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ABSTRACT
The Town of Cavendish, Vermont's Iron and Manganese Treatment Facility was placed in ser vice in May 2009. This facility uses an innovative biological filtration process that is the first of its kind in New England. The biological process has proved effective in treating water with very high concentrations of both iron and manganese. This article describes the treatment process, how it was selected, and lessons learned.
Introduction
Iron and manganese are common elements in the Earth's crust and are common constituents in groundwater. Iron and manganese dissolved in water become oxidized when exposed to air or oxidant chemicals such as bleach and precipitate. The oxidized forms can cause aesthetic problems such as staining and discoloration of fixtures and laundry.
The concentrations of iron and manganese in Cavendish's system were very high in 2009. For reference, the U.S. Environmental Protection Agency (EPA) has established secondary maximum contaminant levels (SMCLs) for iron and manganese of 0.3 mg/L and 0.05 mg/L, respectively. In Cavendish, iron averaged approximately 2.5 mg/L and manganese averaged approximately 1.8 mg/L - and both have increased with time.
Basic Iron and Manganese Treatment Strategies
There are two basic strategies for treating water with high iron or manganese concentrations. The first is sequestration, which consists of adding a polyphosphate chemical to interact with the metal ions, thereby inhibiting precipitation. Sequestration doesn't actually remove the iron or the manganese, but it can be effective in reducing the negative effects, and it is inexpensive compared to other methods. Sequestration becomes less effective as the metal ion concentration increases.
The other strategy consists of removing the iron and manganese from the water. Removal processes typically used to remove these constituents include ion exchange, lime softening and chemical oxidation. The chemical oxidation process is the most common in municipal water treatment applications and uses an oxidant such as chlorine or permanganate to convert soluble metal ions into insoluble precipitate that can be removed from the water by filtration.
Another less common process utilizes bacteria to assist in oxidizing the metal ions, which are then removed by filtration. Biological filtration has been used extensively in Europe and Canada, but the process used in this case study was the first of its kind in New England.
The Biological...