Worked design example: duplex automatic multimedia pressure filters treating mains make-up water for three hospital cooling towers at 45 m³/hr. Turbidity consistently ≤ 1.0 NTU for ACOP L8 and HSG274 Legionella risk compliance, continuous inline turbidity monitoring, automatic DP-triggered backwash.
A regional acute hospital operates three open-circuit cooling towers with a combined peak make-up requirement of 45 m³/hr during summer cooling load. A Legionella risk assessment conducted under HSE ACOP L8 and HSG274 Part 1 identified mains water turbidity spikes as a microbiological risk factor: particulates provide attachment substrate for Legionella biofilm in tower basins and distribution pipework. The Legionella risk control scheme requires make-up water turbidity to be maintained at ≤ 1.0 NTU before entry to any tower basin. The mains supply averages 0.4 NTU but spikes to 2.5–3.0 NTU during post-rainfall events and distribution main disturbances. A multimedia filtration system with continuous automatic backwash is required to guarantee ≤ 1.0 NTU to all three towers at all times, independently of main supply variation.
Why duty/standby? Hospital critical infrastructure requires that cooling tower make-up is never interrupted. A duplex (duty/standby) arrangement ensures that the standby vessel can immediately take over when the duty vessel initiates an automatic backwash cycle. Uninterrupted supply to all three towers is maintained at all times, even during peak turbidity events when backwash frequency increases.
HSG274 Part 1 (Cooling Towers) identifies suspended solids and turbidity as risk factors that promote Legionella colonisation of cooling systems:
Suspended particulate matter provides attachment sites for Legionella and other biofilm-forming bacteria in tower basins, fill packs, and distribution pipework. Turbidity above 1 NTU substantially increases available attachment surface area.
Organic particulate matter and suspended solids quench biocide residuals by reacting with oxidising biocides (chlorine, bromine). Higher turbidity means higher biocide demand to maintain target residuals, and increased risk of sub-lethal concentrations in protected zones.
ACOP L8 (HSG274 Part 1) requires the responsible person to implement a written scheme of control. Where turbidity is identified as a risk factor in the risk assessment, the scheme must specify the treatment and monitoring to control it.
Inline turbidity logging on the multimedia filter outlet with a 4–20 mA signal to the building energy management system (BEMS) provides continuous evidence that the turbidity control measure is functioning. Alarm at > 0.8 NTU, shutdown/divert at > 1.0 NTU.
Media options, duplex arrangements and worked application examples
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