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NITRIFICATION, AND IRON AND ARSENIC REMOVAL IN BIOLOGICALLY ACTIVE FILTERS: A CASE STUDY
Citation:
LYTLE, D. A., T. J. SORG, C. MUHLEN, A. Chen, L. Wang, AND K. French. NITRIFICATION, AND IRON AND ARSENIC REMOVAL IN BIOLOGICALLY ACTIVE FILTERS: A CASE STUDY. Presented at 2007 AWWA WQTC Annual Meeting, Charlotte, NC, November 04 - 08, 2007.
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Description:
The effectiveness of arsenic removal from water is largely dependent on the oxidation state of the arsenic. As (III) is much more difficult to remove relative to the oxidized As(V) form. Unlike Fe(II) that can be oxidized by oxygen, efficient As(III) oxidation requires a strong oxidant such as permanganate or free chlorine. Ammonia in the source water complicates matters because it creates a demand for chlorine and forms chloramines. Chloramines are not effective As(III) oxidants. Furthermore, ammonia in the distribution system can lead to microbiological nitrification, corrosion, and taste issues. There are also bacteria capable of oxidizing As(III) to As(V) although they have not been reported to be important in drinking water systems. The purpose of this study was two-fold: (1) to monitor and evaluate nitrification, As(III) oxidation and arsenic removal in a full-scale iron removal (aeration), filtration plant with biologically active granular media filters located in Ohio, and (2) to determine how to most efficiently regain biological properties in new filters following filter re-bedding with new media. Results showed that the microbiologically active filters in the water plant consistently oxidized all of the 1.2 mg/L NH3-N in the raw water to nitrate. The full-scale plant also observed effective iron (2.3 mg/L in raw water) and arsenic (46 mg/L in raw water) removal, and regularly meets the new arsenic standard. The majority of arsenic (37 mg/L) in the raw water was in the reduced As (III) form. The interesting observation was that a significant amount of the arsenic was removed without the addition of a strong chemical oxidant (only aeration) such as free chlorine to convert As (III) to the As (V) form. Although some As (III) may be removed during iron removal, the observed arsenic reduction through the plant was far greater than expected. Detailed bench- and pilot-scale investigations (which will be discussed) supported the conclusion that oxidation of As (III) (and ammonia) took place within the filters by microorganisms which explained the greater than expected arsenic removal. Seasonal variations in ammonia and As(III) oxidation effectiveness were not observed likely because yearly changes in water temperature and other water quality of the groundwater source were minimal. Pilot filter tests were used to assess whether seeding the filters with nitrifying bacteria reduced the timeframe to achieve compete nitrification. The results showed seeding approaches did not reduce the timeframe which was approximately 70 days. Microbiological analysis using traditional and molecular techniques confirmed the presence and identity of oxidizing bacteria. Microbiological oxidation of ammonia and arsenic is a simple, robust and an effective way to oxidize convert ammonia to nitrate and oxidize As(III) in full-scale water treatment systems.