Health Stream Literature Summary - Issue 55 - September 2009

Solar disinfection of drinking water (SODIS): An investigation of the effect of UV-A dose on inactivation efficiency.
Ubomba-Jaswa, E., Navntoft, C., Polo-Lopez, M.I., Fernandez-Ibanez, P. and McGuigan, K.G. (2009) Photochemical and Photobiological Sciences, 8(5); 587-595.

Solar disinfection (SODIS) has been demonstrated to be an effective household water treatment method for developing countries that is practical and low cost. This method involves exposure of water in poly(ethylene) terephthalate (PET) bottles to sunlight for 6 hours. Microbial pathogens are killed by the synergistic effects of mild heat and UV-A light. However a range of methods have been used in studies of this technique and it is difficult to compare results and draw conclusions about the mechanisms and efficiency of UV-A inactivation. Also the results obtained for small systems under experimental conditions may not be applicable to natural sunlight and larger volumes of water. The aim of this study was to determine the limitations of SODIS when it is scaled-up through use of larger batch volumes or continuous flow recirculation reactors.

Three solar reactor systems were used: borosilicate glass tubes used as batch systems (batch reactor, 2.5 L total volume), and two higher volume (14 L and 70L) solar photoreactors fitted with compound parabolic collectors (CPCs) and recirculation systems at two flow rates (2 and 10 L min-1). The effects of several parameters on inactivation by natural sunlight were studied: (i) the total volume of the treated water; (ii) the irradiated area of the solar collector in the photoreactor; and (iii) the flow rate. The aim was to determine whether inactivation of E. coli K-12 is driven by solar UV irradiance, the total solar UV dose received or by a combination of both. Experiments were also done to assess the effect of water temperature under dark conditions, irradiated collector surface area, and water flow rate. Experimental work was conducted in Almeria, Spain.

Complete inactivation of E. coli K-12 (6-log reduction) to below the limit of detection occurred at both low and high UV intensities provided an uninterrupted solar UV dose greater than 108 kJ per square metre was reached. This indicates that inactivation depends on the UV dose rather than the UV irradiance. However higher UV irradiance results in more rapid inactivation and is also believed to reduce the capacity of the bacteria to repair UV damage. Inactivation was dependent not only on the UV dose received but on the manner in which the dose was delivered (continuous vs intermittent). The borosilicate tube system had no water flow and therefore provided continuous UV exposure, while the continuous flow system and irradiated collector surface resulted in intermittent exposure. Following a 5 hour exposure to sunlight, complete inactivation of bacteria was observed only in the borosilicate tube. In the continuous flow system, an accumulated UV dose of greater than 108 kJ per square metre was also deposited to the bacteria but intermittently, resulting in a 2-log concentration of residual viable bacteria remaining after the 5 hours. Studies of the irradiated collector surface also showed survival of viable bacteria probably due to switching on of repair mechanisms during dark periods which confer increased UV resistance.

These results indicate that attempts to scale-up SODIS through the use of pumped, re-circulatory, continuous-flow reactors need to consider flow rate, water volume, temperature and solar energy to ensure that an adequate uninterrupted UV dose is delivered to microorganisms.

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