 are taken.

Claims: CLAIMS 1. A method of monitoring a process water for the presence of unwanted metal cations comprises abstracting samples sequentially from a stream of said process water, urging each sample to flow forwards and backwards through a passage defined between confronting, non-conducting solid surfaces, whilst in contact with two electrodes spaced apart in the directions of the flows and incorporated in an electric current measuring circuit, thereby generating a streaming electrical current characteristic of each sample and monitoring said streaming currents, characterised in that the process water is substantially free from particles above 10 microns. 2. A method as claimed in claim 1, characterised in that the water sample is substantially free of solid particles of size greater than 2 microns. 3. A method as claimed in claim 1, characterised in that the cations are from a flocculating agent and at least a substantial proportion of the floes generated by the flocculating agent are removed prior to measuring the streaming current. 4. A method as claimed in claim 1, characterised in that the source of the process water is a cation exchange column and the metal cations sensed are selected from calcium and sodium. 5. A method as claimed in claim 1, characterised in that the process water is a filtered industrial waste water. 6. A method of monitoring a treated water for the presence of excess flocculating agent comprises abstracting samples sequentially from a stream of said treated water; urging each sample to flow forwards and backwards through passage defined between confronting, non-conducting solid surfaces, whilst in contact with two electrodes spaced apart in the directions of the flows and incorporated in a electric current measuring circuit, thereby generating a streaming electrical current characteristic of each sample and monitoring said streaming currents, characterised in that at least a substantial proportion of the floes created in the treated water by the said agent are removed prior to the samples being abstracted therefrom. 7. A method as claimed in claim 6, characterised in that the floes are at least partly removed by use of a hydrocyclone. 8. A method as claimed in claim 6, characterised in that the floes are at least partly removed by a fine meshe screen adapted to be self-cleaning in use. 9. Apparatus for carrying out the method of claim 1 , characterised in the combination of a clarifier adapted to clarify a process water, means to impose substantially constant flow conditions on a stream of clarified water from the clarifier and an SCD disposed to receive the stream of water. 10. Apparatus as claimed in claim 9 characterised i that the clarifier is a hydrocyclone. 11. Apparatus as claimed in claim 9 characterised in that the clarifier is an angled screen. 12. Apparatus as claimed in claim 9 characterised in that the clarifier is a filter. 13. Apparatus as claimed in claim 9, characterised in that the clarifier is a graded combination of two or more clarifiers selected from the group hydrocyclone, angled screen and filter.

Inventors: BLANCHARD MICHAEL JOHN, BLANCHARD, MICHAEL, JOHN
Applicants: ANALAQ LTD, ANALAQ LIMITED
Application year: 1988
Main classification: G01N27
IPC codes: G01N27