At Class B, you stop treating chlorination as a single process step and start treating it as a system to optimize — one where basin geometry, chemistry, and downstream water quality regulation are all connected. This guide covers the actual reaction chemistry behind breakpoint, how contact time is really calculated (not just assumed), the chemistry and risk of nitrification in chloraminated systems, what drives disinfection byproduct formation, and where chlorine dioxide, ozone, and UV fit as alternatives.
Full Breakpoint Chemistry
When chlorine gas dissolves in water it hydrolyzes to hypochlorous acid:
Cl2+H2O→HOCl+H++Cl− If ammonia (NH3) is present, HOCl reacts with it in a stepwise sequence, forming three chloramine species depending on how much chlorine relative to ammonia is present and the water's pH: $$HOCl + NH_3 \rightarrow NH_2Cl \text{ (m