High Velocity Flushing

Optimal performance and longevity for your Horne thermostatic mixing products is a key priority for us.  It is designed in; material selection, thermostatic accuracy and reliability, ease of installation and maintenance, alongside low cost spare parts and free maintenance training.

One often-overlooked accessory, however, that plays a critical role for a long and reliable operational life, is our Flushing Kit. It's a small investment, but is invaluable, especially when you consider the comparative cost of a quantity of Horne showers, Optitherm taps or mixing valves, and then consider the improvements it can also bring to your water system. Just one or two kits are needed per site, and its regular use ensures the system operates at peak efficiency from first commissioning; removing contaminants, thus protecting precision components, but also for managing biofilm and maintaining water quality. Read on to understand why now may be the time to invest in a Flushing Kit for every installation.

What is high velocity flushing?

Elevated or high velocity flushing involves the use of high-speed water flow to clean the internal surfaces of plumbing systems. It should not be confused with a daily or weekly Duty Flush, via the tap / shower fitting, which simply runs water (at the rate governed by any outlet flow regulator) to drain, to reduce stagnation. Instead, this mechanical cleaning method bypasses the thermostatic mixing or shower valve, and helps to remove debris, particulate matter, scale, biofilm, and other contaminants that accumulate within the pipes.

The benefits of this method include:

1. Mechanical Disruption: High velocity flushing increases shear forces at the pipe walls, a change that can physically remove accumulated inorganic and organic matter from pipework.
2. Enhanced Water Quality: By removing biofilm, sediments, corrosion products, mineral deposits and other contaminants, high velocity flushing helps maintain high water quality, clean the system, reduce the risk of damage to downstream components and manage the risk of pathogen growth and contamination.
3. Prevention of Biofilm Reformation: Regular flushing removes corrosion and precipitation products and deters the re-establishment of biofilm, helping maintain the long-term cleanliness of the water system.

It's a crucial step in the commissioning process of new plumbing installations and is required by BS EN 806 and BS 8558. High velocity flushing must be performed before commissioning any thermostatic mixing or shower valve (or any other precision plumbing component) to ensure the system and its components perform and operate optimally.

Mechanical Cleaning of Pipe Surfaces

The primary function of high velocity flushing is to mechanically clean the internal surfaces of pipes. When a new system is constructed, sediments and particulate matter, swarf and other construction compounds may enter the pipe work of the water system, posing the risk of damage to downstream fittings and precision components.  Once the building is occupied and the water system in use, corrosion products can also accumulate as sediments, providing a key location for biofilm attachment and development over time, leading to reduced water quality and flow efficiency. High velocity flushing, introduces a temporary hydraulic regime that is a considerable departure from flow conditions of normal water usage. 

As a long-established and traditional method for mechanically cleaning inaccessible internal pipe surfaces, the change in water velocity increases shear forces at the pipe walls causing mobilisation of adhered materials and accumulated sediments.  Scouring and dislodging, materials are moved into suspension in the passing water and can then be flushed away to drain.

Commissioning new / refurbished water systems

BS EN 806-4 and its complementary guidance BS 8558, require a cleansing flush, with wholesome water, of new domestic water system (DWS) pipework after a new installation, and before required commissioning of any specialist componentry. This also applies to existing water systems that have had componentry and pipework replacements, or those that have been substantially altered or refitted, and is required to eradicate any new contamination introduced during those works. Systems with historically reduced, low or no usage (for example, during Covid-19) are also recommended for recommissioning, including a cleansing, high speed flush. Specific precautions and guidance are:

1. A minimum water velocity of 2 meters per second should be reached during the flushing process, to ensure effective removal of debris and contaminants generated or introduced during the new or refitted installation.  Where there is heavy contamination or particulate matter, velocities may need to reach 4-6 m/s.
2. Suitable precautions and actions should be taken to protect sensitive valves and precision equipment from the flush-mobilised debris.*
3. To maximize water movement, remove any flow regulators and shower handsets that may exert a loss of pressure and flow velocity reduction.*
4. For concealed thermostatic valves and other sensitive valves, adhere strictly to the manufacturer's instructions.

* The design of all Horne thermostatic mixing valves, taps and showers, uniquely and specifically, assists installers and commissioners to comply easily with the above British Standards.

Catering for both points 2 and 3 above, every Horne design gives access points to both hot and cold water supplies, strategically placed upstream of any component that would contribute to pressure / velocity loss: check valve, mixing valve, flow regulator or flow-limiting shower head, and downstream of the integral isolating servicing valves. The full-bore isolating ball valve and flushing kit adaptor, discharging from that location add no restrictions to achieving a maximum, effective, system flow velocity.

This design configuration and the associated Flushing Kit accessory specifically allows for maximised water flow rates to enhance the removal of sediments and, once operational, erode biomass from internal pipe surfaces, thus ensuring a cleaner and more efficient system.

References:

• BS EN 806-4:2010 Specifications for installations inside buildings conveying water for human consumption. Installation
• BS 8558:2015 Guide to the design, installation, testing and maintenance of services supplying water for domestic use within buildings and their curtilages - complementary guidance to BS EN 806

Impact on biofilm

Following a commissioning, cleansing flush, ongoing DWS water quality can be managed through regular HV flushing - this assists with the control of waterborne and retrograde pathogens that reside in biofilm that inevitably forms on surfaces in continual contact with non-sterile water supplies.

Understanding Biofilm Formation

Biofilm is a complex aggregation of microorganisms marked by the secretion of a protective and adhesive matrix. This slimy layer can form on various surfaces, including the interior of water pipes. Biofilm development occurs in five distinct stages:

1. Initial Attachment: Free-floating, 'planktonic' microorganisms adhere to a surface.
2. Irreversible Attachment: The initial attachment strengthens as cells secrete extracellular polymeric substances (EPS).
3. Maturation I: The biofilm begins to develop a more complex structure as cells multiply.
4. Maturation II: The biofilm matures and becomes more resilient, forming a robust, three-dimensional structure.
5. Dispersion: Cells or clumps of cells detach from the biofilm to colonise new surfaces downstream.

The Bio-Load in Water Systems

Hans-Curt Flemming, a prominent researcher into biofilms, highlights that 95% of a water system's bio-load resides on its pipe walls. Only 5% are suspended in the water and termed as 'planktonic'. This significant concentration of biofilm on pipework underscores the importance of targeted cleaning methods.

Opportunistic Premise Plumbing Pathogens (OPPP)

OPPP are a group of pathogens that thrive within biofilm in plumbing systems, posing health risks in institutional and commercial settings. Studies have identified several OPPPs, including:

• Legionella pneumophila: Known for causing Legionnaires' disease, it thrives in stagnant and warm water environments like those found in hot water systems.
• Pseudomonas aeruginosa: Common in hospital environments, and in particular at the water system periphery by retrograde contamination, this pathogen can cause severe infections, particularly in immunocompromised individuals.
• Non-tuberculous mycobacteria (NTM): These bacteria are resilient to disinfectants and can cause lung infections, especially in people with pre-existing lung conditions.

Low flow fittings - unintended consequences?

The trend towards using lower flow fittings to conserve water can create challenges, especially in older water systems that were originally designed for higher demand and flow rates.

In institutions like hospitals, where plumbing systems are often extensive and aged, slower flow rates can lead to benign hydraulic regimes. These conditions favour additional build-up of biofilm, necessitating more frequent high velocity flushing to maintain water quality. 

Similarly retrospective low flow rates can also significantly impact drains designed for higher flow rates - making it harder for accumulating mass to move through the drainage system. The unintended consequence of subjecting domestic water and drainage systems to a lower flow regime than originally designed for leads to increased biofilm development, which may be easily disturbed and aerosolised with each use of an upstream tap. Consequently, this also exacerbates the risk of retrograde contamination in the ward environment and the vulnerable water system periphery.

HV Flushing, however, can also periodically help to move accumulations within the drainage system.  It therefore provides a dual benefit of cleaning by mass transfer, both the water and drainage systems... a win-win situation.

The incompetency of chemicals

Chemical treatments are most effective for managing the free-floating, planktonic bacteria within a water system.  As established by researcher Flemming, however, this affects such a small portion of the system load (only 5%) that it actually does more harm than good.

Chemical treatments aimed at controlling biofilm growth in water systems fall short of their intended goals due to several inherent and resilient properties of biofilms. Understanding these characteristics highlights the advantages of alternative methods, such as high velocity flushing, which physically removes biofilm as a bulk material transfer.

Protective Biofilm Layer

As mentioned above, biofilms are complex matrices of microorganisms embedded within an extracellular polymeric substance (EPS). This EPS layer acts as a protective shield, significantly limiting the penetration of chemical agents.

• Limited Penetration: The EPS matrix restricts the diffusion of antimicrobials, resulting in lethal concentrations only reaching the outer layers of the biofilm. In more developed, aged or fast growing, biofilms, the older layers will receive a sub-lethal and declining dose of chemical, allowing it to survive and potentially develop resistance mechanisms​. 
• Heterogeneous Structure: The biofilm's heterogeneous nature, with varying densities and microenvironments, complicates the effective distribution of chemical agents across the biofilm.

Promotion of Antimicrobial Resistance (AMR)

Chemical treatments mostly fail to remove biofilms and inadvertently promote antimicrobial resistance:

• Sub-lethal Doses: Bacteria exposed to sub-lethal concentrations of antimicrobials can develop resistance mechanisms, making them more resilient to future treatments​.
• Resistance Mechanisms: These include:
  • Efflux Pumps: Proteins that actively pump antimicrobial agents out of bacterial cells, reducing their efficacy.
  • Plasmid Transfer: Horizontal gene transfer of resistance genes via plasmids allows bacteria to share resistance traits.
  • Enzyme Formation: Production of enzymes that break down / deactivate antimicrobial agents, such as β-lactamases​ or carbapenemases.

Unintended Consequences

Chemical treatments can lead to several unintended ecological and health consequences:

• Species-Specific Effects: Chemical treatments may disproportionately affect certain bacterial species, disrupting the natural microbial balance and potentially allowing more pathogenic species to dominate. This can clear out natural competition, providing a niche for more harmful bacteria to actually thrive.
• Nutrient recycling: The dead remain. Without an additional method to remove susceptible dead cells, their remaining mass/material can provide a nutrient source for surviving bacteria within the biofilm.
• Pathogenic Transformation: Initially benign bacteria within a biofilm can acquire pathogenic traits through horizontal gene transfer, posing future health risks​.

Indiscriminate cleaning, no chemical residues

High velocity flushing offers a more effective and comprehensive alternative by physically removing biofilm:

• Bulk transfer: Flushing dislodges and erodes excess biofilm (stages 3-5), irrespective of pathogen species, dead cells and the EPS matrix - significantly reducing the overall bio-load, and minimising shedding (and downstream seeding) potential.
• No Chemical Residues: This method avoids the use of costly consumable chemicals, preventing the discharge of potentially harmful substances into the environment. The flushed water, being potable, does not adversely affect environmental water quality or contribute to the spread of antimicrobial resistance in natural ecosystems.
• Preventing AMR Spread: With easy removal of the bulk material biofilm to drain, regular high velocity flushing can reduce residency time and limit the risk of resistance mechanisms being shared between bacteria species.

Is it time to invest in a Flushing Kit?