Zhenjia Xu‡
ab,
Jun Zhou‡b,
Yongdi Liub,
Lifeng Guc,
Xujun Wuc and
Xueying Zhang*a
aCollege of Environmental Sciences and Engineering, Nanjing Tech University, Nanjing 211816, China. E-mail: xueyingzhang@njtech.edu.cn; Tel: +86-25-58139929
bCollege of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing 211816, China
cZhejiang Heze Environmental Technology Co., Ltd., Changxing, China
First published on 15th November 2018
The effects of hydrothermal carbonization (hydrothermal carbonization temperature, hydrothermal carbonization time, pH) on the dehydration performance of dyeing sludge were studied. The specific resistance, viscosity and floccular morphology of sludge before and after hydrothermal carbonization were analyzed. The physical and chemical properties of the liquid were also determined. The results showed that the dehydration performance of sludge was optimum, when the reaction temperature was 180 °C, the reaction time 4 h and the pH was 5.0. Here the specific resistance to filtration and viscosity were 93.69% and 96.78% lower, respectively, than the control group. When the sludge was hydrothermally carbonized, the sludge flocs were broken due to extreme conditions of high temperature and high pressure, which formed a porous mesh structure with better water permeability. The cohesion of the sludge colloidal structure was reduced, the capillary suction time was reduced by 88.89%, and the sludge dewatering performance was improved. This study shows the feasibility of the use of hydrothermal carbonization in sludge reduction.
At present, common methods used to improve sludge dewatering performance include chemical flocculant conditioning,4 heat treatment,5 ultrasonic treatment,6 electrolysis,7 and Fenton oxidation.8,9 The sludge dewatering process widely used in domestic sewage treatment plants involves the addition of conditioning agents such as calcium oxide or polyacrylamide to the sludge and the conditioned sludge undergoes mechanical dehydration. In theory, the sludge can be dewatered mechanically to reduce the moisture content to 60%. In practice, the moisture content of the treated sludge is 70–76%.10,11 Due to the addition of conditioning agents, the volume of the cake after dewatering can increase by 0.5–0.8 times, which is not conducive to subsequent disposal. The use of flocculants increases environmental threats and health risks. Polyacrylamide, the most common chemical flocculant, has been shown to be harmful to the environment and presents a carcinogenic risk.12 Not only that, due to the use of aluminium salts, it may also lead to human health problems including Alzheimer's disease.13 During heat treatment of sludge, malodorous gases are easily generated, and the resulting sludge has a low calorific value. Ultrasonic treatment and electrolysis do not produce odorous gases when used to treat sludge, but they do require specialized equipment, high energy consumption, and high maintenance costs. However, some low-molecular compounds (such as acetic acid, propionic acid, methanol, ethanol, and acetaldehyde) are continuously accumulated in the liquid phase during the Fenton oxidation process and cannot be completely mineralized. As H2O2 in the Fenton oxidation process has strong oxidizing properties, strict requirements related to equipment materials, high energy consumption, and high cost limit the practical engineering application of the process.
In recent years, research on hydrothermal carbonization of harmless renewable resources or energy has reached high interest globally.14,15 Hydrothermal carbonization is a thermochemical process used to produce materials with a heating value similar to brown coal.16 It is a complex reaction, performed in a closed system using wet, carbohydrate rich biomass as raw materials under conditions of temperature (180–260 °C) and high autogenous pressure.17 At present, hydrothermal carbonization is predicted a large potential in the preparation of carbon materials or carbon composites with different properties. Some researchers use hydrothermal carbonization to hydrolyze viscous organic substances in sludge at a certain temperature and pressure. The colloidal structure of the sludge is destroyed, and intracellular macromolecular organics are released, so that sludge dewatering performance is enhanced and the sludge is further converted into a carbon-based material that can be used as a fuel or i.e. biochar. In addition, the hydrothermal carbonization method overcomes various deficiencies of the above-mentioned conventional sludge treatment processes. Hydrothermal carbonization requires a small site area, short reaction period, is a stable operation with high relative efficiency and low energy consumption, and is a very promising sludge dewatering process. However, the optimum parameters of this technology and its performance are currently unclear. Based on a literature review, the present study is the first to adopt hydrothermal carbonization technology to treat dyeing wastewater sludge. By optimizing the processing conditions, an efficient sludge dewatering process was developed.
pH | TS (%) | VS (%) | SRF (1010 m kg−1) | CST (s) | Viscosity (mPa s) |
---|---|---|---|---|---|
a TS, total solid; VS, volatile solid; SRF, specific resistance to filtration; CST, capillary suction time. | |||||
7.05 ± 0.01 | 17.29 ± 0.15 | 49.15 ± 1.08 | 5.89 ± 0.12 | 274.3 ± 3.2 | 2950 ± 50 |
Hydrothermal carbonization was carried out in a 2 L high-temperature and high-pressure reactor. The tested sludge and deionized water (the mass ratio of sludge to water is 2.4:1) were mixed into a slurry using a glass rod and poured into a high-temperature and high-pressure reactor. The sludge was agitated at 160 rpm for 15 min, and nitrogen was purged for 5 min to remove the air. The reactor was heated to the target temperature with the heating rate of 10 °C min−1, and the reaction time started when the target temperature was reached.
In addition, considering the strong acid and alkali conditions, although the dewatering performance of the sludge was greatly improved, it also had serious environmental consequences; thus, weak acid and weak alkaline conditions were selected: pH = 5, pH = 7.0 (sludge pH after slurry was approximately 7.05), and pH = 9.0 (the original pH value of the sludge was 7.0. When adjusting the pH value, deionized water was replaced with a dilute sulfuric acid solution at pH 5.0 or a dilute sodium hydroxide solution at pH 9.0, and mixed with sludge in the same ratio as deionized water. Due to the strong buffering property of the sludge, a concentrated sulfuric acid or sodium hydroxide solution of 50% was added to the mixed sludge to adjust the sludge to the target pH. At this time, the experimental group showed an increase in volume compared with the control group, and an equal volume of deionized water was added to the control group to eliminate any experimental errors).
During the hydrothermal carbonization experiment, the corresponding pressure was monitored using a gas pressure gauge. After the reaction time passed, the reactor was allowed to cool to room temperature. When the experiment was complete, the reaction product was collected (the hydrochar suspension), labelled, and stored at 4 °C. Two sets of parallel experiments were performed for each set of experiments. All experimental data were averaged.
The standard CST instrument consists of two plastic blocks, a stainless-steel cylindrical funnel, a Whatman no. 17 filter paper (which is a standard grade of chromatography paper), and three electrodes fixed in the upper block and connected to an electrical timer. The test is carried out by pouring a small amount of sludge into the cylindrical tube. Then under the effect of capillary pressure, the sludge filtrate flows radially through the filter paper until it reaches the first two sensors that activate the timer. The timer stops when the flow reaches the third sensor, giving the CST value in seconds. The capillary suction pressure is much greater than the hydrostatic pressure inside the funnel and thus the test does not depend on the amount of sludge, but there must be a sufficient quantity to perform the test.22
It can be seen that the CST is also a function of various parameters such as filter paper properties, instrument properties and sludge-related properties in the following form:
(1) |
(2) |
The CST of the hydrochar suspension showed the opposite trend compare to viscosity Fig. 1(B). The CST of the hydrochar suspension treated at the four different temperatures was 40.5, 55.4, 62.6, and 66.7 s, respectively. The CST was shortest at 180 °C reaction time. Compared with the original sludge, the hydrochar suspension CST decreased by 85.24%. However, the hydrochar suspension CST increased with increasing reaction temperature, indicating that the water holding capacity of the suspension was enhanced. From the viscosity curve of the suspension in Fig. 1(B), the viscosity of suspension after treated at the four different temperatures was 150, 69, and 64, and 56 mPa s, respectively, and the viscosity was lowest when the reaction temperature was 240 °C. Compared with the original sludge, suspension viscosity decreased by 98.10%. The viscosity of the suspension decreased with increasing temperature. During hydrothermal carbonization first the carbohydrates are hydrolyzed. This leads to a partial solubilization of the material. Than the sugars formed show an elimination of water to unsaturated compounds like furfurals. These furfurals polymerize to from micro sphere or other small particles.28 However, the carbon microspheres were too small to increase the water holding capacity of the suspension and caused a significant increase in the suspension CST. Under the 180 °C reaction conditions, the organic matter released from the sludge was hydrolyzed to small molecules. At this temperature, carbon microspheres were not formed in large numbers. Therefore, the main mode of sludge dehydration at present is mechanical dehydration. If the carbon microspheres are too small, they are likely to clog the filter, resulting in incomplete dewatering of the sludge. Therefore, 180 °C was the best choice. These findings are consistent with those shown in Fig. 1(A).
As shown in Fig. 1(C), the SRF of sludge treated at the four different temperatures was 5.73 × 109, 6.42 × 109, 6.89 × 109, and 6.93 × 109 m kg−1, respectively, and the SRF of sludge was lowest at 180 °C reaction temperature. Compared with the original sludge, suspension SRF decreased by 90.42%. Excessively high temperatures also have an adverse effect on the suspension SRF. Higgins and Novak29 demonstrated that super colloidal particles with a particle size of 1–100 μm had the most significant effect on sludge dewatering. Carbon microspheres formed by the dehydration with consecutive condensation reaction of organic matter can easily block sludge filter cake or filter media, further affecting dewatering efficiency. In addition, a high proportion of super colloidal particles can also significantly increase the particle surface area/volume ratio, which not only enhances the degree of hydration of the sludge particles and impairs their dehydration performance, but also causes a significant increase in the dosage of dehydration conditioner. It can be concluded that a higher reaction temperature does not improve the dewatering performance of sludge Fig. 1(A and B). When the substantial cost and dehydration effect are considered, the reaction at 180 °C was optimal.
Fig. 2 Change of sludge properties after hydrothermal carbonization at different time reaction conditions: settling performance changes (A), CST and viscosity changes (B) as well as SRF changes (C). |
When the reaction time was 1–4 h, dewatering performance of the sludge increased gradually with prolongation of the reaction time; when the reaction time was 4–6 h, dewatering performance of the sludge decreased Fig. 2(B and C). Based on the original optimization shown in Section 3.2, when the reaction time was 4 hours, the suspension CST, viscosity, and SRF were further reduced by 7.41%, 20.01%, and 25.49%, respectively. However, when the reaction time was longer than 4 h, the hydrothermal carbonization treatment did not improve the dewatering performance of the sludge. This is due to the fact that the cell structure of sludge microorganisms (including cell walls and cell membranes) is further destroyed as the hydrothermal carbonization time (longer than 4 h) increases. The intracellular organic compounds were released and converted into soluble substances. These organic substances included proteins, carbohydrates and lipids. As a result, the viscosity of the sludge increased, further deteriorating the dewatering performance of sludge occurs. Concluding, these findings indicate that the appropriate reaction time was 4 h.
A correlation between the CST and viscosity change was observed Fig. 3(B). In relation to the CST changes, the initial pH values were pH = 5, 7, and 9. After hydrothermal carbonization, the CST was 30.5, 37.5, and 36.2 s, respectively. When the initial pH was 5, the CST was shortest, and further decreased by 18.67% on the basis of the study shown in Section 3.3, which was 88.89% lower than that of the original sludge. In terms of changes in sludge viscosity, the viscosity of sludge after hydrothermal carbonization was reduced to 95 mPa s when the initial pH was 5. On the basis of the study shown in Section 3.3, a further reduction of 20.83% was noted, and the viscosity of the original sludge phase was reduced by 96.78%. This was due to catalysis by H2SO4, accelerating the hydrolysis of a part of the less reactive organic materials (such as cellulose), which gradually began to hydrolyze into small molecules and further reduced the sludge viscosity. Ye33 demonstrated a positive correlation between sludge dewatering performance and viscosity, that is, dehydration performance deteriorated when viscosity increased. Therefore, hydrothermal carbonization at an initial pH of 5 is the best choice, which is consistent with the results shown in Fig. 3(A).
According to the results shown in Fig. 3(C), when the initial pH was 5, the minimum specific resistance of the sludge was 3.77 × 109 m kg−1, which was 11.72% lower than the experimental results shown in Section 3.3. Compared with the original sludge, the reduction was 93.69%. This is because H2SO4 is a strong electrolyte that acts as an electrification neutralizer and compresses the electric double layer, which can affect the zeta potential of the colloidal particles in the sludge, destroy the gel structure of the sludge, enhance the fluidity, and thus improve the dewatering performance of the sludge. In Fig. 3(A and B). It can be observed that pH = 5 was effective in enhancing sludge dewatering performance.
pH | COD (mg L−1) | BOD (mg L−1) | BOD/COD | NH4+–N (mg L−1) | TS (%) | VS (%) | |||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
After | Before | After | Before | After | After | Before | After | Before | After | Before | After | ||
pH 7 3 h | 180 °C | 6.69 | 736 | 12505 | 313 | 5627 | 0.45 | 242 | 6555 | 17.29% | 14.11% | 49.15% | 52.97% |
200 °C | 6.93 | 703 | 11245 | 330 | 5285 | 0.47 | 259 | 7190 | 17.31% | 13.06% | 49.92% | 51.44% | |
220 °C | 6.55 | 720 | 5150 | 309 | 2678 | 0.52 | 235 | 5195 | 17.30% | 12.27% | 50.01% | 50.45% | |
240 °C | 6.59 | 746 | 4413 | 315 | 2339 | 0.53 | 251 | 5115 | 17.43% | 12.11% | 49.52% | 51.87% | |
pH 7 180 °C | 1 h | 6.81 | 703 | 27215 | 261 | 11158 | 0.41 | 247 | 4013 | 17.42% | 15.21% | 50.13% | 55.80% |
2 h | 6.73 | 718 | 22065 | 257 | 9267 | 0.42 | 234 | 5470 | 17.37% | 14.63% | 49.33% | 53.81% | |
3 h | 6.69 | 708 | 12505 | 291 | 5627 | 0.45 | 252 | 6555 | 17.30% | 14.11% | 49.17% | 52.97% | |
4 h | 6.44 | 737 | 16180 | 314 | 8252 | 0.51 | 260 | 5975 | 17.30% | 13.62% | 49.45% | 52.53% | |
5 h | 6.36 | 708 | 17650 | 325 | 9178 | 0.52 | 230 | 5555 | 17.37% | 13.36% | 49.27% | 53.88% | |
6 h | 6.06 | 732 | 19125 | 298 | 9945 | 0.52 | 242 | 5375 | 17.40% | 13.17% | 49.97% | 54.37% | |
180 °C 4 h | pH 5 | 4.81 | 2945 | 25745 | 1228 | 14932 | 0.58 | 382 | 6114 | 16.96% | 13.32% | 49.95% | 51.23% |
pH 7 | 6.44 | 736 | 16180 | 300 | 8252 | 0.51 | 231 | 5975 | 17.29% | 13.62% | 49.15% | 52.53% | |
pH 9 | 7.44 | 2207 | 18390 | 1022 | 9931 | 0.54 | 372 | 5880 | 16.78% | 13.44% | 49.21% | 53.77% |
During the hydrothermal carbonization treatment of sludge at 200 °C to 240 °C and a reaction time of 4–6 h, the levels of ρ(NH4+–N) and ρ(COD) in the sludge filtrate gradually decreased with reaction temperature. This was due to the dehydration polymerization of organic matter (cellulose, protein) during hydrothermal carbonization to produce carbon microspheres (0.4–6 μm in diameter).36 The formation of carbon microspheres also indirectly leads to the deterioration of sludge dewatering performance, which is consistent with the results shown in 3.2. Excessive reaction time and excessive temperature did not improve the dewatering performance of sludge.
As shown in Fig. 4(A and B), the larger size, compact and dense distribution of the original sludge particles was more compact compared with Fig. 4(C and D), which shows the sludge after hydrothermal carbonization. The structure of the particles was loose, and formed a porous mesh structure with better water permeability. This also verified the aforementioned conclusions. Under the conditions of a reaction temperature of 180 °C, a reaction time of 4 h, and a pH of 5.0, sludge flocs were broken down, the sludge particle size was reduced, and the degree of hydration was reduced, which in turn led to an increase in sludge dewatering performance.
Fig. 4 SEM photograph of sludge before hydrothermal carbonization (A) and (B); SEM photograph of sludge after hydrothermal carbonization (C) and (D). |
(2) Hydrothermal carbonization time higher than 4 h and reaction temperature higher than 180 °C did not improve sludge dewatering performance. This was due to the fact that sludge broke down more thoroughly at this time and temperature, and although the viscosity of sludge started to decrease, the sludge particles were very fine and clogged the filter. Therefore, sludge dewatering performance deteriorated.
(3) The structure of sludge particles following this treatment process changed from dense to loose, and organic matter was released into the supernatant to form a porous mesh structure with better water permeability, thereby improving the compressibility and dehydration of the sludge.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra05350b |
‡ These authors contributed equally to the article. |
This journal is © The Royal Society of Chemistry 2018 |