Method for treating an air flow to be treated through a biofilter and associated installation

Abstract

The method comprises the following steps, implemented by a control unit (26) of a biofilter (20) after each spray activation i: • - determining at least one current value of a parameter representative of the acidity of the aqueous liquid discharged from the biofilter (20), • - maintaining the current spray activation period constant when the current value of the parameter representative of the acidity is between a minimum acidity value and a threshold acidity value; • - increasing the current spray activation period when the current value of the parameter representative of the acidity is between the threshold acidity value and the maximum acidity value; • - decreasing the current spray activation period when the current value of the parameter representative of the acidity decreases below the minimum acidity value or increases above the maximum acidity value.

Description

[0001] DESCRIPTION

[0002] TITLE: Process for treating an air stream through a biofilter and associated installation

[0003] The present invention relates to a method for treating an air stream to be treated through a biofilter, comprising the following steps: bringing an air stream to be treated through a volume of porous filter medium of the biofilter, the porous filter medium comprising microorganisms; recovery of a treated air stream downstream of the volume of porous filter medium; successive spraying of the porous filter medium with an aqueous liquid at successive spraying trigger times; evacuation of the aqueous liquid from the volume of porous filter medium.

[0004] Such a process is intended in particular to treat an airflow from a space in which air extraction must be carried out continuously, for example for reasons of corrosion, safety or comfort of operators moving in the space.

[0005] The space to be treated is, for example, located in a building, particularly in an industrial building such as a factory, especially in a water treatment plant.

[0006] Such a process is particularly suitable for installations in which odorous and / or potentially dangerous compounds are generated, particularly from wastewater, in order to deodorize the air flows exiting these installations.

[0007] Examples of compounds to be removed include sulfur compounds such as hydrogen sulfide or mercaptans, nitrogen compounds such as ammonia, and volatile organic compounds.

[0008] To treat such air streams, particularly those loaded with sulfur compounds, it is known to pass the air stream to be treated through a biofilter, in order to significantly reduce or eliminate the sulfur compounds in contact with the biofilter, and produce a treated air stream.

[0009] The biofilter comprises a porous filter medium on which microorganisms are present, in particular bacteria fixed to the porous filter medium.

[0010] Natural selection favors microorganisms that break down sulfur and nitrogen compounds, transforming them into odorless gases and other components that are harmless to operators when exposed to appropriate humidity and acidity levels. To ensure effective treatment over time, microorganisms must therefore be maintained under these suitable conditions. Regularly moistening the filter medium by spraying it with an aqueous liquid, such as water or water with added nutrients, is a known method for this purpose.

[0011] However, humidifying the biofilter by spraying leads to partial clogging of the porous filter medium. This results in a significant increase in the biofilter's pressure drop. The airflow extraction rate from the treated area is then reduced, which can cause corrosion, safety, or residual odor problems.

[0012] To overcome this problem, the fan supplying the biofilter with airflow to be treated is oversized, so that, during each spraying sequence, the extraction is sufficient to compensate for the pressure loss related to the spraying.

[0013] Oversizing the fan is generally not economically advantageous, as it leads to excessive electricity consumption, or even degraded treatment.

[0014] Furthermore, the spraying trigger period is fixed and does not take into account whether or not the appropriate conditions for microorganisms are met.

[0015] One aim of the invention is therefore to obtain a method of treating an airflow through a biofilter, which is particularly efficient and safe with respect to the treated space, while being economical and simple to implement.

[0016] To this end, the invention relates to a method of the aforementioned type, characterized by the following steps, implemented by a biofilter control unit after each spraying activation i:

[0017] - determination of at least one current value of the parameter representing the acidity of the discharged aqueous liquid from at least one measurement of the parameter representing the acidity of the discharged aqueous liquid;

[0018] - maintaining the current spraying triggering period constant with respect to the spraying triggering period preceding the spraying triggering i when the current value of the parameter representing acidity is between a minimum acidity value and a threshold acidity value;

[0019] - increase in the current spraying triggering period compared to the spraying triggering period preceding the spraying triggering i when the current value of the representative acidity parameter is between the threshold acidity value and the maximum acidity value;

[0020] - decrease in the current spray triggering period compared to the spray triggering period preceding spray triggering i when the current value of the representative acidity parameter decreases below the minimum acidity value or increases above the maximum acidity value.

[0021] The method according to the invention may comprise one or more of the following features, taken individually or in any technically feasible combination:

[0022] - the parameter representing the acidity of the discharged aqueous liquid is the pH of the discharged aqueous liquid;

[0023] - the minimum acidity value is between 2 and 3, the threshold acidity value is between 2.5 and 3.5, the maximum acidity value is between 8 and 9;

[0024] - the current value of the parameter representing acidity is the minimum value of the parameter representing acidity of the aqueous liquid discharged during the spray triggering period preceding the spray triggering i;

[0025] - the increase in the spraying trigger period is implemented as long as the spraying trigger period is less than a maximum spraying trigger period, the decrease in the spraying trigger period is implemented as long as the spraying trigger period is greater than a minimum spraying trigger period;

[0026] - it includes, prior to determining the current value of the parameter representing acidity, the determination of a current value of a parameter representing the humidity of the treated airflow from at least one measurement of the parameter representing the humidity of the treated airflow, the process comprising the following steps: if the current value of the parameter representing the humidity of the treated airflow is greater than a threshold humidity value, maintaining the triggering of a next spraying at the spraying triggering period; if the current value of the parameter representing the humidity of the treated airflow is less than the threshold humidity value, immediate triggering of a spraying i;

[0027] - in the case of an immediate triggering of a spray i occurring when the current value of the parameter representing the humidity of the treated airflow is less than the humidity threshold value, the spray triggering period preceding the triggering of spray i is equal to the time elapsed between the immediate triggering of spray i and the triggering of spray i-1 preceding spray i;

[0028] - the supply of the airflow to be treated includes the passage of the airflow to be treated through a fan comprising a rotating air pumping element having a controllable rotation speed;

[0029] - it includes determining a current value of a parameter representative of the pressure loss through the volume of porous filter medium from at least one measurement of the parameter representative of the pressure loss through the volume of porous filter medium, and adjusting the rotational speed of the rotating part of the fan using the current value of the parameter representative of the pressure loss to maintain constant the flow rate of air to be treated circulating in the volume of porous filter medium;

[0030] - the parameter representing the pressure loss is a pressure of the airflow to be treated measured downstream of the fan and upstream of the volume of porous filter medium and / or is a pressure difference determined from the pressure of the airflow to be treated measured downstream of the fan and upstream of the volume of porous filter medium;

[0031] - the porous filter medium includes mineral particles, in particular expanded clay particles, the microorganisms advantageously including heterotrophic or autotrophic bacteria.

[0032] The invention also relates to an installation comprising:

[0033] - a biofilter comprising:

[0034] * a volume of porous filter medium, the porous filter medium comprising microorganisms;

[0035] * an inlet for the supply of the airflow to be treated through the volume of porous filter medium;

[0036] * a recovery outlet for a treated airflow downstream of the porous filter medium volume;

[0037] * a system configured to perform successive waterings of the porous filter medium at successive spraying trigger times, the system comprising a spray inlet of an aqueous liquid into the volume of porous filter medium;

[0038] * an outlet for draining the aqueous liquid from the porous filter medium volume, characterized by a biofilter control unit configured to, after each spraying:

[0039] - determine at least one current value of the parameter representing the acidity of the discharged aqueous liquid from at least one measurement of the parameter representing the acidity of the discharged aqueous liquid;

[0040] - maintain the current spraying triggering period constant with respect to the spraying triggering period preceding the spraying triggering i when the current value of the representative acidity parameter is between a minimum acidity value and a threshold acidity value;

[0041] - increase the current spraying triggering period compared to the spraying triggering period preceding the spraying triggering i when the current value of the representative acidity parameter is between the threshold acidity value and the maximum acidity value; - decrease the current spraying triggering period compared to the spraying triggering period preceding the spraying triggering i when the current value of the representative acidity parameter decreases below the minimum acidity value or increases above the maximum acidity value.

[0042] The installation according to the invention may comprise one or more of the following features, taken individually or in any technically possible combination:

[0043] - the biofilter includes a column containing the volume of porous filter medium;

[0044] - The control unit is configured to:

[0045] - prior to determining the current value of the parameter representing acidity, determine at least one current value of a parameter representing the humidity of the treated airflow from at least one measurement of the parameter representing the humidity of the treated airflow,

[0046] - if the current value of the parameter representing the humidity of the treated airflow is greater than a humidity threshold value, maintain a trigger for the next spraying at the spraying trigger period;

[0047] - if the current value of the parameter representing the humidity of the treated airflow is less than the humidity threshold value, immediately trigger a spraying;

[0048] - The control unit is configured to:

[0049] - determine at least one current value of a parameter representative of the pressure loss through the volume of porous filter medium from at least one measurement of the parameter representative of the pressure loss through the volume of porous filter medium, and

[0050] - adjust the rotation speed of the rotating part of the fan using the current value of the parameter representing the pressure loss to maintain a constant flow rate of air to be treated circulating in the volume of porous filter medium.

[0051] The invention will be better understood upon reading the following description, given solely by way of example, and made with reference to the attached drawings, in which:

[0052] - [Fig.1] Figure 1 is a schematic view, partially in perspective, of an air flow treatment installation according to the invention, equipped with a biofilter;

[0053] - [Fig.2] Figure 2 is a view of a flowchart for implementing a process for treating an airflow according to the invention, within the installation of Figure 1.

[0054] In all that follows, the terms "upstream" and "downstream" are normally understood in relation to the normal direction of airflow in the installation.

[0055] A first treatment installation 10 according to the invention is illustrated in the figure

[0056] 1. The installation 10 is intended to collect an air stream to be treated 12 in a space 14, in order to treat compounds contained in the air stream to be treated 12 and to discharge a treated air stream 16 having a reduced content of these compounds compared to the air stream to be treated 12.

[0057] Space 14 is for example an air volume within an industrial unit, particularly within a wastewater treatment plant, for example in pretreatment, primary settling, biological treatment or sludge treatment facilities.

[0058] The compounds to be treated are, for example, sulfur compounds, such as hydrogen sulfide, or mercaptans, including methyl mercaptan, dimethyl disulfide, or dimethyl sulfide.

[0059] Alternatively or in addition, the compounds to be treated are nitrogen compounds, such as ammonia or volatile organic compounds, as described in Article 2 of European Council Directive 1999 / 13 / EC of 11 March 1999.

[0060] The flow rate of the air to be treated 12 by the installation 10 is for example greater than 300 m3 / h and is generally between 300 m3 / h and 100,000 m3 / h for a biofilter, the number of biofilters in the installation being able to be chosen according to the capacity required for treatment.

[0061] The treatment in installation 10 reduces the concentration of the aforementioned compounds in the treated air stream 16 to a value below 1 mg / m³ 3 , in particular less than 0.5 mg / m³ 3 .

[0062] As illustrated in Figure 1, the installation 10 includes a biofilter 20 in which the treatment of the airflow to be treated 12 is carried out. It further includes a fan 22 to supply the biofilter 20 so that it receives the airflow to be treated 12 coming from space 14.

[0063] The installation 10 further includes a sensor system 24, and a control unit 26 to control in particular a spray triggering period in the biofilter 20 and advantageously, to maintain a constant flow rate at the discharge of the supply fan 22.

[0064] The biofilter 20 here comprises a vertically extending column 30. More generally, the biofilter 20 can comprise a plurality of columns 30 mounted in series and / or in parallel, the columns 30 being able to extend vertically, horizontally, or obliquely. Similarly, the airflow can circulate from top to bottom or from bottom to top. In the installation 10 shown in Figure 1, it circulates from top to bottom in the biofilter 20. The column 30 delimits an internal volume 32 containing a porous filter medium 34, on which microorganisms are present.

[0065] The biofilter 20 also includes an inlet 36 for supplying the airflow to be treated 12 into the column 30, which is here an upper inlet, an internal porous sieve 38 for supporting the porous filter medium 34, and an outlet 40 for discharging the treated airflow 16, which is here a lower outlet.

[0066] The biofilter 20 further includes a system configured to carry out successive waterings of the porous filter medium, comprising a spray inlet 42 of an aqueous liquid opening into the column 30.

[0067] The biofilter 20 also includes an outlet 44 for recovering aqueous liquid that has passed through the porous filter medium 34 and the porous sieve 38.

[0068] As mentioned above, in the example in Figure 1, column 30 extends vertically. Here, it has an outer wall (made of plastic, concrete, metal, or other material) that internally delimits the inner volume 32.

[0069] The interior volume 32, for example, is between 0.7 m 3 and 12 m 3 .

[0070] It contains the porous filter medium 34, arranged above the porous sieve 38, with a delimitation of a gaseous space 46 between the air flow inlet 36, the aqueous liquid injection inlet 42 and the upper surface of the porous filter medium 34.

[0071] In an advantageous example, the porous filter medium 34 is formed of a bed of mineral particles, in particular expanded clay, for example with a density greater than 1.2.

[0072] The density and particle size of the porous filter medium 34 are chosen according to the use. Generally, the particles of the porous filter medium have an effective size between 2 mm and 5 mm, in particular between 2.5 mm and 2.9 mm, or between 2.3 mm and 3.8 mm.

[0073] Microorganisms are attached to the porous filter material 34. They include, for example, bacteria, in particular heterotrophic or autotrophic bacteria.

[0074] The porous filter medium 34 is preferably inert.

[0075] Microorganisms are capable of degrading sulfur compounds, particularly hydrogen sulfide. The degradation of hydrogen sulfide is given by the equation:

[0076] H2S + 2 O2 -> 2 H+ + SO4 2 -

[0077] In parallel, bacterial synthesis is carried out according to the following reaction:

[0078] H2S + CO2 + HCO3- + PO4 3 - + NH4 + -> C5H7NO2(S, P) + 2 H + + SO4 2 -

[0079] Alternatively or in addition, they are capable of degrading mercaptans, ammonia, nitrates, and / or volatile organic compounds. Examples of microorganisms include T. thioparus, T. thiooxidans, and Nitrosomonas.

[0080] Preferably, the treatment takes place under appropriate humidity and pH conditions. The relative humidity of the air exiting the treatment is preferably between 95% and 99%.

[0081] The pH should preferably be between 2 and 8.

[0082] The airflow inlet 36 is provided in an upstream region of the column 30, upstream of the surface of the porous filter medium 34. In the example of Figure 1 the upstream region is a top region of the column and the inlet 36 is located above the surface of the porous filter medium 34.

[0083] In this example, the airflow inlet 36 includes an upstream bent pipe 50 carrying the airflow to be treated 12. It opens vertically into the gaseous space 46.

[0084] The porous sieve 38 is positioned in a lower region of the column 30. It retains the porous filter medium 34. It delimits, downstream of the porous filter medium 34, a chamber 52 for separation and evacuation of the treated air stream 16 and the aqueous liquid stream which has passed through the porous filter medium 34 which is located here below the porous filter medium 34.

[0085] The evacuation outlet 40 opens here into the chamber 52 under the porous sieve 38, downstream of the porous filter medium 34 in the direction of the air flow to be treated 12. It opens transversely with respect to a vertical axis of the column 30.

[0086] In this example, the evacuation outlet 40 has a downstream elbow pipe 54 going up along the column 30, outside of it.

[0087] The aqueous liquid injection inlet 42 has at least one spray head 56, introduced axially into the gaseous space 46 of the column 30 through the supply inlet 36. The head 56 opens above and away from the porous filter medium 34. It is connected to a water source by an inverted J-shaped connecting pipe which here extends along the column 30.

[0088] The aqueous liquid injection inlet 42 is equipped with or connected to at least one valve 62 for controlling the flow of aqueous liquid introduced through the liquid injection inlet 42.

[0089] The control valve 62 is connected to the control unit 26. It is controllable by the control unit 26 between a closed position, in which no liquid is brought through the aqueous liquid injection inlet 42, and at least one open position in which aqueous liquid is brought through the aqueous liquid injection inlet 42 to spray the porous filter medium 34.

[0090] The aqueous liquid is preferably water or water containing nutrients. The liquid recovery outlet 44 is located below the porous sieve 38 and below the porous filter medium 34. It opens into the chamber 52, downstream of the porous filter medium 34.

[0091] In this example, it opens transversely and is connected to an aqueous liquid drain via a downstream elbow fitting 60.

[0092] The supply fan 22 includes a rotary air-pumping element 70 activated by a motor 71, and a variator 72 connected to the motor 71, configured to vary the speed of the rotary element 70 on command from the control unit 26.

[0093] The sensor system 24 includes a sensor 80 for measuring a parameter representative of the acidity of the aqueous liquid discharged downstream of the porous filter medium 34, a sensor 82 for measuring a parameter representative of the humidity of the treated air stream 16, and at least one sensor 84A, 84B, 84C for measuring a parameter representative of the pressure drop through the porous filter medium 34 in the column 30.

[0094] In the example shown in Figure 1, the parameter representing acidity is pH. Sensor 80 is a pH measurement sensor for the aqueous liquid discharged from the recovery outlet 44.

[0095] In this example, the parameter representing humidity is hygrometry. The measuring sensor 82 is a hygrometry measuring sensor, mounted downstream of the exhaust outlet 40, to determine the water content of the treated air stream 16.

[0096] The parameter representing the pressure loss is, for example, a pressure at the discharge of the fan 22, a pressure difference between the suction and discharge of the fan 22, or a pressure difference between the pressure of the treated airflow 16 and the discharge of the fan 22.

[0097] The sensor system 24 advantageously includes a sensor 84A for measuring the discharge pressure of the fan 22, located between the fan 22 and the air flow inlet 36. It optionally includes a sensor 84B for measuring the suction pressure of the fan 22 located upstream of the fan 22 between the space 14 and the fan 22.

[0098] In an advantageous variant, the sensor system 24 includes a pressure measurement sensor 84C in the treated airflow 16 arranged for example downstream of the downstream bent pipe 54.

[0099] All sensors 80, 82, 84A to 84C of the sensor system 24 are connected to the control unit 26 which receives the signals from these sensors 80, 82, 84A to 84C.

[0100] The control unit 26 comprises at least one computer including a processor 90 and a memory 92 comprising software modules, intended to be executed by the processor 90 to implement functions of the control unit

[0101] 26.

[0102] Alternatively, the computer is made in the form of a programmable logic component, such as an FPGA (Field Programmable Gate Array), or a dedicated integrated circuit, such as an ASIC (Application Specific Integrated Circuit).

[0103] The control unit 26 thus comprises a module 94 for receiving data from sensors 80, 82, 84A to 84C of the sensor system 24, and a module 96 for determining a trigger period Ti for spraying aqueous liquid into the column 30 and for initiating this spraying once the period Ti has elapsed. The control unit 26 advantageously includes a module 98 for controlling the speed of the rotary element 70 of the fan to direct the airflow 12 supplied by the fan 22.

[0104] The sensor data receiving module 94 is designed to receive signals from the various sensors 80, 82, 84A to 84C, and to process them to obtain, at each measurement instant (for example at a frequency greater than 0.03 Hz), current values ​​of the parameters representative of acidity, humidity and pressure loss.

[0105] The determination module 96 is configured in particular to maintain, after a spraying trigger i, the spraying triggering period Ti constant and equal to the spraying triggering period Ti-1 preceding the spraying trigger i when at least one current value pHi of the representative acidity parameter determined from measurements of the representative acidity parameter, in particular a minimum value of the representative acidity parameter during the spraying triggering period Ti-1 preceding the spraying trigger i, is between a minimum acidity value pHmin and a threshold acidity value pHseuil.

[0106] Furthermore, the determination module 96 is advantageously configured to increase the spray triggering period Ti relative to the spray triggering period Ti-1 preceding the spray triggering i when at least one current pHi value of the representative acidity parameter determined from measurements of the representative acidity parameter, in particular a minimum value of the representative acidity parameter during the spray triggering period Ti-1 preceding the spray triggering i, is between the threshold acidity value pHseuil and the maximum acidity value pHmax

[0107] Furthermore, the determination module 96 is advantageously configured to decrease the spray activation period Ti relative to the spray activation period Ti-1 preceding the spray activation i when at least one current pHi value of the representative acidity parameter determined from measurements of the representative acidity parameter, in particular a minimum value of the representative acidity parameter during the spray activation period Ti-1 preceding the spray activation i, is below the minimum acidity value pHmin or above the maximum acidity value pHmax

[0108] The determination module 96 is also configured to trigger an aqueous liquid spray in the column at time i by controlling the control valve 62 from the closed position to the open position, if the current value of the representative humidity parameter at time i has a value less than a threshold humidity value RHmin, and to wait until the end of the spray triggering period to trigger the spray, as long as the current value of the representative humidity parameter at time i is greater than a threshold humidity value RHmin.

[0109] The control module 98 is configured to increase the speed of the rotary element 70 when the current value of the pressure drop parameter increases, and to decrease the speed of the rotary element 70 when the current value of the pressure drop parameter decreases. This control is advantageously implemented to maintain a constant airflow rate, that is, preferably with a variation of less than 10% from its average value.

[0110] A first method for treating an airflow to be treated 12 according to the invention will now be described from a start-up 108.

[0111] The fan 22 is activated to extract the airflow to be treated 12 from the space 14 and bring it to the airflow inlet 36.

[0112] The air stream to be treated 12 then flows into the column 30 through the porous filter medium 34. Upon contact with the bacteria present in the porous filter medium 34, the compounds to be treated, in particular compounds with an odor or presenting a risk, are treated by the microorganisms present on the porous filter medium 34.

[0113] The treated airflow 16 is collected in chamber 52, then extracted through exhaust outlet 40 to be conveyed to the atmosphere, or to another space.

[0114] With reference to Figure 2, in step 110, to maintain effective activity of the microorganisms present on the porous filter medium 34, the control unit 26 determines an initial spray triggering period T0, for example by extracting this value from a memory 92 of the control unit 26.

[0115] At step 112, the control unit 26 starts a timer to determine the time t elapsed since the start 108 before the first spraying, or since the last spraying triggering i-1. The control unit 26 collects and stores in memory 92, successive measurements of the parameter representing the acidity in the aqueous liquid discharged at the recovery outlet 44, obtained via the sensor 80 over time.

[0116] At step 114, the control unit 26 reads the stopwatch to determine if the elapsed time t is less than the current spray trigger period Ti-1 which is the initial spray trigger period T0 defined before the first spray or which is the spray trigger period defined after the last spray trigger i-1.

[0117] If time t is less than the current spray trigger period Ti-1, at step 116, the control unit 26 receives the current value RHi of the representative humidity parameter obtained from sensor 82 and determines whether this current value RHi is greater than the minimum humidity threshold value RHmin.

[0118] If this current value RHi is greater than the minimum humidity threshold value RHmin, the control unit 26 returns to step 114 to determine the time elapsed since the last spraying, until the time corresponding to the spraying trigger period Ti is reached.

[0119] At step 118, when the elapsed time corresponds to the current spray triggering period Ti-1, the control unit triggers a spray i of aqueous liquid for a given spray duration, for example greater than 1 minute and in particular between 1 minute and 10 minutes, for example 5 minutes, by piloting the control valve 62 from the closed position to the open position, and then from the open position to the closed position.

[0120] The aqueous liquid is poured into the porous filter medium 34 through the head 56 for the specified spray duration. The aqueous liquid flows through the porous filter medium 34 and through the porous screen 38 and collects in the chamber 52. The aqueous liquid is then discharged through the liquid recovery outlet 44.

[0121] On the contrary, at step 116, if the current value RHi is less than the minimum threshold value of humidity Rhmin, before the time t elapsed since the last spraying i-1 reaches the current spraying trigger period Ti-1, the control unit 26 then immediately triggers the spraying step 118 i, without waiting for the time t elapsed to equal the current spraying trigger period Ti-1.

[0122] In step 124, the control unit 26 stores the elapsed time Di between the spray activation i and the timer activation in step 112. This elapsed time corresponds to the period Ti-1 when the current value Rhi has remained above the minimum humidity threshold value RHmin for the entire period Ti-1, or is less than the period Ti-1 when liquid spraying was triggered before the end of period Ti-1 because the current value Rhi fell below the minimum humidity threshold value RHmin. The control unit 26 then updates the spray activation period Ti-1 preceding the spray activation i to be equal to the elapsed time Di.

[0123] At step 126, the control unit determines a current value pHi of the representative parameter of the determined acidity, from measurements of the representative parameter of the acidity in the aqueous liquid discharged at the recovery outlet 44, obtained via the sensor 80, in particular during the spray triggering period Ti-1 preceding the spray triggering i.

[0124] The current pHi value is, for example, the minimum value of the representative acidity parameter measured during the spray triggering period Ti-1 preceding the spray triggering i.

[0125] At step 128, if at least one current value pHi of the representative acidity parameter determined from measurements of the representative acidity parameter, in particular when the minimum value of the representative acidity parameter during the spray trigger period Ti-1 preceding the spray trigger i is between the minimum acidity value pHmin and the threshold acidity value pHthreshold, the control unit 26 maintains the spray trigger period Ti constant and equal to the spray trigger period Ti-1 preceding the spray trigger i (which is equal to the elapsed time Di).

[0126] In step 130, if at least one current pHi value of the representative acidity parameter determined from measurements of the representative acidity parameter, particularly when the minimum value of the representative acidity parameter during the spray trigger period Ti-1 preceding the spray trigger i is greater than the threshold value pHseuil, and advantageously less than a maximum acidity value pHmax, the control unit 26 increases the current spray trigger period Ti relative to the spray trigger period Ti-1 preceding the spray trigger i (which is equal to the elapsed time Di), for example by multiplying the elapsed time value Di by a coefficient K greater than 1, as long as this period is less than a maximum threshold period Tmax. The maximum threshold period Tmax is, for example, less than 4 hours.

[0127] In step 132, if at least one current pHi value of the representative acidity parameter determined from measurements of the representative acidity parameter, particularly when the minimum value of the representative acidity parameter during the spray trigger period Ti-1 preceding the spray trigger i is less than the minimum acidity value pHmin or is greater than the maximum acidity value pHmax, the control unit reduces the current spray trigger period Ti relative to the spray trigger period Ti-1 preceding the spray trigger i (which is equal to the elapsed time Di), for example, by dividing the elapsed time value Di by a coefficient K greater than 1, as long as this period is greater than a minimum threshold period Tmin. The minimum threshold period Tmin is, for example, greater than 0.25 hours.

[0128] At step 129, control unit 26 then resets the timer and returns to step 112.

[0129] In common examples where the pH of the discharged aqueous liquid is the representative parameter of acidity, the minimum acidity value pHmin is advantageously between 2 and 3, the threshold acidity value pHseuil is advantageously between 2.5 and 3.5, and the maximum acidity value pHmax is advantageously between 8 and 9.

[0130] In common examples where the hygrometry of the treated airflow 16 is the representative parameter of humidity, the minimum value of humidity RHmin is advantageously between 95% and 99%.

[0131] Furthermore, at least during each spraying operation in step 118, the control unit 26 receives measurements from at least one sensor 84A to 84C and determines a current value for the parameter representing the pressure drop. The control unit 26 drives the drive 72 to increase the rotational speed of the rotating element 70 and maintain the flow rate of the air stream to be treated 12 constant, particularly during the spraying operation, until the current value of the parameter representing the pressure drop decreases again.

[0132] Thanks to the invention just described, the spraying of the porous filter medium 34 is carried out at an adjustable time interval Ti, making it possible to maintain the acidity of the aqueous spray liquid within a chosen preferred range or to return to this chosen range in case of deviation. Within this range, the microorganisms present in the porous filter medium 34 are in favorable conditions to flourish and effectively process the compounds to be treated in the airflow 12.

[0133] When the acidity falls outside this range, the control unit 26 adjusts the watering trigger period Ti. This ensures that watering is carried out only when necessary.

[0134] Furthermore, the humidity measurement of the treated airflow instantly establishes that the humidity in the porous filter medium 34 is above a threshold acceptable for microorganisms. This allows for immediate irrigation to be triggered in case of deviation and for the irrigation trigger period to be redefined.

[0135] Finally, the indirect measurement of the fluid flow rate downstream of the fan 22 allows for adjusting the rotational speed of the fan 22's rotating element 70, according to the current clogging status of the biofilter 20, particularly to compensate for pressure drop during aqueous liquid spraying. This makes it possible to maintain a constant extraction flow rate in space 14, ensuring the safety of personnel in space 14 and efficient extraction to reduce odors or remove hazardous compounds, while also reducing the investment and operating costs of the fan 22.

Claims

DEMANDS 1. A method for treating an air stream (12) through a biofilter (20), comprising the following steps: bringing an air stream (12) through a volume of porous filter medium (34) of the biofilter (20), the porous filter medium (34) comprising microorganisms; recovering a treated air stream (16) downstream of the volume of porous filter medium (34); successive spraying of the porous filter medium (34) with an aqueous liquid at successive spray trigger times; evacuation of the aqueous liquid from the volume of porous filter medium (34), characterized by the following steps, implemented by a control unit (26) of the biofilter (20) after each spray trigger i: - determination of at least one current value of the representative parameter (pHi) of the acidity of the aqueous liquid discharged from at least one measurement of the representative parameter (pHi) of the acidity of the aqueous liquid discharged; - maintaining the current spraying triggering period (Ti) constant with respect to the spraying triggering period (Ti-1) preceding the spraying triggering i when the current value of the representative parameter of acidity (pHi) is between a minimum acidity value (pHmin) and a threshold acidity value (pHthreshold); - increase in the current spray triggering period (Ti) compared to the spray triggering period (Ti-1) preceding the spray triggering i when the current value of the representative acidity parameter (pHi) is between the threshold acidity value (pHthreshold) and the maximum acidity value (pHmax); - decrease in the current spray triggering period (Ti) compared to the spray triggering period (Ti-1) preceding the spray triggering i when the current value of the representative acidity parameter (pHi) decreases below the minimum acidity value (pHmin) or increases above the maximum acidity value (pHmax).

2. A method according to claim 1, wherein the parameter representing the acidity of the discharged aqueous liquid is the pH of the discharged aqueous liquid.

3. A method according to claim 2, wherein the minimum acidity value (pHmin) is between 2 and 3, the threshold acidity value (pHthreshold) is between 2.5 and 3.5, and the maximum acidity value (pHmax) is between 8 and 9.

4. A method according to any one of the preceding claims, wherein the current value (pHi) of the representative acidity parameter is the minimum value of the representative acidity parameter of the aqueous liquid discharged during the spray triggering period (Ti-1) preceding the spray triggering i.

5. A method according to any one of the preceding claims, wherein the increase in the spray triggering period is implemented as long as the spray triggering period is less than a maximum spray triggering period (Tmax), the decrease in the spray triggering period being implemented as long as the spray triggering period is greater than a minimum spray triggering period (Tmin).

6. A method according to any one of the preceding claims, comprising, prior to determining the current value (pHi) of the representative parameter of acidity, determining a current value of a representative parameter (Rhi) of the humidity of the treated airflow (16) from at least one measurement of the representative parameter (Rhi) of the humidity of the treated airflow (16), the method comprising the following steps: if the current value of the representative parameter (Rhi) of the humidity of the treated airflow (16) is greater than a threshold humidity value (Rhmin), maintaining a triggering of a next spraying at the spraying triggering period; if the current value of the representative parameter (Rhi) of the humidity of the treated airflow (16) is less than the threshold humidity value (Rhmin), immediate triggering of a spraying i.

7. Method according to claim 6, wherein, in the case of an immediate triggering of a spray i occurring when the current value of the representative parameter (Rhi) of the humidity of the treated airflow (16) is less than the threshold humidity value (Rhmin), the spray triggering period (Ti-1) preceding the spray triggering i is equal to the elapsed time (Di) between the immediate triggering of the spray i and the triggering of the spray i-1 preceding the spray i.

8. A method according to any one of the preceding claims, wherein the supply of the airflow to be treated (12) comprises the passage of the airflow to be treated (12) through a fan (22) comprising a rotating air pumping element (70) having a controllable rotational speed. 18 9. Method according to claim 8, comprising determining a current value of a parameter representative of the pressure loss through the volume of porous filter medium (34) from at least one measurement of the parameter representative of the pressure loss through the volume of porous filter medium (34), and adjusting the rotational speed of the rotating member (70) of the fan (22) using the current value of the parameter representative of the pressure loss to maintain constant the flow rate of air to be treated (12) circulating in the volume of porous filter medium (34).

10. Method according to claim 9, wherein the parameter representing the pressure drop is a pressure of the airflow to be treated (12) measured downstream of the fan (22) and upstream of the volume of porous filter medium (34) and / or is a pressure difference determined from the pressure of the airflow to be treated (12) measured downstream of the fan (22) and upstream of the volume of porous filter medium (34).

11. A method according to any one of the preceding claims, wherein the porous filter medium (34) comprises mineral particles, in particular expanded clay particles, the microorganisms advantageously comprising heterotrophic or autotrophic bacteria.

12. Installation for treating an air stream to be treated (12), comprising: a biofilter (20) comprising: * a volume of porous filter medium (34), the porous filter medium (34) comprising microorganisms; * an inlet for the supply of the air flow to be treated (12) through the volume of porous filter medium (34); * a recovery outlet for a treated airflow (16) downstream of the porous filter medium volume (34), * a system configured to carry out successive waterings of the porous filter medium (34) at successive spraying trigger times, the system comprising an inlet (42) for spraying an aqueous liquid into the volume of porous filter medium (34); * an outlet for draining the aqueous liquid from the volume of porous filter medium (34), characterized by a control unit (26) of the biofilter (20) configured to, after each spraying: - determine at least one current value of the representative parameter (pHi) of the acidity of the discharged aqueous liquid from at least one measurement of the representative parameter (pHi) of the acidity of the discharged aqueous liquid; 19 - maintain the current spraying triggering period (Ti) constant with respect to the spraying triggering period (Ti-1) preceding the spraying triggering i when the current value of the representative parameter of acidity (pHi) is between a minimum acidity value (pHmin) and a threshold acidity value (pHthreshold); - increase the current spray triggering period (Ti) compared to the spray triggering period (Ti-1) preceding the spray triggering i when the current value of the representative acidity parameter (pHi) is between the threshold acidity value (pHthreshold) and the maximum acidity value (pHmax); - decrease the current spray triggering period (Ti) compared to the spray triggering period (Ti-1) preceding the spray triggering i when the current value of the representative acidity parameter (pHi) decreases below the minimum acidity value (pHmin) or increases above the maximum acidity value (pHmax).

13. Installation according to claim 12, wherein the control unit (26) is configured to: prior to determining the current value (pHi) of the representative parameter of acidity, determine at least one current value of a representative parameter of the humidity of the treated airflow (16) from at least one measurement of the representative parameter of the humidity of the treated airflow (16); if the current value of the representative parameter (RHi) of the humidity of the treated airflow (16) is greater than a humidity threshold value (RHmin), maintain a triggering of a next spraying at the spraying triggering period; if the current value of the representative parameter (RHi) of the humidity of the treated airflow (16) is less than the humidity threshold value (Rhmin), trigger a spraying immediately.

14. An installation according to any one of claims 12 or 13, wherein the control unit (26) is configured to: determine at least one current value of a parameter representative of the pressure drop through the volume of porous filter medium (34) from at least one measurement of the parameter representative of the pressure drop through the volume of porous filter medium (34), and 20 adjust the rotation speed of the rotating part (70) of the fan (22) using the current value of the parameter representing the pressure loss to maintain constant the flow rate of air to be treated (12) circulating in the volume of porous filter medium (34).

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