BAC conversion in a potable water treatment plant: feasibility and characterization of biofilm growth on granular activated carbon using partially clarified water

The work presented here are the results and analysis of experiments conducted to assess the viability of biological conversion of an activated carbon filter at the Croton Filtration Plant (CFP), a drinking water treatment plant in New York City. The plant uses their activated carbon filter to remove taste and odor contaminants, and the New York City Department of Environmental Protection (NYCDEP) is looking to improve filter media longevity through the development of a biofilm on the surface of the activated carbon granules inside of the filter. Batch and semi-batch experiments were carried out at the bench scale using used and unused activated carbon from the plant, as well as water sourced from the plant, immediately prior to activated carbon filtration, to determine the ability of the system to accommodate the growth of a biofilm. Prior to the activated carbon filtration, raw water sourced from the Croton basin is treated with sedimentation and dissolved air flotation (DAF), leading to low dissolved organic carbon concentrations, and potentially hindering bacterial growth in the filter. As such, the effects of nutrient additions, in the form of Tris-Acetate-Phosphate (TAP), a defined growth media, and lysogeny broth (LB), an undefined growth media, were also explored. These media were introduced to the system through pre-soaking in batch experiments, and through simulated backwash in semi-batch experiments. Batch experiments found that soaking the activated carbon in the growth media prior to testing lead to significant bacterial growth within the first fourteen days, followed by an overall decay in bacterial growth indicators in the following thirty days. These tests also indicated that an even mixture of used carbon, which had spent more than twelve months in active use in the CFP filter, and pristine carbon, lead to greater growth than experiments with entirely used or entirely pristine carbon. Semi-batch experiments were carried out by replacing DAF-clarified water at regular, three day intervals and introducing nutrients during simulated backwashing at regular, nine day intervals. These experiments observed no growth trends, with bacterial presence observed through polymerase chain reaction (PCR) testing, but no indicators of increases in biomass adsorbed to the carbon surface, even in the tests backwashed with 8% strength growth media. As such, these experiments indicate that the CFP could develop a biofilm ex situ, using LB growth media and a mixture of pristine and used carbon, but that such a biofilm would not continue to grow in the active filter conditions. Further, these experiments indicate that the biofilm would lose density over time in the filter, do to the lower availability of nutrients when compared to the conditions in which it was grown.