Small public water systems play a vital role in providing safe drinking water to many rural areas in many states in the U.S. In view of the growing amounts and types of pollutants, providing safe drinking water is becoming increasingly difficult for small public water systems because of their unique geographical, financial, technical and operational constraints. The objective of this study was to develop a drinking water treatment technology for resource-constrained small public water systems using chitosan modified diatomaceous earth (DE) to remove a group of dissolved contaminants (natural organic matters, arsenic, and nitrate). Chitosan is an effective biosorbent for various dissolved contaminants mainly due to its high density amino groups and hydroxyl groups. DE of different sizes and permeability was modified by a chitosan to achieve the uniform thin coating on DE surfaces. The new adsorbent had the unique properties of both DE (good mechanical strength, large surface area, and good permeability) and chitosan (ubiquitous biopolymer with outstanding versatile adsorption capacity). The adsorption performance in removing the target contaminants was examined by batch adsorption tests.