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7th International Conference on Sewer Processes & Networks
Wednesday 28 - Friday 30 August 2013
The Edge Conference Centre, Sheffield
Erosion resistance and
behaviour of highly organic
in-sewer sediment
Irene Seco
Manuel Gómez
Alma Schellart
Simon Tait
Wet weather pollution from combined sewer systems
Release of in-sewer sediment deposits
accumulated during dry-weather constitute a
major source of pollutants that affect the
water quality of receiving natural water
bodies
Wet weather pollution from combined sewer systems
Particularities in the Mediterranean region in Spain:
Rain regime
Long dry-weather periods
Intense precipitation events
cumulative precipitation = 600 mm/year
(concentrater 50 days/year)
average dry-period between rain: 11 days
≈ 40% rainfall registered P(mm)>10mm
Wet weather pollution from combined sewer systems
Particularities in the Mediterranean region in Spain:
Rain regime
Long dry-weather periods
Intense precipitation events
In-sewer sediment accummulation and
consolidation
Wet weather pollution from combined sewer systems
Particularities in the Mediterranean region in Spain:
Rain regime
Long dry-weather periods
Intense precipitation events
Flow regime in rivers
Low circulating flow
(0.56 m3/s average Congost River.Vallès
Oriental. Catalonia. Spain)
Low dilution capacity
Wet weather pollution from combined sewer systems
Particularities in the Mediterranean region in Spain:
Rain regime
Flow regime in rivers
Urban pattern
Long dry-weather periods
Low circulating flow
Low dilution capacity
High percentage of
impervious surface in
Urban areas
Intense precipitation events
Wet weather pollution from combined sewer systems
Particularities in the Mediterranean region in Spain:
Rain regime
Flow regime in rivers
Urban pattern
Long dry-weather periods
Low circulating flow
Low dilution capacity
High percentage of
impervious surface in
Urban areas
Intense precipitation events
Sewer solids mainly from
wastewaters
Relevant Organic
composition
Wet weather pollution from combined sewer systems
Particularities in the Mediterranean region in Spain:
Rain regime
Flow regime in rivers
Urban pattern
Long dry-weather periods
Low circulating flow
Low dilution capacity
High percentage of
impervious surface in
Urban areas
Intense precipitation events
Overflows during wet-weather
Significant volumes discharged in a short
time from sewer network (CSO)
Significant organic Pollution in natural streams
and sea
Objectives of this study
Particular conditions of Build-up/Wash-off affects the initiation of sediment
motion
Objectives of this study
Particular conditions of Build-up/Wash-off affects the initiation of sediment
motion
Investigate erosion
behaviour of real insewer organic-rich
sediment collected in
Spain
Analyse changes in transport potential for
different lengths of antecedent dry-weather
periods
Consider potential incidence of the
environmental conditions in-sewer on the
transport loads and initiation of motion
Objectives of this study
Improve prediction of
in-sewer sediment transport loads
Suitable to be applied to the particular
Mediterranean climate and urban pattern
conditions
• Long dry-weather
period/Build-up
• Intense rainfall/Wash-off
Real in sewer sediment characteristics
Sediment collected from a sewer system
(residential and commercial area in Catalonia, Spain)
Real in sewer sediment characteristics
Sediment collected from a sewer system
(residential and commercial area in Catalonia, Spain)
relevant Organic content
(O.M. around 80%)
Cohesive properties
organic nature of solid particles from domestic
wastewaters, and the presence of greases
Real in sewer sediment characteristics
Sediment collected from a sewer system
(residential and commercial area in Catalonia, Spain)
relevant Organic content
Cohesive properties
d50 = 0.31 mm (310 µm)
Density = 1310 kg/m3
Related with the organic nature of solid particles
from domestic wastewaters, and the presence of
greases
% accumulated on
the sieve
(O.M. around 80%)
Sieving analysis of sewer sediment in natural state
100
80
60
40
20
0
0
0.25
0.5
0.75
1
1.25
1.5
mesh width (mm)
1.75
2
2.25
2.5
Real in sewer sediment characteristics
Sediment with high organic content and cohesive properties
Consolidation, microbiological degradation, chemical-biological
interactions (bonding forces between particles )
Effects on the transport of solids
significant influence on the incipient motion
Laboratory erosion measurement
Erosion meter devised
(based on a design by Liem et al. 1997).
Evaluation Erosion
Rate
Assessment critical
threshold of motion
at solid-fluid interface
Liem et al. (1997) investigation on erosional proecess of
cohesive sediment using an in-situ measuring device.
International Journal of Sediment Research, 12 (3), 139-147
Laboratory erosion measurement
A prepared sample is exposed to a consolidation
period and subsequently subject to increased
shear stress, to simulate increased flows through
sewer pipes at the start of a storm event.
Consolidation period
Simulation of dry-weather periods
• Different lengths: 16, 64, 140 hours
• Environmental conditions:
Anaerobic and Aerobic
• Constant low shear stress:
0.15 N/m2 (dry-weather flows inside conduits)
Erosion phase
Sampling and
Analysis
Laboratory erosion measurement
A prepared sample is exposed to a consolidation
period and subsequently subject to increased
shear stress, to simulate increased flows through
sewer pipes at the start of a storm event.
Consolidation period
Erosion phase
Simulation of dry-weather periods
Simulation flows at
start of storm event
• Different lengths: 16, 64, 140 hours
• Environmental conditions:
Anaerobic and Aerobic
• Constant low shear stress:
0.15 N/m2 (dry-weather flows inside conduits)
Increasing shear
stress is applied in a
stepwise way
Sampling and
Analysis
Laboratory erosion measurement
A prepared sample is exposed to a consolidation
period and subsequently subject to increased
shear stress, to simulate increased flows through
sewer pipes at the start of a storm event.
Consolidation period
Erosion phase
Simulation of dry-weather periods
Simulation flows at
start of storm event
• Different lengths: 16, 64, 140 hours
• Environmental conditions:
Anaerobic and Aerobic
• Constant low shear stress:
0.15 N/m2 (dry-weather flows inside conduits)
Increasing shear
stress is applied in a
stepwise way
Sampling and
Analysis
Sediment samples
collected during
erosion tests at each
shear stress interval
Remaining sediment
after tests collected
Main Results
Erosion rate of sediments monitored in terms of
Suspended Sediment concentration
Average erosion rate (q) linked to the applied shear stress (tb)
1.60
1.20
T1
16h
0.80
0.40
0.00
0
0.2
0.4
0.6
0.8
1
Applied shear stress (Tau_b)
[N/m2]
2.00
1.60
1.20
T2
64h
0.80
0.40
0.00
0
0.2
0.4
0.6
0.8
1
Applied shear stress (Tau_b)
[N/m2]
1.20
T4
16h
0.80
0.40
0.00
0
0.2
0.4
0.6
0.8
Applied shear stress (Tau_b)
[N/m2]
1
erosion rate (E) [g/m2/s]
erosion rate (E)
1.60
2.00
1.60
1.20
T5
64h
0.80
0.40
0.00
0
0.2
0.4
0.6
0.8
Applied shear stress (Tau_b)
[N/m2]
2.00
1.60
1.20
T3
140h
0.80
0.40
0.00
0
0.2
0.4
0.6
0.8
Applied shear stress (Tau_b)
[N/m2]
Aerobic
2.00
erosion rate (E) [g/m2/s]
erosion rate (E) [g/m2/s]
2.00
[g/m2/s]
erosion rate (E) [g/m2/s]
Anaerobic
1
1
Main Results
Erosion Rate values
Comparison between tests with increasing length of periods of consolidation.
2.0
16h (T1)
1.6
erosion rate [g/m2/s]
erosion rate [g/m2/s]
2.0
64h (T2)
1.2
140h (T3)
0.8
0.4
1.6
16h (T4)
1.2
64h (T5)
0.8
0.4
0.0
0.0
0
0.2
0.4
0.6
Applied shear stress (Tau_b) [N/m2]
Anaerobic
0.8
0
0.2
0.4
0.6
Applied shear stress (Tau_b) [N/m2]
Aerobic
0.8
Main Results
Erosion Rate values
Comparison between tests with increasing length of periods of consolidation.
2.0
16h (T1)
1.6
erosion rate [g/m2/s]
erosion rate [g/m2/s]
2.0
64h (T2)
1.2
140h (T3)
0.8
0.4
1.6
16h (T4)
1.2
64h (T5)
0.8
0.4
0.0
0.0
0
0.2
0.4
0.6
Applied shear stress (Tau_b) [N/m2]
Anaerobic
0.8
0
0.2
0.4
0.6
Applied shear stress (Tau_b) [N/m2]
Aerobic
drop in overall values of Erosion Rates as length dry-period increase
0.8
Main Results
Tests with the same consolidation period and different environmental ambience.
2.0
(T1) Anaerobic
1.6
(T4) Aerobic
1.2
0.8
0.4
0.0
0
0.2
0.4
0.6
0.8
erosion rate (E) [g/m2/s]
erosion rate (E) [g/m2/s]
2.0
1.6
(T2) Anaerobic
1.2
(T5) Aerobic
0.8
0.4
0.0
0
0.2
0.4
0.6
0.8
Applied shear stress (Tau_b) [N/m2]
Applied shear stress (Tau_b) [N/m2]
16h consolidation period
64h consolidation period
Main Results
Tests with the same consolidation period and different environmental ambience.
2.0
(T1) Anaerobic
1.6
(T4) Aerobic
1.2
0.8
0.4
0.0
0
0.2
0.4
0.6
0.8
erosion rate (E) [g/m2/s]
erosion rate (E) [g/m2/s]
2.0
1.6
(T2) Anaerobic
1.2
(T5) Aerobic
0.8
0.4
0.0
0
0.2
0.4
0.6
0.8
Applied shear stress (Tau_b) [N/m2]
Applied shear stress (Tau_b) [N/m2]
16h consolidation period
64h consolidation period
Conclusions
Laboratory tests to estimate erosional resistance from highly organic
sediment beds under storm runoff conditions
Improvements in prediction of in-sewer sediment transport loads
(Mediterranean conditions)
Conclusions
Environmental conditions
influence over sediment-bed nature
and structure
Conclusions
Environmental conditions
influence over sediment-bed nature
and structure
Increment of resistance against erosion
(as consolidation dry-period lengthen
and oxygen is available)
Increase of erosional strength with
depth
Conclusions
Environmental conditions
influence over sediment-bed nature
and structure
Increment of resistance against erosion
(as consolidation dry-period lengthen
and oxygen is available)
Increase of erosional strength with
depth
Lesser magnitudes of
Critical Shear Stresses
comparing with results obtained with
low-organic sediment and synthetic
sediment (differences in sediment
properties)
Conclusions
Future investigations aimed to:
-asses critical shear stress in highly organic sediment beds
- analyse influence of temperature in dry-period
Concerned about pollution control
Thank you for your attention
Wet weather pollution from combined sewer systems
Particularities in the Mediterranean region climate in Spain:
Rain regime
Long dry-weather periods
Intense precipitaction events
100
cumulative precipitation = 600 mm/year
80
50
20
40
15
30
10
20
Dec
Nov
Oct
Sept
Aug
0
Jul
0
Jun
5
May
10
Apr
≈ 40% rainfall registered P(mm)>10mm
25
Mar
Intense precipitation events following long dryperiods
30
60
Feb
Max. dry-period: 70 days (2010-2012)
35
70
Jan
average dry-period between rain: 11 days
90
average cummulative
Precipitation (mm)
(concentrater in spring and fall)
40
Pm (mm)
mean T (°C)
Max T (°C)
mean Temperature (°C)
day-precipitation = 4 day/month
Main results
Erosion of sediments from bed during tests were monitored in terms of
Suspended Sediment concentration and related with Erosion Rate
q : average Erosion Rate in a applied Shear Stress
step (tb) linked to Suspended Sediment
Concentration (CSS)
V : water volume of the column over
sediment sample
AS : surface area bed subjected to erosion
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