FR2678734A1 - Improvement to the continuous measurement of the redox potential of waste water - Google Patents

Abstract

The invention relates to a device for improving the continuous measurement of the redox potential of waste water and biological sludges. REDOX pairs, (REF1, Pt1) and (REF2, Pt2), each composed of a reference electrode and of a noble metal electrode, alternately measure the redox potential in the water to be measured, while the first pair measures the REDOX potential and displays it on the voltmeter (V), the noble metal electrode of the second pair is subjected to an activation-regeneration treatment by means of a current electrode (A/R) and a volt-ampere gram generator (G). This electrochemical treatment has the purpose of desorbing the substances contaminating the active surface of the REDOX electrodes. At the end of the working cycle, the pairs are switched over using a sequencer (S) and change-over switches (C1), (C2) and (C3). The device according to the invention is in particular intended for continuously measuring the REDOX potential in a purification station.

Description

 Improvement in the continuous measurement of the oxy-reduction potential of wastewater.

 The present invention relates to an improvement device for the continuous measurement of the oxidation-reduction potential of waste water, in particular in treatment plants.

The measurement of the redox potential in water is the measure of the equilibrium potential of the redox reaction

du couple étudié (soit son potentiel REDOX) peut être calculée au moven de la relation de NERNST (1864-1941, prix Nobel 1920)

In this case, where we have a perfectly reversible thermodynamic equilibrium, we demonstrate that the tension

of the couple studied (i.e. its REDOX potential) can be calculated from the moven of the NERNST relation (1864-1941, Nobel Prize 1920)

pour 1 mole de réactif (soit le nombre dlectrons mis en jeu lors de la réaction)

activités en

de l'oxydant et du réducteur, souvent confon- dues avec les concentrations dans le cas de solutions extremement diluées.

T: temperature in Kelvin n: number

per 1 mole of reagent (i.e. the number of electrons involved in the reaction)

activities in

oxidizer and reducing agent, often confused with concentrations in the case of extremely dilute solutions.

= (RED) fred

et (RED) sont les concentrations , fox et fred coefficients d' activité inférieurs à 1).= (OX) fox and

= (RED) fred

and (RED) are the concentrations, fox and fred activity coefficients less than 1).

F = Faraday constant

Eo = Normal potential (value of E when

Note
For the general relationship

The general form of the NERNST equation is

However, an unassailable indicator electrode, which does not participate in the reaction, plunges into a solution containing a REDOX system takes on an equilibrium potential.

The electrode works only as an electron acceptor and donor (corresponding to the permanent exchange of electrons during the reversible oxidation-reduction reaction

This electrode is therefore used to measure the REDOX potential in water,
It should be noted that when multiple systems
REDOX are found in the same equilibrium solution, the potentials are all at the same value (so-called global potential).

 In most cases, the electrode is made of platinum or gold (noble metals, inert and unassailable).

 This type of electrode is used in particular in treatment plants to continuously indicate the oxidation-reduction state of wastewater and biological sludge used to purify wastewater.

 Their reliability is biased by contamination of the ma (over time) by organic matter, metal cations, sulfur, impurities strongly linked to the active surface of the metal (monocrystalline platinum in general).

 These species adsorbed on the metal deactivate it and distort the measurement of the "memory" potential, of the oxidation-reduction sorting states, poor repeatability, shift and drift of the potentials, increasingly slow variations, etc.

The electrode is said to be "deactivated".

 These defects considerably worsen the phenomenon which makes that the REDOX equilibria take longer to establish themselves than the proportion of non-reversible systems is greater in the medium studied, assumed to be stable.

 It is therefore necessary to clean the metal electrode from time to time and to activate the platinum (or gold) section in contact with the water.

 However, this treatment is tedious and relatively "violent" for the electrode (at the metal / water interface) which does not repolarize immediately. with good potential.

 This explains why even automatic abrasive cleaning systems are not entirely satisfactory.

 However, fairly recent work has shown that the methods of electrochemical regeneration of the metal surface would be more effective and me "traumatic" than this abrasive cleaning.

 Because, much more than conventional soiling in waste water (such as yraisses, bio-organic materials, etc.), it is especially the impurities adsorbed on the metal that deactivate it: these methods, called activation-regeneration, therefore have the rattle of de-adsorbing the contaminating species of the active surface. of the electrode. They have in common the fact of periodically subjecting the electrode (always immersed in the measurement water) to voltages and electric currents) variable over time (at the very level of the active metal / water interface), according to more or less complex protocols depending on the measurement media and the contaminating chemical species.

 Unfortunately, while the electrode is subjected to this electro-chemical treatment (voltametric and amperometric), it can no longer measure the overall redox potential of the measurement medium. And even after activation, at rest, it takes a relatively long time for it to resume its correct equilibrium potential (at zero current), hence a discontinuous measurement.

 The device according to the invention allows the continuous use of the REDOX potential measurement system in the sample water while benefiting from the advantages of the activation-regeneration treatment of the REDOX electrodes.

 The basic idea is based on the alternation between measurement / activation and regeneration of two identical REDOX electrodes while the first REDOX electrode is in the "measurement" phase, the second electrode is in the "activation-regeneration" phase and vice versa.

 Each REDOX electrode therefore undergoes, in turn, and during the same time, an identical electrochemical treatment and is then put back into measurement each in turn for a similar time in the same measurement medium.

There is perfect symmetry of treatment, and the two electrodes, of strictly identical design (same noble metal, same surface condition, etc.), immersed in the same sample water (practically in the same place. To within a few cm) will alternately give the same potential continuously, as if we were dealing with a single REDOX electrode which would not contaminate itself. (more exactly, there is no continuity solution in the measurement of the global REDOX potential of the mi
place of measurement, because obviously this potential can evolve over time ~).

The device according to the invention comprises according to a first characteristic
- Five electrodes; either the two REDOX electrodes in noble metal (platinum for example), their associated reference electrodes (at constant potential: example Ag / AgCl) and finally a common activation-regeneration electrode (in soft graphite for example).

 - An electronic voltmeter with very high input impedance which measures and displays the REDOX potential for example.

 - Possibly a signal transmitter with very high input impedance which generates a low impedance signal that can be transmitted remotely.

 - Three analog signal switches routing the various signals (measurements and activation-regeneration) and a control logic.

 - A sequencer controlling the switches via the control logic so as to alternate measurement / activation at equal times of the two REDOX electrodes.

 - Finally a programmable generator of voltage and currents (conductive measuring water) variable in time, each identical cycle of which is triggered by the sequencer at each alternation of REDOX probe.

 The sequenced operation of the device according to the invention will be explained in its detailed description.

According to particular embodiments
- Reference electrodes can be common (removal of an analog switch)
- The two reference electrodes are preserved and the activation / regeneration electrodes are deleted.

Each becomes an activation / regeneration electrode with respect to the associated REDOX electrode. Then just add an analog switch
This solution, less satisfactory in theory, is nevertheless suitable in practice in many cases because the reference electrode (Ag / AgOl for example) has a relatively low impedance and can therefore work in ampXrometry (within certain limits). The alternation is long enough for this electrode to regain its stable and constant reference potential because each activation / regeneration cycle generally ends with a rest period.

 - Each REDOX electrode / reference electrode pair can be galvanically isolated from the rest of the circuit (other electrodes and associated electronics) during the measurement phase. This precaution prevents false measurements due to parasitic (if any) loopback currents (by water, earth, electrical supply, signal transmission output, etc.).

 This "floating" measurement section also escapes any influence (via water) of the variable voltages and currents (and fairly large amplitudes) applied to the other electrodes in the activation-regulation phase.

 - The voltammograms generated during the activation-rdyénérati.on phase take the form of a succession of cycles (work) / (rest at fixed imposed potentials) ending over a long period of absolute rest, where the couple d 'electrodes (REDOX, REF) is "floating (in the galvanic sense), the REDOX electrode gently resuming its equilibrium potential (measurement phase), as well as the reference electrode (REF) in the case where it is used as working electrode.

 - Finally technologically, all five (or four) electrodes can be grouped into a single sensor head (monoprobe) of small diameter and all electronic circuits in a single measurement / regeneration activation / transmission device.

The accompanying drawings illustrate the invention
Figure 1 shows the general block diagram of the device according to the invention.

FIG. 2 represents the voltammograms undergone by the REDOX electrodes and the associated chronograms.

FIG. 3 represents the technological structure of the combined probe head.

With reference to these drawings, the device firstly comprises a set of five electrodes immersed in the measurement water comprising the two REDOX electrodes (Ptl) and (Pt2) (in platinum in the embodiment according to Figures 1 and 3 ), the two reference (REFI) and (REF2) electrodes and the activation-regeneration electrode (A / R).

The device then includes a digital voltmeter (V), a signal transmitter (T), three electronic analog changeover switches (Cl) (C2) (C3), a logic inverter (IL), a sequencer (S) and finally an activation-regeneration generator (G), these electrode circuits can be grouped together in a single device (A)
The sequenced operation of the device according to the invention is as follows
The sequencer (S) delivers the logic signal (which takes the value "l" for a time>. The switches (Cl) and (C2) are then in position (1), which connects the pair of electrodes (REFl , Ptl) to the digital voltmeter (V): this is their measurement phase. During this time the electrode (REF2) is disconnected from the voltmeter and remains floating. The electrode (Pt2) is also disconnected from the voltmeter
The switch (C3), controlled by the logic signal
(5) via the logic inverter (IL) is therefore passed over the po
sition (2), which connects the electrode (Pt2) to the generator (G), the working electrode (A / R) which ensures the return of the current being always connected to the activation generator (G) Wrégénératron, whose triggering is also ensured by Le-sequencer at each transition of the Logic signal (u)
When S) takes the logic value "0", and this for a time To identical to the previous phase, the switches (cul) and (C2) come to position (2), which connects the pair of electrodes (REF2 , Pt2) to the voltmeter (V), thus ensuring their measurement phase. In turn, and for the same duration, the electrode (REFl) is disconnected from the voltmeter and
stays floating. The electrode- (Ptl) is also disconnected from the voltmeter.

The switch (C3) is at the same time turned on.
location (1) and therefore connects the electrode (Ptl) to the generator (G) which has triggered a work cycle identical to the previous one.

The alternation of the two pairs of electrodes has therefore been achieved and the previous sequences are repeated indefinitely.
According to a variant not illustrated, the two reference electrodes (REF1) and (REF2) can be common and the
switch (C1) is then deleted, the reference potential 4 being permanently connected to the voltmeter (V).

According to another variant not illustrated, the two
electrodes (REFl) and (REF2) are retained and are deleted
the working electrode (A / R): each reference electrode
plays the role of the electrode (A / R) with respect to the electrode
Associated REDOX (Ptl) or (Pt2). It suffices to add an analog switch identical to), alternately connecting the generator (G) to (REF1) and (REF2).

According to a last variant not illustrated, each
torque (REF1, Ptl) and (REF1, Pt2) is alternately galvanically isolated from the rest of the circuit in the measurement phase.

 This isolation can be done by relays, isolation transformers and amplifiers or opto-electronic circuits.

 One of the types of voltampêrogrammes delivered by the generator (G) of activation-regeneration is detailed in figure 2 (with the time on the abscissa and the scale (U) of the potentials on the ordinate).

Each work sequence begins with a salvo of
N identical cycles with a total duration TA / R; each activation-regeneration cycle breaks down itself into a series of ramps (Ra) (slow over time) going up and down with speed VRa and duration TRa, amperometric phase undergone by the REDOX electrodes, followed by a fixed fixed potential (Uo) at minimum current over time

It should be noted that each initial ramp of a given cycle restarts from the same place and with the same slope as the final ramp of the previous cycle (thus completing an interrupted ascent or descent).

 These N work cycles end with a long rest period Te, where the REDOX-electrode is floating and resumes its equilibrium potential.

tandis que l'autre élec trode (Pt2) est au travail. Lorsque ( # ) = "0" c'est l'inver- se, l'électrode (Pt2) étant en phase mesure (potentiel et l'autre au travail.

While (+) = "l", the REDOX electrode (Ptl) is in the measurement phase (potential

while the other electrode (Pt2) is at work. When (#) = "0" is the reverse, the electrode (Pt2) being in the measurement phase (potential and the other at work.

les N cycles de l'ordre de 3 à 45 minutes (TA/R), et la période de repos Tr de l'ordre de 15 à 30 minutes. (Il y a donc de 15 à 45 cycles).To fix an order of magnitude, the duration To of a phase is of the order of one hour, each cycle of the order of one to two minutes

the N cycles of the order of 3 to 45 minutes (TA / R), and the rest period Tr of the order of 15 to 30 minutes. (There are therefore 15 to 45 cycles).

de l'ordre de

As for the work potentials (ramps (Ra)), they have maximum amplitudes of 1 to 2 volts and speeds

of the order of

Waveforms can be reprogrammed by EPROM type memory swap.

 Technologzquernent, Figure 3 shows an embodiment of the combined probe head, ie combining the five electrodes into a single probe. The probe head (TS), ending with a protective strainer (CR), is of circular section and includes the five electrodes in a symmetrical arrangement: in the center the working electrode (A / R) whose core central (GR) is a soft graphite rod, laterally the two REDOX probes themselves combined, because they consist of the pairs (REF1, Ptl) and (REF2, Pt2). (SC2) represents the exploded view and in section following (AA) of this last combined probe: the platinum wire (Pt) is sheathed with glass (GV) (glass / Pt welding) and the polished end of the metal constitutes the surface REDOX electrode active. Around this electrode (Pt2), the reference (REF2) is constructed by symmetry of revolution: either a filament (F) of Ag / Agel (chlorinated silver) twisted on the glass sheath (GV) and immersed in the electrolyte (EL), usually KCl with precise concentration. This. electrolyte can be gelled or on porous plastic support (no maintenance).

 The sealing tube (TE) in PVC or DELRIN closes this volume, and ends at its end with a centering and sealing ring in elastic silastene (BCE). Between this ring and the tube (TE) runs a capillary connection bridge (PLU) through which the electrolyte flows at almost zero flow, thus ensuring an electrical connection bridge with the very long measurement water in the weather.

 The assembly is sealed with a sealing resin in the probe body (CS) in DELRIN. Note: the combined probe (REFI, Ptl), symmetrical with respect to (A / R), is of exactly the same construction.

 The device according to the invention is particularly intended for continuously monitoring the state of oxidation-reduction of waste water and. biological sludge in treatment plants

Claims (6)

REDOX characterized by the alternation of two REDOX couples, (REF1, Ptl) and (REF2, Pt2), each composed of a reference electrode and a REDOX electrode made of noble metal, the first couple measuring the REDOX potential in l sample water and displaying it on the voltmeter (V) with its possible remote transmission by the transmitter (T), while the noble metal electrode of the second couple is subjected to an deactivation-regeneration treatment via a current electrode (A / R) and a reconfigurable generator (G) of voltammograms, this electrochemical treatment having the role of de-adsorbing the contaminating species from the active surface of the REDOX electrodes (Ptl) and (Pt2), the two couples being swapped at the end of the cycle thanks to the sequencer (S) which.  1) Device for continuously measuring the potential generates the logic signals (#), controlling the switches (C1) and (C2) of the measurement switch, and (4) by a logic inverter (IL) controlling the switch (C3) of the activation-regeneration switch (all of these electronic circuits constituting the device (A)).  2) Device according to claim 1 characterized in that the two reference electrodes (REFI) and (REF2) are common, the switch (Cl) being deleted.  3) Device according to claim 2 characterized in that we keep the two electrodes (REF1) and (REF2) separate and independent and we remove the working electrode (A / R), each of the reference electrodes playing the role of the electrode (A / R) with respect to the associated REDOX electrode (Ptl) or (Pt2).  4) Device according to claim 1 characterized in that each pair of electrodes (REFl, P-tl) and (REF2, Pt2) is alternately galvanically isolated from the rest of the circuit in the measurement phase.  5) Voltampêrogramme according to claim 1 carac merise in that it consists of a succession of N cycles each consisting of a series of ramps (Ra) slow in time rising then descending, excursion of the order d '' one to two volts, followed by an imposed potential (UO) at minimum current, all of these cycles ending with a long period of rest where the REDOX electrode is floating and resumes its equilibrium potential .  6) Device according to claim 1 characterized by the realization of the probe head (TS) in combined form, grouping the five electrodes in a monoprobe water in the center the working electrode (A / R) in soft graphite (GR) , laterally the two electrode pairs (REF1, Ftl) and (REF2, Pt2) also forming two probes combined with symmetry of revolution, the second of which (SC2), perfectly identical to the first, is composed of a reference electrode Ag / AgCl (filament (F)). whose electrolyte (EL) is gelled or on a porous plastic support with micro-flow by capillary connection bridge (PLC), and a REDOX electrode proper in platinum (Pt) sheathed in glass, the probe body (CS ) being in DETJRIN.