A method for odor recovery from wastewater treatment plants and its dedicated device

Abstract

This invention discloses a method and dedicated device for odor recovery from wastewater treatment plants, belonging to the field of wastewater and waste gas treatment technology. The invention includes a covered aerobic aeration tank, a pretreatment unit, a low-resistivity self-mixing device, a control system, and an aeration system. The method involves collecting odorous gas escaping from the aerobic tank in a sealed environment. After dust removal and dehumidification, an online detection device identifies the pollutant concentration and automatically switches between desulfurization and adsorption pretreatment paths. The system utilizes pressure difference to achieve non-powered mixing of odorous gas and fresh air. A PLC controller links the frequency converter of the blower to maintain the oxygen concentration of the mixed gas at 18.5%~19.0%. The treated gas is reused for aeration, while excess gas is discharged after end-of-pipe treatment. This invention achieves resource utilization of odorous gas, reduces deodorization air volume by 40%~50%, saves over 105,000 kWh of electricity annually, reduces carbon source addition by 15%, and is safe and controllable, reducing energy consumption and ensuring long-term stable operation of the biochemical treatment process.

Description

Technical Field

[0001] This invention belongs to the field of wastewater and waste gas treatment technology, specifically relating to a method for recycling odorous gases from a wastewater treatment plant and a dedicated device thereof. Background Technology

[0002] The odor generated during wastewater treatment mainly originates from units such as screens, grit chambers, and sludge dewatering. Besides inorganic odorous substances like H2S and NH3, it often contains volatile organic compounds (VOCs) such as alcohols, ketones, organic acids, and thiols. Traditional treatment methods often employ independent biological filters, activated carbon adsorption, or chemical scrubbing processes. While these can achieve emission standards, they generally involve high equipment investment, large land area requirements, complex operation and maintenance, and may generate secondary pollution such as spent activated carbon or waste liquid, resulting in poor economic efficiency and sustainability.

[0003] To reduce treatment costs, recent studies have explored introducing odorous gases into the main wastewater treatment system for in-situ disposal; for example:

[0004] 1. Chinese invention patent CN112047472A discloses a "system for simultaneous treatment of odor and wastewater", which uses a fan to introduce odor into A. 2 The anoxic zone of the / O-MBR reactor uses micro-aeration to achieve partial removal of dissolved oxygen, claiming to alleviate membrane fouling. However, this approach has certain limitations in practical applications: the dissolved oxygen concentration in the anoxic zone is typically controlled below 0.2 mg / L, while the efficient mineralization of typical volatile organic compounds (VOCs) such as ethanol, acetone, and ethyl acetate relies on aerobic metabolic pathways. Under anoxic conditions, these VOCs can only be converted to a limited extent through adsorption or slow facultative metabolism. Therefore, the removal efficiency for medium to high concentrations of VOCs is relatively low, and the additional organic load introduced may have a certain impact on the stability of the denitrification process.

[0005] 2. Chinese invention patent with publication number CN103041696A proposes to connect the odor pipeline to the existing blower aeration system and send it directly into the aerobic aeration tank, using activated sludge to degrade odorous substances, thereby reducing the construction of independent deodorization facilities.

[0006] While this solution has some value in simplifying engineering processes, it still has the following limitations in practical applications:

[0007] (1) The odor composition is complex and the oxygen content is unstable: Although the collected odor comes from a semi-enclosed space, its oxygen concentration is usually lower than the atmospheric level (16-20%) and fluctuates with the working conditions; if there is no real-time monitoring and compensation of oxygen supply, it is easy to cause insufficient dissolved oxygen in the aeration tank, which will affect the nitrification effect.

[0008] (2) Toxic components inhibit microbial activity: Odor often contains high concentrations of H2S, NH3, etc., which may inhibit nitrifying bacteria when directly introduced into the biochemical system, leading to a decrease in ammonia nitrogen removal rate;

[0009] (3) Risk of localized flammable gas enrichment: Although the system is open as a whole, in local enclosed areas such as the fan cavity and the low point of the pipeline, if the CH4 concentration is high and the ventilation is poor, there is still a risk of short-term enrichment to the lower explosive limit.

[0010] (4) Lack of excess gas emission mechanism: The scheme treats odor as a "fully usable gas source" without considering the dilution effect of non-degradable components (such as N2 and CO2) on the oxygen partial pressure of the mixed gas. Long-term operation may reduce aeration efficiency.

[0011] In summary, although existing technologies attempt to introduce odors into biochemical systems for in-situ disposal, they still suffer from the following common drawbacks:

[0012] (1) Inappropriate injection location - either high concentration of VOCs is sent into the anoxic area (cannot be effectively mineralized), or it is directly mixed into the aerobic aeration gas source (lacking component screening and purification).

[0013] (2) Gas source not distinguished - the gas source (such as the screen room, sludge dewatering room) was not classified and treated, resulting in high H2S and high NH3 gas directly impacting the biochemical system;

[0014] (3) Lack of oxygen supply regulation - The oxygen concentration in the recycled gas is not monitored and dynamically compensated in real time, making it difficult to maintain the stability of dissolved oxygen;

[0015] (4) The dilution effect of odor on oxygen supply capacity was not assessed: Odor oxygen content is usually lower than that of air and contains inert components such as N2 and CO2. Directly using it as an aeration source will reduce the oxygen partial pressure of the mixed gas. Due to the lack of O2 monitoring and air volume linkage control, long-term operation is prone to insufficient oxygen supply. If the odor contains CH4, it may also accumulate in local areas of the pipeline or fan for a short time, bringing the risk of combustion and explosion.

[0016] Therefore, there is an urgent need for a safe, controllable, and intelligent method for odor recovery; that can prioritize the use of low-toxicity, oxygen-enriched VOC-containing odor gas generated by the aerobic biological treatment tank itself as an auxiliary gas source, and achieve resource utilization of waste gas, reduce aeration energy consumption, and ensure the safe and stable operation of the odor recovery system by means of deep purification, oxygen mixing, closed-loop O2 control, and excess gas bypass, while ensuring the wastewater treatment effect. Summary of the Invention

[0017] The technical problem to be solved by this invention is to provide a method for recycling odorous gas from a wastewater treatment plant. This method involves purifying the odorous gas escaping from the aeration tank and recycling it in situ for aeration, thereby significantly reducing the total amount of gas that needs to be sent to the terminal deodorization system of the plant. Another technical problem to be solved by this invention is to provide a special device for recycling odorous gas from a wastewater treatment plant. This device adopts a low-resistance self-mixing device, which actively adjusts the oxygen content in the mixed gas by controlling the exhaust volume of the bypass fan and discharges excess gas, preventing the accumulation of inert components and preventing their circulation and accumulation in the system, thereby significantly improving oxygen transfer efficiency and nitrification stability.

[0018] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0019] A special device for odor recovery from a wastewater treatment plant includes a covered aerobic aeration tank, an odor collection pipe, a pretreatment unit, a control system, a low-resistance self-mixing device, a pipeline mixer, a negative pressure collection fan, a high-pressure fan, a bypass fan, an aeration system, and a plant odor treatment system. The covered aerobic aeration tank has a sealed cover plate on top, with an exhaust port connected to the odor collection pipe. The odor collection pipe branches into a main line and branch lines, with the branch lines directly connected to the plant odor treatment system. The pretreatment unit includes a bag filter and a cooling system. The condensate dehumidifier, dry desulfurizer, activated carbon adsorption box, dry ammonia filter, and emergency flare burner are connected in series or branched via pipelines. The low-resistance self-mixing device is installed between the outlet of the negative pressure collection fan and the high-pressure fan, and adopts a coaxial nested structure. The pipeline mixer connects the low-resistance self-mixing device to the high-pressure fan and the bypass fan. The aeration system is connected to the outlet of the high-pressure fan and extends to the bottom of the covered aerobic aeration tank. The control system is electrically connected to each detection device, electric valve, and fan frequency converter.

[0020] Furthermore, the main odor collection pipe is configured with an air volume of 100% of the standard aeration volume, and the branch pipes are configured with 10% of the aeration air volume to maintain a slight negative pressure of -50 to -100 Pa in the covered aerobic aeration tank.

[0021] Furthermore, the control system includes a front-end online detection device, a rear-end online detection device, a pipeline switching electric valve, a bypass fan outlet calorific gas flow meter, a pressure sensor, a PLC controller, and a frequency converter; the front-end online detection device includes an H2S online monitor, a CH4 online monitor, a VOC online monitor, and an NH3 online monitor; the rear-end online detection device includes an oxygen concentration analyzer and a VOC online monitor; the PLC controller is electrically connected to the front-end online detection device, the rear-end online detection device, the pipeline switching electric valve, the bypass fan outlet calorific gas flow meter, the pressure sensor, and the frequency converter.

[0022] Furthermore, the low-resistance self-mixing device includes a tangential odor inlet, a swirling odor buffer chamber, an air inlet, a nested annular jet mixing section, a condensate drain valve, and a mixed gas outlet; the tangential odor inlet is located on the upper side wall of the swirling odor buffer chamber, the bottom of the swirling odor buffer chamber is coaxially connected to the nested annular jet mixing section, the air inlet is located on the middle side wall of the nested annular jet mixing section, and the condensate drain valve is installed at the lowest point of the bottom of the swirling odor buffer chamber.

[0023] Furthermore, the electric valve includes K2 to K9, wherein K2 and K3 are the inlet and outlet pipes of the dry desulfurizer, K4 and K5 are the inlet and outlet pipes of the activated carbon adsorption box, K6 and K7 are the inlet and outlet pipes of the dry ammonia filter, and K8 and K9 are the inlet and outlet pipes of the emergency flare burner.

[0024] Furthermore, the negative pressure collecting fan is driven by a variable frequency motor, with a rated air volume of 110% of the aeration volume and an outlet pressure of +1~+3 kPa; the bypass fan is equipped with a throttling orifice plate on its pipeline.

[0025] Furthermore, the dry desulfurizer is filled with iron oxide packing, and the dry ammonia filter contains sulfuric acid impregnated packing; the aeration system includes microporous aeration discs or perforated pipes, which are evenly distributed at the bottom of the covered aerobic aeration tank through branch pipes.

[0026] Furthermore, the method for recycling odorous gases from a wastewater treatment plant includes the following steps:

[0027] (1) Odor collection in a closed loop: After the negative pressure collection fan is started, a micro negative pressure of -50~-100Pa is generated in the covered aerobic aeration tank. The odor is transported through the main odor collection pipe, and the branch pipe maintains the negative pressure balance.

[0028] (2) Preliminary pretreatment: The odorous gas is sequentially filtered by a bag filter for dust removal and then dehumidified by a condenser dehumidifier, reducing the relative humidity to ≤70%;

[0029] (3) Staged pretreatment: The online detection device monitors the pollutant concentration and automatically switches to the corresponding pretreatment unit. When CH4 ≥ 1.5%, it is introduced into the emergency flare burner;

[0030] (4) Non-powered mixing: The low-resistance self-mixing device uses pressure difference to draw in fresh air and mix it with odorous gas. The control system adjusts the frequency of the bypass fan according to the monitoring signal of the online detection device to maintain the oxygen concentration of the mixed gas at 18.5%~19.0%;

[0031] (5) Reuse of aeration: The mixed gas is pressurized by a high-pressure blower and introduced into the covered aerobic aeration tank through the aeration system;

[0032] (6) Excess gas treatment: Excess gas is transported to the odor treatment system in the plant area via a bypass fan and discharged in compliance with standards.

[0033] Further, in step (3), when H2S≥50ppm, switch to dry desulfurizer, when VOCs≥50ppm, switch to activated carbon adsorption box, and when NH3≥50ppm, switch to dry ammonia filter; in step (4), when VOCs concentration exceeds 50ppm again, the bypass fan frequency is increased to the high limit for forced exhaust.

[0034] Further, in step (4), the pressure sensor monitors the negative pressure of the mixing chamber of the low-resistivity self-mixing device as -30~-250Pa, and triggers an alarm when the deviation of the flow rate of the hot mass gas flow meter at the outlet of the bypass fan exceeds ±15%; the pressure difference of the low-resistivity self-mixing device is not less than 2kPa.

[0035] Compared with the prior art, the present invention has the following advantages:

[0036] (1) Odor recovery is safe and controllable: This invention effectively removes harmful components by using online monitoring of multiple parameters (H2S, CH4, NH3, VOC) and modular pretreatment units (desulfurizer, activated carbon, ammonia filter, flare) that automatically switch, avoiding the risk of toxic inhibition on the activated sludge system and ensuring long-term stable operation of biochemical treatment.

[0037] (2) Original low-resistance blending and tail-end diversion scheme: The present invention adopts a low-resistance self-blending device, which actively adjusts the oxygen content (18.5%~19.0%) in the blended gas by controlling the exhaust volume of the bypass fan, and discharges excess gas to prevent the accumulation of inert components and prevent them from circulating and accumulating in the system, thereby significantly improving oxygen transfer efficiency and nitrification stability.

[0038] (3) Reduce the load and energy consumption of the deodorization system: This invention significantly reduces the total amount of gas that needs to be sent to the deodorization system at the end of the plant by purifying the odor gas escaping from the aeration tank and then reusing it in situ for aeration; only the excess gas (about 15% of the total odor gas) exceeding the aeration requirements is directed into the deodorization system, which can reduce the overall deodorization air volume of the plant by 40% to 50% and reduce the additional energy consumption caused by the repeated transportation of odor gas. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the overall process flow of this application;

[0040] Figure 2 This is a schematic diagram of the low-resistivity self-mixing device of this application;

[0041] In the diagram, 1. Bag filter; 2. Condensation dehumidifier; 3. Control system; 4. Dry desulfurizer; 5. Activated carbon adsorption box; 6. Dry ammonia filter; 7. Negative pressure collection fan; 8. Emergency flare burner; 9. Low-resistance self-mixing device; 91. Tangential odor inlet; 92. Swirl odor buffer chamber; 93. Air inlet; 94. Nested annular jet mixing section; 95. Condensate drain valve; 96. Mixed gas outlet; 10. Pipeline mixer; 11. High-pressure fan; 12. Bypass fan; 13. Aeration system; 14. Covered aerobic aeration tank; 15. Odor collection pipe; 16. Plant odor treatment system. Detailed Implementation

[0042] The present invention will be further illustrated below with reference to specific embodiments. These embodiments are implemented based on the technical solutions of the present invention, and it should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0043] Process flow description:

[0044] like Figure 1-2 As shown, the present invention provides a special device for the odor recovery method of a wastewater treatment plant, including a bag filter 1, a condenser dehumidifier 2, a control system 3, a dry desulfurizer 4, an activated carbon adsorption box 5, a dry ammonia filter 6, a negative pressure collection fan 7, an emergency flare burner 8, a low-resistance self-mixing device 9, a tangential odor inlet 91, a swirling odor buffer chamber 92, an air inlet 93, a nested annular ejector mixing section 94, a condensate drain valve 95, a mixed gas outlet 96, a pipeline mixer 10, a high-pressure fan 11, a bypass fan 12, an aeration system 13, a covered aerobic aeration tank 14, an odor collection pipe 15, and a plant odor treatment system 16.

[0045] The top of the aerobic aeration tank 14 is equipped with a sealed cover to form a closed space for collecting gases that escape during the biochemical reaction. The exhaust port on the cover is fixedly connected to one end of the odor collection pipe 15 by flange or welding. The odor collection pipe 15 branches into a main line and a branch line. The main line is set according to the standard aeration volume of 100%, and the branch line is set according to 10% aeration volume. The branch line is directly connected to the plant odor treatment system 16 to maintain a slight negative pressure inside the enclosure. The main odor collection pipe 15 connects a bag filter 1 and a condenser dehumidifier 2 in series. The pipe connections are sealed with gaskets. The bag filter 1 removes dust and particulate matter from the odor, protecting other equipment in the system. The condenser dehumidifier 2 removes excess moisture from the odor, preventing condensate corrosion of the pipes and equipment. The outlet pipe of the condenser dehumidifier 2 branches via a tee connector, connecting to the inlets of the dry desulfurizer 4, activated carbon adsorption box 5, dry ammonia filter 6, and emergency flare burner 8. The dry desulfurizer 4 is filled with iron oxide packing, and the dry ammonia filter 6 contains sulfuric acid impregnated packing. The outlet pipes of the dry desulfurizer 4, activated carbon adsorption box 5, and dry ammonia filter 6 are merged via a manifold and connected to the inlet flange of the negative pressure collection fan 7. The outlet of the emergency flare burner 8 is directly open to the atmosphere.

[0046] The outlet pipe of the negative pressure collecting fan 7 is fixedly connected to the tangential odor inlet 91 of the low-resistance self-mixing device 9. The tangential odor inlet 91 is located on the upper side wall of the swirling odor buffer chamber 92 and is fixedly connected by tangential welding or flange, so that the odor enters the buffer chamber tangentially, forming a rotating airflow. The bottom outlet of the swirling odor buffer chamber 92 is coaxially connected to the upper inlet of the nested annular ejector mixing section 94 and fixed by sealed welding to ensure that the rotating airflow enters the annular channel of the mixing section smoothly and avoids airflow turbulence. The air inlet 93 is located on the middle side wall of the nested annular ejector mixing section 94 and is connected to the middle of the mixing section. The central channel is connected via pipe welding or quick-release flanges, allowing fresh air to directly enter the central channel and mix with the odorous gas in the annular channel through convection. The lower outlet of the nested annular jet mixing section 94 is coaxially and fixedly connected to the upper inlet of the mixed gas outlet 96. The connection is sealed with a corrosion-resistant gasket to ensure that the homogeneous gas after turbulent mixing can be stably discharged to the subsequent pipeline mixer. The condensate drain valve 95 is installed at the lowest point of the bottom of the swirling odor buffer chamber 92 and is connected by threads or flanges. It is used to drain the water accumulated by the airflow condensation during the operation of the device to prevent the accumulated liquid from affecting the mixing efficiency or corroding the equipment.

[0047] The mixing gas outlet 96 of the low-resistance self-mixing device 9 is connected to the inlet of the pipeline mixer 10 via a pipeline. The inner wall of the pipeline is smoothed, and the connection uses a quick-release flange. The air inlet 93 of the low-resistance self-mixing device 9 is connected to the ambient atmosphere via a pipeline. An inspection port is reserved on the pipeline. The annular gap width of the low-resistance self-mixing device 9 is 5-15mm. The ratio of the cross-sectional area of ​​the odor inlet 91 to the cross-sectional area of ​​the air inlet 93 is 1:0.7-1.2. The entire device is made of corrosion-resistant and smooth material. The outlet pipeline of the pipeline mixer 10 is divided into three sections via a tee joint. Two air intakes are connected: one to the inlet of the high-pressure blower 11 and the other to the inlet of the bypass blower 12. The bypass blower 12 has a throttling orifice plate 16, the diameter of which is calculated based on the rated airflow ratio (2.0-3.0:1) between the negative pressure collecting blower 7 and the bypass blower 12. This is used to maintain a stable flow ratio without a regulating valve and to prevent interference between the two blowers. The outlet pipe of the bypass blower 12 is connected to the plant odor treatment system 16, and a throttling orifice plate is connected in series on the pipe. The outlet pipe of the high-pressure blower 11 is connected to the main air intake pipe of the aeration system 13. The main air intake pipe of the aeration system 13 extends to the bottom of the covered aerobic aeration tank 14, and is evenly connected to several microporous aeration discs or perforated pipes through branch pipes. The branch pipes are arranged parallel to the bottom of the tank.

[0048] Differential pressure driven mixing mechanism: The mixing gas outlet 96 of the low-resistivity self-mixing device 9 is connected to the inlet of the pipeline mixer 10 through a pipe with a smooth inner wall (roughness Ra≤0.4μm), and the connection is made of a quick-release flange for easy maintenance; the air inlet 93 of the low-resistivity self-mixing device 9 is connected to the ambient atmosphere through a pipeline, and an inspection port is reserved on the pipeline.

[0049] 1. Key structural parameters

[0050] The annular gap width of the low-resistance self-mixing device 9 is set to 5-15 mm. This range is based on calculations of boundary layer thickness and shear force balance, ensuring sufficient air intake while maintaining the stability of the high-speed jet. The ratio of the cross-sectional area of ​​the odor inlet 91 to the cross-sectional area of ​​the air inlet 93 is controlled at 1:0.7-1.2. This ratio ensures that, under target operating conditions, the mass flow ratio of odor to fresh air is close to the stoichiometric ratio, avoiding local hypoxia or excessive dilution. The entire device is made of corrosion-resistant, smooth materials (such as FRP lined with PTFE or 316L stainless steel) to reduce frictional resistance and prevent corrosion products from flaking off and clogging the flow channels.

[0051] 2. The "Two-Stage Collaboration" Fluid Dynamics Principle and Low-Resistance Mechanism

[0052] The core mechanism by which this system achieves efficient mixing without consuming additional electrical energy lies in constructing a two-stage synergistic flow field: a "swirling ejector main mixer (low-resistance self-mixing device 9) + a short-range low-resistance pipeline mixer (pipeline mixer 10)". The specific principle is as follows:

[0053] First stage: Pressure difference driven and swirling main mixing (low-resistance self-mixing device 9)

[0054] The system utilizes the positive pressure (+1~3kPa) at the outlet of the front-end negative pressure collecting fan 7 and the negative pressure at the inlet of the rear-end high-pressure fan 11 to form a specific total pressure difference (ΔP≥2kPa). This pressure difference drives the odorous gas to pass through the tangential air inlet of the low-resistance self-mixing device 9 in a high-speed jet (flow velocity can reach 20-40m / s), instantly forming a strong rotating flow field.

[0055] Entrainment effect: According to Bernoulli's principle, high-speed rotating airflow generates a significant low-pressure vortex core at the nested annular gap, actively entraining fresh external air into the mixing core region at extremely high speeds without the need for additional power.

[0056] Turbulent breaking mechanism: The coupling of tangential and axial velocity components generates three-dimensional strong turbulent vortices within the low-resistivity self-mixing device 9. These large-scale vortices rapidly break down into micro-scale vortices, increasing the gas-phase interface area by several orders of magnitude, thereby completing 85%-90% of the macroscopic mixing task within an extremely short distance.

[0057] Low resistance characteristics: Since the low resistance self-mixing device 9 has no fixed blocking blades inside, it mainly relies on the kinetic energy conversion of the fluid itself and the streamlined swirling flow, and its local resistance loss is extremely low (<200Pa).

[0058] Second stage: Inertial continuation and secondary homogenization (pipe mixer 10)

[0059] The gas flowing out of the low-resistivity self-mixing device 9 is already in a state of high turbulence and high swirling intensity, and directly enters the series-connected pipe mixer 10.

[0060] Synergistic effect: Unlike traditional applications, the pipeline mixer 10 in this system no longer undertakes the main mixing work. Instead, it uses the residual kinetic energy and rotational inertia of the airflow given by the front-end low-resistance self-mixing device 9 to perform secondary averaging and flow field rectification of the airflow.

[0061] Resistance decoupling: Since the airflow entering the pipe mixer 10 is already in a highly turbulent state, the flow guiding element inside the pipe mixer 10 only needs to apply a very small disturbance to eliminate the local concentration gradient and improve the mixing uniformity from 90% to ≥95%. This design cleverly avoids the defect of traditional static mixers that "rely on high resistance shear to obtain mixing degree", so that the total resistance of the entire series system is still maintained at an extremely low level.

[0062] 3. Intelligent traffic diversion and stable operation mechanism

[0063] The outlet pipe of the pipe mixer 10 is divided into two paths via a tee connector: one path connects to the inlet of the high-pressure blower 11, and the other path connects to the inlet of the bypass blower 12. To solve the problem of mutual interference when the two blowers are operating in parallel, a throttling orifice plate 16 is installed on the pipe of the bypass blower 12. The orifice diameter of this orifice plate is precisely calculated based on the rated airflow ratio (2.0-3.0:1) between the negative pressure collecting blower 7 and the bypass blower 12. The throttling orifice plate acts as a "fluid resistance" here, matching the characteristic curves of the two blowers by increasing local resistance, thereby forcibly maintaining a stable flow split ratio under the condition of no dynamic regulating valve, preventing the other blower from surging or backflow due to fluctuations in the operating conditions of one blower. The outlet pipe of the bypass blower 12 is connected to the odor treatment system of the plant area, and a throttling orifice plate is connected in series on the pipe for further flow stabilization; the outlet pipe of the high-pressure blower 11 is directly connected to the main air inlet pipe of the aeration system 13. The main air inlet pipe of the aeration system 13 extends to the bottom of the covered aerobic aeration tank 14 and is evenly connected to several microporous aeration discs or perforated pipes through branch pipes. The branch pipes are arranged parallel to the bottom of the tank to ensure uniform distribution of dissolved oxygen.

[0064] Instrument control instructions:

[0065] The detection probes of the pre-online detection device K1 (including H2S online monitor AIT001, CH4 online monitor AIT002, VOC online monitor AIT003, and NH3 online monitor AIT004) are inserted into the manifold before the outlet of the condenser dehumidifier 2 through the reserved interface in the pipeline, and the probes are sealed to the pipeline wall; the pipeline switching electric valves K2~K9 are installed on the inlet pipelines of each pretreatment branch respectively. K2 and K3 correspond to the inlet and outlet pipelines of the dry desulfurizer 4, K4 and K5 correspond to the inlet and outlet pipelines of the activated carbon adsorption box 5, K6 and K7 correspond to the inlet and outlet pipelines of the dry ammonia filter 6, and K8 and K9 correspond to the inlet and outlet pipelines of the emergency flare burner 8. The control circuits of each electric valve are connected to the PLC controller. The pre-online detection device K1 analyzes the pollutant components of H2S, CH4, NH3, and VOC in the odor gas online, and controls the pipeline switching electric valves K2~K9 according to the set value to switch the odor gas pipeline for pretreatment. The detection probes of the post-online detection device K10 (containing oxygen concentration analyzer AIT005 and VOC online monitor AIT006) are inserted into the main pipeline after the outlet of the pipeline mixer, and the signal lines are connected to the PLC controller. The pressure sensor PT007 is installed on the swirling odor buffer chamber 92 of the low-resistance self-mixing device 9, and its signal lines are connected to the PLC controller. The hot mass gas flow meter K11 at the outlet of the bypass fan is connected in series on the outlet pipeline of the bypass fan, and its signal lines are connected to the PLC controller. The post-online detection device K10 monitors the oxygen concentration and VOC concentration of the gas at the outlet of the mixer, controls the frequency converter of the bypass fan, and connects the signals of the fan inlet pressure sensor PT007 and the bypass fan outlet flow meter K11 to achieve safe, stable, and efficient odor recovery. The PLC controller is connected to the frequency converters of the negative pressure collection fan 7, the high pressure fan 11, and the bypass fan 12 through control lines, and is also connected to the signal output terminals of each online detection device, pressure sensor PT007, and flow meter to realize signal reception and command issuance.

[0066] The working steps or principle of this device are as follows:

[0067] First, a sealed cover is installed on the top of the aerobic aeration tank 14 to form a closed space. After the negative pressure collection fan 7 is started, a micro negative pressure of -50~-100Pa is generated in the tank, which draws the odorous gas emitted from the biochemical reaction through the odor collection pipe 15 to the treatment link. At the same time, the branch of the odor collection pipe supplies gas to the plant odor treatment system 16 at 10% of the aeration air volume to maintain the negative pressure balance in the enclosure. The odorous gas first passes through the bag filter 1 to remove particulate matter, and then passes through the condenser dehumidifier 2 to reduce the relative humidity to ≤70%, completing the preliminary pretreatment. The online detection device monitors the gas in real time. The concentrations of H2S, CH4, NH3, and VOCs in the odor gas are automatically switched via a PLC controller to pretreatment paths, specifically removing the corresponding pollutants exceeding the standards. When H2S ≥ 50 ppm, electric valves K2 / K3 open, and the odor gas is desulfurized by dry desulfurizer 4. When CH4 ≥ 1.5%, electric valves K8 / K9 open, and the gas is introduced into the emergency flare burner 8 for combustion and emission. When NH3 ≥ 50 ppm, electric valves K6 / K7 open, and the odor gas is removed by dry ammonia filter 6. When VOCs ≥ 50 ppm, the electric valve... When K4 / K5 is activated, the odorous gas is adsorbed by the activated carbon adsorption box 5. The pretreated clean odorous gas is then transported to the low-resistance self-mixing device 9. Utilizing the pressure difference (≥2kPa) between the slightly positive pressure (+1~+3kPa) at the outlet of the negative pressure collecting fan 7 and the slightly negative pressure (≥2kPa) at the inlet of the high-pressure fan 11, fresh air is automatically drawn in, achieving turbulent mixing within the device. An online monitoring device then monitors the oxygen and VOC concentrations of the mixed gas. The oxygen concentration is maintained at 18.5%~19.0% by adjusting the frequency of the bypass fan 12. If VOCs exceed the standard, excess gas and residual gases are forcibly discharged. The remaining pollutants are discharged to the odor treatment system 16 in the plant area; the pressure sensor PT007 monitors the negative pressure of the mixing chamber, and the thermal mass gas flow meter K11 at the outlet of the ventilator verifies the actual air volume. The multi-source signal coordination ensures the stability of the system; the mixed and homogenized gas is pressurized by the high-pressure fan 11 and evenly transported to the bottom of the aeration tank through the aeration system 13 for use in biochemical reactions. The residual VOCs in the odor gas are used as a supplementary carbon source and degraded by microorganisms; the excess gas exceeding the aeration requirements is transported to the odor treatment system 16 in the plant area by the bypass fan 12 and discharged in compliance with standards after terminal purification.

[0068] Example 1

[0069] A method for reusing odor from a wastewater treatment plant includes the following steps:

[0070] (1) Odor gas containing 18% O2, 35ppm VOCs, 30ppm H2S, 25ppm NH3, and 0.8% CH4, escaping from the aerobic tank of a municipal wastewater treatment plant (100,000 tons / day treatment capacity), is collected in a sealed space formed by a sealed cover plate on the top of the aerobic aeration tank. After the negative pressure collection fan (rated air volume 8800 Nm³ / h, outlet pressure +2 kPa) is started, a slight negative pressure of -70 Pa is generated in the tank. The odor gas is transported to the treatment link through the main odor gas collection pipe (designed air volume 8000 Nm³ / h), while the branch pipe is aerated at 10% air volume (800 Nm³ / h). 3 / h) supplies gas to the odor treatment system in the plant area to maintain the negative pressure balance inside the enclosure;

[0071] (2) The odor first enters the bag filter to remove particulate matter such as fibers and dust, and then enters the condenser dehumidifier to reduce the relative humidity to 65% to complete the preliminary pretreatment;

[0072] (3) The online detection device detected that the concentration of each pollutant did not exceed the standard, so there was no need to switch to the targeted pretreatment unit. The clean odor gas was transported to the low-resistance self-mixing device through the manifold.

[0073] (4) The low-resistance self-mixing device utilizes the 3kPa pressure difference formed by the slightly positive pressure (+2kPa) at the outlet of the negative pressure collecting fan and the slightly negative pressure (-1kPa) at the inlet of the high-pressure fan to automatically draw in fresh ambient air, which is then mixed with odorous gases in a turbulent flow within the device. The online detection device then monitors the oxygen concentration and VOCs concentration of the mixed gas in real time, and adjusts the frequency of the bypass fan through the PLC controller to maintain the oxygen concentration at 18.7%.

[0074] (5) The pressure sensor monitors the negative pressure of the mixing chamber as -100Pa, and the flow meter at the outlet of the bypass fan verifies that the deviation between the actual air volume and the theoretical air volume is 5%, and the system is operating stably;

[0075] (6) The gas after mixing and homogenization is further mixed by the pipeline mixer, and after being pressurized by the high-pressure blower, it is evenly transported to the bottom of the aerobic tank through the microporous aeration disc of the aeration system. The residual VOCs in the odor gas are degraded by microorganisms as a supplementary carbon source.

[0076] (7) Excess gas exceeding the aeration requirements (accounting for about 15% of the total odor) is transported by a bypass fan to the plant’s biological filter for treatment and then discharged in compliance with standards.

[0077] The oxygen content of the mixed gas is stable at 18.7%, the standard deviation of DO fluctuation in the aeration tank is ±0.4mg / L, the terminal deodorization air volume is reduced to 4800Nm³ / h (a 40% reduction), the annual power saving reaches 105,000kWh, the carbon source addition is reduced by 15%, the ammonia nitrogen removal rate is ≥90%, and the system operation reliability is 99%.

[0078] Example 2

[0079] The difference from Example 1 is that in step (1), the micro negative pressure inside the covered aerobic aeration tank is set to -50Pa.

[0080] The mixed gas oxygen content was 18.6%, the standard deviation of DO fluctuation in the aeration tank was ±0.5 mg / L, the terminal deodorization air volume was 4900 Nm³ / h (reduced by 38.75%), the annual power saving was 102,000 kWh, the carbon source addition was reduced by 14%, the ammonia nitrogen removal rate was ≥90%, the system operation reliability was 98%, and the odor collection efficiency was slightly lower than that of Example 1, but there was no obvious overflow phenomenon.

[0081] Example 3

[0082] The difference from Example 1 is that in step (1), the micro negative pressure in the covered aerobic aeration tank is set to -100Pa.

[0083] The mixed gas oxygen content is 18.8%, the standard deviation of DO fluctuation in the aeration tank is ±0.4mg / L, the terminal deodorization air volume is 4800Nm³ / h (reduced by 40%), the annual power saving is 97,000 kWh (fan energy consumption increased by 8%), the carbon source addition is reduced by 15%, the ammonia nitrogen removal rate is ≥90%, the system operation reliability is 99%, there is no odor overflow but the economic efficiency is slightly reduced.

[0084] Example 4

[0085] The difference from Example 1 is that in steps (1) and (3), the online detection device detects that the H2S concentration in the odorous gas rises to 55ppm, and the PLC controller automatically opens the electric valves K2 and K3. The odorous gas enters the dry desulfurizer filled with iron oxide packing to remove H2S (the H2S concentration after treatment is ≤10ppm) and is then transported to the low-resistance self-mixing device.

[0086] The mixed gas oxygen content is 18.7%, the standard deviation of DO fluctuation in the aeration tank is ±0.4mg / L, the terminal deodorization air volume is 4800Nm³ / h (reduced by 40%), the annual power saving is 105,000 kWh, the carbon source addition is reduced by 15%, the ammonia nitrogen removal rate is ≥90%, the system operation reliability is 99%, and the inhibitory effect of high concentration H2S on nitrifying bacteria is effectively avoided.

[0087] Example 5

[0088] The difference from Example 1 is that in steps (1) and (3), the online detection device detects that the VOCs concentration in the odor rises to 58ppm, and the PLC controller automatically opens the electric valves K4 and K5. The odor enters the activated carbon adsorption box to adsorb VOCs (the VOCs concentration after treatment is ≤20ppm) and then is transported to the low-resistance self-mixing device.

[0089] The mixed gas oxygen content is 18.7%, the standard deviation of DO fluctuation in the aeration tank is ±0.4mg / L, the terminal deodorization air volume is 4800Nm³ / h (reduced by 40%), the annual power saving is 105,000 kWh, the carbon source addition is reduced by 15%, the ammonia nitrogen removal rate is ≥90%, the system operation reliability is 99%, and there is no sludge aging problem caused by VOCs accumulation.

[0090] Example 6

[0091] The difference from Example 1 is that in steps (1) and (3), the online detection device detects that the NH3 concentration in the odorous gas rises to 52ppm, and the PLC controller automatically opens the electric valves K6 and K7. The odorous gas enters the dry ammonia filter containing sulfuric acid impregnated packing to remove NH3 (the NH3 concentration after treatment is ≤8ppm) and then is transported to the low-resistance self-mixing device.

[0092] The mixed gas oxygen content is 18.7%, the standard deviation of DO fluctuation in the aeration tank is ±0.4mg / L, the terminal deodorization air volume is 4800Nm³ / h (reduced by 40%), the annual power saving is 105,000 kWh, the carbon source addition is reduced by 15%, the ammonia nitrogen removal rate is ≥90%, the system operation reliability is 99%, and the stability of the nitrification reaction is not affected by high concentration of NH3.

[0093] Example 7

[0094] The difference from Example 1 is that in step (4), the oxygen concentration of the mixed gas maintained by the low-resistivity self-mixing device is set to 18.5%.

[0095] The oxygen content of the mixed gas is stable at 18.5%, the standard deviation of DO fluctuation in the aeration tank is ±0.45mg / L, the terminal deodorization air volume is 4800Nm³ / h (reduced by 40%), the annual power saving is 103,000 kWh, the carbon source addition is reduced by 15%, the ammonia nitrogen removal rate is ≥88%, the system operation reliability is 99%, and the oxygen supply meets the basic needs of biochemical reactions.

[0096] Example 8

[0097] The difference from Example 1 is that in step (4), the oxygen concentration of the mixed gas maintained by the low-resistivity self-mixing device is set to 19.0%.

[0098] The oxygen content of the mixed gas is stable at 19.0%, the standard deviation of DO fluctuation in the aeration tank is ±0.4mg / L, the terminal deodorization air volume is 4800Nm³ / h (reduced by 40%), the annual power saving is 100,000 kWh (the energy consumption of fresh air mixing increases by 10%), the carbon source addition is reduced by 15%, the ammonia nitrogen removal rate is ≥90%, the system operation reliability is 99%, and there is no additional treatment benefit.

[0099] Example 9

[0100] The difference from Example 1 is that in step (4), the pressure difference of the low-resistance self-mixing device is adjusted to 2 kPa (negative pressure collecting fan outlet pressure +1 kPa, high pressure fan inlet negative pressure -1 kPa).

[0101] The mixed gas oxygen content is 18.7%, the standard deviation of DO fluctuation in the aeration tank is ±0.4 mg / L, the terminal deodorization air volume is 4800 Nm³ / h (reduced by 40%), the mixing efficiency is slightly reduced due to the reduced pressure difference, the bypass fan needs to increase the operating frequency by about 5% to maintain stable oxygen concentration, the annual power saving is slightly reduced to 103,000 kWh, the carbon source addition is reduced by 15%, the ammonia nitrogen removal rate is ≥90%, the system operation reliability is 98%, and the terminal treatment reagent consumption is about 20% higher than that of the biological filter.

[0102] Example 10

[0103] The difference from Example 1 is that in step (7), the excess gas is transported to the activated carbon adsorption box in the plant area for end treatment.

[0104] The mixed gas oxygen content is 18.6%, the standard deviation of DO fluctuation in the aeration tank is ±0.5mg / L, the terminal deodorization air volume is 4800Nm³ / h (reduced by 40%), the annual power saving is 100,000 kWh, the carbon source addition is reduced by 15%, the ammonia nitrogen removal rate is ≥90%, the system operation reliability is 99%, and the flue gas tracer test shows that the mixing uniformity of the system of this invention is ≥95% under standard operating conditions (Examples 1-9); in Example 10, due to the resistance fluctuation of the activated carbon adsorption box, the pressure disturbance of the bypass pipeline was caused, and the mixing uniformity dropped to 85%, which required slight compensation of energy consumption through the frequency conversion of the fan.

[0105] Comparative Example 1

[0106] (1) The odorous gas with the same composition as in Example 1, which escaped from the aerobic tank of a municipal sewage treatment plant, was deodorized using a traditional independent biological filter process without odor reuse. All the odorous gas (8000 Nm³) was directly expelled by an induced draft fan. 3 ( / h) is transported to the biological filter for treatment;

[0107] (2) The biological filter is equipped with a dedicated spray circulation system, packing material and chemical dosing device. During operation, the deodorizing agent is continuously consumed and the packing material is replaced regularly.

[0108] (3) The aeration system of the aerobic tank is equipped with fresh air supplied by a blower to maintain the stability of dissolved oxygen in the tank;

[0109] (4) The gas treated by the biological filter meets the emission standards.

[0110] The standard deviation of DO fluctuation in the aeration tank is ±0.8 mg / L, the terminal deodorization air volume is 8000 Nm³ / h (without reduction), the annual power saving is 0 kWh, the carbon source addition is not reduced, the ammonia nitrogen removal rate is ≥85%, the system operation reliability is 95%, but it generates secondary pollution such as waste packing material, and the operating cost is significantly higher than that of this invention.

[0111] Comparative Example 2

[0112] (1) The odorous gas with the same composition as in Example 1 that escaped from the aerobic tank of a municipal sewage treatment plant was directly connected to the aeration system through a pipeline in the existing "full reuse and direct discharge" mode without setting up a pretreatment unit and intelligent control system.

[0113] (2) The blower continuously inputs fresh air to mix with the odor, without monitoring or controlling the oxygen concentration and pollutant concentration of the mixture;

[0114] (3) The mixed gas is introduced into the aerobic tank through the aeration system, and there is no mechanism for bypassing and disposing of excess gas;

[0115] (4) During operation, the biochemical reaction needs are maintained solely by the characteristics of the aeration system itself.

[0116] Because H2S, NH3, and inert VOCs continuously circulate and accumulate in the biological treatment tank, the following results:

[0117] 1. H2S concentration exceeding 20 ppm on the 3rd day significantly inhibited nitrifying bacteria;

[0118] 2. NH3 enrichment leads to pH fluctuations and deterioration of sludge flocs;

[0119] 3. Long-term accumulation of VOCs leads to sludge aging.

[0120] Meanwhile, N2 and CH4 could not be discharged, and the oxygen content of the mixed gas dropped to 16.2% on day 5, with large fluctuations in DO (±1.1 mg / L). The ammonia nitrogen removal rate dropped from 85% initially to 52% on day 10, while SVI rose to 180 mL / g. On day 12, the system was forced to shut down due to a CH4 concentration reaching 2.3%, during which time multiple odor complaints were received from the plant area. Although the terminal deodorization airflow was 0 Nm³ / h, the biological system could no longer operate stably, and the treatment effect deteriorated significantly.

[0121] Comparative Example 3

[0122] (1) Using the technical solution of Chinese patent CN103041696A, the odor gas with the same composition as in Example 1 is directly connected to the original blower aeration system and sent into the aerobic aeration tank.

[0123] (2) No pollutant pretreatment unit or oxygen concentration monitoring and compensation device was installed; the air supply was adjusted only by the existing blower.

[0124] (3) There is no bypass exhaust mechanism, and all odorous gas is introduced into the aeration tank;

[0125] (4) Monitor the changes in dissolved oxygen and ammonia nitrogen removal rates in the aeration tank during operation.

[0126] On day 7, the oxygen content of the mixed gas decreased to 16.8%, and the dissolved oxygen (DO) fluctuated by ±1.1 mg / L. On day 15, the ammonia nitrogen removal rate decreased from 88% to 65%, and the total nitrogen in the effluent exceeded the standard. On day 18, sludge floated to the surface due to lack of oxygen, forcing the system to reduce its load. Although no air was vented to the deodorization system (air volume 0 Nm³ / h), the wastewater treatment function had clearly deteriorated, and there were safety risks.

[0127] In summary, this invention, through integrated design, achieves a stable oxygen content in the mixed gas at 18.6%~18.9%, reduces the standard deviation of DO fluctuation in the aeration tank from ±1.1 mg / L in existing technologies to ±0.4 mg / L, reduces the treatment air volume of the end-of-pipe deodorization system by 40% (from 8000 Nm³ / h to 4800 Nm³ / h), saves 100,000-105,000 kWh of electricity annually, and reduces carbon source dosage by 15%. It has significant effects in ensuring stable operation of the biochemical system, reducing energy consumption, and realizing the resource utilization of odor, and is safe and controllable, solving many defects of existing technologies.

[0128] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A device for the reuse of odorous air from a sewage plant, characterized in that: The application relates to a sealed aerobic aeration tank, an odor collection pipe, a pretreatment unit, a control system, a low-resistance self-mixing device, a pipeline mixer, a negative pressure collection fan, a high-pressure fan, a bypass fan, an aeration system and a plant area odor treatment system.

2. The dedicated apparatus for reusing odor gas of a sewage plant according to claim 1, characterized in that: The main road and the branch road of the odor collection pipe are connected with the plant area odor treatment system; the pretreatment unit comprises a bag filter, a condensation dehumidifier, a dry desulfurizer, an activated carbon adsorption box, a dry ammonia gas filter and an emergency flare combustor, which are connected in series through pipelines; the low-resistance self-mixing device is installed between the outlet of the negative pressure collection fan and the high-pressure fan and adopts a coaxial nested structure; the pipeline mixer is connected with the low-resistance self-mixing device, the high-pressure fan and the bypass fan; the aeration system is connected with the outlet of the high-pressure fan and extends to the bottom of the sealed aerobic aeration tank; and the control system is electrically connected with detection devices, electric valves and fan frequency converters.

3. The dedicated apparatus for reusing odor of sewage plant according to claim 1, characterized in that: The main road of the odor collection pipe is provided with air volume selected according to 100% of standard aeration amount, and the branch road is provided with air volume selected according to 10% of aeration amount, so as to maintain micro negative pressure of-50 to-100 Pa in the sealed aerobic aeration tank.

4. The special device for reusing odor of sewage plant according to claim 1, characterized in that: The control system comprises front and rear online detection devices, a pipeline switching electric valve, a bypass fan outlet thermal mass gas flowmeter, a pressure sensor, a PLC controller and a frequency converter; the front online detection devices comprise H2S, CH4, VOC and NH3 online monitors; the rear online detection devices comprise an oxygen concentration analyzer and a VOC online monitor; and the PLC controller is electrically connected with the front and rear online detection devices, the pipeline switching electric valve, the bypass fan outlet thermal mass gas flowmeter, the pressure sensor and the frequency converter.

5. The special device for reusing odor of sewage plant according to claim 1, characterized in that: The low-resistance self-mixing device comprises a tangential odor air inlet, a cyclone odor buffer cavity, an air inlet, a nested annular ejection mixing section, a condensate drainage valve and a mixed gas outlet; the tangential odor air inlet is arranged on the upper sidewall of the cyclone odor buffer cavity, the bottom of the cyclone odor buffer cavity is coaxially connected with the nested annular ejection mixing section, the air inlet is arranged on the middle sidewall of the nested annular ejection mixing section, and the condensate drainage valve is arranged at the lowest point of the bottom of the cyclone odor buffer cavity.

6. The special device for reusing odor of sewage plant according to claim 1, characterized in that: The electric valve comprises K2-K9, wherein K2 and K3 are dry desulfurizer inlet and outlet pipelines, K4 and K5 are activated carbon adsorption box inlet and outlet pipelines, K6 and K7 are dry ammonia gas filter inlet and outlet pipelines, and K8 and K9 are emergency flare combustor inlet and outlet pipelines.

7. The special device for reusing odor of sewage plant according to claim 1, characterized in that: The negative pressure collection fan is driven by a frequency conversion motor, the rated air volume is 110% of the aeration amount, and the outlet pressure is +1 to +3 kPa; and the pipeline of the bypass fan is provided with a throttle orifice plate.

8. The method according to any one of claims 1 to 7, wherein the method is a method for recycling odorous air from a sewage plant, characterized by: The dry desulfurizer is filled with iron oxide fillers, and the dry ammonia gas filter contains sulfuric acid impregnated fillers; and the aeration system comprises a microporous aeration disc or a perforated pipe which is uniformly distributed at the bottom of the sealed aerobic aeration tank through branch pipes. The application further discloses a sealed aerobic aeration tank operation method. (1) Odor closed collection: After the negative pressure collection fan is started, -50~ -100 Pa micro negative pressure is generated in the covered aerobic tank, and the odor is transported through the main road of the odor collection pipe, and the branch road maintains negative pressure balance; (2) Preliminary pretreatment: The odor is sequentially dedusted by a bag filter and dehumidified by a condensation dehumidifier, and the relative humidity is reduced to ≤70%; (3) Hierarchical pretreatment: The online detection device monitors the pollutant concentration, and automatically switches to the corresponding pretreatment unit, and when CH4≥1.5%, it is introduced into the emergency flare burner; (4) Unpowered mixing: The low-resistance self-mixing device inhales fresh air and odor for mixing by using pressure difference, and the control system adjusts the frequency of the bypass fan according to the monitoring signal of the online detection device to maintain the oxygen concentration of the mixed gas at 18.5%~19.0%; (5) Reuse aeration: The mixed gas is pressurized by a high-pressure fan and introduced into the covered aerobic tank through an aeration system; (6) Excess gas disposal: The excess gas is transported to the plant area odor treatment system by a bypass fan for standard discharge.

9. The method of claim 8, wherein: In step (3), when H2S≥50ppm, switch to a dry desulfurizer, when VOCs≥50ppm, switch to an activated carbon adsorption box, and when NH3≥50ppm, switch to a dry ammonia gas filter; In step (4), when the VOCs concentration exceeds 50ppm again, the bypass fan frequency is increased to the high limit value to force exhaust.

10. The method of claim 8, wherein: In step (4), the pressure sensor monitors the negative pressure of the mixing cavity of the low-resistance self-mixing device to be -30~ -250 Pa, and the bypass fan outlet thermal mass gas flow meter triggers an alarm when the air volume deviation exceeds ±15%; The pressure difference of the low-resistance self-mixing device is not less than 2kPa.

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