FR2756273A1 - Effluent treatment in a single active sludge purification vat - Google Patents

Abstract

Process for controlling the treatment of effluents in a vat (3) of an active sludge purification plant, with alternating periods of aeration and non-aeration (pause) by a turbine (4), comprises measuring the Redox potential of the effluent in the vat (3), continuously calculating the values of the first and second derivatives of the curve Redox potential = f(time) and stopping the turbine (4) when the values are respectively positive and greater than a predetermined threshold value. Also claimed is an apparatus for carrying out the process having \- 1 sensor (7) to measure the Redox potential in the vat (3) and optionally a sensor (7') to measure the concentration of dissolved oxygen in the effluent, a microprocessor which continuously registers the readings from the sensor(s) and calculates the required parameters which are used for automatically switching on and off the aeration turbine (4) at the appropriate times. The microprocessor also controls pumps which feed effluent into the treatment vat (3), pass treated effluent into a clarifier (5) and recycle active sludge (B) to the biological treatment vat (3) and to excess sludge storage vessel (6).

Description

PROCESS AND AUTOMATED PROCESSING CONTROL
EFFLUENTS IN A PURIFICATION STATION TANK BY
THE TECHNIQUE OF ACTIVATED SLUDGE
The present invention relates to a process for controlling the treatment of effluents in a treatment plant tank using the technique known as activated sludge.

 It also relates to an automaton for implementing this method.

 As is well known, the technique of purifying effluents by activated sludge consists in injecting air into the mass of effluents to be transformed, which results in a rapid development of the bacterial flora naturally present in said effluents. This microbial growth results in a very noticeable reduction in the content of polluting organic materials, that is to say a progressive transformation of these materials into other non-polluting compounds.

 To cause satisfactory bacterial development, an adequate supply of oxygen and sufficient mixing must be ensured to avoid settling of the sludge.

 Nitrogen compounds are the main source of pollution from industrial, household and agricultural effluents.

 The biological nitrogen removal processes using activated sludge use specific bacteria which oxidize ammoniacal nitrogen (NH4 +) to nitrates (NO3-) in several stages: this is nitrification.

 The nitrates are then reduced to non-polluting nitrogen gas (N2), which is eliminated in the atmosphere: this is the denitrification step.

 These two reactions are preceded by ammonification.

The overall process significantly reduces the DIBO5 (Demand
Biological in Oxygen for 5 days, which translates the quantity of oxygen necessary for the oxidation of biological pollutants by microorganisms, at 20 "C) and allows the elimination of nitrogen (DEGREMONT, Memento technique on the water, Lavoisier, Paris, 1989).

 Ammonification corresponds to a hydrolysis of organic nitrogen (urea and amino acids) into ammoniacal nitrogen (NH4 +).

Nitrification is oxidation by microorganisms (Nitrosomas,
Nitrobacter) from ammoniacal nitrogen (NH4 +) to nitric nitrogen (NO3-). Here is the overall reaction:

Taking into account the simultaneous synthesis of biomass (C5H7N02), the reaction becomes:
NH4 + + 1.8302 + 1.98HC03- or 0.021C5H7NO2 + 0.98N03- + 1.041H20 + 1.88H2C03.

 The nitrifying bacteria corresponding to this biomass are strict aerobic. They are autotrophic and have a fairly low growth rate. Two substrates are decisive for these bacteria: dissolved oxygen and NH4 + ions.

 During the denitrification process, the nitrates (NO3-) and the nitrites (NO2-) are reduced to gaseous nitrogen (N2) sparingly soluble in water. This non-polluting gas is released into the atmosphere. The reaction is very interesting because it effectively eliminates nitrogen and does not produce any troublesome residue.

Denitrification is therefore carried out in two stages: denitratation and denitration. The global denitrification equation is written:
2NO3- + 2H + N2 + 512 2 + H20.

 The reaction produces oxygen, hence the advantage of associating oxygen-consuming nitrification with denitrification.

 This association is justified for other reasons. In fact, the optimum pHs are close and the reaction temperatures are compatible.

 In view of these results, logic wanted the development of time alternation or phase alternation processes (MATHIEU J.L., 1986) where the aerobic and anaerobic periods follow one another in the same basin. These methods allow the balance between nitrifying strains and denitrifying strains in a single basin.

 This is why we have developed an effluent purification technique which consists of alternating, within the same tank containing the effluents, periods of aeration using turbines, and periods of non-aeration (pauses ).

 However, the duration of these periods has been determined empirically, in particular by occasional physico-chemical analyzes, and up to now, difficulties have been encountered in optimizing the duration of the aeration and pause periods.

 In addition, a direct measurement of the dissolved oxygen in the aeration tank where the activated sludge is located seemed a priori to correspond to the simplest principle possible to best manage the operation of aeration turbines.

 However, in the field of biological purification (MATHIEU J.L., 1986), some researchers could not be satisfied with measuring the consumption of dissolved oxygen to control the purifying activity of bacteria. They have been practicing for a few years the measurement of the redox potential (Redox).

 The continuous measurement of Redox allows better management of treatment plants. Indeed, the different stages of the wastewater treatment cycle are characterized by specific Redox values. These are therefore suitable for regulating stations.

 It will simply be recalled that the measurement of the Redox potential consists in evaluating the potential difference in volts, taken by two blades (platinum in general), one being immersed in the standardized oxidizing / reducing system (reference electrode) and the other in the solution to be measured.

The equilibrium potential taken by the measuring electrode is given by the NERNST equation:
E = Eo + RT / nF Log (Ox) / (Red)
where Eo: normal potential of the system (Volt) given by the electrode
standard hydrogen (EH)
R: ideal gas constant (8.31 J / K / mole)
T: absolute temperature (T = 273 + t) with t in "C
n: number of electrons involved
F: Faraday constant (96,500C)
Log: natural logarithm
Ox: activity of the oxidized form of the species (mole / l)
Red: activity of the reduced form of the species (mole / l)
In biological medium as in activated sludge, the redox couples are represented in particular by Fe2 + / Fe3 +; Mn2 + / Mn3 +; 02 / OH NH4 + / N02- / N03-, etc.

 On the basis of these observations, the applicant has discovered that it is possible to rationally and economically control the treatment of effluents in a tank of a purification station by the technique known as activated sludge.

 Thus, the present invention relates to a process for controlling the treatment of effluents in a treatment plant tank by the so-called activated sludge technique, which comprises alternating, within said tank, periods d 'aeration and non-aeration (pause) by means of a turbine and to measure, within said tank, the redox potential (Redox Potential) of said effluents.

 In a particularly remarkable manner, this method consists in continuously calculating the values of the first and second derivatives of the Redox Potential curve = f (time) and in stopping said turbine when these values are positive and the value of the second derivative is greater than a predetermined threshold.

 In practice, when said values are positive, this corresponds to an upward inflection point in the Redox Potential curve = f (time) which translates the disappearance of ammonium ions from the medium and a maximum concentration of nitrates (end of the nitrification reaction ).

 Mathematically, an inflection point is characterized by a cancellation of the second derivative of the function describing the graph studied, with change of sign.

What macroscopically corresponds to a clear crossing is, in fact, a zone of instability around zero.

Furthermore, according to other advantageous but non-limiting characteristics of this process:
- it also consists of continuously measuring the concentration of dissolved oxygen in said effluents, calculating the values of the first derivative of the concentration of dissolved oxygen curve = f (time) and, when said concentration is greater than 1.5 mg 02 / 1, to stop said turbine only if and only if said first derivative value is positive and greater than a predetermined threshold;
- the restarting of said turbine is carried out when the first and second derivatives of the Redox Potential curve = f (time) are negative and that the value of the second derivative is less than a predetermined threshold.

 The invention also relates to an automaton for implementing the method described above.

It is essentially characterized by the fact that it comprises:
- at least one Redox potential measurement probe within said tank;
- a microprocessor at least capable of continuously recording the potential measurements given by the probe as a function of time and of calculating the first and second derivatives thereof, then sending to the actuating motor of the aeration turbine information aimed at cause it to stop when the values of these derivatives are positive and the value of the second derivative is greater than a predetermined threshold.

Furthermore, according to other advantageous but non-limiting characteristics of this automaton:
- It also includes a probe for measuring the concentration of dissolved oxygen within the tank and the processor is also capable of continuously recording the concentration measurements given by the probe, calculating the first derivative thereof and, when said concentration is greater than 1.5 mg 02/1, to send to the actuating motor of the aeration turbine information intended to cause it to stop if and only if said first derivative value is positive and greater than a predetermined threshold;
the processor is also able to send information to the actuating motor of the aeration turbine intended to cause it to start up only when the first and second derivatives of the Redox Potential curve = f (time) are negative and that the value of the second derivative is less than a predetermined threshold.

 Other characteristics and advantages of the invention will appear on reading the detailed description which follows of a preferred embodiment.

This description will be made with reference to the accompanying drawings in which:
- Figure 1 is a diagram showing the layout of the various equipment of a treatment plant operating by the activated sludge technique;
- Figure 2 is a more precise diagram showing the structure of so-called aeration - mixing and clarification basins;
- Figure 3 is a front view of an aeration turbine;
- Figure 4 shows typical curves of redox potential measurements and dissolved oxygen concentration in the effluents, as a function of time;
- Figure 5 is a diagram showing the location of the machine within a treatment plant and its connections to the various equipment from which it receives or communicates information.

 Throughout the present application, COD is understood to mean the "chemical oxygen demand" which translates the oxygen consumption of the effluents under oxidation conditions in the presence of sulfuric acid.

 The purification station, of known type, within which the method of the present invention is capable of being implemented, is shown diagrammatically in FIG. 1.

 It includes a first so-called lifting tank 1, the function of which is to receive and store the polluted effluents E to be treated.

 Downstream is provided a screening device 2 which consists of a tray provided with a grid. The purpose of this grid is to eliminate bulky pollutants which could cause operating problems in the different parts of the treatment plant. These grids are manual or automatic cleaning.

 The different connections between the tanks and vats which equip the station have been represented by arrows f.

 The effluents thus "screened" are directed to a tank or tank "aerationbrushing" 3 in which the activated sludge carry out the process of transformation of polluting materials.

 As is particularly visible in Figures 1, 2 and 3, the tank 3 is equipped with a ventilation turbine 4 of known type, provided with a motor 40, a drive shaft 41 and mixing blades 42. The assembly is fixed above the tank 3 by means not shown, so that the blades 42 are always slightly below the level N of effluents contained in the tank.

 The tank 3 is connected by a pipe to a clarifying tank 5, into which a portion of the treated effluents E and of the activated sludge B are admitted periodically, with a view to separation by decantation. The treated effluents are then evacuated (arrow h, FIG. 2) for possible additional treatments, while a part of the sludge B is brought back into the tank 3 (arrow g) and the remaining part evacuated in storage tank 6 (arrow g ').

 In a known manner, it has been proposed, as has been mentioned in the introduction to the present description, to follow the process of transformation of the pollutants of the effluents by measuring the redox potential (Redox) within the effluents.

 We briefly recall that the voltage of a measurement electrode is expressed by reference to that of a second electrode that is chosen so that its potential remains constant under the measurement conditions. It is called "reference electrode" or "impolarizable electrode".

 The potential difference is measured using a millivoltmeter with high input impedance (more than 1012 Q). In practice, this function is ensured by a "Redox transmitter" box. This technique is practiced so that the equilibrium conditions of the electrode are preserved and that the reference electrode keeps a well-defined potential.

 The three main reference electrodes are: - the normal hydrogen electrode (EHN) where the equilibrium is located: H + + e-> 1 / 2H2; - the calomel electrode Hg / Hg2Cl2, seat of the balance: Hg2Cl2 + 2e- -, 2Hg + 2Cl- - the electrode Ag / AgCl, seat of the balance: AgCl + e- Ag + Cl-.

 The universal reference electrode is the standard hydrogen electrode (EHN), the use of which is limited to test laboratories. For current uses, more manageable electrodes are preferred.

In the tests described below, a silver chloride (Ag / AgCl) reference electrode was used. The relationship between EH (EHN) and E (Ag / AgCl) measured at 20 "C and for a concentration of 3M KC 1 is as follows:
EH = E (AglAgcl) + 0.210.

 there is therefore an offset of +210 mV in favor of the electrode (Ag / AgCl).

 The measuring electrode is often made up of platinum, the function of which is isolated from the measuring medium by a sealed tube (MATHIEU J.L., 1986).

 The electrode voltages are often reduced to the potential of the normal hydrogen electrode: Eo = OmV by convention.

 Electrodes like the Ag / AgCl electrode for example have the disadvantage of being attackable electrodes. They consist of a metal covered with a sparingly soluble salt of this metal, immersed in a solution containing the anion of this salt at a well defined concentration (HEDUIT A. et al, 1987).

 In FIG. 4 is shown in solid lines, a typical curve recorded within a treatment plant such as that which has just been described, reflecting the variation in redox potential (ordered) as a function of time. (abscissa).

Overall, it has been observed that the curve of the Rédox follows the variations due to the succession of aeration (rise of the Rédox) and non-aeration or pause (fall of the
Redox).

 The Applicant has discovered that there is a correlation between the aeration periods, the Redox, the concentration of ammonium ions and of nitrate.

 In other words, the Applicant has discovered that it was possible to control the stopping of the operation of the aeration turbine at the point of upward inflection of the Redox curve = f (time).

 This is why the method according to the invention consists in continuously calculating the values of the first and second derivatives of the Redox potential curve = f (time) and in stopping said turbine when these values are positive and when the value of the second derivative is greater than a predetermined threshold.

 The threshold is chosen so as to not take into account non-significant variations in potential. Those skilled in the art will determine the value of this threshold on a case-by-case basis, in particular as a function of the pollution rate of the effluents to be treated.

At the end of nitrification, when the NH4 + ions have been eliminated, the oxygen demand of the effluents is less important. In the basin, there is an increase in the concentration of dissolved 2, which induces a sudden variation in the
Redox potential.

 In certain operating cases of a station, the nitrification phase can be carried out at concentrations of 2 of the order of 2 to 3 mg 02/1. In these aeration conditions, at the end of nitrification, the elimination of NH4 + ions induces a visible rise in the 02 concentration, but the inflection point on the Redox Potential is hidden. This is what is represented by the arrow I in FIG. 4.

Indeed, there is a logarithmic type relationship between the Potential
Redox and the 02 concentration, of the type: Redox potential = a + b Log (02).

 So when you leave the high sensitivity range of the Redox Potential between 0 and 1 mg 02/1, a variation in the oxygen concentration may not be listed on the Redox Potential.

 This is why, according to a particular embodiment of the invention, the method also consists in continuously measuring the concentration of dissolved oxygen in said effluents, in calculating the values of the first derivative of the curve dissolved oxygen concentration = f (time) and, when said concentration is greater than 1.5 mg 02/1, to stop said turbine only if and only if said first derivative value is positive and greater than a predetermined threshold.

 In a way, this detects the "dropout" of oxygen, characteristic of the end of nitrification whatever the level of oxygenation.

 Figure 4 also shows, in the form of a dashed line curve, the measurement of the dissolved oxygen concentration. The arrow J identifies the zone in which said first derivative is positive.

According to another embodiment of the invention, the restarting of the turbine is carried out when the first and second derivatives of the Potential curve
Redox = f (time) are negative and the value of the second derivative is less than a predetermined threshold.

 This corresponds to the inflection point I 'in FIG. 4.

 The thresholds mentioned above will be determined, like the previous case by case, by the skilled person.

 The invention also relates to an automaton for implementing this method.

 it has the function of ensuring itself the management and the piloting of the process according to the invention.

To do this and according to a particular characteristic of the invention, it comprises:
- at least one Redox potential measurement probe within said tank;
- a microprocessor at least capable of continuously recording the potential measurements given by the probe, of calculating the first and second derivatives thereof and of sending to the actuation motor of the aeration turbine an information aiming to cause it to stop when the values of these derivatives are positive and that the value of the second derivative is greater than a predetermined threshold.

 FIG. 5 shows the location of such an automaton A within a treatment plant implementing the activated sludge technique.

 The reference numbers 1 to 6 used in this figure correspond to the equipment described with reference to Figures 1 to 3.

 References P1 and P2 respectively designate pumps which equip pipes between the clarifying tank 5 and the tank 3 on the one hand, and between the clarifying tank 5 and the tank 6 for storing discarded sludge.

 References 7 and 7 'designate the probes which are fitted to the tank 3.

 The arrows in dashed lines show the connections of the controller to the various equipment of the station.

 Thus, the automaton is able to check the correct operation of the lifting station or that of the pump P2. it also includes means capable of delivering a diagnosis in the event of a problem with the operation of one of these pieces of equipment.

 With the aid of the microprocessor, the automaton is also capable of performing all of the abovementioned calculation functions and in particular driving the turbine 4 in place in the tank 3.

 In this way, we provide automated and complete control of the treatment plant.

Claims (6)

 1. Method for controlling the treatment of effluents in a tank (3) of a purification station by the technique known as activated sludge, which comprises alternating, within said tank (3), periods of aeration and non-aeration (pause) by means of a turbine (4) and to measure, within said tank (3), the redox potential (Redox potential) of said effluents, characterized in that it consists in continuously calculating the values of the first and second derivatives of the Potential curve Redox = f (time) and stopping said turbine (4) when these values are respectively positive and the value of the second derivative is greater than a predetermined threshold.  2. Method according to claim 1, characterized in that it also consists in continuously measuring the concentration of dissolved oxygen in said effluents, in calculating the values of the first derivative of the curve dissolved oxygen concentration = f (time) and, when said concentration is greater than 1.5 mg 02/1, in stopping said turbine only if and only if said first derivative value is positive and greater than a predetermined threshold.  3. Method according to claim 1 or 2, characterized in that the starting of said turbine (4) is carried out when the first and second derivatives of the Redox Potential curve = f (time) are negative and that the value of the second derivative is less than a predetermined threshold.  - a microprocessor at least able to continuously record the potential measurements given by the probe (7), to calculate the first and second derivatives and to send to the actuation motor (40) of the aeration turbine (4) information intended to cause it to stop when the values of these derivatives are positive and the value of the second derivative is greater than a predetermined threshold.  - at least one probe (7) for measuring the Redox potential within said tank (3);  4. Control automaton for implementing the method according to one of claims 1 to 3, characterized in that it comprises:  5. Automate according to claim 4, characterized in that it also includes a probe (7 ') for measuring the dissolved oxygen concentration within said tank (3) and that said processor is also capable of recording continuously the concentration measurements given by the probe (7 '), in calculating the first derivative thereof and, when said concentration is greater than 1.5 mg 02/1, to be sent to the actuation motor (40) of the turbine. aeration (4) information intended to cause it to stop only if and only if said first derivative value is positive and greater than a predetermined threshold.  6. Automate according to claim 4 or 5, characterized in that said processor is also able to send to the actuating motor (40) of the aeration turbine (4) information aimed at causing it to start up only when the first and second derivatives of the Redox Potential curve = f (time) are negative and that the value of the second derivative is less than a predetermined threshold.