A Dental Guide to RO: Disinfection, Sterilisation, and Inactivation

Date: 4/12/2024
Read Time: 6 minutes
Author: Dr Surani McCaw, B.E. (Chemical), Ph. D.

Key points:

• Disinfection, sterilisation, and inactivation are distinct processes, each with specific applications and limitations.
• Semi-critical dental items are considered low risk only when sterilised using heat or high-level disinfection
• Proper RO plant design is crucial for preventing biofilm formation and enhancing the effectiveness of sanitisation methods.
• While monitoring is essential, it should not be the sole reliance for assessing system performance. A robust design is the foundation of long-term microbial control.
• Consulting with qualified professionals is vital to ensure appropriate DSI practices and compliance with relevant standards.

In the realm of dental practice, adhering to a rational approach to disinfection and sterilisation of reusable patient care equipment and/or devices, such as the Spaulding classification scheme is paramount. The scheme divides reusable patient care items into three distinct categories: critical, semi-critical, or non-critical based on the risk of infection transmission, also as outlined in “Guidelines for infection control in dental health-care settings” published by US Centres for Disease Control and Prevention in 2003 and AS 5369:2023.

Semi-critical items are considered low risk only when sterilised using heat or high-level disinfection. Ensuring this level of infection control demands a rigorous approach to disinfection, sterilisation, and inactivation (DSI) procedures within RO systems. Although often used interchangeably, these terms refer to distinct processes, each with specific applications and limitations. This article delves into the complexities of DSI, emphasising the importance of proper RO system design, the integration of appropriate DSI processes, and ongoing maintenance to achieve optimal microbial control in dental settings.

In my previous article, “AS 5369:2023 – Safeguarding Smaller Healthcare Facilities,” I stressed the importance of choosing the right disinfection processes and maintaining consistent routines to meet microbial safety standards in healthcare.

Microbes exist in two main forms, planktonic (free-floating in fluids) and bound to surfaces within biofilms. The choice of sanitisation method, as well as its effectiveness, depends on the microbial state. In reverse osmosis (RO) systems, most bacteria exist as biofilms, with their extent influenced by the hydraulic conditions within the plant.

Sites for microbial attachment and biofilm formation often arise from issues such as substandard process design, poor material selection, or the degradation of pipework due to corrosion or pitting caused by chemical changes in the water. The condition and configuration of the RO unit itself can significantly impact the effectiveness of sanitising agents and methods. Therefore, it is vital to carefully plan, procure, commission, and operate systems to optimise sanitisation and prevent conditions that encourage microbial growth.

In the white paper “Disinfection, Sterilisation, and Inactivation – Not Knowing the Difference Could Cost You a Life”, we explain that disinfection is just one method of sanitisation. Other common sanitisation methods include sterilisation and inactivation, each with distinct outcomes. The paper also describes the sanitisation terms, methods and the outcomes:

• Disinfection: A process that eliminates vegetative forms of microorganisms, excluding spore forms, with low levels of viable microorganisms potentially remaining. Common methods in healthcare and pharmaceuticals include water temperatures between 80°C and 90°C, and chemicals like peracetic acid or ozone.

• Sterilisation: A process that completely eliminates or destroys all forms of microbial life (both vegetative and spore forms). Steam is a commonly used method for sterilisation in healthcare and pharmaceutical settings.

• Inactivation: The process of eliminating microbial replication without necessarily destroying the microorganism’s surface structure. Viable microorganisms may still remain, and DNA may be repaired, potentially making microorganisms viable again. UV light is an inactivation process.

Each method is suited to different contexts and achieves specific results. The white paper includes a detailed table that outlines these methods and their respective applications.

UV light is commonly used in dental facilities and day hospitals. However, as UV light is an inactivation process, it should never be used as a primary or sole method for controlling bacteria.

• UV only inactivates planktonic microorganisms, not surface-bound microorganisms/biofilm.
• The effectiveness of UV light depends on various factors such as turbidity, particulate concentration, contact time, colour, organic content, water temperature, and the susceptibility of specific microorganisms.
• Any microorganisms that escape the inactivation process can continue to replicate and colonise the downstream plant and equipment.

It is important to note that when using low or medium-pressure UV lamps, their efficiency is significantly affected by the temperature of the water. UV lamp intensity decreases as the water temperature increases. This temperature dependence means that in systems with higher water temperatures, such as those using thermal disinfection or in areas where ambient water temperature can be high, the UV lamps may not perform at their optimal intensity, reducing their effectiveness in inactivating microorganisms. To maintain effective microbial control, proper temperature control and monitoring are essential to ensure that UV systems operate within the optimal temperature range for maximum efficiency.

Several key technical considerations for RO system design, as outlined in the article “AS 5369:2023 – Safeguarding Smaller Healthcare Facilities”, can influence the extent of biofilm formation within the RO plant:

1. Choosing an Appropriate Disinfection Process: Implementing an appropriate disinfection method and routine, is essential for meeting microbial limits in healthcare settings. It’s also important to consider Occupational Health and Safety (OHSE) risks associated with each method.
2. Maintaining Appropriate Water Temperature: Keeping water temperatures below 20°C or above 65°C inhibits bacterial growth. Effective temperature management is crucial for controlling microbial activity.
3. Correct Sizing of Heating Elements: Properly sized heating elements are essential for maintaining supply and return temperatures above 80°C, ensuring effective thermal disinfection.
4. Preventing Temperature Gradients: Incorrectly sized heating elements or lengthy pipe runs can create temperature variations that encourage thermophilic bacterial growth.
5. Temperature Monitoring: For thermal disinfection, maintaining temperatures between 80°C and 90°C for at least 60 minutes is vital to control thermophilic bacteria. A reliable temperature monitoring system is essential to ensure compliance.
6. Pipe Insulation: Proper insulation of ringmain pipes helps reduce heat loss and maintain consistent water temperatures, thereby minimising bacterial growth. However, excessive insulation of components such as the UV unit and endotoxin filter in an RO unit with thermal disinfection can create conditions that promote the growth of thermophilic bacteria
7. Hydraulic Balance: Maintaining adequate water velocity is crucial to prevent stagnation, which can promote biofilm formation. Proper pump selection and pipe sizing should ensure a minimum velocity of 1 m/s, avoiding excessive velocities above 1.5 m/s, particularly during peak operations.

When key design considerations are overlooked in the pursuit of low CAPEX, excessive and uncontrolled biofilm can take hold of the RO plant, significantly undermining the effectiveness of sanitisation methods. Biofilm acts as a protective barrier, shielding microorganisms from chemical or ozone-based sanitising agents, thus reducing their efficacy. Although thermal disinfection is not affected by the shielding effect of biofilm, improper implementation can still promote the growth of thermophilic bacteria. This issue is exacerbated when piping materials, such as thermoplastics, are used, as they do not absorb heat efficiently. Without maintaining temperature between 80 at 90°C at the most vulnerable points in the system, thermophilic bacteria can thrive. Once the system becomes contaminated with such microorganisms, it increases the risk of failing to meet microbial health standards. Prevention is always more effective than reacting to contamination after it occurs.

Biofilm colonisation in an RO plant is a gradual process, and design shortcomings typically do not become apparent within the first 12 months after commissioning. These deficiencies often surface only once the CSSD is fully operational, with most equipment in use and reprocessing loads increased. At this stage, key technical considerations in the RO system design can significantly influence the extent of biofilm formation.

Evidence-based practices suggest that it typically takes more than three years of CSSD operation to properly assess the RO plant’s fitness for purpose. If you aim to secure a performance guarantee for at least 10 years, it is crucial not to ignore the critical design elements that control biofilm formation. Basing your decision solely on cost, without considering these essential factors, is not recommended.

Although monitoring is an essential component of your ‘detective’ controls (along with preventive, reactive, informative, and supportive controls), relying solely on monthly grab samples is not an adequate way to demonstrate that your system is fit for purpose. A compliant grab sample does not guarantee that the system will remain compliant throughout its operational lifespan. Additionally, grab samples should not be taken within 24 hours following a disinfection process.

Typically, a ‘point-in-time’ grab sample measures microorganisms in their planktonic (free-floating) state, capturing culturable organisms, but not viable but non-culturable organisms. Grab samples also do not assess microorganisms within biofilm communities. Depending on the system design, operating conditions, sanitisation method, and frequency of sanitisation, your system may harbour biofilm communities that go undetected by monthly grab samples, leaving the system at risk. Over time, any undetected microbial activity or colonisation could pose a significant risk to your dental facility’s ability to maintain a fit-for-purpose system.

If critical design elements that control biofilm formation are overlooked, but the majority (though not all) of the monthly samples show bacterial concentrations within the limits stipulated in AS 5369:2023, should the facility feel confident in the RO unit provider? The answer is no. While regular sampling is essential, it is not a complete measure of the system’s long-term performance and microbial control.

The pillar of confidence in AS 5369:2023 compliance must always be on the correct design of the RO plant. Given that microbiology is a specialised field, it is essential to consult with qualified and experienced professionals when addressing microbial control.

The importance of carefully designed and maintained RO systems for effective microbial control cannot be overstated in dental settings. Regular disinfection routines and robust system monitoring are necessary, but the long-term performance depends on the system’s design, which must be tailored to prevent conditions that promote microbial growth. By understanding the intricacies of DSI and adhering to best practices, dental professionals can significantly reduce the risk of infection and safeguard the health of patients.