Abstract
Waterborne pathogens are a major cause of diarrheal diseases in developing communities. The World Health Organization (WHO) recognizes point-of-use (POU) technologies as effective methods to improve water quality and prevent diarrheal diseases at the household level. This work describes a novel method to embed silver in ceramic porous media in the form of metallic silver nanopatches. This method has been applied to develop a new POU technology, a silver-embedded ceramic tablet (SCT) that provides long-term water disinfection. The work presented here investigates the development, technical performance, social acceptance and economic sustainability of this novel POU technology.
When dropped into a household water storage container, the SCT releases silver ions at a controlled rate that in turn disinfect microbial pathogens. Characterization of the silver-embedded ceramic media was performed using transmission electron microscopy. Spherical-shaped patches of metallic silver were observed at 1− 6 nm diameters and confirmed to be silver with energy dispersive spectroscopy. Disinfection experiments in a 10 L water volume demonstrated a 3-log reduction of Escherichia coli (E. coli) within 8 h while silver levels remained below the WHO drinking water standard (100 μg/L). Silver release rate varied with clay mineralogy, sawdust particle size, and initial silver mass. Silver release was repeatable for daily 10 L volumes for 179 days. Results suggest the ceramic tablet can be used to treat a range of water volumes.
Field performance of the technology was evaluated among 79 rural households in Limpopo Province, South Africa over one year. In 50 households, the SCT was evaluated as a secondary POU method in combination with the ceramic water filter (CWF). Twenty-five households were given CWFs with lower plastic receptacles and spigots, and the other half was given CWFs with receptacles and a SCT for the lower receptacle reservoir. The effectiveness of the SCT as the primary POU technology was evaluated among 29 households in a blinded study. For households evaluating the SCT as the primary POU method, each household received two 20-L plastic water-storage containers with a cover and spigot. One container per household held a SCT, and the other container in each household held a ceramic tablet without any silver. This latter container and tablet served as a control. Both tablets appeared to be identical, and the residents could not distinguish between the SCT and the control tablet. Residents were instructed to fill both containers at night before going to bed and using water from both containers equally throughout the following day. All 79 households were visited weekly over the ensuing 5-week period and again at weeks 37 and 52. Water samples were collected from the source water and spigot of each container for analysis of turbidity, total silver, coliform bacteria, and E. coli.
Results demonstrated that all three approaches significantly reduced total coliform bacteria and E. coli relative to the controls (for silver-ceramic-tablet households) or the source water (for filter and filter-plus-tablet households). The SCT’s performance was comparable to current inexpensive, single-use POU methods and worked best when used in combination with the CWF to provide a 100% reduction in E. coli after 1 year. Silver levels in all treated water samples were less than 20 µg/L, significantly below the drinking water standard of 100 μg/L. User surveys indicated that all the technologies were simple to use and culturally appropriate.
In addition to being effective against bacteria, the disinfection efficiency of the SCT against other harmful waterborne pathogens, such as Cryptosporidium parvum (C. parvum) and MS2 phage, was also evaluated. Results showed excystation of C. parvum ocysts reduced to 14% from 74% among samples treated with ceramic tablets embedded with 500 mg of silver, and 89% reduction of MS2 bacteriophage. Silver concentrations ranged from 85.7 to 172.9 μg/L, demonstrating that silver is an effective disinfecting agent against these pathogens where current methods such as chlorination are ineffective.
Finally, market demand and operational costs for a start-up venture based on the technology was investigated to determine the economic sustainability of the SCT. Through the NSF I-Corp program, 100 customer discovery interviews were conducted to determine market demand, and a prototype manufacturing facility at the University of Virginia was established to evaluate operational costs.
Results suggest that the SCT has a competitive advantage. The ease of use, reusability, effectiveness against waterborne pathogens and ease of transport and distribution make it a promising and affordable technology that could improve human health. The work described here provides a fundamental understanding of this novel method to impregnate silver in porous ceramic media, which has great potential to provide safe drinking water for billions of people who are in need of it around the world.
