Enhanced dewaterability of waste-activated sludge with zero-valent iron-activated persulfate oxidation under mild hydrothermal conditions Open Access

Waste-activated sludge (WAS) is a byproduct in wastewater treatment plants, which is composed of microorganisms, organic and inorganic matters with extracellular polymers (EPS) as the skeleton (Wang et al. 2018). It is difficult to dewater WAS due to the gel-like and porous fractal structure (Wang et al. 2014). Efficient conditioning and dewatering technologies are considered the key approaches for reducing the cost of sludge treatment and disposal.

Recently, hydrothermal treatment (HT) has gained attention due to its advantages of high efficiency and high value-added products (Yu et al.2018b). HT could promote the decomposition of sludge flocs, especially EPS, and reduce viscosity (Wang et al. 2017). However, a positive effect on dewaterability only occurs when the HT temperature exceeds 180 °C (Wang et al. 2014). The α-helix of protein in soluble EPS is considered the main factor during HT of sludge (Liu et al. 2019). However, the produced hydrolysate with high organic concentrations and refractory organics, such as melanoids, hinder HT application in the field of sludge conditioning (Wang et al. 2017). Synergistic effects combining HT with chemical conditioning, such as acid, alkali and advanced oxidation processes (AOPs), have been utilized to improve sludge dewaterability under mild conditions; that is, a lower temperature, which can help reduce energy consumption and secondary pollution (Wang et al. 2017). Over the past few years, persulfate (PDS)- or peroxymonosulfate (PMS)-based AOPs have been widely developed in the field of environmental remediation, including for wastewater treatment and sludge conditioning (Liu et al. 2020; Qiao et al. 2020). In general, sulfate radicals (⁠⁠) can be generated from the activation of PDS or PMS by heat, transition metal activation, ultrasound, and so on which have been reported to improve dewaterability (Kim et al. 2016; Zhen et al. 2018). The original structure of EPS can be destroyed by a series of radical reactions involved in the PDS conditioning process (Liu et al. 2020).

Compared to ferrous minerals, zero valent iron (ZVI, i.e., Fe0) is more recyclable and cheaper (Li et al. 2018), which can slowly release ferrous ions and directly activate PDS on the surface of ZVI (Equation (3)) (Qiao et al. 2020). The capillary suction time (CST) value of sludge was reduced by 50% under ZVI-activated PDS process (Zhou et al. 2015). The dissolution rate and specific surface area could restrict the conditioning efficiency in ZVI/PDS technology (Zhen et al. 2018). Increasing the temperature can also promote the dissolution of ZVI, which can improve the activation efficiency of ZVI during AOP conditioning (Li et al. 2018). In addition, hydrothermal treatment with high temperature and high pressure is more conducive to free radical generation, compared with conventional heat treatment. Chen et al. (2020) made use of the sodium persulfate coupled thermal hydrolysis (200 °C, 101 min) process to improve sludge dewatering performance. However, few previous studies have reported the effects of temperature on the PDS/PMS activation efficiency by ZVI during sludge conditioning. Knowledge of the evolution of the dewaterability of hydrothermal sludge during ZVI-activated PDS combined with HT at different temperatures and the potential mechanisms are lacking.

In this study, the collaborative process of ZVI/PDS and HT was applied for sludge conditioning, and the synergistic effects on sludge dewatering performance were identified. In detail, the main purpose of this study was to (1) determine the optimal ZVI/PDS content and HT temperature for sludge conditioning and (2) elucidate the synergistic effects on sludge dewaterability during HT-ZVI/PDS and the potential mechanisms by exploring the sludge characteristics in hydrolysates and solids.

Concentrated WAS obtained from a wastewater treatment plant in Nanjing, China, was used in this study. The obtained WAS was stored in the 4 °C refrigerator before use. The water content of WAS was 95.8%, the pH was 6.91, the total suspended solid (TSS) was 45.21 g/L, the ratio of volatile suspended solid (VSS) to TSS was 35.46%, CST was 190.60 s, the specific resistance of filtration (SRF) was 5.95 × 10¹² m/kg (Table S1). Chemicals, including PDS and ZVI, were all purchased from Sinopharm Group Co. Ltd (Shanghai, China).

The HT process took place in a stainless-steel reactor (MCD-250, Sen Long Co., Beijing, China) with a volume of 300 mL and a maximum pressure and temperature of 20 MPa and 300 °C, respectively. A sludge sample of 250 ml was used in each experiment.

After the sludge was loaded into the reactor, it was heated to the designated temperature and kept at that temperature for 30 minutes. A series of runs with different amounts of PDS and ZVI additions were performed at 120 °C and 160 °C, respectively, to obtain the best performance for WAS dewatering (25 °C was set as the control group). After the hydrothermal reaction, the temperature in the reactor was reduced to room temperature by internal circulating water bath cooling. The dewatering performance of the WAS was evaluated immediately after the hydrothermal process. The treated WAS was also kept in the refrigerator at 4 °C until to characterization.

CST and SRF were considered as good indicators of sludge filterability (Li et al. 2019). The dewatering performance of sludge was negatively correlated with the values of CST and SRF. The CST was measured by the capillary suction time apparatus (Type 304M, Triton Electronics, UK).

Centrifugation and thermal extraction methods were used to extract soluble EPS (S-EPS), loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) from sludge (Wang et al. 2016). All EPS were filtered through a 0.45 μm microfiber filter for component analysis. Three-dimensional fluorescence excitation emission matrix spectroscopy (3D-EEM) was applied to analyze organic compounds in different fractions of EPS by three-dimensional fluorescence spectrometer (F-7000, Hitachi, Japan). The range of excitation wavelength and emission wavelength were 200–400 nm and 280–520 nm, respectively. The scanning rate of the spectrum was 1,200 nm/min, and the scanning interval was 5 nm. Parallel factor analysis (PARAFAC) was performed on the fluorescence spectral data of EPS (Kowalczuk et al. 2009).

Soluble chemical oxygen demand (SCOD) and ammonium nitrogen (NH₄⁺-N) were analyzed using standard methods (APHA 1998). Soluble polysaccharides and proteins were determined with the methods of Bradford (1976) and Miller (1959), respectively.

The sludge samples were dried in a vacuum freeze dryer at −60 °C for 12 h (Biocool FD-1-50, China). The functional groups of the WAS were estimated by Fourier transform infrared spectroscopy (FTIR; JASCO FT/IR-300, Japan) at spectral wavelengths ranging from 500 to 4,000 cm⁻¹. The region 1,800–800 cm⁻¹ of the FTIR spectra was further determined by Peak Fit v4.12 to fit the curve. X-ray photoelectron spectroscopy (XPS) was analyzed using an ESCA Lab 250Xi photoelectron spectroscopy system (Thermo Fisher Scientific, USA) with Al-Kα radiation to determine hydrophilic and hydrophobic functional groups.

Three repeated samples were measured, and their average values were calculated. The possible significant differences were examined by one-way analysis of variance tests.

Structural equation model (SEM) was used for determined the direct and indirect relationship between dissolved organic matter (DOM) and sludge dewatering performance (Gao et al. 2019). SPSS 20.0 software was used to analyze the key factors affecting sludge dewatering performance by Pearson correlation method.

The WAS dewatering performance outcomes after being treated by ZVI/PDS and HT conditioning are shown in Figure 1. The dewaterability of sludge improved slightly in the presence of PDS. The CST decreased from 190.6 s to 141.1 s after treatment with 200 mg/g TSS PDS. There was no significant difference (p > 0.05, based on t-tests) in SRF compared with raw sludge (RS) in the control. In contrast, CST (p < 0.01, based on t-tests) and SRF (p < 0.01, based on t-tests) were improved significantly after treatment with the combination of PDS and ZVI, which was consistent with the research of Li et al. (2019).

The impacts of HT temperature on WAS dewatering performance with ZVI/PDS conditioning are shown in Figure 1(a). The SRF increased from 5.95 × 10¹² to 26.17 × 10¹² m/kg at 120 °C compared with the RS in the control and decreased to 14.54 × 10¹² m/kg at 160 °C. The dewaterability deteriorated at 120 °C and improved slightly at 160 °C. Liu et al. (2019) reported a similar result that the sludge dewaterability deteriorated with the increase of hydrothermal temperature, but the deterioration decreased above a certain temperature threshold (120–150 °C). HT caused the release of negatively charged EPS, which hindered sludge dewatering (Wang et al. 2017). However, the SRF decreased to 2.13 × 10¹² m/kg after PDS addition and further declined to 0.79 × 10¹² m/kg after the combination of PDS and ZVI at 120 °C. Hence, PDS and ZVI addition helped reverse the deterioration of dewaterability at lower temperatures (120 °C). Compared with RS, the sludge dewaterability was enhanced by HT-PDS conditioned at 120 °C, and it was further improved with ZVI addition on this basis (p < 0.01, based on t-tests) (Figure 1(a) and 1(b)). This was because the synergistic effect could further destroy the sludge floc, reduce the hydrophilicity of the sludge and improve the sludge dewaterability. Interestingly, the SRF was 1.36 × 10¹² m/kg under HT-ZVI/PDS conditioned at 160 °C. The dewatering performance did not further improve as the HT temperature increased to 160 °C. In terms of the CST and SRF, the optimal HT temperature for the following tests was selected as 120 °C.

The effect of PDS dose on WAS dewaterability was evaluated during HT at 120 °C with a ZVI dose of 100 mg/g TSS, which is shown in Figure 1(c). The SRF decreased from 4.07 × 10¹² to 3.47 × 10¹² m/kg when the PDS dose increased from 50 to 100 mg/g TSS and then declined to 0.79 × 10¹² m/kg as the dose of PDS increased to 200 mg/g TS (Figure 1(c)). The sludge dewaterability increased significantly with increasing PDS concentration because more produced from PDS can oxidize the sludge flocs (Equation (3)) (Zhen et al. 2012).

The CST declined from 104.5 to 89.8 s when the ZVI dose increased from 10 to 50 mg/g TSS and was further reduced to 52.7 s at a ZVI dose of 100 mg/g TSS (Figure 1(d)). At the same time, the release of dissolved iron from ZVI increased from 447.56 mg/L to 708.1 mg/L (Fig. S1(b)), which played an important role in the activation of PDS. Compared to RS, the CST decreased marginally when the ZVI concentration was 10 mg/g TSS, which may be due to the low content of ZVI limiting the AOPs (Zhen et al. 2018). The increased ZVI concentration provided more Fe²⁺ (up to 532.95 mg/L) during the AOPs (Fig. S1(c)) when the ZVI concentration was 100 mg/g TSS, which finally led to more generation of (Equation (1)). After that, the produced oxidized EPS and released bound water into free water (Zhen et al. 2012).

As a result, a PDS dose of 200 mg/g TSS and a ZVI dose of 100 mg/g TSS were chosen as the optimal conditions for HT-ZVI/PDS treatment at 120 °C. According to Chen et al. (2020), SRF reduction by thermal hydrolysis (200 °C, 101 min) coupled with sodium persulfate (100 mg/g of dry solids) conditioning process was 91.65%, which is similar to our results (86.72%). In addition, mild hydrothermal conditions could effectively reduce energy consumption and also avoid the production of substances such as melanoids.

The variations in DOM, including SCOD, NH₄⁺-N, protein and polysaccharide, in the hydrolysates are shown in Figure 2. DOM was used to evaluate the decomposition of sludge flocs (Zhen et al. 2018; Xiao et al. 2020).

The SCOD concentration was 590.23 mg/L in the presence of PDS, which was higher than that in RS (300.56 mg/L) (Figure 2(a)). However, SCOD was reduced to 325.12 mg/L after treatment with the combination of PDS and ZVI (Figure 2(a)). The SCOD content was not all the soluble organic matters released during the ZVI/PDS process, but the residual after secondary oxidation (Zhen et al. 2018). This indicated that massive produced after the activation of PDS by ZVI can not only cause the release of biopolymers but also simultaneously nonselectively mineralize the extracted organics (Kim et al. 2016). Likewise, Zhen and coworkers reported that the Fe(II)/PDS process produced more ^, resulting in a decrease in SCOD (Zhen et al. 2018).

The increase in HT temperature could cause a significant increase in the concentration of SCOD (Figure 2(a)). COD solubilization was linked to the temperature of HT, indicating sludge flocs were decomposed (Zhen et al. 2018). After HT-PDS conditioned at 120 °C, the SCOD was 2,255.43 mg/L, which was much higher than that under HT conditioned at 120 °C (1,610.27 mg/L). After the interaction of HT and ZVI/PDS, the SCOD concentration was reduced to 1,632.50 mg/L, indicating that the mineralization of organics seemed to occur as the produced by activating the PDS increased (Zhen et al. 2018). In contrast, when the HT temperature was 160 °C, compared with HT-PDS conditioning, SCOD increased from 6,250.74 to 6,430.53 mg/L under the combination of HT and ZVI/PDS, which may be because the release of organic matter was greater than mineralization.

The protein and polysaccharide contents in the hydrolysates increased significantly with increasing HT temperature (Figure 2(c) and 2(d)). After adding PDS at 120 °C, the protein content decreased from 40.13 to 15.51 mg/L; however, at the same time, the polysaccharide content increased from 724.75 to 961.72 mg/L. Compared with HT-PDS treatment at 120 °C, the protein content further decreased to 10.36 mg/L but the polysaccharide content declined to 736.68 mg/L under HT-ZVI/PDS conditioned at 120 °C (Figure 3(d)). The combination of HT and PDS conditions promoted the dissolution of polysaccharides, while the synergetic effects of HT and ZVI/PDS promoted the degradation of polysaccharides. The protein content did not further decrease under HT-ZVI/PDS treatment at 160 °C, which was consistent with the trend of SCOD. The proteins degraded into amino acids and finally NH₄⁺-N, resulting in a higher NH₄⁺-N concentration. NH₄⁺-N increased from 52.43 mg/L for RS to 131.32 mg/L and 305.84 mg/L after HT and HT-ZVI/PDS conditioned at 120 °C, respectively. The decline in protein content indicated the breakup of peptide bonds and the decrease in polysaccharides suggested the decomposition of carbonyl functional groups, which suggested that a change in the number of hydrophilic functional groups and hydrophobic functional groups (Yu et al. 2014). More proteins and polysaccharides were degraded as a consequence of radical oxidation under the synergetic pretreatment of HT and ZVI/PDS, which enhanced the dewaterability of WAS (Kim et al. 2016; Chen et al. 2020). SEM was conducted to analyze the relationships among sludge dewatering performance indexes (CST and SRF) and organic components in the hydrolysate (Figure 4). The results showed that protein was significantly correlated with SRF (p < 0.001) and CST (p < 0.01).

3D-EEM spectroscopy was applied to characterize the effects of ZVI/PDS and HT on EPS composition and transformations. The detailed results are shown in Figure 3. Regions I, II, III, IV, and V in the 3D-EEM were aromatic protein-like, tryptophan-like proteins, fulvic acid-like materials, soluble microbial product, and humic acid-like, respectively (Yu et al. 2018a). The 3D-EEM results indicated that regions I, II and IV were predominant in the EPS fraction of RS (Figure 3(a)). The fluorescence intensity of all areas in the S-EPS, LB-EPS and TB-EPS of the WAS with PDS addition were significantly lower than that in the RS. However, the fluorescence intensity of regions III and V in the S-EPS was enhanced with ZVI/PDS addition, and all the fluorescence regions in TB-EPS were weakened significantly. This result indicated the migration of organics from TB-EPS to S-EPS. In addition, EPS were decomposed by radicals from the activation of PDS (Zhen et al. 2012).

The fluorescence intensity of regions I, II and IV decreased after HT at 120 °C and almost disappeared at a higher HT temperature (160 °C), which indicated that HT could accelerate the decomposition of LB-EPS and TB-EPS effectively (Chen et al. 2020). In contrast, the fluorescence intensity of regions III and V was strengthened after HT at 120 °C and further enhanced at 160 °C, suggesting that increasing temperature could promote the transformation of biopolymers into fulvic and humic acids (Wang et al. 2021). The fluorescence intensity in regions III and V was strengthened under HT-PDS but weakened after the combination of HT and ZVI/PDS addition (Figure 3(b) and 3(c)), which indicated fulvic acid and humic acid were released and then removed (Li et al. 2018).

Four fluorescent components were identified based on the PARAFAC model, which were microbial activity-related humic materials, terrestrial humic-like substances, tryptophan and fulvic-like (Fig. S2) (Kowalczuk et al. 2009). Statistical correlation analysis showed that CST was significantly positively correlated with the intensities of fulvic-like in TB-EPS (p < 0.01), which indicated that fulvic-like substances in TB-EPS impeded sludge dewaterability (Table S2). It can be inferred that fulvic-like in TB- EPS was removed after the synergistic effect of HT and ZVI/PDS addition, thus sludge dewaterability was improved.

In order to reveal the typical functional groups in the raw and conditioned WAS, FTIR was employed. The broad band at approximately 3,305 cm⁻¹ referred to the stretching vibrations of O-H and N-H in proteins, and the band at 2,933 cm⁻¹ was C-H stretching vibrations (Liu et al. 2019). Bands of the amide I associated with proteins appeared at 1,700–1,600 cm⁻¹, and bands of the amide II associated with proteins were detected at 1,600–1,500 cm⁻¹ (Chen et al. 2020). Specifically, the bands at approximately 1,660 cm⁻¹ (C = O) and 1,538 cm⁻¹ (C = C and N-H) corresponding to amides I and II were related to proteins. The peaks at 1,500–1,300 cm⁻¹ are the COO- and C = C stretching vibrations in the aromatic ring carbons (Wang et al. 2018). Bands of amide III region associated with proteins appeared at 1,300–1,200 cm⁻¹, which was attributed to the C-N stretching vibrations. Peaks at 1,200–900 cm⁻¹ was for ring vibrations C-O-C, C-O-P and O-P-O in polysaccharides and nucleic acid (Chen et al. 2021). After conditioning, the peak intensity of functional groups decreased and then increased (Fig. S3), which was consistent with the variation of protein and polysaccharide contents in the sludge. Decomposition of EPS and microbial cells were further analyzed by infrared self-deconvolution with curve fitting (Wang et al. 2018). Compared with RS, the peak at 1,243 cm⁻¹ of the amide III region disappeared under the synergistic effect of HT and ZVI/PDS at 120 °C (Figure 5), which could cause a decrease in hydrophilic functional groups such as C-N. In addition, the O-P-O stretching vibration at 927 cm⁻¹ indicated the disintegration of the cells and the release of intracellular nucleic acid (Figure 5) (Chen et al. 2021). Hence, water could be released easily and sludge dewaterability was improved.

The mechanism of sludge dewatering performance under collaborative treatment of HT and ZVI/PDS at 120 °C could be further revealed by investigating the hydrophilic and hydrophobic characteristics of WAS. The C 1s peak related to the hydrophilic and hydrophobic functional groups were classified into C-(C/H), C-(O/N) and C = O by XPS spectra (Figure 6 and Table S3) (Wang et al. 2018; Yu et al. 2020). Hydrophilic substances (comprising C-(O/N) and C = O) in the WAS by HT conditioned at 120 °C accounted for 52.55%, which was much higher than that in the WAS conditioned by HT-ZVI/PDS (30.86%) and HT-PDS (45.27%) and the RS (49.14%) (Table S3). These results implied that the number of hydrophilic substances in the WAS increased after HT, which caused the deterioration of the dewatering performance of sludge. The hydrophobic components (comprising C-(C/H)) of the organic matters in WAS were different: 47.55% (HT) <50.86% (RS) <54.73% (HT-PDS) <60.14% (HT120-ZVI/PDS) (Table S3). The hydrophobic substances increased after combining HT with PDS addition and further increased under HT-ZVI/PDS. Thus, the enhanced hydrophobicity of sludge contributes to improve sludge dewatering performance, which also was in accordance with the results shown in Figure 1.

The distributions and fractionations of iron in the solids and hydrolysates were determined to reveal the effects of ZVI on PDS activation at different HT temperatures. Fe²⁺ released from ZVI played an important role in homogeneous AOP efficiency (Zhen et al. 2018). Dissolved iron ions from the RS were not detected and remained constant after HT at 120 °C and 160 °C, although the total iron in the RS was 1,218.25 mg/L (Fig. S1(a)). PDS addition had a limited effect on iron dissolution at room temperature. However, the dissolved iron ions increased to 191.27 mg/L after HT-PDS treatment and increased sharply to 708.25 mg/L after the HT-ZVI/PDS process at 120 °C (Fig. S1(a)). More specifically, the Fe²⁺ concentration increased to 532.95 and 974.12 mg/L after the HT-ZVI/PDS process at 120 °C and 160 °C, respectively (Fig. S1(c)). However, excess Fe²⁺ may directly consumed (Equation (2)), which was unfavorable to the degradation of organic matters (Zhen et al. 2012). The remaining amount of PDS was analyzed to evaluate the usage efficiency. 58.00% and 18.52% of PDS was consumed in the presence or absence of ZVI, respectively (Fig. S1(c)). Almost all persulfate was consumed after the interaction of HT at 120 °C and 160 °C, indicating that since PDS can be activated effectively during HT, the dewatering performance of sludge can be further enhanced.

In this study, ZVI-activated PDS oxidation was evaluated to improve sludge dewaterability during hydrothermal treatment at moderate temperatures. Under the optimal condition of PDS dose of 200 mg/g TSS, ZVI dose of 100 mg/g TSS and HT at 120 °C, SRF reduced to 0.79 × 10¹² m/kg and CST decreased to 52.7 s, respectively, compared to that of raw sludge (5.95 × 10¹² m/kg and 190.60 s). Statistical correlation analysis showed that soluble protein (p < 0.001) and fulvic-like (p < 0.01) in EPS were the main factors limiting sludge dewaterability. The synergistic effect of HT and ZVI/PDS caused the chemical bonds of C-N and O-P-O to break down, which increased the hydrophobicity of sludge, ultimately improving sludge dewatering.

The authors declare that they have no competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The supports from National Key Research and Development Program of China (2020YFC1,908,700), the Natural Science Foundation of Jiangsu Province (BK20200407), Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX20_0027) and the Fundamental Research Funds for the Central Universities (3203002110D) are gratefully acknowledged.

All relevant data are included in the paper or its Supplementary Information.