The primary food production consumes more than two thirds of all fresh-water abstraction world-wide including the irrigation of crops, livestock watering, and aquaculture production. During this process, water has been identified as a vehicle for the transmission of many harmful agents. This raises emerging needs for solutions that can provide clean water that is essential to human health and is a major resource across a variety of food industrial and agricultural processes. In this research, we present, for the first time, an ultrasonic assisted synthesis method for creating ultrasmall porous MnO_2/Fe₃O nanocomposites templated on poly(amidoamine) (PAMAM) dendrimer. The prepared nanocomposites were characterized by ultraviolet-visible spectroscopy, dynamic light scattering and transmission electron microscope, showing a uniform size distribution as small as 2 nm. The ultrasmall porous nanostructure can provide high surface to volume ratio, thus high accessible surface area, rendering such nanocomposite a promising candidate removing heavy metal ions, organics and pathogens for clean water in food processing. We examined the hosting and dispersing capacities of dendrimers for heavy metal ions, linear, cyclic, and aromatic hydrocarbons, and major waterborne pathogens, using comprehensive biophysical and analytical methods. The outcomes of this research are expected to provide a rapid, low-cost technology for effective removal of multiple contaminants in the water used for food processing that serve the long-range improvement and sustainability of the US agriculture and food systems.