John E. Tobiason
For drinking water with low to moderate levels of iron (Fe) and manganese (Mn) (at or somewhat above the SMCL), sequestration is a less expensive treatment alternative compared to metal oxidation and removal. Sequestration complexes Fe and Mn to prevent precipitation and subsequent water quality problems (turbidity, color, staining, etc.). Despite the widespread use of sequestering agents, research has not resulted in a successful method to directly assess the complexation of Mn and Fe. This study was conducted to develop a method for assessing sequestering agent effectiveness and to assess the effectiveness of several phosphate based sequestering agents for several groundwater sources with varying iron and manganese concentrations.
The effectiveness of three different phosphate based sequestering agents was determined at three dosages (100%, 150%, and 200% stoichiometric). Source waters had a pH between 7 and 8 as well as varying concentrations of iron, manganese, alkalinity, calcium, magnesium, and total organic carbon. For this study, an operational method to measure sequestering agent effectiveness was developed based on metal fractionation. Raw water samples were carefully collected under anoxic conditions to mimic system operating conditions and revent inadvertent iron oxidation. Raw waters were dosed with polyphosphate followed by free chlorine. pH adjustment was also conducted when necessary (pH less than 7). After a reaction period of 48 hours at 12°C, samples were fractionated to measure total, particulate, colloidal, and dissolved iron and manganese. Metals in the colloidal and dissolved forms were assumed to be effectively sequestered.
Results show that sequestering agents were more effective for manganese than for iron. As much as 85% of total manganese at concentrations as high as 0.68 mg/L was sequestered whereas for iron 66% of total iron was sequestered at a total iron concentration of 0.18 mg/L. The majority of the sequestered manganese was in the dissolved form, whereas the majority of the sequestered iron was in the colloidal form. As polyphosphate dosages increased in concentration, generally more metal was sequestered. Sequestration was ineffective for source waters containing high total hardness (273 mg/L as CaCO3) probably due to competition between cations (e.g. Ca2+, Mg2+ vs. Fe2+ and Mn2+) for the sequestering agents. Conversely, sequestration was most effective for source waters containing low hardness levels (30 mg/L as CaCO 3). Although there were differences in effectiveness between sequestering agents, no consistent trends with respect to water quality were observed. Ultimately the effectiveness of sequestering agents depends on source water quality in addition to Mn and Fe levels. Bench or pilot scale testing must be completed prior to addition of sequestering agents.