Using electrolytic zero-valent iron-activated sodium hypochlorite (EZVI-NaClO) to pretreat sludge, the capillary suction time (CST) was utilized to evaluate sludge dewaterability. Ammonia nitrogen (NH4-N), dissolved phosphorus, and total phosphorus in the supernatant were used to analyze sludge disintegration. This approach aimed to evaluate the effectiveness of the pretreatment process and its impact on the sludge composition. The migration and transformation of extracellular polymeric substances (EPS), including dissolved EPS (S-EPS), loosely boundEPS, and tightly bound-EPS (TB-EPS), were analyzed by detecting protein and polysaccharide concentrations and three-dimensional fluorescence excitation-emission spectroscopy (3D-EEM). The sludge particle properties, including sludge viscosity and particle size, were also analyzed. The results suggested that the optimal pH value, NaClO dosage, current, and reaction time were 2, 100 mg/gDS (dry sludge), 0.2A, and 30 min, respectively, with a CST reduction of 43%. Protein and polysaccharide contents in TB-EPS were significantly reduced in the EZVI-NaClO group. Conversely, protein and polysaccharides contents in S-EPS increased, suggesting that EZVI-NaClO treatment could disrupt the EPS. Besides, the viscosity of the treated sludge decreased from 195.4 to 54.9 mPa·S, indicating that sludge fluidity became better. ZEVI-NaClO could enhance sludge dewaterability by destructing protein and polysaccharide structure and improving sludge hydrophobicity.

  • Electrolytic zero-valent iron-activated sodium hypochlorite (EZVI-NaClO)-modified extracellular polymeric substance (EPS) extracted from WAS.

  • EZVI-NaClO can reduce O = C-NH- that affects protein strength and stiffness.

  • EZVI-NaClO enhances the flocculation and hydrophobicity of EPS.

Electro-catalytic sludge condition is an innovative method for enhancing sludge dewatering performance, by the application of an electric field that propels water moving from the sludge toward the cathode (Yu et al. 2017; Gao et al. 2021). Electro-catalytic dewatering demonstrates the ability to decrease sludge moisture content from 85 to 60% rapidly with minimal energy consumption. When achieving a 50% drying fraction, the energy consumed by electro-catalytic conditioning is 50% of the traditional thermal drying method. Therefore, electro-catalytic dewatering technology exhibits significant potential for widespread industrial utilization (Mahmoud et al. 2018; Sha et al. 2019).

Hypochlorite as a potent oxidant can generate highly effective free radicals with remarkable oxidation properties. The predominant form found in the NaClO solution is hypochlorous acid (HOCl) within a pH of 2–6, which is a highly oxidizing substance that could attack the microorganism cells from their internal components and result in cell lysis. So its predominant application lies in water treatment, where it is widely utilized as both disinfectant and bleach (Cheng et al. 2022). The O-Cl bond in NaClO can be easily cleaved at room temperature, resulting in the activation of ClO-. This facilitates the formation of oxygen free radicals, including O2· and ·OH, and augments the oxidizing capacity (Guo et al. 2023). This effect can be intensified by adding an activating agent that generates a significant amount of other reactive free radicals, such as ·OH, Cl·, and ClO·. Several studies found that employing NaClO as a replacement for H2O2 in Fenton reaction for wastewater treatment was cost-effective and can enhance efficiency in the removal of chemical oxygen demand (COD) (Tunçal et al. 2015; Behin et al. 2017; Tunçal et al. 2018). Similarly, Ca(ClO)2 with Fe2+ not only facilitated the degradation of macromolecular substances but also promoted the subsequent mineralization of small molecular substances (He et al. 2022). Besides, it has been confirmed that the combination of Fe(VI) and NaClO could prove effectively oxidized ability in degrading phenolic substances when pH was set at 8, and with a reduction of 22.8% in the formation potential of disinfection by-products (He et al. 2023). The findings suggested that hypochlorite had a significant potential for pollutant degradation in wastewater treatment. When the Fe2+/Ca(ClO)2 system was compared to the Fenton reagent. It was found that the Fenton reaction occurred only on the surface and outside the sludge floc. However, the HClO produced by Fe2+/Ca(ClO)2 system resulted in the release of intracellular organic matter into the liquid phase (Yu et al. 2019b). However, excessive hypochlorite can result in an overproduction of HClO and OH-, leading to an overly alkaline environment and excessive decomposition of extracellular polymeric substance (EPS), which exacerbates dewatering performance (Liang et al. 2019). Conversely, the Fe2+/NaClO system, which produces less HClO, is extensively researched in sludge conditioning. It decomposes macromolecular substances effectively in sludge without deteriorating dewaterability (Yang et al. 2021).

It is important to note that Fe(II) is readily converted into trivalent iron, whereas the conversion rate of trivalent iron to Fe(II) is quite slow. This necessitates adding a significant amount of ferrous salts to activate sodium hypochlorite. When zero-valent iron (ZVI) is employed as the electrode, the sequential conversion between ZVI, Fe(II), and Fe(III) can be easily achieved (Lin et al. 2014). The research on using electrolytic ZVI-activated hypochlorite to pretreat sludge is relatively rare. Therefore, it is worthwhile to conduct more in-depth studies on the application of hypochlorite in sludge dewatering treatment.

The reaction of iron-activated sodium hypochlorite is as follows:
formula
(1)
formula
(2)
formula
(3)
formula
(4)
formula
(5)
formula
(6)

The main objectives of this study are to (1) investigate the improvement of sludge dewaterability and disintegration performance using electrolytic ZVI-activated sodium hypochlorite (EZVI-NaClO) system and optimize experimental parameters; (2) examine the effects of the EZVI-NaClO on the migration, transformation, and property changes of EPS; (3) and obtain more comprehensive insights into the mechanism of EZVI-NaClO in conditioning sludge and EPS property changes.

Raw sludge

The raw sludge (RS) used was obtained from the reflux sludge of the secondary sedimentation tank at a sewage treatment plant in Wuhan. The sludge was filtered using a 30 mesh sieve, then settled for 2 h, and the supernatant was removed. The thickened sludge was stored at 4 °C (Guo et al. 2021). The range of raw sludge pH, moisture content, capillary suction time (CST), suspended solids, and volatile suspended solids were 6.5–7.4, 96.5–98%, 60–80 s, 27–35 g/L, and 18–23 g/L, respectively. CST was determined by the CST instrument (304M model, Triton, UK).

Procedure of sludge conditioning

The bench-scale test for sludge conditioning was conducted in a batch mode using beakers under stirring by a magnetic stirrer. First, 600 mL of sludge samples were poured into a 1 L beaker, using iron plates as electrodes with a gap between two electrodes of 5 cm and an immersion depth of 6 cm. pH ranges between 1 and 6. NaClO dosage was controlled in the range of 60–140 mg/gDS with a gradient of 20 mg/gDS, and current was controlled at 0.1, 0.2, and 0.3 A by using a DC-regulated power supply (HY3005B, HYELEC, China). Sludge sample of 5 ml was taken to detect CST every 15 min (Guo et al. 2021). The initial CST of raw sludge was defined as CST0, and the ratio of conditioned sludge CST to CST0 was used to evaluate dewaterability. Raw sludge, sludge with dosing NaClO only, and sludge with electrolytic ZVI and pH of 2 (regarded as electro flocculation (EF)) were all set as control groups.

Extraction method of EPS

EPS were extracted by thermal extraction, and detailed procedures were as follows: sludge sample (25 mL) was centrifuged at 4000 r/min for 5 min. The supernatant was filtered through a 0.45 μm membrane to obtain dissolved EPS (S-EPS). The centrifuged precipitate was diluted to 25 mL with 0.05% NaCl solution heated to 70 °C. After mixing for 1 min, it was centrifuged for 10 min, and the supernatant was filtered to obtain loosely bound-EPS (LB-EPS). The residue was diluted to 25 mL with NaCl solution once again, then heated at 80 °C for 30 min, and centrifuged for 15 min. The filtrate was defined as tightly bound-EPS (TB-EPS) (Li et al. 2016; Guo et al. 2019). Protein (PN) and polysaccharide (PS) contents in EPS were detected by the anthrone – sulfuric acid method and the fast Lowry method, respectively.

Other analysis methods

Ammonia nitrogen (NH4-N), dissolved phosphorus (SRP), total phosphorus (TP), COD, and UV254 were detected by the standard method. The iron ion contents in the supernatant and sludge cake were determined using an o-phenanthroline method by spectrophotometer (UV-6100S, KunKe, China) (Yu et al. 2019a). The sludge viscosity was determined using a viscometer (NDJ-9S, Shanghai Precision Instrument, China). The sludge pH was monitored using a digital pH meter (ST3100, Ohous, USA). The concentration, atomic structure, and chemical composition of C, O, and N in sludge were analyzed via X-ray photoelectron spectroscopy (XPS; Thermo Scientific K-Alpha, Thermo Scientific, USA). The structure and composition of minerals presented in sludge were analyzed by X-ray diffraction (SmartLab SE, Japan).

Experimental parameters optimization

The changes of CST/CST0 under different treatment conditions are illustrated in Figure 1. The CST decreased at first, then increased with the decreasing pH when dosing 80 mg/gDS NaClO under 0.2A current (Figure 1(a)) and different conditioning time. CST decreased significantly with a maximum reduction of 60% when conditioned for 15 min at a pH of 2. These results suggested that a pH of 2 was the most effective to reduce sludge CST. It would produce a negative effect on sludge dewaterability when pH was higher than 4, and the deteriorating effect was more significant with a higher pH value. This is attributed to the predominant oxidation of HClO produced in the reaction. In addition, precipitation of OH with Fe2+ and Fe3+ weakened flocculation function. The CST/CST0 value was 0.9 approximately under different conditioning time when the NaClO dosage was 60 and 80 mg/gDS with a pH of 2 and current of 0.2 A (Figure 1(b)), indicating that CST was reduced in low NaClO dosage negligibly. The lowest CST/CST0 value was observed at a dosage of 100 mg/gDS, reaching around 0.5, resulting in CST decreasing from 78.2 to 42.1 s. At dosage more than 120 mg/gDS, the CST reduced no further, and on the contrary, the reduction extent became less obvious compared to a dosage of 100 mg NaClO/gDS. Besides, CST was increased with time under this dosage. The results suggest that 100 mg NaClO/gDS was the optimal dosage, and a high dosage would deteriorate dewaterability. Therefore, NaClO dosage was set at 100 mg/gDS for the subsequent experiments. It can be observed that there is only a slight decrease in CST when the current was set at 0.1 A (Figure 1(c)) with pH and NaClO dosage fixed at 0.2 A and 100 mg/gDS, respectively. The maximum reduction appeared at 30 min, with CST/CST0 dropping 0.23. In contrast, larger reductions were observed at 0.2 and 0.3 A, with CST reductions to around 40% when conditioned for 30 min. Therefore, the optimal current and conditioning time was 0.2 A and 30 min, respectively, which were selected for the subsequent tests.
Figure 1

The change of CST/CST0 under different pH values (a), NaClO dosage (b), and current (c).

Figure 1

The change of CST/CST0 under different pH values (a), NaClO dosage (b), and current (c).

Close modal

Substances in supernatant

The concentrations of NH3-N, SRP, TP, COD, and UV254 in the supernatant are shown in Figure 2. The NH4-N contents increased significantly after conditioning (as shown in Figure. 2(a)). Especially, EZVI-NaClO conditioned sludge had the highest content of 137.3 mg/L in supernatant, which was four times higher than that of RS (31.6 mg/L). The result suggested that the EZVI-NaClO treatment was the most effective approach in breaking down nitrogen-containing substances. As for SRP (Figure 2(b)) and TP (Figure 2(c)), the contents of both were increased and the change tendency was similar. NaClO oxidation resulted in both the SRP and TP contents being the highest, reaching 174.2 and 206.4 mg/L, respectively. The results demonstrated that NaClO, EF (pH = 2), and EZVI-NaClO could react with organic substances containing phosphorus and make phosphorus releasing from the sludge effectively, possibly through the reaction and precipitation of iron ions or the mineralization facilitated by electrolysis and oxidation. It will be analyzed in Section 3.5. COD increased obviously after NaClO, EF, and EZVI-NaClO conditions (as shown in Figure 2(d), reaching to 696.4, 671.0, and 653.2 mg/L, respectively). The result indicated that a large amount of organic matter was released from the sludge to supernatant by the oxidation. The slight COD reduction after the EZVI-NaClO condition may be due to organic matter decomposition and mineralization (Behin et al. 2017). The UV254 (as shown in Figure 2(e), the supernatant diluting10 times before testing) absorbance increased visibly with NaClO, EF (pH = 2), and EZVI-NaClO condition, which were 0.34, 0.24, and 0.25, respectively. The results demonstrated that NaClO oxidation releases and dissolves humus-like organics and aromatic compounds into the supernatant. However, the content of the substances was reduced after EZVI-NaClO treatment, which is likely due to the enhanced oxidation capabilities of EZVI-NaClO.
Figure 2

The change of NH4-N (a), SRP (b), TP (c), COD (d), and UV254 (e) in the supernatant.

Figure 2

The change of NH4-N (a), SRP (b), TP (c), COD (d), and UV254 (e) in the supernatant.

Close modal

EPS analysis

Research studies have confirmed that the main components of EPS, namely, PN, polysaccharides, and humus, have significant effects on sludge dewatering (Sheng et al. 2010). Different substances have different effects on sludge dewatering performance (Wu et al. 2016; Ge et al. 2019). The LB-EPS content has a positive correlation with sludge dewaterability (Zhen et al. 2018), while the TB-EPS content was negatively correlated with sludge dewatering (Fan et al. 2021). Specifically, excessively high PN content in sludge TB-EPS can hinder the dewatering process (You et al. 2017).

It can be seen from Figure 3 that PS (Figure 3(a)) and PN (Figure 3(b)) concentrations in TB-EPS decreased sharply while increasing greatly in S-EPS of conditioned sludge compared with that of RS. The PS content in raw sludge TB-EPS was 116.8 mg/L, which was significantly higher than that in S-EPS. After NaClO, EF(pH = 2), and EZVI-NaClO treatment, the decline of PS in TB-EPS was similar, with values of 25.8, 25.1, and 27 mg/L, respectively. However, the PS in S-EPS increased slightly lower in the EZVI-NaClO group compared to the other two groups.
Figure 3

The changes of polysaccharide (a) and protein (b) contents of EPS in different pretreatments.

Figure 3

The changes of polysaccharide (a) and protein (b) contents of EPS in different pretreatments.

Close modal

Raw sludge TB-EPS had a PN content of 738.2 mg/L, which was much higher than that of 67.4 mg/L in S-EPS. NaClO and EZVI-NaClO reduced 85% PN in TB-EPS approximately, with contents of 118 and 100.8 mg/L respectively. However, EF (pH = 2) only reduces PN content by around 67.4%. Conversely, PN in NaClO, EF (pH = 2), and EZVI-NaClO conditioned sludge S-EPS increased much higher than that of raw sludge reached to 589.65, 620.56, and 560.24 mg/L, respectively. The result indicated that PS and PN migrated and transformed from TB-EPS to S-EPS. Notably, EZVI-NaClO led to the largest decrease of PN in TB-EPS and the smallest increment in S-EPS, which demonstrated that the PN content was significantly influenced by sludge treatment (Zhang et al. 2017), and PS and PN in EPS might have been transformed into other substances.

Figure 4 displays the 3D-EEM diagram of the three layers of EPS. The majority of five types of fluorescent substances was found in raw sludge TB-EPS, with more prominent peak. While S-EPS just contained a small amount of soluble microbial by-products and tryptophan (Bourven et al. 2012), and LB-EPS mainly contains humic acid and glutamic acid (Wang et al. 2009). After EZVI-NaClO treatment, the fluorescence peaks of TB-EPS and LB-EPS became unconspicuous, leaving only small amounts of tryptophan, humic acid substances, and soluble microbial by-products, while more distinct for S-EPS. NaClO and EZVI-NaClO treatment resulted in a similar change in 3D-EEM diagram of TB-EPS (Figure 5(f) and 5(i)), but a big difference for LB-EPS and S-EPS.
Figure 4

The 3D-EEM diagram of raw sludge S-EPS (a) LB-EPS (b), and TB-EPS (c); EZVI-NaClO-related sludge S-EPS (d), LB-EPS (e), and TB-EPS (f); NaClO-treated sludge S-EPS (g), LB-EPS (h), and TB-EPS (i); and EF-treated sludge S-EPS (j), LB-EPS (k), and TB-EPS (l).

Figure 4

The 3D-EEM diagram of raw sludge S-EPS (a) LB-EPS (b), and TB-EPS (c); EZVI-NaClO-related sludge S-EPS (d), LB-EPS (e), and TB-EPS (f); NaClO-treated sludge S-EPS (g), LB-EPS (h), and TB-EPS (i); and EF-treated sludge S-EPS (j), LB-EPS (k), and TB-EPS (l).

Close modal
Figure 5

The content of Fe2+ and Fe3+ in sludge cake (a), the content and in the supernatant (b), and sludge viscosity under different treatments (c).

Figure 5

The content of Fe2+ and Fe3+ in sludge cake (a), the content and in the supernatant (b), and sludge viscosity under different treatments (c).

Close modal

NaClO oxidation causes a more pronounced peak value of fluorescent substances in S-EPS compared to EZVI-NaClO treatment. Electroflocculation (pH = 2) treatment results in more fluorescent substances in TB-EPS, suggesting that it does not deeply disrupt the sludge structure or sufficiently oxidize substances in the deeper layers. The results indicated that NaClO could breakdown sludge structure and make substances to releasing from TB-EPS to S-EPS. However, the substances in S-EPS could not be oxidized due to lack of sufficient oxidation capacity. On the other hand, EZVI-NaClO treatment enhanced the oxidation capacity, allowing for the release of various substances from TB-EPS to S-EPS. These substances could be directly oxidized or converted into nonfluorescent substances in the supernatant. The decreasing of substance concentration in S-EPS facilitated the separation of sludge and water (Liang et al. 2021; Sha et al. 2022).

Distribution of iron ions and sludge viscosity

As shown in Figure 5(a), Fe2+ and Fe3+ contents in raw sludge cake were only 0.25 and 0.2 mg/gDs, respectively. After EZVI-NaClO conditioning, Fe2+ and Fe3+ content increased significantly, both reaching around10 mg/gDs, which was higher than that in the EF (pH = 2) group. While for in sludge supernatant (Figure 5(b), the contents in EZVI-NaClO conditioned group were nearly equal to the EF (pH = 2) group. As for Fe2+ and Fe3+ in sludge supernatant, the contents of EZVI-NaClO-treated group were 40.8 and 7.8 mg/gDs, respectively, which were slightly higher than that of the EF (pH = 2) group. These results indicated that electrolytic EZVI produced much Fe2+, which was oxidized by NaClO into Fe3+. Moreover, the presence of Fe3+ effectively contributed to sludge flocculation and sedimentation.

Sludge viscosity is shown in Figure 5(c). The initial viscosity of raw sludge was remarkably high at 195.4 mPa·s. After NaClO, EF (pH = 2), and EZVI-NaClO treatment, the viscosity decreased to 42.9, 43.7, and 54.9 mPa·s, respectively. These results indicated that sludge fluidity was enhanced by the pretreatment. The relatively higher viscosity in EZVI-NaClO treatment might be due to the production of significant Fe3+ that promotes flocculation and particle aggregation.

Functional group in sludge analysis

The XPS spectra of N1s for sludge are shown in Figure 6(g)–6(l). There were two peaks of PN-N and pyrrole-N in the RS (Figure 6), which was similar to the finding of the study by Sha et al. (2022). The PN-N and pyrrole-N values of RS were 69.7 and 30.3%, respectively. After NaClO, EF(pH = 2), and ZEVI-NaClO treatment, the PN-N values of sludge decreased to 53.1, 48.8, and 23.8%, respectively, while the pyrrole-N values increased to 46.9, 51.2, and 76.2%, respectively (Figure 6(i) and 6(k)), indicating that the PN-N converted to pyrrole-N, especially in the ZEVI-NaClO group. The results indicated that ZEVI-NaClO had the highest degradation effect of PN-N, which provided information on the disintegration of PN in sludge.
Figure 6

The XPS analysis of C1s, N1s, and O1s analysis of sludge under different treatments.

Figure 6

The XPS analysis of C1s, N1s, and O1s analysis of sludge under different treatments.

Close modal
Figure 7

Mechanism analysis diagram.

Figure 7

Mechanism analysis diagram.

Close modal

The XPS spectra of C 1 s in Figure 6 show that there were three peaks in RS, i.e., C = C at 283.97 eV, C-C at 284.57 eV, and C-O at 285.57 eV, with values of 36.4, 36.4, and 27.2%, respectively. After NaClO, EF(pH = 2), and ZEVI-NaClO treatment, C = C disappeared, while the values of C-C increased in NaClO and ZEVI-NaClO, and the values of C-O increased in EF(pH = 2) significantly. Besides, the peak at 287.8 eV corresponded to O = C-NH- appeared in carbonyl and amide. C–C belonged to the nonpolar hydrocarbons, which exhibited surface hydrophobicity (Chen et al. 2020). These results indicated that the ZEVI-NaClO could significantly increase the hydrophobic group and decrease the hydrophilic group, which prevented the combination of water and sludge, enhancing the sludge dewaterability.

The XPS spectra of O 1 s are shown in Figure 6. The peaks at 530.8 eV and 532.5 eV were related to O-H and O = C (Cao et al. 2020). The peaks for O-H and C-O could represent polysaccharides, while the PN could be represented by C-C, C-H, C = O, and O = C-NH-. The value of O-H in ZEVI-NaClO conditioned sludge decreased from 57.1% of raw sludge to 38.7%, indicating that the affinity of O-H bonds and moisture became weak (Jin et al. 2004).

It could be concluded from the aforementioned analysis that EZVI-NaClO treatment had a more significant effect on breaking both PNs and polysaccharides structure in the sludge.

Possible mechanism analysis

The mechanism analysis is shown in Figure 7. The electron spin resonance (ESR) spectrogram presented that there were seven equidistant peaks with a height ratio of 1:2:1:2:1:1:2:1 in the EZVI-NaClO system. This specific peak pattern corresponded to the characteristic of chlorine dioxide (ClO2·) (Xu et al. 2022), which was derived from the activation of NaClO by Fe2+ generated from electrolytic ZVI with strong oxidizing property. In addition, it could be combined with other oxidizing groups (such as HClO) to attack sludge EPS and the internal material of sludge cells. So that the sludge and organic matter properties changed, and the separation effect of sludge and water was enhanced. Meanwhile, bound water was released and became free water with the cracking of sludge floc and cell structure. Besides, many trivalent irons were produced during the electrolysis process, which has excellent flocculation function, making the sludge floc more compact and reducing the combination ability of treated sludge and water. Comprehensively, the aforementioned effects enhanced the sludge dewatering performance.

This study utilized electrolytic ZVI to activate sodium hypochlorite to improve sludge dewatering performance, and the action mechanism was explored. The optimal pH value, NaClO dosage, current, and reaction time were 2,100 mg/gDS, 0.2A, and 30 min, respectively, with smallest CST/CST0 of around 0.5 after ZVI-NaClO treatment. The system could make sludge release NH3-N and COD into liquid. PN and polysaccharides in TB-EPS were oxidized and destructed and then migrated to S-EPS. Especially, the increase of nonpolar bonds in organic substance and changes in PN structure and properties reduced the affinity between water and sludge. Besides, the Fe(III) produced in the ZVI-NaClO process could aggregate with sludge. The series of functions improved sludge dewaterability significantly.

This work was jointly supported by the National Natural Science Foundation of China (Grant number 52200173).

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

The authors declare there is no conflict.

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