wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 1 3e1 2 4

Avai lab le a t www.sc iencedi rec t .com

journa l homepage : www.e lsev ie r . com/ loca te /wat res

Asset deterioration and discolouration in waterdistribution systems

P.S. Husband*, J.B. Boxall

Pennine Water Group, Department of Civil and Structural Engineering, University of Sheffield, Mappin street, Sheffield S1 3JD,

United Kingdom

a r t i c l e i n f o

Article history:

Received 18 June 2010

Received in revised form

4 August 2010

Accepted 10 August 2010

Available online 17 August 2010

Keywords:

Hydraulics

Shear stress

Cohesive material

Regeneration

Asset deterioration

* Corresponding author. Tel.: þ44 114 222541E-mail addresses: s.husband@sheffield.ac

0043-1354/$ e see front matter ª 2010 Elsevdoi:10.1016/j.watres.2010.08.021

a b s t r a c t

Water Distribution Systems function to supply treated water safe for human consumption

and complying with increasingly stringent quality regulations. Considered primarily an

aesthetic issue, discolouration is the largest cause of customer dissatisfaction associated

with distribution system water quality. Pro-active measures to prevent discolouration are

sought yet network processes remain insufficiently understood to fully justify and optimise

capital or operational strategies to manage discolouration risk.

Results are presented from a comprehensive fieldwork programme in UK water distribu-

tion networks that have determined asset deterioration with respect to discolouration.

This is achieved by quantification of material accumulating as cohesive layers on pipe

surfaces that when mobilised are acknowledged as the primary cause of discolouration. It

is shown that these material layers develop ubiquitously with defined layer strength

characteristics and at a consistent and repeatable rate dependant on water quality. For UK

networks iron concentration in the bulk water is shown as a potential indicator of dete-

rioration rate. With material layer development rates determined, management decisions

that balance discolouration risk and expenditure to maintain water quality integrity can be

justified. In particular the balance between capital investment such as improving water

treatment output or pipe renewal and operational expenditure such as the frequency of

network maintenance through flushing may be judged. While the rate of development is

shown to be a function of water quality, the magnitude (peak or average turbidity) of

discolouration incidents is shown to be dominated by hydraulic conditions. From this it

can be proposed that network hydraulic management, such as regular periodic ‘stressing’,

is a potential strategy in reducing discolouration risk. The ultimate application of this is the

hydraulic design of self-cleaning networks to maintain discolouration risk below accept-

able levels.

ª 2010 Elsevier Ltd. All rights reserved.

1. Introduction and Wales, 124,671 (80%) were about discoloured water. Yet

Discolouration remains the single largest water quality issue

facing water companies worldwide. Of the 154,985 customer

complaints about drinking water quality in 2007 for England

6; fax: þ44 114 2225700..uk (P.S. Husband), j.b.Boier Ltd. All rights reserved

with a 99.96% compliance with the European Drinking Water

Directive, the overall result for England and Wales is compa-

rable with the best reported in Europe. In 2007, 33% of all

incidents investigated by the UK Drinking Water Inspectorate

xall@sheffield.ac.uk (J.B. Boxall)..

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 1 3e1 2 4114

(DWI) were due to discolouration, Fig. 1a. Of significance it was

found that 48% of these incidents were a result of ‘planned

work’, Fig. 1b (DWI, 2007; DWI, 2008; DWI, 2009). This is a clear

indictment that current operational and management strat-

egies are insufficiently informed and that there is inadequate

understanding of the processes and mechanisms that lead to

discolouration events. In addition the DWI questioned the UK

industry’s reactive, failure driven attitude towards the main-

tenance of assets, which it had aimed to improve through

instigating Distribution Operation and Maintenance Strate-

gies, DOMS (DWI 2002). To justify efficient implementation of

DOMS, water companies need to balance justifiable capital

expenditure against the long term operational costs of main-

tenance interventions with respect to maintaining water

quality. From 2010 UK water supplier’s statutory duties to

manage assets will also include discolouration complaints as

a quantitativemeasure reflecting consumers’ expectations set

in a framework of maintaining serviceability (OFWAT, 2008).

Furthermore implementation of Water Safety Plans (WSP),

regarded as the most effective way to ensure a water supply

safe for human consumption, require comprehensive risk

assessment and risk management of the network (WHO,

2005). To achieve these statutory obligations within the

distribution network it is evident that understanding of the

processes governing discolouration is essential.

2. Background

For a discolouration event to occur a three-phase processmay

be conceptualised consisting of a material supply to the

network, a period during which material accumulates and

ultimately the mobilisation of material into the bulk flow

(Boxall and Saul, 2005). Material sources linked with dis-

colouration include oxidation of iron mains, metal precipita-

tion, biological activity and poor system maintenance or

treatment practices (McCoy and Olsen, 1986; Gauthier et al.,

2001; Slaats et al., 2002; Sarin et al., 2004). Trials in a plastic

only network with particle-free water produced by 0.1 mm

ultra-filtration demonstrated that particles responsible for

discolouration can originate solely from the supplied water

(Vreeburg et al., 2008). Most likely it was proposed through

intrinsic processes such as precipitation and biological

Fig. 1 e DWI incidents in En

activity. With the inevitable presence of material within

a distribution system, it could be assumed that localised

settling and the subsequent mobilisation of loose deposits

may account for discolouration incidents (Barbeau et al.,

2005). Research analysing material entrained during flushing

operations has however repeatedly demonstrated that it is

particulate in nature, with a size predominately around 10 mm

and a specific gravity in the range 1e1.3 (Boxall et al., 2001;

Gauthier et al., 2001; Seth et al., 2004; Verberk et al., 2006;

Vreeburg and Boxall, 2007). Sedimentation of these particles

is restricted to quiescent conditions; therefore in general they

remain as a permanent wash load or solute within distribu-

tion systems (Smith et al., 1999; Boxall et al., 2001; Hossain

et al., 2003; Ryan and Jayaratne, 2003). Based on such obser-

vations, Boxall et al. (2001) proposed and subsequently vali-

dated for field data (Boxall and Saul, 2005; Husband and Boxall,

2010) that it is the generation of cohesive-like material layers

that are responsible for discolouration. In their empirical

model, Predicting Discolouration in Distribution Systems

(PODDS), Boxall et al. (2001) suggest that these material layers

behave in a similar way to cohesive estuarine muds and

cohesive-like sewer sediments (Parchure and Mehta, 1985;

Mehta and Lee, 1994; Skipworth et al., 1999). In the PODDS

model however the concept of cohesive layers was specifically

applied with the assumption that the layers are conditioned

by system hydraulics. The erosion of material layers with

distinct shear stress characteristics and their subsequent

regeneration has since been demonstrated under laboratory

conditions in a full scale pipe loop (Husband et al., 2008).

The practically recognised trigger for discolouration event

initiation is an increase in hydraulic conditions, for example

a rise in flow (Twort et al., 2000; Ackers et al., 2001; Marshall,

2001; Slaats et al., 2002; Ryan and Jayaratne, 2003). It was

recognised by Boxall et al. (2001) in the PODDS model that

increasing the flow raises the force acting tangentially to the

pipe surface, the boundary wall shear stress. If this exceeds

the conditioned shear strength of the cohesive layers then

mobilisation of discolouration material would occur. An

appreciation of the non-linear relationship between shear

stress and velocity can explainwhy some valve operations, re-

zoning or other network interventions may have resulted in

discolouration incidents when planning indicated only

marginal increases in flow velocity. By analysing for boundary

gland and Wales, 2006.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 1 3e1 2 4 115

shear stress, hydraulically simulating ‘planned works’ as part

of a pro-active management strategy, it is likely that such

events may be anticipated.

In the Netherlands a 0.4 m/s flow velocity has been iden-

tified as a design criterion for self-cleaning networks (Slaats

et al., 2002). The value of 0.4 m/s is a pragmatic one

proposed to prohibit the accumulation of material within

Dutch water distribution systems.

With progressively more stringent regulations, increasing

customer expectations and acceptance that discolouration is

a widespread international issue, it is essential that water

suppliers have the ability to assess, quantify and proactively

manage this risk of water quality failure. The concept of

cohesive material layers with a relationship linking shear

stress and discolouration potential can accurately describe

the mobilisation process and has been verified through

modelling of distribution network discolouration events.

What is not known however is the rate these layers regenerate

following disturbance or the controlling variables, although

laboratory and associated field studies suggest that it is

a continuous, repeatable and linear accumulation process

(Cook, 2007; Husband and Boxall, 2008; Husband et al., 2008).

Turbidity profiles from such fieldwork have shown distinct

responses from unlined cast iron and plastic pipes and

interpretation has indicated internal pipe condition in moni-

tored sections (Husband and Boxall, 2010). However, the

significance of upstream pipes, in particular unlined cast iron,

and the impact of this on bulk water quality and hence

material accumulation and discolouration risk in downstream

sections has not been investigated. If the rate at which

material layers regenerate was known and the influential

factors understood it would be possible to make strategic

management decisions that justify cost benefits from capital

or operational expenditure. In addition the knowledge gained

of system behaviour may be used to support risk assessment

methodologies such as WSP and aid prediction of dis-

colouration response from planned works or changes in

network operation. The aim of this work is therefore to

quantify material layer regeneration and assess the signifi-

cance of potential influencing factors such as source water

type, coagulation treatment processes and pipe material. The

work is based on the results of repeated field operations in the

UK, spanning a number of years, where turbidity data has

been collected from hydraulically induced discolouration

events. From this data, regeneration values have been deter-

mined and correlation of these with site information has been

used to assess influencing factors.

3. Fieldwork

Collection of field data during uni-directional flushing oper-

ations was planned in partnership with nine collaborating

water companies who serve over 75% of the registered

consumers in England and Wales (40 million customers).

Fieldwork objectives involved the implementation and

monitoring of incremental hydraulic flushing operations to

obtain the temporal and spatial turbidity response of pipes

within live distribution systems. The incremental approach

of stepped flow increases above typical peak flows has been

successfully applied to demonstrate the cohesive nature and

variable shear strength properties of material layers shown

to cause discolouration and how these layers are conditioned

by the hydraulic regime (Husband and Boxall, 2010). The

results presented here are for repeated visits to sites such

that the amount of material accumulated over time can be

investigated and correlated with other system information.

The knowledge gained by investigating material quantities

and shear strength characteristics of developing layers will

help inform management strategies to mitigate discoloura-

tion risk.

Selection of test sites was planned to facilitate coverage of

a range of factors hypothesised as potentially influential in

discolouration processes and the development of material

layers. Factors included:

� pipe material, significant as the surface on which the

material layers develop,

� pipe diameter and roughness that determine boundary

shear stress identified to condition material layers under

hydraulic loading,

� factors known to influence bulk water quality such as

B water source,

B water treatment processes and

B presence of upstream unlined cast iron pipes, known to

contribute to corrosion products to the bulk water.

To establish a comparable dataset between sites required

a consistent baseline, technically difficult to achieve due to

the inherent variables in distribution networks. Ideally the

initial flushing operation would mobilise fully developed

material layers that represent a maximum discolouration

potential. With hydraulics known to condition the material

layers, priority was therefore given to sites for which histor-

ical records spanning back 5 years or more indicating no

hydraulic events in excess of typical demands. Operation

dates were typically set by company maintenance pro-

grammes yet seasonal variations are an unknown factor in

layer development, although work indicated minimal impact

for one specific groundwater site (Boxall et al., 2003). To

mitigate for possible seasonal effects repeat flushing opera-

tions were planned to allow a full year layer development

period. For the year between the first and repeat operations,

collaborating water companies were requested not to sanc-

tion non-critical interventions within the monitored site that

may compromise the development of discolourationmaterial.

The two operations (initial visit and repeat) were planned to

be completed under identical conditions (same time of day,

flushing flow rates and duration) but a year apart, thereby

producing two sets of comparable turbidity data. From this an

annual regeneration of erodable discolouration material, irre-

spective of operational date or location, could be determined.

Subsequent site specific details could then be correlated to

identify influencing factors.

To capture the required data at the wash out hydrant

a specially adapted standpipe was used. This incorporated

a valve at the discharge point to control the flow; using the

hydrant valve results in a rapid pressure drop in water pres-

sure entering the standpipe causing the dissolution of air into

bubbles that critically disrupts turbidity measurements. Prior

Fig. 2 e Regeneration fieldwork sites.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 1 3e1 2 4116

to the valve a tapping point directedwater through a sampling

cell housing the turbidity probe, a CENSAR CT�, whilst

a second probe was placed directly in the standpipe riser. In

addition to the two turbidity probes logging at 10 s, manual

samples were collected for laboratory water quality analysis

and by a HACH Pocket Turbidimeter�. This replication of the

recorded turbidity data ensured a high degree of confidence

with the results. In addition to the turbidity probes the

standpipewas also fittedwith a Burkert Easyflow� flowmeter,

pressure tapping and pressure gauge to monitor the hydraulic

conditions, essential for hydraulic model and pipe roughness

calibration (critical in calculating the hydraulic gradient and

therefore pipe wall shear stress). In all examples shown

upstream pipe sections were flushed to ensure a clean supply

and an inlet hydrant with attached sampling standpipe and

turbidity monitoring were connected to confirm that material

mobilised at thewash out could be solely attributed to the pipe

length under investigation.

Fig. 3 e Pipe diagram and h

The study utilised 15 locations from across the UK with 67

monitored pipe sections. Site locations are shown in Fig. 2

with the following water supply details:

� 5 Groundwater supply e no coagulation treatment

� 2 Groundwater/surface water blended supply e iron coag-

ulation treatment

� 8 Surface water supply e 5 iron and 3 aluminium coagula-

tion treatment

Pipe sections monitored were primarily of typical distri-

bution pipe diameters around 100 mm but did range from

53 mm to 306 mm with the following split in pipe materials:

� 23 Cast Iron (CI, unlined)

� 17 PE (or polyethylene derivative)

� 15 uPVC

� 12 Asbestos Cement (AC) or cement lined CI

The results reported here generally refer to the annual

regeneration and are therefore not directly an indication of the

risk or potential magnitude of a discolouration event. The

annual regeneration is a percentage value relating material

mobilised from the repeat operation to that measured during

the initial operation. In addition to an annual regeneration, an

average turbidity value is calculated for each flushing opera-

tion. As flushing duration and hydraulic steps were variable

between pipe lengths, this value is not directly comparable

but it may be considered to indicate the possible discoloura-

tion potential should a shear force similar to those imposed by

the flushing and in excess of typical conditioning hydraulics

occur for that pipe length.

4. Site results

Results from three fieldwork sites are presented that illustrate

the turbidity features typical to all flushing studies conducted.

Individual field results also demonstrate how pipe length

specific characteristics need to be considered when analysing

ydrant locations, Site 1.

Fig. 4 e Site 1 initial and 12 month return turbidity results with flushing flow rate.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 1 3e1 2 4 117

the flushing turbidity response. Following the examples,

overall trends governing the regeneration of discolouration

material in distribution systems are explored based on anal-

ysis of site characteristics.

In order to determine a useful regeneration index, three

methods were trialled to establish if there is an optimum for

future studies. These calculations have been undertaken for

all pipe lengths, but are only presented here for the six

example pipe lengths across the three sites. The first method

uses the peak turbidity, which is a subjective measure based

on manually selecting a turbidity feature that can be clearly

identified from both initial and repeat operations (not neces-

sarily the highest single value although typically a dominant

turbidity peak). This peak value does however convey

a potential risk rating as it is possible that a consumer may be

subjected to water with this turbidity as a ‘worst case’. The

secondmeasure is the average turbidity, simply themean of all

data within the measured time frame. The third measure is

based on integration of the timeeturbidity plots: effectively

a step in calculating an amount of material (Gauthier et al.,

2001; Boxall et al., 2003). However the material mobilised

from sites is not consistent due to differences in water quality

Table 1 e Site 1 regeneration flushing results.

Flush Ø mm L m Pipe Peak NTU

1 2 %

A 89 380 PE 22.5 6.1 27

B 72 282 PE 25.6 7.8 30

so multiplication by a common conversion factor to sus-

pended solids, a mass of material, is not undertaken. Should

site specific conversion factors be determined however, mass

of material mobilised would probably be the optimal and

operationally pertinent comparison. With the appropriate

score determined for each operation, a percentage regenera-

tion value, indicating the rate material returns to a fully

developed and maximum discolouration risk, is calculated by

dividing the repeat by the initial results (flush 2/flush 1).

4.1. Example site 1

The first site is shown in Fig. 3 and comprises two sections of

polyethylene (PE) pipe of different internal diameters (72 and

89 mm) that form a looped section of an urban District

Metered Area (DMA e a term used to denote a management

sub-zone of a distribution network in the UK). The site is

supplied aluminium coagulation treated river water and has

asset records indicating no upstream unlined iron pipe

sections.

This operation used the planned approach of stepped

increases in flushing flow rate to investigate discolouration

Average NTU Amount NTU.s

1 2 % 1 2 %

10.5 3.7 35 28038 9988 36

17.1 5.3 31 33857 10650 31

Fig. 5 e Pipe diagram and hydrant locations, Site 2.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 1 3e1 2 4118

material shear strength properties, in particular the regen-

erated material mobilised during the repeat flushing opera-

tion. Fig. 3 shows the network location of the two monitored

sections A and B. Fig. 4 shows the temporal turbidity response

(primary y-axis) and the flushing flow (secondary y-axis). Each

plot shows two turbidity datasets; the initial field visit flush 1

and the repeat operation 1 year (12 months) later, flush 2.

For both plots in Fig. 4 measured turbidity increases are

evident in response to the flushing showing discolouration

material is eroded and propagates to the wash out. This

mobilisation of material is universal to every flushing opera-

tion undertaken during the course of these studies and high-

lights the ubiquitous nature of discolouration. Crucially at

every increase in applied shear stress there is a further release

of discolourationmaterial, an expected phenomenonwith the

cohesive layer concept. The same response characteristics to

the stepped increases in flow are observed during the initial

and repeat operations. This demonstrates that within a year

cohesive layers accumulate with a range of shear strengths,

not solely weakly or strongly adhered material. The mobi-

lisation of the regenerated material can be seen to produce

a scaled version of the initial data, indicating distinct accu-

mulation patterns and a reproducible process. Due to the

hydraulic and temporally equivalent consecutive operations

the rate at which this material has regenerated can be derived

by comparing the two datasets.

The percentage regeneration results are shown in Table 1

and demonstrate a consistency between the three calcula-

tion methods for these two pipe lengths. A combined average

of 30% represents a potential maximum discolouration risk,

assuming linear development, in less than 4 years for these

plastic pipes. Examination from UK industry standard

hydraulic model EPANET software using 24 h extended period

simulation with top down demand allocation (pattern derived

fromDMAflowmeter data and distributed according to billing

data) of the network shows a peak daily velocity of 0.1 m/s in

both pipes with the turbidity observed showing no evidence of

self-cleaning, as expected based on the Dutch value of 0.4m/s.

During the flushing the shear stress at each of the three steps

is approximately double for the smaller 72mmpipe at 1/2/8 N/

m2 compared to 0.6/1.3/3.6 N/m2 for the 89 mm pipe (flow

velocities 0.6/0.9/1.9m/s and 0.5/0.7/1.2m/s respectively). This

difference in shear stress (and not velocity) is reflected in

more discolouration material mobilised during the initial trial

from the smaller diameter and 100 m shorter section of pipe

(so less material might have been expected).

4.2. Example site 2

The second site presented here is supplied with blended

ground/surface water having undergone iron coagulation

based treatment. Results are from stepped flushing of two

parallel pipe lengths of 79 mm PE that form a loop within

a further looped residential DMA, Fig. 5. The turbidity/flow

results are shown in Fig. 6.

Flush C produced trends of similar magnitude to those

from Site 1, Fig. 4 at about 30 NTU, with the exception of a 200

NTU spike on start-up not seen during the repeat operation.

Flush D, the second and shorter pipe section however

produced turbidity of over 900 NTU (instrument limit), well in

excess of values seen elsewhere in plastic pipes. Although

most turbidity was mobilised during the first flow step, the

two further increases in flow produced turbidity peaks of 80

NTU and 10 NTU respectively. This demonstrates material

attachment with low cohesive shear strength and therefore

presents a significant discolouration risk. It was suggested

that an additional intrusion of material may have been

responsible yet the flush a year later produced a scaled replica

of the turbidity trend, peaking this time at 300 NTU. This

indicates that the accumulation is most likely a chronic

process due to a continual material source and favourable

hydraulics. No source of material to generate this level of

turbidity was identified, but investigations to examine the

accountability of the blended ground/surface iron coagulated

supply water in Site 2 have been proposed.

FromTable 2 it can be seen that although the peak turbidity

is high for Flush D, all three scores show regeneration of

material to be around 30%, indicating over 3 years required for

a return to maximum risk assuming linear regeneration. This

is akin to the plastic pipes in Site 1. Due to the non-replicated

turbidity feature on start-up during the first visit of Flush C

having an adverse effect on the material regenerated calcu-

lations, the results are non-conclusive. This second trial in

a looped network however indicates that the presence of

potential tidal points where two flow paths converge (not

confirmed due to sensitivity of hydraulic solutions in such low

flow situations) may induce unusually high material accu-

mulation, speculatively with a low shear strength and there-

fore high discolouration risk.

4.3. Example site 3

The third example site is a groundwater supplied rural DMA

with no coagulation treatment; Fig. 7. Operational constraints

limited flushing to a single step from the two predominately

cast iron (CI) pipe length operations, Flushes E and F. The

majority of Flush E (2 km in total), with the exception of the

final 594mdead-end section, and thewhole of Flush F (1.5 km)

from the supply to a village a further 1.5 kmdownstream. Peak

daily flows exceed 0.4 m/s, the proposed self-cleaning velocity

(Slaats et al., 2002). The turbidity results are shown in Fig. 8.

Fig. 6 e Site 2 initial and 12 month return turbidity results with flushing flow rate.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 1 3e1 2 4 119

The turbidity results show that regeneration of dis-

colouration material has occurred in both pipe sections with

material accumulation again following distinct and repeatable

patterns. Regeneration values for mobilised turbidity are

shown in Table 3 and indicate a combined 30% regeneration in

12 months for these predominately CI pipes in this ground-

water supplied network. It should be noted however that

a discolouration risk may exist long before a ‘full’ accumula-

tion is attained, evident from these operationswhere turbidity

exceeds the UK regulatory limit (at the customers tap) of 4

NTU. From an operational viewpoint the turbidity spikes

between 12:20 and 12:30 in Fig. 8b are due to propagation of

material entrained during the Flush E. This reinforces the

need to plan flushing of all connecting pipe lengths to prevent

particulate material, which once suspended, remains as

a solute leading to potential customer complaints.

By considering travel times, sections from the temporal

turbidity plots can be assigned to specific lengths of the

flushed pipe sections, such as dead ends and different pipe

materials. Segregation of the results for the dead-end CI

section of Flush E, the initial turbidity observed in Fig. 8a up to

Table 2 e Site 1 regeneration flushing results.

Flush Ø mm L m Pipe Peak NTU

1 2 %

C 79 254 PE 34 11 31

D 79 97 PE 900 313 35

time 10:05, indicates 48% regeneration or effectively only two

years until full discolouration risk, assuming the deterioration

is linear. Analysis of the period 10:11 to 10:25, associated to the

900m CI section of this flush and Flush F Fig. 8b, also a CI pipe,

both show lower regeneration. The bulk water quality and

pipe materials are the same for these sections; however the

normal daily flow is lower in the smaller diameter final dead-

end section of Flush E. This suggests that the daily hydraulic

loading is a factor that influences regeneration. The period

10:05 to 10:11 shown in Fig. 8a represents the turbidity

response from the 101 mm asbestos cement (AC) pipe section

with a modelled peak daily flow of 0.49 m/s. As little turbidity

(less than 4 NTU) is initially mobilised or regenerated this

supports the Dutch proposal that a self-cleaning velocity may

be a simple and effective management tool to control dis-

colouration in non CI pipes. After this there is a 10 NTU

turbidity step that is associated with the 900 m of equivalent

diameter CI pipe. Both these sections experience the same

daily demand and therefore flow velocity, yet the CI section

exhibits a significant discolouration risk. The increased

accumulation of material may be anticipated if in-situ

Average NTU Amount NTU.s

1 2 % 1 2 %

27.7 3.3 12 25186 2991 12

162.3 51.1 31 195392 62275 32

Fig. 7 e Pipe diagram and hydrant locations, Site 3.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 1 3e1 2 4120

corrosion processes are considered contributory but the

results also reveal CI pipes, typically rougher than plastic or

AC pipes and therefore undergoing greater changes in shear

stress, are not self-cleaning at this velocity. Epanet (Rossman,

2000) hydraulic model calibration, determined through the

collection of flow and pressure data during the flushing

operations, set the DarcyeWeisbach pipe roughness at

0.1 mm and 1 mm in the AC and CI pipes respectively.

The majority of sites reported here only underwent one

repeat visit. However Husband and Boxall (2008) presented

results fromasitewithmultiple repeat visits thatdemonstrated

linear asset deterioration. Combined with the repeatable, and

therefore predictable, turbidity trends observed during these

trials and a consistent accumulation rate at each site, this

indicates a regular, yet site specific, supply of discolouration

material. Of the three site examples the lowest average regen-

erated turbidity came from the groundwater site, even

accounting for CI pipes and the likely presence of corrosion by-

Fig. 8 e Site 3 initial and 12 month return t

products. All this evidence implicates water quality as the key

factor governing regeneration. It is therefore proposed that

discolouration material accumulation can be simplified as two

processes;primarilyanindirectprocesswithmaterialadsorbing

to the boundary surfaces from the bulk water and secondly

a direct precipitation process due to corrosion of iron assets.

5. Site fieldwork summary

By collating the data from all sites it is possible to explore the

relative influence of the factors identified as possibly influ-

encingassetdeterioration, in the formofmaterial accumulation

leading to potential discolouration. Key factors acknowledged

as influencing thewater quality include the sourcewater origin,

type of coagulation process during treatment and presence of

unlined upstream cast iron pipes. In addition the interaction at

the boundary of bulk water and the pipe wall is intrinsically

important for any study of accumulation processes and hence

pipematerial isdeemedafactor.Due to thenumberandrangeof

interactions possible in live networks scatter is expected when

summarising the data from all the sites. To investigate the

statistical significance of this variance the coefficient of varia-

tion (Cv), a dimensionless number has been determined where;

Cv ¼ s

m(1)

with s the standard deviation and m the mean of the sample

groups. Cv values of less than one are low-variance, whereas

values greater than one are considered high-variance.

From Table 1 to Table 3 it can be seen that the average and

integration method for calculating regeneration percentages

returns comparable results while the peak method, subject to

manual interpretation, is more variable. As the average

urbidity results with flushing flow rate.

Table 3 e Site 3 regeneration flushing results.

Flush Ø mm L m Pipe Peak NTU Average NTU NTU/s

1 2 % 1 2 % 1 2 %

E 101 900 CI 17.2 3.1 18 10.9 2.0 18 9300 1550 17

101 440 AC No feature 4 0.9 23 1079 255 24

74 594 CI 31.4 14.3 46 14.3 6.8 48 6832 3298 48

Combined Not applicable 10.4 2.8 27 21880 6020 28

F 101 1617 CI 13.7 4.1 30 8.0 2.4 30 24585 7660 31

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 1 3e1 2 4 121

method is simpler and returns turbidity values likely to be

indicative of that which may be observed by a consumer

should a discolouration event occur, this has been used as the

basis to compare factors between sites. The annual

percentage and magnitude of regeneration results for all sites

tested across the UK are shown in Table 4. An overall appraisal

indicates that water quality factors are statistically significant

in determining the annual percentage of material regen-

erated, with an average variance of only 0.5 and no single

result greater than 1. Conversely with an average variance of

1.2 this analysis suggests that water quality does not influence

the potential discolouration magnitude.

From Table 4 information of practical benefit to water

suppliers can be derived. For example it indicates CI pipes have

the greatest deterioration, presenting a maximum discoloura-

tion risk after only 2 years from flushing (52% regeneration).

With an average of over 20 NTU mobilised within a year these

pipes also pose the greatest potential turbidity risk to

customers. Combinedwith the knowledge that no self-cleaning

effect has been identified and the detrimental effect on water

quality (and subsequent layer development downstream) due

to the release of corrosion by-products, effective management

of these pipes is essential. If CI pipes are split between surface

water and groundwater supplied sites the regeneration is 66%

and 31% respectively, with crucially the former generating

significantly higher average turbidities. Non ferrous, or

‘smooth’ pipes with no corrosion by-products, present an

average regeneration percentage of 28%, suggesting 4 years

after cleaning for full regeneration. After a year the average

Table 4 e Coefficient of variation determination of annual regfactors influencing water quality.

Factor Regeneration per annum, %

m s

CI pipe (all sites) 52 30.7

Non-ferrous 28 15.7

CI/surface water 66 32.0

CI/groundwater 31 11.4

Surface water 55 32.5

Blended water 24 14.2

Groundwater 32 10.5

Iron coagulation 49 33.2

Aluminium coagulation 40 29.2

No coagulation 32 10.5

Upstream unlined CI pipes 54 30.1

No unlined CI pipes 27 14.2

Average Cv

turbidity during flushing in these pipes is 5 NTU, just exceeding

UK regulatory limits. The results show surface water sites

regenerate discolouration material quicker than other water

sources, although some blended water sites with looped

connections have produced very high turbidity, Fig. 6b. Typi-

cally surfacewaters are regardedashavingpoorerqualitywater

so this result fits with the understanding that regeneration is

governed by water quality. Examination of correlation with

treatment processes suggest that sites supplied with iron

coagulatedwater have a greatermaterial layer regeneration yet

of lower magnitude than aluminium coagulated waters.

However treatment processes are linked to source water char-

acteristics, so thismeasure inparticularmay be capturingmore

than simply treatment process effects. The presence of

upstreamunlined cast ironasapotential independent source of

material was also examined. Not surprisingly sites with this

feature present a higher rate of material regeneration, on

average 50% or full risk in two years, twice the rate or half the

time of sites with no upstream unlined CI pipes. This may

explain the anomaly demonstrated between the lower regen-

eration recorded from the blended water sites compared to the

expected higher quality groundwater sites. In these trials the

latter siteshad significant lengthsofupstreamCIpipeswhereas

the blended sites are plastic only networks. This complex

interaction of variables is intrinsic when dealing with opera-

tional networks and indicates the need for ongoing assessment

and a requirement for laboratory trials to study isolated factors.

In the UK it is acknowledged that iron is a dominant

constituent of discolouration material and it is common for

eneration and average regenerated turbidity for identified

Average regenerated turbidity, NTU

Cv m s Cv

0.6 24 47.4 2.0

0.6 5 3.6 0.8

0.5 36 58 1.6

0.4 4 2.0 0.5

0.6 20 41.8 2.1

0.6 13 16.6 1.29

0.3 3 2.1 0.6

0.7 13 12.6 1

0.7 27 61.5 2.2

0.3 3 2.1 0.6

0.6 9 7.8 0.6

0.5 9 13.2 1.4

0.5 1.2

Fig. 9 e Bulk water iron concentration against a) annual material regeneration and b) mean annual regenerated turbidity.

Fig. 10 e Factors regulating pipe discolouration potential

(asset deterioration).

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 1 3e1 2 4122

water suppliers to receive red water complaints due to precipi-

tated ferric oxides. It is therefore proposed that bulk water iron

concentrations could potentially be a singlemeasure capturing

the dominant influence of the water quality factors such as

source water, coagulation treatment processes and presence of

unlined upstream iron assets. To investigate this, bulk water

iron concentrations (not the concentrations exiting treatment

works) for each pipe length have been determined using data

from regulatory sampling at the DMAs studied. These are

plotted against the annual percentage regeneration, Fig. 9a, and

the average regenerated turbidity, Fig. 9b.

Although not conclusive with a proportional variance R2 of

0.57, Fig. 9a indicates a potential relationship exists between

bulk water iron concentration and material regeneration. The

general trend is for regeneration to increase with iron levels,

whilst of operational benefit it appears a greater discoloura-

tion risk is likelywhen concentrations rise above 25e40 mgFe/l.

This reinforces the findings that water quality is a key factor

governing cohesive layer development and suggests that for

the UK bulk water monitoring of iron may be used to justify

capital investments such as improvements to treatment to

lower iron concentrations versus the necessity of operational

interventions such as flushing. Although only statistically

indicative, linear regression implies that for every microgram

per litre increase in iron concentration the annual regenera-

tion increases by just over 1%. It also suggests that even with

negligible iron concentration levels, annual regeneration

remains at about 15% or effectively 6 years for material layers

to reach their maximum discolouration risk. This indicates

that although iron may be a primary source of discolouration,

there are other contributory factors requiring investigation,

such as the prevalence of other metals or microbiological

interactions and biofilm development. If the linear regression

is forced through the origin of the graph (with a resulting

R2 ¼ 0.44), effectively considering iron as a vital component of

discolouration material, an increase of just 1 mgFe/l leads to

a 1.5% increase in annual regeneration rate. Compared to the

regeneration water quality relationship in Fig. 9a,b indicates

no relationship between bulk water iron concentrations and

quantity of material mobilised from the pipe wall, R2 ¼ 0.01,

matching the result from Table 4 and indicating water quality

does not influence the discolouration magnitude.

6. Discussion

This research demonstrates that discolouration, or the accu-

mulation of cohesive layers that can lead to discolouration

upon mobilisation, is an unavoidable and continuous process

that occurs throughout UK water distribution systems. The

magnitude or severity of a potential discolouration event

however may be controlled throughmaintenance or as a result

of network design and operation. Fortunately in some cases the

operational hydraulics under the daily demand patterns may

already be sufficient to limit the discolouration risk to accept-

able levels and this may have led to the general complacency

surrounding discolouration. In time water companies who

wish to demonstrate pro-activewater qualitymanagementwill

effectively need to assign a risk rating for every pipe and this

will necessitate an understanding of the controlling factors.

Throughwork such as this these elementsmay be defined such

that each network and its unique characteristics may be taken

into account and suitable management strategies imposed.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 1 3e1 2 4 123

Primarily thisworkdemonstrates thatwaterqualitydefines

the rate at which discolouration causing layers regenerate.

From the sites tested it has been possible to propose deterio-

rationpercentagesandevensuggest target ironconcentrations

to limit material accumulation. With this type of data it now

becomes a possibility forwater companies to optimise costs by

balancing capital expenditure to improve water quality justi-

fiedonaquantified risk and reduction in customer complaints.

Not surprisingly, due to the particulate nature of discoloura-

tion, it is shown that surface waters with the addition of

coagulation treatment processes, supply water that leads to

the quickest deterioration. Groundwaters however, generally

accepted to be of a higher quality, see a reduced rate of dete-

rioration. The presence of additionalmaterial sources, such as

unlined cast iron pipes has a negative impact on deterioration.

Cast iron pipes, most likely due to the direct corrosion

processes are shown to deteriorate quicker than other pipes

only subjected to the indirect, bulk water, material source.

Results show that water quality does not influence the

magnitude of a potential discolouration incident. Therefore if

a pipe is known to pose a risk, no improvements to water

qualitywill ameliorate thishazard completely, although itmay

extend the interval prior to an event occurrence. It is therefore

proposed that systemhydraulics are crucial indetermining the

magnitude of mobilised turbidity. A basic understanding then

would be that a reduction in the discolouration potential may

be achieved by increasing the peak daily flow, and hence shear

stresson thepipewalls.Networkmanagersmayfind itpossible

to change demand and/or flow patterns such that regular peak

boundary shear stress is increased to limit the ultimate accu-

mulation of material and the associated risk to customers.

These twooverarchingfindings relating toaccumulationand

ultimate volumeofmaterial are encompassed in the conceptual

model shown in Fig. 10. In this model water quality is shown to

govern the gradient of deterioration while hydraulic conditions

define the ultimate state achieved in a particular pipe.

The real significance ofhydraulic patterns, the conditioning

shear stress regimes, and how they affect the accumulation of

discolouration material has yet to be thoroughly investigated.

It is possible to make assumptions based on examining

hydraulic records, yet the number of variables involved, even

within the samenetwork,will always be an issue in comparing

results. To enable progress in the understanding of the role of

hydraulics in discolouration it is suggested that researchneeds

to be conducted within a controlled environment whereby

specific, individual factors may be adjusted and the influence

evaluated. It may then be possible to extrapolate results to live

systems which could see an improvement in management

strategies; reducing the risk of water quality failure and

subsequent customer complaints, extending maintenance

intervals and reducing operational costs.

7. Conclusions

Fundamentals:

� Water quality influences the rate of discolouration material

regeneration within water distribution systems but shows

no significant correlation to magnitude of potential

discolouration

� Bulk water iron concentrations may effectively be used as

an indicator of water quality influencing discolouration

regeneration/deterioration

� System hydraulics are significant in determining the

magnitude of potential discolouration

Network components:

� On average CI pipes would present a fully regenerated dis-

colouration risk after 2 years

B Surface water sourced CI pipes present a fully regen-

erated discolouration risk after 1½ years

B Groundwater sourced CI pipes present a fully regenerated

discolouration risk after 3 years

� On average plastic pipes present a fully regenerated dis-

colouration risk after 4 years

Operational:

� Lower bulk water iron concentration reduced the rate of

asset deterioration, with an apparent lower limit of 25 mg/l

after which that data becomes scattered as other factors

also become significant

� Higher normally or routinely experienced pipe wall shear

stress reduces the magnitude of potential discolouration

� Looped networks in some instances may create hydraulic

flow patterns that favour the development of material layers

leading to an increased discolouration risk

� Network flow patterns may be managed to reduce dis-

colouration risk

� The hydraulic design of networks to maintain discoloura-

tion risk below acceptable thresholds (self-cleaning) is

feasible

Issues:

� The relative significance of peak and variable daily

hydraulics are not clear

� The influence of other system variables are not determined,

e.g. hardness, chlorine residuals, organic components,

microbiological interactions, seasonal variations, pH,

temperature, etc.

r e f e r e n c e s

Ackers, J., Brandt, M., Powell, J., 2001. Hydraulic Characterisationof Deposits and Review of Sediment Modelling. DrinkingWater Quality and Health e Distribution Systems e DW/03.UKWIR, pp. 109.

Barbeau, B., Julienne, K., Carriere, A., Gauthier, V., 2005.Dead-end flushing of a distribution system: short and long-term effects on water quality. Journal Water SRT e Aqua 54,371e383.

Boxall, J.B., Saul, A.J., Skipworth, P.J., 2001. A novel approach tomodelling sediment movement in distribution mains basedon particle characteristics. In: Ulanicki, B., Coulbeck, B.,Rance, J.P. (Eds.), Water Software Systems: V. 1: Theory andApplications (Water Engineering & Management), vol. 1.Research Studies Press, Hertfordshire, UK, pp. 263e273.

Boxall, J.B., Saul, A.J., Gunstead, J.D., Dewis, N., 2003. Regenerationof Discolouration in Distribution Systems. ASCE/EWRI/WorldWater and Environmental Resources Conference,Philadelphia.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 1 3e1 2 4124

Boxall, J.B., Saul, A.J., 2005. Modelling discolouration in potablewater distribution systems. Journal of EnvironmentalEngineering ASCE 131 (5), 716e725.

Cook, D.M., 2007. Field Investigation of Discolouration MaterialAccumulation Rates in Live Drinking Water DistributionSystems. Civil and Structural Engineering. Sheffield,Sheffield. Ph.D.

DWI Information Letter 15/02 DWI, 2002. In: Rouse, M. (Ed.),Distribution Operation and Maintenance Strategies (DOMS) eDWI Requirements and Expectations. DWI.

DWI, 2007. Drinking Water 2006; Drinking Water in England andWales 2006. A report by the Chief Inspector Drinking WaterInspectorate. Drinking Water Inspectorate, London, pp. 89.

DWI, 2008. Drinking Water 2007; Drinking Water in England andWales 2007. A report by the Chief Inspector Drinking WaterInspectorate. Drinking Water Inspectorate, London, pp. 83.

DWI, 2009. Drinking Water 2008; Drinking Water in England andWales 2008. A report by the Chief Inspector, Drinking WaterInspectorate. Drinking Water Inspectorate, London.

Gauthier, V., Barbeau, B., Millette, R., Block, J.-C., Prevost, M., 2001.Suspended particles in the drinking water of two distributionsystems. Water Science and Technology: Water Supply 1 (4),237e245.

Hossain, A., Naser, J., McMannus, K., Ryan, G., 2003. CFDinvestigation of particle deposition and dispersion ina horizontal pipe. In: Third International Conference on CFDin the Minerals and Process Industries, Melbourne, Australia.

Husband, P.S., Boxall, J.B., 2008. Water Distribution System AssetDeterioration and Impact on Water Quality e A Case Study.World Environmental and Water Resources Congress, May2008. ASCE, Ahupua’a, Hawaii.

Husband, P.S., Boxall, J.B., Saul, A.J., 2008. Laboratory studiesinvestigating the processes leading to discolouration in waterdistribution networks. Water Research 42 (16), 4309e4318.

Husband, P.S., Boxall, J.B., 2010. Field studies of discolouration inwater distribution systems: model verification and practicalimplications. Journal of Environmental Engineering 136 (1),86e94.

Marshall, G.P., 2001. Understanding and Preventing DiscolouredWater. Drinking Water Quality and Health e DistributionSystems DW-03. UKWIR/Thames Water Utilities Limited.pp. 75.

McCoy, W.F., Olsen, B.H., 1986. Relationship among turbidity,particle counts and bacteriological quality within waterdistribution lines. Water Research 20 (8), 1023e1029.

Mehta, A.J., Lee, S.-C., 1994. Problems in linking the thresholdcondition for the transport of cohesionless and cohesivesediment grain. Journal of Coastal Research 10 (1), 170e177.

OFWAT, 2008. In: Dunn, A. (Ed.), PR09/09 OFWAT’S Review of theOPA and Regulation of Service to Consumers. OFWAT PR09.

Parchure, T.M., Mehta, A.J., 1985. Erosion of soft cohesivesediment deposits. Journal of Hydraulic Engineering 111 (10),1308e1326.

Rossman, L.A., 2000. In: U.S.E.P. Agency (Ed.), EPANET 2 UsersManual. EPA, Cincinnati.

Ryan, G., Jayaratne, A., 2003. Particles in distribution systems andassessment of discoloured water. In: Maintenance andAssessment of Distribution Sstems to Improve Water Quality.C.G. Workshop. Sydney.

Sarin, P., Snoeyink, V.L., Bebee, J., Jim, K.K., Beckett, M.A.,Kriven, W.M., Clement, J.A., 2004. Iron release from corrodediron pipes in drinking water distribution systems: effect ofdissolved oxygen. Water Research 38, 1259e1269.

Seth, A., Bachmann, R., Boxall, J., Saul, A.J., Edyvean, R., 2004.Characterisation of materials causing discolouration inpotable water systems. Water Science and Technology 49 (2),27e32.

Skipworth, P.J., Tait, S.J., Saul, A.J., 1999. Erosion of sediment bedsin sewers: model development. Journal of EnvironmentalEngineering 125 (6), 566e573.

Slaats, N., Rosenthal, L.P.M., Siegers, W.G., Boomen, M.v. d.,Beuken, R.H.S., Vreeburg, J.H.G., 2002. Processes Involved in theGeneration of Discolored Water. KOA 02.058. Kiwa. AmericanWater Works Association/Kiwa, The Netherlands, pp. 116.

Smith, S.E., Holt, D.M., Delanoue, A., Colbourne, J.S.,Chamberlain, A.H.L., Lloyd, B.J., 1999. A pipeline testing facilityfor the examination of pipe wall deposits and red-waterevents in drinking water. Journal of Chartered Institution ofWater and Environmental Management 13, 7e15.

Twort, A.C., Ratnayaka, D.D., Brandt, M.J., 2000. Water Supply.Arnold, London.

Verberk, J.Q.J.C., Hamilton, L.A., O’Halloran, K.J., Van DerHorst, W., Vreeburg, J., 2006. Analysis of particle numbers, sizeand composition in drinking water transportation pipelines:results of online measurements. Water Science & Technology:Water Supply 6 (4), 35e43.

Vreeburg, J.H.G., Schippers, D., Verberk, J.Q.J.C., van Dijk, J.C.,2008. Impact of particles on sediment accumulation ina drinking water distribution system. Water Research 42 (16),4233e4242.

Vreeburg, J.H.G., Boxall, J.B., 2007. Discolouration in potable waterdistribution systems: a review. Water Research 41, 519e529.

WHO, 2005. In: Davison, A., Howard, G., Stevens, M., et al., (Eds.),Water Safety Plans Managing Drinking-water Quality fromCatchment to Consumer. World Health Organization, Geneva,p. 244.