Evaluation of manganese control in the source water and microfiltration plant of a South Carolina water utility.

摘要

Although present in most water supplies, manganese is a difficult-to-treat nuisance contaminant in drinking water. The US Environmental Protection Agency (EPA) has set a Secondary Maximum Contaminant Level (SMCL) of 0.05 mg/L but experts have recommended a goal of 0.02 mg/L or less to avoid chronic issues with the metal. This work is a report on manganese control for the Startex-Jackson-Wellford-Duncan (SJWD) Water District, a water utility in northwestern SC. Fourteen locations were sampled from June 2010 through October 2011. Manganese, iron, dissolved organic carbon, dissolved oxygen, and other relevant parameters were measured.;The lake-fed Middle Tyger River is the main water source for SJWD. Sampling revealed that although the lake has high summer manganese concentrations (August soluble Manganese in the river just below the dam was 0.643+/-0.121 mg/L), the river effectively oxidized 72+/-7% of the manganese over the first six miles (+/- represents standard deviation). In the last two miles, the manganese concentration increased slightly, resulting in 0.16+/-0.05 mg/L at the plant intake from June to September. The rest of the year, there was little manganese from the lake, but there was a slight increase in manganese concentration with distance from the lake so that it averages 0.09+/-0.05 mg/L October--April at the plant. In-river processes, not manganese inputs from the lake, play the primary role in controlling manganese concentrations in the river at the treatment plant intake. SJWD also has a secondary source, the North Tyger reservoir. Sampling showed that this reservoir experienced elevated soluble manganese concentrations from January to May. This was unexpected, given the oxic conditions of the lake during this time.;The second objective was to study manganese treatment processes for membrane plants, particularly to test a manganese contactor concept. In a manganese contactor, manganese oxide coated media sorb soluble Mn2+ ions which are then oxidized in place by the addition of an oxidant. The newly formed oxide can then sorb another Mn2+ ion, allowing the process to be repeated.;Bench-top batch experiments compared contactor-before-filter and contactor-after-filter process trains to conventional direct oxidation with KMnO4. Processes and process conditions were designed to simulate the proposed SJWD membrane treatment plant. Manganese, iron, dissolved organic carbon concentrations, UV254 absorbance, filter flux, disinfection byproduct (DBP) formation potential, and DBP formation potential under uniform formation conditions were measured.;All three processes were capable of removing manganese to well below the SMCL of 0.05 mg/L. The manganese contactor processes always removed manganese to low levels, generally 0.004 mg/L. The direct oxidation process achieved manganese removal to 0.008 mg/L on occasion, but required very careful attention to the KMnO4 dose. Incorrect doses resulted in high manganese levels in the effluent. Membrane filter flux decline was more severe for the contactor-after-filter configuration than for the other two processes. All three processes were comparable in removing turbidity and organic carbon and in DBP formation potential.