CN111024860B - Method for measuring different forms of moisture in sludge by using headspace gas chromatography - Google Patents
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Abstract
The invention discloses a method for measuring different forms of moisture in sludge by utilizing a headspace gas chromatography, which comprises the following steps: (1) placing the sludge into a headspace bottle, sealing, and balancing in the headspace bottle; (2) placing the obtained sample into a headspace gas chromatograph, balancing the sample in a headspace sample injector, and detecting the sample by using a gas chromatograph to record a gas phase signal value A of water; (3) after multiple extractions, the logarithm of the gas phase signal value logA and the extraction times n are plotted, and the obtained logA constant speed area is free water evaporation; the first linear descending area of logA is capillary water evaporation; the second linear descending area is surface bound water evaporation; (4) calculating the mass of each part of water according to the established standard curve; and measuring the water content of the sludge to obtain the total water mass of the sludge, and subtracting the previously measured water masses of 3 different forms to obtain the combined water mass of the sludge. The invention can efficiently and accurately measure the distribution condition of different forms of water in the sludge.
Description
Technical Field
The invention relates to the field of measurement of different water morphological characteristics of sludge. Specifically, the invention relates to a method for measuring different forms of moisture in sludge by using a headspace gas chromatography.
Background
Along with the enhancement of environmental awareness of people and the increasingly strict environmental requirements of governments, the treatment rate and the treatment degree of sewage are continuously improved, and the output of sludge, which is the residual sediment after sewage treatment, is sharply increased. The water content of the sludge is generally as high as 95-99%, and a lot of inconvenience is brought to transportation and subsequent disposal. Therefore, the reduction of the water content of the sludge to realize the reduction of the sludge is an essential link for sludge treatment and disposal.
The sludge has complex components and high organic matter content, and consists of inorganic particles and organic particles, zoogloea, single cells, filamentous bacteria and other substances which are adsorbed or generated by flocs in wastewater, the substances are embedded in a net structure formed by Extracellular Polymeric Substances (EPS), and the coated bacteria and adsorbed water molecules also form an oxidation film, so that the surface water content of the sludge colloid is too high, and the solid-liquid separation is difficult.
At present, many researchers adopt means such as adding chemical reagents, biological enzymes and ultrasound, microwave and fenton to condition sludge, and mainly destabilize and agglomerate sludge colloidal particles through the actions of compressing double electric layers, adsorbing and bridging and net catching and rolling, so that water on the surface of sludge flocs is converted into free water to realize solid-liquid separation, or destroy the structure of Extracellular Polymers (EPS), degrade microorganisms, and convert the wrapped bound water into free water to improve the dehydration performance of the sludge. Fundamentally, the conditioning mode influences the water distribution form of the sludge by influencing the property of the sludge, so that the water in the sludge is changed from 'difficult removal' to 'easy removal'. Therefore, the water distribution of the sludge is closely related to the dehydration performance of the sludge, and the accurate measurement of the water distribution of the sludge has important significance for the reduction and resource utilization of the sludge.
The existence of moisture in sludge can be theoretically classified into four types- -free water, capillary water, adsorbed water, and bound water, according to the interaction of water with a solid phase. At present, a filter pressing method, a centrifugal method, a differential scanning calorimetry method and an expansion measurement method are all used for analyzing the moisture distribution of sludge, but the methods can only distinguish free water and bound water in the sludge and cannot realize the quantification of the moisture in four states. Chen et al used thermogravimetry-differential thermal analysis (TG-DTA) to determine the binding energy of water and sludge particles, and differentiated the water distribution of the sludge according to the difference of the binding energy, and the results show that the method uses very few sludge samples (10-15mg), the reproducibility of the experimental results is poor, and the error is about 30 kJ/kg. Then, researchers determine the moisture content of four different forms according to the drying weight loss curve of the sludge by adopting a thermal drying method, and the results show that the municipal sludge with the water content of 90 percent, the capillary water account for 64.93 percent, the surface bound water account for 33.11 percent and the bound water account for 1.96 percent, but the precision of the thermal drying method is low, and for the sludge with low water content, the drying weight loss curve can lack a constant-speed evaporation area, so that the thermal drying method can not accurately determine the moisture of each part. Therefore, there is a need to develop a new detection method to accurately measure the water distribution of sludge, which provides effective technical assistance for factory production and laboratory research.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide a high-efficiency and accurate determination method for different forms of water in sludge.
The purpose of the invention is realized by the following technical scheme:
a method for measuring different forms of moisture in sludge by utilizing a headspace gas chromatography comprises the following steps:
(1) placing the sludge into a headspace bottle, sealing by a gland, and balancing in the headspace bottle for 30-120 min;
(2) putting the sample obtained in the step (1) into a headspace gas chromatograph, balancing the sample in a headspace sample injector, detecting the sample by using a gas chromatograph, and recording a gas phase signal value A of water;
(3) the headspace detection mode is multiple extraction, logarithm logA of the water-gas phase signal value in the step (2) and extraction times n are plotted, and the obtained logA constant speed area is free water evaporation; the first linear descending area of logA is capillary water evaporation; the second linear descending area is surface bound water evaporation;
(4) calculating the mass of each part of water according to the established standard curve; measuring the water content of the sludge to obtain the total water mass of the sludge, and subtracting the mass of 3 types of water in different forms measured by headspace gas chromatography from the total water mass of the sludge to obtain the fourth form of water of the sludge, namely the combined water mass;
establishing the standard curve: deionized water with different masses is taken as a standard sample, a standard curve is established, the abscissa of the standard curve is the signal value of the standard sample, and the ordinate represents the mass of the standard sample.
Preferably, the headspace injector conditions are as follows: the equilibrium temperature is the measurement temperature, the equilibrium time is 5-25min, the vibration condition is set as multiple times of extraction, strong oscillation, the pressurization time of a headspace bottle is 5-20s, the quantitative ring filling time is 5-30s, and the transmission time to GC is 10-50 s.
Preferably, the mass of the sludge added into the headspace bottle is between 0.04 and 0.06 g.
Preferably, the conditions of the gas chromatograph: a Thermal Conductivity Detector (TCD), wherein the carrier gas is nitrogen, the flow rate is 1-5mL/min, the temperature of a column box is 80-150 ℃, the flow distribution is carried out, and the flow distribution ratio is 0.05-0.3: 0.5-1.5, and the detection time is 1-10 min.
Preferably, step (4) uses a halogen analyzer to measure the water content of the sludge.
According to the invention, a water vapor signal in a gas phase is taken as a basis, free water in sludge is water dissociated in the sludge and has no binding force with sludge flocs, and when a test is started, the free water is mainly evaporated, so that the water vapor content in the gas phase of a headspace bottle is mainly contributed by the free water, and the Henry's law is not suitable for the condition; according to the saturated vapor pressure Klebsiel-Clausis equation and the ideal gas state equation:
the relation between the water content in the gas phase in the headspace bottle and the temperature can be obtained through the equation, and under the condition that the temperature is not changed, the water content in the gas phase is not changed any more, so that the constant-speed area of the signal value is free water evaporation.
The sludge mainly comprises evaporation capillary water and surface bound water, and gas-liquid two-phase balance exists in water balance in a system. Therefore, the water distribution coefficient in headspace bottles follows henry's law, namely:
Kn＝CL n/Cg n
in the formula CgConcentration of the substance to be detected in the gas phase, mol/m3；CL-the concentration of the substance to be detected in the liquid phase at equilibrium temperature, mol/m3(ii) a n is the number of extractions.
When the vapor content in the gas phase of the headspace bottle is mainly contributed by capillary water, the desorption of water molecules in the solid phase of the sludge to the gas phase needs to overcome the capillary force
Q is capillary force; r is a gas constant; t is the temperature of the sludge sample; p0Is the saturated vapor pressure; p is the equilibrium vapor pressure;
when the water vapor content in the gas phase of the headspace bottle is mainly contributed by surface bound water, the desorption of water molecules in the solid phase of the sludge to the gas phase needs to overcome the intermolecular physical force of sludge flocs
H＝2πρsεσ2▽
Q is intermolecular physical force of moisture; rhosThe amount of sludge flocs; ε is the minimum latent heat; σ is a critical distance when the physical adsorption force is 0; is the distance between the crystal lattices; and z is the distance from water molecules to the surface of the sludge flocs.
The natural logarithm of the GC signal detected in each extraction sampling and the extraction times are in a linear relation:
Log(An)＝kn+c
because the different forms of water evaporation need different forces to overcome, the evaporation difficulty is different, and the generated vapor pressure is different, the natural logarithm of GC signals is different from the slope k generated by extraction times.
The method for establishing the standard curve in the step (4) comprises the following steps: and (2) performing headspace gas chromatography measurement on the standard sample to obtain a signal value, extracting for multiple times, and establishing a standard curve conversion equation y between standard samples with different qualities and a standard sample total signal value, wherein y represents the quality of the standard sample, x represents the standard sample total signal value, a is a slope, and b is a constant.
Preferably, the number of extractions in step (3) (4) is 20.
Preferably, the step (4) is specifically: respectively carrying out headspace gas chromatography measurement on the sludge with different water contents to obtain signal values A1, A2, A3, … and A20 of the sample, dividing the water form of each part according to an inflection point, and calculating the total signal value of each part of water, namely S1, S2 and S3; and substituting S1, S2 and S3 into a standard curve conversion equation, calculating to obtain the mass of free water, capillary water and surface bound water in the sludge, subtracting the mass of the former 3 kinds of water from the total mass of the sludge to obtain the mass of the bound water, and finally calculating the proportion of each part of water.
Compared with the prior art, the invention has the following beneficial effects:
the water content of the sludge is combined with the sludge flocs in different ways, and the steam pressure generated by evaporation of different forms of water in the sludge is also different. Since the evaporation process of the water in the sludge is a dynamic process, the traditional HS-GC analysis method is difficult to describe the dynamic change. The invention adopts multiple headspace extraction technology (MHE), continuously samples a fixed headspace bottle at intervals, and can carry out in-situ detection on the concentration change of volatile components in gas phase, thereby continuously observing the water vapor pressure change of a sludge sample in the MHE process. Therefore, the invention can efficiently and accurately measure the distribution of different forms of water in the sludge.
Drawings
FIG. 1 is a fitted standard curve plot of a standard sample as measured by headspace gas chromatography.
FIG. 2 shows the top-air gas chromatograms of municipal sludge and paper sludge (a) municipal sludge and (b) paper sludge.
FIG. 3 shows the relationship between the logarithm of the peak area of municipal sludge and papermaking sludge and the number of extractions (a) of municipal sludge and (b) of papermaking sludge.
FIG. 4 shows the results of measuring the water distribution of municipal sludge and paper sludge (a) and (b) in the municipal sludge and paper sludge.
FIG. 5 is a graph showing the thermal drying of municipal sludge after mechanical dewatering.
Detailed Description
The present invention will now be described in further detail with reference to the following examples, which are illustrated in the accompanying drawings.
Instrumentation and reagents used: thermo HS TriPlus 300 type automatic headspace sampler, agilent a7890 gas chromatograph (thermal conductivity detector, GS-Q type capillary chromatography column), headspace bottle (21.6mL), white teflon/white silica gel spacer (containing iron lid).
Example 1
The sludge comes from municipal sewage treatment plants and paper mills
1. Preparation of a standard sample: preparing standard samples from deionized water (10 μ L, 20 μ L, 30 μ L, 40 μ L, 50 μ L) with different masses;
2. establishing a marking line equation: and (3) placing the headspace bottle containing the standard sample into a headspace sample injector, setting the sample injection mode to be MHE, extracting for 20 times, balancing the temperature to be 80 ℃, balancing the time to be 15min, and balancing the loop for 12 s. Gas chromatography operating conditions: a Thermal Conductivity Detector (TCD), wherein the carrier gas is nitrogen, the flow rate is 3.8mL/min, the temperature of a column box is 105 ℃, the flow distribution is carried out, and the flow distribution ratio is 0.1: 1, the detection time is 5 min. Then, extraction is carried out, the water vapor peak area is recorded, a standard curve is obtained according to the relationship between the total peak area extracted for 20 times by different standard samples and the mass of the total peak area, as shown in fig. 1, a good linear relationship can be found between the mass of the standard sample and the water vapor peak area, and the correlation is 0.998.
3. And (3) measuring the total moisture content of the sludge: and (4) measuring the water content of the municipal sludge and the papermaking sludge, and calculating the total water content of the sludge.
4. Sample detection: and (3) after the sludge sample to be detected is determined in the step (3), placing the empty bottle filled with the sample to be detected in a headspace sampler, performing headspace gas chromatography analysis by adopting the same operation conditions of the headspace sampler and the gas chromatograph in the step (2), and recording the chromatographic peak area signal value of the sample.
5. Dividing water in different forms: for sample 1 (municipal sludge), as can be seen from the change of the number of extractions and the HS-GC signal in FIG. 2a, the signal value is almost unchanged before 6 extractions, and as can be seen from the relationship between logA and n in FIG. 3a, the relation between the natural logarithm of the GC signal detected at the time of sampling and the number of extractions is a piecewise linear function, so that water vapor is mainly contributed by free water when n is greater than or equal to 1 and less than or equal to 6, and is mainly evaporated by capillary water when n is greater than or equal to 9 and less than or equal to 13, and is mainly evaporated by surface bound water when n is greater than or equal to 14 and less than or equal to 20.
For sample 2 (paper sludge), as can be seen from the change of the number of extractions and the HS-GC signal in FIG. 2b, the signal value is almost unchanged before 7 extractions, and as can be seen from the relationship between logA and n in FIG. 3b, the relation between the natural logarithm of the GC signal detected during the extraction sampling and the number of extractions is a piecewise linear function, so that the water vapor is mainly contributed by free water when n is greater than or equal to 1 and less than or equal to 7, and is mainly evaporated by capillary water when n is greater than or equal to 10 and less than or equal to 12, and is mainly evaporated by surface bound water when n is greater than or equal to 13 and less than or equal to 20.
6. And (4) calculating a result: substituting the total peak area S1 of free water, the total peak area S2 of capillary water and the total peak area S3 of surface bound water into a standard curve equation, calculating the mass of each part of water, subtracting the contents of the three forms of water from the total content of sludge to obtain the mass of bound water, and finally calculating the proportion of different forms of water, as shown in FIG. 4.
7. Method reproducibility and accuracy
The reproducibility evaluation of the method was that 3 parallel samples were prepared according to the method described in the application example, and by detecting the water distribution of the sludge, the detection results are shown in table 1, and the RSD was less than 3%. Therefore, the method can be considered to have better repeatability for accurate detection of the sludge water distribution.
The accuracy of the method is that the moisture distribution detected by the method is compared with the moisture proportion (table 2) detected by the traditional thermal drying method, the result is shown in table 3, and the RSD of the moisture content in the sludge measured by the two methods is not more than 2%, which shows that the method has better accuracy. Since the thermal drying method is only suitable for sludge with high water content, for sludge with low water content, such as sludge after mechanical dehydration, the drying weight loss curve can lack a constant-speed evaporation area, so that the thermal drying method cannot accurately determine the water content of each part (as shown in fig. 5). The method directly measures the moisture content based on the phase equilibrium principle, and improves the detection precision, so that the measured moisture distribution data is more reasonable.
TABLE 1 relative standard deviation of MHE-GC methods
TABLE 2 measurement of sludge moisture distribution by thermal drying
TABLE 3 relative standard deviation of MHE-GC method from the thermal drying method
The embodiments of the present invention are not limited to the above-described embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and they are included in the scope of the present invention.
Claims (6)
1. A method for measuring different forms of moisture in sludge by utilizing a headspace gas chromatography is characterized by comprising the following steps:
(1) placing the sludge into a headspace bottle, sealing by a gland, and balancing in the headspace bottle for 30-120 min;
(2) putting the sample obtained in the step (1) into a headspace gas chromatograph, setting a sample injection mode to be MHE, the equilibrium temperature to be 80 ℃, the equilibrium time to be 5-25min, setting the vibration condition to be extraction for multiple times, and strongly oscillating; after being balanced in a headspace sample injector, the gas phase signal value A of water is recorded through gas chromatography detection; gas chromatography operating conditions: a thermal conductivity detector, wherein the carrier gas is nitrogen, the flow rate is 1-5mL/min, the temperature of a column box is 80-150 ℃, the flow is divided, and the flow dividing ratio is 0.05-0.3: 0.5-1.5, and the detection time is 5-10 min;
(3) the headspace detection mode is multiple extraction, logarithm log A of the gas phase signal value in the step (2) and extraction times n are plotted, and the obtained log A constant speed area is free water evaporation; the first linear descending area of logA is capillary water evaporation; the second linear descending area is surface bound water evaporation;
(4) calculating the mass of each part of water according to the established standard curve; measuring the water content of the sludge to obtain the total water mass of the sludge, respectively carrying out headspace gas chromatography measurement on the sludge with different water content to obtain signal values A1, A2, A3, … and An of the sample, dividing the water form of each part according to An inflection point, and calculating the total signal value of each part of water, namely S1, S2 and S3; substituting S1, S2 and S3 into a standard curve conversion equation, calculating to obtain the mass of free water, capillary water and surface bound water in the sludge, subtracting the mass of the former 3 kinds of water from the total mass of the sludge to obtain the mass of the bound water, and finally calculating the proportion of each part of water;
establishing the standard curve: deionized water with different masses is taken as a standard sample, a standard curve is established, the abscissa of the standard curve is the signal value of the standard sample, and the ordinate represents the mass of the standard sample.
2. The method of claim 1, wherein the headspace injector conditions are as follows: the headspace bottle was pressurized for 5-20s, the quantitative ring fill time was 5-30s, and the transfer-to-GC time was 10-50 s.
3. The method of claim 1, wherein the mass of sludge added to the headspace bottle is between 0.04 and 0.06 g.
4. The method according to claim 1, 2 or 3, wherein the standard curve in step (4) is established by:
and (2) performing headspace gas chromatography measurement on the standard sample to obtain a signal value, extracting for multiple times, and establishing a standard curve conversion equation y = ax + b between standard samples with different qualities and a standard sample total signal value, wherein y represents the quality of the standard sample, x represents the standard sample total signal value, a is a slope, and b is a constant.
5. The method of claim 4, wherein the number of extractions in step (3) (4) is 20.
6. The method of claim 1, 2 or 3, wherein step (4) uses a halogen analyzer to measure the moisture content of the sludge.
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