"Evaluation of Nitrification in a Large Regional Rural Water System" by Lisa M. Snyders

Document Type

Thesis - University Access Only

Award Date

2009

Degree Name

Master of Science (MS)

Department / School

Civil and Environmental Engineering

Abstract

This thesis investigates the potential for nitrification and alternatives for preventing nitrification in the Mni Wiconi Rural Water Distribution Supply System located in central South Dakota. Chlorine and ammonia are used to form a chloramine residual for disinfection and are dosed at a 3.5:1 Cb:NH3-N ratio in the water leaving the water treatment plant. Because the Mni Wiconi distribution system is large and the water travels great distances, water age is a concern. Water age leads to chloramine decay which results in increased ammonia residuals. Depleted chlorine residuals decrease the resistance of the system to grow nitrifying organisms. Water quality models were used along with distribution system water quality analyses and laboratory experiments to evaluate nitrification prevention strategies and water quality at locations where potential nitrification episodes may occur. A previous version of the EPANET2 model for the Mni Wiconi distribution system was updated to reflect recent system modifications and was calibrated with a fluoride study. The model was used to predict water age and ammonia residuals. The results indicated that the water in the core line was much older than expected, reaching 500 hours at Kadoka and near 700 hours on the far west side of the north core line. A nitrification action plan was developed. Based on values of water quality parameters experienced in the distribution system and found in the literature, action levels were proposed for parameters that would trigger responses by system operators to prevent nitrification or remove nitrification from the system. Two nitrification episodes were encountered in the Mni Wiconi distribution system during the study period. In both cases, the episode occurred in water storage tanks where the water age was high and mixing was inadequate. Operating personnel took the necessary steps to rid the tank and surrounding distribution system of elevated nitrite levels and reestablish the chlorine residual. As a result of these episodes, nitrification prevention strategies were hypothesized and evaluated. Nitrification prevention strategies that were experimentally examined included adjusting the chlorine to ammonia dosage ratio, booster chlorination, and adjusting the pH of the treated water. Dosing a chloramine to ammonia-nitrogen ratio of 4: 1 rather than 3.5: 1 produces higher total chlorine residuals, lower free ammonia residuals, and lower nitrite levels. The water quality at a 4: 1 chlorine to ammonia ratio is more preventative of nitrification than a 3 .5: 1 ratio. Increasing the pH of the water at the water treatment plant from 7.3 to 8.3 would help to maintain a higher chloramine residual in the distribution system. Although increasing the pH would slightly increase the Trihalomethanes (THMs) produced in the clearwell and in the distribution system, the THMs produced initially in the clearwell and one week into the distribution system would be less than the MCL of 80 μg/L.

Library of Congress Subject Headings

Nitrification -- South Dakota -- Prevention

Water quality management -- South Dakota

Water -- South Dakota -- Distribution -- Safety measures

Mni Wiconi Rural Water Supply Project (U.S.)

Format

application/pdf

Number of Pages

197

Publisher

South Dakota State University

Download

Share

COinS