Discolouration in potable water distribution systems PODDS ... · PDF...
Transcript of Discolouration in potable water distribution systems PODDS ... · PDF...
Prof. Joby BoxallPennine Water GroupUniversity of Sheffield
Discolouration in potable water distribution systems
PODDS: from theory to practice
• Most apparent water quality issue to customers
• A major cause of customer contacts
• Often a ‘mask’ for other water quality issues
• Discolouration incidents prosecuted in UK found ~50% associated with ‘planned works’ (2009 data)
Discolouration
• 146 significant events reported in 2009
• 34% discolouration• ~50% of these due to planned
operations
Sediment deposit or cohesive layers?
∞
∞?
Layer Shear strength , τ (N/m2)
1.2
Dis
colo
urat
ion
pote
ntia
lC
(N
TUm
)
τ’ τa
τexcess
Cplastic
Ciron
CI pipe
Plastic pipe∞
∞?
Layer Shear strength , τ (N/m2)
1.2
Dis
colo
urat
ion
pote
ntia
lC
(N
TUm
)
τ’ τa
τexcess
Cplastic
Ciron
CI pipe
Plastic pipe
0
5
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30
0.000 -0.004
0.004 -0.008
0.008-0.016
0.016-0.031
0.031-0.063
0.063-0.125
0.125-0.250
0.250 -0.500
bin size ranges (mm)
% c
ount
s
mean
mean + standarddeviationmean - standarddeviation
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00:38 00:46 00:53 01:00 01:07 01:14 01:22 01:29 01:36 01:43Time (hrs:mns)
Turb
idity
(NTU
)
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Flow
rate
(l/s
)
FlushIntermediateInletHachFlow rate
The PODDS approach..
• Proposes that processes in addition to gravitational settling lead to material accumulation and retention within pipes of distribution systems• pipe and fitting corrosion, lining erosion, biological
growth, flocculation, chemical reactions, electrochemical interactions etc
• Specific processes not explicitly considered in PODDS approach, approximated to ‘cohesive forces’
The PODDS model
• An empirical computational tool to predict the turbidity response of distribution networks to changes in hydraulic conditions• Describes discolouration potential as a function of
normal daily or recent hydraulic forces (shear stress)• Calculates mobilisation by comparison of this with
imposed event hydraulic force (shear stress)• Coded as addin to EPANET – readily applicable to any
1D network modelling software
Verification for Cast Iron pipes
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idity
(NTU
)
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idity
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)
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e (l/
s)
Measured Turbidity
Modelled Turbidity
Flow Rate
660m 4” CI; 2003 90m 3” CI; 2005
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6740 6940 7140 7340 7540 7740 7940 8140 8340
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idity
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idity
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s)
Measured TurbidityModelled TurbidityFlow Rate
230m 3” CI 2005 360m 12” CI; 2006
Verification for plastic pipes
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idity
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Flow Rate
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idity
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Rat
e (l/
s)
Measured TurbidityModelled TurbidityFlow Rate
447m 146mm PE with τultimate limited to
1.2N/m2; 2006 380m 89mm PE with τultimate limited to
1.2N/m2; 2005
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idity
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Measured TurbidityModelled TurbidityFlow Rate
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idity
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Measured TurbidityModelled TurbidityFlow Rate
170m 106mm uPVC with τultimate limited to
1.2N/m2; 2005280m 72mm PE with τultimate limited to
1.8N/m2; 2005
• Typical operational scenario: increase demand at outlet by 10 l/s
• 100% flow/velocity increase downstream
• 10% flow/velocity increase upstream
• Where is the risk of material mobilization?
Example based on actual discolouration incident in the UK, 2009
100 l/s → 110 l/s
10 l/s → 20 l/s
Material mobilisation: where is the risk?
110 l/s
20 l/s
Areas of customer
complaints
Discolouration material mobilised in
upstream section
Material mobilisation
Example based on actual discolouration incident in the UK, 2009
A
B
C
Velocity or Shear Stress?
PODDS history
• PODDS 1, 2001-2003 EPSRC funded. Detailed research into material characterisation, model formulation and initial field verification
• PODDS II, ‘Realising the potential of the PODDS model for the UK water industry’, 2004-2006. Nation wide validation through extensive field studies
• PODDS III, ‘Managing Discolouration: Research informing practice’, 2007-2009. Risk based computational tools, repeat testing to start exploring asset deterioration.
• PODDS IV, ‘Discolouration in trunk mains’. 2010-2013. Research focused on trunk main applications.
14
Site Bridgwater Northern Ring
Main
Coker Hill to Maiden Beech
Bowden to Devizes
Properties 4km, 600 to 400mm, various
materials
7km, 450mm AC 6km 350mm Unlined Ductile
IronOriginal proposal & estimated cost
Swabbing £490K
Swabbing£530K
Main replacement £2M
Revised proposal (PODDS mediated) & cost
Overnight flushing of main £227K
Trunk main conditioning
£150K
Trunk main conditioning
£40K
Savings £263K £380K *£2M*Expenditure deferred
Examples of benefits
Trunk Main: PODDS fit / calibration
A
A
B
B
Conditioned layer strength
Prediction Of Discolouration in Distribution Systems
- Quantifying discolouration impact of flow changes
- Maintain TM flexibility by conditioning procedures (maintenance schedules with no water loss)
- Simulate discoloured water event potential
• High impact industry driven research
• New conceptual understanding
• Predictive empirical model• But process knowledge……0
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Peak
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bidi
ty (N
TU)
*Full pipe conditioning to higher demand requires new flow to be maintained in excess of 24 hours.
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00:00 12:00 00:00 12:00 00:00
Time (hours)
Turb
idity
(NTU
) Turbidity response curve for 1.5 Ml/d flow conditioned pipe
with 0.5 Ml/d flow increase.
Dec
reas
ing
Laye
r she
ar
stre
ngth
Time
Time
Erosion 1Regeneration
Modes of regeneration
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ing
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r she
ar
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ngth
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Modes of regeneration
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Modes of regeneration
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r she
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ngth
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Modes of regeneration
• Vast surface area, high residence times, uncertain and variable conditions• Physically, chemically and biologically complex and active systems
NG – Nutrient Gradient PI – Protozoan Interaction C – Corrosion E – Erosion S – Sloughing © Kat Fish 2010
Distribution system as reactors, not inert networks of pipes
Unique experimental facility
• Temperature controlled, re‐circulating system• 600m of HDPE pipe, 79mm diameter• Full scale system flows and pressures• Coupons to allow analysis of accumulation directly on the pipe wall
designed to limit distortion of boundary layer flow
SEM Images – 28 day biofilm development, 16oC 0.2 l s‐1
SEM Images – 28 day biofilm development, 16oC 0.2 l s‐1
Primary Attachment
SEM Images – 28 day biofilm development, 16oC 0.2 l s‐1
Primary Attachment
Developed Biofilm
Quantify the distribution of biofilm (cells) with respect to depthInsight into the impact of environmental conditions (extremes of hydraulic condition and temperature during growth phase shown)Knowledge regarding the stability of the biofilm (change in distribution due to imposed flushing regime)
High varied conditioning shear stress at 8⁰C
0.1 N/m² steady state conditioning shear stress at 16⁰C
Conclusive association of discolouration and
microbiology / biofilms
Biofilm
Summary• PODDS simple, cost effective water
quality tool
• Management strategies can be developed for:• safe operating flow limits • short term flow increases• long term conditioning and
operation
• Capital vs. Operational costs can be evaluated
• Accurate flow measurement essential!
• HOWEVER...• Regeneration rates?
• Biofilms/particulates?Water Engineering Research
University of SheffieldDepartment of Civil & Structural Engineering
www.PODDS.co.uk
Water Distribution System Operational Strategies:Implications from PODDS Verified Research
The PODDS concept to Water Distribution Systems (WDS) management is based on scientific research that hasshown discolouration is a predictable response to increases in system shear stress. Applying an hydraulic shear force(τa) above the peak daily (τ’) creates an excess shear (τexcess) leading to discolouration (figure 1). The response is afunction of the strength characteristics and the accumulated discolouration (C) of material layers attached to thepipewalls.
Cohesive material layers develop throughout WDS with asset deterioration primarily being determined by waterquality (amount of particulate material entrained) and conditioning hydraulics. Optimal pro‐active managementstrategies are required to maintain assets yet minimise network interventions, thus saving time, money and limitdiscolouration risk to customers. To achieve this pipe material needs to be considered during operational planningas material accumulation processes, layer strength characteristics and discolouration potential are different inplasticand cast iron pipes
Plasticor smooth-walled
pipes
Cast Ironor rough-walled
pipes
Figure 1 – PODDS model of layer shear strength vs. discolouration potential
1Average UK value; actual deterioration rate variable based on water quality and network hydraulics.Warning: potential discolouration risk can be posed much sooner.
2Value shown based on work in the Netherlands by KIWA.
Prof. Joby Boxall Tel: 0114 2225760email: [email protected]
Dr. Stewart Husband Tel: 0114 2225416email: [email protected]
August 2013
Modelling has shown that a flushing induced force of 1.2 N/m2 is sufficient to
mobilise all material layers and clean pipe. MAINTENANCE
Modelling has shown that for any increase in applied shear force (e.g. flushing), material will continue to be mobilised.
"6/11"4/5"3/3
/6.0/2.1 2
slslsl
smmN TARGET
FLUSHING VELOCITY
Risk based value. Criteria: available flow, pipe discolouration
status and risk of unplanned hydraulic disequilibria.
4 Years 1DETERIORATION
(from clean to maximum risk)
1.5 years surface water 3 years ground water
a) Research suggests a peak daily flow of 20.4m/s will promote “self-cleaning” b) Improving water quality (reduces
deterioration rate).
OPERATION
a) Higher daily flows reduce potential discolouration event magnitude.
b) Improving water quality (reduces deterioration rate).
Biofilms believed integral to cohesive layers formation; liable to ‘slough’ increasing
discolouration risk. NOTES
Corrosion by-products increase pipe deterioration rate and ‘feed’ downstream
pipes (irrespective of material). Implementation of self-cleaning velocity
criteria to maintain residential water quality (networks characterised by a branched structure with
downstream declining diameter)
NETWORK DESIGN Not applicable
∞
∞?
Layer Shear strength , τ (N/m2)
1.2
Dis
colo
urat
ion
pote
ntia
lC
(N
TUm
)
τ’ τa
τexcess
Cplastic
Ciron
CI pipe
Plastic pipe∞
∞?
Layer Shear strength , τ (N/m2)
1.2
Dis
colo
urat
ion
pote
ntia
lC
(N
TUm
)
τ’ τa
τexcess
Cplastic
Ciron
CI pipe
Plastic pipe
PODDS V Translating laboratory based microbial / biofilm research into practicable field based discolouration knowledge to deliver the next paradigm in discolouration management New directions•Field studies linking biofilm development and discolouration•Develop rapid, cultivation-independent assays to quantify microbial viability•Multivariate analysis to associate asset deterioration and explanatory factors•Develop strategies to manage biofilms and discoloration•Validate and deliver the next generation of PODDS toolsAdditional on-going benefits;•Laboratory based experimental programme•Water network specific sensor technologies•PODDS network, forum and user groups
Acknowledgements
• Dr Stewart Husband• All the collaborating companies and
individuals who have supported and enable the research over the years
• For more information, please visit:www.PODDS.co.uk