Nomenclature
C_{s}	concentration of sodium chloride in sludge (M)	r	correlation coefficient (-)
D	fractal dimension for self-similar structure (-)	SV	sludge volume defined as ratio of interface height H to initial height H_{0} (-)
d_{50}	median diameter of flocs (m)
d_{f}	floc diameter (m)	SV_{300}	sludge volume defined as ratio of interface height H_{300} after 5 h (300 min) to initial height H_{0} (-)
E	specific ultrasonic energy dissipated into sludge (J/kg)
g	acceleration of gravity (m/s^{2})	Tl	turbidity index defined as the ratio of the turbidity in the supernatant to the concentration of solids in sludge represented by the same unit (mg/L) as the turbidity (-)
H	height of interface plane between sludge and supernatant (m)
H_{0}	initial height of sludge (m)	t	net ultrasonic exposure time (s)
H_{300}	interface height after 5 h (300 min) (m)	u_{0}	sedimentation velocity during hindered settling period (m/s)
I	ultrasonic power dissipated into sludge (W)
M	mass of sludge sample (kg)	0	sedimentation time (s)
P	load power (W)	\mu	viscosity of supernatant (Pa s)

cut by the ultrasonic pretreatment, leading to a better settling behavior.

Some research has been dedicated to the role of salt addition to the ultrasonicated sludge. Kakii et al. $ [17] $ reported that salt addition promoted reflocculation of ultrasonicated sludge. Yin et al. $ [21] $ found that ultrasonic pretreatment of activated sludge from petrochemical plant reduced the necessary flocculant dosage by approximately 25–50% because it decreased the specific cake resistance in filtration. Likewise, Hakata et al. $ [22] $ reported that the ultrasonic pretreatment with $ Al^{3+} $ -based coagulation improved the permeate flux in microfiltration of municipal wastewater treated by an activated sludge-lagoon process. In contrast, Feng et al. $ [19] $ demonstrated that sludge suffered from ultrasonication and cationic polymer addition provides no clear advantage over polymeric conditioning alone for improving sludge dewaterability. Dewil et al. $ [14] $ reported that the required dosage of flocculant increased proportionally with the level of ultrasonic energy to reach the same dryness as the untreated cake in vacuum filtration. Therefore, it is of particular importance to reveal whether the combined pretreatment of ultrasonication and flocculant addition is effective for the improvement of solid-liquid separation efficiency of sludge.

In the present article, the combined pretreatment of ultrasonication and salt addition is examined in order to improve the solid-liquid separation efficiency. In particular, the paper focuses on the synergy effect of ultrasonication and salt addition on settling behaviors such as the sedimentation velocity, sediment volume, and supernatant quality under various operational conditions of the ultrasonic power, sonication time, and added salt concentration using excess activated sludge produced from municipal sewage treatment works.

### 2. Materials and methods

#### 2.1. Materials

The excess activated sludge mixed liquor employed in this study was sampled at the Ueda Sewage Treatment Works (Nagoya City, Japan). The solid concentration ranged from 3.6 to 4.9 g/L, the mean value being 4.3 g/L, during the course of our experimental work. The sludge was concentrated at 5.0 g/L by decantation for 20 h in the refrigerator kept at 5°C to minimize change in its property, and used in the experiments within 4 days. The true density of solids in the activated sludge measured by a pycnometer is $ 1.45 \times 10^{3} $ kg/m $ ^{3} $ [23]. It is reported that the original activated sludges contain approximately 6–7 kg bound water per kg of dry solid mass [24]. The viscosities of sludge and supernatant were measured by using a capillary viscometer. The concentrations of Na $ ^{+} $ and Cl $ ^{-} $ in the sludge were measured by an ion chromatography system (Dionex ICS-1100/2100, Thermo Fisher Scientific Inc., USA). The electric conductivity was measured with a conductivity meter (DS-52, Horiba Ltd., Japan). The zeta potential of particles in the sludge was determined by a particle microelectrophoresis apparatus (Model Mark II, Rank Brothers Ltd., UK). The properties of the activated sludge used in the experiments are listed in Table 1. Sodium chloride was employed as inorganic flocculants.

#### 2.2. Experimental apparatus and technique

The ultrasonic apparatus employed was an ultrasonic homogenizer (UP-200S, Dr. Hielsher GmbH, Germany) equipped with a tip with an operating frequency of 24 kHz and a nominal load power output ranging from 50 to 200 W. After the sludge sample was warmed to room temperature (20 °C), the ultrasonic tip was immersed in the sample of 80 g to a depth of approximately 5 mm above the bottom of a 100-mL beaker. The sample was processed with the tip for different total operating times by pulsed ultrasonic irradiation in which one cycle consisted of both the operating time of 0.5 s and the downtime of 0.5 s in order to avoid the rise in temperature as far as possible. Thus, levels of sonication were varied by changing the load power and sonication time. The ultrasonicated sludge was adjusted to a variety of salt concentrations by the addition of the concentrated solution of sodium chloride. Thereafter, the sludge was conditioned by the rapid mixing at a speed of 150 rpm for 3 min using an agitator (Three-One Motor, BL 600, Shinto Scientific Co., Ltd., Japan) followed by the slow mixing at a speed of 50 rpm for 20 min to allow reflocculation to occur. This agitation condition was determined based on the results obtained for different agitation conditions.

Batch gravity sedimentation experiments were conducted using vertical Plexiglass cylinder with 2.9-cm internal diameter and 20-cm height in order to evaluate the settleability of the treated sludge. The settleability was evaluated by three criteria: the sedimentation velocity, the sludge volume, and the quality of the supernatant. Once the treated sludge of 80 g (corresponding to

Properties of activated sludge.

Activated sludge
pH	6.9
Viscosity (mPa s)	2.62
Na $ ^{+} $ (mg/L)	26
Cl $ ^{-} $ (mg/L)	31
Electric conductivity (mS/m)	45.1
Zeta potential (mV)	-26.9