Term of Award

Fall 2015

Degree Name

Master of Science in Applied Engineering (M.S.A.E.)

Document Type and Release Option

Thesis (open access)

Copyright Statement / License for Reuse

Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 License.

Department

Department of Mechanical Engineering

Committee Chair

Francisco Cubas

Committee Member 1

Mike Jackson

Committee Member 2

Peter Rogers

Abstract

The Occoquan Reservoir is a eutrophic reservoir that is part of an indirect potable reuse system. To protect the reservoir water quality, a high quality nitrified product water from a water reclamation facility and an oxygenation system are used during periods of thermal stratification to prevent the onset of anaerobic conditions above the sediments. During the stratified warmest months of the year, oxygen depletion rates exceed nitrate and artificial oxygen supply rates near the dam resulting in ammonia accumulation in the water column. Field observations and laboratory experiments revealed that sediment ammonia release rates ranged from 170-542 mg/m2∙day. At such rates, ammonia concentrations above the sediments reach values as high as 5.6 mg-N/L in the absence of nitrate and oxygen, and values as high as 2 mg-N/L when the oxygenation system is operational. A thorough analysis on ammonia cycling revealed that for the years studied, hypolimnetic ammonia oxygen demand may reach values as high as 77 metric tons of oxygen during a stratification period of 140 days. Furthermore, ammonia oxygen demand represented 20-100%, and in some cases more than 100% of the hypolimnetic oxygen demand estimated from oxygen depletion curves, which are commonly used to design oxygenation systems. Finally, it was determined that to satisfy ammonia oxygen demand in the reservoir, it is necessary to provide three times the oxygen demand estimated from the oxygen depletion curves. These results highlight the importance of estimating benthic fluxes of reduced substances into the water column (e.g. ammonia) when designing oxygenation systems.

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