Solar Disinfection Studies
Oral Rehydration Solutions
As previously discussed in the section "Disinfection by Boiling," neither boiling nor the use of chemical agents are suitable for the disinfection of oral rehydration solutions because of the inherent handicaps that preclude their use. One possible way to overcome this problem is to take advantage of the germicidal properties of sunlight. We have found that exposure to sunlight in transparent containers renders these solutions bacteriologically safe, without deterioration of the ingredients.
In the course of our study on the small-scale disinfection of drinking water for home use, we experimented with standard ORS solution prepared with chlorine-free tap water deliberately contaminated with fresh sewage. The contaminated ORS solution was distributed in portions of one litre into sterile polyethylene bags, some of which were placed in direct sunlight, some kept indoors under natural and artificial light, and some in the dark inside a cabinet. The initial coliform counts, typical for heavily contaminated water from village sources, ranged from 7,100 to 16,500 per 100 millilitres.
The results, reported in a letter to The Lancet (2:1257, 1980) and reproduced in Table 2, indicate that zero coliform and Streptococcus faecalis counts were obtained in one and two hours respectively, in samples kept in direct sunlight. That the action of sunlight is irreversibly lethal was demonstrated by the inability of the coliforms to regrow when the exposed solutions were kept in the dark for 24 hours. Coliform reductions of around 80% took place in two hours at room conditions and in the dark, but heavy regrowth occurred upon further storage for one day.
The temperature of the test ORS solution did not rise much beyond 30°C after two hours of exposure to sunlight, thus supporting the hypothesis that heat was not a factor involved in the destruction of microorganisms. As was shown in the case of solar disinfection of water, the germicidal action seems to be due mostly to solar radiation in the near UV region (A) of the spectrum (315 nm to 400 nm). The fact that the sodium bicarbonate concentration and the pH of 8.33 did not change indicates that there was no detectable decomposition of the constituents.
These experiments clearly demonstrate that ORS solutions in transparent containers will lead to the complete destruction of such enteric bacteria as coliforms and Streptococcus faecalis, as well as 90% reduction in the total bacterial count, within a period of two hours.
These results are substantially supported by those obtained for plain water as described previously. This is expected because in both cases the procedure and experimental conditions applied were similar in every respect. A salient difference, however, pertains to the additional constituents present in ORS solutions, as well as a slightly higher pH. The differences in composition of the two media have apparently not altered significantly the ultimate lethal action of sunlight, which in both cases was irreversible. It can be concluded, moreover, that even the presence of a carbohydrate, salts, and some nitrogenous substances from the added sewage would not hinder the solar germicidal action. This is in sharp contrast to the legitimate expectation that ORS solutions, being a more appropriate medium for the support of microorganisms, might somehow inhibit the action of sunlight.
The lack of regrowth of the affected bacteria suggests the practicality of storage of both drinking water and ORS solutions for use when needed provided these commodities are protected from recontamination. When storage is desired, it would be wise to keep them in the same containers used in the disinfection process to eliminate the possibility of postcontamination from other vessels, or through handling. Along the same vein, it appears feasible for health care centres and dispensaries involved in oral rehydration therapy to prepare stock, concentrated solutions of ORS, disinfect them by exposure to sunlight, and appropriately dilute them with decontaminated water just before distribution to villagers. Such a system will facilitate matters, reduce efforts and costs, and take the burden off the shoulders of housewives. Wherever such an arrangement is not possible, then housewives need to be instructed on the proper method for the preparation and decontamination of home- made oral rehydration solutions.
Our experimental work was designed merely to demonstrate the feasibility of decontamination of an ORS solution freshly prepared according to the WHO-UNICEF standard formula, using available plastic bags with screw caps. No attempt was made to expand and diversify the work as all the essential experimental information had already been derived from the much more extensive study on decontamination of water. Accordingly, it would be logical to assume that the specific conditions pertaining to containers, fluids, and exposure would be applicable in the two cases. It would be possible, for instance, to use any other type of container which is available and meets the requirements.
In highly endemic rural areas, it would be useful for families to practice routinely solar disinfection of drinking water, in which case the decontaminated water would be available for the preparation at home of ORS solutions when needed. If this procedure is adopted, at least in emergencies, then the time required for disinfection of the ORS solutions after their preparation would be saved in favour of an early start of oral rehydration therapy. In such an event, the only disadvantage is the potential risk of contamination of the ORS solutions from the ingredients or containers.
Indeed, this technology provides a good deal of flexibility which allows for the possibility of adaptation to suit local conditions and needs.