Pollutant release, pore structure, and thermal effect of sewage sludge during anaerobic fermentation were investigated. Results showed that the pH value firstly declined and then increased during anaerobic fermentation. The BOD5 and organics of sewage sludge declined, and the BOD5 of samples which was originally neutral declined as much as 53.6%. The micropore of samples was relatively developed. The biggest adsorption amount was 69.2 cm3/g. The average pore size was enlarged about 16.0–19.8% under anaerobic fermentation. There existed endothermic valley during heating procedure of 0–200∘C because of the dehydration, and the mass loss was 60.9–72.5%. The endothermic valley of the sample fluctuated at the 14th day in the anaerobic fermentation. During the heating procedure of 200–600∘C, there existed exothermal peaks because of the oxidation and burning of the organics. The curve of sample which was originally neutral had comparatively large endothermic valley and exothermal peak.
As a kind of byproduct produced in the treatment of the municipal wastewater, the sewage sludge has properties such as high moisture content, perishability, instability, and putrid odor. The environmental problem caused by the sewage sludge is also one of the most highlighted ones at present and in the future [
Nowadays, the treatments of sewage sludge were mainly sanitary landfill and incineration [
In this page, the effect of the original pH value on the efficiency of the anaerobic fermentation was determined by analyzing the pore structure and the thermal effect in the anaerobic fermentation. The pH value, organic content, and BOD5 of samples with differently original pH value were determined by pollutant releasing test; the change of pore structure in the anaerobic fermentation was determined by static nitrogen adsorption test. The component and phase deformation rules in the process were determined by TG-DTA test.
In order to observe the influence of differently original pH value of samples on the releasing rules of sewage sludge leachate and its microstructure, the samples used in the tests were separately taken from the Tangxun Lake wastewater treatment plant (as sample I), the Hanxi wastewater treatment plant (as sample II), and the Sha Lake wastewater treatment plant (as sample III). Since the sewage sludge was influenced by factors such as the numbers of the light industry enterprises and the numbers of residents around the plant and the original moisture content, the testing of samples’ pH values strictly followed the Chinese standard: water quality-determination of pH value-glass electrode method (GB/T 6920-86). And the original pH values of samples were 5.89, 6.96, and 8.13 which were named as sample I, sample II, and sample III, respectively. The physical properties of sewage sludge were shown in Table
The physical properties of samples.
Pollutant | Sample I | Sample II | Sample III |
---|---|---|---|
Proportion (mg/L) | 1.24 | 1.24 | 1.27 |
Organics (%) | 46.6 | 43.2 | 41.8 |
Moisture content (mg/L) | 69.7 | 80.3 | 78.4 |
Pore ratio | 3.17 | 3.36 | 3.50 |
Permeability coefficient (cm/s) | 1.44 × 10−8 | 1.20 × 10−8 | 1.07 × 10−8 |
Compressive strength (kg/cm2) | 0.059 | 0.068 | 0.066 |
BOD5 (mg/L) | 238 | 194 | 190 |
COD (mg/L) | 618 | 555 | 584 |
Since the scanning electron microscope (SEM) pictures and the curves of pore volume-pore size of three kinds of samples were similar, the SEM pictures and curves of sample II were chosen as model for analyzing. The SEM pictures of sample II were shown in Figure
The microstructure of original sewage sludge.
The curve of pore-volume and pore size of original sewage sludge.
Before the tests, a small hole was made on the upper end of the conical robber plug, and the anticontaminating rubber tubing was inserted into that hole, making it strongly connected with each other. Then, the big sundries in the sewage sludge were removed manually. In the test, sewage sludge with differently original pH values was separately put into the 500 mL conical flasks. In every flask, the weight of the sewage sludge samples was all 240 g. Then Vaseline was brushed on the bottleneck of the conical flasks, and the flasks were sealed by the rubber plugs prepared before. The other end of the rubber tubing in the plug was put in a container containing distilled water, ensuring that the conical flask was under anaerobic and airtight condition. After that, the conical flasks were kept in thermostatic water bath box (Jintan Chenghui Instrument Factory), controlling the temperature at 95°C to improve the efficiency of the anaerobic fermentation of the sewage sludge [
The conical flasks were taken out and put into a container with oxygen at the 2th, 7th, 10th, 14th, 17th, 21th, and 28th day after the beginning of the anaerobic fermentation, respectively. When the flasks were taken out, a little of the sewage sludge was taken out and a filter screen was put on the bottleneck of the flask. Then, the leachate in the flask was poured into a small beaker. Then the flasks were sealed again and put back into the water bath box.
The pH value and the organic content of the sewage sludge and the relationship between the pH value, organic content, and the time of the anaerobic fermentation were tested according to determination method for municipal sludge in wastewater treatment plant (CJ/T 221-2005). According to water quality-determination of biochemical oxygen demand after 5 days (BOD5) for dilution and seeding method (HJ 505-2009) and water quality-determination of the chemical oxygen demand-fast digestion-spectrophotometric method (HJ/T 399-2007), the BOD5 and COD of the leachate from the samples were determined and the relationship between them and the time of anaerobic fermentation was analyzed.
The properties of pore structure of the sewage sludge with differently original pH value at the 14th and 28th days in the anaerobic fermentation were determined by the static nitrogen test, using the JW-BK static nitrogen adsorption instrument (Beijing JWGB Sci.&Tech. Co., Ltd.). In the test, the temperature was the saturation temperature of the liquid nitrogen, and nitrogen (99.9%) was used as the adsorbing medium. The relative pressure ratio of
In the tests, three kinds of samples were, respectively, taken out at 14th and 28th days in the anaerobic fermentation. To test the thermogravimetry (TG) and the differential thermal tnalysis (DTA) of those samples, the TG-DTA simultaneous thermal analyzer (Beijing Henven Scientific Instrument Factory) was applied to the test to analyze the result of the samples and the phase deformation. The range of the temperature of the analyzer was 0–1000°C and the hearting rate of it was controlled at 10°C/min.
The changing relationship between the pH value, the organic content, and the time of the anaerobic fermentation was shown in Figure
The curve of pH value.
The curve of organic matter.
At the beginning, under the coaction of the organics and the acidogenic bacteria, the protein and part of the insoluble organics in the samples were transformed to water soluble matter, such as monosaccharide amino, lower fatty acid, by the ectoenzyme released by the bacteria. Then, the water soluble matter produced above was transformed to plenty of carbon dioxide (CO2), acetic acid (CH3COOH), and energy by the endoenzyme, making the acidity of leachate stronger. Thus, the pH value of the samples began to decrease and the organic content also rapidly declined at the start of the fermentation. With the continual of the anaerobic fermentation and the action of methanogens, the organic acid was resolved and methane was released. In this procedure, it was easy for the organic acid to combine with the various forms of nitrogen (such as
The changing relationship between BOD5 and the time of the anaerobic fermentation was shown in Figure
The curve of BOD5 of sewage sludge.
With the continual of the anaerobic fermentation, the insoluble organics in the sewage sludge were also resolved to mass of soluble organics (such as CH3COOH) in the allogenic water, making the second digestion of the microorganism more difficult. Thus, degrading rate was greatly influenced by the outside factors ultimately. When the living environment was not suitable for the microorganism, the breeding and multiplying abilities of them were relatively weaker, influencing the degradation efficiency of the BOD5 in the leachate.
Since the adsorption-desorption isotherms of the different samples in the anaerobic fermentation were similar, the adsorption-desorption isotherms of sample II at the 14th day of the fermentation were chosen as model for analysis. From Figure
The adsorption-desorption isotherms of sample II.
Since the inside of the pore was rough and the pores were bottleneck like, it was easier to adsorb nitrogen and form obstacles at the top of the pore to help stop the fast releasing of nitrogen gas in the desorption procedure. When the ratio of
The distribution curves of pore volume-pore size of samples I, II, and III were given in Figures
The average pore size and specific surface area of samples.
Days of anaerobic fermentation | Sample I | Sample II | Sample III | |||
---|---|---|---|---|---|---|
14 | 28 | 14 | 28 | 14 | 28 | |
Average pore size (nm) | 7.14 | 8.28 | 8.65 | 10.1 | 8.09 | 8.69 |
Specific surface area (m2/g) | 76.0 | 72.8 | 64.3 | 59.5 | 73.9 | 62.3 |
The distribution curve of pore volume and pore size of sample I.
14th day of the anaerobic fermentation
28th day of the anaerobic fermentation
The distribution curve of pore volume and pore size of sample II.
14th day of the anaerobic fermentation
28th day of the anaerobic fermentation
The distribution curve of pore volume and pore size of sample III.
14th day of the anaerobic fermentation
28th day of the anaerobic fermentation
Compared with the pore size of samples at the 14th day of the anaerobic fermentation, the average pore size of samples at the 28th day increased about 13.3–16.7%, and the maximum values of pore volume of the sample at 14th and 28th days both had slight increase. The reason behind this was that, at the beginning of the anaerobic fermentation, the sewage sludge was mainly made up of a lot of organics macromolecule compounds. When the pore was formed, the outside and inside walls of pore were both rough. The microorganism parasitized in the allogenic water contained in the pores. With the lasting fermentation and the continual flow of the water in the pores, the degradable organics inside the pores and on the surface of the sewage sludge particles were consumed by the microorganism. The microorganisms nourished themselves and multiplied by this way, leading to the decreasing of BOD5 and smoothing the surface of the pore. Thus, at the 28th day of the fermentation, the specific surface areas of three kinds of samples were all smaller than that of the 14th day. The pore volume and the average pore size at the 28th day were both increased compared with those of the 14th day.
The rule of the distribution of the pore volume-pore size was closely related to the environment condition of the anaerobic fermentation. When the experiment was completed, the total pore volume of sample II was 0.178 cm3/g increasing by 33.8% and 10.6%, respectively, when compared with samples I and III. Meanwhile, the average pore size of sample II in the fermentation was also larger than that of samples I and III. The specific surface area of sample II was smaller, indicating that the living environment of sample II was more acidic, which led to the weakening of the activity of methanogens at the end of the fermentation. Thus, during this period, the ability of methanogens to consume organics was weakened, resulting in much organic remaining on the pore walls of sewage sludge particles at the end of the test. Because of that, the total pore volume and average pore size were small, and the specific surface area was large.
The rules of the thermal effect at 14th, 28th day of the anaerobic fermentation were shown in Figures
The percentage of quality loss.
Days of anaerobic fermentation | Sample I | Sample II | Sample III | |||
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14 | 28 | 14 | 28 | 14 | 28 | |
<200°C | 61.9% | 66.8% | 67.0% | 72.5% | 60.9% | 70.1% |
200–600°C | 8.48% | 7.70% | 9.20% | 12.7% | 7.33% | 9.31% |
>600°C | 0.130% | 0.200% | 0.0600% | 0.390% | 0.680% | 0.150% |
Total mass loss |
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The curve of the thermal effect at 14th day of the fermentation.
The curve of TG
The curve of DTA
The curve of the thermal effect at 28th day of the fermentation.
The curve of TG
The curve of DTA
From Figures
From the TG curve of the samples at the 14th day of the anaerobic fermentation, it is shown that the obvious fluctuation happened at the endothermic valley at 100–200°C, and it disappeared at the 28th day of the fermentation. Since the microorganism had relatively high activity at the 14th day, the reaction of resolving the organics was more severe. According to the analysis of the pore structure, there existed a little free water in the pore and the hole of the pore was small. When the temperature became higher, the heat conductivity of the wall of the pore differed from outside to the inside. Therefore, the temperature of the inside wall was lower than the outside, resulting in the fact that the dehydration of the free water firstly happened in the outside wall then in the inside wall of the pore. After 28 days of the anaerobic fermentation, since the organic on the surface of the inside and outside walls of the pore and on the hole of the pore was degraded, the wall became thinner and the hole became larger. Therefore, the heat conductivity efficiency was significantly improved and the fluctuation of the endothermic valley disappeared.
From the DTA curves, it can be drawn that the peak value at 480°C of the exothermal peak at 14th day was 1.81–14.50
In addition, the mass loss of sample II was larger than that of samples I and III, in the same reaction time. The minimum value of the endothermic valley and the peak value of the exothermal peak of sample II were also larger. The original moisture content of sample II was relatively higher and the microorganism took in the water when the degradation of organics happened in the anaerobic fermentation. Thus, the moisture loss in the heating procedure was more obvious. Moreover, the outside environment of sample II was more suitable for the anaerobic fermentation; the number and the activity of the microorganisms of sample II were both larger than those of samples I and III. Therefore, the endothermic valley and the exothermal peak of sample II were bigger and more significant.
To study the releasing rules of the pollutant, microstructure, the phase deformation, and the component of the sewage sludge in the anaerobic fermentation and to find theoretical foundation for the use and harmless disposal of the sewage sludge, the pollutant releasing test, the static nitrogen adsorption test, and the TG-DTA test were performed to determine the concentration of pollutant, pore structure, and properties in the thermal effect of the sewage sludge with differently original pH value on the anaerobic fermentation. From the test results, the conclusions can be drawn as follows.
(1) The pH value of samples declined firstly and then increased with the continual of the fermentation. The difference values between the original pH value and the final pH value of samples I and III were both smaller than 2%, while the difference value of sample II was 4.5%. The contents of organic and the BOD5 both gradually decreased with the continual of the fermentation, and the decreasing rate during 2–15 days of the fermentation was relatively high. At the end of the test, the content of the organic and the BOD5 of sample II declined most which were 37.3% and 53.6%, respectively, and they were still slowly decreasing.
(2) The adsorption isotherms of sewage sludge in the anaerobic fermentation belonged to kind IV, and there existed a
(3) When the temperature was between 0 and 200°C, the mass loss of sewage sludge samples was 32.9–39.1% and there existed significant endothermic valley since the dehydration of free water. When the temperature was between 200 and 600°C, the mass of sewage sludge decreased a little and there existed two exothermal peaks because of the oxidative cleavage and the burning of organics. When the temperature was higher than 600°C, the TG and DTA curves were more smoothly. In the curves of the samples at 14th day of fermentation, there appeared an obvious fluctuation at the endothermic valley and the peak value of the exothermal peak was comparatively large. At the same time of the heating procedure, the mass loss and endothermic valley and the exothermal peak of sample II were all larger than those of samples I and III.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to express their great appreciation for funding provided by the National Natural Science Foundation of China (51474168) and Nature Science Foundation of Hubei Province (2014CFB889).