Sewage sludge disruption through sonication to improve the co-
preparation of coal–sludge slurry fuel: The effects of sonic frequency
*, Jianzhong Liu
, Yukun Lv
, Xuemin Ye
Department of Power Engineering, North China Electric Power University, Baoding, Hebei Province 071003, China
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
Sonication with different frequencies is used to disrupt sludge ﬂocs.
The disrupted sludge is mixed with coal to prepare coal–sludge slurry.
Effects of sonic frequency on reducing slurry viscosity are studied.
Low-frequency sonication generates more intense cavitation effects.
Low-frequency sonication has more obvious effects of reducing slurry viscosity.
Received 28 August 2015
Accepted 10 January 2016
Available online 29 January 2016
Coal water slurry
The sewage sludge volume has been increasing annually, and if not treated properly, this sludge endan-
gers the environment and human health. Sludge can be considered as a carbon-containing material, and
the energy within is utilized easily and economically by blending it with coal to prepare a slurry fuel
called coal–sludge slurry (CSS). However, sludge is always disrupted before CSS preparation because of
its high bound water content and viscosity. In this study, sonication was performed to disrupt sludge
and to enhance co-slurrying with coal. The effects of sonic frequency were highlighted, and the results
showed that low-frequency sonication signiﬁcantly reduces viscosity. Moreover, this process improved
CSS slurrying. The characteristic viscosity (the apparent viscosity measured at a shear rate of 100 s
CSS prepared with raw sludge was 1663.6 mPa·s; following sludge disruption via sonication (the spe-
ciﬁc energy was 30 kJ/g dry sludge) at 15, 25, and 35 kHz, the characteristic viscosities of prepared CSSs
decreased to 1121.6, 1194.8, and 1234.3 mPa·s, respectively. On the basis of the dynamic model of a sonic
cavitation bubble, low-frequency sonication intensiﬁed cavitation such that the radius and impact ve-
locity of a cavitation bubble increased. This ﬁnding was consistent with experimental results.
© 2016 Elsevier Ltd. All rights reserved.
Sewage sludge is a solid condensate generated via wastewater
treatment processes and comprises approximately 0.5%–1% of waste-
water volume. In fact, sludge output has increased considerably with
the recent increase in wastewater volume. Sludge contains high
amounts of salts, heavy metals, pathogens, nutrient substances, and
organic pollutants, among others; thus, untreated sludge poses pol-
lution hazards when discharged into the environment
China is among the few countries that consider coal to be a
primary energy source, and coal consumption in this country reached
3.51 billion tons in 2014. Oil demand also increased continuously
as a result of rapid economic development; therefore, the nation’s
dependency on imported oil increased to 59.5% in 2014. At present,
China faces the two serious problems of energy supply and envi-
ronment pollution. In light of this situation, sludge recycling is of
The use of coal–water slurry (CWS) is advantageous in that it
clearly protects the environment against storage, transport, and com-
bustion emissions. Thus, this liquid fuel has become an energy-
saving, environment-friendly oil substitute in China. The addition
of sludge into CWS was proposed recently to produce a sludge-
containing slurry fuel called coal–sludge slurry (CSS) [3,4] that is
used to fuel power plant boilers, industrial boilers, and furnaces.
This slurry fuel is also regarded as a gasiﬁcation material
CSS technology is advantageous over other sludge disposal tech-
nologies (landﬁll, agricultural usage, digestion, and incineration) in
the following main ways. First, CSS is cost eﬃcient because complex
and expensive sludge pretreatments are unnecessary. Second, the
high moisture content in sludge can be transformed into free water
* Corresponding author. Tel.: +86 13567119806; fax: +86-571-87952884.
firstname.lastname@example.org (R. Wang).
1359-4311/© 2016 Elsevier Ltd. All rights reserved.
Applied Thermal Engineering 99 (2016) 645–651
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