Abstract
Ceramic water filters, a common point-of-use technology used to treat water in the home, are produced around the world using local labor and materials. Current production methods frequently use silver nanoparticles as an antimicrobial agent to prevent fouling in the filter and release silver at safe concentrations into the effluent for residual disinfection. Silver nanoparticles are expensive and difficult to acquire in developing world countries, so the use of silver nitrate was investigated.
In the lab, filter disks made with silver nanoparticles (AgNP) and disks made with silver nitrate (AgNO3) were evaluated using miscible displacement flow-through experiments with pulse and continuous-feed injections of E. coli. Experiments using pulse injections of E. coli showed similar performance in breakthrough curves between the two application methods. Long-term challenge tests performed with a continuous feed of E. coli and growth medium resulted in similar log removal rates, but the removal rate by AgNP filters decreased over time. AgNO3 filters provided consistent removal with lower silver levels in the effluent and effective bacterial disinfection. Results from continued use with synthetic groundwater over 4 weeks, with a pulse injection of E. coli at 2 and 4 weeks support similar conclusions—AgNP filters perform better initially, but after 4 weeks of use, AgNP filters suffer larger decreases in performance.
To build on this, full-size filters were produced with AgNP, AgNO3, and with copper nitrate (Cu(NO3)2) (applied with the same method as the silver nitrate) at PureMadi Filter Production Facilities in South Africa and evaluated for total coliform and E. coli removal, and metal concentrations in the effluent. Results of these experiments showed total coliform and E. coli removal was greater for AgNO3 filters (log reductions of 4.06 and 4.11) relative to AgNP filters (log reductions of 3.85 and 3.92). Cu(NO3)2 filter had log total coliform and E. coli removals of 3.33 and 3.54, respectively. AgNO3 filters produced with the same method and the same amount of silver as AgNP filters release smaller amounts of silver into the treated water (average 2 µg/L versus 12 µg/L).
MS2 bacteriophage is commonly used as a model virus for evaluating the antiviral abilities of point-of-use technologies. Both Ag and Cu were tested for disinfection ability against MS2. Ag and Cu, at the concentration of their drinking water standard, showed ¬¬¬¬0.46 and 1.87 log removal, respectively after 24 h of exposure. Additionally, ceramic filters were made with AgNO3 and with Cu(NO3)2 and compared for MS2 removal in a miscible displacement experiment. When comparing performance in breakthrough curves between the two application methods, Cu(NO3)2 filters performed slightly better. Concerns exist as to the validity of MS2 as a model virus for disinfection performance, so disinfection experiments were performed with adenovirus. Silver and copper, at the concentration of their drinking water standard, showed 0.91 and 1.61 log removal, respectively after 8 h of exposure, with little added inactivation afterwards.
Results suggest that the silver nitrate method may effectively reduce costs, improve silver retention in the filter, increase effective lifespan and maintain effective pathogen removal while also eliminating the risk of exposure to inhalation of silver nanoparticles by workers in developing-world filter production facilities.
