Retrofitting side stream filtration to existing systems

Before selecting equipment, a thorough assessment of the existing system capacity and water condition is vital. Unlike primary filtration, side stream filtration operates by continuously diverting a portion of the circulating water—typically 5% to 15%—through a high-efficiency filter. To size the unit correctly, engineers must calculate the total system volume. If original O&M manuals are unavailable, volume can be estimated based on the pipework diameters and radiator/fan coil counts, or by measuring the chemical dilution rate.

Consultants should refer to BSRIA BG50 (Water Treatment for Closed Heating and Cooling Systems) which emphasises that mechanical filtration is as critical as chemical dosing. In retrofit scenarios, the goal is often 'remediative' rather than just 'preventative'. This means the initial filtration load will be significantly higher than in a new build. Sizing should also account for the required turnover rate; a standard benchmark is to ensure the entire system volume passes through the filter every 24 hours.

The physical connection of the filtration unit to the existing network is critical for performance. In a retrofit context, connecting across the main flow and return headers is common, but this relies on the system’s differential pressure, which may fluctuate with variable speed pumps. For consistent results, a UKGP side stream filtration skid with an integral pump is preferred. This ensures the filter operates at a constant flow rate irrespective of the main system's hydraulic state, providing reliable 'round-the-clock' cleaning even when the main pumps are modulation down.

Connection points should ideally be located in the plant room on the main return pipework, prior to the boilers or the UKGP plate heat exchanger. This protects the most expensive primary assets from incoming debris. Use 'wet tapping' or 'hot tapping' techniques if the system cannot be drained down, though most commercial retrofits are scheduled during low-demand periods where a partial drain-down of the plant room headers is feasible. Ensure that the take-off and return points are spaced at least 500mm to 1000mm apart to prevent 'short-circuiting' of the filtered water.

When specifying the unit for a retrofit, a dual-stage approach is the industry gold standard. Magnetite, a byproduct of internal corrosion in steel pipework, is often the primary culprit for seized pump rotors and blocked control valves. Therefore, the unit must incorporate high-intensity magnetic rods within the flow path. These rods should be easily removable for cleaning without requiring a complete system shutdown.

Following the magnetic stage, a physical bag filter captures non-magnetic particles such as scale, weld slag, and Bio-wrap. For a heavily contaminated system, engineers should start with a 50-micron bag to avoid immediate blinding, and progressively move down to a 5-micron or 1-micron bag as the water clarifies. Specifying a unit with clear pressure gauges on the inlet and outlet allows facility managers to monitor the 'dirty' status of the filter, ensuring maintenance is proactive rather than reactive.

Effective side stream filtration is part of a wider ecosystem of water quality management. During a retrofit, it is common to find that existing air removal is inadequate. Micro-bubbles contribute to ongoing corrosion, which produces the very magnetite the filter is trying to remove. Incorporating UKGP air & dirt separators alongside the filtration skid ensures both dissolved gases and heavy particles are addressed, creating a comprehensive treatment strategy.

Furthermore, once the mechanical filtration has removed the bulk of the 'sludge', the system's chemical balance must be restored. A UKGP chemical dosing pot should be installed to allow for the easy introduction of corrosion inhibitors and biocides. Removing the debris often reveals the true state of the system chemistry; with the solids gone, chemicals can work more effectively to passivate the metal surfaces and prevent further degradation. Balancing mechanical removal with chemical protection is the only way to ensure long-term system stability.

The first 48 hours after commissioning a retrofit side stream filter are critical. In a dirty system, the filter bag may reach capacity within hours. Maintenance staff should be briefed to check the differential pressure gauges frequently during this period. As the system cleans up, the intervals between bag changes will lengthen. It is a common mistake to remove the filter once the water 'looks clear'; however, microscopic particles continue to cause erosive wear, so the unit should remain in continuous operation.

Performance should be validated through periodic water analysis in accordance with BSRIA BG50 guidelines. By tracking the reduction in Total Suspended Solids (TSS) and iron levels, M&E contractors can provide tangible evidence of system improvement to the end client. Not only does this protect the plant, but it also frequently results in lower energy consumption, as clean heat exchanger surfaces and improved flow characteristics allow the system to operate much closer to its original design efficiency.