The Potential of Ultrasound Treatment for Sludge Reduction

306 Pages·2009·2.22 MB·English

Cranfield University

Davide Minervini

The Potential of Ultrasound Treatment
for Sludge Reduction

School of Applied Sciences
Department of Sustainable Systems
Centre for Water Science

PhD Thesis

Cranfield University

School of Applied Sciences
Department of Sustainable Systems
Centre for Water Science

PhD

2008

Davide Minervini

The Potential of Ultrasound Treatment
for Sludge Reduction

Supervisors: Dr. Elise Cartmell and Professor Simon A. Parsons

September 2008

This thesis is submitted in partial fulfilment of the requirements for the Degree of
Doctor of Philosophy.

©Cranfield University, 2008. All rights reserved. No part of this publication may be
reproduced without the written permission of copyright holder

Abstract
Abstract

The potential of ultrasound treatment for sludge reduction during the activated sludge
process was assessed. Batch and dynamic disintegration studies were completed using
an activated sludge pilot-scale plant fed with settled sewage that was comprised of
two 1.2 m3 lanes, operated as a test and control. A 1 kW ultrasound system was
integrated into the test lane of the pilot-scale plant to allow continuous in-line
ultrasound treatment of the return activated sludge (RAS). Seven dynamic trials were
completed over an 8-month period, treating from 1.7 to 12.5% of the RAS with
energy densities between 42 and 168 kJ L-1.
During the batch disintegration studies, it was observed that ultrasound treatment at
42 to 186 kJ L-1 caused floc breakage and sludge solubilisation. Floc size was reduced
by 88% at 42 kJ L-1 while the degree of soluble COD release increased almost linearly
from 11 to 36 % between 42 to 168 kJ L-1. A change in the biological activity was
observed only at 168 kJ L-1 with an 8.5 % increase in the specific oxygen
consumption of the treated RAS samples in comparison to the untreated ones.
During the dynamic studies, a 20% degree of sludge reduction was observed treating
10% of the RAS at 42 kJ L-1. At these operational conditions, there was no significant
difference in the total COD and nitrogen removal between the control and test lanes.
However, a 5.5-fold increase in the capillary suction time and a 3.6-fold increase in
the specific resistance to filtration in the RAS from the test lane indicated a
detrimental impact on dewaterability. Increasing the energy input, by treating 12.5%
of the RAS at 84 kJ L-1, did not result in a significant increase in sludge reduction,
which indicates that there might be limits to the degree of reduction achievable with
the ultrasound treatment.
Results from the dynamic studies suggested that lysis-cryptic growth was the main
mechanism behind the observed sludge reduction. Based on modelling, lysis-cryptic
growth could justify 98 % of the sludge reduction observed. Cost analysis results
indicated that sludge reduction by ultrasound disintegration was not currently
economically viable unless the cost for sludge treatment and reuse increased to £ 961
from the reference price of £ 423 per tonne of dry solids.
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Acknowledgments
Acknowledgments

First and foremost, I am extremely grateful to my supervisors, Dr. Elise Cartmell and
Prof. Simon Parsons, who have managed to channel my passion for water/wastewater
processing towards a fruitful end. Otherwise, without their help, I would have become
lost following my curiosity without finishing my PhD.
Special thanks to Dr. Elise Cartmell for her advice and comments during the most
arduous phase of my PhD - the final draft, when I was, among other things, working
on a three-shift system as a WTP technical operator.
I am also grateful to my industrial sponsors at Anglian Water, Emily Payne and
Joanne Callan, for their support and patience during the more problematic periods of
my research.
Many thanks to Lawrence for wiring up my ultrasound control panel, which was the
brain of my ultrasound system. I would also like to express my heart-felt thanks to my
cousin Roberto for manufacturing and shipping to the UK my flow cell, the beating
heart of the system. I must also express my gratitude to Rukhsana and Jane for their
help in the lab and pilot hall.
I owe a great deal to all my friends in the department for cheering me up during trying
times at Cranfield. In particular, I want to thank Max for amusing us with his
Luxembourgese outlook on life, Nicandro for the useful information about water
processing and Nicola for the mystical Friday night spent sieving primary sludge in
the fermentation lab till 5am: one of the few nights in my life I am sure I will never
forget!
A huge and sincere embrace goes to Pantelis. Thank you so much for your help, for
our endless discussions about sludge and other more romantic issues during our hard
times in the Cranfield pilot hall and for your much needed, stinking rig.
Thanks to my beloved MUM, my sisters and my niece Matilde for giving me the
energy and the reason to do something good with my life.
Finally … my Louise! The best side effect of my PhD research. Thanks for your
support and for removing the last chances of a peaceful yet boring life from my
horizon. I do not know if we will be able to achieve our goals, but what I do know is
that we will give our all trying!
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To the memory of my father
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Consider ye the seed from which ye sprang;
Ye were not made to live like unto brutes,
But for pursuit of virtue and of knowledge.

Dante Alighieri (1265-1321)
The Divine Comedy (Inferno, Canto XXVI 118-120)
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Table of contents
Table of contents
ABSTRACT..............................................................................................................................I
ACKNOWLEDGMENTS......................................................................................................II
TABLE OF CONTENTS......................................................................................................VI
LIST OF FIGURES..............................................................................................................XII
LIST OF TABLES.............................................................................................................XXII
ABBREVIATIONS...........................................................................................................XXV
CHAPTER 1: INTRODUCTION..........................................................................................1
1.1 PROJECT BACKGROUND.................................................................................................1
1.1.1 THE ACTIVATED SLUDGE PROCESS................................................................................1
1.1.2 SLUDGE PRODUCTION....................................................................................................2
1.1.3 SLUDGE TREATMENT AND REUSE..................................................................................3
1.1.4 SLUDGE REDUCTION STRATEGIES.................................................................................4
1.1.5 ULTRASOUND POTENTIAL FOR SLUDGE REDUCTION.....................................................6
1.2 PROJECT DEVELOPMENT................................................................................................7
1.3 PROJECT HYPOTHESIS....................................................................................................7
1.4 PROJECT AIM AND OBJECTIVES.....................................................................................7
1.5 THESIS PLAN....................................................................................................................8
CHAPTER 2: LITERATURE REVIEW............................................................................11
2.1 OUTLINE........................................................................................................................11
2.2 ULTRASOUND TECHNOLOGY........................................................................................12
2.2.1 PHYSICS AND BACKGROUND THEORIES.......................................................................12
2.2.2 OPERATIONAL PARAMETERS AND MECHANISMS OF ACTION......................................14
2.2.3 ULTRASOUND APPLICATIONS IN ENVIRONMENTAL ENGINEERING..............................16
2.2.4 ULTRASOUND SYSTEMS...............................................................................................21
2.3 REDUCING SLUDGE PRODUCTION DURING WASTEWATER TREATMENT...................28
2.3.1 BACTERIAL METABOLISM IN BIOLOGICAL WASTEWATER TREATMENT......................28
2.3.2 SLUDGE CHARACTERISATION AND PRODUCTION........................................................33
2.3.3 SYNTHESIS AND OBSERVED YIELD..............................................................................36
2.3.4 ESTIMATION OF OBSERVED YIELD...............................................................................37
2.3.5 SLUDGE REDUCTION MECHANISMS.............................................................................40
2.4 ULTRASOUND POTENTIAL FOR SLUDGE REDUCTION..................................................43
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