Material Degradation

A variety of materials stability tests have been completed for the stainless steel lined PVC UV-Tube, with mixed results. Concerns about PVC degradation under UV light initially led to the lining of the tube with stainless steel, and have subsequently led to the search for new material options. Even with the lining, with extended UV exposure of stagnant water, some by-products are produced at detectable levels (most likely due to the exposed end caps). The levels, however, are well below levels that would lead to a person to consume a daily dose above the US Environmental Protection Agency’s reference daily oral dose (see table 1). Serious concerns about unlined PVC and maybe ABS

Although many researchers have investigated the effects of UV-A and UV-B from sunlight on various materials, less is know about the affects of UV-C (shorter wavelength UV radiation) used in water disinfection. To ensure that UV-Tubes do not produce harmful disinfection by-products, we conduct the following tests to simulate possible household operating conditions. The inlet and outlet water samples were analyzed by an outside laboratory for metals and 72 common volatile organic compounds (VOCs):

o Flow through: The water passes through the tube at a minimal flow rate.
o Overnight test: The tube is left on for 16 hours with stagnant water. Then the flow is turned on and the first outlet water is collected.
o Vacation test: The tube is left on for 8 days with stagnant water. Then the flow is turned on and the first outlet water was collected.
o Total Evaporation: The tube is left on for 35 days with stagnant water, which eventually evaporates. Then the flow is turned on and the first outlet water is collected.
  UV Exposure pH Temp (°C) Absorbance (cm-1) Acetone (µg/L) Bromomethane (µg/L) 2-butanone (µg/L) Chloromethane (µg/L)
Detection Limit         5 0.5 5 0.5
US EPA Ref Dose       1,000 14 6,000 NA
Inlet 0 7.8 20.7 0.200 0 0 0 0
Flow-Through (0.35 L/min) 9 min 7.8 21.1 0.194 0 0 0 0
Overnight 16 hour 7.6 22.9 0.086 160 0 0 0
Vacation 8 day 6.7 27 -- 250 1.4 7.7 0
Total Evaporation 35 day -- -- -- 250 0 9.0 0.51

Note: Oral Reference Dose (RfD) is an estimate of the acceptable daily exposure made by the Integrated Risk Information System. The RfD is given in mg/lg-day, we have converted to µg/L by assuming a 50 kg person consuming 5 liters of water daily (EPA, 2003). Samples were analyzed for 72 Purgeable Organic Compounds forllowing EPA Method 524.2. All compounds not listed were non detect in all sampes.

Although even the highest levels are well below the EPA reference dose for a 50 kg person drinking 5 liters of water a day, anecdotal evidence suggests there is still a taste and odor problem when stagnant water is subjected to long exposures in the stainless steel lined UV-Tube. Confident that by-product levels were too low to cause any health consequences, we began using the stainless steel lined tube to disinfect tap our own tap water during our trip to Cuernavaca in January 2003. We noticed that after 12 hours of exposure, the water coming out of the UV-Tube had a significant taste and odor of plastic. PVC is, therefore, not the most ideal material for a UV-Tube.

However, because of its worldwide availability, low cost, interconnecting parts, and workability, we believe that PVC is still an appropriate option for some users. To mediate both the problem of generating by-products and the problem of taste and odor, we have modified the instructions for using the stainless lined PVC version of the UV-Tube. Instead of leaving it on 24 hours a day and taking water whenever it is needed, users will be instructed to turn the UV-Tube on only once or twice a day to disinfect water. After use it will be drained and turned off. In tests conducted with flowing water, no by-products, taste or odors were detected, and we used this method for disinfecting our own drinking water during the Cuernavaca trip.