Online volatile fatty acid monitoring system for water quality

Abstract

The invention provides an on-line volatile fatty acid monitoring system for water quality, which can effectively eliminate or reduce the influence of main interferents such as ammonia nitrogen to the greatest extent, realizes high-precision, high-selectivity and full-automatic on-line continuous monitoring, and has good economical efficiency and stability. The system comprises: a sampling and pretreatment unit, which is used for extracting a water sample and removing solid particles and suspended solids in the water sample through an on-line filtering device; the ammonia nitrogen separation unit is used for carrying out first distillation stripping operation on the pretreated water sample; the VFA separation unit is used for acidizing the water sample from which ammonia nitrogen is removed, and then carrying out secondary distillation stripping operation in an accurately controlled acidic environment; the VFA quantifying unit is used for absorbing VFA steam separated from the secondary distillation process by using quantitative set absorption liquid, measuring the conductivity change and real-time temperature of the absorption liquid before and after absorption through a sensor, and calculating the VFA concentration based on the change; and a control and data processing unit.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of water quality analysis, in particular to an online volatile fatty acid monitoring system for water quality. BACKGROUND

[0002] As an important organic parameter, the concentration monitoring of volatile fatty acid (VFA) has key significance in anaerobic digestion efficiency evaluation, sewage treatment process control, fermentation process optimization, and environmental pollution indication. At present, the determination methods of VFA mainly include laboratory analysis methods and partial online monitoring technologies.

[0003] Laboratory analysis methods include gas chromatography and titration method. The gas chromatography has high accuracy and sensitivity, but it is particularly expensive and complicated to operate, and requires professional analysts and long analysis period, which is difficult to meet the demand of online detection. The titration method is difficult to realize online continuous detection due to multiple operation steps, time and labor consumption, and low automation degree. Therefore, both the gas chromatography and the titration method are analyzed in the laboratory.

[0004] The existing partial online monitoring technology is online colorimetric method or spectroscopic method. The colorimetric method utilizes the color change generated by the reaction of VFA with a specific color developing agent, and measures the absorbance by a photometer for quantification. The spectroscopic method (such as NIRS) utilizes the absorption or scattering characteristics of VFA molecules in a specific spectral region (such as near-infrared region), and analyzes by establishing a chemometrics model of spectrum and concentration. However, in actual use, the reagent consumption of the colorimetric method is large, and the long-term operation cost is high. Moreover, the color developing reaction may be interfered by water sample turbidity, color, and other coexisting reducing or oxidizing substances. The selectivity of some color developing reactions is not high, and may still be affected by ammonia nitrogen and other substances, which is a problem to be solved.

[0005] The spectroscopic method (such as NIRS) has a complex model establishment and is highly dependent on a large number of representative samples for calibration. The model is very sensitive to changes in water sample matrix (such as temperature, pH, salinity, and suspended solids content). Once the actual sample matrix deviates from the calibration set, the prediction accuracy of the model may decrease significantly. It is still a great challenge to accurately remove specific interferents (such as high concentration of ammonia nitrogen) in complex water samples.

[0006] Therefore, there is an urgent need to develop a VFA monitoring system that can effectively eliminate or minimize the influence of ammonia nitrogen and other main interferents, achieve high precision, high selectivity, full-automatic online continuous monitoring, and has good economy and stability, in order to meet the increasing demand for fine process control and environmental regulation. SUMMARY

[0007] In view of the above problems, the present application provides an online volatile fatty acid monitoring system for water quality, which can effectively eliminate or minimize the influence of ammonia nitrogen and other main interference substances, and realize high-precision, high-selectivity, full-automatic online continuous monitoring, and has good economy and stability.

[0008] An online volatile fatty acid monitoring system for water quality, characterized in that it comprises:

[0009] a sampling and pretreatment unit for automatically, regularly and quantitatively extracting water samples from a monitoring point according to a preset program, and removing solid particles and suspended solids in the water samples through an online filtering device;

[0010] an ammonia nitrogen separation unit for performing a first distillation stripping operation on the pretreated water samples, so that the ammonia nitrogen in the water samples is efficiently converted into free ammonia and volatilized with a carrier gas to be separated and removed, while the VFA is stably retained in the water samples in the form of its non-volatile salt;

[0011] a VFA separation unit for performing acidification treatment on the water samples from which the ammonia nitrogen has been removed, and then performing a second distillation stripping operation in an accurately controlled acidic environment, so that the VFA in the water samples is volatilized and separated to obtain VFA vapor for subsequent absorption and quantification;

[0012] a VFA quantification unit for absorbing the VFA vapor separated from the second distillation process using a set amount of absorption liquid, and measuring the change in conductivity and real-time temperature of the absorption liquid before and after absorption through a sensor, and calculating the VFA concentration based on the change;

[0013] and a control and data processing unit for integrated control of the automatic operation;

[0014] The control and data processing unit controls and connects the electrical elements in the sampling and pretreatment unit, the ammonia nitrogen separation unit, the VFA separation unit, and the VFA quantification unit.

[0015] Further characterized in that:

[0016] The sampling and pretreatment unit comprises a corrosion-resistant sampling pump, an online filter, a multi-channel switching valve, and a liquid level sensor;

[0017] The multi-channel switching valve connects the water sample, the cleaning liquid, and the calibration liquid through a water circuit, so that the water circuit can be cleaned or calibrated online, and the liquid level sensor is used to confirm the success of sampling to prevent empty pumping;

[0018] The ammonia nitrogen separation unit comprises a first distillation still, an alkali supply system, an online PH monitoring and control system, a carrier gas supply and control system, a high-efficiency condenser, and an ammonia nitrogen capture / treatment device;

[0019] The distillation temperature of the ammonia-nitrogen separation unit is controlled at 85-95℃, the pH value is controlled at 9.5-11.0, and the carrier gas purging time is 5-15 minutes;

[0020] The first distillation kettle is made of high-temperature-resistant and corrosion-resistant material, is equipped with an external heating module, a temperature sensor, a liquid level sensor, and an optional stirring device;

[0021] The alkali supply system includes an alkali storage tank and a precision metering pump;

[0022] The online pH monitoring and control system includes a high-temperature-resistant and alkali-resistant online pH electrode and a transmitter, which is used to monitor the pH value of the water sample in the first distillation kettle in real time, and to accurately maintain the pH at the set target alkalinity value by controlling the injection rate of the alkali metering pump through closed-loop feedback;

[0023] The carrier gas supply and control system includes an inert gas source, a pressure reducing valve, a gas filter, and a mass flow controller, which is used to accurately control the flow rate of the carrier gas entering the distillation kettle;

[0024] The high-efficiency condenser is used to condense the NH3-containing steam volatilized from the first distillation kettle;

[0025] The ammonia-nitrogen capture / treatment device directs the condensed NH3 gas to an ammonia-nitrogen absorption trap or directly to a waste gas treatment system for safe discharge;

[0026] The VFA separation unit includes a second distillation kettle, an acid supply system, an online pH monitoring and control system, a carrier gas supply and control system, and a high-efficiency condenser;

[0027] The VFA separation unit operates at a distillation temperature of 90-105, slightly higher than the boiling point of water under normal pressure to enhance volatilization, or uses reduced pressure distillation, and a pH value of 2.0-3.0, with a carrier gas purging time of 10-20 minutes;

[0028] The second distillation kettle is made of high-temperature-resistant and corrosion-resistant material, is equipped with an external heating module, a temperature sensor, a liquid level sensor, and an optional stirring device;

[0029] The acid supply system includes an acid storage tank and a precision metering pump;

[0030] The online pH monitoring and control system includes an acid-resistant online pH electrode and a transmitter, which is used to monitor the pH value of the water sample in the second distillation kettle in real time, and to accurately maintain the pH at the set target acidity value by controlling the injection rate of the acid metering pump through closed-loop feedback;

[0031] Preferably, the first distillation kettle and the second distillation kettle are the same distillation kettle, and the distillation kettle enters the VFA separation operation after the ammonia-nitrogen separation operation is completed, which saves equipment cost;

[0032] The VFA quantification unit comprises a VFA absorption cell, a high-precision flow-through conductivity electrode, a quantitative absorption liquid supply module, and a waste liquid discharge valve.

[0033] The VFA absorption cell adopts a structure of a serpentine tube, a porous medium packed column, a bubble column, or a micro-channel reactor, and is made of an inert material that does not adsorb VFA;

[0034] The high-precision flow-through conductivity electrode is selected from a conductivity sensor that has fast response, good stability, and strong anti-pollution ability, and is provided with an automatic temperature compensation function.

[0035] The quantitative absorption liquid supply module comprises an absorption liquid storage tank and a precision pump.

[0036] The waste liquid discharge valve is used to discharge the absorption liquid that has absorbed VFA after measurement is completed.

[0037] The control and data processing unit integrates control of all automatic operations, and collects raw data from various sensors such as a pH electrode, a temperature sensor, a conductivity electrode, and a liquid level sensor in real time: executes an embedded VFA concentration calculation algorithm, and displays real-time data, historical trends, alarm information, and device status on a human-machine interaction interface; stores data of measurement results and operation logs; executes a fault self-diagnosis and alarm program; and performs bidirectional communication with an upper computer monitoring system, a distributed control system, or a cloud data platform through a standard industrial communication protocol.

[0038] The control and data processing unit comprises an industrial-grade PLC or a high-performance embedded microprocessor, a high-precision multi-channel analog input / output module, a digital input / output module, a color touch screen, a large-capacity non-volatile data storage, and a plurality of standard communication interface modules.

[0039] After the above technical scheme, the quantitative water sample is introduced into the ammonia-nitrogen separation unit after online automatic sampling, and the pretreated water sample is subjected to first distillation stripping operation under the alkaline environment of the ammonia-nitrogen separation unit, so that the ammonia-nitrogen (NH3 / NH4+) in the water sample is efficiently converted into free ammonia (NH3) and is separated and removed with the carrier gas, while ensuring that the VFA is stably retained in the water sample in the form of its non-volatile salt under this pH condition; then the water sample (or its residual liquid) from which the ammonia-nitrogen has been removed is subjected to acidification treatment in the VFA separation unit, and then subjected to second distillation stripping operation under the precisely controlled acidic environment, so that the VFA (which has been converted into its volatile molecular form at this time) in the water sample is efficiently volatilized and separated to generate VFA vapor, and then a specific and quantitative absorption liquid (such as high-purity deionized water or a very dilute NaOH standard solution) is used to efficiently absorb the VFA vapor separated from the second distillation process, and the change in conductivity and real-time temperature of the absorption liquid before and after (or during) absorption is precisely measured by a high-precision conductivity sensor, and the VFA concentration is calculated based on the change; it can effectively eliminate or minimize the influence of ammonia-nitrogen type main interference substances, and realize high-precision, high-selectivity, full-automatic online continuous monitoring, and has good economy and stability. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 is a structural schematic diagram of a specific embodiment of the present application;

[0041] Figure 2 is a result graph of VFA detection of simulated wastewater A by GC method using the device of the specific embodiment of the present application;

[0042] Figure 3 is a result graph of VFA detection of simulated wastewater B by GC method using the device of the specific embodiment of the present application;

[0043] Figure 4 is a result graph of VFA detection of three sewage samples by CG method using the device of the specific embodiment of the present application. DETAILED DESCRIPTION

[0044] An online volatile fatty acid monitoring system for water quality, see Figure 1 , which comprises a sampling and pretreatment unit, an ammonia-nitrogen separation unit, a VFA separation unit, a VFA quantification unit, and a control and data processing unit.

[0045] The sampling and pretreatment unit is used to automatically, regularly and quantitatively extract water samples from monitoring points according to a preset program, and remove solid particles and suspended solids in the water samples through an online filtration device.

[0046] The ammonia-nitrogen separation unit performs a first distillation stripping operation on the pretreated water sample, so that the ammonia-nitrogen in the water sample is efficiently converted into free ammonia and volatilized with the carrier gas to be separated and removed, while the VFA is stably retained in the water sample in the form of its non-volatile salt;

[0047] The VFA separation unit acidifies the water sample from which the ammonia-nitrogen has been removed, and then performs a second distillation stripping operation in a precisely controlled acidic environment, so that the VFA in the water sample is volatilized and separated to obtain VFA vapor for subsequent absorption and quantification;

[0048] The VFA quantification unit uses a set amount of absorption liquid to absorb the VFA vapor separated from the second distillation process, and measures the change in conductivity and real-time temperature of the absorption liquid before and after absorption through a sensor, and calculates the VFA concentration based on the change;

[0049] The control and data processing unit is used for integrated control of automatic operation;

[0050] The control and data processing unit controls and connects the electrical elements in the sampling and pretreatment unit, ammonia-nitrogen separation unit, VFA separation unit, and VFA quantification unit, respectively.

[0051] In specific implementation, the sampling and pretreatment unit is integrated with a pretreater, which is further connected with an air compressor, and includes a corrosion-resistant sample pump, an online filter, a multi-flow channel switching valve, a liquid level sensor, and a backflow adjusting pump;

[0052] The multi-flow channel switching valve connects the water sample, cleaning liquid, and calibration liquid through a water circuit, so that the water circuit can be cleaned or calibrated online, and the liquid level sensor is used to confirm successful sampling to prevent empty pumping.

[0053] In specific implementation, the ammonia-nitrogen separation unit and the VFA separation unit are integrated in a distillation kettle, which is connected with an alkali solution supply system, an acid solution supply system, an online PH monitoring and control system, a carrier gas supply and control system, a high-efficiency condenser, an ammonia-nitrogen capture / treatment device, and a pure water pump-in mechanism;

[0054] The distillation temperature of the ammonia-nitrogen separation unit is controlled at 85-95°C, the PH value is controlled at 9.5-11.0, and the carrier gas blowing time is 5-15 minutes;

[0055] The distillation temperature of the VFA separation unit is controlled at 90-105, which is slightly higher than the boiling point of water under normal pressure to enhance volatilization or uses reduced pressure distillation, the PH value is controlled at 2.0-3.0, and the carrier gas blowing time is 10-20 minutes;

[0056] The distillation kettle is made of high-temperature-resistant and corrosion-resistant materials, is equipped with an external heating module, temperature sensors, and liquid level sensors, and is selected with a stirring device.

[0057] The alkali solution supply system comprises an alkali solution storage tank and a precision metering pump;

[0058] The acid solution supply system comprises an acid solution storage tank and a precision metering pump;

[0059] The on-line PH monitoring and control system comprises a high-temperature-resistant, acid-resistant and alkali-resistant on-line PH electrode and transmitter, which is used for monitoring the PH value of the water sample in the first distillation kettle in real time, and controlling the injection rate of the alkali solution metering pump through closed-loop feedback to accurately maintain the PH at the set target PH value, i.e., 10.0±0.2 to 11.5±0.2 when ammonia-nitrogen is separated, and 2.0±0.2 to 3.0±0.2 when VFA is separated;

[0060] The carrier gas supply and control system comprises an inert gas source, a pressure reducing valve, a gas filter and a mass flow controller, which are used for accurately controlling the flow rate of the carrier gas into the distillation kettle;

[0061] The high-efficiency condenser is used for condensing the NH3-containing steam volatilized from the first distillation kettle;

[0062] The ammonia-nitrogen trapping / treatment device guides the condensed NH3 gas into an ammonia-nitrogen absorption trap or directly into a waste gas treatment system for safe discharge;

[0063] The distillation kettle enters the VFA separation operation after the ammonia-nitrogen separation operation, which saves the equipment cost.

[0064] In specific implementation, the VFA quantification unit comprises a VFA absorption pool, a high-precision flow-through conductivity electrode, a quantitative absorption liquid supply module and a waste liquid discharge valve:

[0065] The VFA absorption pool adopts a serpentine tube, a porous medium packed column, a bubble column or a micro-channel reactor structure, and the material is selected to be inert and not to absorb VFA;

[0066] The high-precision flow-through conductivity electrode selects a conductivity sensor with fast response, good stability and strong anti-pollution ability, and is provided with an automatic temperature compensation function;

[0067] The quantitative absorption liquid supply module comprises an absorption liquid storage tank and a precision pump;

[0068] The waste liquid discharge valve is used for discharging the absorption liquid having absorbed VFA after measurement is completed;

[0069] The control and data processing unit controls all automated operations and collects raw data from various sensors such as pH electrode, temperature sensor, conductivity electrode, liquid level sensor in real time: executes built-in VFA concentration calculation algorithm and displays real-time data, historical trends, alarm information, device status on the human-machine interface; stores measurement results and operation logs; performs fault self-diagnosis and alarm program; and communicates with the upper computer monitoring system, distributed control system or cloud data platform through standard industrial communication protocol.

[0070] The control and data processing unit includes an industrial-grade PLC or a high-performance embedded microprocessor, a high-precision multi-channel analog input / output module, a digital input / output module, a color touch screen, a large-capacity non-volatile data storage, and a variety of standard communication interface modules.

[0071] The core process steps of the online VFA automatic analysis method are as follows:

[0072] S1. Sample collection and pretreatment stage:

[0073] S101. Automatic sampling: the control unit starts the sampling pump at a preset time interval or an external trigger signal, selects the water sample flow path through the multi-channel switching valve, and extracts a set volume (e.g. 50-100 mL) of water sample;

[0074] S102. Online filtration: the extracted water sample flows through the online filter to remove solid particles and larger suspended solids, and the purified water sample is sent to the first distillation kettle.

[0075] S2. First distillation-ammonia nitrogen selective exclusion stage:

[0076] S201. Water sample alkalization: the control unit instructs the alkali precise metering pump to inject an appropriate amount of strong alkali solution (such as 1 mol / L NaOH) into the first distillation kettle, the online pH electrode in the kettle monitors the pH value in real time, and the alkali injection amount is accurately adjusted through the PID feedback control algorithm until the water sample pH reaches and stabilizes at the preset strong alkaline target value (e.g. pH 10.5±0.1),

[0077] S202. Heating and stripping distillation: start the heating module of the first distillation kettle to quickly heat the alkalized water sample in the kettle to a preset distillation temperature (e.g. 90℃±1℃), at the same time, start the carrier gas supply system to blow off inert carrier gas (such as N2) into the kettle at a constant, preset flow rate (e.g. 0.5 L / min), this process lasts for a preset time (e.g. 10 minutes). Under this pH and temperature condition, most of the ammonia nitrogen (NH4+) in the water sample is converted into free ammonia (NH3), and is volatilized out with the carrier gas and water vapor; VFA exists in the liquid phase in the form of its non-volatile salt (such as sodium acetate);

[0078] S203. Ammonia nitrogen vapor treatment: the NH3-containing steam volatilized from the first distillation kettle is condensed by a high-efficiency condenser, and if an ammonia nitrogen synchronous determination unit is configured, it is introduced into an ammonia nitrogen absorption trap containing a known volume of dilute acid absorption liquid for absorption and conductivity measurement; otherwise, it is directly introduced into a waste gas treatment system for safe discharge.

[0079] S3. Second distillation-VFA selective extraction stage:

[0080] S301. Water sample acidification: after the first distillation is completed (the residual liquid in the kettle is the water sample with ammonia nitrogen removed), the control unit instructs the acid liquid precision metering pump to inject an appropriate amount of non-volatile strong acid solution (such as 10% H3PO4) into the second distillation kettle, the online pH electrode in the kettle monitors the pH value in real time, and the acid liquid injection amount is accurately adjusted through the PID feedback control algorithm until the water sample pH reaches and stabilizes at the preset strong acid target value (for example, pH 2.5±0.1);

[0081] S302. Heating and stripping distillation: start the heating module of the second distillation kettle, quickly heat the acidified water sample in the kettle to the preset second distillation temperature (for example, 98℃±1℃), at the same time, start the carrier gas supply system, and introduce inert carrier gas into the kettle at a constant, preset flow rate for stripping. This process lasts for a preset time (for example, 15 minutes), under this pH and temperature condition, VFA (such as acetate CH3COO-) is converted into its volatile molecular form (such as acetic acid CH3COOH), and is efficiently volatilized with carrier gas and water vapor;

[0082] When the first distillation kettle and the second distillation kettle are the same distillation kettle, the first distillation and the second distillation of the two distillation kettles are operated sequentially and at intervals.

[0083] S4. VFA absorption and conductivity precision measurement stage:

[0084] S401. VFA vapor absorption: the VFA-containing steam (mainly VFA and water vapor, and carrier gas) volatilized from the second distillation kettle and condensed by a high-efficiency condenser is directly introduced into a VFA absorption pool pre-injected with a precise volume (for example, 50mL) of high-purity deionized water (or extremely dilute NaOH standard solution) with known initial conductivity (oinitial) by the absorption liquid supply module, the special structure of the absorption pool (such as bubbling, spraying, serpentine channel, etc.) ensures that the VFA vapor and the absorption liquid are in full contact, realizing efficient, rapid and complete absorption of VFA;

[0085] S402. Conductivity and temperature synchronous measurement: During or after the VFA absorption process, the high-precision flow-through conductivity electrode and precision temperature sensor in the VFA absorption cell measure the conductivity value (o final) and real-time temperature (T) of the absorption liquid in real time and synchronously. The measurement can continue until the conductivity reading stabilizes or the preset maximum absorption time is reached.

[0086] S5. VFA concentration calculation and result output stage:

[0087] S501. Raw data acquisition and processing: The control and data processing unit acquires the conductivity values before and after absorption (o initial, o final) and corresponding temperature values (T initial, T final or average temperature T avg), and calculates the net conductivity change value of the absorption liquid

[0088] o raw = o final - o initial;

[0089] S502. Temperature compensation correction: Using the built-in temperature compensation algorithm optimized for the absorption liquid and target VFA ion system (for example, a linear compensation formula based on standard temperature 25℃:△o_compensated=△o_raw / [1+α(T_avg-25)], where α is the conductivity temperature coefficient of the absorption liquid), the original net conductivity change value is corrected to the conductivity change value at the standard reference temperature△o_compensated;

[0090] S503. VFA concentration calculation: According to the calibration curve (for example, C_VFA=k×△o_compensated+b, or a more complex polynomial fitting model) established and stored in the system through a series of calibration experiments with known concentration VFA standard solutions (usually in terms of acetic acid equivalent or specific VFA components), the corrected conductivity change value△o_compensated is used to calculate the concentration of VFA in the water sample (unit: mg / L or mmol / L);

[0091] S504. Result display, storage and communication: The calculated VFA concentration value is displayed in real time on the human-machine interface of the device, and can be displayed in the form of charts or lists with historical data. Measurement results, raw data, operating parameters and alarm information are automatically recorded and stored in the internal large capacity memory for reference and traceability. At the same time, the measurement results can be uploaded in real time to the user's upper monitoring system (SCADA / DCS) or cloud data platform through the configured industrial communication interface (such as RS485 Modbus, Ethernet).

[0092] S6. System automatic cleaning and reset stage:

[0093] S601. Automatic cleaning procedure: After a complete measurement and analysis cycle, the control unit automatically initiates a preset cleaning procedure. Through precise switching of the multi-channel valve system, one or more cleaning solutions (e.g., 0.1M HCI solution for removing alkaline residues and carbonate scale, 0.1M NaOH solution for removing organic matter and acidic residues, and finally a large amount of deionized water for thorough rinsing) in the storage tanks are used in sequence (or in combination) to fully and effectively clean the first distillation still, the second distillation still, the VFA absorption tank, all related fluid lines, and key components such as pH electrodes and conductivity electrodes, to remove sample residues, reaction byproducts, reagent traces, and possible scale or biofilm formation, preventing cross-contamination and system blockage.

[0094] S602. System emptying and resetting: After cleaning is complete, the system automatically empties all residual liquids in the lines and reaction stills into the waste liquid collection container. All valve, pump, and other actuator mechanisms return to the initial standby state, and the pH electrode and conductivity electrode readings return to the baseline level (or close to zero), preparing the entire device for the next automatic measurement and analysis.

[0095] S603. 3. Automatic calibration procedure: According to the preset time interval (e.g., once every 24 hours) or under certain conditions (such as operator instructions, continuous multiple measurement results deviating from the expected range), the system can automatically draw zero-point calibration liquid (such as high-purity water) and one or more VFA standard solutions of known concentration stored in the calibration liquid storage tank for measurement, and automatically verify or adjust (single-point or multi-point calibration) the calibration curve parameters stored in the system to ensure the accuracy and reliability of long-term measurement.

[0096] Specific embodiments: Composition and key operating parameter settings of the online VFA monitoring system, main component selection and parameter settings are as follows:

[0097] The sampling pump of the sampling and pretreatment unit is selected from the Masterflex L / S series peristaltic pump with PTFE material pump tube; the online filter uses a self-backflushing filter with a pore size of 0.45um. The single sampling volume is set to 80mL.

[0098] The first distillation kettle is made of 500 mL quartz glass material, external PID temperature control heating jacket, built-in Pt100 temperature sensor and Hamilton Polilyte Plus H VP pH electrode; the alkali solution is 1.0 mol / L NaOH solution, which is injected by KNE SIMDOS02 precision metering pump; the carrier gas is N2 with a purity of 99.999%, which is controlled by Brooks SLA5850S mass flow controller, and the flow rate is set to 0.4 L / min; the first distillation target pH is controlled at 10.8±0.1, the distillation temperature is controlled at 92±1℃, and the distillation purge time is set to 12 minutes; the NH3 vapor is directly discharged into the laboratory fume hood after being cooled by the water-cooled serpentine condenser;

[0099] VFA separation unit: after the first distillation kettle completes the removal of ammonia nitrogen, it is sequentially used as the second distillation kettle after automatic cleaning program, the acid solution is 20% concentration H3PO4 solution, which is injected by another KNFSIMDOS 02 precision metering pump, and the pH electrode is still Hamilton Polilyte Plus H VP (note its wide pH range adaptability), the second distillation target pH is controlled at 2.2±0.1, the distillation temperature is controlled at 99±1℃ (slightly higher than the normal pressure boiling point of water to strengthen the volatilization), the carrier gas N2 flow rate is still 0.4 L / min, and the distillation purge time is set to 18 minutes;

[0100] VFA quantitative unit: the VFA vapor is introduced into a specially designed bubble-type glass absorption cell after being cooled by the water-cooled serpentine condenser, and 50 mL of high-purity deionized water (resistivity > 18 MΩ·cm) is pre-injected into the absorption cell as the absorption liquid; conductivity measurement uses Mettler Toledo InPro7108-VP / CPVC four-electrode flow-through conductivity sensor, combined with M800 multi-parameter transmitter; the temperature is compensated by the built-in Pt1000 of the conductivity sensor.

[0101] Control and data processing unit: Siemens S7-1200 series PLC is used as the main controller, equipped with corresponding analog / digital input and output modules, the human-computer interaction interface is Siemens KTP700 Basic color touch screen, and the data is uploaded through the Profinet interface of the PLC.

[0102] It is also configured with an automatic cleaning and calibration unit: the cleaning liquid includes 0.1M HCl, 0.1M NaOH and deionized water, and the calibration liquid includes zero point calibration liquid (deionized water) and 100mg / L, 500mg / L mixed VFA standard solution (acetic acid: propionic acid: butyric acid mass ratio is 3:1:1, calculated based on total acetic acid equivalent).

[0103] The application is used for determining and verifying the VFA in simulated wastewater and actual anaerobic digestion liquid of wastewater treatment plant.

[0104] (1) System calibration:

[0105] A series of mixed VFA standard solutions with known concentrations (acetic acid, propionic acid and butyric acid are mixed according to the mass ratio of 3:1:1, and the total VFA concentration (calculated by acetic acid equivalent) is 0, 10, 25, 50, 100, 250, 500, 750 and 1000 mg / L respectively) are prepared.

[0106] The device constructed by the embodiment is used to automatically measure the above VFA standard series without ammonia nitrogen in the whole process, and the corrected conductivity change value (△o_compensated) of the absorption liquid after stable absorption at each concentration is recorded. The calibration curve between VFA concentration (C_VFA) and △o_compensated is established.

[0107] The obtained calibration curve is C_VFA (mg / L) = 2.5 x △o_compensated (uS / cm) - 0.5, and the linear correlation coefficient R2 is greater than 0.999. The detection limit (MDL) of the method is 3 mg / L, and the lower limit of quantification (LOQ) is 10 mg / L. The 100 mg / L VFA standard solution is repeatedly measured for 10 times, and the relative standard deviation (RSD) is 2.8%.

[0108] In order to verify the ammonia nitrogen removal effect, a mixed standard solution containing 500 mg / L VFA and different concentrations of ammonia nitrogen (0, 200, 500, 1000 and 2000 mg / L NH4+-N) is prepared. The device constructed by the embodiment is used for measurement, and the results show that even in the presence of 2000 mg / L NH4+-N, the measured VFA concentration has a deviation of less than ±3% compared with the determination result of the 500 mg / L VFA standard solution without ammonia nitrogen, which indicates that the ammonia nitrogen interference is effectively eliminated.

[0109] (2) Determination of simulated wastewater samples containing high concentration of ammonia nitrogen:

[0110] Two simulated wastewaters A and B are prepared.

[0111] Simulated wastewater A: containing VFA 150 mg / L (acetic acid equivalent), without ammonia nitrogen;

[0112] Simulated wastewater B: containing VFA 150 mg / L (acetic acid equivalent), and containing NH4+-N 800 mg / L.

[0113] The VFA of the simulated wastewater A and the simulated wastewater B were determined by the device constructed by the specific embodiment and the laboratory standard gas chromatography (GC, Agilent 7890B, FID detector, HP-FFAP chromatographic column) respectively, and each method was repeated for 3 times.

[0114] The results are shown in Figure 2 , Figure 3 .

[0115] As can be seen from the data in Figure 2 , Figure 3 , for the simulated wastewater containing no ammonia nitrogen and containing up to 800 mg / L ammonia nitrogen, the VFA concentration determined by the device of the present application is in good agreement with the results of the GC method, and the relative error is within ± 3%, indicating that the device can effectively eliminate the interference of high-concentration ammonia nitrogen and accurately determine the VFA.

[0116] (3) Determination and comparison verification of actual anaerobic digestion liquid samples in a sewage treatment plant:

[0117] Three water samples (sample 1, sample 2, sample 3) were taken from the outlet of the anaerobic digestion tank of a municipal sewage treatment plant and filtered by a 0.45 um filter membrane for standby.

[0118] The laboratory determined that the ammonia nitrogen concentration of the samples was: sample 1: 1250 mg / L; sample 2: 980 mg / L; sample 3: 1550 mg / L.

[0119] The VFA concentration in the three actual samples was determined by using the online VFA monitoring device of the present application and the laboratory standard GC method respectively, each method was repeated for 3 times for each sample, and the average value was taken, and the results are shown in Figure 4 .

[0120] Comparison of VFA concentration determination results of actual anaerobic digestion liquid

[0121] Figure 4 It can be seen that for the actual anaerobic digestion liquid samples with high ammonia nitrogen background, the VFA results determined by the device constructed by the specific embodiment of the present application are in good agreement with the reference results of the GC method.

[0122] For sample 1 (ammonia nitrogen 1250 mg / L), the VFA determined by the present application was 455.2 mg / L, and the GC method was 460.5 mg / L, and the relative error was -1.15%;

[0123] For sample 2 (ammonia nitrogen 980 mg / L), the present application determined 188.7 mg / L, and the GC method was 192.0 mg / L, and the relative error was -1.72%;

[0124] For sample 3 (ammonia nitrogen 1550 mg / L), the present application measured 870.5 mg / L, and the GC method measured 879.2 mg / L, with a relative error of -0.99%. These results fully demonstrate the accuracy and reliability of the device of the present application in online monitoring of VFA under actual complex matrix conditions, especially high ammonia nitrogen.

[0125] Compared with the prior art, the present application has the following remarkable beneficial effects:

[0126] 1. Efficiently eliminates ammonia nitrogen interference, significantly improves measurement accuracy: The present application innovatively adopts a double distillation technology based on precise pH regulation. In the first alkaline distillation process, by adjusting the pH of the water sample to strong alkalinity and heating to blow off, almost all ammonia nitrogen in the water sample can be selectively volatilized in the form of NH3 and removed, while VFA remains in the form of stable non-volatile salt under this condition. This pretreatment step fundamentally eliminates the interference of ammonia nitrogen (usually the most important interference in VFA conductivity or titration measurement) on the subsequent second-stage acid distillation extraction of VFA and the final conductivity measurement. Therefore, the present application makes the accuracy and reliability of VFA measurement results far superior to those of traditional methods or partial online technologies that do not effectively separate ammonia nitrogen, especially when dealing with complex water samples (such as anaerobic digestion liquid, landfill leachate, high-nitrogen industrial wastewater) with ammonia nitrogen concentration much higher than VFA concentration, the advantage is more prominent.

[0127] 2. Achieve fully automatic online continuous monitoring, convenient operation and reduce dependence on manpower: The present application integrates all functions such as complex sample pretreatment, accurate reagent addition, precise pH control, double distillation separation, VFA efficient absorption, high-precision conductivity measurement, intelligent data calculation and result output, as well as key system automatic cleaning and automatic calibration into an integrated and modular device. The entire analysis process is automatically completed by the central control unit (PLC or embedded system) according to the preset program, which can realize 7x24 hours unattended continuous online monitoring. This not only greatly reduces the intensity and frequency of manual operation, avoids errors that may be introduced by human operation, improves the real-time and consistency of monitoring data, but also significantly reduces the daily maintenance workload and improves the long-term operation stability of the system through effective automatic cleaning and fault self-diagnosis functions.

[0128] 3. High measurement accuracy, low detection limit, fast response, and meeting diversified requirements: Through fine optimization of double distillation conditions (temperature, pH, carrier gas flow rate, and distillation time), efficient removal of ammonia nitrogen and efficient extraction and recovery of VFA are ensured. The special design of the VFA absorption unit ensures that the VFA vapor is quickly and completely absorbed by the absorption liquid; the high-precision and high-stability conductivity sensor (such as a four-electrode type with precise temperature compensation) is used, combined with advanced signal processing and calibration algorithms, to ensure the sensitivity and accuracy of the conductivity measurement. It is expected that the present application can achieve ppm level (mg / L) VFA measurement accuracy, with a repeatability (RSD) better than 3-5%, and a detection limit expected to reach a low level (e.g. 1-5 mg / L VFA as Acetic Acid). The complete analysis cycle (from sampling to result) of a single sample can be flexibly controlled within 30-60 minutes according to the characteristics of the water sample and the accuracy requirements, which can meet the needs of dynamic control of most industrial processes and rapid warning of the environment.

[0129] 4. Wide application range and strong resistance to matrix interference (in addition to the solved ammonia nitrogen interference): Since the core separation step of the present application is based on the physical volatilization characteristics (distillation) of substances at a specific pH, rather than relying on specific chemical reaction color change or complex spectral absorption mode, the present technology is basically insensitive to common physical interference factors in water samples (such as turbidity, color, and suspended solids) (most solids have been removed by the pretreatment unit). In addition to the already efficiently solved ammonia nitrogen interference, for other non-volatile inorganic salt ions or most non-volatile organic matter in the water sample, as long as they do not produce volatile acid-base substances under distillation conditions to interfere with the final conductivity measurement, the present technology also has good resistance. This enables the present application to be widely used in various types of water bodies with complex composition and variable matrix, such as municipal sewage, industrial wastewater (especially high-concentration organic wastewater generated in food processing, pharmaceutical, chemical, and aquaculture industries), digestion liquid of anaerobic digestion system (biogas engineering), landfill leachate, fermentation liquid of fermentation industry, as well as surface water and groundwater.

[0130] 5. System stability and reliability, potential economic and environmental friendliness: The key components of the device are made of corrosion-resistant and high-temperature-resistant high-quality materials, combined with modular design, which is convenient for daily maintenance, troubleshooting and component replacement. The perfect automatic cleaning and calibration function effectively prevents system blockage, sensor contamination and measurement drift, ensuring the long-term stable and reliable operation of the system. Compared with large and precise analytical instruments such as gas chromatograph, the structure of the device is relatively simplified, and the cost of the core sensor (conductivity electrode) is moderate. The reagents consumed during operation are mainly conventional acids (such as phosphoric acid), bases (such as sodium hydroxide) and a small amount of high-purity water (as absorbent), which are widely available and low in price, and the amount can be effectively controlled through optimized design. By using efficient heating and condensation technology, the energy consumption per unit of measurement can be reduced. In summary, the present application is expected to achieve relatively low equipment purchase cost and long-term operation and maintenance cost under the premise of ensuring high performance, and has good application prospect. At the same time, through proper collection and treatment of waste gas and waste liquid, environmental protection requirements can be met.

[0131] 6. Potential for expanding to realize simultaneous online monitoring of ammonia nitrogen and VFA: By specially absorbing the ammonia nitrogen vapor generated in the first-stage alkaline distillation process and measuring the conductivity (or other suitable parameters) of the absorbent, the device has technical interfaces and possibilities for realizing simultaneous or sequential online monitoring of the ammonia nitrogen concentration in the water sample. If this function is realized, one instrument can provide both VFA and ammonia nitrogen, two key water quality parameters, which will further enhance its application value and cost performance, and provide users with more comprehensive water quality information and process insight.

[0132] It is obvious to those skilled in the art that the present application is not limited to the details of the above exemplary embodiments, and can be implemented in other specific forms without departing from the spirit or essential characteristics of the present application. Therefore, the embodiments should be regarded as exemplary and non-limiting, and the scope of the present application is defined by the appended claims rather than the above description, and it is intended to encompass all changes falling within the meaning and scope of the equivalent elements of the claims. Any reference signs in the claims should not be regarded as limiting the claims involved.

[0133] In addition, it should be understood that although the present specification is described in terms of embodiments, not every embodiment contains only one independent technical solution, and the description of the specification is only for the sake of clarity, and those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can be combined appropriately to form other embodiments that those skilled in the art can understand.

Claims

1. An on-line monitoring system for volatile fatty acids in water, characterized in that It comprises: a sampling and pretreatment unit for automatically, regularly and quantitatively extracting water samples from monitoring points according to preset procedures and removing solid particles and suspended solids in the water samples through an online filtering device; an ammonia nitrogen separation unit for performing a first distillation stripping operation on the pretreated water samples, so that ammonia nitrogen in the water samples is efficiently converted into free ammonia and separated and removed with a carrier gas, while VFA is stably retained in the water samples in the form of non-volatile salt; a VFA separation unit for performing acidification treatment on the water samples from which ammonia nitrogen has been removed, and then performing a second distillation stripping operation in an accurately controlled acidic environment, so that VFA in the water samples is volatilized and separated to obtain VFA vapor for subsequent absorption and quantification; a VFA quantification unit for absorbing VFA vapor separated from the second distillation process using a set amount of absorption liquid, and measuring the change in conductivity and real-time temperature of the absorption liquid before and after absorption through a sensor, and calculating the VFA concentration based on the change; and a control and data processing unit for integrated control of automatic operation; The control and data processing unit controls and connects the electrical elements in the sampling and pretreatment unit, ammonia nitrogen separation unit, VFA separation unit, and VFA quantification unit, respectively.

2. The online monitoring system for volatile fatty acids in water according to claim 1, wherein: The sampling and pretreatment unit includes a corrosion-resistant sampling pump, an online filter, a multi-channel switching valve, and a liquid level sensor.

3. The system for online monitoring of volatile fatty acids in water according to claim 1, wherein: The ammonia nitrogen separation unit includes a first distillation still, an alkali supply system, an online pH monitoring and control system, a carrier gas supply and control system, a high-efficiency condenser, and an ammonia nitrogen capture / treatment device.

4. The online monitoring system for volatile fatty acids in water according to claim 1, wherein: The ammonia nitrogen separation unit operates at a distillation temperature of 85-95℃ and a pH value of 9.5-11.0, with a carrier gas purging time of 5-15 minutes.

5. The online monitoring system for volatile fatty acids in water according to claim 3, wherein: The VFA separation unit includes a second distillation still, an acid supply system, an online pH monitoring and control system, a carrier gas supply and control system, and a high-efficiency condenser.

6. The online monitoring system for volatile fatty acids in water according to claim 1, wherein: The VFA separation unit operates at a distillation temperature of 90-105, slightly higher than the boiling point of water under normal pressure to enhance volatilization or using reduced pressure distillation, a pH value of 2.0-3.0, and a carrier gas purging time of 10-20 minutes.

7. The online monitoring system for volatile fatty acids in water according to claim 5, wherein: The first distillation still and the second distillation still are the same distillation still, which enters the VFA separation operation after completing the ammonia nitrogen separation operation.

8. The online monitoring system for volatile fatty acids in water according to claim 1, wherein: The VFA quantification unit includes a VFA absorption pool, a high-precision flow-through conductivity electrode, a quantitative absorption liquid supply module, and a waste liquid discharge valve.

9. The online monitoring system for volatile fatty acids in water according to claim 1, wherein: The control and data processing unit integrates control of all automatic operations and real-time acquisition of raw data from various sensors such as pH electrodes, temperature sensors, conductivity electrodes, and liquid level sensors: executes built-in VFA concentration calculation algorithms, and displays real-time data, historical trends, alarm information, and device status on a human-machine interaction interface; stores measurement results and operation logs; performs fault self-diagnosis and alarm programs; and communicates bidirectionally with an upper computer monitoring system, a distributed control system, or a cloud data platform through standard industrial communication protocols.

10. The online monitoring system for volatile fatty acids in water according to claim 1, wherein: The control and data processing unit comprises an industrial PLC or a high-performance embedded microprocessor, a high-precision multi-channel analog input / output module, a digital input / output module, a color touch screen, a large-capacity non-volatile data storage, and a plurality of standard communication interface modules.

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