AWWA WQTC69433

The objective of this study was to compare the effect of corrosion inhibitors on the level of heterotrophic bacteria in bulk water and biofilm in 14 parallel pilot-scale water distribution systems (PDSs). The study took one year with 12 PDSs receiving different types and doses of corrosion inhibitors. Water and pipe coupon samples were taken and examined during the experiments. There was a statistically significant increase relative to the control lines (PDS 13-14) in bulk water bacterial densities (measured as heterotrophic plate count, HPC) with the addition of any of the four inhibitors. For bulk HPC this increase was relatively small, being 15.4% (0.42 log) when using phosphate based inhibitors (PDS 1-9), and 11.0% (0.31 log) for the silica based inhibitor (PDS 10- 12). The data also showed a statistically significant increase of 0.34 log in biofilm bacterial densities (measured as HPC) with the addition of any of the phosphate based inhibitors (ortho-phosphorus, blended ortho-poly-phosphate, and zinc ortho-phosphate). The silica based inhibitor resulted in an increase of 0.36 log. It was determined that phosphorous stimulated growth in separate experiments, however the compound causing the Si based inhibitors to increase bulk and biofilm density was never identified. Analysis of all 14 PDSs showed that bulk HPC was affected most by temperature and residual. A sensitivity test showed that maintaining higher residual levels can counteract the relatively modest increases in bulk HPC due to inhibitors. Biofilm HPC was most affected by pipe material, effluent residual and temperature in addition to the small increase due to inhibitor. Biofilm density on polyvinyl chloride (PVC) pipes were consistently much lower than the other materials (0.66, 0.92, and 1.22 log lower than lined cast iron (LCI), unlined cast iron (UCI), and galvanized steel (G), respectively). Includes 14 references, tables, figure.