Pipe Section Reactor to Evaluate Chlorine–Wall Reaction

A bench‐scale, pipe section reactor (PSR) was developed and tested to measure the decay rate of chlorine (Cl 2 ) at the pipe wall, commonly described as “wall demand” but herein referred to as the chlorine–wall reaction. This experimental protocol is more convenient than pipe loop reactors that require a large laboratory and a large volume of water. The design allows investigation of the effect of velocity on chlorine–wall reaction. Two 6‐in. (152‐mm) diameter sections of old cast‐iron (CI) pipe and new cement‐lined ductile‐iron (DI) pipe were used. In addition to water velocity, the effects of water quality (corrosion rate, dissolved oxygen [DO], and pH) on the chlorine–wall reaction were also evaluated. The Cl 2 decay rate was described by zero‐order kinetics for CI pipe. The zero‐order rate constant was larger at higher velocity (because of higher mass transfer at the pipe surface although a limit was reached), lower pH (because of faster corrosion rate), and lower DO (because of greater release of ferrous iron after removal of the oxidized layer at the surface). For the DI pipe section, the Cl 2 decay kinetics were first‐order with respect to chlorine concentration.