The effects of using water velocity as a technique to control biofilm development in water supply systems P. Angus (1), S. Ingle (2), D. King (3), J. Turner (4) (1) Paul.Angus@WSPGroup.com (2) steven.ingle@btconnect.com (3) D.C.King@ljmu.ac.uk (4) John.Turner@WSPGroup.com Abstract A biofilm may be defined as a colony of living micro-organisms, which exists in a layer or clump on a particular surface. Biofilms are self-perpetuating and difficult to remove, and will form on virtually any surface including glass, metals and plastics, especially if such surfaces are wet or damp. It is known that biofilms exist widely on the inside of pipework walls and in the components of plumbing systems, and that they can support a large variety of pathogenic bacteria. Techniques used to control biofilm in plumbing systems generally include chemical dosing, heat treatments (pasteurisation) and flushing with clean water. It is known that no single treatment regime is entirely effective against biofilms. This paper investigates whether the technique of increasing water velocities in plumbing systems (either permanently by decreasing pipe diameters, or intermittently by use of variable speed pumps) to scour the inside of pipework is feasible. Smaller pipe diameters and variable speed pumping may also be seen to contribute to sustainability of systems. It is recognised, however, that raised water velocities may result in increased pipework noise and accelerated erosion of pipework materials. It is argued that if increased velocity is deemed desirable, then careful design and installation techniques could control such factors. T he evidence presented here appears to suggest that increased velocities may well be a useful technique to add to any biofilm treatment regime. Keywords Biofilm; bacteria; velocity; hydraulic; laminar, noise, turbulent; flow; variable speed pumping;

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1 Introduction Biofilm formation is a major factor in the growth and transport of undesirable bacteria, as well as contributing to fouling and corrosion of pipe networks, with the associated health consequences1. Biofilms in plumbing systems which house and protect pathogens, are often treated using various biocides, flushing and pasteurisation techniques; however, it is evident that some bacteria tend to develop a resistance to a wide variety of biocides and to heat treatments2

. This leads to higher dosages of biocide and higher temperatures being specified, and these factors are subsequently seen to have a negative effect on public health and sustainability, and contribute to damage of components in plumbing systems.

This paper investigates the possibility of using an alternative technique: with the widespread introduction of variable speed motors for pumping, it is suggested that fluid velocity may be increased, thus producing a scouring effect on the pipe walls, and so hinder the initial attachment of bacteria which would in time form a biofilm. It is known, however, that this increase in fluid velocity will lead to an increase in pipework noise, and this can be a problem, especially in domestic buildings and areas of public buildings where noise control is critical. An increase in water velocity can also accelerate erosion of pipework materials. However, the recommendations offered in common UK design guides for limiting water velocities in plumbing systems seem based on somewhat inconclusive and dated research relating to noise control, though rates of pipework erosion due to excessive water velocities can certainly be predicted with more confidence. It is possible to argue, however, that design and installation techniques and pipework materials have changed in recent years, with many systems being boosted by variable speed pumps, and there is a case for re-examining the likely effects of higher water velocities in plumbing systems. A range of literature featuring measurements of noise levels from plumbing systems at various flow rates has been consulted to assess whether high flow rates and high velocities do indeed cause a noise nuisance. It is also questioned whether erosion of pipework materials by such phenomena as cavitation, a function of high velocities, may be significantly reduced by careful system design. There are several other benefits of using the approach of allowing higher water velocities in plumbing systems: It would be possible to reduce pipe diameters, which in turn reduces the actual surface areas of pipework material to support biofilm growth, whilst also reducing the sizes of bracketing, ancillaries and insulation material, thus contributing to a more sustainable use of materials and reduced installation costs.

1 Edstrom Industries Inc. (2003) ‘Biofilm – key to understanding and controlling bacterial growth in automated drinking water systems’ http://www.edstrom.com/DocLib/4230-DS3100_CompleteBiofilm.pdf . 2 Wadowsky RM et al. (1985) Effect of Temperature, pH ,and Oxygen levels on the multiplication of naturally occurring Legionella pneumophila in Potable Water. Applied & Environmental Microbiology May 1985 p 1197-1205

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2 The formation of biofilms A biofilm is a layer or clump of micro-organisms in which the individual cells adhere to each other and to an available surface, and these are commonly to be found in pipework or plumbing components1. Once cells have settled and begun to multiply, the colony of micro-organisms protects itself by excreting a slimy film of extracellular polymeric substance (EPS), which binds the organisms together, sticks to the surface and is very difficult to remove3

. Biofilms can form on living or non-living surfaces, and represent a prevalent mode of microbial life.

There are five stages of biofilm development in pipes (refer to fig. 1)4

.

1. Initial attachment occurs where a microbe (or a small colony) settles on a surface which has the correct conditions to sustain life. These first colonists adhere only weakly to the surface initially, mainly due to van der Waals forces. It is at this stage that the biofilm may be most easily removed.

2. Irreversible attachment then takes place as the micro-organsims multiply in number. The microbe colony forms its own surface upon which other micro-organisms can adhere. This stage is typically accomplished in a matter of minutes or hours at most.

3. Maturation I: The new biofilm begins protecting itself against environmental factors which may be harmful to it by excreting a sticky and slimy film of EPS. In water systems the EPS protects the microbes fairly well against such chemicals as chlorine and bromine, and elevated temperatures, which are commonly used in water treatment.

4. Maturation II: The biofilm now becomes bigger and tougher. ‘Super colonies’ of biofilm now begin to make use of the chemicals that are meant to destroy them – outer layers of the biofilm may actually be killed by chemical attack, but the dead cells amalgamate with the protective EPS slime to form a strong layer that protects the colony below.

5. Dispersion: As the biofilm colony increases in size, it becomes more unstable and begins to break apart; these broken parts then begin to attach to other available surfaces and the cycle re-commences.

Fig. 1 The five stages of biofilm formation (Soure: Monroe 2010)

3 Hall-Stoodley L, Costerton JW, Stoodley P (2004) Bacterial biofilms: from the natural environment to infectious diseases. Nature Reviews. Microbiology 4 Monroe D (2010) ‘Looking for Chinks in the Armor of Bacterial Biofilms’ PLoS Biology Vol. 5, No. 11

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The public health risk presented by biofilms in water supply systems is considerable, since biofilms are known to be associated with many human diseases. For example, waterborne disease outbreaks documented in the USA in a 2002 E nvironmental Protection Agency (EPA) White Paper include typhoid fever paratyphoid fever, gastroenteritis from various microbiological causes, bacillary dysentery, cholera, Legionnaires Disease and peptic ulcers5. By far the most common serious waterborne malady, however, is Legionnaires Disease, and Legionella bacteria are known to thrive in water pipework where biofilms protect colonies from disinfectants

6

. Hence the efforts of researchers and design, installation and maintenance engineers to find ways of controlling biofilm formation and growth.

3 Hydraulics, system design and sizing Hot and cold water supply systems are required to deliver particular volume flow rates of water for the proper operation of appliances, fixtures and fittings. Design engineers, taking note of required flow rates, must then make decisions about system pressures and flow velocities to determine the diameters of pipework and associated equipment. The over-riding factor for designers is to sustain adequate flow rates to fixtures and equipment. Failure to provide this means, not only that appliances or equipment may not function correctly, but in extreme cases potential damage to property or life may result. Design engineers must make engineering decisions to select pipe diameters with an eye on the relationships between volume flow rate, frictional resistances of pipework and water velocities in the system7. Under-sizing of pipework can also lead to excessively high water velocities, which can cause a noise nuisance and may accelerate the rate of erosion of pipework materials. Conversely, over-sizing of pipework, despite affording adequate pressures and flow rates, will result in low velocities, and these can promote conditions ideal for corrosion by solvency and the formation of biofilms, both of which can cause fouling of the system by debris or scale and consequent public health risks8

.

Presently the design of most hot and cold water supply systems in the UK is based on a limiting water flow velocity of 1 m/s, though for larger installations a limiting velocity of up to 3m/s may often be used9

5 U.S. Environmental Protection Agency Office of Ground Water and Drinking Water Standards and Risk Management Division (2002) Health Risks from Microbial Growth and Biofilms in Drinking Water Distribution Systems, EPA, Washington

. Alternatively a limiting pressure drop due to friction per unit pipe length may be applied: 360 Pa/m is advised by the Building Services

6 Rogers J, Dowsett A, Dennis, Lee J, and Keevil C (1994) Influence of temperature and plumbing material selection on biofilm formation and growth of Legionella pneumophila in a model potable water system containing complex microbial flora, Applied & Environmental Biology, American Society for Microbiology. 7 BSI (2006) BS6700 Design, installation, testing and maintenance of services supplying water for domestic use within buildings and their curtilages. 8 Lehtola, MJ et al (2006) ‘The effects of changing water flow velocity on the formation of biofilms and water quality in pilot distribution system consisting of copper or polyethylene pipes’ Elsevier. 9 CIBSE (2007) Guide C Reference Data, CIBSE, London

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Research and Information Association (BSRIA)10 and 250 Pa/m by American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE)11. These values are endorsed by the standard design guides used in industry, in particular the Chartered Institution of Building Services Engineers (CIBSE) Guide C and the Chartered Institution of Plumbing and Heating Engineers (CIPHE) Design guide12

, and it is generally accepted that if these values are applied, there will be no undue noise from the plumbing system or unexpected erosion of pipework materials. It is a matter of conjecture whether designers ever question these methods or simply apply them arbitrarily.

To prevent under- or over-sizing when deciding upon a design flow rate, design engineers must first consider the diversity of use of sanitary appliances. It is accepted that the demand on virtually all water supply systems will be variable throughout any 24 hour period with peaks and troughs of high and low demand. T hus a ‘loading unit’ approach, which takes account of the diversity of use of sanitary appliances and equipment, is recommended by the design guides to refine the method of pipe sizing. To ensure proper operation of sanitary appliances, the loading unit method calculates a notional design flow rate based upon expected peak use conditions. It is accepted, however, that due to the variable nature of water usage, that for the majority of the time, most systems will operate at well below peak capacity. T his therefore unavoidably exposes the system to flow rates lower than the notional design flow rates, and consequently conditions which may be conducive to biofilm growth and accelerated corrosion. Since water supply systems are, by their very nature, subject to intermittent and variable patterns of use, it is difficult to see how low velocities can be prevented entirely. Although opportunity may now present itself for better flow control, with the introduction of variable speed pumps. A constant challenge for engineers is to design systems that will provide optimum flow rates for the proper functioning of system components and satisfy statutory regulations without over- or under-sizing of pipework. Today’s design engineer also faces the constant quest for sustainability and energy efficiency in all building services systems, and this has led to certain modern design innovations on water supply systems: • Pumped systems can be either retro-fitted or designed from new to operate with

variable speed pumps. Through the use of system sensors monitoring demand, the pumps can speed up and slow down to satisfy the water flow demand as it fluctuates throughout the day. This is, however, not quite such a simple solution as it would seem, since the selection of system control valves is normally based upon a n assumed constant flow rate. If this flow rate is varied the unfortunate knock-on effect is that the authority of control valves changes, and thus flow rates, velocities

10 Building Services Research and Information Association (1995) Rules of Thumb UK/France TN18/1995, BSRIA, Bracknell 11 American Society of Heating, Refrigerating and Air Conditioning Engineers (2005), Fundamentals 2005 ASHRAE Handbook, Atlanta GA 12 CIPHE (2002) Plumbing Engineering Services Design Guide, CIPHE, Hornchurch.

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and frictional resistance become more difficult to predict. This means that variable speed pumping must be combined with a quite sophisticated control system that recognizes varying system conditions, such that when pump speed reduces during part-load periods, the control valves must set at a position such that pressure in the system reduces to a value that satisfies the current load flow requirements13

.

Whilst this approach inevitably cuts down energy use, the problem of low velocities is even more prevalent and conditions for the formation of biofilms persist. It is possible, however, to use the pumps to produce controlled pulses, thus flushing the system with high velocity surges at regular intervals. This technique is known to restrict (though not prevent) biofilm formation14

(see section 3).

• Since hot water supply systems tend to lose heat from long runs of pipework, standard design practice is to select pipe diameters to compensate for this loss. It therefore follows that thermal insulation is of vital concern. M odern thermal insulation materials and increased insulation thicknesses reduce this heat loss, and thus allow smaller diameter pipework to be selected. Smaller pipework then leads to increased velocities, which are also known to inhibit biofilm formation (see section 3).

If it is accepted that increased water velocity may limit the formation of biofilm then the consequences of higher velocities must be examined. 3.1 Nature of water flow in pipes When fluid flows through a channel, pipe, or duct, there are two basic forms of motion: smooth laminar motion and complex turbulent motion. In turbulent flow, there is an irregular, random motion of the particles in directions transverse to the direction of the main flow, as illustrated in Figure 2 below. In laminar flow, also illustrated in Figure 2, the liquid moves in such a way that individual fluid particles move along paths parallel to one another and in the general direction of motion.

13 Egan T (2010) White Paper: Conversion from Constant Flow System to Variable Flow, S. A . Armstrong Ltd. Toronto. 14 Crozes, G. F., and R. S. Cushing (2000) Evaluating Biological Regrowth in Distribution Systems. Water Quality Technology Conference. Denver, CO.

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Fig. 2 Turbulent and laminar flow The main property which governs whether laminar or turbulent flow exists is the dimensionless quantity, Reynold’s number (Re). This is calculated using the formula:

𝑅𝑒 =𝑑 𝑣 𝜌𝜇

where d is the pipe diameter (m), v is the flow velocity (m/s), ρ is the density of the fluid (kg/m3

), and µ is the absolute viscosity of the fluid (kg/m s).

For Reynolds numbers less than 2000, the flow is laminar and for Reynolds numbers greater than 4000, t he flow is turbulent. Between these two values lies a transition region in which the flow may exhibit the properties of either laminar or turbulent flow. The discontinuous transition between the two states is a fundamental problem that has been studied by a great many physicists and engineers over a long period of time15

.

In most plumbing engineering systems, velocities are high enough to result in turbulent flow, although in larger pipe sizes at low velocities the Reynold’s number may occasionally be between 2000 a nd 4000 and conditions associated with the transient region may be in evidence. To take a typical practical case, consider a 15mm diameter pipe carrying water at 16oC (density of 1000 kg/m3 and absolute viscosity of 1.14 x 10-3

𝑅𝑒 = 0.015 × 1 × 1000

1.14 × 10−3 = 13158

kg/m s) at a velocity of 1 m/s, Reynolds number will be calculated as:

i.e. the flow is turbulent.

15 Moxey D & Barkley D (2010) Distinct large-scale turbulent-laminar states in transitional pipe flow, Proceedings of the National Academy of Sciences, New York

Equation 1

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In reality, because of its low absolute viscosity, water flow only becomes truly laminar in nature (i.e. having a Reynolds number lower than 2000) in large pipe sizes with low velocities, and therefore turbulent flow is much more likely. Turbulent flow is, however, a very complex process and numerous researchers have devoted considerable efforts to attempt to understand the variety of baffling properties and phenomena associated with it. Although a considerable amount of knowledge about the topics has been developed, the field of turbulent flow still remains the least understood area of fluid mechanics. Certain empirical data then must be used from laboratory studies to make predictions of likely noise and pipework erosion caused by high velocity and this is discussed in Section 5. 4 Methods of controlling biofilm Common treatments used to inhibit the formation and growth of biofilms in water systems, include the use of high temperatures (pasteurisation), chemical dosing and flushing. It is well known and universally accepted, however, that none of these methods can entirely eradicate biofilm development, at best they only inhibit it. In addition, a number of studies cast doubt over the usefulness of the accepted methods: It would appear that many species of micro-organism are not actually killed by chemical disinfection, they are merely rendered dormant and remain ready to return to active life when environmental conditions are more favourable to them. This is unfortunately true of several species of pathogenic bacteria, which cause human water borne diseases16

.

Heat treatment is also known not to be an entirely successful control mechanism, since many micro-organisms can mutate to develop resistance to high temperatures, and may simply lie dormant in the biofilm until conditions are more ideal. There are certain other factors which might be considered to affect the formation of biofilms.

16 Xu K, McFeters G and Stewart P (2000) ‘Biofilm resistance to antimicrobial agents’, Microbiology Comment, Montana State University, Bozeman, Montana.

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4.1 Pipework material type and surface smoothness Some studies indicate that choice of pipework material has a particular effect on biofilm formation. In particular copper pipes have been shown to be less susceptible to biofilm growth in the initial operations until the surface oxidises than steel and plastics materials, though this is disputed by other studies17. It is, however, agreed generally that steel (particularly if corrosion is present) is the pipework material most prone to rapid biofilm growth. It is noted in several studies that smoother surfaces delay the initial build-up of attached micro-organisms, though eventually smoothness plays no p art in significantly affecting the total amount of biofilm that will attach to a surface within just a few days. Mayette (1992) concludes that the material of the surface has little or no effect on biofilm development and opines that ‘piping material that microorganisms cannot adhere to has yet to be discovered. Studies have shown that microbes will adhere to stainless steel, Teflon, PVC and PVDF (Kynar) with nearly equal enthusiasm.’18

Research carried out by Meltzer (1993) reinforces this view. Surface structure, whilst appearing initially to influence the rate of fouling (smooth surfaces tending to foul at a slower initial rate than rough surfaces) has no long term benefit: biofilm formation after a period of days is inevitable and virtually impossible to prevent19

.

It is known that at a microscopic level stainless steel is a smoother pipework material than other metals. F igure 3 be low gives an indication of the relative size of micro-organisms as they relate to the roughness of three common grades of stainless steel pipe. It can be noted from these pictures that a even a material considered to be particularly smooth is still rough enough to provide refuge for microbes.

17 Camper A (2010) ‘Biofilm Microbiology, Control and Release’ http://www.epa.gov/safewater/disinfection/tcr/pdfs/presentations/meeting_presentation_tcr-revisions_biofilmmicrobiology.pdf 18 Mayette, D.C. (1992) ‘The Existence and Significance of Biofilms In Water’, WaterReview, pp. 1-3, Water Quality Research Council, Lisle Il 19 Meltzer, T.H. (1993) ‘High-purity Water Preparation for the Semiconductor, Pharmaceutical, and Power Industries’ Tall Oaks Publishing, Inc., Littleton CO

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Figure 3 Diagrammatic comparison of the size of a cell to the roughness of three

typical stainless steel surfaces (Source: Metzer, 1993) 4.2 Flow velocity Flowing water flow is known to exhibit a particular velocity profile as it f lows along pipes, regardless of flow rate or whether turbulent or laminar flow is taking place. Due to the frictional effect of the pipe walls, where velocity approaches zero, water flows more slowly in the layers adjacent to pipe surfaces.

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This effect is illustrated in Figure 4.

Fig. 4 Velocity profile in pipework A thin layer adjacent to the pipe wall exists where the velocity is low enough for laminar flow, and this layer is called the boundary layer or laminar sublayer. The thickness of the laminar sublayer was calculated by Pittner & Bertler (1988) for various flow velocities and for five pipe size pipes (shown below in Table 1) 20

. It can be seen that for each pipe size, the laminar sublayer becomes thinner as water velocity increases.

Table 1: Laminar Sublayer Thickness (microns) (Source: Pittner & Bertler 1988) Pittner & Bertler also calculated that the shear forces present within the laminar sublayer are much less than the force that would be required to dislodge a micro-organism cell from the pipe surface. It must therefore be accepted that due to the presence of this slow moving laminar sublayer, higher velocities will not develop shear forces sufficient to remove biofilm close to pipe walls. Higher velocities do, however, result in a thinner laminar sublayer and therefore velocity could be perhaps utilised as a tool to restrict the thickness of biofilms. High water flow rates then may alter or control biofilm growth but will not prevent the attachment of bacteria to pipe surfaces altogether.

20 Pittner G and Bertler G (1988) ‘Point-of-use Contamination Control of High Purity Water Through Continuous Ozonation’ Ultrapure Water 5(4), pp. 16-22

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4.3 Flushing Where high water velocities are present, biofilms which extend beyond the laminar sublayer are subject to the shear forces of the turbulent water. These forces have been found to cause the detachment of some of the cells from the biofilm21. It has also been found that detachment increases with fluid velocity and mass of biofilm22

.

Flushing with clean water at high velocity is a technique frequently used in water supply systems in conjunction with heat treatment and chemical dosing, and is generally intended to remove loose debris after such treatments. T his technique has, however, been found to limit biofilm thickness since shear forces caused by flushing tend to slough off biofilm which extends beyond the laminar sublayer and out into the turbulent flow area of the pipe23

In systems that have fluctuating water flow, such as systems with variable speed pumping, it has been found that bacteria are sloughed off during periods where the velocity is highest and biofilm thickness is thence limited to the thickness of the laminar sublayer.

. Therefore, the maximum thickness of the biofilm is limited to approximately the same thickness as the laminar sublayer for a particular water velocity.

5 Noise from pipework Noise can be a very subjective property and it is consequently difficult to set limiting values. There is a great deal of research into acceptable noise levels in buildings, and many design guides (see for example CIBSE Guide A) recommend acceptable conditions to sustain human comfort. It is known from several pieces of research that the extent to which noise from building services prompts complaints from building occupants depends upon a range of factors, such as the type of building, the level and type of activity taking place therein, the nature of the noise (pitch, quality etc.), the sensitivity of occupants to the noise and so on24

.

It is also extremely difficult to predict noise levels that result from plumbing systems, since so many different physical processes are in evidence. A mong the factors to evaluate are: • Noise is caused by the motion of water in pipes and radiated directly into the space.

One of the objectives of this paper is to attempt to predict the likely increase in this

21 Lawrence JR et al (1995) ‘Behavioural strategies of surface colonising bacteria’ Advanced Microbiology Ecology Vol 14. pp, 111-145. 22 Trulear BE and Characklis W (1982) ‘Dynamics of biofilm processes’ Journal of Water Pollution Control Fed. 54(9) PP, 1288-1301. 23 Patterson MK, Husted GR, Rutkowski A and Mayette DC (1991) ‘Isolation, Identification, and Microscopic Properties of Biofilms in High-Purity Water Distribution Systems’ Ultrapure Water 8(4), pp. 18-24. 24 Wise A and Swaffield J (2002) ‘Chapter 10: Noise’, Water, Sanitary and Waste Services for Buildings, 5th Edition, Butterworth Heinemann

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type of noise due to increased water velocities, and it is therefore suggested that noise levels at raised velocities can be predicted by reference to the empirical data in Table 2 (below), refined by use of Equation 2 (also below).

• Noise can be transmitted by vibration of pipes against building fabrics via pipe brackets and supports (flanking paths). T his type noise is likely to vary considerably with different building types, materials and different installation techniques. Noise levels via flanking paths are therefore likely to be impossible to predict with any confidence without carrying out specific field measurements for each application where raised velocities are mooted25

.

• Noise can result from the vibration of pumps and other items of plant propagating along pipes and radiating into spaces either directly or via flanking paths. T hese noise levels are unlikely to alter due to increased water velocity. However, where more powerful pumps are specified to create the increase in velocity, the noise generated by these would need to be included in the noise predictions on a case by case basis.

• Noise is caused by the operation of sanitary equipment such as taps, float operated

valves and so on, which radiate directly into spaces. There is a considerable amount of research as to how much noise various items of sanitary equipment generate and how much of a disturbance these cause. W ise & Swaffield (2002) provide an excellent summary of such work24. In any system employing higher than usual velocities, at the point of use the flow rate and velocity would have to be returned to levels required for correct operation of the appliance, since water hammer can and damage to equipment could result.

There are various pieces of research available which attempt to evaluate the noise generated directly by the flow of water in the system pipework. Some representative results for small bore pipework are presented by Wise & Swaffield (2002)24 and reproduced in Table 2 below. T o complete the picture measurements for larger diameter pipework would need to be carried out, though it is generally the case that small pipes are noisier – they have higher frictional resistances and therefore higher water velocities, more turbulent flow and thus more noise. Table 2 Noise levels, dBA, in a reverberant test room from pipework of 15-16mm

bore (Source: Wise & Swaffield, 2002)

Velocity of flow (m/s)

Pipe material Copper Steel Lead Plastics

0.1 24 26 25 29 0.55 27 30 29 30 3.4 46 38 39 41 5.2 46 38 38 41

25 Romeu J, Jimenez S and Capdevila R (2004) ‘Noise emitted by water supply installations’ Applied Acoustics 65, pp 401–419

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Noise arising from changes in fluid flow rate in pipes has been found by empirical studies to follow the 6th power of velocity26

2

1log60VVLp =

, i.e.

Thus if water velocity is doubled the resulting increase in noise could be predicted by calculation as:

dBLp 1812log60 ==

And if water velocity were tripled the increase in noise could be predicted as 28.6 dB. If it is borne in mind that the normal sound level evident in a typical quiet suburban living room is around 40 dBA and the normal sound level in a conversation is around 60 dBA27

, then it seems unlikely that pipework noise is likely to cause undue disturbance at higher velocities than the 1 m/s and 3 m/s limiting values normally applied in design.

It is known that where pipework distribution routes are straight, even at high velocities and turbulent flow conditions, the noise resulting from fluid flow is relatively insignificant28

. However, where there are changes of direction such as elbows and tees or restrictions to flow such as valves (especially electronic and pneumatic rapid closer types) and diameter reducers, the flow becomes more turbulent and both vibration and noise increase. In a sharp bend, where turbulent flow exists, water will sometimes re-circulate (see fig 3 below) and this contributes not only to noise, but pipework erosion.

Fig 5 Representation of water flow paths at a sharp bend Increased turbulence at fittings may produce local noise up to 10dB higher than the noise generated in straight pipe runs28. The noise nuisance created by this may be tolerable, however, but erosion due to such phenomena as recirculation (refer to figure 5) and cavitation then become a problem (see section 6). 5.1 Attenuation of pipework noise

26 Sound Research Laboratories Ltd (1988) ‘Noise Control in Building Services, First Ed’, Pergamon Press 27 McMullan R (2007) ‘Environmental Science in Buildings’ 6th edition, Palgrave McMillan. 28 Sharp B & Sharp D (1984) ‘Water Hammer: Practical Solutions’, Halsted Pr

Equation 2

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Where pipework noise is expected and control of such noise is desirable, attenuation may be achieved by the use of certain techniques. A simple method, often utilised, is to suddenly increase pipe diameter, so drastically reducing velocity and turbulence, thus providing attenuation using the same principle as that applied to road vehicle exhaust pipes. This, however, rather negates the idea of increased water velocities being used to control biofilm! Insulation of pipework systems to contain noise radiation is not easy to achieve, though the thermal insulation which is routinely installed on all pipework will of course provide some degree of noise insulation. To effect noise insulation at high frequencies a lagging technique may be utilised: steel sheeting wrapped around the mineral wool thermal insulation is found to offer in the region of 10 dB of insulation at a frequency of 1 kHz29

29

. At lower frequencies, however, useful insulation can be achieved only by use of very heavy and expensive lagging sheet or a separately supported enclosure. The attenuation provided by this pipe lagging technique is significantly reduced, however, if the external cladding is connected to the pipe wall at any point . Noise resulting from the flow of water in pipes may be transmitted to occupied spaces in a building if the pipes are in direct contact with large radiating surfaces, such as walls, ceilings, and floors. Isolation of pipework runs from the structure thus provides significant noise reduction. A reduction of 10 to 12 dB may be obtained if pipework is isolated from the building structure using flexible, vibration absorbing mountings rather than rigid brackets, and if appropriate resilient material such as neoprene is used to sleeve pipes and seal around wherever they pass through structural members29. 6 Erosion of pipework materials 6.1 Cavitation Cavitation is an occurrence in plumbing systems having high water velocities, usually in the region of a flow restriction such as a va lve. A restriction can cause a condi tion whereby the local pressure of the water upstream drops to a level below the water’s vapour pressure (around 18 kPa for water at 16ºC). When this occurs the water attains a pressure low enough to support vapour bubbles. Immediately after the flow restriction, pressure suddenly increases resulting in the rapid collapse of the vapour bubbles, and thence extreme local pressure fluctuations. This phenomenon usually results in increased vibration, high noise levels and damage to pipework or valve materials. Typically, cavitation can arise in plumbing systems immediately downstream from a partially open valve or a sharp change in direction. If the vapour bubbles are near to or in contact with the pipe wall when they implode, the forces exerted by the water rushing into the cavities can result in the removal of small quantities of pipework material (termed “pitting”), leading eventually to failure, in addition to the problems of vibration and noise.

29 Van Houten J (2003) ‘Control of Plumbing Noise in Buildings’ Plumbing Systems & Design, http://www.ctaspe.com/docs/techarticles/Plumbing%20noise.pdf

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The average flow velocity in a 90o elbow bend in a piping system is actually about the same as that in a straight pipe. However, because the bend causes excitation of the flowing particles leading to random high speed motion and re-circulation, the region on the inside of the bend has a much higher velocity and there can be a tendency to cavitation and consequently noise and erosion30

.

Manufacturers of components such as pumps and valves take account of cavitation when designing these items and offer guidance to installers of such items. Cavitation noise is typically in the mid to high frequency range, and increases with the 12th power of velocity26, i.e.

2

1log120VVLp =

So, for example if velocity is doubled, the increase in noise level can be predicted by calculation as follows:

dBLp 3612log120 ==

Thus, the noise generated is considerable as is the danger of pipework erosion and eventual failure. Careful design of systems may, however, be sufficient to alleviate cavitation problems. Techniques used include: • Restricting locations where changes of direction are allowed – before a valve or

flow restriction, a region of 10 times the pipe diameter is avoided, and after a valve a region of 20 times the pipe diameter is avoided.

• Avoiding low pressure areas by increasing the total or local pressure in the system. • Locating the item concerned at a low point in the system – where it is not possible to

increase total static pressure, local static pressure in components may be increased by lowering the component. T hus control valves and pumps are generally positioned in the lowest part of systems to maximize available static head.

• Reducing water temperature – since vapour pressure is dependent upon temperature

the likelihood of cavitation increases dramatically with higher water temperatures. • In addition it is good practice to provide adequate pipework support using materials

with a suitable stiffness. 6.2 Erosion corrosion Erosion corrosion (also known as impingement damage) is the corrosion of a pipework material caused or accelerated by the motion of water against the internal pipe walls, and is a particular feature of high velocities in plumbing systems. It is characterized by surface features with a directional pattern (comet tails, horseshoe marks, etc – see figure

30 Sharp B (1981) Water Hammer: Problems and Solutions, Hodder Arnold

Equation 3

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4 below) and is particularly prevalent in copper tube, an extremely common pipework material in water supply services. Copper pipe installed in a plumbing system very quickly forms an internal film of oxide, which protects the copper from further corrosion, and this is partly what makes copper such an ideal pipework material. However, where cavitation or extreme turbulent motion is present, this can remove the protective film and expose bare metal. The exposed surface quickly corrodes and the resulting oxide is in turn eroded away, leading to rapid failure of the pipework or plumbing component31

.

Fig 4. Cutaway of copper pipe showing typical erosion corrosion damage (Source: Roberts 2008)

Fig 5. Diagrammatic representation of typical cause of erosion corrosion in copper pipe (Source: Roberts 2008) As can be seen from figures 4 and 5, a typical cause of erosion corrosion is the over-feeding of soldered joints at installation, which results in an impingement to the water flow. A similar common problem is the failure of installers to remove the internal burrs from the end of a copper pipe before it is inserted into its socket and soldered in place. Consequently increased turbulence and pressure changes leading to cavitation occur downstream of the flow obstruction and the characteristic impingement damage results. Erosion corrosion is known to occur most frequently in pumped circulation hot water distribution systems where the temperature accelerates the process, but it will also occur in cold water distribution systems if water velocities are particularly high. Impingement damage, by its very nature, tends to be highly local in nature and can cause pitting and pin-holing of pipework in quite a short timescale.

31 Roberts C (2008) ‘Water pipe leakage from erosion corrosion’ Subrogator Magazine

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To summarise, increasing water velocity appears to have profound effects on the issues of noise and pipework erosion. However, if it is accepted that increased water velocity is a useful tool for controlling biofilm, then thoughtful design and careful installation will often alleviate these effects. 6 Discussion and conclusion The field of biofilms and the way biofilms interact with hot and cold water supply systems is extremely complex, and this paper has only been able to touch on several of the important issues. Some of the work consulted appears to present some, on the face of it, unhelpful, contradictory views: S ome commentators appear to accept that high velocity flushing is beneficial and may legitimately be used as a technique for inhibiting biofilm formation21, whereas others argue that sudden flow increases or hydraulic disturbances should be avoided since the sloughing off of accumulated biofilm may compromise public health more than if the biofilm were simply left alone22. Bearing in mind the nature of water supply systems, constant high velocities are of course not a viable option. T hus where techniques like variable speed pumping and flushing are employed, measures must be in place to remove sloughed off micro-organisms to prevent this public health risk. This paper has not examined the various types of life which exist in biofilms and the different natures that some of these can exhibit: It is newly postulated, for example, that certain types of bacteria are able to communicate via a capability known as ‘quorum sensing’ and this enables them to find safe locations to colonise21. It has also been observed that water systems with different velocity profiles support different types of microbial life. Those with continuous high velocity flow tend to attract filamentous varieties of micro-organism, especially suited for attachment to surfaces32

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. It is found that at higher flow rates, a denser more tenacious biofilm is formed and as a result these surfaces often appear to be free from foulants, since they are not slimy to the touch . It would be necessary to investigate whether this denser biofilm type is more or less harmful to public health than the type of sticky biofilm seen in lower velocity systems. A deeper understanding of the types of microbe life present in various biofilms and the response of these biofilms to various flow velocities would undoubtedly lead to a better understanding of how velocity may be used to influence the growth and adhesion of biofilms. Further work is necessary to assess whether it is indeed possible to meaningfully inhibit and control biofilm formation by manipulating flow velocities. The case has been examined in a fair amount of detail whether velocities can indeed be increased without undue danger of excessive pipework noise and erosion of pipework

32 Mittelman M (1985). ‘Biological Fouling of Purified-Water Systems: Part 1, Bacterial Growth and Replication’, Microcontamination 3(10), pp. 51-55, 70

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materials. T he conclusion from this part of the work would be that if it is deemed necessary or desirable to increase velocities as part of a biofilm control regime, then it would be possible to achieve this as long as the careful design considerations outlined earlier were followed. This, which could also result in smaller pipe sizes, together with the technique of variable speed pumping, could be considered a step in the right direction as far as sustainability is concerned. 7 Presentation of Authors (1) Paul Angus BEng (Hons), I Eng, FCIPHE, ACIBSE, MSoPHE (2) Steven Ingle DSc, MSc, I Eng, FCIPHE, FSoPHE, ACIBSE, LCG, RPP. (3) Derek King MPhil, BEng (Hons), Cert Ed, CEng, MCIBSE (4) R J Turner MSc, C.Env, MCIWEM, FIHEEM. John Turner is an independent consultant engineer presently working with WSP Consultants Group in Leeds, UK. John is involved in the design of public health services which include hot and cold water, above and below ground drainage, and recycling and treatment systems for wastewater.

Steve Ingle is an independent consultant engineer presently working for Ingle Project Design Consulting Engineers, UK. Steven is involved in the design of all types of Building Services Engineering on commercial and Industrial projects in the UK and Internationally. Paul Angus is a Senior Public Health engineer

working with WSP Group, based in Edinburgh, Scotland. P aul’s project experience includes developments throughout the UK, and he has overseas projects experience. He promotes a s ustainable approach to public health engineering design, including water

conservation and recycling, pollution control and waste management and treatment. Derek King is a senior lecturer in the School of the Built Environment at Liverpool John Moores University, being programme leader for degree programmes in Building Services Engineering. Among his research interests are the practicalities of applying sustainability to public health engineering systems.

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