Defluoridation of typical eastern Virginia groundwater

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Virginia Polytechnic Institute and State University


Groundwater on the eastern coast of the State of Virginia exceeds the Environmental Protection Agency Maximum Contaminate Level (MCL) for fluoride. This thesis explores the use of alum and alum and lime for the reduction of fluoride to the MCL or lower. In order to reduce an initial fluoride concentration of 2.1 mg/l to 1.8 mg/l 125 mg/l alum was required, for reduction from 5.5 mg/l to 1.8 mg/l, 415 mg/l alum was needed, and for reduction from 8.0 mg/l to 1.8 mg/l, 670 mg/l alum was utilized. If the system pH was lowered below 6.0 the fluoride reduction was insufficient and thus lime was required to maintain the pH above 6.0.

However, hardness increased with the lime dose and other methods of pH adjustment such as employing soda ash should be considered.

Aluminum was detected in the unfiltered treated water, perhaps in sufficient concentrations to cause problems in the distribution system, but further testing of filtered samples is needed. Sulfates were low in the raw water, but buildup with the addition of alum. No tests were made for sulfates in the treated water.

Tests of the sludge produced by the addition of alum and lime indicated high volumes, low solids and very poor dewatering characteristics. Polymers should be considered as a possible means to achieve better sludge dewatering.