Abstract
In this study, the conditioning effect of cationic polyacrylamide (CPAM) with different charge densities on raw sludge (RS) and thermo-hydrolyzed sludge (HS) pretreated with or without ferric salt is studied through orthogonal experiments. In addition, this paper uses the principles of rheology and morphology to analyze and clarify the conditioning mechanism of RS and HS, and reveals the mechanism of thermal hydrolysis to improve the dewatering performance of sludge. Compared with the RS, the HS has smaller particle size, better filterability, stronger fluidity and more obvious thixotropy. However, due to the influence of filter pressing time, ferric salt should be added before conditioning. The orthogonal experiment shows that the optimal conditioner is CPAM with charge density of 60, and the specific resistance to filtration and capillary suction time of the adjusted thermo-hydrolyzed sludge are reduced to (1.11 ± 0.07) × 1012 m/kg and 16.1 ± 1.8 s; the particle size increased from 61.2 to 253.5 μm. The moisture content of the sludge cake is about 48%. The structural strength and thixotropy of HS are higher than those of the RS, and can be greatly improved by adding ferric salt. Morphological analysis confirms that thermal hydrolysis can lyse microbial cells in sludge, and the sludge treated with ferric salt will have more porous structure and stronger flocculation strength.
HIGHLIGHTS
Effects of CPAM on conditioning of raw sludge and thermal hydrolysis sludge were studied.
Thermal hydrolysis treatment will improve sludge dewatering capacity and fluidity.
Ferric salt treatment enhanced structural strength and thixotropy of conditioned thermal hydrolysis sludge.
Rheological analysis can be used as a new tool to reflect the dewatering capacity of sludge.
INTRODUCTION
Wastewater treatment plants produce excessive amounts of by-product sewage sludge every day, and the treatment/disposal of these sludges is an urgent problem (Wu et al. 2016). Due to the high content of organic matter in the sludge, the flocculent particles of sludge have a colloidal structure and are highly hydrophilic. They are easy to combine with water molecules in different forms, which makes it difficult to remove part of the water in the sludge and ultimately leads to simple dehydration. The sludge still has a high moisture content. This severely hinders the reduction of sewage sludge. Therefore, there is an urgent need for effective methods to promote sludge dewatering (or improve its dewatering capacity) (Dai et al. 2018). At present, physical and chemical methods are used for sludge dewatering pretreatment. By adding energy or chemical reagents to the sludge, the flocculation structure of the sludge is destroyed, and part of the intracellular water and pore water in the sludge is released, so as to reduce the moisture content in the sludge. Among these methods, the chemical adjustment of polyacrylamide (PAM) is the most widely accepted method in wastewater treatment plants (Ruiz-Hernando et al. 2015a, 2015b). Polyacrylamide is the collective name for thousands of organic polymer electrolytes with acrylamide as the main component. Cationic polyacrylamide (CPAM) is commonly used for dewatering because sludge particles are usually negatively charged. The efficiency of sludge conditioning depends on CPAM and sludge characteristics. Studies have shown that the conditioning effect of CPAM is mainly related to its charge density (Ruiz-Hernando et al. 2015a, 2015b). There have been some studies on conditioning sludge by CPAM, but there is little information about the type of conditioner used for dewatering.
In order to reduce the mass production of sludge in wastewater treatment plants, thermal treatment, hydrothermal treatment, alkaline hydrolysis, ultrasonic treatment and biological methods (Li et al. 2015; Zhou et al. 2015; Park et al. 2017) are applied in large-scale wastewater treatment plants (Romero et al. 2015). Physical and chemical methods can induce the release of intracellular compounds by destroying sludge flocs and microbial cells (Li et al. 2015). Among them, due to the flocculation of Fe3+, the chemical oxidation of potassium ferrate, Fenton reagent and calcium peroxide can improve the settleability and dewaterability of sludge (He et al. 2015; Wu et al. 2018; Wei et al. 2019). In addition, the thermal hydrolysis process (THP) has been proven to have the potential to use hot compressed water and has very good advantages (Akiya & Savage 2002). During the hydrothermal treatment process, the extracellular polymeric substance (EPS) decomposes to release its bound water. The chemical bonds between the cell wall and the cell membrane can even be broken at a sufficiently high temperature to make the components in the cell soluble (Appels et al. 2008). Many studies have shown that the non-particulate matter released by cell lysis has an adverse effect on the filtration of sludge (Qiang et al. 2009), but it can be significantly improved by chemical conditions (Park et al. 2003). Rheological analysis can serve as a bridge between physicochemical behavior and dehydration capacity to better understand the mechanism of sludge dehydration (Wang et al. 2017). Ruiz-Hernando et al. (2014) found that the combination of rheological characterization and thixotropy analysis can be used for quantitative evaluation of sludge dewaterability. Therefore, for the physicochemical sludge reduction process, detailed research on sludge treatment and rheological analysis is needed to clarify the mechanism and provide effective dewatering solutions.
In this study, raw sludge (RS) and hydrolyzed sludge (HS) are selected to compare physicochemical and rheological properties, because thermal hydrolysis process have been extensively studied and applied (Han et al. 2017). The effect of CPAM with different charge densities on the condition of RS and HS with or without ferric salt are studied by orthogonal experiments. Moreover, the rheological characteristics and morphology of the RS and HS are analyzed to clarify the influence of thermal treatment/hydrothermal treatment on the characteristics of sludge. The results are expected to provide new insights into the conditioning mechanism of sludge in thermal hydrolysis treatment and conditioning.
MATERIALS AND METHODS
Sludge and coagulant
Both RS and HS were obtained from Junxin Environmental Sludge Plant in Changsha City, Hunan Province, China. In this experiment, a 200 L cylindrical thermal hydrolysis reaction tank is used for the hydrothermal reaction of RS and HS, the temperature is 180 °C, and the sludge residence time is 120 min. Samples of RS and HS were collected before and after the thermal hydrolysis reaction and transferred to the laboratory to prepare for the experiment within 60 minutes (Figure 1). In order to aggregate small particles, FeCl3 of analytical grade was added to the HS to prepare a ferric-treated HS (FHS) sample and a ferric-treated RS (FRS) sample at an optimized dosage of 30 mg/g DS (dry solids) (refer to Figure 2).
Orthogonal experiment
Studies have shown that when CPAM is used for sludge dewatering, the charge density has a greater impact on the flocculation effect of sludge. In this experiment, five CPAM samples with different charge densities were used to optimize the sludge conditioning, by orthogonal experiment, of RS, HS, FRS and FHS. Taking the charge density of CPAM (20–60) as the variable, specific resistance to filtration (SRF), capillary suction time (CST) and the particle size of conditioned sludge were used as the response variable. The experimental results are shown in Table 1.
FHS | Particle size (μm) | 275.3 ± 12.4 | 242.5 ± 18.7 | 266.1 ± 23.8 | 253.5 ± 16.1 | 197.9 ± 18.4 |
CST (s) | 21.8 ± 2.3 | 25.2 ± 3.1 | 20.4 ± 2.1 | 16.1 ± 1.8 | 18.7 ± 2 | |
SRF (1012 m/kg) | 2.06 ± 0.2 | 2.11 ± 0.18 | 1.63 ± 0.11 | 1.11 ± 0.07 | 1.22 ± 0.09 | |
FRS | Particle size (μm) | 386.4 ± 15.7 | 302.3 ± 23.5 | 275.4 ± 26.3 | 297.2 ± 21.4 | 243.2 ± 18.6 |
CST (s) | 51.7 ± 4.3 | 46.3 ± 5.2 | 46.5 ± 3.8 | 38.8 ± 3.4 | 39.4 ± 2.8 | |
SRF (1012 m/kg) | 2.42 ± 0.11 | 3.86 ± 0.16 | 2.13 ± 0.14 | 4.04 ± 0.08 | 1.27 ± 0.04 | |
HS | Particle size (μm) | 246.4 ± 23.2 | 213.2 ± 18.3 | 220.4 ± 13.8 | 262.4 ± 17.6 | 211.6 ± 20.4 |
CST (s) | 41.6 ± 5.6 | 36.3 ± 4.9 | 43 ± 3.7 | 32.8 ± 3.1 | 38.7 ± 3.4 | |
SRF (1012 m/kg) | 2.35 ± 0.16 | 3.27 ± 0.14 | 2.36 ± 0.18 | 2.21 ± 0.11 | 3.03 ± 0.09 | |
RS | Particle size (μm) | 494.2 ± 23.2 | 472.4 ± 28.6 | 503.1 ± 30.5 | 395.2 ± 40.3 | 450.6 ± 18.6 |
CST (s) | 75.6 ± 4.3 | 63.1 ± 2.6 | 65.5 ± 2.8 | 58.4 ± 3.7 | 62.6 ± 2.5 | |
SRF (1012 m/kg) | 5.88 ± 0.12 | 6.02 ± 0.17 | 4.31 ± 0.2 | 3.43 ± 0.11 | 1.35 ± 0.08 | |
Charge density | 30 | 40 | 50 | 60 | 70 | |
Run | 1 | 2 | 3 | 4 | 5 |
FHS | Particle size (μm) | 275.3 ± 12.4 | 242.5 ± 18.7 | 266.1 ± 23.8 | 253.5 ± 16.1 | 197.9 ± 18.4 |
CST (s) | 21.8 ± 2.3 | 25.2 ± 3.1 | 20.4 ± 2.1 | 16.1 ± 1.8 | 18.7 ± 2 | |
SRF (1012 m/kg) | 2.06 ± 0.2 | 2.11 ± 0.18 | 1.63 ± 0.11 | 1.11 ± 0.07 | 1.22 ± 0.09 | |
FRS | Particle size (μm) | 386.4 ± 15.7 | 302.3 ± 23.5 | 275.4 ± 26.3 | 297.2 ± 21.4 | 243.2 ± 18.6 |
CST (s) | 51.7 ± 4.3 | 46.3 ± 5.2 | 46.5 ± 3.8 | 38.8 ± 3.4 | 39.4 ± 2.8 | |
SRF (1012 m/kg) | 2.42 ± 0.11 | 3.86 ± 0.16 | 2.13 ± 0.14 | 4.04 ± 0.08 | 1.27 ± 0.04 | |
HS | Particle size (μm) | 246.4 ± 23.2 | 213.2 ± 18.3 | 220.4 ± 13.8 | 262.4 ± 17.6 | 211.6 ± 20.4 |
CST (s) | 41.6 ± 5.6 | 36.3 ± 4.9 | 43 ± 3.7 | 32.8 ± 3.1 | 38.7 ± 3.4 | |
SRF (1012 m/kg) | 2.35 ± 0.16 | 3.27 ± 0.14 | 2.36 ± 0.18 | 2.21 ± 0.11 | 3.03 ± 0.09 | |
RS | Particle size (μm) | 494.2 ± 23.2 | 472.4 ± 28.6 | 503.1 ± 30.5 | 395.2 ± 40.3 | 450.6 ± 18.6 |
CST (s) | 75.6 ± 4.3 | 63.1 ± 2.6 | 65.5 ± 2.8 | 58.4 ± 3.7 | 62.6 ± 2.5 | |
SRF (1012 m/kg) | 5.88 ± 0.12 | 6.02 ± 0.17 | 4.31 ± 0.2 | 3.43 ± 0.11 | 1.35 ± 0.08 | |
Charge density | 30 | 40 | 50 | 60 | 70 | |
Run | 1 | 2 | 3 | 4 | 5 |
Sludge conditioning experiment
First, it was necessary to cool the sludge to below 60 °C. In the orthogonal experiment, the CPAM solution was added to the sludge at a dose of 5 mg/g DS. After orthogonal optimization, the selected PAM was added to the sludge sample at doses of 0, 1, 2, 3, 4, 5, and 6 mg/g DS to determine the optimal dose for conditioning. And in the batch experiment, the CPAM solution was quickly poured into a 10 L sludge sample in a 15 L sampling tank and the sample was mixed at a speed of 850 r/min for 10 minutes. Next, by determining SRF, CST and particle size distribution, sludge samples were carefully collected to evaluate the filterability of sludge. Through rheological and morphological analysis, the sludge under the optimal conditions selected by the orthogonal experiment was further studied, and the mechanism of sludge conditioning was explored. Then, the sludge sample to be observed by scanning electron microscopy (SEM) needed to be protected with liquid nitrogen to keep the flocculation morphology intact, and then the sample needed to be freeze-dried in vacuum at −45 °C for 48 hours. Finally, the remaining sludge was separated by a plate and frame filter press with a maximum pressure of 2.0 MPa and a filter press with a diameter of 5 μm for 3 hours, and then the resulting sludge cake was taken out to obtain the moisture content of the sludge cake.
Rheological experiment
Analytical methods
Total sludge solids (TS) and volatile solids (VS) were measured by weight method at 105 °C and 600 °C respectively. The pH was monitored by using an HQ30d portable meter (Hach, USA). SRF was measured according to the pressure filtration method. CST was measured with an HDFC-10A capillary suction time instrument. Particle size was monitored using a Mastersizer 3000 laser diffraction particle size analyzer (Malvern Panalytical, UK). Sludge morphology was analyzed using Zeiss Sigma 300 field emission SEM (Zeiss, Germany).
RESULTS AND DISCUSSION
Characteristics of sludge before and after thermal hydrolysis
The characteristics of RS and HS are shown in Table 2. During the thermal hydrolysis process, large organic molecules (including nuclear inclusions) in the sludge were degraded, and the resulting soluble components were transferred to the liquid phase, so the HS had a smaller particle size (Huang et al. 2017). Compared with the RS, the organic acids with small molecular weight produced by organic matter degradation in the process of sludge thermal hydrolysis will lead to the decrease of pH (Feng et al. 2015). In addition, the values of SRF and CST decrease in the process of thermal hydrolysis, which is consistent with the better dehydration capacity of the sludge after thermal hydrolysis treatment. As shown in Table 2, due to the hydrolysis of FeCl3, ferric salt improves the filtration performance of RS and HS, and lowers the pH.
Sample . | TS (%) . | VS (%) . | pH . | SRF (1012 m/kg) . | CST (s) . | Particle size (μm) . |
---|---|---|---|---|---|---|
RS | 10.01 | 43.51 | 7.5 ± 0.2 | 34.5 ± 4.2 | 1863.6 ± 122.3 | 106.5 ± 2.1 |
HS | 8.98 | 34.02 | 6.4 ± 0.3 | 5.9 ± .0.2 | 602.4 ± 42.5 | 61.2 ± 1.3 |
FRS | 6.7 ± 0.1 | 12.2 ± 2.6 | 503.1 ± 27.8 | 131.2 ± 3.5 | ||
FHS | 5.8 ± 0.2 | 3.1 ± 0.1 | 130.7 ± 20.5 | 98.3 ± 2.5 |
Sample . | TS (%) . | VS (%) . | pH . | SRF (1012 m/kg) . | CST (s) . | Particle size (μm) . |
---|---|---|---|---|---|---|
RS | 10.01 | 43.51 | 7.5 ± 0.2 | 34.5 ± 4.2 | 1863.6 ± 122.3 | 106.5 ± 2.1 |
HS | 8.98 | 34.02 | 6.4 ± 0.3 | 5.9 ± .0.2 | 602.4 ± 42.5 | 61.2 ± 1.3 |
FRS | 6.7 ± 0.1 | 12.2 ± 2.6 | 503.1 ± 27.8 | 131.2 ± 3.5 | ||
FHS | 5.8 ± 0.2 | 3.1 ± 0.1 | 130.7 ± 20.5 | 98.3 ± 2.5 |
Selection of CPAM type and dosage
Analysis of orthogonal experiment results
Table 1 shows the results of orthogonal experiments performed on RS, HS, FRS and FHS by CPAM with different charge densities. According to the results in Table 1, the optimal formulations were as follows: P1 for RS and FRS (charge density 70), P2 for HS and FHS (charge density 60). Through the verification of experiments, it was found that P1 and P2 were the optimal CPAM charge density types of the corresponding sludge, resulting in the smallest SRF and CST as follows: SRF = (1.35 ± 0.08) × 1012 m/kg, CST = 62.6 ± 2.5 s for RS; SRF = (1.27 ± 0.04) × 1012 m/kg, CST = 39.4 ± 2.8 s for HS; SRF = (2.21 ± 0.11) × 1012 m/kg, CST = 32.8 ± 3.1 s for FRS; SRF = (1.11 ± 0.07) × 1012 m/kg, CST = 16.1 ± 1.8 s for FHS. It is clear that the optimal CPAM type significantly enhances the filtration performance of the corresponding sludge. For the SRF and the CST, RS and FRS were reduced by more than 80%, and HS and FHS also reduced by more than 71%. The results also show that Fe3+ had a negative effect on the conditioning of RS, but it was beneficial to the conditioning of HS.
Through the analysis of the results, it can be found that the charge density of CPAM has a significant effect on the SRF of RS and FRS, but has no effect on CST. This is because the influence of CPAM conditioning on RS can only be reflected by changing the pressure provided by the vacuum pump in the SRF test. And ferric salt can flocculate CPAM, and the flocculated sludge particles were positively charged, which is greatly affected by CPAM with different charge densities (Saveyn et al. 2008). However, for HS, the charge density of CPAM had no significant effect on SRF and CST, which is due to the decrease of sludge particle size after thermal hydrolysis treatment. Small particles of sludge block the passage of microbial cell debris and macromolecular substances. In the absence of CPAM conditioning, Fe3+ reduces the small particles in HS through flocculation, resulting in a certain increase in the size of the sludge in FHS (Table 2), so the charge density of CPAM has a significant impact on CST. However, during the SRF test, the remaining small particles of sludge also block the filter paper, resulting in no significant change in the time of filtrate removal. Therefore, the charge density of CPAM has no obvious effect on SRF.
From the change of sludge particle size, the charge density of CPAM has little effect on the particle size of FRS and HS. The former was attributed to the flocculation of Fe3+, which forms large-molecule flocs in the sludge, while the latter was due to the breakdown of sludge cells after thermal hydrolysis treatment, resulting in poor flocculation of small particles of sludge. The charge density of CPAM also has a significant effect on the particle size of FHS, indicating that neutralization was the main mechanism of particle aggregation.
The influence of CPAM dosage on sludge filtration performance
As shown in Figure 3, the dosage of CPAM was evaluated and calculated by CST, SRF and particle size values of four kinds of sludge. The SRF and CST of the RS decrease rapidly from the initial (34.5 ± 4.2) × 1012 m/kg and 1,863.6 ± 122.3 s to the minimum value (1.06 ± 0.05) × 1012 m/kg and 59.2 ± 3.6 s when the dosage was 4.0 mg/g DS, and then increase with the increase of CPAM dosage. When the dosage was 5.0 mg CPAM/g DS, the SRF and CST of HS decrease to (2.21 ± 0.11) × 1,012 m/kg and 32.8 ± 3.1 s, and then flatten as the CPAM dose continues to increase. When the dosage of CPAM was 4.0 mg/g DS and 5.0 mg/g DS, the particle size of RS and HS increase to 403.5 + 22.3 and 262.4 + 17.6 μm respectively. However, when the dosage exceeds this value, the trend tends to be flat. Therefore, the optimal CPAM dosage for RS and HS was 4.0 mg/g DS and 5.0 mg/g DS respectively.
As shown in Figure 3, due to the flocculation of Fe3+, the changes in SRF and CST of FRS and FHS were flatter than those of RS and HS. The SRF and CST of FES decrease first and then increase with the increase of CPAM dosage. When the CPAM dosage was 4.0 mg/g DS, the values of SRF and CST reach the lowest. When the CPAM dosage was 5.0 mg/g DS, the SRF and CST of FHS reach the minimum value of (1.11 ± 0.07) × 1012 m/kg and 16.1 ± 1.8 s. And the particle size increases from 131.2 ± 3.5 and 98.3 ± 2.5 μm to 201.4 ± 7.2 and 253.5 ± 16.1 μm. Therefore, it was obvious that the optimal CPAM dosages for FRS and FHS were 4.0 and 5.0 mg/g DS respectively. Compared with RS, FRS has a smaller particle size at the optimal dosage, indicating that ferric salt had an adverse effect on the aggregation of flocs after CPAM treatment. And ferric salt was essential for HS because the SRF in FHS was lower and the CST was shorter.
Rheological properties of sludge
In this study, by changing the shear rate, the rheological behavior of the initial sludge and the sludge treated with Fe3+ and CPAM was analyzed. The relationship between rheological parameters and dewatering performance was established through the obtained rheological diagram, and the influence of different treatment conditions on the dewatering capacity of sludge was analyzed (Xiao et al. 2017). The rheological behavior of initial sludge and conditioned sludge is shown in Figure 4. The viscosity of all sludge samples will decrease at a lower shear rate, and then gradually stabilize at a higher shear rate (500–1,000 s−1) (Cao et al. 2020). The yield stress is the minimum applied stress required for continuous material flow, which is highlighted by the importance of determining the best operating conditions for various sludge treatment operations (especially mixing and pumping) (Eshtiaghi et al. 2013). Spinosa & Lotito (2003) pointed out that lower yield stress means better sludge treatment operations, especially in terms of dewatering performance. By using the flow curves fitted by different rheological models, it was found that the Herschel–Bulkley model can describe the initial sludge well, and the thermo-hydrolyzed sludge follows the power law model. On the other hand, Marinetti et al. (2010) observed that the sludge treated by CPAM had an upward peak in the rheogram and was not suitable for curve fitting of the Herschel–Bulkley model.
In the Herschel–Bulkley model, a higher n-value parameter (flow behavior index) indicates that the sludge was more like a Newtonian fluid (Feng et al. 2016), and a lower k value parameter (consistency) indicates that the strength of the sludge structure related to the apparent viscosity change was weak (Liu et al. 2016). As shown in Table 3, the n value and k value of FRS were lower than RS. This is because Fe3+ aggregates negatively charged sludge flocs and reduces the fluidity of sludge. HS has a lower n value and a higher k value than RS. This is because under high temperature thermal hydrolysis conditions, most of the proteins in the sludge will undergo irreversible denaturation, resulting in the increase of bound water of protein molecules and the decrease of interaction between particles, thus reducing the yield stress of sludge (Farno et al. 2014). At the same time, the release of intercellular substances after EPS breaks at high temperature in turn leads to an increase in sludge viscosity. FHS has better fluidity and lower viscosity than HS, because Fe3+ will aggregate part of the protein and polysaccharides in the HS (Wang et al. 2006).
. | Power law (τ=kγn) . | Bingham law (τ = mγ + τy) . | Herschel–Bulkley (τ = kγn +τy) . | |||||||
---|---|---|---|---|---|---|---|---|---|---|
Model . | k . | n . | R2 . | m . | τy . | R2 . | k . | n . | τy . | R2 . |
RS | 44.1781 | 0.1701 | 0.9657 | 0.0857 | 80.6532 | 0.9382 | 3.3002 | 0.4782 | 62.9190 | 0.9950 |
HS | 9.5495 | 0.2814 | 0.9821 | 0.0514 | 26.2546 | 0.8797 | 5.7117 | 0.3429 | 6.7280 | 0.9834 |
FRS | 2.0190 | 0.3649 | 0.9934 | 0.0220 | 7.4407 | 0.9473 | 0.6833 | 0.5071 | 3.4096 | 0.9998 |
FHS | 1.7347 | 0.0904 | 0.9521 | 0.0171 | 85.5545 | 0.9837 | 0.6914 | 0.5225 | 2.4587 | 0.9997 |
. | Power law (τ=kγn) . | Bingham law (τ = mγ + τy) . | Herschel–Bulkley (τ = kγn +τy) . | |||||||
---|---|---|---|---|---|---|---|---|---|---|
Model . | k . | n . | R2 . | m . | τy . | R2 . | k . | n . | τy . | R2 . |
RS | 44.1781 | 0.1701 | 0.9657 | 0.0857 | 80.6532 | 0.9382 | 3.3002 | 0.4782 | 62.9190 | 0.9950 |
HS | 9.5495 | 0.2814 | 0.9821 | 0.0514 | 26.2546 | 0.8797 | 5.7117 | 0.3429 | 6.7280 | 0.9834 |
FRS | 2.0190 | 0.3649 | 0.9934 | 0.0220 | 7.4407 | 0.9473 | 0.6833 | 0.5071 | 3.4096 | 0.9998 |
FHS | 1.7347 | 0.0904 | 0.9521 | 0.0171 | 85.5545 | 0.9837 | 0.6914 | 0.5225 | 2.4587 | 0.9997 |
The RS with CPAM shows a typical rheological curve with the maximum shear stress (as shown in Figure 4(e)), which indicates that a certain amount of energy was required to break the flocculated structural bond after CPAM flocculation. For HS, the typical peak of shear stress disappears. This is because the network strength of the sludge matrix was reduced by the hydrothermal treatment, and the intercellular organic molecules competing with the flocs for the binding sites with CPAM were released (Feng et al. 2009), so less energy was required to break the structural bonds of the flocs. The same phenomenon occurs in Figure 4(g), indicating that Fe3+ interferes with the flocculation of CPAM and reduces the structural strength (Shih et al. 2001).
Thixotropy can be defined as when the sludge is sheared, the viscosity continues to decrease with time, and when the shear is not interrupted, the viscosity recovers with time (Mewis & Wagner 2009), usually used to describe the resilience of the internal structure of a non-Newtonian fluid after shearing. The structural model of Equation (4) is usually used to characterize thixotropy. Figure 5 shows the evolution of normalized viscosity ((η − ηe)/(ηi − ηe)) at a shear rate of 1,000 s−1 for 15 minutes. As the shear progresses, the normalized viscosity of all samples decreases. It is worth noting that when applied to a low shear rate, the normalized viscosity drops sharply, but gradually slows down over time, and finally reaches a steady state. After shearing, the flocculent structure of sludge was destroyed rapidly due to the action of shearing force. Some EPS and other organic matter were dissolved into small particles by thermal hydrolysis treatment. Ruiz-Hernando et al. (2015a, 2015b) emphasized that small particles were more resistant to shear forces. Therefore, the rate of decrease of the normalized viscosity becomes slower with time. As shown in Figure 5, the normalized viscosity of HS was higher than RS. This is because the sludge accelerates the destruction of the sludge structure and the release of organic matter after thermal hydrolysis, which leads to the increase of the normalized viscosity of the sludge. For FRS and FHS, the sludge flocs formed by the flocculation of Fe3+ were difficult to re-aggregate after breaking at high shear rate, especially for FRS.
Analysis of sludge morphology
As shown in the SEM images of Figure 6, there are obvious differences between RS and HS. Compared with HS with many broken microbial cells, the surface of RS is relatively smooth. Compared with FHS (Figure 6(g)), FRS (Figure 6(c)) also has a smoother surface, and both show a more compact structure than RS and HS due to the flocculation effect of Fe3+. For the sludge treated by CPAM, the overall appearance of RS and HS flocs (Figure 6(b) and 6(f)) shows fewer pores and more compact structures than FRS and FHS (Figure 6(d) and 6(h)), which can visually show the flocculation effect of Fe3+ on sludge particles. At the same time, the specific internal structure of the floc can also be observed. The phenomena shown in Figure 6(b) and 6(d) indicate that RS flocculation strength was weak after Fe3+ was added. However, the voids of FHS in Figure 6(h) were larger than those of HS in Figure 6(f), indicating that FHS has a higher flocculation strength.
Effect of conditioner on sludge structure and dewatering performance
Tang et al. (2017) pointed out that after adding conventional CPAM conditioner to the sludge, the sludge will have large particles with high compressibility and loose structure, and the moisture content is about 80% after preliminary dehydration. In addition, the thixotropic behavior is more obvious, indicating that the fluid in the sludge is difficult to flow and a huge amount of energy is consumed during dewatering. In this experiment, the influence of Fe3+ and CPAM can be explained by the potential relationship between rheological behavior and changes in sludge particle size. In Table 3, it is clear that the k value of the sludge treated with Fe3+ was significantly higher than that of the untreated sludge, indicating that the sludge treated with Fe3+ has stronger thixotropy, which means that it has good fluidity and transportation capacity. However, the bridging effect of Fe3+ was weakened due to the large dispersibility, which eventually leads to the decrease of sludge particle size treated by Fe3+. Therefore, by comparing the time of the normalized viscosity of the sludge, the influence of the conditioner on the flocculation of the sludge can be known. In addition, the smaller particle size of Fe3+ treated sludge means that the structure of sludge was harder, which reduces its compressibility and energy consumption.
As a traditional test method for sludge dewatering performance, CST is closely related to soluble biopolymer in sludge after CPAM conditioning, and the SRF measurement is closely related to the vacuum pump pressure. Both CST and SRF have successfully measured the filtration rate, but it was impossible to predict the maximum solids content of sludge cake in the dewatering process (Wu et al. 2019). Therefore, the dewatering tests of the initial sludge and the conditioned sludge were carried out using a plate and frame filter press. The maximum moisture content of sludge cake is shown in Figure 7. It can be found that the moisture content of the RS cake (90%) was much higher than that of the HS (58%), indicating that thermal hydrolysis treatment can help improve the dewatering performance of the sludge. And the moisture content of the sludge cake treated without the conditioner (58%) was also higher than that of the sludge with the conditioner (48–55%). At the same time, when the content of conditioner was optimal, moisture content can reach the minimum (48%), which was lower than the minimum moisture content of raw sludge after ultrasonic treatment (68.4%), alkali treatment (49.5%) and heat treatment for 30 min at 80 °C (65.2%) (Ruiz-Hernando et al. 2015a, 2015b). Moisture content (48%) verified the accuracy of the selection of the conditioner content. In general, the charge density and dosage of CPAM have a certain impact on the moisture content of the remaining sludge and thermo-hydrolyzed sludge. In actual operation, it is necessary to choose CPAM with different charge density and dosage according to different sludge.
CONCLUSIONS
Compared with RS, HS has smaller particle size, better filterability, stronger fluidity and more obvious thixotropy. Fe3+ was required before CPAM conditioning, and the optimal conditioner was CPAM with charge density of 60. And the moisture content of the sludge cake was 48%. In addition, the normalized viscosity, structural strength and thixotropy of the HS conditioned by the conditioner were higher than that of the RS conditioned by the conditioner. Rheological analysis showed that Fe3+ improved the filtration performance of HS by improving its flocculation strength and flowing capacity. Morphological analysis confirmed that Fe3+ makes the HS treated by CPAM have a more porous structure and stronger floc strength. Finally, the results show that rheological behavior analysis can be used as an effective way to explain the mechanism of sludge dewatering. The change of sludge rheological behavior can be used as a new direction for studying sludge dewatering methods.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.