Abstract
Point-of-use water treatment technologies that use silver as a disinfectant have been shown to be effective by killing disease-causing bacteria and preventing potential recontamination during transport and storage in rural areas. However, current commercially available technologies require up to 8 hours for disinfection or use silver nanoparticles, which is not as efficient for disinfection as silver ions. In an effort to find solutions to these shortcomings, the release of silver and copper through electrolytic generation as a new point-of-use disinfection mechanism was examined. Electrolysis has previously been used in water treatment, but never in a low-resource, point-of-use setting.
In the laboratory a series of experiments were conducted to establish a proof of concept for an electrolytic point-of-use device. Two voltages common to commercially available batteries, 4.5 volts and 9 volts, were applied to a point-of-use apparatus with either two silver or copper wires submerged 1 inch into 10 liters of synthetic groundwater. In addition, the effects of wire diameter, ionic strength of groundwater, and other possible POU parameters on metallic ion release were examined. Silver levels measured over time by graphite furnace atomic absorption spectroscopy established a proof of concept that this kind of technology could be practically implemented in a point-of-use water treatment device in a rural setting. It was determined that the apparatus including only silver wire should be run for only 2 minutes at 9 volts to yield the target 50 ppb concentration for water treatment. Further, this conclusion was supported when 50 ppb electrolytically generated silver from the apparatus yielded up to a 5 log reduction of E. coli bacteria in synthetic groundwater. Copper was less effective in disinfection and also required 62 minutes to release the target 500 ppb for disinfection when 9 volts were applied to the system.
Based on this work, an electrolysis POU prototype was developed and evaluated in 20 households in Limpopo, South Africa over four weeks. The electrolysis prototype achieved a 2 log reduction in total coliform bacteria in household drinking water, which is comparable to field performance of other point-of-use devices in low-resource settings. It also consistently released enough silver sufficient for disinfection but below the WHO drinking water guideline. The use of electrolysis in a POU water treatment device is promising technology, and the field performance of the prototype suggests that such a technology could be incorporated into a low resource setting.
In tandem with the work on this new technology, the long-term performance of two established POU technologies were also evaluated. The first technology, a silver-impregnated ceramic tablet (MadiDrop), disinfect water by releasing silver ions into household water-storage containers. The second, a silver ceramic water filter, mechanically removes pathogens through filtration. It is also painted with a silvr nanoparticles solution that reduces live pathogens and provides a residual disinfectant to reduce the risk of recontamination in the lower reservoir.
404 homes in Limpopo, South Africa were randomized to receive a MadiDrop, silver ceramic water filter, safe-storage water container, or no intervention. The disinfection of total coliform and E. coli bacteria for each intervention was measured every six months over two years. The MadiDrop’s disinfection of total coliform bacteria (3.22 ± 0.27 log reduction) exceeded the performance of silver ceramic water filters (1.80 ± 0.35 log reduction) and filters without silver (1.18 ± 0.25 log reduction). Safe-storage water containers did not improve water quality (0.01 ± 0.27 log reduction). After intervention adjustments, silver concentrations in treated water were 31.8 ± 36.7 µg/L for the silver ceramic filter intervention arm and 27.4 ± 39.1 µg/L for the MadiDrop intervention arm. These mean silver concentrations were less than the 100 µg/L World Health Organization guideline for silver in drinking water. MadiDrop longevity, based on consistent silver-ion release rate, was determined to be at least 12 months of daily use.
Results suggest that an electrolytic apparatus is promising technology for point-of-use water treatment and warrants further optimization of the device. Although copper by itself is not suitable for electrolytic disinfection, it still has potential to be introduced in a silver and copper system, as it would allow for more disinfection potential while still remaining under the EPA limit for metals in drinking water. In addition, this body of work affirms that MadiDrops and filters are effective in disinfecting household drinking water in low-resource settings, but there are opportunities to optimize the products by decreasing silver release.
