Simultaneous Optimization of Manganese Removal and DBP Formation Minimization

Last Modified: Aug 10, 2018

Authors:

  • Meric Selbes, Aaron Duke, William Becker - Hazen and Sawyer

In general, Potomac River provides water quality with relatively low manganese levels. However, manganese concentrations can increase with natural organic matter (NOM) following storm events. If not removed, color associated with manganese would result in customer complaints in the distribution system. Oxidation or greensand filtration is commonly used for the removal of manganese. Chlorine is a common oxidant used prior to filters to oxidize manganese and create an induced greensand filter affect. However, use of chlorine leads to formation of regulated disinfection by-products (DBPs). In the presence of high NOM levels (i.e., a storm event), chlorine dose needs to be increased due to chlorine demand exerted by NOM which exacerbates the formation of DBPs.

In this study, existing treatment process was reviewed at this large-scale WTP for the removal of manganese and DBP precursors. While the treatment process is capable of lowering manganese and DBP precursors under normal raw water conditions, presence of high manganese and NOM levels presents a greater challenge. Several treatment alternatives were evaluated including application of PAC, optimization of coagulation, switching to alternative coagulants, switching to biological filtration and GAC adsorption. Preliminary analysis has indicated that leveraging the existing systems would improve simultaneous removal of both manganese and DBP precursors. Implementation of enhanced or low-pH enhanced coagulation would increase the removal NOM. Consequently, a higher dose of chlorine prior to the filters can be implemented to enhance the removal of manganese. Furthermore, based on the profiling study results, existing coagulant ferric chloride was found to introduce notable levels of manganese within the treatment process. Consequently, switching to ferric sulfate as the coagulant would decrease the manganese load on the induced greensand filters. This presentation will present the approach utilized for selecting the appropriate treatment strategies for simultaneously attaining multiple water quality goals at this treatment plant.

For more information, please contact the author at mselbes@hazenandsawyer.com.

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