Disinfectant Decay and Corrosion

The primary objective of this research was to develop and test a pipe-section reactor to measure the decay rate of disinfectant at the pipe wall. The pipe-section reactor is more convenient than pipe loop reactors which require a large laboratory and large supply volume of water. It is also a more realistic representation of the pipe surface than provided in pipe coupon experiments or annular reactors. The effects of water velocity and corrosion rate can be investigated using a pipe-section reactor, while still affording the simplicity of laboratory-scale, batch reactor operation. The secondary objective was to collect field data from two distribution systems in North Carolina (North Chatham County and Durham) to measure the decay of free chlorine by two alternative methods and to explore a relationship with corrosion rate. The ultimate goal would be to achieve a better understanding of disinfectant decay at the pipe walls such that a set of default values for decay rate coefficients could be specified for free chlorination and chloramination, the two most common secondary disinfectants. The pipe-section reactor consisted of a 20-inch (508 mm) length of either a six-inch (152 mm) diameter section of cast iron (CI) or cement-lined ductile iron (DI) pipe. An inner cylinder of Plexiglas was inserted to create a narrow annular space between pipe wall and the Plexiglas wall. A large, propeller-type stirrer was inserted into the inner cylinder to circulate water from the inner cylinder to the annular space. The water velocity adjacent to the pipe wall was controlled by the rotational speed of the stirrer motor; the velocity range was from about 0.5 to 2.0 ft/s (0.16 to 0.62 m/s). The pipe-section reactor can be operated in both continuous flow and batch modes of operation. In either mode, experiments can be designed easily to test different rate models for wall reactions. The reactor was filled with the tap water distributed by the Orange Water and Sewer Authority (OWASA) which on annual average is of very low alkalinity (3.1 mg/L as CaCO 3 ) with a pH of 7.4 and a total organic carbon (TOC) of 3.2 mg/L. The average annual concentration of a phosphate-based corrosion inhibitor was 1.3 mg/L. Except in experiments in which dissolved oxygen (DO) was removed, the DO was between 7 and 8.5 mg/L in the test water within the reactor at the beginning of the experiment. The decay rate in bulk water was measured in independent experiments in which 2 L of OWASA tap water was placed in a 2-L clean glass bottle in a dark place at room temperature (21C). Sample volumes of 25 mL were withdrawn for each chlorine measurement, giving a total withdrawal of 200 to 300 mL in each rate test. The experiments with the CI pipe-section reactor measured the effects of initial chlorine concentration, water velocity, pH, Cl - and DO on the rate of chlorine decay. The same parameters with the exception of DO were investigated in a DI pipe-section reactor.