Urban Water 2 (2000) 21±27
www.elsevier.com/locate/urbwat
Resuspension and oxygen uptake of sediments in combined sewers
Jes Vollertsen *, Thorkild Hvitved-Jacobsen
Environmental Engineering Laboratory, Aalborg University, Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark
Received 7 June 1999; received in revised form 2 May 2000; accepted 11 May 2000
Abstract
Combined sewer sediment properties were studied in small-scale laboratory ¯umes. Physical and biological characteristics of
freshly remoulded sediment beds and beds undisturbed for 1±4 days were examined. When sediment beds were incubated for 1±2
days, the beds became covered with bio®lm and the bed volumes increased by 20±40% due to the formation of methane-®lled
cavities. The bed strength decreased with increasing incubation time. The oxygen uptake rates of bio®lm-covered sediment beds were
on average ®ve times higher than those for bio®lm-free beds. Ó 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Combined sewers; Methane; Resuspension; Sediment; Sediment oxygen uptake rate
1. Introduction
Solids with di€erent physical, chemical and biological
properties are transported into combined sewers during
wet as well as dry weather periods and to some extent
deposited as sewer sediments. During storms, the sediments in the sewers can be eroded or remoulded into
new bed structures with new properties. During successive dry weather periods, physical and microbial
transformations in the sewer sediment bed will change
the sediment bed properties. The newly formed sediment
beds may either be undisturbed, i.e., with an undisturbed and well-de®ned sediment bed surface, or be
continuously changing due to bed movement and near
bed transport of solids (Ashley & Verbanck, 1996).
The type and magnitude of chemical and biological
processes within a sediment bed will di€er between an
undisturbed bed and a constantly remoulded bed because of, e.g., di€erences in electron acceptor and electron donor conditions. A self-weight consolidation of
the sediments might be expected and microbial activities
in sediment beds do occur. The understanding of the
processes in sediment deposits and their in¯uence on the
surface resistance to erosion is still only in a preliminary
state (Ashley & Verbanck, 1996).
*
55.
Corresponding author. Tel.: +45-963-58-080; fax: +45-98-1425E-mail address: [email protected] (J. Vollertsen).
If the sediment bed is constantly remoulded, the
sediment has little opportunity to consolidate and a
continuous bio®lm will not be able to develop on the
bed or grow in the bed. If the sediment bed, however,
has a stable surface, bio®lms can be formed and gradients of electron donors and gradients of electron acceptors may play a larger role for the microbial
transformations of sediments in an undisturbed sediment bed than in a continuously remoulded sediment
bed.
Some physical and biological properties of freshly
remoulded sediment beds and sediment beds which have
been kept undisturbed for up to four days, are addressed
in this study. These sediment bed properties are studied
in small-scale laboratory ¯umes and di€erences between
freshly remoulded and undisturbed sediments are investigated qualitatively.
2. Methods
Sediment bed resistance against erosion was assessed
qualitatively using small-scale laboratory ¯umes operated in parallel. The strength of sediment beds was assessed using the amount of sediment that could be
resuspended under well-de®ned conditions as an inverse
measure of the sediment bed strength. Sediment oxygen
uptake rates and kinetics were determined in a closed
¯ume before and after incubation with wastewater of the
sediment beds.
1462-0758/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved.
PII: S 1 4 6 2 - 0 7 5 8 ( 0 0 ) 0 0 0 3 6 - 4
22
J. Vollertsen, T. Hvitved-Jacobsen / Urban Water 2 (2000) 21±27
Sewer sediments were collected from three adjacent
manholes in a combined interceptor sewer in the city of
Aalborg, Denmark. The sewer is a 1.00 m diameter
concrete pipe located in an old residential area, Hasseris.
The sewer serves approximately 20 households upstream
of the sampling location. Around 10 l of sewer sediments were brought back to the laboratory where surplus water was gently drained-o€. Larger particles, e.g.,
stones, leaves and cigarette ®lters, were removed from
the sediments. The sediments were mixed thoroughly
before laying them in six ¯umes within one hour of
collection.
2.1. Resuspension of sewer sediments
The collected sediments were divided among parallel
operated ¯umes. Each ¯ume was 1000 mm long and held
a 400 mm long, 95 mm wide and 25 mm deep sediment
bed. Tap water was circulated over each ¯ume by means
of a centrifugal pump taking water from a 12 l reservoir.
Tap water was chosen for practical reasons and because
it was expected that the use compared with combined
sewage would not signi®cantly a€ect the qualitative
outcome of the study. Two grids were placed in each
¯ume adjacent to the arti®cial base close to the inlet in
order to reduce incoming turbulence and distribute the
water ¯ow evenly over the ¯ow cross-section.
The ¯umes were operated at room temperature in two
modes: incubation mode and resuspension mode. In the
incubation mode, the hydraulic conditions were adjusted to avoid erosion of the sediments. The ¯ow was
0:083 l sÿ1 and the slope was slightly negative ()0.6%)
resulting in an average ¯ow velocity over the sediment
bed of 0:20 m sÿ1 . In the resuspension mode ± which
was maintained for 300 s ± the ¯ow was increased to
0:17 l sÿ1 and the slope changed to 3.2% resulting in an
average ¯ow velocity over the sediment bed of
0:45 m sÿ1 . The ¯ow was turbulent with a Reynolds
number of about 1700 and a water depth of approximately 4 mm. The average shear-stress was approximately 1:2 N mÿ2 . As the ¯umes were short and water
depths small, the accuracy of the hydraulic measurements was low. The resuspended sediments were allowed
to settle in the reservoirs and the total solids (TS) of the
settled matter subsequently used as an inverse measure
of the bed strength. The ¯umes were operated in incubation mode between 3 and 101 h, and subsequently in
resuspension mode.
2.2. Sediment oxygen uptake rates
The sediment oxygen uptake rate (SOUR) was determined by measuring the ¯ux of dissolved oxygen
(DO) from the bulk water into the sediment bed. The
sediments were placed in a water-®lled, closed ¯ume
where water was recirculated over the sediment bed and
the DO depletion of the bulk water was measured.
Subsequently the SOUR was calculated taking into account the volume of the water in the ¯ume and the
sediment bed surface area. SOUR was found for fresh
sediments and for sediments that had been incubated in
wastewater for one day.
Prior to SOUR measurement the sediments were laid
in a stainless steel box with the inner dimensions: length
748 mm, width 92 mm and height 25 mm resulting in a
sediment surface area of 0:069 m2 . The box containing
the sediments was placed in an incubation ¯ume of
similar design as the resuspension ¯ume. Prior to the
®rst SOUR measurement, tap water was circulated for
about 30 min in order to wash-o€ small amounts of
sludge formed at the sediment bed surface during
placement of sediments in the steel box.
The closed ¯ume for measuring SOUR had the same
dimensions as the incubation ¯ume. The depth of the
bulk water above the sediment bed and the arti®cial base
was 10 mm. The measuring ¯ume was operated in an
aeration mode and in a measuring mode. In the aeration
mode, the water was circulated through the ¯ume via a
reservoir where the water was aerated. In the measuring
mode the reservoir was bypassed. In both modes the
¯ume was completely ®lled with water. The sediment
surface area-to-water volume ratio in the measuring
mode was 43 mÿ1 . To avoid erosion of the bed the ¯ow
velocity over the sediment bed had to be kept at
0:071 m sÿ1 . At any time the water pressure over the
sediment bed was kept close to atmospheric pressure. In
the aeration mode it was adjusted by changing the elevation of the reservoir and opening a valve built into the
lid of the ¯ume. In the measuring mode a short opening
of the valve ensured atmospheric pressure. DO was
measured using an INGOLD oxygen sensor with a 12
mm diameter Te¯on membrane and DO vs. time was
recorded.
The ¯ume for SOUR measurement was cleaned mechanically prior to use with hot water and a brush. In
order to ensure that no bio®lm existed, the ¯ume was
operated without sediments and with 1% hydrogen
peroxide solution for 600 s and then ¯ushed at least ®ve
times. After having prepared the sediment bed and reactor system, the steel box containing the bed was
transferred to the measuring ¯ume.
The measuring ¯ume was ®lled with tap water and 1
ml of 32% acetic acid was added in order to ensure that
non-limiting organic substrate conditions were established. The resulting bulk water COD concentration in
the ¯ume was 100 gCOD mÿ3 . Acetic acid was used
because it is known to be a common readily biodegradable component in wastewater (Raunkjr, Nielsen,
& Hvitved-Jacobsen, 1997) and because aerobic biomass
in sewer sediments can utilise it for exponential growth
at high growth rates and without initial lag phase
(Vollertsen & Hvitved-Jacobsen, 1999). The ¯ume was
J. Vollertsen, T. Hvitved-Jacobsen / Urban Water 2 (2000) 21±27
operated in a thermostatic water bath at 20 1°C. It
was operated in aeration mode for approximately one
hour to ensure DO concentrations close to saturation
and steady-state di€usion conditions in the sediment
bed. After switching to the measuring mode, the decrease in DO concentration was recorded until the DO
concentration reached zero. The DO uptake rate was
calculated as the slope of the DO concentration versus
time from a linear regression on adjacent measurements.
SOUR was then found by multiplying the bulk water
DO uptake rate by the ratio of sediment surface area to
¯ume volume. After measurement, the box containing
the sediment bed was returned to the incubation ¯ume
where it was incubated with fresh wastewater for 26±30
h to ensure that the sediment was covered with a bio®lm.
The procedure of SOUR measurement was repeated
with the incubated sediment bed.
2.3. Physical and chemical analysis
Total solids (TS) and volatile solids (VS) were measured according to APHA, AWWA, WEF (1995).
Methane concentrations were determined by gas chromatography. The gases were separated on a Hyasep Q
column …2 m 2 mm† and detected with a ¯ame ionisation gas chromatograph with nitrogen as carrier
30 ml mÿ1 . The concentrations were quanti®ed using
known methane standards.
3. Results and discussion
The physical and microbial properties of the combined sewer sediment beds were studied; in particular
resistance to erosion and sediment bed DO uptake.
These properties were addressed using small-scale laboratory ¯umes, which allowed the same sediments to be
subjected to di€erent well-de®ned physical and chemical
conditions. The conditions under which the sediment
beds were studied di€er to some degree from in situ
conditions, i.e., temperature, ¯ow conditions and the
way the beds were built up. The results in this study
should hence be treated as qualitative results rather than
quantitative results.
3.1. Incubation time vs. resuspension
Undisturbed sediment beds were studied with respect
to erosion compared with freshly remoulded sediments.
After the ®rst day of incubation the average amount of
resuspended solids decreased slightly ± but not signi®cantly (Fig. 1). The amount of solids that became resuspended after about 100 h of incubation was on
average 70% larger than the amount of solids that became resuspended when sediments were freshly remoulded. That the amount of resuspended solids on
23
Fig. 1. In¯uence of the incubation period on resuspension. Seven experiments were performed.
average increased during the experiments shown in Fig. 1
is statistically signi®cant at a 1.6% signi®cance level. If
the resuspension experiments performed with freshly
remoulded sediments are omitted and a linear regression
analysis is made only on results from ¯umes incubated
for one day or longer, the signi®cance level becomes as
low as 0.6%.
On the second day of incubation an early development of a bio®lm was observed on top of the sediments
as a colour change from black to brown. Schmitt and
Seyfried (1992) also found that bio®lm developed on top
of undisturbed sediment beds. They investigated sulphate reduction in sewer sediments under anaerobic
conditions, and observed that the sediments had to be
raked once a day to avoid bio®lm development. In this
study, the bio®lms developed an estimated thickness of
0.5±1.0 mm after some days of incubation. When the
¯umes were operated in resuspension mode, the bio®lms
were often seen to resist resuspension for some period.
In this period the older, thicker bio®lms decreased
somewhat in thickness and loose, ¯u€y bio®lm material
was eroded. After some time of resuspension the bio®lm
was observed to break up and shreds of bio®lm were
torn-o€. This could happen anywhere on the sediment
bed surface. The underlying sediments were thereafter
eroded continuously and the remaining bio®lm undermined progressively in the upstream direction resulting
in the development of trenches in the sediment bed.
When the resuspension mode was terminated, parts of
the bio®lm-covered sediment bed typically remained
intact.
During incubation of the sewer sediments, the level of
the sediment bed increased by some 5±10 mm i.e., the
sediment bulk volume increased by 20±40%. It could be
observed that this increase was mainly due to the development of gas cavities within the sediment bed, by
taking samples of the sediment bed with a 16 mm
24
J. Vollertsen, T. Hvitved-Jacobsen / Urban Water 2 (2000) 21±27
diameter circular glass tube. Some gas cavities could be
seen in the extracted sediment core but most of the gas
was released during this sampling procedure, where gas
was observed to bubble up at the wall of the sampling
tube. The volumes of the single gas cavities in the sediment beds were estimated by extraction of gas from two
sediment beds by inserting a syringe into the sediment
and emptying cavities. The cavity volumes found in
these beds were up to 2 ml and an analysis of the extracted gas showed the methane content to be 14%.
According to Chanton, Martens, and Kelley (1989) it is
reasonable to assume the rest of the volume to be nitrogen that had di€used into the cavities. Gas cavities
and the emission of gas from sewer sediment beds were
observed by Iversen (unpublished) during extraction of
sediment cores in combined sewers, where they found
the methane production from the sediments to be substantial.
Gas cavities are probably not an uncommon phenomenon in combined sewer sediments, which have remained undisturbed for a couple of days. As a large
volume of free gas in a sediment bed results in a decreased bulk density, gas cavities have to be taken into
account when determining in situ sediment bulk density
for use in e.g., sediment transport models.
Several processes were observed to in¯uence the
sediment bed resistance to resuspension. Consolidation
and bio®lm growth tended to increase the resistance
while development of gas cavities tended to decrease the
resistance. The same processes are believed likely to
occur in combined sewer sediments in situ. However,
which processes will dominate depends on bed properties, such as the quantity and quality of the organic
matter deposited as well as the physical, chemical and
microbial conditions of the sediments and their incubation. In this study the behaviour of an arti®cially laid
sediment bed was studied under laboratory conditions
and the result was a decrease in resistance to resuspension due to development of gas cavities after some days
of incubation. From ®eld investigations, other investigators found increasing resistance to resuspension due
to prolonged dry weather periods (Ristenpart & Uhl,
1993).
3.2. Sediment oxygen uptake rate
When sediment beds have been left undisturbed for
some time, bio®lms tend to develop on the beds resulting
in e.g., changed biomass densities and di€usion coecients. Hence, transformations of bulk water components are likely to di€er, when the bulk water is exposed
to a bio®lm-covered sediment bed compared with a
bio®lm-free sediment bed. Development of bio®lms on
sediment beds was observed in the present study for
aerobic bulk water conditions and Schmitt and Seyfried
(1992) observed it for anaerobic bulk water conditions.
Addressing wastewater transformations of organic
matter, a signi®cant part of the transformations of bulk
water components takes place in the bio®lm and not in
the bulk water itself (Hvitved-Jacobsen, Vollertsen, &
Nielsen, 1998). If sediment deposits are large, the sediment bed forms a substantial part of the wetted sewer
surface. For modelling of wastewater organic matter
transformations in sewers it is hence important to understand how a sediment bed behaves compared to a
bio®lm, with respect to organic matter transformation.
An often-used approach in assessment of bio®lm and
sediment bed organic matter transformations under
aerobic conditions is the determination of the rate and
the kinetics of the DO uptake.
DO uptake kinetics for natural sediments from more
or less polluted environments have been reported to depend on the bulk water DO concentration (SO ) in di€erent ways. For example, Graneli (1977) reports, for
Swedish lake sediments, that SOUR is linearly dependent
on SO in the investigated interval 20±100% air saturation.
For four eutrophic lakes he reports SOUR for 100% air
saturation as being in the interval 1:0±1:6 g O2 mÿ2 dÿ1 .
Provini and Marchetti (1976) ®nd for wastewater polluted river sediments without macro-invertebrates, oxygen uptake to be independent of SO above 2±3 g O2 mÿ3
at 15°C with a level about 4±5 g O2 mÿ2 dÿ1 . Belanger
(1980) reports similar results for lake sediments with
levels of around 3 g O2 mÿ2 dÿ1 . Edberg and Hofsten
(1973) ®nd the DO dependency to be somewhat like an
exponential function with an exponent slightly below 1.
They ®nd SOUR at about DO saturation for the Baltic
sea, lakes and running waters in the range
0:3±3 g O2 mÿ2 dÿ1 . For a section of the Milwaukee
river polluted from combined sewer over¯ows, Kreutzberger, Race, Meinholz, Harper, and Ibach (1980) ®nd
SOUR ranging from 2.8 to 6:4 g O2 mÿ2 dÿ1 .
DO uptake rates and kinetics for sewer bio®lms have
been found to be 7.2 SO0:54 dÿ1 for bio®lms grown at low
wastewater loads and 4.1 SO0:77 dÿ1 for bio®lms grown at
high wastewater loads (Raunkjr et al., 1997). Bjerre,
Hvitved-Jacobsen, Schlegel, and Teichgraber (1998)
found rates and kinetics from 1.7 to 2.6 SO0:35 to 0:53 dÿ1
for bio®lms grown on wastewater.
When the resistance to mass transfer through the
di€use boundary layer is insigni®cant, DO uptake rate
kinetics for partly penetrated bio®lm, theoretically and
experimentally, is half-order in the bulk water DO
concentration (Harremoes, 1978; Jansen & Harremoes,
1984). When the mass transfer resistance through the
di€usive boundary layer is large compared with the reaction rate inside the bio®lm, the overall reaction rate
increases and ultimately approaches a ®rst-order rate in
the bulk water DO concentration (Harremoes, 1978;
Characklis & Marshall, 1989). Jùrgensen and Marais
(1990) showed that the di€use boundary layer thickness
over a bacterial mat sampled from a hypersaline pond,
J. Vollertsen, T. Hvitved-Jacobsen / Urban Water 2 (2000) 21±27
25
Fig. 2. Sewer sediment oxygen uptake rates (SOUR). The curves showing the highest rates correspond with incubated sediments; the lowest with fresh
sediments (Table 1).
decreased from a thickness of about 0.9 mm at a ¯ow
velocity of 0:003 m sÿ1 to 0.2 mm at a ¯ow velocity of
0:077 m sÿ1 . This decrease in di€use boundary layer
thickness resulted in a DO ¯ux into the bio®lm that
seemed to reach a constant level asymptotically, somewhat higher than the DO ¯ux measured at 0:077 m sÿ1 .
Belanger (1980) observed a similar behaviour for benthic oxygen uptake of lake sediments. This indicates that
for ¯ow velocities typical in sewers the limitation of DO
uptake due to di€usion through the boundary layer is of
minor importance. However, e€ects of ¯ow velocities on
bio®lm DO uptake have been reported for laboratory
studies of sewer bio®lms. For example, for ¯ow velocities increasing from 0:2 m sÿ1 to 0:9 m sÿ1 Nielsen,
Raunkjr, Norsker, Jensen, and Hvitved-Jacobsen
(1992) report an increase in DO uptake rates of 10±40%.
For studies in real sewers, Pomeroy and Parkhurst
(1973), Boon and Lister (1975), and Matos and de Sousa
(1991) indicate that higher ¯ow rates result in higher DO
uptake rates.
In this study, the ¯ow velocity in the ¯ume was kept
at 0:071 m sÿ1 to ensure that no bio®lm was eroded and
circulated with the bulk water as this would have introduced an unacceptable error in the SOUR measurements. Due to the low ¯ow velocity, the SOUR rates
found in this study are probably underestimated compared with real systems.
SOUR was determined twice on six di€erent sewer
sediment samples. Firstly for sediment beds without
bio®lm cover, i.e., immediately after application of the
sediment in the box, and then for the same sediment
beds incubated in wastewater. After this period they
were completely covered with a clearly visible bio®lm.
Results are given in Fig. 2 and Table 1.
SOUR was found to be exponentially related with SO
with an exponent n between 0.57 and 0.88 ± average for
all experiments was 0.72. No signi®cant di€erence was
found between average exponents for non-incubated
and incubated sediment beds (Table 1). Exponents signi®cantly higher than 0.5 indicate that the mass transfer
Table 1
Parameters corresponding with Fig. 2a
VS (%)
Non-incubated
1.8
1.5
2.6
2.5
3.0
2.6
Average
a
±
The units of kn are g
kn
n
k0:5
k0:72
kn
n
k0:5
k0:72
0.33
0.26
0.28
0.36
0.30
0.43
0.57
0.72
0.79
0.68
0.70
0.65
0.37
0.35
0.40
0.46
0.40
0.53
0.27
0.26
0.30
0.34
0.30
0.39
1.45
1.50
1.56
1.11
1.92
1.38
0.79
0.79
0.62
0.65
0.78
0.88
2.14
2.37
1.89
1.40
3.05
2.58
1.61
1.68
1.31
0.99
2.12
1.82
0.69
0.42
0.31
1.49
0.75
2.24
1.59
0.33
…1ÿn†
O2
…3 nÿ2†
m
Incubated
ÿ1
d
including the columns where n ˆ 0:5 and n ˆ 0:72:
26
J. Vollertsen, T. Hvitved-Jacobsen / Urban Water 2 (2000) 21±27
resistance through the di€usive boundary layer is important and hence higher SOUR would have been obtained if ¯ow velocities in the ¯ume had been higher.
A clear di€erence between non-incubated and incubated sediment beds is seen when comparing the nth
order rates based on the best statistical ®t. For nonincubated beds, the average rate constant, kn , is
m0:07 dÿ1 whereas for incubated sediments it
0:33 g O0:31
2
0:25
is 1:49 g O2 m0:25 dÿ1 . However, when comparing
di€erent rate constants it is problematic to use the rate
constants directly when the exponents di€er, as this results in di€erent units of kn ; these units are generally
…1ÿn†
m…3 nÿ2† dÿ1 . Therefore, the rate constants for
g O2
the half-order reaction, k0:5 , and for the average exponent ± n ˆ 0:72 nth order ± are also given in Table 1. For
both these values of n, the average values of kn between
incubated and non-incubated sediment beds di€er at a
0.2% signi®cance level.
Comparing the rates found for bio®lm-covered sediment beds with the earlier-mentioned ®ndings for bio®lms grown on wastewater shows that the rates for
incubated sediment beds are lower than those found by
Raunkjr et al. (1997), but in the same range as the
®ndings by Bjerre et al. (1998). Nielsen et al. (1992)
found DO uptake rates several times higher than in this
study. The relatively low rates presented here might be
caused partly by the lower bulk water velocity and,
therefore, it seems reasonable to assume that the bio®lm-covered sediment beds had DO uptake rates and
kinetics comparable with sewer bio®lms. Comparison
between SOUR for bio®lm-free sediment beds and
natural but more or less polluted sediment beds shows
the SOUR is found in this study to be in the lower range
of the reported values. Again this might be partly due to
the relatively low ¯ow velocities.
Bio®lm-covered sediments beds showed SOUR signi®cantly higher than bio®lm-free sediment beds. The
rates were on average ®ve times higher for bio®lm-covered sediments (Table 1). Therefore, it is important to
take into account these di€erences in SOUR when
simulating aerobic transformations of wastewater organic matter in sewers that contain considerable
amounts of sediments. Simulating transformations of
wastewater organic matter, it might be acceptable to
treat an undisturbed, bio®lm-covered sediment bed in
the same way as the bio®lm that covers the sewer wall.
However, if the sediments are freshly remoulded or in
constant movement it might be necessary to take a
corresponding lower DO uptake rate into account.
considered an inverse measure of the bed strength increased during incubation and the bulk density of the
sediment bed decreased during incubation. The decrease
in bulk density was mainly due to a large methane
production in the sediment bed, resulting in a bed volume increase of 20±40%. The development of a bio®lm
covering the bed was, on the other hand, seen to
strengthen the bed.
The study has shown that when addressing resuspension of sewer sediments, sediment age and bio®lm
cover should be taken into account as they signi®cantly
in¯uence the strength and density distribution of the bed
deposits. Several other factors, e.g., organic matter
content and composition, which have not been dealt
with in this study, probably also play a role for the
microbial transformations and their in¯uence on the
physical sediment bed properties. It is, therefore,
strongly recommended that studies on microbial processes in the sewer systems be taken into account in the
description and modelling of strength and resuspension
of biologically active sewer sediments.
The sewer sediment oxygen uptake rate (SOUR) increased by about ®vefold from the SOUR of freshly
remoulded sediments to the SOUR of sediments which
had been aerobically incubated to allow a bio®lm to
develop. This placed the incubated, bio®lm-covered
sediment beds in the range of surface oxygen uptake
rates reported for sewer bio®lms. Sediment beds that
were freshly remoulded had a SOUR in the range of that
reported for lake sediments. The DO uptake kinetics of
bio®lm-covered sediment beds and bio®lm non-covered
sediment beds was seen to behave exponentially with an
average exponent of 0.72 at a bulk water ¯ow velocity of
0:071 m sÿ1 . Therefore, when simulating wastewater
transformations of organic matter under aerobic conditions, di€erent DO uptake rates and kinetics should be
used if the sediments are constantly remoulded compared with sediment beds which have been undisturbed
for some time.
4. Conclusions
APHA, AWWA, WEF (1995). Standard methods for the examination
of water and wastewater (19th ed.). APHA, AWWA, WEF
Washington.
Ashley, R. M., & Verbanck, M. A. (1996). Mechanics of sewer
sediment erosion and transport. Journal of Hydraulic Research,
34(6), 753±769.
Physical as well as microbial properties of sewer
sediment beds were found to change with the duration
of bed incubation. The amount of resuspended solids
Acknowledgements
Financial support for this research project was provided by the Danish Technical Research Council, the
framework programme on ``Solids in Sewage Systems''.
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