Investigation of biologically active granular activated carbon filters.

Abstract

Geosmin is naturally produced by numerous cyanobacteria and actinomycetes in surface waters. Although it is non-toxic, it causes an unpleasant taste and odor even at very low concentrations. Water utilities, therefore, often must expend great effort and funds to remove geosmin to avoid customer complaints when the compound is present in the source water. Saint Paul Regional Water Services in St. Paul, MN successfully uses granular activated carbon (GAC) filters to remove geosmin from its drinking water, but, curiously, the useful life of their full-scale GAC filters has exceeded estimates based on batch sorption isotherms and AdDesignS modeling. It has been hypothesized that geosmin-degrading microorganisms on the GAC filters degrade geosmin, thereby extending the GAC filter bed life. In this study, the microbial communities growing in full-scale, biologically active GAC filters at Saint Paul Regional Water Services were characterized using automated ribosomal spacer analysis (ARISA) and high throughput DNA sequencing (Illumina sequencing) of 16S rRNA gene fragments. This study showed that Saint Paul Regional Water Services has highly diverse bacterial filter communities that are functionally stable throughout year. Illumina sequencing revealed that a dominatnt bacterial phylum on the GAC filters was Nitrospira and that pathogen levels (e.g., Enterobacteria) were negligible. Additionally, the effects of mediatype and inoculation on the development of a geosmindegrading bacterial community on GAC filters was investigated using a pilot-scale column system that was fed geosmin. A geosmin-degrading biofilm developed after 40 days of being enriched with 100 ng/L of geosmin. Additionally, the geosmin-degrading organisms proved to be robust in that they were able to resume geosmin degradation after 6 weeks when geosmin was absent. The effects of GAC type and inoculation did not impact the biomass levels or geosmin removal.