Enhancement in Dewatering Efficiency of Disrupted Sludge through Ultrasonication and Re-Flocculation—Sustainable Sludge Management

Abstract

The solids in sewage sludge are primarily composed of organic matter and offer new possibilities for sustainable sludge management, if considered as a stable biomass source in terms of quantity and quality. Reducing the volume of sludge with an extremely high moisture content is challenging, and enhanced dewatering through mechanical treatment is crucial from an environmental and sustainability perspective because it alleviates the reliance on thermal treatment. This study employed ultrasonication to enhance the dewatering efficiency of activated sludge. The disruption of sludge induced by ultrasonication notably facilitated the elimination of intracellular water during mechanical expression. Additionally, the ultrasonicated sludge was verified to be re-flocculated by introducing inorganic electrolytes such as Ca²⁺ (divalent cations), Al³⁺ (trivalent cations), and polyferric sulfate. Conversely, no re-flocculation of disrupted sludge was observed upon applying organic polymer flocculant. Under optimized conditions, the sludge re-flocculation progressed to form large flocs, leading to a decreased suspended solids (SS) value from 1423 to 73 mg/L and reduction in capillary suction time (CST) from over 2000 to 18 s. Following pretreatment, the moisture content of the mechanically expressed cake at 500 kPa decreased significantly from 76 wt% (untreated sludge) to less than 60 wt% (treated sludge) due to the elimination of intracellular water.

1. Introduction

The volume of discharged sludge within total industrial waste is increasing with the proliferation of wastewater treatment facilities. Sewage sludge constitutes the primary component of this discharged sludge, posing a significant challenge in contemporary wastewater treatment due to its exceptionally high moisture content, exceeding 99 wt%. Nonetheless, the solids within sewage sludge primarily consist of organic matter, rendering it a stable biomass source in terms of quantity and quality. Global research has demonstrated that solids can be transformed from waste into valuable resources, such as soil additives or fertilizers [1,2]. Moreover, this can yield energy equivalent to that of low-grade coal [3], and presents new possibilities for sustainable sludge management. Sewage sludge research is an indispensable element of sustainable development activities, and it combines aspects of environmental protection and a circular economy [2].

Various treatments have been deployed to mitigate sludge transport expenses. Thermal treatments, such as incineration and hydrolysis [4], represent common methods for reducing sewage sludge volume. However, they pose environmental concerns due to carbon dioxide emissions. Other methods, such as freezing and thawing [5,6], ozonation [7], acid and alkali treatment [8], microwave irradiation [9,10], osmotic pressure [11], and electric fields [12], have also shown promise in sludge dewatering. In recent years, sludge deep dewatering technologies have been suggested as high-efficiency dewatering methods for sewage sludge. The main methods include chemical preconditioning with high-pressure filtration and electrical mechanical dewatering [13,14,15,16]. Chemical preconditioning can destroy the floc structures and microbial cells. Conditioning agents that improve the dewatering performance of sewage sludge are attracting attention, and biochar and other materials are being used [17]. Nevertheless, costs and environmental impact challenges persist.

Ultrasonication has garnered considerable attention in sludge dewatering due to its advantages of minimal secondary pollution and simplicity within the dewatering system as a pretreatment [18]. The advantages of ultrasonication, such as its high efficiency, stability, and cleanliness, may also be advantageous when applied to sewage sludge management. Some studies have indicated that under specific conditions, ultrasonication reduces the requisite amount of flocculant [19]. Ultrasonic waves disrupt sludge flocs and microbial cells [18], converting bound water to free water and enhancing sludge dewatering performance [20,21]. However, this pretreatment often compromises sludge settleability and filterability due to the release of proteins, polysaccharides, and other substances, collectively termed extracellular polymeric substances (EPSs), from cells [22,23,24]. EPSs, present inside and outside cells, exhibit high water retention when combined with bound or free water [25]. Moreover, ultrasonic treatment results in fine-particle generation owing to sludge disruption. It has been noted that the fine particles generated as a result of excessive ultrasonic treatment have a negative effect on the reduction in cake moisture content during mechanical expression [26].

Fine particles clog the filter during filtration. Coagulation or flocculation is an effective technology used to improve dewatering performance [27], and inorganic salts and/or polymeric flocculants are used prior to sludge dewatering. This study implemented post-treatment flocculation following ultrasonic irradiation to enhance solid–liquid separation by incorporating fine particles generated during ultrasonic disruption. This enhancement is pivotal for efficient dewatering. A mechanical expression was employed to assess the dewatering rate and dewatering degree of disrupted sludge subjected to ultrasonication and re-flocculation. Generally, it is quite difficult to substantially reduce the moisture content of compressed cakes via mechanical expression because of the liquid contained in the microorganism cells [28]. However, the results from this study demonstrated that the moisture content of the mechanically expressed cake at 500 kPa decreased significantly from 76 wt% (untreated sludge) to less than 60 wt% (disrupted sludge through ultrasonication and re-flocculation) owing to the elimination of intracellular water. Furthermore, it was confirmed that the dewatering rate could be improved by selecting an appropriate flocculant. Therefore, establishing an efficient dewatering technology has the potential to increase the conversion rate of sewage sludge to biomass, which contributes toward sustainable sludge management.

2. Materials and Methods

2.1. Excess Sludge and Flocculants

Experiments were conducted using excess activated sludge obtained from a sewage treatment facility in Nagoya, Japan. The initial solid concentration ranged from 3.5 to 4.5 g/L and was concentrated to 5 g/L through decantation for 24 h in a refrigerator maintained at 4 °C. Subsequently, it was allowed to equilibrate to room temperature (23 ± 3 °C) before immediate utilization to minimize changes during experimentation. The true density of solids in the activated sludge, determined via a pycnometer, was 1.45 × 10³ kg/m³, following established procedures by prior researchers [26]. The electrical conductivity of the sludge was assessed using a conductivity meter (DS-52, Horiba Ltd., Kyoto, Japan). The characteristics of the sludge samples are listed in

Table 1. Characteristics of sampled sludge.

Floc diameter df,50 (µm)	103.5
Electrical conductivity (µS/cm)	497.4
Density of solid (kg/m3)	1450
Solid content (g/L)	5
CST (s)	31.4

. The floc diameter of the sludge was evaluated using the median diameter d_f,50. Four distinct flocculants were employed for sludge flocculation: CaCl₂ and Al₂(SO₄)₃ (Fujifilm Wako Pure Chemical Corp., Osaka, Japan) as inorganic salts, and polyferric sulfate (Danpower, Taki Chemical Co., Ltd., Kakogawa, Japan) and cationic polymers (Kurifix CP-802, Kurita Water Ind., Ltd., Tokyo, Japan) as inorganic and organic polymer flocculants, respectively. These salts and flocculants were combined in various concentrations and combinations to optimize sludge flocculation.

2.2. Ultrasonication and Re-Flocculation

The pretreatment was conducted using an ultrasonic homogenizer (UP-200S, Dr. Hielsher GmbH, Teltow, Germany) operating at 24 kHz and 200 W, following the warming of the sludge sample to room temperature (23 ± 3 °C). The ultrasonic tip was immersed in a sample comprising 80 g to a depth of approximately 15 mm above the bottom of a 100 mL beaker. A single cycle, consisting of an operating time of 0.5 s and downtime of 0.5 s, was employed [26]. The ultrasonication levels were varied by changing the sonication time from 60 to 150 s. The range of net ultrasonication exposure time was 30–75 s. The net ultrasonic power, which was evaluated by the ultrasonic power dissipated into a liquid rather than the load power, was 44.8 W [29], and the specific ultrasonic energy ranged from 16.8 to 42 J/g. Subsequently, the sample was mixed at 50 rpm [19] for 5–30 min using an agitator (Three-One Motor, BL 600, Shinto Scientific Co., Ltd., Tokyo, Japan). At this point, salts or flocculants were added under various conditions to promote sludge re-flocculation. Five different salt addition methods were employed: divalent cation Ca²⁺, trivalent cation Al³⁺, the stepwise addition of Ca²⁺ followed by Al³⁺, the stepwise addition of Al³⁺ followed by Ca²⁺, and the simultaneous addition of Ca²⁺ and Al³⁺. Furthermore, inorganic flocculants (polyferric sulfate, Danpower, Taki Chemical Co., Ltd., Kakogawa, Japan) or organic flocculants (cationic polymer, Kurifix, Kurita Water Ind., Ltd., Tokyo, Japan) were introduced to the ultrasonicated sludge. The floc diameters were measured using a laser diffraction particle size analyzer (SALD-2200, Shimadzu Corp., Kyoto, Japan). Additionally, the suspended solids (SS) of the supernatant were quantified using a spectrophotometer (DR6000, Hach Co., Loveland, CO, USA) to assess sample turbidity. A digital photomicroscope (BA210EINT, Shimadzu Rika Corp., Japan) was used to capture images of the flocs and measure the Feret diameters using ImageJ software 1.53e for larger flocs beyond the measurement range of the laser diffraction particle size analyzer. The supernatant used for SS measurements was obtained after standing for 1 h. The capillary suction time (CST) was determined using a CST instrument (RCST304M, Trion Electronics Ltd., Dunmow, UK) under various conditions. The CST denotes the time required for the sludge filtrate to flow by a certain amount owing to the capillary suction pressure of the filter paper. The CST instrument comprised CST paper (Trion Electronics Ltd., UK) and a funnel with a diameter and a filtrate volume of 1.8 cm and 5 mL, respectively, enabling substantial data collection in a brief period. Experiments were also conducted on untreated and ultrasonicated sludge samples, collected on the same dates. Multiple experiments confirmed the similarity of treatment effects.

2.3. Filtration and Mechanical Expression

Mechanical expression was conducted using the apparatus depicted in Figure 1 to elucidate its impact on sludge dewatering, including the effects from ultrasonication and re-flocculation. A dead-end filtration cell with a filtration area of 9.48 cm² was used. Constant-pressure filtration was initiated by adjusting the applied filtration pressure. Once all of the sludge formed a filter cake, the filtration period was completed and, subsequently, consolidation of the filter cake proceeded. The ratio of the cake thickness to the cell cylinder was maintained at relatively low values under 0.6 to justify the assumption that the sidewall friction is negligible [30,31]. A microfiltration membrane (mixed cellulose ester; nominal pore size (0.1 µm), Advantec Toyo Co. Ltd., Tokyo, Japan) served as the filter medium to prevent gas leakage during mechanical expression. However, assessing whether pretreatment enhances sludge filterability proves challenging due to the tendency of fine particles generated by ultrasonication to clog the membrane filter. Hence, an additional filtration experiment employing filter paper (5B; particle retention size (4 µm), Advantec Toyo Co., Ltd., Japan) was conducted to clarify the filtration behavior of each treated sludge. Nitrogen gas was consistently applied at a pressure (p) of 500 kPa in both experiments. The expression pressure in sludge treated with polyferric sulfate ranged from 500 kPa to 1000 kPa to analyze consolidation behavior under varying pressures. Although the application of high pressure can produce sludge cake with low moisture content [26], in this study, the dewatering effect was confirmed under relatively low pressure. The time course of the squeezed liquid and moisture content of the compressed cake were measured using an electronic balance (UX6200H, Shimadzu Corp., Japan) and an infrared moisture meter (MOC-120H, Shimadzu Corp., Japan), respectively, in each expression experiment. Furthermore, filtration and consolidation behaviors were analyzed using these values [32]. Untreated samples were included in each experiment for comparative analysis, collected on the same dates.

3. Results and Discussion

3.1. Self-Flocculation by Ultrasonication

The duration of ultrasonic irradiation and stirring following ultrasonication were initially investigated to gain a comprehensive understanding of the impact of ultrasonication on excess sludge. Figure 2 suggests that slow stirring at 50 rpm for 20 min post-disruption proved most effective in promoting floc growth and indicates the occurrence of self-flocculation during stirring after ultrasonication treatment. Self-flocculation occurs when the floc size surpasses that observed prior to ultrasonication and stirring without adding any coagulants or flocculants [26]. This phenomenon arises owing to the release of intracellular substances akin to extracellular polymeric substances (EPSs) and salts from microbial cells after sludge ultrasonication. The results confirm that ultrasonic disruption increases the electrical conductivity of sludge. EPSs are widely recognized as the primary sludge components after microorganisms and water [18] and are frequently associated with microorganisms such as flocculants. Consequently, EPSs have been investigated as environmentally friendly bioflocculants [33,34]. Considering the pivotal role of EPSs in binding various substances within sludge flocs, such as inorganic particles and organic fibers, they are essential materials for maintaining the stable structure of sludge flocs. It is noteworthy that when the ultrasonication duration exceeds 120 s, the floc diameter is less likely to increase because of the increase in small fragments. It has also been reported that the size of flocs immediately after ultrasonication decreases with increasing load power, but there is little effect on floc size after stirring [26].

3.2. Salt Addition to Ultrasonicated Sludge

The salt addition conditions were scrutinized to enhance the filterability of the ultrasonicated sludge. Pretreatment via ultrasonication generates numerous fine particles, posing a primary challenge during filtration. Hence, cationic salts, such as Ca²⁺ and Al³⁺, were introduced to the ultrasonicated sludge. Like the prior experiment, the impacts of ultrasonication and stirring duration were investigated. An ultrasonication duration of 90–120 s proved most effective (Figure 3a). However, a shorter duration was favored for practical applicability, leading to the adoption of a 90 s duration for all conditions. The specific ultrasonic energy for this condition was 25.2 J/g, which increased the water temperature by 6 °C; however, the water temperature during stirring was in the range of 26–29 °C. Figure 3b illustrates the impact of Ca²⁺ concentration on the floc diameter of the sludge. The largest floc formed when the ratio r_Ca, representing the added Ca²⁺ to solid mass in the sludge, was 0.080, resulting in a floc size of 497 µm, significantly surpassing that achieved through self-flocculation (165 µm, Figure 2).

Various samples created under different salt addition conditions were measured to maximize the floc size of the re-flocculation sludge resulting from ultrasonic disruption (Figure 4). A notable value was observed under the stepwise addition of Ca²⁺ initially followed by Al³⁺, with concentrations of r_Ca = 0.080 and r_Al = 0.0096, respectively (Figure 4f). Even when compared with the case where Ca²⁺ and Al³⁺ were added stepwise to the non-ultrasonicated sludge, as indicated by the two-dot chain line, significantly larger flocs were formed after ultrasonic disruption. Simultaneously, the SS content of the supernatant was measured under each condition, with the stepwise addition method yielding a much lower SS content (340 mg/L) than the other methods. Therefore, the stepwise addition of Ca²⁺ followed by Al³⁺ was selected as the salt addition method for this study. The sequence of salt addition is also crucial, with Ca²⁺-coagulated sludge further coagulated by Al³⁺, forming large flocs. Unexpectedly, flocs coagulated by the trivalent cation Al³⁺ were not larger than those coagulated by the divalent cation Ca²⁺ (Figure 4e). This discrepancy is attributed to increased steric hindrance between polymer substances and Al³⁺ during coagulation. To clarify the ultrasonicated sludge supernatant, the quantity of Al³⁺ added to the sludge was reassessed by measuring the SS and CST. CST is commonly used to assess sludge filterability [12,35]. The SS and CST values decreased from 340 mg/L to 208 mg/L and from 87.6 s to 22 s, respectively, as the concentration of Al³⁺ increased from r_Al = 0.0096 to r_Al = 0.036. This outcome aligns with the increase in NaCl concentration in ultrasonicated sludge, which augments the incorporation of fine particles into the floc formation [36]. Consequently, the stepwise addition of Ca²⁺ (r_Ca = 0.080) followed by Al³⁺ (r_Al = 0.036) proved most effective in promoting floc growth and clarifying the supernatant. Essentially, this method represents the optimal approach to enhancing sludge dewaterability.

Figure 5 shows photographs of flocs before and after ultrasonication and salt addition (Ca²⁺ and Al³⁺). Post-treatment, floc density increased, while floc size was notably enlarged (Figure 5b). Typically, floc density decreases with increasing floc diameter [37]. This occurrence can be attributed to electrostatic coagulation between the negatively charged sludge and the cationic salt. Unlike bridge flocculation, this mechanism resulted in high-density flocs with reduced water content.

3.3. Effect of Polymeric Flocculants

Polymeric flocculants are widely acknowledged for their efficacy in sludge separation and dewatering. Therefore, they are frequently used in contemporary water treatment processes [38]. Both inorganic flocculants (polyferric sulfate, Danpower) and organic flocculants (cationic polymer, Kurifix) were introduced to the ultrasonicated sludge and untreated sludge for comparison to further improve filterability and investigate the efficacy of polymeric flocculants for ultrasonicated sludge. Figure 6 compares the floc diameters of the non-ultrasonicated and ultrasonicated sludges after the addition of flocculants and stirring under various conditions. The organic flocculant Kurifix exhibited unsuitability for ultrasonicated sludge. As depicted in Figure 6a,b, r_K = 0.0075 was identified as the most effective concentration when Kurifix was added to the non-ultrasonicated sludge, while r_D = 0.15 emerged as the optimal concentration of Danpower added to ultrasonicated sludge.

The CST and SS of the supernatant were measured to compare the filterability of the stepwise salt-added sludge and the polymeric flocculant-added sludge (Figure 7). The effects of flocculation conditions exhibited similarities for CST and SS, with the values being higher for ultrasonicated sludge. SS represents the number of fine particles in the sludge supernatant and is a crucial factor influencing filterability evaluation. Therefore, these trends are closely aligned. The CST and SS decreased upon adding inorganic salts and inorganic polymer flocculants, facilitating the coagulation of the ultrasonicated sludge through charge neutralization. As illustrated in Figure 7a,b, CST decreased from over 2000 s (non-added) to 22 s (Ca²⁺ and Al³⁺) and 18 s (Danpower), while SS decreased from 1423 mg/L (non-added) to 208 mg/L (Ca²⁺ and Al³⁺) and 73 mg/L (Danpower). However, no reduction in CST or SS was observed when an organic polymer flocculant (Krifix) was introduced to the ultrasonically treated sludge. Adding Danpower proved to be the most effective method for re-flocculating and enhancing the filterability of ultrasonicated sludge.

3.4. Filtration and Mechanical Expression Properties

The experimental data obtained under 500 kPa using nitrogen gas and 5B filter paper as the filter medium are plotted in Figure 8 in the form of the reciprocal filtration rate (dθ/dv) against filtrate volume (v) per unit medium area based on the Ruth filtration rate equation [39] to analyze the filtration details of the sludge. The four different conditioned samples (untreated, ultrasonicated and stepwise salt-added, ultrasonicated and Danpower-added, non-ultrasonicated and Kurifix-added) are compared in the graph, displaying a trend similar to that shown in Figure 7. Although it did not reach the level of flocculating sludge with an organic polymer flocculant, a faster filtration rate was obtained for the ultrasonicated sludge than for the untreated sludge. The downward convex behavior of the plot with the stepwise addition of salt indicated that the filter medium was clogged with fine particles [40]. As mentioned previously, these fine particles were produced by ultrasonication, and no clogging was observed in the untreated sample or in the ultrasonicated and Danpower-added sample. Subsequently, mechanical expression was assessed under the same pressure, with an MF membrane filter used instead of filter paper.

Figure 9a shows a plot of the moisture content (R) of the sludge cake on a mass basis as a function of consolidation time for four different conditioned samples, similar to the filtration experiments. The key observations include the degree and rate of dewatering. The moisture content of the ultrasonicated samples (Danpower, Ca²⁺ and Al³⁺) was significantly lower than that of the non-ultrasonicated samples (Untreated and Kurifix). The effect of ultrasonic irradiation on reducing the moisture content of the sludge was confirmed, as described in our previous paper [26]. Furthermore, the values of the ultrasonicated samples were below 66 wt%, corresponding to the moisture content of microbial cells in activated sludge [41]. This indicates that the intracellular water in the sludge was discharged by ultrasonic disruption. The Kurifix-added sludge exhibited the highest filterability among the four samples (Figure 8), but the final water content was 82.9 wt%, higher than that of the untreated sludge (76.2 wt%). This suggests that organic polymer flocculant has high water retention [42]. The dewatering rate during the consolidation period was the fastest for Danpower, mitigating the drawbacks of ultrasonication. It is clear that the incorporation of fine particles generated by ultrasonic treatment into flocs significantly affects the dewatering rate. The constant pressure during the mechanical expression of sludge conditioned by the addition of Danpower was increased to 750 kPa and 1000 kPa and is plotted alongside 500 kPa in Figure 9b for a more comprehensive analysis of the dewaterability of ultrasonicated sludge. A clear trend emerges, indicating that an increase in expression pressure significantly improves expression performance.

4. Conclusions

This study employed ultrasonication and re-flocculation as pretreatments prior to mechanical expression to enhance the dewaterability of excess sewage sludge. The floc diameter, SS, and CST were measured in the differently treated sludge samples to assess filterability. Furthermore, we verified that adding Danpower (inorganic polymeric flocculant) or the stepwise addition of Ca²⁺ followed by Al³⁺ (inorganic cation salt) effectively re-flocculated the ultrasonicated sludge. Even when compared to the case where inorganic cation salts were added to non-ultrasonicated sludge, significantly larger flocs were formed after ultrasonic disruption. Typically, floc density decreases with increasing floc diameter, but the re-flocculated flocs produced by electrostatic coagulation between the negatively charged sludge and cationic salt resulted in high density. Interestingly, commonly used organic polymer flocculants were not suitable for flocculating ultrasonicated sludge. Four differently conditioned samples were analyzed under a constant pressure of 500 kPa in the filtration and mechanical expression experiments. While the non-ultrasonicated samples exhibited poor dewatering, the moisture content of the ultrasonicated samples was below 66 wt%, consistent with the intracellular water content of activated sludge. This indicates that intracellular water was converted into free water via the ultrasonic disruption of microbial cells. Furthermore, Danpower-added sludge significantly improved in the degree and rate of dewatering compared to conventional technology. The results obtained in this study are comparable to those of sludge deep dewatering technology and have many advantages, such as low secondary contamination and simple operation. In addition, the proposed method also provides a solution to the drawbacks of conventional ultrasonic treatment methods, such as the generation of fine particles owing to sludge disruption, which reduces the solid–liquid separation properties. Finally, the proposed dewatering technology is expected to significantly contribute toward biofuel production from sewage sludge, and demonstrates new research directions for sustainable sludge management.