UV Disinfection in RAS: Dose, Transmittance, Placement and Lamp Ageing

How UV-C disinfection works in recirculating aquaculture: the dose concept in mJ/cm², published doses for pathogens, the effect of turbidity and UV transmittance, loop placement and lamp ageing.

How UV disinfection works

The FAO guide explains that UV disinfection uses wavelengths that destroy the DNA of microorganisms, targeting pathogenic bacteria and single-celled organisms, and that it does not affect the fish because treatment takes place outside the rearing area. Lazado et al. (2026) add that UV-C forms pyrimidine dimers in DNA or RNA, so organisms are inactivated rather than necessarily killed; the most effective wavelengths are 250–270 nm, and no harmful by-products or residuals for fish have been identified. Low-pressure lamps, the most common type, emit almost all their energy at 254 nm, while medium-pressure lamps emit a broad 220–300 nm spectrum with high output in a compact unit. Wikipedia notes that cells can repair UV damage by light and dark repair, and that UV-treated water has no protection against recontamination.

The UV dose concept

The UV dose (fluence) is irradiance multiplied by exposure time: µW/cm² × seconds = µJ/cm² (Wikipedia). The FAO guide uses µWs/cm². Because 1 W·s equals 1 J, 1,000 µWs/cm² equals 1 mJ/cm². The mJ/cm² values in brackets below are unit conversions, not separate measurements.

90 percent kill of bacteria and viruses: roughly 2,000–10,000 µWs/cm² (2–10 mJ/cm²); fungi 10,000–100,000 µWs/cm² (10–100 mJ/cm²); small parasites 50,000–200,000 µWs/cm² (50–200 mJ/cm²) (FAO).
90 percent kill of most bacteria and viruses: 2,000–8,000 µJ/cm² (2–8 mJ/cm²) (Wikipedia).
Spironucleus salmonicida: medium-pressure UV at 50 mJ/cm² or more inactivated the parasite immediately in a controlled exposure test; the salmon RAS trial used low-pressure UV at an estimated 70–80 mJ/cm² at 100 percent UV transmittance (Lazado et al. 2026).
Giardia intestinalis, cited by the same authors: trophozoite growth was delayed at 1–2 mJ/cm², and about 10 mJ/cm² prevented cysts from establishing viable trophozoites.
Freshwater RAS: low-dose ozone followed by about 50 mJ/cm² of UV reduced bacteria counts to near zero (Sharrer and Summerfelt 2007).

SRAC Publication 453 states that disinfection rates are generally proportional to light intensity, which depends on bulb wattage and on the flow treated, and that sizing criteria for bacterial and algal kill are well published. Wikipedia describes reactor sizing as a function of flow rate, lamp power and UV transmittance, and notes that drinking-water reactors are typically validated to deliver 40 mJ/cm² within a stated range of flow and transmittance. If flow is too high, water passes with too little exposure; if too low, heat can damage the lamp.

Turbidity and UV transmittance

The FAO guide states that UV efficiency depends on the size and species of the target organism and on water turbidity, and that UV works best where mechanical filtration and biofiltration have already removed organic matter. SRAC 453 notes that UV penetration is controlled by clarity, so relatively clear water is required. According to Wikipedia, microorganisms embedded in particles are shielded and pass through unaffected, which is why pre-filtration improves transmittance. Sharrer and Summerfelt suggested that recirculating systems favour bacteria that embed in particles or form aggregates that shield them.

A 2026 review in Frontiers in Microbiology lists high turbidity, lamp fouling and insufficient exposure time as the main limits and recommends routine UV transmittance (UVT) monitoring, with UV used alongside solids removal rather than as a stand-alone barrier. In the Lazado et al. trial, UVT in the untreated control systems fell below the 70 percent threshold from day 8, while UV- and ozone-treated systems kept higher UVT. Davidson et al. (2011) also found that ozone raised UVT in trout RAS (see the ozone guide).

Placement in the loop

In the FAO principle drawing, UV disinfection is an optional unit on the return side, after the mechanical filter, biofilter and degasser. The 2026 review describes UV in finfish RAS as a downstream or side-stream barrier, and Sharrer and Summerfelt applied ozone first and UV second, before the water returned to the tanks. FAO adds that lamps must operate in the water: lamps mounted above the surface have little or no effect because of reflection. Both closed in-pipe units and open-channel units are used, and in closed units the lamp sits inside a quartz sleeve (Wikipedia).

SRAC 453 states that modern design calls for internal disinfection only when stock is especially valuable or disease risk is high. Broodstock maturation systems are usually fitted with UV, and virtually all shrimp maturation systems use it. Surface water entering a RAS may be pre-treated by chlorination, ozone or repeated UV, while groundwater or chlorinated mains water usually needs no intake disinfection.

Lamp ageing and maintenance

SRAC 453: bulbs have a limited effective life, typically several months, so UV works only if bulbs are replaced routinely.
FAO maintenance schedule: clean the UV unit and change lamps yearly (6–12-month tasks).
Wikipedia: size the dose for end of lamp life, defined as the hours at which output falls to 80 percent of initial output; fouled sleeves, lamp age and outages reduce intensity, and shatter-proof coatings can cut output by up to 20 percent.
Wikipedia: amalgam low-pressure lamps last up to 16,000 hours; low-pressure lamps convert about 35 percent of input to UV-C, medium-pressure lamps 10 percent or less.
UV-C ages plastics, insulation and gaskets near the lamp (Wikipedia).