In commercial HVAC and industrial process cooling, water is at the centre of the system. The quality of that water determines its longevity and health. For many facility managers and building owners, water treatment is often viewed through the lens of chemical dosing alone. However, chemicals can only do so much when the water is impacted by corrosion by-products and sludge. Side stream filtration (SSF) ensures that your water remains clear and your equipment stays protected.
At ClearFlow Water Treatment, we believe that understanding the mechanics of your water system is the first step toward optimising it. In this guide, we will explore why side stream filtration is an important pillar of water management.
What is side stream filtration?
To understand side stream filtration, we must first look at how primary water systems operate. In a typical closed loop heating or cooling system, 100% of the water circulates through the chillers, boilers, and heat exchangers.
Side stream filtration for closed loop systems is a method of continuously cleaning the water within a heating or chilled system without interrupting its operation.
In a closed loop system, a small portion of the circulating water (typically 5–10%) is diverted through a filtration unit (the ‘side stream’) and then returned to the main system. Over time, this process removes contaminants from the entire system volume.
Although closed systems are sealed, they are not completely free from contamination. Side stream filtration helps remove these impurities before they can cause problems.
Types of contamination build up
1. Corrosion products
The most common culprit is magnetite (black sludge). This occurs when internal steel components react with oxygen. These fine, heavy particles are highly abrasive and settle in low-flow areas, creating a thick, insulating sludge that chokes heat transfer. Magnetite is particularly troublesome because it is magnetic and tends to clump together, sticking to the surfaces you need to keep clean.
2. Dirt and debris
New systems often contain welding slag and casting sand left over from the manufacturing and installation process. Even in established systems, every time a seal is replaced or a minor leak is topped up, new particulates and oxygen are introduced. Over time, these small introductions add up to a significant debris load.
3. Scale formation
Even though a system is ‘closed’, it still contains water that originally had dissolved minerals, mainly calcium, magnesium and bicarbonates. These are naturally present in mains water, especially in hard water areas.
These hard deposits attach to heat exchanger surfaces, acting as a thermal barrier.
4. Biological fouling
Bacteria such as Pseudomonads thrive in the dark, warm environments of closed loop systems. They produce a sticky slime known as biofilm. Once a biofilm is established, it becomes difficult to remove with chemicals alone, as the outer layer of slime protects the bacteria from the biocide.
How the side stream filtration process works
The beauty of side stream filtration lies in its simplicity. It works by continuously extracting, cleaning, and returning a portion of the system water, gradually improving overall water quality without stopping the system.
Step 1: Controlled diversion
A small portion of the circulating water is diverted from the main pipework to the filtration unit. Typically, 5 – 10% of total system flow is used for side stream filtration. Diversion is carried out via a branch connection on the main flow or return pipe, fitted with isolation valves to allow for maintenance. Sometimes a dedicated side stream pump is used if the system’s main circulation doesn’t provide sufficient pressure. Only a small portion of system water is treated at a time to avoid overloading the filter, while ensuring gradual cleaning of the full system volume.
Step 2: High-efficiency removal
The water diverted from the systems enters the filtration unit, which removes impurities. The type of filtration required depends on the system’s specific needs and contaminants to be removed. We utilise different technologies, from centrifugal separators that use cyclonic action to spin heavy particles out of the water, to magnets and cartridge filters to capture magnetite (iron oxide sludge) and incredibly fine particles, often down to one micron. Clean water then exits the filter back to the system.
Step 3: Continuous polishing and return
Unlike a one-off system flush, which only cleans the system at a single point in time, SSF runs 24/7. This is crucial because contamination is a dynamic, ongoing process. As corrosion continues and water is added, the filter is always there to catch the new arrivals. The cleaned water is then returned to the system, gradually lowering the overall particle count of the entire system volume.
Supporting chemical treatment performance
There is a powerful connection between mechanical filtration and chemical dosing. One clears the path so the other can do its job effectively.
Chemical inhibitors and biocides are added to your water to act as a protective shield for the pipes and to keep bacteria at bay. However, when water is clouded with suspended solids and debris, these chemicals are forced to react with that floating waste first.
Furthermore, when heavy debris settles at the bottom of a pipe, it creates a physical barrier. This prevents corrosion inhibitors from touching the metal underneath, which can lead to corrosion under the layer of sludge, resulting in eventual leaks.
By continuously removing these solids, side stream filtration ensures your chemical treatment is focused exactly where it belongs. This leads to a more stable environment, often allowing for lower chemical usage and significantly better long-term protection for your entire system.
Benefits of side stream filtration for system hygiene
Standard circulation allows water to become turbid and cloudy, catching only large debris, which leads to a gradual decline in energy efficiency and high risks of biofilm. In contrast, side stream filtration maintains clear water by removing fine particles down to generally one micron.
1. Enhanced heat transfer and energy savings
Closed loop systems are designed for maximum thermal conductivity. Even a 0.5mm layer of scale or sludge can reduce heat transfer efficiency by as much as 10% to 15%. By keeping heat exchange surfaces clean, SSF ensures the system operates at its peak design efficiency, directly lowering energy consumption.
2. Reduced microbiological contamination
Magnetite sludge, rust, and other debris provide surfaces and nutrients for bacteria. Biofilms can form on these surfaces, protecting microbes from chemical treatment. By removing the organic matter and the silt that bacteria hide in, you make the environment far less hospitable for colonies of bacteria, including Pseudomonads.
3. Extended equipment lifespan
Modern boilers and chillers feature narrow waterways and high-efficiency heat exchangers. These are incredibly sensitive to debris. Sludge and magnetite can cause abrasive wear on pumps, valves, and other components. By continuously removing these contaminants, side stream filtration reduces mechanical stress, extending the lifespan of system equipment and preventing premature corrosion.
4. Lower maintenance and operational costs
Dirty systems often require reactive labour-intensive maintenance, like flushing, chemical cleaning, or component replacement, such as mechanical seals on pumps. Side stream filtration minimises the buildup of deposits, reducing the frequency of these interventions and associated downtime.
Integrating side stream filtration into ongoing water management
It is important to remember that side stream filtration does not replace other water treatment methods. Instead, it is a vital component of a holistic water hygiene strategy.
A professional water management plan should also include:
- Initial cleaning and flushing for new or heavily fouled systems to remove the bulk of existing debris.
- Chemical dosing to provide a protective film on metal surfaces and prevent biological growth.
- Regular testing of TSS (Total Suspended Solids), iron levels, and bacterial counts to ensure the side stream filtration is performing as expected and the system remains in balance.
How we can support side stream filtration installation
At ClearFlow Water Treatment, we provide a turnkey service to ensure your side stream filtration is perfectly matched to your system needs.
We begin by analysing your current water quality. By measuring the total suspended solids (TSS) and identifying the types of contaminants present, we can determine the exact filtration technology required for your closed loop system.
We design the filtration loop to fit your physical space and system requirements, ensuring the unit is easy to access and maintain. We look at flow rates, pressure drops, and power requirements to ensure a seamless fit.
Our specialist engineers then manage the installation process to minimise disruption to your operations. We ensure the system is commissioned correctly, with flow rates balanced for maximum filtration efficiency.
A filter is only effective if it is maintained. We provide ongoing support, including performance reporting. We ensure that your investment continues to deliver energy savings and system protection year after year, aligning with industry guidance such as BSRIA BG 29 and BG 50.
Contact us to request a complimentary site survey to determine if side stream filtration is the best solution for your system.
