Sewer Diversion System Conditions Flow to Limit Odors, Corrosion

Faced with the need to divert wastewater down a 10-foot vertical drop, California's Orange County Sanitation District (OCSD) used a flow conditioning insert to minimize odor and corrosion problems associated with the diversion.

OCSD is the third largest wastewater agency west of the Mississippi River, collecting and treating about 230 million gallons of wastewater daily from more than 2.5 million residents, 21 cities, and more than 450 square miles of service area.

By installing the Vortex Flow Insert, the Orange County Sanitation District ended up with more flexibility in how they manage flow, and they saved a significant amount of money by not having to deal with odors or upgrade pump size on the 30-inch sewer line.

With more than 600 full-time staff, 580 miles of sewer pipe, and two treatment plants, OCSD releases treated water into the Pacific Ocean through a 120-inch diameter offshore pipeline that extends five miles from shore about 200 feet below the surface.

As a recipient of the National Environmental Achievement Award for excellence in operations, OCSD is known for its environmentally friendly projects and methods, including the daily reclamation of 10 million gallons of treated wastewater for landscape irrigation, groundwater protection, and industrial use.

Diversion Strategy

At the south side of the John Wayne Airport employee parking lot, which lies just north of Main Street in Irvine, OCSD needed to divert sewage flow from a 30-inch diameter sewer line to the 66-inch Main Street trunk located 10 feet below.

"On the downstream end of the 30-inch pipe, we had a pump station that needed to be refurbished and upgraded. We knew that the potential flow during a wet weather event was more than the existing size pumps could handle, yet we didn't want the cost of installing larger pumps," said Chuck Winsor, collection facilities manager for OCSD. "We decided to build a diversion structure that would allow us to intercept the flow and send it either down the 30-inch pipe as usual or divert some or all of the flow to the larger capacity 66-inch pipe below."

California's Orange County Sanitation District collects and treats 230 million gallons of wastewater daily from more than 2.5 million residents, 21 cities, and more than 450 square miles of service area.

To drop the flow from the 30-inch pipe down to the 66-inch pipe, OCSD needed to deploy a vertical drop in the alternate sewer pipe alignment inside a new 84-inch- diameter manhole downstream from the diversion structure. Hydrogen sulfide gas and other odors are common in conventional sewer drops due to turbulence, causing many municipalities to ban them or use more costly methods such as a gradual grade, which increases sewer pipe length.

To avoid odors and additional expense, OCSD installed a Vortex Flow Insert from IPEX USA, a supplier of thermoplastic piping systems. With no moving parts and requiring virtually no maintenance, the insert is designed to eliminate odors and minimize corrosion in sewer drops.

Flow Insert

The Vortex Flow Insert's patented spiral flow design creates a downdraft that traps odorous gases and sucks them down toward the bottom where they are entrained back into the sewage flow. Wastewater flows into the top of the insert where it is directed around a channel of decreasing radius that accelerates the wastewater flow to a higher velocity. Once the flow is channeled downward, that velocity causes the flow to hug the inside walls, creating a negative air core that draws airborne gases downward.

"By installing the new diversion structure and the Vortex Flow Insert, we ended up with a lot more flexibility in how we manage the wastewater flow. In dry weather we can still use the 30-inch sewer, and in a really high-flow situation or for maintenance purposes, we can instead divert flown down through the Vortex to the 66-inch sewer," Winsor said. "We still refurbished the pumps at the downstream end of the 30-inch sewer, but with the diversion and Vortex, we didn't need to upgrade the size of the pumps or deal with odors. That was a significant cost reduction."

To intercept flow and send it down the existing 30-inch pipe or divert some or all flow to the larger capacity 66-inch pipe, the Orange County Sanitation District installed a diversion structure upstream of the Vortex Flow Insert.

IPEX custom designs and builds the inserts to meet the unique needs of each application. The drop height can be as little as five feet or more than 100 feet, and they are sized based on the peak flow that the unit is required to handle.

"When we began the Irvine diversion project north of Main Street, we originally provided incorrect information on the peak flow, and our first Vortex Insert was undersized. When we put the flow through, it splashed and created turbulence and odors," Winsor said. "Even though we provided wrong information, IPEX worked with us to determine the correct size, and they replaced the unit with a larger Vortex Insert able to handle the peak flow and prevent splashing. In the end, we ended up with no odors and a better space where we can lower a person down to a platform above the water level that allows maintenance to be performed."

The insert was ultimately sized for 10 million gallons per day for peak wet weather flow.

OCSD also used the system at its Huntington Beach Wastewater Treatment Plant No. 2., where the district is installing a large new headworks area and replacing one of the off-site pump stations. The pump station needs to ensure that the flow is discharged at higher elevation at the downstream end and then dropped down to the treatment plant into the new headworks. OCSD has installed a 40 mgd Vortex Flow Insert for the sewer drop at the treatment plant, which will be operational in 2011 when the new pump station is complete.

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