Water Reuse in Paper Mills. Measurements and Control Problems in Biological Treatment

138 Pages·4.819 MB·English

by  Alexandersson Tomas.

Water Reuse in Paper Mills
Measurements and Control Problems
in Biological Treatment
Tomas Alexandersson
Licentiate Thesis
Department of Industrial Electrical Engineering and Automation
Department of Industrial Electrical Engineering and Automation
Lund University
Box 118
SE-221 00 Lund
Sweden
http://www.iea.lth.se
ISBN 91-88934-28-4
CODEN:LUTEDX/(TEIE-1036)/1-138/(2003)
 2003 Tomas Alexandersson
Printed in Sweden by Media-Tryck, Lund University
Lund 2003
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Abstract
Paper manufacturing is a complex and multidisciplinary science due to the
diversity of paper products, used raw materials and different production
processes. Besides fibres different chemicals, water and energy are needed to
produce paper. The use of fresh water has decreased significantly during the
last decades and there are several reasons for this, such as: limited availability
of fresh water, increased cost for effluent treatment and marketing benefits.
This decreased consumption has been made possible by the reuse of process
water instead of fresh water. However, at a certain degree of closure different
problems occur. Many of them are in some way related to the growth of
microorganisms in the system. One method to solve the problems is to
implement an internal kidney consisting of at least a biological treatment
step. Since nutrients, such as nitrogen and phosphorous, normally are
limited in the whitewater these have to be added in order to have an efficient
biological treatment process. One major challenge is to operate the biological
system with low concentrations of nutrients in the effluent otherwise the
conditions in the whitewater system will be negatively affected.
Consequently, there is a need for automatic control of the nutrient addition.
It is possible to control the flow of whitewater to the treatment process but
not the actual concentrations of organic compounds in the whitewater,
which therefore can be regarded as a process disturbance. An investigation
was made at two different paper mills with different degrees of closure to
determine the variation of chemical oxygen demand (COD) in the
whitewater. The results showed that the whitewater concentration in an open
mill could vary a lot whereas the conditions were more stable in a closed mill.
For the control there is a need for information about the process state and
output from the system. In this case, for controlling a biological treatment of
whitewater, different on-line instruments are needed. First of all, a market
survey, limited to instruments for measurements of organic matter,
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ammonium and orthophosphate, was conducted. The experiences gathered
about use of on-line instruments at several of the Swedish municipal
treatments plants were explored in a telephone survey. One interesting
observation was that most on-line instruments were only used for
monitoring. The number of instruments used for direct control was low but
this number was increasing as new and better instruments are becoming
available. As a conclusion of these two surveys, three different brands of
instruments were deemed suitable for measurements in whitewater.
Computer simulation is an important tool for evaluation of different
controllers but requires a mathematical model of the system. Laboratory
experiments were initiated to determine important parameters for such a
model. Both mesophilic and thermophilic treatment of recycled fibre
whitewater with a fluidised anaerobic reactor and an aerobic suspended
biofilm process resulted in high removal of COD of around 90%. The
nutrient requirement for the anaerobic mesophilic reactor was determined to
19 mg N/g COD and 2.5 mg P/g COD . For thermophilic
reduced reduced
degradation the requirement was determined to 24.5 mg N/g COD and
reduced
4.4 mg P/g COD for the anaerobic process and the corresponding
reduced
values for the aerobic process were 37.1 mg N/g COD and 5.5 mg P/g
reduced
COD . A decrease of the added amount of nitrogen to 77% of what was
reduced
originally consumed did not have any immediate effect on the COD
reduction.
Pilot tests with the purpose to study both the stability of a biological
treatment process and evaluate two different on-line instruments were
conducted at a packaging board mill. The results demonstrated that the
removal efficiency was not markedly affected from variations of the load to
the combined anaerobic/aerobic treatment process and that both instruments
failed to provide stable results. Experiences from other instruments have been
gathered during the assembly of a complete system consisting of a pilot plant
of a biological treatment process, on-line instruments and data-acquisition
equipment.
It has been demonstrated that it is possible to use on-line instruments for
measurements in whitewater to acquire information about the biological
treatment process. This information could be used in several different ways
for the control of the addition of nutrients. Different control structures are
suggested ranging from feed forward of the organic load with corrective
feedback of concentrations in the anaerobic effluent to more complex model-
based control structures with automatic update of model parameters.
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Acknowledgements
I would like to first express my gratitude towards Dr. Thomas Welander who
gave me the opportunity to continue my education. He performed an
inhuman effort when he, in a very short time, wrote the major part of the
project application for the ClosedCycle project, which I have been working
on. The ClosedCycle project is financially supported by the European
Commission, which is gratefully acknowledged.
I am also very grateful for the support and encouragement throughout my
project from my supervisor Prof. Gustaf Olsson and co-supervisor Assoc.
Prof. Ulf Jeppsson. On numerous occasions I was very frustrated and felt
rather lost. The feeling I had was the same feeling you would have if you
were asked to put together a bicycle and your starting materials were some
seeds for a rubber tree and pieces of iron ore. Although both of my
supervisors are usually very busy people, they always had time to discuss the
rubber tree and the iron ore and so with their help I managed to create the
bicycle in the end.
As a graduate student I had the privilege of attending various courses and
met a lot of nice fellow Ph.D. students, such as the people from the
department of Water and Environmental Engineering. Special thanks go to
Michael Ljunggren, who provided me with pictures and practical
information about some of the processes used in wastewater treatment.
Michael also shared my interest for training and so during breaks there was
always some stimulating discussion about strength training, pulse intervals,
nutrition or something else in this area. Michael and I still do not
understand why the others always started to shake their heads and looked so
strangely at us when we started in on these discussions and distractions.
My own department is filled with nice people. You have all made me feel
welcomed although, as a chemical engineer, I was a long way from home
(KC). Thanks for the stimulating atmosphere all of you created together. I
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would especially like to mention Carina Lindström who provided delicious
morning coffee or tea and Getachew Darge who assisted me with his
technical knowledge. It is easy to take your services for granted and not
appear to give them enough show of appreciation during hectic times. Thank
you.
A lot of people both at Anox AB and Cenox helped me in various ways. Dr.
Anders Ternström and Dr. Alan Werker proof read parts of my manuscript,
Åsa Malmqvist always had time for creative discussions, and Stig Stork made
those very tedious runs for new batches of whitewater. I was encouraged by
everyone's anticipation of when I was going to come back to Anox AB.
Hopefully, I interpreted the concern in the right way and was missed; it
could be that you just wanted to figure out how many happy days you had
left.
I am very grateful to Prof. Erik Dahlquist who took the time to read my
thesis and travelled to Lund to discuss the work with me during my licentiate
seminar.
Finally, my thoughts turn to my room-mate at IEA, Sabine Marksell, who
has become very dear to me and a part of my life. Thanks Sabine for always
encouraging me and boosting my self-confidence.
Lund, July 09, 2003
Tomas Alexandersson
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Contents
CHAPTER 1 INTRODUCTION .......................................................1
1.1 PROBLEM DEFINITION....................................................................1
The project................................................................................................2
Other projects............................................................................................2
Challenges.................................................................................................3
1.2 OVERVIEW......................................................................................3
1.3 MAIN RESULTS................................................................................4
CHAPTER 2 PROCESSES INVOLVED..............................................7
2.1 PAPER..............................................................................................8
History of paper.........................................................................................8
Paper products...........................................................................................8
Paper production.......................................................................................9
2.2 PULPING..........................................................................................9
Raw material............................................................................................9
Mechanical pulping.................................................................................11
Chemical pulping....................................................................................11
Bleaching................................................................................................12
2.3 PAPER MAKING..............................................................................13
The paper machine..................................................................................13
The whitewater system.............................................................................14
Composition of the whitewater.................................................................15
Mass balances in the whitewater system....................................................17
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2.4 VARIATIONS IN THE WHITEWATER..............................................17
Mill no. 1...............................................................................................18
Mill no. 2...............................................................................................19
Sampling and storage..............................................................................19
Production disturbances...........................................................................20
Analyses..................................................................................................20
Results.....................................................................................................20
2.5 WASTEWATER TREATMENT (WWT)...........................................23
Introduction............................................................................................23
Internal versus external WWT.................................................................24
Wastewater composition..........................................................................24
2.6 MECHANICAL/PHYSICAL/CHEMICAL METHODS..........................25
Settling...................................................................................................25
Flotation.................................................................................................27
Sand filtration........................................................................................29
Membrane filtration................................................................................29
Chemical treatment.................................................................................29
Ozonation...............................................................................................30
2.7 BIOLOGICAL DEGRADATION........................................................31
Different energy and carbon strategies......................................................31
Microorganisms.......................................................................................32
Environmental demands..........................................................................33
Nutrient requirements.............................................................................34
2.8 AEROBIC BIOLOGICAL WWT.......................................................35
Activated sludge......................................................................................36
Biofilm...................................................................................................37
2.9 ANAEROBIC TREATMENT.............................................................38
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CHAPTER 3 ON-LINE ANALYSERS...............................................41
3.1 INTRODUCTION...........................................................................41
3.2 MEASUREMENT PRINCIPLES.........................................................43
Organic compounds.................................................................................43
Ammonia................................................................................................44
Phosphate................................................................................................45
3.3 MARKET SURVEY...........................................................................46
Information sources.................................................................................47
Discussion...............................................................................................52
3.4 EXPERIENCES................................................................................53
Introduction............................................................................................53
Telephone survey.....................................................................................54
WWT plants...........................................................................................55
On-line instruments................................................................................55
Service and calibration............................................................................57
Discussion...............................................................................................57
3.5 CONCLUSIONS..............................................................................58
CHAPTER 4 CLOSURE OF PAPER MILLS......................................61
4.1 WATER USAGE...............................................................................61
4.2 BENEFITS.......................................................................................62
4.3 PROBLEMS.....................................................................................63
Microbial growth....................................................................................64
Corrosion................................................................................................64
Explosions...............................................................................................65
Interfering substances...............................................................................65
4.4 QUALITY RELATIONSHIPS.............................................................66
4.5 SOLUTIONS AND EXPERIENCES....................................................67
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Biocides...................................................................................................67
Advanced water recycling.........................................................................68
Evaporation............................................................................................68
Fixing agents...........................................................................................69
Enzymes..................................................................................................69
Membrane filtration................................................................................69
Sand filtration........................................................................................70
Cost for water re-use................................................................................70
4.6 CONCLUSIONS..............................................................................72
CHAPTER 5 THE INTERNAL KIDNEY..........................................75
5.1 THE INTERNAL KIDNEY................................................................75
5.2 MOTIVATION FOR SELECTION OF PROCESS................................77
5.3 THE PROCESS................................................................................79
Biological process.....................................................................................79
Separation process....................................................................................81
Additional treatment process....................................................................81
5.4 IMPORTANT DESIGN AND OPERATIONAL PARAMETERS..............82
5.5 EXPERIMENTAL EXPERIENCES......................................................84
Biological process in lab scale...................................................................85
Pilot test and on-line instruments.............................................................89
Other experiences.....................................................................................91
5.6 INDUSTRIAL EXPERIENCES............................................................94
Zülpich Papier........................................................................................94
Westfield mill..........................................................................................94
Gissler & Pass paper mill.........................................................................95
Hennepin Paper Co.................................................................................95
AssiDoman Lecoursonnois........................................................................95
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