Comparison of stannous chloride and phosphate for lead corrosion control

A pipe-loop system with five parallel lines composed of new ductile-iron and lead pipes was operated for 13 months to investigate the effects of corrosion control chemicals on lead release and bacterial regrowth. Four of the pipe loops were treated with corrosion control chemicals - orthophosphate, polyphosphate, a blend of orthophosphate and polyphosphate, and stannous chloride (SnCl₂) - and one served as an untreated control. The pipe-loop system received chloraminated and filtered surface water from a full-scale lime-softening plant and was monitored for total lead, dissolved lead, heterotrophic plate count (HPC) bacteria, and coliforms. Total lead concentrations in each of the treated loops were significantly lower (95th percentile confidence level) than in the untreated control, and orthophosphate (1 mg/L as P) consistently yielded the lowest lead concentrations of all of the chemicals tested, including SnCl₂ (0.125 mg/L). Nevertheless, none of the chemicals was able to consistently maintain total lead concentrations below the 15-μg/L action level for the 8-h stagnation time sample in the new lead pipes used for this study. In addition, all of the phosphate-containing chemicals caused HPC populations to exceed those in the untreated control loop during the warm summer months, raising concerns about regrowth and the potential for increased coliform detections. Conversely, HPC levels in the SnCl₂ loop were significantly lower than in the control during the same period.