WO1997012840A1 - Method and device for controlling a device for purifying waste water - Google Patents

Abstract

The invention relates to a method for controlling a device for purifying waste water by anaerobic culture. Waste water is supplied into a reactor (1) and a purified liquid (23) and a decomposition gas (24) are extracted therefrom. In parallel to the regular supply of the reactor (1), supply inpulses are generated, and at least one impulse responsive parameter is measured. Application to a purification device by methane fermentation.

Description

 Method for controlling a wastewater treatment device and corresponding device

The present invention relates to a method for controlling a device for cleaning up waste water and to a corresponding device.

Anaerobic techniques such as methane fermentation or denitrification are frequently used as an effective depollution process. Anaerobic cultures are indeed a treatment system very well suited to the elimination of carbonaceous pollution or based on nitrates from industrial effluents, corresponding respectively to anaerobic digestion and denitrification.

Anaerobic digestion makes it possible to treat loads easily reaching 15 to 30 kg of chemical oxygen demand (COD) per m3 of reactor and per day, with purification yields traditionally of 90-95%. It is applied in particular to the treatment of agro-food effluents.

Anaerobic digestion is often considered a pretreatment of wastewater, followed by an aerobic finish. It is thus possible to reach very low levels in concentration of pollutants, and thus allow a discharge of treated water into the natural environment.

The purification processes by anaerobic digestion use increasingly intensive reactors, which make it possible to confine large concentrations of biomass in very compact volumes. Such reactors working at very high loads have a significant technological advance contributing to more performance. However, the ecosystems present in the reactors are constantly under heavy pressure in order to work under boundary conditions compatible with a good microbial balance. Manual control of such pollution control devices thus proves difficult and unreliable, and leads often to use reactors below their capacity for fear of organic overload.

To overcome the drawbacks of manual procedures, there has been proposed in document FR-2,672,583 a method for regulating and automatically operating a device for cleaning up waste water by methane fermentation. This process consists in simultaneously measuring in the gas phase of the digesters of a fermentation reactor the following three parameters: the gas flow rate resulting from the transformation of organic matter during fermentation, the ratio of the percentages of methane and carbon dioxide , and the hydrogen gas content, then processing the information thus collected in real time in order to obtain signals reflecting the instantaneous state of the ecosystem of the pollution control device.

The process described in document FR-2,672,583 is applicable exclusively to anaerobic digestion by methane fermentation. On the other hand, it requires the simultaneous measurement of three parameters, and using their variations requires calibration of their responses.

American patent US Pat. No. 4,349,435 relates to an anaerobic reactor system in which the COD supply and methane generation rates are measured, the COD of the effluent is estimated from these measurements and the rate is controlled of power in connection with this estimate.

US patent US-A-4,986,916 describes a method for controlling a biologically catalyzed reaction. According to this method, a trace gas is identified which is a metabolic intermediate or is balanced with such an intermediate. Its concentration is measured and an evaluation of the metabolic state of the reaction is obtained by a deterministic relationship.

The object of the invention is a method for controlling a device for depolluting organic, soluble or no, by anaerobic culture, reliable, easy to implement, and allowing stable and optimal operation.

The object of the invention is therefore to be able to treat polluted loads that are much higher than those usually obtained in industry.

The object of the invention is in particular an automatic regulation process for a device for depolluting waste water having the above advantages.

The invention also relates to a device for cleaning up waste water by anaerobic culture which is reliable and efficient, and which allows automatic control.

The present invention thus relates to a method for controlling a device for cleaning up waste water by anaerobic culture. In this process, at least one reactor is supplied with waste water at a feed rate, the waste water is decomposed into a purified liquid and a decomposition gas, and the purified liquid and the decomposition gas are extracted from the digester.

The originality of the process according to the invention lies in that superimposed on a regular supply of the reactor with supply pulses, and in that at least one parameter of response to the pulses is measured.

The application of pulses superimposed on the regular supply is the basis of an original control process, based on a combination of disturbances and analyzes.

The pulses applied can be positive or negative, depending on whether they increase or decrease the regular feed rate. They thus make it possible to evaluate whether the biological system can accept an additional load, or if it is on the contrary necessary to reduce the treated loads.

The feed pulses can have a significantly higher amplitude than variations in the regular feed rate. The process according to the invention thus makes it possible to obtain an increased detection capacity, and to be satisfied with measuring a single response parameter while obtaining very good reliability. The process according to the invention is suitable for any type of reactor and for any effluent to be treated. Indeed, the disturbances are directly applied to food, and the analyzes are not linked to a particular type of fermentation. Preferably, the regular feed rate is automatically adjusted according to the results of the analyzes.

This automatic control is simple in mathematical terms and robust in practical terms, because it is based on notions of real biological functioning.

The automatic adjustment practiced in the method according to the invention thus makes possible an optimal functioning of the depollution device with respect to biological treatment, by pushing it to work to the maximum of its capacities. In addition, the method according to the invention is also applicable to lower loads.

The feed pulses advantageously consist of positive feed steps.

It is advisable that the response parameters belong to a set comprising the flow rate of the decomposition gas and the pH of the purified liquid.

The supply pulses are preferably applied periodically.

They advantageously have heights between 1/2 and 10 times the height of the regular supply flow, and have durations advantageously between 5 min and 2 h.

In a preferred embodiment of the method according to the invention, the response parameters are the rate of the decomposition gas and the pH of the purified liquid, and the analyzes include the following steps:

a smoothing as a function of time of the gas flow is practiced, a measured volume of the decomposition gas produced in response to at least one positive pulse is calculated by temporal integration of the smoothed gas flow,

- when the measured volume is negative or zero, the pH is compared to an intervention threshold value, so that when the pH is greater than the threshold value, the regular supply flow is left unchanged, and when the pH is less than or equal to the threshold value, the regular supply flow is reduced,

- When the measured volume is positive, it is compared to a theoretical volume obtained under the effect of these pulses under nominal conditions of anaerobic culture, and the regular feed rate is increased according to the result of the comparison.

A negative or zero value of the volume measured corresponds to a decrease or stagnation of the anaerobic culture under the effect of an overload of waste water.

The invention also relates to a device for depolluting waste water by anaerobic culture capable of decomposing waste water into a purified liquid and into a decomposition gas. This device includes:

- at least one reactor,

- at least one waste water supply duct opening into the reactor, - at least one first duct for extracting the purified liquid and at least one second duct for extracting the decomposition gas,

- at least one sensor disposed on at least one of the first and second extraction conduits, - at least one main pump fitted to the supply duct,

- at least one electronic processing unit connected to the sensors and to the main pump, which can act on the main pump.

According to the invention, the electronic processing unit is capable of producing supply pulses in the supply duct superimposed on a regular supply, of analyzing parameters measured by the sensors, of determining a regular supply flow. adjusted to analyzes and produce that regular adjusted feed rate.

In a preferred embodiment of the device according to the invention, it comprises at least one secondary pump fitted to the supply duct and connected to the electronic processing unit, this unit producing supply pulses by means of the secondary pump.

The invention will be better understood with the aid of the following description of an exemplary embodiment and implementation, with reference to the appended drawings in which:

Figure 1 is a schematic representation of a device for depolluting waste water according to the invention. Figure 2 represents the evolution as a function of time, expressed in hours, of a first regular supply flow, expressed in liters / hour, of the device of Figure

1.

Figure 3 represents the evolution as a function of time, expressed in hours, of the decomposition gas flow, expressed in liters / hour, produced by the device of Figure 1 supplied with the supply flow of Figure 2.

Figure 4 represents the evolution as a function of time, expressed in hours, of a second feed rate regular, expressed in liters / hour, of the device in Figure 1.

FIG. 5 represents the evolution as a function of time, expressed in hours, of the decomposition gas flow, expressed in liters / hour, produced by the device of FIG. 1 supplied with the supply flow of FIG. 4.

A device for cleaning up waste water, as shown in Figure 1, comprises a digester 1 or reactor capable of decomposing waste water into a purified liquid and a decomposition gas.

The digester 1 can for example consist of a fluidized bed type device, allowing high performance.

A supply duct 2 supplies the digester 1 with waste water. This supply duct 2 is equipped with a main pump 5 making it possible to dose a regular supply flow, and a secondary pump 6 in parallel with the main pump 5, making it possible to apply disturbances to this flow of regular feeding. The digester 1 is also connected to a first extraction conduit 3 for the purified liquid and to a second extraction conduit 4 for the decomposition gas. A recycling duct 7 mounted on the first extraction duct 3 makes it possible to recycle in the digester 1 a portion of the treated liquid. A pH meter 13 positioned on the recycling duct

3 and a flow meter 14 placed on the second extraction duct

4 allow the pH of the purified liquid and the gas flow formed in the digester 1 to be measured respectively.

The pollution control device according to the invention also comprises an electronic processing unit 10 capable of receiving signals from the sensors 13 and 14. The electronic processing unit 10 is capable of acting on the main pump 5 via of a first control system 15, and on the secondary pump 6 via a second control system 16. In operation, the digester 1 receives beforehand an ecosystem made up of a microbial population capable of decomposing waste water. This population is made up of anaerobic organisms, capable of carrying out, for example, methane fermentation or denitrification. The ecosystem forms a biomass included in a liquid phase 20 with, above it, a gaseous phase 21.

A supply stream 22 of waste water is conducted in the liquid phase 20 of the digester 1 at a regular feed rate, adjusted by means of the main pump 5. The waste water arriving in the liquid phase 20 of the digester 1 is then broken down by the microbial population, and produce a stream of purified liquid 23 and a gas stream 24 respectively extracted from the digester

1 via conduits 3 and 4. The measurements made by the pH meter 1 3 on the purified liquid and by the flow meter 14 on the decomposition gas are transmitted to the electronic processing unit 10 In an example of application represented on the

Figure 2. the feed stream 22 has a regular feed rate 37 of the order of 0.35 l / h

The electronic processing unit 10 applies flow pulses to the supply stream 22 by means of the second control system 16 The electronic processing unit 10 is preferably programmed to periodically apply these pulses. It can also apply the pulses irregularly over time, including during manual intervention. These last two possibilities may in particular be judicious in the presence of variations which may occur on an industrial site, for example concerning the regular feed rate 37, the concentration of the effluent to be treated, or the nature of the organic pollution. The pH of the purified liquid and the flow rate of the gas flow 24 are preferably measured continuously by the pH meter 13 and the flow meter 14, respectively.

In a first example of implementation of the method according to the invention, shown in FIGS. 2 and 3, the flow rate of the feed stream 22 is required to change over time along a curve 32 (FIG. 2) The evolution of the feed rate related to a feed rate axis 31 relative to a time axis 30 corresponds for the first eight hours to a regular feed rate 37 of height HO = 0.35 l / h This feed rate 37 is controlled by the electronic processing unit 1 0 acting on the first control system 1 5

During a duration D 1 of approximately one hour, a positive step 33 of a height H 1 approximately equal to 0.35 l / h is superimposed on the regular supply flow. This step of supply is imposed by the unit. electronic processing 10 by means of the second control system 16 acting on the secondary pump 6 The feed stream 22 is then restricted to the regular feed rate of height H0.

The supply flow shown diagrammatically by the curve 32 leads to an evolution of the flow of the gas flow 24 represented by the curve 36 (FIG. 3), related to a gas flow axis 35 relative to the time axis 30

From 0 to 8 hours, the curve 36 undergoes fluctuations around an average value G0 substantially equal to 3.85 l / h. During this entire period, the flow rate of the measured gas flow 24 remains less than 4 l / h, at times undergoing drops in flow rate of up to 0.7 l / h. From the 8th hour, under the effect of step 33, an increase in the gas flow rate of the order of 0.8 l / h relative to the average value G0 is observed. The flow rate of the gas flow 24 remains significantly higher than the average value G0 for a period D4 slightly less than 2 h. Integrating during the duration D4 the increase in the gas flow caused by step 33, a volume V1 of gas is obtained due to the additional charge treated in response to step 33. The duration D4 of integration can for example start at l 'instant of application of the step 33, and end at the moment when the increase in the gas flow rate becomes less than an evolutionary average of this increase.

The increase in the load treated under the effect of the feed level 33 shows that the biological system is capable of admitting organic overload. The electronic processing unit 10 analyzes the curve 36 of the gas flow as a function of the curve 32 of the supply flow, and compares the volume V1 with a theoretical volume that would be obtained under the effect of the curve 32 of the flow d in nominal anaerobic culture conditions. This theoretical volume is calculated by assuming that the digester 1 operates with an identical processing capacity during the regular feed rate 37 and the step 33. The maximum increase in the regular feed rate 37 is set at 20% of regular feeding.

Since the volume V1 obtained is less than the corresponding theoretical volume, the electronic processing unit 10 calculates an increase in the regular feed rate of less than 20%. In a simplified form, this increase is proportional to the ratio of the volume measured V1, to the theoretical volume.

The electronic processing unit 10 imposes an increase in the supply flow 22 by acting on the first control system 15 actuating the main pump 5. The increase can be imposed by a conventional automatic method, such as a proportional integral and derivative regulation (PID). This method can be supplemented by a technique such as fuzzy logic with recognized industrial applicability.

When the digester 1 is not capable of supporting an additional charge introduced in the form of a positive feeding step, the flow rate of the gas flow 24 remains stable or even decreases at the time of increasing This behavior is indicative of a limit of processing capacity or excessive load A reduction in the regular feed rate 37 may then be necessary Such a decision is made depending on the pH of the purified liquid Indeed, the pH is indicative of an insufficient In particular, the device presented operating by methane fermentation, the organic matter present in the liquid phase 20 of the digester 1 is in a first step transformed into volatile fatty acids by the action of acidogenic bacteria In a second step, the acids g low volatiles thus obtained are degraded into methane and carbon dioxide, and therefore no longer influence the pH of the medium if the latter has sufficient buffering capacity ant By cons, in case of organic overload for example, volatile fatty acids can accumulate and lower the PH

In the case where the application of positive steps does not cause an increase in the gas flow, the pH is compared to an intervention threshold value When the pH is above this threshold value, the supply flow is left regular unchanged When, on the contrary, the pH is less than or equal to this threshold value, the regular supply flow is reduced 37 The electronic processing unit 10 calculates this reduction according to the response of the system in terms of gas flow and pH, and imposes it on the feed stream 22 via the first control system 15 acting on the main pump 5. The system described thus makes it possible to continuously adjust the regular feed rate 37 to the maximum processing capacity of the digester 1. It may face operating constraints in the treatment of waste water, manifested by various variations that may occur on an industrial site.

Thanks to the device and method according to the invention, the applied load and the hydraulic residence time used can be optimally chosen. The electronic processing unit 10, combined with the sensors 13 and 14 and the control systems 15 and 16, provide overall supervision of the system and choose the most appropriate local actions.

The device according to the invention is suitable both for operating at maximum capacity, and at lower loads, this choice being decided by the user.

The nature of the effluent does not intervene directly, the control system can be generalized to all depollution of waste water by anaerobic culture.

Generally, the positive feeding steps applied have durations typically between 5 min and 2 h. They are advantageously separated by durations at least equal to 2 h, and typically between 2 h and 6 h in the case of a periodic application. The heights of the positive steps applied preferably have values substantially greater than the maximum increase envisaged for the regular feed rate 37. They thus make it possible to overcome problems of detectability due to fluctuations in the measured curves, such as those of the curve. 36. These heights are advantageously between 1/2 times and 10 times the height H0 of the regular supply flow 37. However, they must not exceed critical values which risk disturbing the biological system. It is also possible to apply negative steps instead of positive steps such as that 33 presented by way of example, or alternately. Their use is particularly relevant when the capacities of the biological system prove to be saturated, for example following an increase in the concentration of pollutants or a change in the nature of the effluent to be treated. Analysis of the response curve of the gas flow then allows the electronic processing unit 10 to determine a desirable decrease in the regular supply flow 37, in combination with the analysis of the pH of the purified liquid. The negative steps typically have heights between 20% and 100% of the height H0 of the regular supply flow 37. The electronic processing unit 10 can carry out an analysis of the measured signals and optionally vary the flow of regular feeding 37 during each pulse. It can also carry out each of the analyzes for several pulses, in order to increase the reliability of the decisions taken.

Thus, in a second example of implementation of the method according to the invention, shown in Figures 4 and 5. the flow rate of the feed stream 22 is forced to change over time along a curve 38 (Figure 4). The evolution of curve 38 for the first twelve hours is identical to that of curve 32. Next, in addition to the first step 33, a second step 34 is applied. The latter has a duration D2 approximately equal to 12 min and a height H2 approximately 2.35 l / h, and the two steps 33 and 34 are separated by a duration D3 slightly greater than 3 h. After the second step 34, we return to the regular feed rate.

The supply flow shown diagrammatically by the curve 38 leads to an evolution of the flow of the gas flow 24 represented on the curve 39 (FIG. 5). Curve 39 is identical to curve 36 of the first example of setting works during the first twelve hours. It then shows a significant increase in the gas flow for a duration D5 approximately equal to 2 h. By integrating during the period D5 the increase in the gas flow rate, a second volume V2 of gas is obtained due to the additional charge treated in response to the second step 34.

The volumes V1 and V2 are then compared jointly with the theoretical volumes that would be obtained under the effect of curve 38 of the feed rate under nominal anaerobic culture conditions. The electronic processing unit 10 calculates an increase in the regular feed rate taking into account the two volumes V1 and V2 at the same time, for example by an average method. The choice of two very different pulses in duration and in height makes it possible to more surely validate the results obtained. Then, the electronic processing unit 10 imposes the calculated increase in the supply flow 22, by means of the first control system 15. In the example of implementation presented, the decision-making is thus carried out by the electronic processing unit 10 after analysis of the response at the two levels 33 and 34. The choice of the number of steps processed jointly results from a compromise between the speed of execution and the reliability of the system. In general, the analysis of a single step is sufficient.

Although the applied impressions are advantageously constituted by steps, they can also take other forms, such as parabolic, or triangular for example.

Furthermore, the measurement of the pH of the purified liquid 23 can be omitted, the mere knowledge of the flow rate of the decomposition gas 24 sufficient to automatically control and regulate the depollution device. This pH measurement, however, offers increased reliability. It is possible to use any other parameter than the gas flow rate to analyze the response of the biological system to a supply pulse. The pH and COD measured, or the residual pollution concentration, in the purified liquid can, for example, play this role, but their measurements reflect accomplished facts and are often carried out too late to react correctly to critical situations. In the particular case of methane fermentation, the ratio of the percentages of methane and carbon dioxide or the content of hydrogen gas in the decomposition gas can be used.

Furthermore, the analysis of the impulse response can be carried out with several parameters instead of just one. The quality of the results obtained with the method according to the invention does not however seem to require this increase in complexity.

It is clear that the main pump 5 and the secondary pump 6 can be grouped into a single pump controlled by the electronic processing unit 10 at least from a single control system. The separation of pumps 5 and 6, however, allows more flexibility.

Instead of being used for regulation and automatic driving, the device according to the invention can be used for simple control. In this case, the electronic processing unit 10 is connected to an alarm means, which is triggered when the digester 1 is overloaded and is no longer able to properly filter the waste water. A second signal can be provided to indicate that the digester 1 is operating below its capacity. However, it is a good idea to automate the regulation, in order to avoid tedious tasks that could be performed imprecisely, and thus reduce the risk of error. Although PID type regulation is fully mastered and very widely used on an industrial site, any other method of regulation can be used.

The following two examples carried out on residual water from a wine distillery further illustrate the invention in detail:

In the first, the fluidized bed reactor has a working volume of 15 liters, includes a column surmounted by a calming zone and a gas-liquid separator. The reactor is equipped with a water jacket keeping the temperature at around 35 ° C, and includes a clarification device (settling device) separating the bioparticles from the liquid phase and allowing recycling of this liquid phase to by means of a peristaltic pump. The reactor uses support particles from a fine granular material having a specific gravity of 2, and the expansion is produced by a recycling stream.

An effluent of COD equal to 18 g / l is introduced for the first 60 hours and 35 g / l for the following 60 hours. Since the initial regular diet is worth 30 kg COD / m ³ d (per m ³ and per day), positive pulses of approximately 15 kg COD / m ³ d are superimposed on it approximately every 9 hours. The automatic control of the feed rate then increases the latter from 30 kg COD / m ³ d to 40 kg COD / m ³ d during the first 60 hours, the flow stabilizing at this latter value. Then, the automatic control increases the flow from 40 kg COD / m ³ d to more than 90 kg COD / m ³ d during the following 60 hours, after the increase in COD in the inlet effluent.

In the second, the reactor has a working volume of 120 I, and its temperature is kept close to 35 ° C by means of a variable power heat exchanger. The liquid phase is recycled to the reactor by means of a vortex type pump. The other specificities are the same as for the reactor of the first series of applications.

The 120 I of the reactor are filled and allowed to stabilize for one month. Then, the control strategy is applied with an initial feed rate almost zero, the COD being close to 30 g / l. We then obtain a growth in the feed rate which, after oscillations, stabilizes at around 40 kg COD / m ³ d after thirty days. The reference signs inserted after the technical characteristics mentioned in the claims are intended only to facilitate understanding of the latter, and in no way limit their scope.

Claims

1. Method for controlling a device for cleaning up waste water by anaerobic culture, in which at least one reactor (1) is supplied with said waste water (22) at a feed rate (32), said waste water is decomposed in a purified liquid (23) and in a decomposition gas (24), and the purified liquid (23) and the decomposition gas (24) are extracted from said reactor (1), characterized in that it is superimposed on a supply regular (37) of said reactor (1) supply pulses (33, 34), and measuring at least one response parameter (36) to said pulses (33, 34). 2. Method according to claim 1, characterized in that the regular feed rate is automatically adjusted (37) according to the result of said analyzes. 3. Method according to one of claims 1 or 2, characterized in that the supply pulses (33, 34) consist of positive supply steps. 4. Method according to any one of the preceding claims, characterized in that said response parameters belong to a set comprising the flow rate (36) of the decomposition gas (24) and the pH of the purified liquid (23). 5. Method according to any one of the preceding claims, characterized in that the supply pulses (33, 34) are applied periodically. 6. Method according to any one of the preceding claims, characterized in that the supply pulses (33, 34) have heights (H 1, H2) between 1/2 and 10 times the height (H0) of the regular supply flow (37). 7. Method according to any one of the preceding claims, characterized in that the supply pulses (33,34) have durations between 5 min and 2 h. 8. Method according to any one of the preceding claims, characterized in that the response parameters being the flow rate (36) of the decomposition gas (24) and the pH of the purified liquid (23), said analyzes comprise the following steps: a smoothing is practiced as a function of time of said gas flow (36), - a measured volume of the decomposition gas (24) produced in response to at least one positive pulse (33,) is calculated by temporal integration of said smoothed gas flow (36) 34). - when the measured volume is negative or zero, the pH is compared to an intervention threshold value, so that when the pH is greater than said threshold value, the regular supply flow (37) is left unchanged, and when the pH is less than or equal to said threshold value, the regular supply flow (37) is reduced, - When the measured volume is positive, it is compared to a theoretical volume obtained under the effect of said pulses (33, 34) under nominal anaerobic culture conditions, and the regular feed rate (37) is increased as a function of the result of the comparison. 9. Device for depolluting waste water by anaerobic culture capable of decomposing waste water (22) into a purified liquid (23) and into a decomposition gas (24), comprising: - at least one reactor (1), - at least one supply pipe (2) for waste water (22) opening into said reactor (1), - at least one first extraction pipe (3) of the purified liquid (23) and at least one second pipe d 'extraction (4) of the decomposition gas (24), - at least one sensor (13, 14) disposed on at least one of the first (3) and second (4) extraction ducts, - at least one main pump (5) equipping the supply duct (2), - at least one electronic processing unit (10) connected to said sensors (13, 14) and to the main pump (5), capable of acting on the main pump (5), characterized in that the electronic processing unit (10 ) is capable of producing supply pulses (33, 34) superimposed on a regular supply (37) in the supply duct (2), of analyzing measured parameters (36) by the sensors (13, 14) , determining a regular feed rate adjusted to said analyzes and producing said adjusted regular feed rate. 10. Device according to claim 9, characterized in that it comprises at least one secondary pump (6) equipping the supply duct (2) and connected to the electronic processing unit (10), said unit (10) producing supply pulses (33, 34) by means of the secondary pump (6).