Turbidity versus UV Transmittance
In some cases the turbidity is confused with the transmittance of the water. The turbidity of the water influences the transmittance of the water, but the transmittance does not affect the turbidity.
Turbidity
Turbidity of the water is in general measured with infrared radiation with a wavelength of 860 nm. The detector is placed at a 90° angle to the source of the emitter of the light beam. The scattered light is measured and is a measure of the amount of suspended solids (clay, silt, algae, microbes, etc.) in the water.
The unit of measurement for turbidity is:
- NTU (Nephelometric Turbidity Units)
- FNU (Formazin Nephelometric Units, ISO standard)
Sometimes also visible light (400 – 600 nm) is used to measure the turbidity, this give a higher accuracy for smaller particles but is more affected by the colour of the particles.
Total Suspended Solids (TSS)
The measurement of the TSS concentration is done by filtering (pore size ~ 1.5 µm) the water and weighing the amount of dry matter in [mg/l]. Turbidity and total suspended solids can not be compared because:
- very fine particles give a high NTU and low TSS
- a few large particles give a low NTU and high TSS
Transmittance
The UV transmittance is affected by dissolved organic matter (DOC) and suspended solids (TSS) which absorb or scatter the germicidal UV radiation. If the DOC concentration increases the transmittance will drop but the turbidity will not change.
If the turbidity increases the transmittance will also be reduced, but often this is a small part of the measured transmittance because UVC is generally applied on clear water where absorption dominates. A lower UV transmittance can be compensated by more UV power.
High turbidity and total suspended solids concentrations have a negative effect on the UV disinfection process. Particles create shading because of which the UV radiation can not reach the microorganism or virus. Increasing the UV power will not improve disinfection and removing or breaking up particulates is advised instead.
