Ice Pigging Technology to Clean Potable Water Trunk Mains in an

Environmental Friendly and Cost Effective Manner

H Candy1, G Quarini

2, N Haskins

1, E Ainslie

2, M Herbert

2, T Deans

2, and D Ash

2.

1 Agbar Environment Limited, Bridgwater Road, Bristol, BS99 7AU, UK (email hcandy@abar.es;

nick.haskins@bristolwater.co.uk)

2 University of Bristol, University Walk, Bristol, BS8 1TR, UK (email joe.quarini@bristol.ac.uk;

martin.herbert@bristol.ac.uk; tim.deans@bristol.ac.uk; dominic.ash@bristol.ac.uk)

ABSTRACT :This paper presents an innovative technique using thick ice slurries to clean water pipes. The

technology has been developed over 10 years by the University of Bristol and has been successfully scaled up to

a fully operational technology in collaboration with Bristol Water and is now delivered to countries around the

world by Agbar Environment. Ice pigging is a simple and elegant technique that overcomes the limitations of

existing cleaning techniques, benefiting from ease of use, reduced costs, more efficient cleaning results, less

interruption to supply to customers.

Keywords: Cleaning; Discolouration; Ice Pigging; Pipe maintenance; Potable Water; Sediment Removal

INTRODUCTION

Whereas the challenge for the 20th

century was water quantity, the challenge for the 21st century is

water quality and meeting customer satisfaction. With ever increasing use on water, more stringent

control of water, and in the developed world an increased focus on consumer protection, there are

higher demands on water companies to manage their networks in a cost effective way whilst

ensuring water quality and reducing customer complaints.

Pipe cleaning is a desired element of routine maintenance for water companies around the world as

they recognise the benefits it brings, and vast sums of money are budgeted each year as companies

seek to treat problematic pipes which cause them water quality issues. For example, in the UK the

water economic regulator has stipulated that company performance is measured on consumer

experience, specifically discolouration complaints, and measurements of turbidity, iron and

manganese. Where companies fail to meet their network benchmark up to 50% of the value of

capital maintenance is at risk (Ofwat 2009).

Although drinking water standards throughout the Western World tend to be high one of the single

most common complaints relates to discoloured water, thus removing inert fine sediments from

potable water pipes is seen as a priority for the water industry (McKissock et al, Veerberg et al,

2007 and 2008). Discolouration occurs when fine sediments in the distribution system are disturbed

and go into suspension, ultimately arriving at the customers tap. Water utilities have developed a

hierarchy of methods to reduce water discolouration problems (Veerberg et al, 2007 and 2008,

Boxall & Saul 2005); first is to prevent particles from entering the water supply network, second is

to prevent suspended particles from settling and last is to remove the sediments from the mains. The

innovative ice pig technique focuses on the last approach.

There are three cleaning methods currently available, all with their limitations. Flushing involves

forcing high speed water through the pipes so as to carry away particulates. However flushing

tends to use very large volumes of water and for large diameter pipes it may simply be impossible

to achieve high enough flow velocities to obtain any cleaning effect. Air scour can be used where

flushing is not desirable, involves isolating a given length of pipe and blowing high velocity air

through it. In order to increase the shear on the walls, it is common to allow small quantities of

water to be propelled through the pipe by the high-speed air. It is an aggressive method that has

become less popular in recent years as it can cause damage to the pipe structure and has associated

health and safety risks. It is also limited to pipes small to medium diameter pipes. Swabbing

consists of forcing an object (the pig) through the pipe so as to push or wipe away the loose

material. Swabbing, whilst not limited by pipe diameter, is highly problematic due to swabs being

unable to cope with changes in pipe diameter or direction, and are prone getting stuck or lost in the

pipe network. Swabbing is also very expensive due to the extensive enabling works required

facilitating launch and receiving stations.

To address these challenges, an innovative and environmentally friendly cleaning method has been

developed which uses ice slurries as viable semi-solid ‘pigs’ (Quarini 2002, Shire et al 2005, and

Shire 2006). The process is in many ways similar to conventional pigging, however, it does not

require special pig launch or receive stations, the pig is disposable, environmentally friendly and

can never get stuck.

Due to its rheological properties the ice slurry forms an ‘ice pig’ which provides enhanced cleaning

shear on the pipe walls, and easily adapts to the local topology of the pipe. This also means that the

ice can be inserted into the pipe through small entry points and will expand to the topology of the

pipe, thus removing the need for large and expensive enabling works.

The potential advantages of this technology are considerable and include the guarantee that the pig

never gets stuck (it eventually melts!), reduced water consumption (typically orders of magnitude

less than for water flushing) and suitability for any topology, with minimal introduction/removal

costs.

METHOD 1 – ICE PIGGING SCIENCE

Ice Pig technology is based on understanding and harnessing the benefits of the specific and

extremely complex science behind semi solids and in particular ice slurries (Christensen et al 1997,

Knodel et al 2000, Bellas et al 2002, and Egolf et al 2005).

Quarini et al (2010) through a series of laboratory testing proved that when correctly controlled ice

slurries

• can successfully be pumped and have the ability to form a pig

• have cleaning abilities more efficient that just water

• are able to transport fine and heavy sediments at velocities lower than can be achieve with

water

• can maintain enhance cleaning performance when negotiating pipe bends and

accommodating diameter changes whilst sill remaining intact. To date ice pigging has

negotiated a 16:1 increase in cross sectional area with no problems.

In simple terms the ice slurry benefits from being pumpable and conforming to varying

topographies like a liquid, and yet behaves in pipe as a solid, providing an enhance shear stress on

the pipe walls and therefore more efficient cleaning of the walls.

Controlling and maintaining the ice slurry consistency is important. A good quality slurry requires

small individual ice crystals. Over time the ice is subject to Ostwalds ripening whereby ice crystals

tend to stick together and form a solid mass as the ice ages. To overcome these issues a freezing

point depressant and mechanical agitation is used. In ice slurries used to clean water mains a

uniquely designed brine solution is used. This approach has been accepted in the UK water quality

regulator.

Although the thermal hydraulics associated with ice slurries is complex (Christensen et al 1997,

Knodel et al 2000, Bellas et al 2002, and Egolf et al 2005), melting tends to be relative slow so that

the temporal longevity of the ice pig to survive as a viable entity is sufficient to clean significant

lengths of pipe (Evans et al 2008), thus highly suitable to cleaning water mains.

METHOD II - ICE PIGGING OPERATION

To use ice pigging to clean water pipes a purpose-built lorry is used to transport and inject large

quantities of ice slurry into hydrants connected to water mains.

The delivery unit is a 10 tonne stirred horizontal tank mounted on an articulated lorry. The lorry has

its own diesel generator set, delivery pump and geared stirrer. Ice slurry is made using specialist

icemakers. Once sufficient ice slurry has been manufactured and loaded into the delivery tank, it is

driven to the required location for ice pigging operations. Figure 1 shows the ice pig lorry at its

delivery location, ready to undertake an ice pigging operation.

The procedure consists of inserting the

ice pig into the water pipe at the entry

point, using mains pressure to propel

the pig through the pipe and out of the

system via the downstream exit point.

Entry and exit points are typically

hydrants, but allsorts of mains fittings

can be used, air valves, quadrina

points, even meters.

The specific procedure involves

valving off from mains supply pressure

the length of pipe to be cleaned, by

shutting off valves A and C on Figure

2. Hydrant valves B and D are then

opened, and the ice pig delivery tank

attached to hydrant B. The self

contained pump on the delivery unit

forces the ice pig into the hydrant and typically fills many tens of diameter length of pipe with good

quality, high ice fraction ice slurry. The displaced water from the pipe runs to waste from hydrant

D. The valve on hydrant B is then closed and the delivery tank removed.

Valve A is opened, this allows mains water to push the ice pig through the pipe, the waste leaves at

hydrant D. As the ice pig reaches hydrant D, a waste tanker is connected to the hydrant to catch

most of the ice pig and the associated sediment deposits. The water following the ice pig rapidly

turns clear, and can again be allowed to run to waste, with no need to tanker it away. Once the

operation is complete turbidity samples are taken, until the water meets the customer’s

requirements, this usually happens very quickly requiring only an extra 0.25 – 0.5 pipe volume to

achieve. Hydrant D is then valved off and valve C opened, and the cleaned pipe is returned to

service.

Figure 1 The ice-pig delivery lorry ready to insert ice at the entry point

Ice pig

insertion

hydrant

Outlet

hydrant

Hydrant

valve B

Hydrant

valve D Mains

Valve C Valve A

Typically 1 to 2 km

Figure 2 Schematic of the ice pigging procedure

Diagnostic instrumentation is used at both the inlet and outlet hydrants. Typically, pressure, flow

rate, temperature and conductivity (to detect salt concentration) are monitored. This information is

used to control and monitor the ice pig operation. Samples are also taken from the outlet hydrants as

the pigging operation proceeds, these samples are then analysed to obtain particulate removal rates.

RESULTS

To date ice pigging has been used to successfully clean over 100km of mains throughout the UK.

Whilst the original focus was to develop a technique to target pipes in the 150-300mm diameter

range, the envelope of experience has since been extended to pipes up to 450mm in both ferrous and

non ferrous pipes. See figure 3.

The length of each pipe which can be cleaned at a time is dependant on a combination of pipe

diameter and material, flow rates, and ambient temperatures, as well as the available inlet and outlet

fittings, and the volume of ice available. The optimum capacity for the current ice manufacture,

storage and delivery design is 9 tonnes. With this volume of ice the maximum length achieve to

Material Diameter

(mm)

Length

(m) 75 1475

150 1600

200 4412

225 1906

250 1666

300 1380

Cast Iron

375 5050

Spun iron 300 1550

300 516

400 1330 Ductile iron

450 1100

Fer

rou

s

Steel 200 2400

Material Diameter

(mm)

Length

(m) 150 5980

200 23795

250 4922

300 25220

Asbestos cement

450 900

300 4800 MDPE

HPPE 315 450

150 4800

200 1389

No

n f

erro

us

PVC/UPVC

400 3750

Figure 3 Summary of Ice pigging operations to April 2010

date in one run has been 3.6km in 150mm concrete pipe. Based on the experience achieved to date

the figure 4 presents the likely lengths achieved for a 9 tonne volume of ice slurry.

Daily Output Matrix (for guidance only)

750

675

600

525

500

Diameters 450

mm 400

350

300

250

200

150

100

80

10

00

200

0

30

00

40

00

50

00

60

00

Linear m

Non Ferrous

Ferrous

The above chart indicates achievable daily targets where ideal conditions exist i.e. flow rates, ambient air and water temperature.

Diameter Nominal Bore mm's 80 150 200 250 300 350 400 450 500 525 600 675 750

Ideal Flow Rate litres/sec 5 6 12 16 20 20 25 36 40 40 80 120 150

Min Flow rate litres/sec 2 4 6 7 8 15 20 30 35 35 70 100 100

Whilst each job is unique, depending on the pipe material, diameter and length, Ice pigging has

proved to be extremely effective in removing sediment in very short periods of time. The typical

operation takes less than 20 minutes to insert 9 tonnes of ice, and less than 2 hours to complete the

operation. This means the interruption to customers is vastly reduced.

The amount of sediment removed is dependant on the condition and sediment load of the individual

pipe, but to jobs to date have shown typically 10 - 20kg per km is removed. As a typical job

involves 1.25-1.5 pipe volumes of water per operation, this is a significant improvement on

cleaning efficiency per unit of water compared to flushing which uses typically 3-4 pipe volumes to

remove considerable less sediment.

Ice pigging is not only applicable to portable water mains, indeed the maximum sediment load

removed to date was from a raw water main where 200kg was removed from 1200m of 6inch PVC

pipe over 4 runs.

In order to quantify the effectiveness of the ice pig in comparison with water in a real water industry

supply environment, a 1600m section of 200mm diameter asbestos cement trunk main pipe was

carefully instrumented with pressure recording transducers at 6 points along its length. Water flow

along this length was varied and the pressure loss along the pipe monitored. From these data, the

hydrodynamic characteristics of the section of pipe were deduced. An ice pig, approximately 100m

in length, was then introduced at the up-stream end of the pipe. The pig was then propelled through

Figure 4 Likely output for a 9 tonne volume of ice

the pipe by mains water whilst the pressure distribution along the pipe was monitored. The speed of

the ice pig was varied during this trial so as to build up a comprehensive data bank on the hydraulic

performance of the pig. Figure 5 shows the pressure loss across the complete 1.6km length of pipe

as a function of flow rate for both water and the ice pig as it travelled through the pipe.

At the start of the ice pigging trial, the ice pig is fresh and there has been no time for melting or for

the pig to accommodate to the pipe, it therefore is able to provide a larger resistance to flow than

near the end of the trial when the pig is about to leave the pipe section. This ‘aging’ effect is clearly

seen on Figure 5, which suggests that the pressure loss at the start of the ice pigging trial might be

as much as 40% higher than at the end of the trial. However, the pressure drop with the ice pig is

typically an order of magnitude larger than for water (at the same flow velocity).

Further the data shown on Figure 5 refers to the pressure loss across the whole 1.6km length, the

pressure gradient along the ice pig itself will be much much greater than the average pressure

gradient across the whole pipe. The pressure gradients along the ice pig are typically two orders of

magnitude greater than those in a water-filled pipe (with both the pig and the water travelling at the

same velocity). This is an important and commercially valuable finding;

Specifically the ratio of ice pig to water only pressure drop can be regarded as a figure of merit, in

that the pressure loss is directly proportional to the shear on the pipe walls, and the shear on the pipe

walls must be related to cleaning: the higher the shear the better the system is at cleaning, removing

and transporting sediments.

Whilst the ice pigging system was initially been developed to clean fine and coarse loose sediments

it is now being used to tackle a variety of other problems such as sticky manganese and iron

deposits and biofilms. The potential for achieving an even greater shear on pipe walls means that

ice pigging is now being tested for a other problems such as stubborn zebra mussels and to prevent

lime scale build up in pipes

The water quality can be considered to be instantly improved after a successful ice pig run. If any

significant amount of sediment has been removed from a pipe it is clearly an indication of an

improvement in the water quality. The longer term improvements in water quality as a result of ice

0

4

8

12

16

0 5 10 15

volume flow rate l/s

Pre

ssure

dro

p( m

wate

r)

Initial

Middle

End

Water

Figure 5 Comparing

pressure drop of start,

middle, and end of ice

pig with that of water

at various flow rates

pigging are being monitored in the routine water quality sampling. To date this information is still

being collated but the initial work is very encouraging, see figure 6.

Location Arnos Vale Highridge Ashton Road

Material Ductile iron Asbestos cement HPPE

Diameter 300mm 300mm 315mm

Length 516 m 2208 m 450 m

Date pigged 26/01/2009 19/6/2008 & 15/10/2008 28/07/2009

No times pigged 1 2 1

Total sediment removed Not available 32 kg 3.2kg

Before After Before After Before After

No. samples for Fe 112 23 363 36 184 50

No. samples Fe> 50µg/l 44 2 53 1 57 1

No. samples Fe> 100µg/l 14 1 20 1 21 1

No. samples Fe> 200µg/l 3 0 2 0 3 0

Average Fe µg/l 35 17.8 23.7 14.0 52.1 13.2

Maximum Fe µg/l 63 35 217.0 105.0 339.0 107.0

No. Samples for Turbidity 110 9 365 36 175 50

Average turbidity NTU 0.124 0.063 0.305 0.055 0.116 0.040

Maximum Turbidity NTU 3.240 0.140 0.507 0.090 1.400 0.130

No. samples for chlorine 48 4 121 34 66 12

Average free chlorine mg/l 0.30 0.18 0.42 0.51 0.28 0.43

Average total chlorine mg/l 0.45 0.30 0.60 0.65 0.44 0.58

Data to date shows that after ice pigging there is an improvement in iron and turbidity

concentrations. Chlorine residual also improves. In the cases of Highridge and Ashton Gate the

chlorine residual has increased post pigging which is a typical result due to a sediment free pipe

having a lower chlorine demand. The Arnos Vale case appears to have a lower chlorine residual

post pigging, this is due to having such a high residual post pigging that now the dosing regime has

been reduced in line with the reduced demand.

CONCLUSIONS

An innovative cleaning technique using thick pumpable ice slurries acting as pigs has been

presented in this paper. Ice pigging provides benefit to the water supply industry by offering a

gentile, rapid and potentially cost effective way of removing loose sediments from water supply

pipes thus successfully delivering effective cleaning in live mains.

Ice pigging is able to overcome all the limitations of current cleaning techniques, thus ice pigging

has successfully established itself as the technology that can replace the use of water flushing,

swabbing, air scouring and foam pigging whilst providing reduced enabling costs, reducing or

eliminating down-time and reducing man-time costs in a safe, risk–free manner.

Given the potential benefits ice pigging can bring to the water industry, it is likely that the

technology will become more widely adopted. The rate of adoption will be accelerated by

environmental concerns associated with sustainability, water usage, down time and supply

interruptions.

Figure 6 Initial results comparing long term water quality monitoring before and after ice

pigging

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