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In this study, a pilot-scale pipeline reactor and refined total chloramine decay model were employed to study the effects of the highly corrosive pipe wall and three kinds of disinfectant constituents on the chlorine decay behavior. The bulk decay coefficient, kbulk for NaClO, NaClO + NH3·H2O, and NaClO + (NH4)2SO4 were 0.011, 0.004, and 0.004 h−1, respectively. By resorting to the refined total chloramine decay model, the comprehensive wall decay coefficient kwall appeared in the ascending order of NaClO, NaClO + NH3·H2O, and NaClO + (NH4)2SO4. The remarkable contribution of the aged cast iron pipe wall to overall total chlorine residual decay was manifested by the ratio, kwall/kbulk. The pipe wall-induced decay was related to microbe consumption and electrochemical corrosion as indicated by variations in total organic carbon (TOC), NO2−-N, NO3−-N, and Fe residual. The larger kwall for NaClO + NH3·H2O (0.720 h−1) relative to NaClO (0.465 h−1) was mainly attributed to enhanced nitrifier-mediated microbe consumption. The largest kwall for NaClO + (NH4)2SO4 (0.864 h−1) was due to the further promoted microorganism regrowth and metabolization as evidenced by the SO42- declining behaviors. On this basis, it was suggested to minimize extra inorganic salt introduction into treated water to constrain microbial development in drinking water distribution system (DWDS). Temporal-free chlorine disinfection was also recommended for the chloraminated DWDS before the critical temperature. HIGHLIGHTS The new refined total chloramine decay model could well simulate total chlorine residual (TCR) decay behavior and give calibrated kwall.; TCR decay in aged cast iron (CI) pipe is caused by Fe-related corrosion and biotic consumption.; Minimize inorganic salt introduction is recommended to maintain the TCR level in DWDS.;

Deep insight into the forces driving chloramine decay in different pipe materials is the key to taking sound action to cope with pipe water quality deterioration. By using the newly developed RTCDM (refined Total Chloramine Decay Model) and pipe section reactor, the role of four typical pipes in disinfection chemistry was qualitatively and quantitatively compared, and the mechanism of pipe wall mediated chloramine decay was further described. As for the four typical pipes studied, the characteristics of deteriorating water quality, especially for accelerating total chloramine decay was in the order of cast iron pipe > steel pipe > cement lined ductile iron pipe > polypropylene-random pipe. Cast iron pipes, cement-lined ductile iron pipes, and steel pipes of long service age are characterized by one or two driving forces leading to TCR decay. Aged cast iron pipes could take up chloramine by Fe(0) and microbes (especially nitrifiers) spreading over the inner wall. Aged steel pipe is characterized by aggressive electrochemical corrosion and weak nitrification. Lime and gypsum leaching is the main cause, and nitrification/denitrification may also occur in aged cement-lined ductile iron pipe. Polypropylene-random pipes have a minimum effect on disinfection chemistry. This knowledge is of value in speculating the reasons leading to TCR loss in the full scale distribution system. HIGHLIGHTS The developed RTCDM has good performance for predicting TCR decay in the bulk phase.; The ability to accelerate TCR decay was in the order cast iron (CI) > steel > cement lined DI > polypropylene random copolymer (PPR).; Chemical corrosion and nitrification are the main causes of TCR consumption for aged CI pipe.; Aggressive corrosion and weak nitrification are the main causes of TCR lost for aged steel pipe.;