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Biological Oxidation of As (III) in a Full-Scale Iron Removal Plant
Citation:
White, C., D. Williams, C. Muhlen, AND D. Lytle. Biological Oxidation of As (III) in a Full-Scale Iron Removal Plant. Presented at Biological Oxidation of As(III) in a Full-Sclae Iron Removal Plant, Denver, CO, March 28 - 29, 2013.
Impact/Purpose:
The purpose of this study was two-fold: (1) to monitor and evaluate As (III) oxidation and arsenic removal in a full-scale iron removal (aeration), filtration plant with biologically active granular media filters and (2) isolate and characterize the microorganisms responsible for arsenic oxidation.
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. Arsenic in ground waters across the Midwest is typically in the form of As (III), and therefore effective treatment involves the use of an oxidant. 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 (also common in many Midwest ground waters) complicates matters because it creates a demand for chlorine and forms chloramines. Chloramines are not effective As (III) oxidants. There are also microorganisms capable of oxidizing As (III) to As (V) although they have not been reported to be important in drinking water systems. Typically, arsenic oxidizing bacteria have been isolated from environments such as sewage, hot springs, and acid mine drainage. The purpose of this study was two-fold: (1) to monitor and evaluate As (III) oxidation and arsenic removal in a full-scale iron removal (aeration), filtration plant with biologically active granular media filters and (2) isolate and characterize the microorganisms responsible for arsenic oxidation. Results showed that the microbiologically active filters in the water plant consistently oxidized iron (2.3 mg/L in raw water), arsenic (46 g/L in raw water), while concurrently removing arsenic. The utility now regularly meets the new arsenic standard. The majority of arsenic (37 g/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. Results from culture-dependent isolations identified over 200 As (III) resistant isolates. LC-MS was used to confirm pure culture As (III) oxidation and culture-independent methods were used to identify the genera. 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. Microbiological oxidation of arsenic is a simple, robust and an effective way to oxidize As(III) in full-scale water treatment systems.
URLs/Downloads:
http://www.awwa.org/conferences-education/conferences/biological-treatment.aspx
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