Download Quantifying the Performance of Storm Tanks

11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
Quantifying the Performance of Storm Tanks
Dr Will Shepherd*, Professor Adrian Saul and Dr Joby Boxall
Pennine Water Group, Department of Civil and Structural Engineering, The University of
Sheffield, Sheffield, S1 3JD, England, UK.
*Corresponding author, e-mail [email protected]
ABSTRACT
Storm tanks at Wastewater Treatment Works (WwTW) provide storage and sedimentation for
excess flows entering the WwTW as a result of storm events, as such they are an essential but
often neglected component of the sewerage system. Historically in the UK, flows entering the
WwTW are limited to approximately 6 times the mean daily dry weather flow (DWF) through
the use of combined sewer overflows in the system and an emergency overflow at the
entrance to the works. Approximately half of the works inflow (i.e. 3 × DWF) is passed to
full treatment and the remainder is discharged to the storm tanks. Once the tanks are full the
excess flows are spilled to the nearest watercourse or ocean. After the storm event has
subsided and the DWF returns to normal, the storm tanks are emptied, with the effluents
being given full treatment.
The pollution retention performance of storm tanks has generally been omitted from
regulatory guidelines, the tanks being designed solely on the stored effluent volume. As a
result, certainly in the UK, the design philosophy for storm tanks has been static, and has not
moved forward in the same way as other developments associated with integrated quantitative
and qualitative modelling of sewer systems.
The Water Framework Directive, will require all water utility companies (within the EU) to
better understand the magnitude, volume and quality of all intermittent discharges that issue
from sewerage systems into receiving waters. This approach is recommended at the
integrated catchment scale and hence the contribution to receiving waters from storm tanks
may form one of the major elements of pollution.
This paper is based on UKWIR research project WW22B (UKWIR, 2007) and includes a
review of current practice and presents results of a field and laboratory based experimental
programme which improves understanding of the hydraulic and pollutant retention
performance of rectangular storm tanks.
KEYWORDS
Storm tanks, design, hydraulic performance, residence time, pollution retention performance,
fieldwork, laboratory work.
INTRODUCTION
The treatment efficiency of Wastewater Treatment Works (WwTW) is influenced by the
volume and strength of sewage passing through them. As such, flow from combined sewer
systems into the treatment process is limited to approximately 3 times the mean Dry Weather
Shepherd et al.
1
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
Flow (DWF). Flows between 3 and 6 times the DWF are passed into storm tanks, which have
been an integral part of the sewage treatment process since the early 20th century and the basic
functions of storm tanks include:
1.
acting as temporary storage tanks when excess flows enter the WwTW and to retain as
much storm sewage as possible for later treatment;
2.
reducing the number of occasions on which excess flow would have discharged to the
receiving water;
3.
allowing settlement to reduce the strength of the storm sewage that is spilled from
tanks;
4.
increasing the time of concentration of the sewerage system such that the spill flow
occurs at a later time which should coincide with increased flows in the receiving water,
thereby providing additional dilution;
5.
retaining the more highly polluted first flush of sewage that is frequently observed
during the early part of the increase in sewer flow caused by rainfall.
To achieve acceptable performance it is desirable that the tank has sufficient depth to allow
the settlement of solids, that scour and re-suspension of settled solids does not occur and that
the washout of solids to the receiving water is avoided.
In the UK, the design of storm tanks is generally based on the Environment Agency
recommended guidelines of 2 hours retention at 3PG + I + 3E (where P is population, G is
number of litres per head per day, I is infiltration inflow and E is industrial effluent) or 68
litres per head of population. These guidelines result in a design tank volume and it is
common practice to use tanks which are either rectangular and circular on plan. There are
however no guidelines on the length to breadth to depth ratio for rectangular tanks or of the
diameter to depth ratio for circular tanks. Neither guideline specifically considers the need to
retain pollutants in the storm flow (although any first flush may be anticipated to occur within
the early part of a storm) or the pollutant retention processes that occur in the tank. Tank
geometry will influence the hydraulic performance of the tank and this in turn will influence
the retention of the pollutants that enter the tank within the inflow. However, no link is made,
at the time of tank design, between the hydraulic performance and the selected geometry and
dimensions of a tank, such that the pollution retention performance, for a known inflow
(quantity and quality) is optimised. This is primarily due to the fact that the current
regulatory drivers do not require that the quality of the spilled flow from tanks be known.
However, European legislation, in the form of the Water Framework Directive, will require
all member states to better understand the pollutant impact of all intermittent discharges to
receiving waters, including all CSO structures and storage tanks within the sewerage systems
and storm tanks at WwTW.
This paper presents a brief review of previous research and results from a study to better
understand the behaviour of rectangular storm tanks through testing in both field and
laboratory.
REVIEW OF STORM TANK DESIGN AND PERFORMANCE.
A literature review highlighted that little work has been completed to specifically examine the
performance of storm tanks at WwTW. Most of the literature addresses the performance of
conventional on-line and off-line storage tanks and of primary and secondary settlement tanks
and clarifiers. In respect of the latter, many models of tank performance have been
developed. These include theoretically based formulae, regression based methods, and, more
2
Quantifying the Performance of Storm Tanks
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
recently, mathematical simulation of the dynamic processes. The measure of a settling tank’s
performance to retain pollution has been termed its removal efficiency, η , and this has been
defined as the proportion of the inflow load that is retained within the tank over the duration
that the tank is in operation. Historically, the efficiency of tanks has been based on Surface
Load Clarification Theory for an Ideal Rectangular Tank. For example, in Germany a surface
loading rate of 10m/hr and a tank length to width ratio of at least two are specified whilst, in
America the specified loading rates are much lower, ranging from 0.5m/hr for small
populations (US Army Corps of Engineers) to 5m/hr (Metcalf and Eddy, (1991)).
The theory was originally developed by Hazen (1904), and expanded by Camp (1946), who
defined an ideal rectangular continuous flow settling basin as having the following
characteristics:
1.
the direction of flow is horizontal and the velocity is the same in all parts of the
settling zone (hence, each particle of water is assumed to remain in the settling zone for a time
equal to the detention period – namely, the volume of the settling zone divided by the
discharge rate);
2.
the concentration of suspended particles of each size is the same at all points in the
vertical cross section at the inlet end of the settling zone;
3.
a particle is removed from suspension when it reaches the bottom of the settling zone;
4.
for any given discharge, the removal is a function of the surface area and is
independent of the depth of the basin, or, the removal is a function of the overflow rate, and,
for a given discharge is independent of the detention period;
5.
the concentration of suspended matter at any cross section in the settling zone
increases with the depth below the surface, and decreases with the proximity of the cross
section to the outlet end of the basin.
Any particle settling in a moving liquid will move in a particular direction and at a particular
velocity, which is the vector sum of its own settling velocity and the velocity of the
surrounding liquid. In an ideal rectangular tank the paths of all discrete particles will be
straight lines, and all particles with the same settling velocity will move in parallel paths (as a
function of the position that they enter the tank).
In the design of settling tanks, the usual procedure is to select a particle with a terminal
velocity Vc and to design the tank so that all particles that have a terminal velocity equal to or
greater than the design terminal velocity will be removed. The rate, Q, at which clarified
water is produced is then:
Q = AVc
where A is the surface area of the basin.
Effectively the selection of Vc defines the surface overflow rate or surface loading rate,
expressed in cubic metres per square metre surface area. Particles that have a fall velocity of
less than Vc will not be removed during the time provided for settling. If an average settling
velocity (Vs ) is assumed for the suspension, then according to surface load theory the removal
efficiency (η ) for quiescent conditions is given by:
η=
Vs
Q/ A
It is stressed that this expression is only valid under uniform, steady and laminar flow
conditions. Furthermore, Metcalf and Eddy (1991) identified that particles may settle as
discrete particles, as flocculants, by hindered settling or due to compression. Of these
processes discrete particle settling and flocculant settling are most commonly observed in
storm tanks. Hence particle retention is a function of the nature and type of the particles and
Shepherd et al.
3
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
of the surface overflow rate. Clements (1966), studied the velocity variations through a
rectangular storm tank, and he observed that sometimes there were significant velocity
variations across the width of the tank. These changes in velocity would change the surface
overflow rate, and, based on the work of Hazen (1904), he introduced the concept of an
effective velocity – based on the effective settling length of the tank taking due regard of the
velocity variations across the width of the tank. When velocity variations were high, the
effective mean velocity through the tank was also high, with a consequent reduction in the
settling efficiency when compared to that calculated from the actual mean velocity. New
design guidelines for storm tanks were proposed based on the concept of an effective velocity
to allow the particles of sewage to settle out.
Consideration has also to be given to the pollution characteristics of the influents that enter
into a tank. A typical influent contains a distribution of particle sizes and hence a range of
particle terminal velocities, Chebbo and Bachoc (1992) and Michelbach and Weiss (1996)
whilst Madaras and Jarrett (2000) described the spatial and temporal distribution of sediment
concentration and particle size distribution in a full size sedimentation basin. It is usual for
the distribution to be split into a number of discrete terminal velocity bands with the
efficiency calculated for each band. The total removal efficiency is calculated by summing
the respective efficiency for each terminal velocity band.
Lessard and Beck (1991) presented a conceptual model for the simulation of the dynamic
performance of storm tanks. Four modes of behaviour were considered: the filling and
emptying process and the effects of dynamic sedimentation and quiescent settling. The model
was applied to a storm event at Norwich WwTW and the simulated results showed that the
storm tank had a relatively small effect on the spilled load of suspended solids. A sensitivity
analysis showed that this finding was a function of the characteristics and distribution of the
sediments that entered the tank. Similar conclusions were reached by Baumer et al. (1996)
and Lindeborg (1996) who carried out a series of dynamic performance tests in final settling
tanks and sedimentation tanks respectively.
The US EPA (1986) proposed the following methodology to estimate sediment removal under
dynamic conditions:
⎛
η =1 − ⎜⎜1 +
⎝
1 Vs ⎞
⎟
n Q / A ⎟⎠
−n
where n is a turbulence or short circuiting constant that is used to indicate the settling
performance of the pond: n = 1, for poor performance; n = 3, good performance; n > 5, very
good performance; and n = ∞ , ideal performance. The value of n is thus somewhat
subjective, hence there is a need to more accurately define the short circuiting constant.
DETERMINATION OF RESIDENCE TIME IN STORM TANKS
As part of the UKWIR study, testing has been carried out in both full scale storm tanks and
laboratory scale models. This testing was developed to investigate the deviation of measured
residence times from theoretical residence times, which assume idealised plug flow
conditions.
Field testing
The rectangular storm tank used in the study was located at a WwTW in the North West of
England and was of a typical design with a full width weir distributing inflow to the tank and
a similar weir to over which excess flows spill from the tank. The tank tested was 18.5 m
4
Quantifying the Performance of Storm Tanks
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
long, 6.3 m wide and had a mean depth of 1.8 m, giving a volume of 214 m3, this places it
around the mean dimensions of the storm tanks surveyed during the project. Scumboards
were temporarily fitted inside the tank adjacent to the inlet and spill weirs as per the original
tank design, these were then removed to assess their effect on residence times. A photograph
of the tank as tested is shown in Figure 1 and a schematic of the test layout in Figure 2. Full
details of the site and field testing set-up are included in the UKWIR (2007) report.
Figure 1: Photograph of field tank.
FFT Penstocks
Pump
Inlet Channel
Tracer
Injection
Tank 1
Tank 2
Tank 3
Scufa
Sand bag
Figure 2: Schematic drawing of field testing arrangement.
Testing was carried out at a range of constant flow rates, which were provided by pumping
from the works inlet over the abandoned 3 DWF weir. To estimate the true residence time
Rhodamine WT fluorescent tracer was introduced into the inflow to the tank at a location
upstream of the tank. ‘SCUFA’ fluorometers were used to measure the tracer concentration
on entry to the tank, adjacent to the inlet weir and on exit from the tank at the spill weir.
Shepherd et al.
5
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
Inlet
Spill
First arrival 34 minutes
5 percentile 38 minutes
50 percentile 105 minutes
95 percentile 247 minutes
Centroid 121 minutes
Peak 51 minutes
Theoretical 136 minutes
2.0E-06
1.5E-06
1.0E-06
1.0E-07
8.0E-08
6.0E-08
4.0E-08
2.0E-08
5.0E-07
0.0E+00
0
50
100
150
200
Time (minutes)
250
300
Spill Concentration (l/l)
Inlet Concentration (l/l)
2.5E-06
0.0E+00
350
Figure 3: Example solute trace showing different definitions of residence time.
Typical results from field tracer tests are shown in Figure 3, it can clearly be seen that due to
mixing processes within the tank, the spill trace is much more lagged and attenuated than the
inlet trace. There are a number of different techniques that may be used to define the
residence time from the tracer data:- the minimum value is the difference in first arrival times
of the two tracer distributions (in this example 34 minutes), the modal average is the time
between peaks of the two traces (51 minutes), whilst the mean residence time may be
described as the time difference between the centroid of the traces (121 minutes).
Alternatively it is feasible to describe the time when a certain percentile of the tracer has
passed through the tank, for example 5% (38 minutes), 50% (105 minutes) or 95% (247
minutes). These compare with a theoretical residence time, calculated by dividing the tank
volume by the flow rate, of 136 minutes. In this paper the 50 percentile definition was used
as this splits the pollutant load equally, half will have a greater residence time and half the
pollutant load will have a shorter residence time.
Measured fifty percentile residence time
(field scale minutes)
Figure 4 plots the mean fifty percentile residence times derived from the field data. It can be
seen that that these fifty percentile residence times are significantly shorter than the
theoretical residence times. The single test at a low flow-rate (high theoretical residence time)
appears to show enhanced short-circuiting, whilst removing the scumboards was shown to
increase the fifty percentile residence time (at the tested flow rate). The error bars in Figure 4,
plotted to show ± 1 standard deviation from the mean, show that where repeat traces have
been carried out, values of the fifty percentile residence time exhibit little scatter, enhancing
confidence in the results.
200
Theoretical residence time
180
With scumboards
160
Without scumboards
140
120
100
80
60
40
20
0
0
5
10
15
20
Flow (l/s)
25
30
35
40
Figure 4: Windermere Fifty percentile residence times.
6
Quantifying the Performance of Storm Tanks
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
Laboratory testing
Fieldwork testing was limited due to limited time on site, constraints on flow magnitudes and
the fact that only 1 geometrical configuration was available. A series of laboratory tests was
therefore completed to enhance the scope of the study by testing a Froude number scaled tank
within a controlled and repeatable environment.
The design of the laboratory tank, Figure 5, was based upon a dataset of storm tanks
commonly used in the UK and was of modular construction such that a range of different
geometry tanks could be tested. These geometrical arrangements included a scale model of
the storm tank tested in the field and results from this laboratory tank are reported here.
Figure 5: Schematic drawing and photograph of laboratory tank.
The laboratory test methodology was the same as that in the field, whereby a slug of
fluorescent tracer was monitored as it passed through the tank at a range of constant flow
rates. Cyclops fluorometers were used to measure the tracer concentration in the laboratory
tests (these are smaller and more applicable to the laboratory situation than the SCUFAs used
in the field testing). The primary advantages of laboratory testing were that clean water was
used and that the hydraulic conditions were more accurately controlled. This resulted in
variations in the background concentration of the injected tracer and in the magnitude of the
flow rate over the duration of each test being negligible. Figure 6 shows how the shape of the
tracer pulse varies with flow rate and highlighting the magnitude of the attenuation and
dispersion processes affecting time varying pollutant concentrations. It can also be seen that
there is less noise in the measurements than was seen in the field, meaning the start and end of
the tracer pulse can be more accurately determined, and therefore the residence time more
accurately calculated.
0.060 l/s
0.243 l/s
18%
Concentration (% of inlet peak)
Concentration (% of inlet peak)
100%
80%
60%
40%
20%
0.060 l/s
0.243 l/s
16%
14%
12%
10%
8%
6%
4%
2%
0%
0%
0
100
200
300
Time (s)
400
500
600
0
2000
4000
6000
8000
Time (s)
a) Inlet traces
b) Spill traces
Figure 6: Comparison of tracer distributions in laboratory tank with scum-boards at high and
low flow-rate.
Shepherd et al.
7
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
Fifty percentile residence time (s)
Figure 7 shows the mean 50 percentile residence times with error bars of ± 1 standard
deviation for the repeat injections, as a function of flow rate. The results show the expected
power law trend with discharge, however the measured residence times are always
significantly less than the idealised theoretical residence times.
4500
Measured with scum-board
4000
Measured without scum-board
Theoretical
3500
3000
2500
2000
1500
1000
500
0
0
0.05
0.1
0.15
0.2
0.25
0.3
Flow rate (l/s)
Figure 7:Variation in laboratory residence times with flow rate, measured and theoretical.
In addition to the tracer tests, an experiment to directly quantify Total Suspended Solids
(TSS) retention efficiency was conducted. This entailed introducing a constant concentration
of crushed olive stone and collecting discrete samples from the tank inlet and spill. These
samples were then analysed to determine TSS concentrations and allowed retention efficiency
to be estimated based on the difference between inlet and spill concentrations. Crushed olive
stone was used as a surrogate for sewer solids as this sediment, when mixed with water, has
been shown to accurately represent live sewage particles in appropriately scaled laboratory
tests (Ellis 1992).
Figure 8 shows the results of the laboratory TSS experiment together with predicted retention
efficiencies. These were calculated from the measured 50 percentile residence times and the
measured settling velocity distribution of the crushed olive stone. From this it can be seen
that when the residence time is correctly estimated, classical sedimentation theory can
estimate retention efficiency with good accuracy.
100%
Retention efficiency (%)
90%
80%
70%
60%
50%
40%
30%
20%
Predicted
10%
Measured
0%
0
500
1000
1500
2000
2500
3000
Laboratory residence time (s)
Figure 8: Variation in retention efficiency with laboratory residence time, measured and
predicted.
8
Quantifying the Performance of Storm Tanks
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
Verification of laboratory results
In order to verify that the laboratory data is representative of the full scale tanks, a comparison
between the data collected in the laboratory and in the field was carried out. To do this, it was
necessary to apply scale factors to the laboratory results following Froude scale laws. A
comparison of fieldwork and scaled-up laboratory tracer data is shown in Figure 9. Some
differences between the two residence time distributions can be attributed to variations in the
flow rate and background noise in the field tests. However the general shape of the recession
limb is similar in both the laboratory and field.
10%
Field
Laboratory
Concentration (% of inlet peak)
9%
8%
7%
6%
5%
4%
3%
2%
1%
0%
0
50
100
150
200
Time (field minutes)
250
300
350
Figure 9: Comparison of field and laboratory tracer data.
The field and laboratory fifty percentile residence times for a range of flow-rates are
compared in Figure 10. Comparison between the laboratory and fieldwork results show
reasonable agreement in the fifty percentile residence times, particularly when flow variations
present in the field tests are considered. It is immediately apparent that, in both the full-scale
tank and in the scale model, the fifty percentile residence times are always less than the
theoretical residence times, at the flows tested. It is suggested that the ratio of the theoretical
to measured residence times could be used as a correction factor, and hence, in conjunction
with a TSS settling velocity distribution, reliable estimates of solids retention efficiencies may
be made. Such ratios would need to be quantified for a range of tank geometries at various
flow rates in order to be used in storm tank design.
Measured fifty percentile residence time
(field scale minutes)
Theoretical residence time
140
Field
Laboratory
120
100
80
60
40
20
0
0
10
20
30
40
50
Flow (field scale l/s)
60
70
Figure 10: Comparison of field and laboratory residence times.
Shepherd et al.
9
11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008
CONCLUSIONS
Rectangular storm tank 50 percentile residence times measured in the field have been shown
to agree well with those measured in a laboratory scale model. This provides confidence that
residence times in a range of tank geometries can be successfully investigated using Froude
scaled models.
The 50 percentile residence times measured in the laboratory and the field have both been
shown to be less than the theoretical value obtained assuming idealised flow conditions, thus
storm tanks designed without taking this into consideration will over predict pollution
retention. It is suggested that a ratio of measured to theoretical residence times could be used
to more accurately predict solids retention efficiency, when used with a known solids settling
velocity distribution. This is supported by agreement between the results of TSS retention
measurements made in the laboratory and TSS retention calculations based on classical
sediment transport theory, made using measured residence times and settling velocity
distributions.
ACKNOWLEDGEMENT
The authors are indebted to UKWIR for financing the project and allowing the publication of
the research. The opinions expressed are those of the authors and do not necessarily reflect
those of UKWIR or the companies of the authors.
REFERENCES
Camp T. R. (1946). Sedimentation and the design of settling tanks, Trans. ASCE, Vol. 111, 895-936
Chebbo, G and Bachoc, A. (1992). Characterization of suspended solids in urban wet weather discharges. Wat.
Sci. Tech., Vol. 25, No. 8, 171-180
Clements, M. S., (1966), Velocity variations in rectangular sedimentation tanks, Institution of Civil Engineering
Proceedings, Vol. 34, p171-200
Ellis, D.R., 1992. The design of storm drainage storage tanks for self cleansing operation. Thesis, (PhD).
University of Manchester.
Hazen A. (1904). On sedimentation. Trans., ASCE, Vol. 53, 45 – 71
Lessard, P. and Beck, M.B., (1991), Dynamic simulation of storm tanks, Water Research, Vol. 25, No. 4, 375391
Madaras, J.S., and Jarrett, A.R., 2000. Spatial and temporal distribution of sediment concentration and particle
size distribution in a field scale sedimentation basin. Transactions of the American Society of
Agricultural Engineers, 43 (4), 897-902.
Metcalfe and Eddy, Inc. (1991). Wastewater Engineering, Treatment, Disposal and Reuse, 3rd Edition, McGraw
– Hill, Singapore, ISBN 0 – 07-100824 – 1.
Michelbach, S., and Weiss, G.J., (1996). Settleable sewer solids at stormwater tanks with clarifier for combined
sewage. Water Science and Technology, 33(9), 261-267.
UKWIR (2007) Performance of Storm Tanks and Potential for Improvements in Overall Storm Management –
Phase 2. (07/WW/22/5) ISBN: 1 84057 469 0
10
Quantifying the Performance of Storm Tanks