Definition
A slimy layer formed by bacteria, fungi and algae on the inner surfaces of water systems within a polysaccharide matrix; it provides nutrients + protection for Legionella and is up to 1,000× more resistant to disinfectants than free-floating cells.
Detailed Explanation
Biofilm is a slimy layer that bacteria and microorganisms form on the inner surfaces of wet systems in four stages: (1) Attachment — free bacteria stick to the surface (24–72 h); (2) Colonisation — surface colonies form (1–2 weeks); (3) Maturation — a protected 3D structure embedded in a polysaccharide matrix (EPS) (2–4 weeks); (4) Dispersion — fragments break off and seed new biofilm elsewhere. The biofilm layer is 100–500 μm thick, resists flow, and renders most chemical disinfectants 95%+ ineffective.
Why It Matters
Biofilm creates three critical problems in humidification hygiene: (1) DISINFECTANT RESISTANCE — UV cannot reach into the shadow, chlorine does not penetrate the matrix, antibiotic resistance forms under the layer. Legionella is 1,000× more resistant inside biofilm. (2) HEAT-TRANSFER LOSS — biofilm on a heat-exchanger or tank wall acts as insulation, lowering thermal efficiency. (3) PRESSURE DROP + FLOW REDUCTION — pipe ID narrows under biofilm, system pressure drops, nozzle flow suffers. The only effective measure is prevention: continuous flow + RO pre-treatment + UV + periodic chemical cleaning (acid + alkali rotation).
Practical Example
A textile plant in Kayseri saw UV-disinfection efficacy drop from 99% to 70% after 18 months in its evaporative tank; microbiology returned 250 CFU/L of Legionella. NKT inspection found a 2 mm biofilm layer on the inner tank wall. The fix: 2% citric-acid wash + 1% sodium-hypochlorite disinfection + in-tank polish. UV recovered to 99.9% and microbiology returned to zero. A yearly chemical-clean protocol was added.
Engineering Note
Biofilm-prevention protocol: (1) Surface quality — AISI 316L electropolished interior Ra < 0.4 μm; biofilm attaches poorly; (2) Flow velocity > 1.5 m/s — shrinks the laminar layer and adds mechanical shear; (3) Chemical regime — semi-annual acid + alkali rotation (2% citric + 1% sodium hydroxide); (4) Online monitoring — ATP bioluminescence test (10 min) for on-site quick checks; PCR microbiology twice a year. Modern UV reactors integrate ATP sensors for real-time biofilm risk monitoring.
NKT Application Link
In hygienic adiabatic humidification packages NKT applies biofilm prevention as a standard step: AISI 316L electropolished tank + > 1.5 m/s design velocity + annual chemical-clean protocol. The NKT - Climate Track platform tracks recirculation flow + UV-intensity trend + last chemical-clean date together; a biofilm risk index appears in the weekly report.
