Ultraviolet Dosage Requirements in water and wastewater treatment
Ultraviolet Dose Requirements
The energy required to inactivate a microorganism is termed the UV dose or UV fluence and is measured in mJ.cm-2, or mW.s.cm-2, or J.m-2. UV fluence is the modern term, used to represent UV light impacting on a small parcel of fluid from all directions. UV light is scattered by particulate and colloidal material, meaning that it does not merely radiate outwards from the lamp. Light is scattered in all directions, including forward- and back-scattered.
1 mJ.cm-2 = 1000 mW.s.cm-2 = 10 J.m-2
This is defined as being the amount of UV energy reaching the organism (the intensity) multiplied by the contact time (the time the microorganism is in the UV irradiation field).
Dose = Average intensity (eg. mW.cm-2) x contact time (eg. seconds)
Severin (Severin, 1983) investigated the temperature dependence of some bacteria and viruses. Between the temperatures of 5 and 35 ºC. He noted that the inactivation rate increased by less than 20% for f2 bacteriophage, for the above increase in temperature and increased by less than 10% for E. coli and Candida parapsilosis(Is the conclusion correct?). Malley (Malley) (Can someone insert the reference. UVDGM ref is incorrect. Is the conclusion correct?) found that MS2 phage inactivation was temperature independent.
Malley (Malley) (Can someone insert the reference. UVDGM ref is incorrect) found that inactivation is independent of pH, between pH 6 and 9.
Sommer (Sommer, 1999) investigated the reciprocity of time and intensity for E.coli strains, Bacillus subtillis and coliphages. Tests revealed higher inactivation at the higher UV intensities and the same dose, for E. coli. Reciprocity differences for other microorganisms were not statistically significant. It is hypothesized that the difference for E. coli was due to repair mechanisms taking place in the low intensity case. Others (Oliver, 1975) (Rice, 2001) investigating reciprocity over a range of intensity of 1 to 200 mW/cm2 found no difference in inactivation rates