Ultrasonic pretreatment for Enhancing Sludge disintegration and Resource Utilization: A mini review

. As a byproduct of sewage treatment plants, sludge presents dual characteristics of “resource” and “pollution”. While it enriches nutrients from wastewater, it also contains a portion of toxic substances. Moreover, when the organic matter content of sludge is low, it affects the efficiency of resource utilization and energy recovery (sludge digestion efficiency, energy consumption of incineration systems). Developing new physicochemical-biological combined pretreatment to enhance the dissolution of organic matter in sludge (sludge disintegration) and reduce the solid content of sludge is a crucial step to improve sludge dewaterability and digestibility. Ultrasonic, as a novel sludge pretreatment technology, gains the advantages of being environmentally sustainable, efficient, and easily operable. It can disrupt sludge flocs, alter microbial activity, release intracellular organic matter, and be used in combination with other pretreatment processes. This article summarizes the mechanism of ultrasonic pretreatment of sludge and its research progress in improving sludge dewaterability and digestibility, providing a new perspective for the application of ultrasound in sludge resource utilization.


Introduction
Sludge is a refractory waste that typically originates from sewage and industrial wastewater treatment plants. The global annual sludge production is approximately 310 million tons, and most countries still face the problem of unsafe sludge storage and disposal. As a complex mixture, sludge is mainly composed of organic matter, inorganic matter, and microorganisms. It contains abundant organic substances such as ammonia nitrogen, phosphate, organic nitrogen, Soluble Polysaccharide, as well as heavy metals and partially harmful substances. If improperly treated, sludge can cause severe pollution to groundwater and soil, as well as significant impacts on human health and the environment [1]. The existing sludge treatment methods are costly and often require significant amounts of energy and capital investment. Some traditional pretreatment techniques, such as composting and incineration, not only require a large amount of space but also may cause further environmental pollution. Therefore, how to reduce sludge production and turn sludge into a resource has always been an important topic for researchers and the industrial sector. In recent years, ultrasonic pretreatment has emerged as a promising mechanical sludge disintegration technology and has attracted widespread attention. It boasts the advantages of being environmentally sustainable, highly efficient, can disrupt sludge flocs to improve sludge dewaterability, enhance microbial activity, and is easy to use in combination with other pretreatment processes.

Visual Analysis of Sludge Treated by Ultrasonic Processing
In order to gain a better understanding of the current research status and development trends of ultrasonic sludge treatment technology, this article utilizes the Web of Science (WOS) core database and employs bibliometrics to conduct visual and statistical analyses on the relevant research content of global ultrasonic sludge treatment technology. Through the means of visualization analysis, the research status and future challenges faced by ultrasonic sludge treatment technology are sorted, providing scholars with a certain reference for future research endeavors. This article conducts statistical and visual analyses on ultrasonic sludge literature from the years 2000 to 2023 in the WOS core collection database using the search topic "TS=(sonication sludge) OR ALL=(ultrasonic sludge) OR ALL=(ultrasound sludge)". The search was conducted up to February 20, 2023, and a total of 2226 papers were retrieved.

Co-occurrence Frequency and Cluster Analysis of Keywords
Visual analysis using VOSviewer software was conducted on the 7438 keywords that appeared in the 2226 retrieved articles, with a total of 165 keywords occurring more than 20 times. Figure 1 illustrates the high co-occurrence frequency of keywords such as sewage-sludge, ultrasound, disintegration, anaerobic digestion, dewaterability, and solubilization, indicating their frequent appearance in the literature.
The clustering analysis of keywords with a frequency greater than 20 was conducted using the VOSviewer software, as shown in Figure 1. The attributes of the top 10 ranked keywords are presented in Table 1. The analysis revealed a total of 165 keywords with a frequency greater than 20, and their node size represents the total link strength. The proximity between the keyword nodes indicates the degree of correlation between these research topics. "Ultrasound" was the keyword with the highest total link strength, followed by "sewage-sludge" as the second. Ultrasound is primarily used for "pretreatment" of sludge, facilitating subsequent "anaerobic digestion". The keywords "disintegration", "solubilization", and "dewaterability" were ranked fourth, fifth, and sixth, respectively.

Analysis of Hotspot Evolution in Keyword Co-occurrence
The appearance of keywords to some extent reflects the evolution path of research hotspots in the field of ultrasonic sludge treatment. Through the analysis of keyword appearance time using VOSviewer software, as shown in Figure 2, it can be seen that the research keywords on ultrasonic sludge have shifted from "sludge", "wastewater", "ultrasound" to "biogas production", "methane production", "hydrogen production", etc. The evolution process of keywords presents the research progress of ultrasonic sludge treatment: from the establishment of basic theory to the transformation towards engineering application, and then to the detailed analysis of microscopic mechanisms, as well as the introduction of environmental management perspectives to analyze anaerobic digestion technology.

Mechanism of Sludge Disintegration by Ultrasound
Ultrasonic pretreatment of sludge is a technique that utilizes high-frequency sound waves to decompose and dehydrate sludge particles. Sound waves propagate through a transducer in the liquid and cause changes in pressure, resulting in the formation of cavitation bubbles. These bubbles increase as pressure continues to decrease and eventually reach a critical size before suddenly collapsing, releasing a large amount of energy in the form of shock waves, microjets, and heat [2]. This process generates intense pressure and thermal gradients, causing mechanical and thermal stresses to the surrounding liquid and solid particles. Ultrasonic pretreatment can reduce the particle size of sludge, increase the surface area for bacterial degradation and dehydration, and therefore reduce the amount of sludge generated [3]. The reduction in particle size and increase in surface area improve the permeability of the sludge, making dehydration more effective. At the same time, the strong mechanical force generated by the collapse of cavitation bubbles can also release volatile organic compounds and pathogens, thus improving the stability and safety of the treated sludge. Therefore, ultrasonic pretreatment technology is a promising mechanical sludge disintegration technology for sludge treatment.

Research and Application of Ultrasonic Enhancement for Sludge Dewatering
The water in sludge can be classified as interstitial water (about 70% of the total water content), capillary water (about 20% of the total water content), adsorbed water, and intracellular water (which totally account for about 10% of the total weight). Interstitial water is free water existing among sludge particles and can be removed by gravity sedimentation, while capillary water, adsorbed water, and cellular water are relatively difficult for removal and require mechanical dewatering processes [4]. The dewaterability of sludge is closely associated with the structural characteristics of sludge extracellular polymers and the presence of water. The application of low-power ultrasound pretreatment has been found to enhance the dewaterability of sludge, with the magnitude of improvement depending on the degree of sludge disintegration. In cases where the disintegration is too low, little sludge dewaterability was observed. When the degree of sludge disintegration is appropriate, the loose sludge flocs can be broken down into smaller fragments after the adding of a conditioning agent, thereby enhancing the effective collision of sludge particles and subsequently re-agglomerating into more compact particles, which is benefit for enhancing the dewaterability of the sludge [5].
When the power input is overloaded during sonication, the microbial cells and floc structure in the sludge can be disrupted by the fluid shear forces in the cavitation and caused the intracellular and extracellular polymers being released into the liquid phase as smaller particles, which can potentially absorb more free water and interfere the dewatering efficiency. Additionally, these small particles also lower the liquid surface tension, making the separation from the solid more challenging.
In recent years, researchers have been exploring the effects of ultrasonic density and time on dewaterability of sludge. Farshad et al.'s study demonstrated that the optimal ultrasonic density range is 0.375 -1 W/mL for improving sludge dewaterability. At a density of 1 W/mL, the best ultrasonic disintegration results can be achieved the capillary suction time was 86 s for the solids content of 26.4 ± 1.1% and the energy consumption of 4095 kJ/kg TS [6]. However, when the ultrasonic density increased to 1.3 -2.5 W/mL, the CST rate increased rapidly, leading to a decrease in sludge dewaterability and solids content. Serkan et al. conducted experiments on sludge dewatering characteristics and found that the optimal ultrasonic density was 0.75 -1 W/mL (specific energy consumption of 60000 kJ/kg TS) [7]. When the ultrasonic density increased above 1.2 W/mL or below 0.5 W/mL, the CST level increased significantly and the sludge dewaterability decreased. Rumky et al. investigated the performance of ultrasonic-Fenton system effect on sludge dewatering and found that ultrasound can improve the structure and microstructure of sludge flocs, and release capillary water and cellular water with the addition of 320 mM H 2 O 2 and 36 mM Fe 2+ with 30 minutes of sonication [8]. During the ultrasonic process, the small flocculent particles undergo resonance to promote collision and disrupted into denser micro-particles, which can significantly improve the dewatering efficiency.

Enhancing the Digestibility of Sludge via Ultrasound Pretreatment
Ultrasonic pretreatment can enhance the digestion of sludge by increasing the activity of the microorganisms. Low-energy ultrasonic pretreatment can alter the structure of sludge flocs, which hastens oxygen and nutrient transferring rates and contributes to a more conducive environment for microorganisms [9]. Additionally, cultures which are activated by ultrasound maintain high activity for several hours after the cessation of ultrasonic exposure. Suitably dense sonication can induce the release of extracellular hydrolases, lowering the resistance between extracellular hydrolases and substrates, thus augmenting their capability to decompose and absorb organic matter, elevating the activity of proteases, and accelerating the growth of microorganisms [10]. Yan et al. demonstrated that low-intensity ultrasound has the potential to stimulate enzymatic activity and enhance protein hydrolysis in ultrasonic extracting of proteins from sludge, which resulted in a dosage reduction of approximately 38.5% (compared with the using of enzymes only) [9]. In addition, ultrasound technology has been proved to increase the oxidation rate of sodium percarbonate (SPC), leading to increased anaerobic digestion performance of waste activated sludge and reduced SPC dosage. During the pretreatment, ultrasound amplifies significantly enhanced substrate dissolution, hydrolysis, and acidification, and produced abundant intermediate products for the subsequent methanogenic process [11].
However, in certain cases, the digestion of sludge may be slow and inefficient due to the special sludge characteristics as high viscosity, low oxygen contents, and other factors that can inhibit microbial activity. Ultrasonic pretreatment can be applied to break these barriers by providing more nutrients and higher contact surface area for microbiomes and thus increasing the solubility and accessibility of the organic matter. This could due to the micro-scale shear force effects during sonication, which increase the permeability around cell particles and play an important role in strengthening cytoplasmic exchanges inside and outside cells [12]. Additionally, ultrasound energy can help increase the oxygen content in the sludge, which is crucial for microbial growth and the increased activity. Overall, ultrasound energy contributes sludge treatment process by promoting sludge digestion.
Certainly, a significant increase in microbial activity can only be achieved during low strength sonication. Extremely high-energy-input sonication will lead to a sharp decline in microbial activity in the sludge, which even below the efficiency of the cases without ultrasound pretreatment. For instance, after processing for 120 minutes at 0.44 W/mL sonication input, 64% of Escherichia coli in the sludge were killed [11], not mention for bioactivity enhancement.
There are also studies that indicated that increasing the intensity and duration of ultrasonic pretreatment can improve sludge cracking and promote the dissolution of organic matter and the release of N and P in sludge. This can increase the nutrients (N, P and K) levels derived from the sludge, making the sludge disrupting liquid a more effective fertilizer, which can be applied as valuable byproducts and presents increased biological availability. E.g., Tian et al. conducted an experiment on sludge cracking using the alkaline and ultrasonic treatment of WAS (A&UWAS) [13]. The results showed that the A&UWAS has significantly augmented the liberation of DOM byproducts, notably the discharge of free amino acids, which amounted to 1990 ± 12 μmol L -1 , being 3.45 times greater than that of the alkaline and mechanical cutting treatment of WAS (A&MWAS) coupling process, and more readily available for microbial utilization.

Conclusion
Ultrasonic disintegration has emerged as an effective solution to enhance the efficiency of sludge treatment, particularly in the fields of dewatering and digestion. Bing widely coupled with sludge digestion, this technique offers numerous advantages, such as sludge volume reduction, dewaterability and digestibility enhancement, and energy recovery facilitating. When combined with other techniques, benefits originated from ultrasonication can be fully exploited. Moreover, with the development of novel low-energy consumption ultrasonic reactors, the extension of ultrasonic pretreatment to full-scale wastewater treatment plant in enhancing sludge digestion efficiency and resources/energy recovery can be expected.