Cooling Towers: Legionella Prevention and Biocide Replacement with UV-C

Evaporative cooling towers are among the highest-profile Legionella risk systems in industry. They aerosolise warm water and can disperse Legionella bacteria over considerable distances. UV-C disinfection of the recirculating water is increasingly used to reduce — and in well-designed systems, sharply cut back — the chemical biocide load. This article covers the regulatory framework, how UV-C compares with classic biocides, the cost picture, and practical lamp selection.

Regulatory Framework (Germany / EU)

42nd BImSchV (Germany, in force since 2017)

The 42nd Federal Immission Control Ordinance applies to evaporative cooling systems, cooling towers and wet scrubbers. It imposes binding operator duties:

• Registration of the installation with the competent authority.
• Regular microbiological testing — general bacterial count and Legionella, by an accredited laboratory, typically every three months.
• Defined response measures when threshold values are exceeded.
• Documentation of operation, inspections and corrective actions.

VDI 2047 Part 2 (technical guideline)

VDI 2047 Part 2 — the "VDI Cooling Tower Code of Practice" for open recooling systems — gives the technical detail behind the 42nd BImSchV. It requires a documented risk assessment, periodic cleaning and disinfection, microbiological monitoring, and measures to prevent biofilm and deposits.

Legionella Threshold Values

The 42nd BImSchV (Annex 1) defines a three-tier scheme for Legionella spp. in the process water:

Note: a frequent misconception is that 1,000 CFU/100 ml already triggers an immediate shutdown. It does not — 1,000 CFU/100 ml triggers immediate corrective measures, while shutdown belongs to the action-value tier of 10,000 CFU/100 ml. The separate 1,000 CFU figure for make-up water should not be confused with the process-water tiers above.

Why UV-C Is Used Instead of (or Alongside) Classic Biocides

Classic Biocides in the Cooling Circuit

Cooling-water biocides fall into oxidising and non-oxidising groups:

• Chlorine dioxide (ClO2) — oxidising, effective against Legionella and biofilm, but usually requires on-site generation.
• Bromine — oxidising, continuous dosing, generally more expensive to run than chlorine.
• Peracetic acid — oxidising, breaks down to relatively benign products, but aggressive towards some system materials.
• Isothiazolinones — non-oxidising synthetic biocides.
• Copper/silver ionisation — used in some systems; subject to discharge and approval considerations.

Industry guidance (e.g. ASHRAE 188, CTI, AWT) commonly recommends combining an oxidising and a non-oxidising biocide for robust microbiological control.

Pain Points of a Purely Chemical Programme

• Running cost — dosing pumps, chemical resupply, wear parts.
• Biofilm penetration — biofilms need substantially higher biocide concentrations than planktonic cells because of their protective matrix.
• Discharge and approvals — chemical carry-over into wastewater.
• System ageing — aggressive chemistry can attack seals and metals.
• Handling — trained personnel for chemical storage and dosing.

UV-C as a Disinfection Layer

• Physical inactivation — UV-C (254 nm) inactivates microorganisms by damaging their DNA; no chemical is added to the circuit.
• No microbial resistance — unlike chemical biocides, there is no known development of microbiological resistance to UV-C radiation.
• Continuous operation — the lamp runs whenever water flows through the reactor; no batch dosing cycle.
• Lower handling burden — no chemical store, no dosing-pump maintenance.

Important limitation: UV-C acts only as water passes through the reactor and has no residual effect in the bulk water or on tower surfaces. It therefore does not by itself eliminate established biofilm. Documented best practice treats sidestream UV-C as the primary continuous disinfection layer combined with a much-reduced secondary biocide, rather than as a complete chemical replacement.

Cost Structure: Chemical Programme vs. UV-C

The two approaches differ mainly in where the cost sits, not only in how much it is. A chemical-dosing programme is comparatively light on up-front capital (dosing pump, tank, instrumentation) but carries a recurring chemical spend, chemical-handling overhead and potential discharge-approval cost. A UV-C installation is more capital-intensive up front (reactor, lamps, ballasts, installation) but its recurring cost is essentially limited to periodic lamp replacement and quartz-sleeve maintenance, with no recurring chemical purchase.

A hybrid programme — UV-C as the continuous layer plus a reduced secondary biocide — shifts the balance: it lowers chemical consumption and handling relative to a pure chemical programme while keeping a residual chemical capability for peak events.

The economic break-even depends strongly on circuit size, water quality, chemical prices and local discharge rules, and should be evaluated per installation rather than assumed from a generic figure.

Important: UV-C does not replace mechanical cleaning or microbiological monitoring — both remain mandatory under the 42nd BImSchV and VDI 2047 Part 2. What UV-C can do is reduce dependence on continuous chemical biocide dosing.

Lamp Selection in Practice

• Low-pressure 254 nm lamps in IP68 quartz immersion tubes — the common industrial choice: economical and robust in water.
• High-output amalgam 254 nm lamps — higher UV output per lamp, suited to larger basins or heavily loaded water.
• Medium-pressure lamps — broadband UV that penetrates deeper, relevant mainly in special cases such as high turbidity or persistent biofilm problems; less energy-efficient.
• UV-LED — at present not powerful enough for cooling-tower basin duty; more relevant to point-of-entry drinking-water applications.

Because suspended solids and turbidity reduce UV penetration, adequate pre-filtration is important for reliable sidestream UV-C performance.

Cross-References

• UV Economics and ROI — general ROI framing
• Material Pitfalls in Practice — IP protection, ballast placement, quartz-sleeve cleaning
• UV Lamp Technology — low-pressure, amalgam and medium-pressure lamp types
• Standards and Certifications — norms and certification context

Sources

• 42nd BImSchV — Ordinance on Evaporative Cooling Systems, Cooling Towers and Wet Scrubbers (Germany), incl. Annex 1 threshold values.
• VDI 2047 Part 2 — Open recooler systems: securing hygienically sound operation of evaporative cooling systems (VDI Cooling Tower Code of Practice).
• LAI Auslegungsfragenkatalog zur 42. BImSchV — interpretation guidance on the threshold tiers.
• Water Technology — UV Disinfection of Cooling Tower Water (industry article on UV-C scope and the no-residual-effect limitation).
• enviolet — Disinfection of Cooling Towers (UV-C as a biocide-reduction measure; resistance discussion).
• Wallenius Water — Industrial UV prevents Legionella in cooling towers (UV-C suppression of microbial growth in cooling circuits).