Optimisation of sludge pretreatment by low frequency sonication under pressure

• Main Author: Le, Ngoc Tuan
• Other Authors: Institut National Polytechnique de Toulouse - INPT (FRANCE)
• Format: Others
• Language: en
• Published: 2013
• Subjects: Ultrasonic pretreatment; Audible frequency; Hydrostatic pressure; Municipal sludge disintegration; Soluble chemical oxygen demand; Particle size distribution
• Online Access: http://oatao.univ-toulouse.fr/11216/1/le_ngoc_tuan.pdf; http://oatao.univ-toulouse.fr/11216/4/le_ngoc_tuan.pdf; http://oatao.univ-toulouse.fr/11216/7/Le_ngoc_tuan.pdf

The objective of this work is to optimize high-power low-frequency sonication (US) pretreatment of sludge, and especially to investigate for the first time possible improvements by higher pressure and audible frequency. After a preliminary examination of regular process conditions (sludge conditioning, sludge type, prior alkalization, temperature control, etc), effects of US parameters (power -PUS, intensity -IUS, specific energy input -ES, frequency -FS, etc.) and of hydrostatic pressure (Ph) were specifically looked into, separately and in combination, first under cooling at constant temperature (28°C), then under the progressive temperature rise provoked by sonication. First, it was confirmed that specific energy input (ES) plays a key role in sludge US disintegration (i.e. solubilisation of organic matter) and that temperature rise during adiabatic-like sonication is beneficial through additional effects of thermal hydrolysis and cavitation. At a given ES value, low FS (12 kHz vs. 20 kHz) and high PUS enhance soluble chemical oxygen demand (SCOD) due to more violent cavitation, while hydrostatic pressure gives rise to an optimum value due to its opposite effects on cavitation threshold and intensity. One major result is that optimal pressure depends on IUS (P¬US) as well as temperature profile, but not on ES, FS, nor sludge type. Setting the other parameters at the most favorable conditions expected, i.e. 12 kHz, 360 W , 28 gTS/L, and adiabatic conditions, final optimization was achieved by searching for this pressure optimum and examining sequential procedure to avoid too high temperature dampening cavitation intensity and damaging the transducer. Such conditions with sequential mode and Ph of 3.25 bar being selected succeeded in achieving very high SCOD, but only marginally improved subsequent methanization yield.