# Synergy effect of ultrasonication and salt addition on settling behaviors of activated sludge

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Eiji Iritani $ ^{a,*} $ , Masako Nishikawa $ ^{a} $ , Nobuyuki Katagiri $ ^{a} $ , Kenji Kawasaki $ ^{b} $

$ ^{a} $ Department of Chemical Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

$ ^{b} $ Department of Applied Chemistry, Ehime University, Bunkyo-cho 3, Matsuyama 790-8577, Japan

## ARTICLE INFO

Article history:

Received 11 December 2014

Received in revised form 20 February 2015

Accepted 22 February 2015

Available online 28 February 2015

Keywords:

Settling

Ultrasonication

Salt addition

Activated sludge

Reflocculation

## A B S T R A C T

The batch sedimentation tests were conducted for the excess activated sludge treated by ultrasonication and addition of sodium chloride, and the settling behaviors such as the sedimentation velocity, the sludge volume, and the quality of the supernatant were examined for different values of the load power of ultrasonication, the sonication time, and the sodium chloride concentration. The ultrasonication pretreatment significantly increased the sedimentation velocity in the initial stage of sedimentation, and the effect was dramatically facilitated by the addition of sodium chloride even when the effect was unnoticeable in the pretreatment of the salt addition alone. The effect of ultrasonication was increased with increasing load power and sonication time and found to be evaluated by the specific ultrasonic energy dissipated into a liquid. The increase in the sedimentation velocity brought about by the pretreatment was caused by the increase in the floc size, and the sedimentation velocity was related to the median diameter of flocs, based on the Stokes law considering the effect of both the floc size and floc density. Moreover, the combined pretreatment markedly reduced the sludge volume, as compared to the pretreatments of ultrasonication alone and salt addition alone.

© 2015 Elsevier B.V. All rights reserved.

### 1. Introduction

The activated sludge process is extensively used in the world as the typical biological treatment of both wastewater and industrial effluents. A major issue of the operation of activated sludge systems is the removal of biological solids from the liquid phase followed by dewatering of these biosolids. Although settling is used as the initial step to increase the solids content, poor settleability of activated sludge is often problematic.

In many cases, chemical conditioning prior to solid-liquid separation is used in order to enhance the separation efficiency $ [1] $ . Flocculation of activated sludge leads to organic colloids aggregation, thereby improving the ability of sludge to settle $ [2] $ . However, the use of chemical flocculants such as salts and polyelectrolytes increases treatment costs and may also cause secondary environmental pollution. Thus, it is necessary to reduce the specific consumption of flocculants as much as possible.

Various physical and chemical pretreatments, in which sludge is disintegrated and microbial cells are destroyed, have been investigated to reduce the biosolids' volume generated from wastewater treatment plants, including ultrasonication $ [3-5] $ , mechanical disintegration (mills, homogenizers, etc.) $ [6] $ , thermal hydrolysis $ [7] $ , ozonation $ [8] $ , acidification $ [9] $ , alkaline addition $ [10] $ , microwave irradiation $ [11] $ , and their combined treatments $ [12] $ . These pretreatments enhance sludge biodegradability prior to anaerobic digestion or recycling in aeration tank. Among them, ultrasonication is a particularly attractive method because it cannot cause secondary environmental pollution as with other mechanical disintegration. However, these pretreatments frequently deteriorate sludge settleability and filterability mainly due to the floc breakage and the release of extracellular polymeric substances (EPS) from cells $ [13-16] $ .

While ultrasonication has been used for the dispersion or collapse of flocculated particles in the liquid phase, Kakii et al. $ [17] $ revealed a really challenging fact that activated sludge disrupted by ultrasonication flocculated once again. This means that the ultrasonic pretreatment appears promising also as a means to improve the solid–liquid separation efficiency of activated sludge. In fact, on the basis of the results of the sedimentation velocity, capillary suction time (CST) and specific cake resistance in filtration, Feng et al. $ [18,19] $ indicated that ultrasonication with low specific energy is effective for enhancing sludge settleability and filterability. Vaxelaire et al. $ [20] $ reported based on the microscopic observations that numerous filaments of filamentous bacteria were