Glossary term
Ultraviolet Transmittance
Fraction of ultraviolet light transmitted through water over a defined path length, used to validate UV disinfection dose and water-quality limits.
Definition
metricUltraviolet transmittance is the fraction of ultraviolet light that passes through a water sample over a specified optical path length.
Ultraviolet transmittance, commonly abbreviated UVT, is used in UV disinfection, water reuse, wastewater treatment, drinking-water treatment and process validation. It indicates how much germicidal UV light can pass through the water before being absorbed or scattered. Engineering interpretation depends on wavelength, path length, sample handling, turbidity, color, iron, organics, suspended solids, lamp output, sleeve fouling, flow rate, reactor validation and sensor calibration.
Ultraviolet transmittance is the fraction of ultraviolet light that passes through a water sample over a specified optical path length. It is usually abbreviated UVT and reported as a percent.
UVT matters because UV disinfection dose depends on how much germicidal light reaches the water. A UV reactor can have energized lamps and still underdose if water absorbs or scatters too much UV light.
Measurement Basis
At a defined wavelength and path length, UVT can be written as:
where I_0 is incident UV intensity and I is transmitted intensity.
If:
then:
The wavelength, path length and instrument method must be stated. A value measured on one path length or wavelength should not be compared blindly with another basis.
Absorbance Relation
UVT is often related to absorbance:
For:
the absorbance is:
For a cleaner reference condition:
the absorbance is:
The larger absorbance explains why the same lamp system can deliver less useful dose.
Dose Factor
A simple UVT dose factor can compare event and reference conditions:
For:
the factor is:
This screen is useful for diagnosis, but validated UV reactors may use more detailed dose algorithms.
UV Dose Screen
A simplified UV dose screen is:
where I_{eff} is effective intensity and t is exposure time. If:
then:
For:
the dose is:
If the required dose is 40\ \text{mJ/cm2}, the deficit is:
Flow and Contact Time
At the same intensity, the required exposure time is:
For:
the required exposure time is:
If flow increases and hydraulic exposure time falls, a low UVT condition can push the reactor outside its validated envelope even when the lamps are on.
Analyzer Bias Check
If an online UVT analyzer reports:
and a checked laboratory or portable result is:
the bias is:
Relative to the check value:
or about 4.9\%. Near a dose-inhibit limit, that discrepancy should be reconciled.
Validation Evidence
Useful UVT evidence includes wavelength, path length, instrument method, calibration check, online-versus-grab comparison, sample handling, turbidity, TSS, color, iron, organics, lamp status, sleeve fouling, wiper condition, flow, reactor validation curve, intensity sensor calibration, alarms, bypass state and microbial results where required.
Validation should connect UVT to the decision: UV dose credit, reuse release, wastewater disinfection, diversion logic, upstream filtration control, sleeve cleaning, lamp replacement, high-flow operation or post-storm recovery.
Limits and Common Mistakes
UVT is not turbidity, TSS, color or pathogen count. It is an optical transmittance measure over a stated basis. Low turbidity can still coincide with low UVT if dissolved organics or color absorb UV light.
Common mistakes include using lamp-on status as proof of dose, comparing UVT values from different path lengths, ignoring sleeve fouling, using one grab sample for a storm period, treating UVT as independent of flow, accepting sensor data without calibration and claiming compliance when the reactor is outside its validated envelope. A strong UVT review states optical basis, water-quality context, flow condition, dose algorithm, instrument evidence and release criteria.