Nanoparticle-treated particle packs, such as sand beds, may effectively filter and purify liquids such as waste water. When tiny contaminant particles in waste water flow through the particle pack, the nanoparticles will capture and hold the tiny contaminant particles within the pack due to the nanoparticles' surface forces, including, but not necessarily limited to van der Waals and electrostatic forces. Coating agents may help apply the nanoparticles to the particle surfaces in the filter beds or packs.