The Stage 1 Disinfectants and Disinfection Byproducts Rule (Stage 1 DBPR) applies to water systems that add a chemical disinfectant such as chlorine, chloramines, chlorine dioxide, or ozone to their drinking water during any part of the treatment process for any purpose (e.g., for disinfection or oxidation, or for taste, odor, or color). In addition to the basic requirements for all systems that add a chemical disinfectant (covered in the Complying With the Stage 1 Disinfectants and Disinfection Byproducts Rule: Basic Guide EPA Doc. No. 816-B-05-004) this supplement covers the additional requirements for: Subpart H community water systems (CWSs) and non-transient, non-community water systems (NTNCWSs) that add a chemical disinfectant and use conventional filtration. You are a Subpart H system if either (1) your source water is surface water, (2) your source water is ground water under the direct influence of surface water (GWUDI), or (3) your source water is a combination of groundwater and surface water, or groundwater and GWUDI.