Irene Seco Manuel Gómez Alma Schellart Simon Tait

Irene SecoManuel GómezAlma SchellartSimon Tait

Erosion resistance and behaviour of highly organic in-sewer sediment

7th International Conference on Sewer Processes & NetworksWednesday 28 - Friday 30 August 2013The Edge Conference Centre, Sheffield

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

Long dry-weather periodsIntense precipitation events


Rain regime

Particularities in the Mediterranean region in Spain:

cumulative precipitation = 600 mm/year(concentrater 50 days/year)

average dry-period between rain: 11 days

≈ 40% rainfall registered P(mm)>10mm

Long dry-weather periodsIntense precipitation events


Rain regime

In-sewer sediment accummulation and consolidation



Low circulating flow(0.56 m3/s average Congost River. Vallès Oriental. Catalonia. Spain)

Low dilution capacity


Flow regime in riversLong dry-weather periods

Intense precipitation events

Rain regime



Low circulating flowLow dilution capacity



Rain regime

High percentage of impervious surface in

Urban areas

Urban pattern






Rain regime


Urban areas

Urban pattern

Sewer solids mainly from wastewatersRelevant Organic

composition



Significant volumes discharged in a shorttime from sewer network (CSO)Significant organic Pollution in natural streams and sea

Overflows during wet-weather




Rain regime


Urban areas

Urban pattern

Objectives of this study

Particular conditions of Build-up/Wash-off affects the initiation of sediment motion


Consider potential incidence of the environmental conditions in-sewer on the transport loads and initiation of motion

Analyse changes in transport potential for different lengths of antecedent dry-weather periodsInvestigate erosion

behaviour of real in-sewer organic-rich

sediment collected in Spain

Particular conditions of Build-up/Wash-off affects the initiation of sediment motion

• Long dry-weather period/Build-up

• Intense rainfall/Wash-off

Suitable to be applied to the particular Mediterranean climate and urban pattern

conditions


Improve prediction of in-sewer sediment transport loads

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%) organic nature of solid particles from domestic wastewaters, and the presence of

greasesCohesive properties


Catalonia, Spain)


relevant Organic content

(O.M. around 80%) Related with the organic nature of solid particles from domestic wastewaters, and

the presence of greasesCohesive properties

d50 = 0.31 mm (310 µm)

Density = 1310 kg/m3

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.50

20406080

100 Sieving analysis of sewer sediment in natural state

mesh width (mm) % a

ccum

ulat

ed

on th

e sie

ve


Catalonia, Spain)

Consolidation, microbiological degradation, chemical-biological interactions (bonding forces between particles )


Effects on the transport of solidssignificant influence on the incipient motion

Sediment with high organic content and cohesive properties

Laboratory erosion measurement

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

Evaluation Erosion Rate

Assessment critical threshold of motion at solid-

fluid interface

Erosion meter devised (based on a design by Liem et al. 1997).


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.

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)

Consolidation period Erosion phase Sampling and Analysis







Consolidation period

Simulation flows at start of storm

event

Increasing shear stress is applied

in a stepwise way

Erosion phase Sampling and Analysis







Consolidation period

Simulation flows at start of storm

event

Increasing shear stress is applied

in a stepwise way

Erosion phaseSediment samples

collected during erosion tests at each shear stress

intervalRemaining

sediment after tests collected

Sampling and Analysis

Main Results

Average erosion rate (q) linked to the applied shear stress (tb)

Aerobic

0 0.2 0.4 0.6 0.8 10.00

0.40

0.80

1.20

1.60

2.00

Applied shear stress (Tau_b) [N/m2]er

osio

n ra

te (E

) [g/

m2/

s]

0 0.2 0.4 0.6 0.8 10.00

0.40

0.80

1.20

1.60

2.00

Applied shear stress (Tau_b) [N/m2]

eros

ion

rate

(E) [

g/m

2/s]

0 0.2 0.4 0.6 0.8 10.00

0.40

0.80

1.20

1.60

2.00


eros

ion

rate

(E) [

g/m

2/s]

0 0.2 0.4 0.6 0.8 10.00

0.40

0.80

1.20

1.60

2.00


eros

ion

rate

(E) [

g/m

2/s]

0 0.2 0.4 0.6 0.8 10.00

0.40

0.80

1.20

1.60

2.00


eros

ion

rate

(E) [

g/m

2/s]

Anaerobic

T116h

T3140h

T264h

T416h

T564h

Erosion rate of sediments monitored in terms of Suspended Sediment concentration

Main ResultsErosion Rate valuesComparison between tests with increasing length of periods of consolidation.

0 0.2 0.4 0.6 0.80.0

0.4

0.8

1.2

1.6

2.0


eros

ion

rate

[g/

m2/

s]

0 0.2 0.4 0.6 0.80.0

0.4

0.8

1.2

1.6

2.0


eros

ion

rate

[g/m

2/s]

16h (T1)

64h (T2)140h (T3)

16h (T4)

64h (T5)

AerobicAnaerobic

Main ResultsErosion Rate valuesComparison between tests with increasing length of periods of consolidation.

0 0.2 0.4 0.6 0.80.0

0.4

0.8

1.2

1.6

2.0


eros

ion

rate

[g/

m2/

s]

0 0.2 0.4 0.6 0.80.0

0.4

0.8

1.2

1.6

2.0


eros

ion

rate

[g/

m2/

s]

16h (T1)

64h (T2)140h (T3)

16h (T4)

64h (T5)

AerobicAnaerobic

drop in overall values of Erosion Rates as length dry-period increase

Main Results

0 0.2 0.4 0.6 0.80.0

0.4

0.8

1.2

1.6

2.0


eros

ion

rate

(E) [

g/m

2/s]

0 0.2 0.4 0.6 0.80.0

0.4

0.8

1.2

1.6

2.0


eros

ion

rate

(E) [

g/m

2/s]

Tests with the same consolidation period and different environmental ambience.

(T1) Anaerobic

(T4) Aerobic

64h consolidation period


(T2) Anaerobic

(T5) Aerobic

Main Results

0 0.2 0.4 0.6 0.80.0

0.4

0.8

1.2

1.6

2.0


eros

ion

rate

(E) [

g/m

2/s]

0 0.2 0.4 0.6 0.80.0

0.4

0.8

1.2

1.6

2.0


eros

ion

rate

(E) [

g/m

2/s]

Tests with the same consolidation period and different environmental ambience.

(T1) Anaerobic

(T2) Anaerobic(T4) Aerobic

(T5) Aerobic



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

Increase of erosional strength with depth

Increment of resistance against erosion (as consolidation dry-period lengthen and oxygen is available)



Conclusions

comparing with results obtained with low-organic sediment and synthetic sediment (differences in sediment properties)

Lesser magnitudes of Critical Shear Stresses

Increase of erosional strength with depth

Increment of resistance against erosion (as consolidation dry-period lengthen and oxygen is available)



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

Long dry-weather periodsIntense precipitaction events


Particularities in the Mediterranean region climate in Spain:

0102030405060708090

100

0510152025303540Pm (mm)

mean T (°C)

aver

age

cum

mul

ative

Pr

ecip

itatio

n (m

m)

mea

n Te

mpe

ratu

re (°

C)

day-precipitation = 4 day/monthcumulative precipitation = 600 mm/year

(concentrater in spring and fall)average dry-period between rain: 11

daysMax. dry-period: 70 days (2010-2012)

Intense precipitation events following long dry-periods

≈ 40% rainfall registered P(mm)>10mm

Rain regime

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

Documents

Irene Seco Manuel Gómez Alma Schellart Simon Tait