The role of iron oxide-pipe deposits on chlorine disinfection byproduct formation in the distribution system

This study investigates the abiotic role of the pipe deposit surface in chlorine disinfection byproduct (DBP) formation by conducting experiments involving actual and model deposits (magnetite and goethite) and both finished and raw waters as well as simple model natural organic matter (NOM) probe compounds. It is found that actual deposit NOM has the potential to increase DBP formation and contribute to dichloroacetic acid formation. A mechanism for abiotic reaction is promulgated which provides a basis for understanding varied heterogeneous systems which includes reaction mechanisms for aqueous DBP formation, redox reactions with the surface, superoxide formation, and adsorption effects. In addition, the specific effect of the magnetite and goethite surface on finished and raw water as well as NOM probe compounds at varied concentrations is analyzed resulting in a reaction scheme developed to aid in the interpretation of different NOM/pipe deposit systems. In the presence of water treatment plant finished and raw water, the goethite surface increases the observed rate of chlorine decay by as much as 86% over water only and enhances DBP formation by as much as much as 129% with increasing solids concentration after the initial reaction period (two-hours). In the presence of water treatment plant finished and raw water, the magnetite surface significantly increased the observed rate of chlorine decay. The primary mechanism for the observed chlorine decay rate for the 5 g/L magnetite system is the reaction between chlorine and the reduced iron sites and not chlorine with TOC or TOC-surface intermediates. The magnetite surface consistently decreases the net DBP production with increasing solids concentration by as much as 94% when compared to water alone. The presence of the magnetite surface results in a net loss of HAA5, and more specifically DCAA, with an increasing net loss corresponding to an increasing solids concentration. The HAA5 concentration, due to the net loss in production, exhibits the greatest observed reduction after the initial reaction period. This study shows that the pipe deposit can alter chlorine decay, increase DBP formation, and change DBP speciation.