Many reservoirs become naturally stratified during the warmest months of the year. This occurs when the sun warms the surface of the reservoir This warm water is less dense than the cooler water below it, and slowly a temperature/density gradient forms, which effectively divides the reservoir into 3 distinct layers. The upper layer, or epilimnion , the middle layer, or thermocline, where the temperature decreases rapidly with increasing depth, and the lower layer, called the hypolimnion. Hypolimnetic anoxia often occurs because of the depletion of dissolved oxygen in this lowest layer caused by the aerobic decomposition of algae and organic material. This loss of oxygen results in a reduced chemical environment, which causes iron, manganese, and other substances to dissolve out of the sediments into the water column. During the water treatment process, these substances must be reduced to levels safe for drinking. Hypolimnetic oxygenation systems can be installed to help reduce the amount of treatment necessary at water treatment plants.
Oxygen can be introduced using compressed air, or more efficiently using pure oxygen. The three primary means of introducing oxygen to the hypolimnion are the airlift aerator, the Speece cone, and the bubble plume diffuser, which can be linear or circular. Below is a section of the linear diffuser in use at both Carvins Cove and Spring Hollow Reservoirs, which supply water to the city of Roanoke, VA.
We are developing and validating models for the three most common systems: the bubble-plume diffuser, the air-lift aerator, and the Speece Cone. The models predict performance based on the applied air or oxygen flow rate and the initial bubble-size created by the diffuser. Because these systems induce mixing, and potentially enhance sediment oxygen demand, the process models are being coupled with a reservoir model to enable more effective design and economic optimization.
Below is a contour plot showing the oxygen contours of a bubble plume as well as the model predicted plume diameter at one sampling location in Spring Hollow Reservoir.
Hypolimnetic Oxygenation: Improving Design by Incorporating Diffuser-Induced Oxygen Demand
--Paul Gantzer, PhD student
Transient Biogeochemical Cycling and Sediment Oxygen Demand
--Lee Bryant, PhD student
Hypolimnetic Oxygenation with Bubble Plume Diffusers
--Vickie Singleton, PhD student
Impact of Hypolimnetic Oxygenation on Water Treatment
--Kevin Elam, MS student
The weather station is used to correlate climate data to conditions withing the reservoirs
We are also involved in similar projects in Switzerland at Lake Hallwil in conjunction with Eawag, the Swiss Federal Institute of
Aquatic Science and Technology, as well as at Lake Bermejales in Spain.
Lake Hallwil. Also included is a picture of the circular bubble plume diffusers used for oxygenation.
Lake Bermejales, Spain