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RESEARCH NOTE

TEMPERATURE EFFECTS ON THE OXYGEN TRANSFERRATE BETWEEN 20 AND 558C

J. C. T. VOGELAAR*, A. KLAPWIJKM, J. B. VAN LIERM and W. H. RULKENS

Department of Environmental Technology, Wageningen Agricultural University, Bomenweg 2,6703 HDWageningen, The Netherlands

(First received 1 October 1998; accepted in revised form 1 May 1999)

AbstractThe inuence of temperature on the oxygen transfer rate (OTR) was studied in a bubblecolumn. Aeration took place in three dierent liquids: tapwater, anaerobically pretreated paper processwater and thermophilic sludge grown on a mineral medium and volatile fatty acids as carbon source.The OTR was measured in a temperature range of 20558C in case of tap- and process water. TheOTR in the thermophilic sludge was determined at 558C.The OTR remained constant over the specied

temperature range in case of tapwater and showed a slight increase in case of process water. Theconstant OTR in case of tapwater was due to the counteracting eect of an increased overall oxygentransfer coecient versus the decreased oxygen saturation concentration at higher temperatures. At558C the OTR in the thermophilic sludge was comparable to both other liquids at this temperature.# 2000 Elsevier Science Ltd. All rights reserved

Key wordsoxygen transfer, Kla, forest industry, wastewater, thermophilic, aerobic

NOMENCLATURE

y theta factor, typical value approximately 1.024 ()a interfacial surface area (m2 m3)c dissolved oxygen concentration in the bulk liquid

(mg O2 l1)ce apparent oxygen saturation concentration,

observed in the experiment (mg O2 l1)

cs saturation concentration of dissolved oxygen atthe specied temperature, pressure and salinity(mg O2 l

1)db bubble diameter (m)Dol oxygen diusion coecient in the liquid phase

(m2 s1)Kl oxygen transfer coecient (m h

1)Kla(T) overall oxygen transfer coecient at temperature

T (h1)OC oxygenation capacity (mg O2 l

1 h1).OTR oxygen transfer rate (mg O2 l

1 h1)R gas constant (=8.314) (J mol1 K1)ract actual respiration rate of the biomass (mg O2

l

1

h

1

)t time (s)T temperature (8C in equation (2), K in equation

(5))tp response time of the oxygen probe (s)vbs bubble rise velocity relative to the liquid (m s

1)

INTRODUCTION

The paper and board industry is putting a lot of

eort in closing the watersystems in the production

line resulting in the so-called zero discharge papermills. Operation of these zero-discharge mills was

shown to be possible in the board industry but

requires an in-line treatment system to prevent bad

smells in the endproducts. Recently, Habets and

Knelissen (1997) presented a sequenced anaerobic

aerobic treatment system as the most cost-eective

option for in-line treatment of process water from a

paper mill using recycled wastepaper as raw ma-

terial. A disadvantage of their set-up is the required

cooling of the process water to mesophilic con-

ditions prior to biological treatment and subsequent

heating afterwards. The process water needs to be

reheated since the optimal temperature for paper

production is approximately 558C. The current

research project aims at a complete anaerobic

aerobic treatment under thermophilic conditions

(50558C) of paper process water in a closed cycle

mill. Regarding the post treatment step, it is

believed that thermophilic aerobic treatment may

be limited by a poor oxygen transfer due to the

lower oxygen saturation concentrations (cs) at

higher temperatures. The goal of this study is to

determine the OTR under mesophilic and thermo-

philic conditions in tapwater and process water.

Wat. Res. Vol. 34, No. 3, pp. 10371041, 2000# 2000 Elsevier Science Ltd. All rights reserved

Printed in Great Britain0043-1354/00/$ - see front matter

1037

www.elsevier.com/locate/watres

PII: S0043-1354(99)00217-1

*Author to whom all correspondence should be addressed.Tel.: +31-317-484-993; fax: +31-317-482-108; e-mail:Jaap.Vogelaar@algemeen.mt.wau.nl

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The oxygen transfer rate is inuenced by several

factors. Stenstrom and Gilbert (1981) mention the

following:

air ow rate

bubble diameter

temperature

viscosity

basin geometry (aects contact time between gasand liquid)

wastewater composition (salts, surfactants, bio-

mass)

Equation (1) (Lewis and Whitman, 1924)

describes the rate of oxygen transfer from a gas to

a liquid phase.

dc

dt OTR Klacs c 1

The oxygenation capacity (OC) of the aeration sys-

tem is calculated as the OTR assuming a zero oxy-

gen concentration in the bulk liquid.

Relation between OTR and temperature

Increasing temperatures result in a lower oxygen

solubility leading to a smaller driving force (csc )

and hence to a lower OTR. However, the diusion

rate of oxygen increases with increasing tempera-

tures while the liquid viscosity and surface tension

decrease. These eects result in an increased Kla

value that might oset the smaller driving force. An

overview of the oxygen solubility data from various

sources, with and without correction for the vapour

pressure are presented in Table 1.

At higher temperatures the OTR was often esti-

mated by use of the theta factor (y ). The theta fac-

tor is dened by equation (2) and can be used to

estimate Kla values at higher temperatures in case

the value at 208C is known.

KlaT Kla20yT20 2

However, very dierent values were found for this

factor under well dened experimental conditions.

Stenstrom and Gilbert (1981) note that the use of

this factor is inadequate to predict oxygen transfer

at higher temperatures.

Bewtra et al. (1970) measured the OTR in a

40 m3 aeration tank and found a constant OTR

over a temperature range of 10308C. Boogerd et

al. (1990) found that the OTR showed only minor

changes in a temperature range between 15 and

708C in a mechanically mixed fermentor while in an

air-mixed fermentor the transfer capacity for O2decreased slowly but steadily.

Materials and methods

The experiments were performed in three dierentliquids: tapwater, anaerobically pretreated paper processwater and thermophilic sludge. The OTR was determinedin a temperature range of 20558C in case of tapwater andprocess water while the OTR in the sludge was measuredat 558C. The set-up is presented in Table 2.

A cylindrical bubble column with an eective volume of3 l was aerated at dierent ow rates, i.e. 0.15, 0.3, 0.45and 0.56 vvm (volume air volume liquid1 min1). Thecolumn was temperature controlled. Mixing took place byaeration and a magnetic stirring device. Oxygen was

measured with an autoclavable Mettler Toledo Inpro 6000oxygen probe (silicon membrane) connected to a Knick7302-2 oxygen meter. In all three experiments the Klavalues were determined in duplicate with the `start-updynamic method' as described by Linek et al. (1993). Inexperiment 2 the process water was aerated for a shortperiod to oxidise possible interfering compounds (sul-phides) before the Kla determination took place.

The thermophilic sludge in experiment 3 was grown ona mineral medium (Schlegel, 1993) with volatile fatty acidsas organic substrates. The thermophilic sludge was sub-strate depleted to assure a constant respiration rate duringthe Kla determination. The respiration rate was measuredby switching o the aeration while stirring was main-tained. The oxygen saturation concentration (cs) in thethermophilic sludge was estimated by equation (3) whichis similar to equation (1) when a steady state during aera-

tion is reached.

ract Klacs ce 3

The use of an autoclavable stainless steel oxygen probecan lead to additional diculties because of the electroderesponse time. The probe response time, tp is dened asthe time needed to record 63% of a stepwise change. Theresponse time (tp) of this electrode is larger than the con-ventional oxygen probes that are used under moderatetemperature conditions. A large tp can lead to an inaccur-

Table 1. Oxygen solubility data (mg l1) in aqueous solutions at p=p0

Temperature (8C) Oxygen solubility

APHA (1995)a Perry and Green (1984)b Boogerd et al. (1990)c Measured, this studyd

20 9.09 9.43 9.09 9.230 7.56 7.98 7.55 7.4340 6.41 7.11 6.41 6.550 6.50 5.5055 5.1 5.07 5.1560 6.10 4.6770 5.82 3.8480 5.65 2.88

aDemineralized water, vapour pressure corrected.bDemineralized water, not vapour pressure corrected.cx5 mM sulphuric acid, vapour pressure corrected.dTapwater, vapour pressure corrected.

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ate calculation of the Kla if the dissolved oxygen concen-tration changes rapidly in time. Several correction pro-grams were designed in the past to compensate for thedynamic behaviour of the probe (Aiba et al ., 1984;Koizumi and Aiba, 1984). Van 't Riet (1979) states that itis possible to measure Kla values without additional cor-rection programs if equation (4) is met.

tp1

3Kla4

The above mentioned restriction was always met in thisresearch and no additional correction programs were used.

RESULTS AND DISCUSSION

The results of the aeration experiments in tap-

water are presented in Fig. 1. The Kla increases as

expected while cs decreases in the same extent.

Standard errors were less than 1%.

The eect of the air ow rate on the Kla is shown

in Fig. 2. The Kla values and hence the OC are line-

arly related to the air ow rate within the specied

range. Apparently, the bubble characteristics remain

unchanged in this ow range. The Kla values in the

sludge and the process water do not dier signi-

cantly from the values found for tapwater.The cs values that were measured (exp. 1 and 2)

or estimated (exp. 3) are presented in Table 3.

These saturation data were used to calculate the

oxygenation capacity (OC) of the system in the

dierent media at the various temperatures (Fig. 3).

The OC in the tapwater remained constant in the

temperature range of 20558C while the OC in the

process water increased. The OC in the thermophi-

lic sludge at 558C is comparable to the OC in the

process water and slightly lower compared to tap-

water. This is mainly due to a lower cs in the sludge

compared to the tapwater.

Comparable results were found by other research-

ers. Smith and Skidmore (1990) determined gas

hold-ups and Kla values in a concentric tube airlift

reactor at 30, 50 and 728C. Calculation of the OTR

with their Kla data and oxygen saturation concen-

trations from Boogerd et al. (1990), results in a

slightly increasing oxygen transfer rate with increas-

ing temperatures. Krahe et al. (1996) also found

increased oxygen transfer rates with increasing tem-

peratures in a mechanically mixed fermentor. They

determined the Kla values experimentally and used

oxygen solubility data from Perry and Green (1984)

to calculate the OTR. However, the solubility data

from Perry and Green (1984) are not corrected for

the increasing water vapour pressure at higher tem-

peratures and overestimate the actual saturation

concentrations. Boogerd et al. (1990) warns for thiseect since it also leads to an overestimation of the

OTR, especially at higher temperatures.

The present results are, in some extent predictable

by the general mass transfer theories. The increased

temperature will lead to a higher diusion rate for

oxygen (Dol) in the liquid phase due to the direct

temperature eect and indirect by the lower liquid

viscosity. This results in a higher Kl value thus hav-

ing a positive eect on the OTR. With regard to

the diusion rate for oxygen, Wise and Houghton

(1966) give the following relation:

Dol 4X2 106exp18 103aRT 5

For bubbles larger than 2.5 103 m the bubble

surface is always mobile and Kl can be calculated

using the Higbie model Heijnen and Van 't Riet,

1984

Fig. 1. cs (r) and Kla (Q) values measured in tapwaterwith a constant air ow rate of 0.3 vvm.

Fig. 2. Kla values determined at 558C in tapwater (W),process water (q) and thermophilic sludge (r) at air ow

rates between 0.15 and 0.56 vvm.

Table 2. Experimental set-up

Liquid Temperature (8C)

Experiment 1 tapwater 20304055

Experiment 2 process water 2055

Experiment 3 thermophilic sludge 55

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Kl

4Dlvbs

pdb

0X56

Visual inspection showed that the bubble diameter

in this research was larger than 2.5 103 m. This

is in accordance with the theory for tapwater. In a

coalescing medium, (tapwater), the bubbles gener-

ated from a porous disc/stone are approximately

(24) 103 m and tend to coalesce to an ultimate

bubble diameter of 6 103 m (Heijnen and Van 't

Riet, 1984).

The oxygen saturation concentration decreases

44% with a temperature increase from 20 to 558C(this research). Equations (5) and (6) predict a 48%

increase in Kl in the same temperature range thus

completely counteracting the lower saturation

values. However, possible changes in interfacial sur-

face area are not taken into account yet.

It is not expected that the bubble diameter will

change with temperature but a lower liquid vis-

cosity will most probably result in a higher bubble

rise velocity. This leads to a lower gas hold-up and

hence a smaller interfacial surface area. According

to Stokes law the terminal bubble rise velocity in

case of rigid spheres (small bubbles) is inversely

proportional to the liquid viscosity. In case of big-

ger bubbles (1 cm) Deckwer et al. (1982) found no

eect of liquid viscosity on the gas hold-up. So, in

the worst case, when Stokes law applies a 50%

decrease in the interfacial surface area can be

expected with a temperature increase from 20 to

558C. The most optimal case predicts no tempera-

ture eect on the interfacial surface area. The latter

seems more likely when looking at the results pre-

sently found.

In general it can be concluded that the oxygen

transfer rate is only slightly aected by the liquid

temperature. This eect might be slightly positive,

negative or none at all (this research) depending on

the bubble diameter and the reactor type.

CONCLUSIONS

The present research has shown that in tapwaterat higher temperatures the decreasing oxygen satur-

ation concentration (cs) was completely oset by

the increased overall oxygen transfer coecient

(Kla ). This results in a constant oxygen transfer

rate in tapwater within the temperature range of

20558C. The temperature eect on the separate

variables Kl and a has not been revealed yet.

The slightly lower OTR values in the process

water and the thermophilic sludge compared to tap-

water are caused by the lower oxygen saturation

concentrations in these liquids. The Kla values in

these liquids do not dier signicantly from the

values found for tapwater.

AcknowledgementsWageningen Agricultural Universityis co-operating in this project with Paques Water SystemsB.V. and Kappa Packaging. The research is nanced bythe Dutch Ministry of Economic Aairs, Senter EET pro-ject 96003.

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