Environmental pollution has become a serious global problem due to continuous industrialization. Especially, water pollution is a major issue, and regulations for waste-water disposal are being reinforced globally. Consequently, waste-water treatment methods that are both environmentally friendly and effective have become critical needs [1
]. In industrial and municipal waste-water treatment plants (WWTPs), the activated sludge treatment method is the most used biological process. However, this method produces a high amount of excess sludge consuming 18%–57% of total operational cost of WWTPs. To reduce the amount of excess sludge, various techniques have been used to treat waste-water for decades. However, traditional methods generally involve a high processing cost, long treatment time, low treatment capability, or generation of chemical by-products [5
]. To overcome these drawbacks, new methods have been proposed in the literature [6
]. Among them, cavitation has shown a remarkable development potential for large scale applications.
Cavitation refers to the process of bubble generation, growth, and collapse in a liquid due to local pressure drop. The bubble generated by cavitation grows through heat transfer from the nearby liquid. When it reaches maximum size, it collapses releasing tremendous energy in the form of heat and a shockwave that imparts the three following effects [10
]. The first is the physical effect. When the bubble collapses, a shock wave of 550 MPa is emitted at a speed of 2000 m/s, a micro jet generates a 450 MPa water hammer at 100 m/s, and shear stress reaches 3.5 kPa. The second is a thermal effect. The collapse of the bubble generates a local hot spot of 2000–6000 K and induces
K/s heat transfer within one microsecond. The third is a chemical effect. Due to the energy generated by bubble collapse, water molecules are decomposed into H· and OH·, the latter is a strong oxidizing agent, and these effects can treat the sludge. The physical effect of cavitation leads to disintegration of particles and lysis of microorganisms resulting in enhancement of biogas production during anaerobic digestion of biomass [12
]. The thermal effect improves the dewaterability of the sludge and reduces the sludge’s viscosity by causing cell lysis and destroying the cell walls [13
]. In the case of the chemical effect, the free radicals generated by cavitation accelerate chemical reactions and inactivate and remove microorganisms through decomposition of hydrogen bonds [14
Cavitation can be classified as a method inducing pressure perturbation in a liquid. Ultrasonic cavitation (UC) occurs due to the pressure perturbation by ultrasound waves. When ultrasound waves propagate in a liquid medium, the liquid molecules repeatedly contract and expand. In the contracting cycle, the liquid pressure increases, while in the expanding cycle it decreases [16
]. The ultrasonic cavitation reactor (UCR) has a simple structure and easily generates bubble; hence, it has been used to treat sludge [17
]. However, UCR has much low energy transfer efficiencies (10%–40%), and the cavitation intensity rapidly decreases with distance from the ultrasonic generator [18
]. Hence, an ultrasonic cavitation reactor has poor scalability and is unable to treat the sludge uniformly. On the other hand, hydrodynamic cavitation (HC) occurs due to the pressure perturbation induced by changes in the cross-sectional area. For an incompressible fluid, the flow rate is calculated as the product of cross-sectional area and velocity. When the cross-sectional area decreases at constant flow rate, the fluid velocity increases, by Bernoulli’s equation, the pressure decreases. The hydrodynamic cavitation reactor (HCR) induces cavitation using this phenomenon, and due to the generation mechanism, all fluids pass through the cavitation generation region and can be treated uniformly [19
]. The HCR can be classified by way of changing the cross-sectional area. The non-rotation type of HCR lets liquid pass through vena contracta, e.g., the Venturi tube. The rotation type of HCR changes the cross-sectional area using a rotor with an uneven surface, e.g., a shockwave power reactor [21
]. In most literature focusing on waste-water treatment using HC, non-rotation types of HCRs have been utilized. However, to induce sufficient pressure drop, a high-power pump is required, resulting in a substantial cost. In addition, due to the poor cavitation intensity, it needs long treatment times and shows low treatment performance [24
]. On the other hand, the rotor-stator type HCR could overcome such limitations. First, since the rotor-stator type HCR is based on HC, waste-water could be treated uniformly. Also, the rotor-stator type HCR changes cross-sectional area repeatedly using rotation, hence the cavitation region and cavitation intensity are much larger than with the non-rotation type HCR. Some research groups have studied the potential for utilizing this technique for water treatment [19
The development of new highly effective HCR is the key to large-scale commercial applications of waste activated sludge (WAS) disintegration. Various types of conventional HCR such as Venturi, orifice, high-pressure jet, Ecowirl, and high-pressure homogenizer have been widely studied to evaluate their effectiveness [34
]. These devices obtained limited disintegration performance (disintegration degrees (
) of 7.7%–31%) with a considerable treatment duration (several hours). On the other hand, rotor-stator type HCR showed a performance which is far higher than the conventional device, the device proposed by Petkovšek et al. [45
] easily achieved 57% of
for only 20 passes, without any structure optimization. However, the research on rotor-stator type HCR is still very limited, which largely hinders the commercial application of HC sludge treatment. To the best of our knowledge, the comparison of the disintegration performances between rotor-stator type HCR and UCR at the same energy consumption has not been investigated.
The aim of this study was to verify the sludge treatment performance of rotor-stator type HCR and to demonstrate its viability as a real sludge treatment device. The sludge treatment performance depends on the characteristics of the substance requiring treatment [46
]. In light of this, to evaluate the sludge treatment capabilities of rotor-stator type HCR, comparisons of performance with existing techniques should use the same substance. In the present work, the sludge collected at a sewage treatment plant was treated using a rotor-stator type HCR and an ultrasonic bath. The treatment performance of the HCR and UCR containing decomposition, oxidation, and solubilization performances were compared by analyzing particle size distribution, sludge volume index (SVI), total chemical oxygen demand (TCOD), volatile suspended solids (VSS), and soluble chemical oxygen demand (SCOD) under the same energy consumption.
2. Experimental Methods
2.1. Rotor-Stator Type Hydrodynamic Cavitation Reactor
The HCR was constructed using the knowledge and results from a previously designed cavitation reactor [48
]. Since HCR generates bubbles using pressure perturbation induced by variations in cross-sectional area in the flow path, 32 dimples were located on each surface of a rotor and three covers. The rotor and rear cover were made of stainless steel, while the front and side cover were made of transparent polycarbonate to observe generation of cavitation. The shapes of the rotor and covers are shown in Figure 1
. To allow all fluids to uniformly pass through the cavitation region, the inlet is located at the center of the front cover, and the outlet is located at the rear cover. Due to the flow path, all liquids pass through three cavitation generation regions and are treated uniformly.
To turn the rotor, an electrical motor was used. To measure the electric power consumption of the motor, a clamp-on power meter was utilized. The flow rate was measured by an electromagnetic flow meter. All flow rate signals were collected and transformed to digital signals using a data acquisition (DAQ) board. Signals were saved in a computer using LabVIEW (National Instruments Co., Austin, Texas, USA). The sludge treatment system was assembled as a circuit system without temperature regulation using a reservoir, an inverter pump, and the HCR. A schematic of the HCR treatment system is shown in Figure 2
2.2. Ultrasonic Bath
The general ultrasonic bath, which has eight ultrasonic terminals on the bottom, was used to provide ultrasonic sludge treatment and is shown in Figure 3
. Since maximum floc size reduction and increase in solubilization rate are obtained at low frequencies [49
], the operating frequency was set to 28 kHz, and only the operating time was regulated in order to produce the same energy consumption as imparted with the HCR treatment.
2.3. Properties of Sludge
In this study, secondary sewage sludge was obtained from a bioreactor from the Gulpo River Waste-Water Treatment Plant (Bucheon, Republic of Korea). The sludge was stored at 4 °C for 24 h prior to experimentation to avoid changes in physicochemical properties and to stabilize the sludge. The properties of the sludge are shown in Table 1
2.4. Experimental Cases
To compare the sludge treatment performance of the HCR with that of an ultrasonic bath, four experimental cases were established of energy consumption. The specific energy input [51
] which indicates energy consumption of the four experimental cases was calculated using consumed energy every time the sludge passed through the HCR, which occurred five times. The number of passes was calculated using the total sludge amount and flow rate. To exclude other parameters affecting cavitation intensity, the rotational speed and pressure were fixed. The experimental cases are shown in Table 2
2.5. Analytical Methods
The particle size distribution was measured using a particle size analyzer (Malvern Co. Mastersizer 2000, Malvern, United Kingdom). Samples were diluted using tap water before analysis owing to the concentration limits of the analyzer, and five measurements were performed for each sample. The particle size distributions were expressed as a volume fraction graph (total area was constant) and percentile particle sizes (in micrometers) corresponding to 10%, 50%, and 90% in a size histogram; the smallest 10%, 50%, 90% of the particle diameters were represented as d (0.1), d (0.5), and d (0.9), respectively.
The TCOD and SCOD were measured using COD digestion vials and a visible spectrophotometer (Hach Co., DR3900, Loveland, Colorado, USA) according to the HACH 8000 method. To measure the TCOD, the samples were diluted with distilled water owing to the range of the vials, and the TCOD was expressed as the original value considering the dilution. Additionally, to measure the SCOD, the samples were filtered and not diluted. The samples were heated at 150 °C, for 2 h using a digital reactor (Hach Co., DRB200, Loveland, Colorado, USA) for the measurements. Five measurements were performed for each sample, and the results were averaged.
Finally, the SVI was visualized for 100 mL of sludge stored for 30 min using a mass cylinder.