Thermophilic aerobic post treatment of anaerobically pretreated paper process water

156 Pages·2002·2.05 MB·English

Thermophilic aerobic post treatment of anaerobically
pretreated paper process water
Jaap C.T. Vogelaar
Promotor
Prof. dr. ir. G. Lettinga
hoogleraar in de anaerobe zuiveringstechnologie en hergebruik van afvalstoffen
Co-promotoren
Dr. ir. A. Klapwijk
universitair hoofddocent, sectie milieutechnologie
Dr. ir. J.B. van Lier
onderzoeker bij de sectie milieutechnologie en directeur Lettinga Associates Foundation
Samenstelling promotie commissie
Prof. dr. ir. W. Norde Rijksuniversiteit Groningen
Prof. dr. ir W.H. Rulkens Wageningen Universiteit
Prof. dr. ir. J. Rintala University of Jyvaskyla, Finland.
Dr. R. Mulder Paques b.v. , Balk (cid:1)
Thermophilic aerobic post treatment of anaerobically
pretreated paper process water
Jaap C.T. Vogelaar
Proefschrift
Ter verkrijging van de graad van doctor
Op gezag van de rector magnificus
van Wageningen Universiteit,
Prof. dr. ir L. Speelman,
in het openbaar te verdedigen
op vrijdag 4 oktober 2002
des namiddags te vier uur in de Aula
CIP-DATA KONINKLIJKE BIBLIOTHEEK, DEN HAAG
Author: Vogelaar, J.C.T.
Title: Thermophilic aerobic post treatment of anaerobically pretreated paper process water
ISBN: 90-5808-713-1
Publication year: 2002
Subject headings: thermophilic, activated sludge, biological wastewater treatment, anaerobic effluent,
forest industry
Thesis Wageningen University, Wageningen, The Netherlands - with references-with summary in
English and Dutch
Aan mijn vader en moeder
Vogelaar, J.C.T. 2002. Thermophilic aerobic post treatment of anaerobically pretreated paper
process water. Doctoral Thesis, Wageningen University, Wageningen, The Netherlands.
Thermophilic waste- or process water treatment increases in importance as industries shift from end-
of-pipe treatment towards integrated process water treatment. The need for process water treatment
becomes evident as the levels of pollutants in industrial water circuits need to be controlled whereas
the intake of fresh water generally diminishes. In the paper and board industry, high process water
temperatures prevail and thus water treatment needs to take place under thermophilic conditions. In
many cases, an anaerobic pretreatment method can be used but aerobic post treatment is required for
polishing of the anaerobic effluent. This thesis describes research in which the aerobic post treatment
of anaerobic effluent of a board mill was investigated under thermophilic conditions.
As a boundary condition for aerobic conversions, sufficient oxygen needs to be transferred from the
gas phase to the liquid in which the bioconversion takes place. It was shown that although the oxygen
saturation concentration decreases with a rise in temperature, this effect is fully compensated by the
increased oxygen diffusion rate with the same temperature increase. The overall oxygen transfer rate
thus remains constant in the temperature range of 20-55 °C.
Post treatment of anaerobic effluent in activated sludge reactors revealed several fundamental
differences between mesophilic and thermophilic treatment. Firstly, batch and continuous experiments
showed a lesser removal of complex soluble COD under thermophilic conditions when compared to
mesophilic reference experiments. This could not be attributed to a higher production of soluble
microbial products (SMP) at elevated temperatures. It is therefore expected that thermophilic biomass
is unable to oxidize the same variety of complex soluble components as the mesophilic biomass is
capable of.
Secondly, thermophilic effluents are often cloudy while effluents of mesophilic activated sludge
reactors are generally clear. This is caused by smaller cohesion forces within thermophilic activated
sludge flocs resulting in a higher sensivity towards shear forces and smaller floc sizes. Furthermore,
fewer colloidal particles from the influent are adsorbed on the thermophilic sludge flocs and are
washed out with the effluent. However, a clear thermophilic effluent can be obtained provided the
influent contains little colloidal material.
The underlying causes for the weaker cohesion forces within the flocs are still unclear. The absence of
protozoa at 55 °C was shown to be of minor importance regarding the effluent turbidity and could not
account for this effect. Binding of hydrophobic pollutants on a hydrophobic surface was hardly
affected by temperature and could not explain the observed effects either. Calculations using the
DLVO theory showed that bacterial exo-polymers are of crucial importance in the flocculation
process. These polymer interactions are highly temperature dependant and are therefore expected to
be the underlying cause for the differences in flocculation behavior.
Besides differences in removal efficiencies and flocculation behavior, also the kinetics of mesophilic
and thermophilic activated sludge treatment differ. The maximum growth (and thus conversion rate)
of biomass cultivated at 55 °C was a factor 1.5 higher than for a similar type of biomass cultivated at
30 °C. Decay rates are doubled with the same temperature increase whereas the theoretical biomass
yields were similar. As a result, higher substrate conversion rates can be obtained under thermophilic
conditions provided that a high concentration of thermophilic biomass is cultivated in the reactor by
application of a high organic loading rate.
These kinetic advantages are however of little use when polishing the effluent of an anaerobic
bioreactor. Under thermophilic conditions biomass growth will be limited since the organic loading
rate is restricted by the need to retain and convert particulates from the anaerobic effluent and by the
absence of readily biodegradable COD. Furthermore, biomass decay rates have doubled under
thermophilic conditions. The combination of these factors diminishes the amount of active biomass in
the thermophilic reactor and can not be compensated fully by the intrinsic higher conversion rates.
Overall conversion rates in a thermophilic bioreactor can thus be lower as compared to a mesophilic
reference system, depending on the applied loading rates.
Nevertheless, for application in the board industry these disadvantages can be dealt with as the water
quality demands are relatively low. Additional treatment methods are however required in case higher
water quality demands prevail.
TABLE OF CONTENTS
1. General Introduction. 1
2. Temperature effects on the oxygen transfer rate between 20-55 °C. 15
3. Kinetics of mesophilic and thermophilic aerobic biomass grown on acetate. 25
4. Sludge and wastewater characterization using batch experiments. 43
5. Mesophilic and thermophilic activated sludge post treatment of 57
paper mill process water.
6. Assessment of effluent turbidity in mesophilic and thermophilic 69
activated sludge reactors - origin of effluent colloidal material.
7. Biosorption and flocculation properties of mesophilic and thermophilic 81
activated sludge.
8. Combined calcium and BOD removal in a novel thermophilic up-flow 107
fluidized bed reactor.
9. General discussion. 117
Samenvatting 129
References 134
Abbreviations 146
Dankwoord 148
Curriculum Vitae 150
Publication list 151
1

GENERAL INTRODUCTION
Chapter 1
1. INTRODUCTION
Paper and board production has been a very energy intensive and water consuming practice since a
long time. The European paper industry produces 70 million tons of paper annually and at the same
time uses an average of 20 cubic meters of freshwater per tonne of paper produced, in total 1.4 billion
cubic meters of freshwater annually (Kappen et al., 1999). In The Netherlands, the figures are
somewhat lower due to a more efficient water usage. The specific water consumption per tonne of
paper produced very much depends on the type of paper produced, the quality of the mechanical
equipment used in the mill, availability of water and energy and customs/traditions in papermaking.
Additional bleaching steps for production of a high quality product such as graphic papers will
increase the water demands as well. Producing a lower quality product, such as board or newsprint
generally requires less water. In some cases, paper mills are not allowed to discharge any (purified)
wastewater or only in limited amounts whereas in Canada and Scandinavia water consumption is
generally higher due to the abundant availability of water and the lower energy prices. In transition
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countries such as India, specific water consumption can be as high as 110-170 m tonne of paper
caused by a low pulp quality, based on straw, bagasses or jute and by the use of outdated paper
machines (Gupta, 1994; Habets, personal communication). Table 1 depicts the specific water
consumption in the paper and board industry in The Netherlands for each product group and the
estimated values in the near future. In The Netherlands, in nearly all cases, groundwater is used as the
primary water source. In the year 2000, approximately 35 million cubic meters of groundwater was
used by the Dutch paper and board industry.

Table 1. Specific water usage in The Netherlands (Joore, 1999)
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specific water usage in time (m tonne )
product group 1970 1985 1993 2000 2010
massive board 8.9 7.5 3 0 ?
corrugated board 9.2 5.6 3.5 0 ?
graphic paper 29.8 23.1 10 <5 ?
tissue 25.1 21.3 15 < 5 ?
total 80 20.0 14.8 7.5 3 ?

In the paper production process, energy consumption is linked to the specific water consumption.
Heat dissipation takes place from mechanical equipment that is used for grinding, mixing and
pumping of pulp and water. As a result, process water temperatures increase to approximately 30 °C.
However, for optimum runability of the paper machines a higher process water temperature of 50-60
°C is desirable. To attain this temperature, steam is generally injected in the process. Steam is
generated at the mill, partially obtained from combined heat and power systems used to generate
electricity and from burning of natural gas. Consequently, minimizing the specific water consumption
2