Evaluation of corrosion inhibitors in drinking water supply systems

The problem of corrosion has existed for as long as metal pipes have been used for the transportation of drinking water. Though, originally the problem was mostly related to bad tasting or discoloured water but in recent times, the water quality has become the focal issue towards preventing adverse medical effects. In Canada, guidelines to protect consumers are set forth by the Federal-Provincial Subcommittee on Drinking Water of the Federal-Provincial Committee on Environment and Occupational Health. To ensure adherence to these guidelines, the City of Thunder Bay (Ontario) conducted a corrosion study of its drinking water supply systems. The study tested four chemical corrosion inhibitors while monitoring concentrations of 26 metals. However, the study was focussed on six metals, viz lead, copper, aluminum, iron, manganese, and zinc. The study seeks out answers to three specific questions. Firstly, which one of the four corrosion inhibitors worked best within the Thunder Bay water supply systems? Secondly, the nature of potential existence of relationships between the concentrations of the six metals to each of the four-corrosion inhibitors. Thirdly, investigations into causal effects of natural changes within the water system on the corrosion inhibition process. An analysis of data sets on metal concentrations arising due to various corrosion inhibitors treatment was also able to indicate that caustic soda was able to produce the most significant overall reductions in metals corrosion. Additional statistical analyses of the data set confirmed that caustic soda is the best choice for mitigating corrosion within the water supply system of Thunder Bay. Of the remainder three corrosion inhibitors, the polyphosphate was also found to perform well in reducing corrosion rates, while the sodium silicate and zinc orthophosphate were found to have marginal benefits. Investigations on the relationship that may exist between metals concentrations for a given corrosion inhibitor treatment process lead to some useful information. While there were a few strong relationships when corrosion inhibitor treatments were viewed separately, the iron and manganese showed a strong correlation during all four-corrosion inhibitor treatments. The four system indicators (namely, pH, alkalinity, temperature, and chlorine) play varying roles of importance on the corrosion process in water supply systems. While the effects of pH and, to a certain extent, alkalinity are limited, the temperature 290 and chlorine are shown to significantly impact on the corrosion rates experienced within the Thunder Bay water distribution systems.