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Lake Bard Hypolimnion Oxygenation
Thousand Oaks, California

GRAND PRIZE — SMALL PROJECTS

ENTRANT: Kennedy/Jenks Consultants
ENGINEER IN CHARGE: Douglas B. Henderson, P.E., DEE

Project Description

The Calleguas Municipal Water District (CMWD) treats water stored in Lake Bard at the Lake Bard Water Filtration Plant (LBWFP). Each spring, the lake stratifies and by early summer, the available dissolved oxygen (DO) in the lake hypolimnion is depleted. Without oxygen, reduced nutrients and metals released from the lake bottom degrade water quality, making it harder to treat.

CMWD hired Kennedy/Jenks Consultants to improve water treatability and upgrade plant operations by comparing the cost-effectiveness of two approaches: 1) increase ozone system capacity, or 2) improve source water quality. By selecting the latter, CMWD reduced ozone system operating costs at a time when liquid oxygen (LOX) supplies in California are limited and electrical power to produce ozone is expensive. Because ozone generators convert only about 6 percent of LOX to ozone, the project significantly increases the value of LOX by using off-gas to oxygenate the hypolimnion.

1. Integrated approach

Air - To protect air quality, residual ozone in the ozone contactor off-gas is destroyed before venting. LBWFP uses ozone to disinfect water, but electrical power is needed to convert air to LOX and LOX to ozone. Since only 6 percent of LOX is converted to ozone, the 94% vented as off-gas is “wasted” oxygen. The hypolimnion oxygenation system uses the “wasted” resource to improve water quality in Lake Bard, significantly reducing the amount of energy-intensive ozone used to treat the water.

Water - Lake Bard stratifies in spring, forming three layers. The epilimnion (top layer), though well oxygenated, contains warmer water susceptible to algae growth. The thermocline separates the upper layer from the colder hypolimnion. By mid-June, the hypolimnion is depleted of oxygen. This project injects oxygen off-gas from the LBWFP into the Lake Bard hypolimnion. Dissolved oxygen improves water quality, and sequesters nutrients and dissolved metals in the sediments by forming an oxidized scale at the sediment-water interface. The improved water is easier to treat, reduces the likelihood of bromate formation, requires less ozone and other chemicals for treatment, and reduces LOX purchases. Most importantly, the plant produces higher-quality, better-tasting water; saves capital and O&M costs; and reduces environmental impacts by using less energy and making beneficial use of oxygen-rich off-gas.

Land - By improving source water quality, LBWFP uses fewer chemicals and reduces both dissolved solids and sludge. Previously, when the hypolimnion became anoxic, the plant drew water from the epilimnion, which is warmer and can contain high concentrations of algae and other organisms that are filtered and eventually produced sludge.

2. Quality

The new system takes oxygen-rich off-gas from the ozone contactor and oxygenates the lake hypolimnion. Kennedy/Jenks designed the pipeline and feed points to efficiently convey and disperse oxygen throughout the lake. Initial operations began this summer and demonstrate that the system works well, with highly promising preliminary results.

CMWD is pleased with the innovation, improved water quality, and substantial capital and O&M cost savings. Using oxygen-rich off-gas to improve water quality makes the hypolimnion an aerobic environment where contaminants remain sequestered in the lake sediments. As a result, the water is easier to treat.

3. Originality and innovation

Thinking outside the box led to a design that combines existing knowledge with site-specific criteria to create a cost-effective new approach. Lake oxygenation is known to be beneficial. However, none of the 200+ water treatment plants in the U.S. using ozone reuse the oxygen-enriched off-gas in this manner. This plant has an atypical combination of conditions that make it economical to use “waste” off-gas to improve the hypolimnion:

treatment plant uses ozone to oxidize water
plant is located near source lake
hypolimnion is large enough to sustain operations through the stratified season

The cost of a new ozone generator to increase LBWFP capacity was estimated to be about $500,000 more expensive than the cost of improving source water quality with oxygen-rich off-gas from the ozone contactor. This system is less expensive to build and operate and reduces the costs of both energy and use. It also places LBWFP in a better position to comply with water quality regulations by reducing the likelihood of converting bromide (unregulated) to bromate (regulated). Moreover, the pH of the hypolimnion water is more agreeable to treatment, and the water is colder and more palatable.

4. Complexity

Designing this innovative solution required a comprehensive, thorough understanding of source water quality, sediment flux rates, and ozone and LOX systems. This project involved expertise in specialized areas including the impacts of dissolved oxygen on sediment chemistry and microbiology. The project also involved hydraulic and conveyance challenges associated with transferring oxygen to the lake bottom and dispersing it near the bottom and middle of the lake.

5. Contribution to social/economic advancement

The project significantly reduces the ozone needed to oxidize the water at LBWFP and improves control of a regulated drinking water constituent (bromate). It also prevents the release of undesirable constituents from the lake bottom into the hypolimnetic water. The project provides CMWD water customers with safer, higher-quality, better-tasting water that is more pleasing to the palate and makes the water healthier for fish and other aquatic organisms.

This is a “green solution” that cost-effectively takes a “waste product” and uses it to benefit operations and the environment. CMWD provides better water at minimal increased cost. The project saved $500,000 in capital costs compared with the alternative. Additional saving are expected and will be realized over time as operation and maintenance costs will be significantly lower than increased ozone generation and use.

This innovative solution can be used by other agencies with water treatment plants that use ozone, are close to their source water, and have a lake with a hypolimnion layer large enough to maintain a full season of stratified water supply. These agencies could implement this process to improve water treatment operations and provide a higher-quality water supply while saving substantial O&M and capital improvement costs.

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