Enhanced Sewage Sludge Disintegration and Nutrients Release by Catalytic Microbubbles Ozonation Using Sewage Sludge-Based Char as Catalyst

Abstract

Using microbubble ozonation (MO) technique to disintegrate sludge is a promising sludge treatment process. To enhance the lysis and reduction of sludge, the catalytic ozonation consisting of MO and sewage sludge derived char (SC) were combined. Total solids (TS), volatile solids (VS), total nitrogen and phosphate (TN and TP) were selected as main parameters for evaluating the treatment performance both in solids and supernatant. With the utilization of the catalytic MO, the ozone utilization and sludge reduction were largely improved. At a reaction time of 90 min, an ozone utilization efficiency exceeding 99% was achieved by using a MO system. The optical ozone and sludge char dosages of 150 mg/g suspended solids (SS) and 1 g/L were found for sludge lysis, respectively. TS and VS concentrations decreased by 43% and 56%, respectively, as compared to those of 16.7% and 17.9% obtained by the treatment with MO alone under the condition of sludge solution pH 4. The supernatant soluble chemical oxygen demand (SCOD), TN, TP, NH₄⁺-N and NO₃⁻-N increased by 1750%, 205%, 25%, 31% and 43%, respectively. A small amount of additional SC exhibited strong catalytic activity on dissolving organic matter of the sludge, demonstrating the positive effect caused by the heterogeneous catalytic ozonation on sludge disintegration.

1. Introduction

The largely increased sewage sludge produced from wastewater treatment plants has become the burden of the environment. A high cost of 25–60% of the total operating cost is normally needed for the treatment of extra sludge [1,2,3,4,5]. The stabilization and minimization strategy for sludge is a promising way to meet the requirement of zero waste discharge, since the cost of sludge treatment can be reduced and the circumstance of sludge secondary pollution can be avoided. Lysis–cryptic growth is an effective reduction method for excess sludge [6,7,8], and the lysis process determines the rate-limiting step. When the disintegration process is implemented to enhance lysis, cells of microorganism can be destroyed to release the intracellular matters (e.g., extracellular polymeric substances (EPS)), which is utilized again, contributing to the reduction of overall biomass yield [9,10,11]. In recent years, many chemical and physical approaches for sludge destruction have been studied, such as ultrasonic process [1,12,13], mechanical disintegration [14,15], thermal hydrolysis [16,17] and ozonation [18]. In these mentioned strategies, ozonation has attracted intense interest, since 50–100% of the activated sludge production can be reduced [19,20,21]. At the same time, the cost of ozonation for sludge disintegration is lower than those of other methods.

Many researchers have contributed to the application of ozonation for sludge disintegration [15,22,23,24]. Kianmehr et al. found that ozonation could greatly improve the hydrolysis rate of sludge [25]. Braguglia et al. [26] observed that the ozonized sludge (4000 kJ/kg TS) possessed a high initial soluble COD, which greatly improved the digestion performance compared with that of untreated sludge. Considering the low efficiency on mass transfer of ozone molecules, the sludge solubilization by microbubble ozonation has been a subject of recent study. In comparison to common bubbles, the ozone microbubbles have smaller sizes with a diameter less than 50 μm, which increases specific surface area and lowers rising speed [27]. Moreover, that microbubble collapse can also prompt the formation of hydroxyl radicals, which has higher oxidation potential than ozone.

It is important to noted that previous study on sludge disintegration mainly focused on the ozonation itself, while few studies focused on the heterogeneous catalytic ozonation, which can provide fast degradation of sewage sludge and its reduction, especially in the system of sludge char catalyzed microbubble ozonation. In the past, the conversion of sewage sludge into carbonaceous catalyst has been one of the emerging attempts [28,29,30,31]. Wen et al. [31] found that a strong synergistic effect could be exhibited on the integration of SC with ozone for the improvement of oxalic acid removal. In general, the decorating of active iron species on raw sludge is expected to result in higher catalytic efficiency on catalytic ozone disintegration [32]. Huang et al. [33] had successfully used sewage sludge activated carbon/MnO_x to improve the aqueous oxalic acid degradation performance through ozonation. However, until now, the utilization of microbubble ozonation catalyzed by SC on the behavior of sewage sludge disintegration and nutrient release was scarcely reported. Therefore, in this work, the synergetic effect of SC and microbubble ozonation was investigated in heterogeneous microbubble disintegration of sewage sludge. The enhancement mechanism of sludge solubilization by the combined system of sludge char and micro-ozone bubbles was also discussed.

2. Materials and Methods

The sewage sludge used was collected from the secondary sentimental tank of Dongqu municipal wastewater treatment plant in Shanghai, China. The raw sludge was stored in a plastic container at 4 °C prior to use, and its basic characteristics are given in Table 1.

The raw sewage sludge was immediately centrifuged at 4000 rpm and dried at 105 °C to a constant weight. Then, the samples were carbonized in a furnace under 600 °C with N₂ protection. The resultant powder was washed several times, and the catalyst marked as SC was obtained. A volume of 20 L of sludge was added into an ozonation reactor designed in semi-batch mode. Before experiment, the ozone concentration and stability of the ozone-containing gas in the KI solution were confirmed by a three-way valve. The SC (0.1–1 g/L) and the continuous ozone gas were introduced into the reactor. At a certain interval, the ozonated sludges were collected for measurement. For the off-gas, its ozone concentration was also detected during the ozonation process. The millibubbles were produced by a stainless-steel diffuser (MBO-75S, Foshan, China) and the size range of the millibubbles was estimated by analyze the snapshot using Nanoparticle Tracking Analysis Technology. The output flow rate of inlet ozone concentration was controlled at 90 mg/L and ozone concentration is determined by the iodometric method. In the ozonation experiments, for comparison, the microbubble generator (contact time: 100 min) and the conventional bubble contactor (contact time: 120 min) were used, wherein the contact time is different due to their different ozone utilization efficiencies. The diagram illustration of the experimental set-up is shown in Figure 1. The whole distribution diagram of bubbles particles in the range of 0 to 600 nm within 20 min after Nanoparticle Tracking Analyzer (LM10-HSBFT14, Malvern Co, Britain) start can be seen in Figure 2; most particle were 100 nanometers, which is also relatively stable.

3. Results

3.1. Characterizations of SC

Figure 3a shows the X-ray diffraction (XRD) pattern of SC to determine the phases and crystallinity. Through the high temperature carbonization process, several crystal phases are obtained in SC, in which the peaks at 2θ = 26.2° correspond to the (002) plane of the graphite structure, which contributes to the carbonization of sewage sludge. Besides, the massive presence of minerals, such as Fe-based oxide, quartz and illite, is expected, as they remain in existence under the carbonization conditions employed. The N₂ adsorption–desorption isotherm of SC is given in Figure 3b, and the mesoporous structure can be exhibited due to it being type-IV with the H₂ hysteresis loop. The specific surface area is calculated by the BET method, and a high value of 121.1 m²/g is obtained for SC. These results demonstrated that SC with good crystallinity can provide many stable active sites for the catalytic reaction induced by microbubble ozonation.

3.2. Performance of Catalytic Ozonation with SC on Sludge Disintegration

With the catalytic treatment, the organics in sludge can be released into the liquid phase, leading to the reduction of the TSS and VSS in sludge [34]. Meanwhile, the disintegration efficiency is evaluated by the SCOD value. As shown in Figure 4a, with the combination of SC and microbubble ozone, the largely enhanced degree of disintegration (DD_SCOD) and SCOD are present. During the 160 min treatment, the SCOD increases from 0 to 1991 mg/L as the dosage changes from 0.5 to 275 mg/g SS. Then, the balance between the sludge lysis and the oxidation of the soluble substances is built, and the stable SCOD value is obtained. It is noted that, the introduction of 0.5 g/L SC substantially increased the SCOD in the first 60 min, indicating the strong catalytic effect of SC on ozone particles and the synergetic effect on both microbubbles and SC catalyst.

The cumulative ozonation efficiency of SC catalyzed sludge oxidation was further compared by using microbubble and SC catalyzed microbubble systems (Figure 4b). It is obvious that in the beginning of the reaction, the cumulative ozonation efficiency in SC catalyzed ozone system is much higher than that of the microbubble system, which demonstrates the synergetic effect of SC and microbubble oxidation. The obvious synergetic effect on both microbubbles and SC catalyst was mainly attributed to the high ozonation utilization efficiency in the SC catalyzed sludge oxidation system, which produced the formation of hydroxyl radicals for sludge disintegration and lysis.

3.3. Comparative Performance of Phosphorus and Nitrogen Release by Microbubble Ozonation with and without SC Addition

Ozone oxidation is able to decompose the microorganism cells, causing the release of cytoplasm compounds, and then the soluble protein and phosphorus concentrations in the bulk solutions increase. The rapid release of TN (Figure 5a) and ammonium (Figure 5b) is discovered for SC catalyzed ozone system when the dosage of O₃ is below 75 mg/L. Different from the continuous increase of TN concentration, the ammonium concentrations are initially increased rapidly and then increased slightly. The initial rapidly increasing content for amine is the result of the oxidative transformation of organic nitrogen. However, the slight increment of ammonium in SC catalyzed microbubble systems when O₃ dosage exceeded 150 mg/g SS further suggests the oxidation of ammonium to nitrogen. As O₃ alone cannot oxidize NH₃ to N₂, the indirect oxidation must contribute to the reactions. As some inorganic anions such as chloride and sulfate can exist in sludge solutions, the side reactions between SC catalyzed hydroxyl radicals and Cl⁻ or SO₄²⁻ can prompt the formation of Cl• and SO₄²⁻•. As most literature has suggested, the Cl⁻ ions are capable of oxidizing the ammonium into N₂ [35,36,37] thus resulting in the slight reduction of TN as the O₃ input reaches 200 mg/g SS. In comparison to microbubble ozonation in the absence of SC, the continuous increase of ammonium and TN in bulk solutions are expected due to the destruction and release of cytoplasm of sludge cells.

Cl⁻ + HO• → OH⁻ + Cl•(1)

2Cl• + O₂ → 2ClO• + O₂⁻(2)

The concentration of TP (Figure 5c) proceeds to grow when ozone dosage is below 150 mg/g SS and reaches equilibrium with a further increase to the ozone dosage. It is attributed, when the ozone dose exceeds 150 mg/g SS, to the rection between the organic substance released from sludge flocs when the hydroxyl radicals can be produced, leading to the smaller increasing tendency of TP at higher ozone input.

3.4. Effect of pH on the Performance of SC Catalyzed Microbubble Sludge Ozonation

In the SC catalyzed ozonation systems, the influences of pH contain the decomposition of ozone molecules, the surface properties of SC, and the charge of solubilized organic molecules, which inhibit prediction of the behavior of sludge decomposition and destruction. Thus, the effect of the initial pH on the removal efficiencies of MLSS and MLVSS removal efficiency in the SC catalyzed microbubble systems are investigated. Figure 6 shows that the highest MLSS and MLVSS removals are achieved at alkaline pH of 10. When the pH descends to 7, the MLSS and MLVSS removal efficiencies decrease rapidly, especially when the ozone dosage exceeds 150 mg/g SS. As pH decreases to 4, it is interesting to find that the MLSS and MLVSS removal efficiencies recover. It is reported that alkaline pH is favorable for O₃ decomposition. The OH^- assisted H₂O⁻ formation contributes to produce the highly oxidative hydroxyl radicals, which play a dominant role in sludge destruction due to the higher O₃ dosage.

O₃ + HO⁻ → HO₂⁻ + O₂(3)

O₃ + HO₂⁻ → HO• + O₂•⁻ + O₂(4)

The measured pH_pzc of SC catalyst is 4.1 by titration, and the higher MLSS removal is achieved at pH 4.0 in the alkaline region (Figure 6). The best organic removal is often achieved at the pH near pH_pzc in catalytic ozonation process. pH_pzc refers to the pH at which the catalyst surface is zero charged. When the solution pH is higher than this point, the electrophilic H in the surface hydroxyl groups will be ionized to decrease the surface OH⁻ groups. The SC surface is positively charged and the protonation process is strengthened when pH = 4, which promotes the decomposition efficiency of microbubble ozone due to the decreased nucleophilicity of the O in surface hydroxyl groups. In this case, the SC catalyzed ozonation probably occurred at the catalyst surface, and the radical reaction in bulk phases should be subordinate in the SC catalyzed microbubble ozonation process.

It can be observed from Figure 6 that the SC catalyzed ozonation of sewage sludge can be divided into two phases with the increase in the ozone dosage. The first phase occurs at the ozone dosage below 150 mg/g SS, which is the process of sludge particle dissolution by the direct attack of ozone molecules. In this period, the sludge floc structure is firstly disrupted by ozone, and then a portion of zoogloeal was dispersed into individual cells. In the second stage, namely when the ozone dosage exceeds 150 mg/g SS, the accumulated ozone in aqueous phase reacts with SC to produce hydroxyl radicals, thus promoting the damage of cell membrane, releasing and degrading the extracellular and intracellular matters in liquid phase. As a result, the MLSS and MLVSS in this study present a sharp increase. The dissolved organics released from EPS and cells are also mineralized by the oxidative radicals, and then the ozone quantity that reacts with the sludge is decreased. Moreover, as some volatile fatty acids (VFA) are recalcitrant to radicals’ attack, the VFA can be produced from the reaction of the mater containing C, leading to the lower pH values. As shown in Figure 7, the pH decreases slightly as the reaction proceeds, especially for the initial pH at 10.

Figure 8 shows the fluorescence intensity of organics in SB-EPS (slightly bonded extracellular polymeric substance) with different initial pH which is greatly reduced over the combined SC and microbubble ozonation. SB-EPS is the substance dissolved in the supernatant [38]. The fluorescence peaks related to the compositions of EPS can be categorized into five zones, shown in Figure 9. The I and II zones, which have the excitation wavelength (ex) and emission wavelength (em) lower than 250 and 350 nm, correspond to the protein-like substances having aromatic structures. The III zone, which has the ex and em between 200–250 and lower than 380 nm, is indicative of fulvic acids. The IV zone, which has the ex between 250–280 nm and the em < 380 nm, corresponds to the dissolved cellular substance. The V zone, having the ex and em higher than 280 and 380 nm, respectively, is the range representing humic acids.

Only the aromatic protein-like substance is found in SB-EPS for the raw sludge. Differently, with the introduction of O₃, aromatic protein-like, fulvic acid-like, soluble microbial by-product-like, and humic acid-like substances can be detected in the spectra. Compared with the treated sludge at initial pH 7, the fluorescence peaks of SB-EPS at pH 10 and 4 are mainly covered in the II–V zones, confirming the release of large number of EPS and its degradation into volatile acids. Furthermore, the SB-EPS content in supernatant increases due to the release of more fulvic acid-like substances at pH 4.

According to above results, with the combination of SC and microbubble ozone, the sludge networks have been effectively destroyed to release organics into supernatants, and then the SB-EPS content increases. Meanwhile, the pH serves an important role in determining the release of LB-EPS into supernatant, as shown the Figure 9. The proportion of humic acids obviously increases when initial pH is 4, which demonstrates that the surface catalyzed ozone oxidation can further destroy and degrade the protein into small acids in bulk solutions. However, for pH 10, the formation of fulvic acids can be primarily induced by the ozonation.

3.5. Mechanism Discussion

As shown in Figure 10, the sludge flocs, particularly the Zoogloea structures, are first disrupted by ozone through the oxidation of TB-EPS and LB-EPS, and bridging matter and individual cells are formed from sludge flocs. Since microbubble ozone has a strong oxidative efficiency, the extracellular and intracellular matters can be released into the bulk phase from the destruction of the cell membrane.

In addition, the microbubbles at acidic pH are protonated and could be adsorbed onto SC surface. The surface catalyzed ozone decomposition promotes the formation of hydroxyl radicals. It is reported that O₃ can be decomposed to produce •O, which can react with the surface hydroxyl surface groups of SC to form O₂H⁻ [39], and then •O₂H can be produced by the reaction of O₂H⁻ with another O₃. These •O₂H radicals subsequently react with another O₃ to generate •O₃⁻ or transfer into •O₂⁻ and react with O₃ to yield •O₃⁻, and finally the O₂ and free •OH are produced. These highly oxidative radicals can react with the soluble maters such as proteins, C, N, P from EPS and cells. The reaction of the mater containing C promotes the formation of the volatile fatty acids. The radicals can mineralize the dissolved organics which is released from the destruction of EPS and cells to form CO₂ and H₂O, resulting in removal of MLSS and MLVSS.

To clarify the contribution of SC and microbubble ozonation, the hydroxyl radical scavenger of thiobarbituric acid (TBA) is introduced. It had been confirmed that TBA can react quickly with hydroxyl radicals with a rate constant of 6 × 10⁸ M⁻¹ S⁻¹, but react slowly with ozone [40]. The added TBA inhibits the MLSS and MLVSS removals in SC catalyzed systems and had no obvious adverse effect in microbubble ozone system (5.1% reduction of MLSS removal). Therefore, the surface catalyzed oxidation from SC and ozone molecules can contribute to the degradation and mineralization of soluble substances. Both ozone and soluble substances released from EPS and cells are adsorbed onto the SC surface, where ozone is complexed with surface-OH and reduced by transition metals such as Fe (II) in a carbon matrix, resulting in the formation of hydroxyl radicals and thus destroying the adsorbed EPS into final humic acids.

4. Conclusions

Sludge disintegration by SC catalyzed microbubble ozonation (MO) was conducted in this study to obtain the enhanced oxidative performance on sludge decomposition and destruction. The SCOD, TN, NH₄-N and TP were selected as main parameters to compare the catalytic effect on sludge lysis and reduction. The effect of initial pH was discussed on the removal efficiency of MLSS and MLVSS. The results show that when ozone dosage is below 150 mg/g SS, the ozone destruction of sludge flocs dominate the reaction. When ozone dosage exceeds 150 mg/g SS, the degradation of the released SCOD plays a key role in the process. The mechanism of surface catalyzed degradation is further discussed by TBA inhibiting experiments, and the results show a strong surface catalyzed effect at pH = 4. Therefore, the novel combination of SC and microbubble ozonation has proven to be a promising solution for sewage sludge disintegration.

Author Contributions

X.Z. (Xin Zhang): Investigation, writing draft, Writing—Reviewing and Editing; G.L.: Investigation, writing draft, Writing—Reviewing and Editing; Y.L.: Investigation, Writing—Reviewing and Editing; Y.M.: Investigation, Writing—Reviewing and Editing; X.H.: Conceptualization, Supervision, Investigation, Writing—Reviewing and Editing; X.Z. (Xinyu Zhou): Conceptualization, Supervision, Investigation, Writing—Reviewing and Editing. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Table

Figure 1. The diagram illustration of catalytic ozonation of sewage sludge. (1. Reactor; 2. Pump; 3. Microbubble generator; 4. Ozone generator; 5. In/off-gas detector).

Figure 2. Particle size distribution map of bubbles nanoparticles.

Figure 3. (a) XRD pattern and N2 adsorption–desorption isotherm for SC; (b) relative pressure of SC.

Figure 4. (a) Effect of ozone dosage on SCOD increment in the combined process of microbubble ozone and SC. (b) Comparative results of cumulative ozonation efficiency by using microbubble and SC catalyzed microbubble.

Figure 5. Effect of ozone dosage on variation of TN (a), ammonium (b) and TP (c) concentration in microbubble and SC catalyzed microbubble ozonation systems.

Figure 6. Effect of initial pH on MLSS (a) and MLVSS (b) removal efficiencies in SC catalyzed microbubble ozonation.

Figure 7. The pH variation in SC catalyzed ozonation system with the function of ozone dosage.

Figure 8. EEM spectra of SB-EPS fractions in sludge conditioned in SC catalyzed ozone systems with different pH (4, 7 and 10) and ozone dosage.

Figure 9. The proportion of different substance distribution (a) 55 mg/L, (b) 110 mg/L, (c) 140 mg/L in sludge supernatant by SC catalyzed ozonation.

Figure 10. Schematic diagram of SC catalyzed ozonation of sewage sludge.

Table 1

Basic characteristics of raw sewage sludge used.

Parameters	Data (mg/L)
Mixed liquid suspended solids (MLSS)	4500–6500
Mixed liquid volatile suspended solids (MLVSS)	3000–3500
pH	6.5–7.5
Soluble chemical oxygen demand (SCOD)	40–250
TP	1.5–6.0
TN	4.5–10.3
NH4+-N	2.0–6.2
NO3−-N	0.12–0.67

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