Minneapolis Water Treatment and Distribution Services (MWTDS) is a surface water treatment plant that experiences occasional earthy/musty taste and odor problems caused by geosmin, an algal metabolite, during the spring and summer. Currently, MWTDS is using powdered activated carbon (PAC) to remove geosmin. PAC is inefficient, however, because only about 2 hours of contact time is available and the carbon is equilibrated with the low effluent concentration. Thus, large doses of PAC are needed, which can be costly. Granular activated carbon (GAC) as a filter media is an alternative process that can be used to remove geosmin. The GAC filter media becomes biologically active and can effectively remove taste and odor compounds to below their taste and odor threshold via a combination of sorption and biological degradation. To investigate the performance of biologically-active GAC filters for geosmin removal, in addition to overall filter performance, an eight-column pilot-scale set-up at MWTDS was operated from November 2013 to May 2014. Three columns contained Calgon F300 GAC, two columns contained Norit 300 GAC, and three columns contained anthracite (non-sorptive control). Additionally, the microbial communities on these biofilters were characterized and the biological activity was measured. Interestingly, anthracite and Calgon F300 GAC had a high amount of bacteria (106-108 16S rRNA genes copies per gram of dry weight) throughout the entire study, while Norit GAC did not reach ~106 16S rRNA gene copies per gram of dry weight until February. All three media types developed highly diverse microbial communities (Shannon index 3.5-4.5) that were dominated by Proteobacteria. At the genera level, anthracite was dominated by Hydrogenophaga, Sphingomonas, and Flavobacterium. Hydrogenohpaga is a known hydrogen-oxidizing bacteria and Sphingomonas is a strict aerobic that is widely distributed in nature and shown to survive in low concentrations of nutrients. Flavobacterium is a facultative anaerobic bacterium that is commonly found in soil and water. Both the Calgon F300 GAC and Norit 300 GAC were dominated by Hydrogenophaga, Rhodobacter, and Rhizobacter. Rhodobacter species have a variety of metabolic capabilities and are found in freshwater and marine environments. Rhizobacter are known to be root-colonizing bacteria that can fix nitrogen. Bacteria from the genus Legionella were detected on the filter media but they comprised less than 0.3% of the microbial community. This genus contains known pathogenic species of bacteria but we do not know whether any of the strains on the filter media are virulent.The influent geosmin concentration varied from the target concentration (100 ng/L), however, the GAC effluent samples were consistently lower than the taste and odor threshold. The particle removal efficiency ranged from 52-59% for all three media types and was worse during the spring runoff when the filter effluent turbidity was above 0.1 NTU consistently. Overall, this study showed highly diverse microbial communities on filter media that can provide stability to the filtration process. Both media types had similar particle removal performances and both had difficultly during spring runoff. An advantage of the GAC media is its physical and biological ability to remove a portion of the organic carbon creating a more biologically stable water.
University of Minnesota M.S. thesis. January 2015. Major: Civil Engineering. Advisors: Raymond Hozalski and Timothy LaPara. 1 computer file (PDF); x, 91 pages, appendices A-B.
Strait, Jacqueline Marie.
Investigation of the performance of biologically-active GAC filters for taste and odor removal.
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