A water filtration system is provided that comprises a combination of two components: silver nanoparticles immobilized on a porous carbon solid matrix and calcium carbonate silver nanoparticles. The silver nanoparticles immobilized on the porous carbon solid matrix are prepared in a one-step wet ball milling process that does not use an environmentally hazardous reducing agent or an organic stabilizer. The calcium carbonate in the calcium carbonate silver nanoparticles is preferably isolated from egg shells. The two filter components can be present in any ratio but an approximate 50:50 ratio is preferred. Also provided is an in situ method of preparing silver nanoparticles on active charcoal. Powdered activated charcoal and silver nitrate are mixed together in a mixture of ethanol and water to form a charcoal-silver nitrate solution which is then subjected to ball milling in the presence of polypropylene glycol to produce silver nanoparticles on active charcoal.