Successful Siphon Bypass
In wastewater applications, siphons work to divert sewage under large obstructions such as rivers or streams. In such applications, a siphon structure serves as a basin that collects sewage on one side, then delivers it through underground pipes to a slightly more elevated basin on the other side using atmospheric pressure.
A Leaky Siphon
The San Antonio Water System (SAWS) makes use of a siphon capable of 92 million gal per day (mgd) in its Historic Mission Reach area.
Installed in the 1940s, this 54-in. double-barrel siphon diverts roughly 21 to 40 mgd of sewage under the San Antonio River. Though it had been successfully repaired throughout its years of service, the siphon showed signs of leaking in late 2006. SAWS determined that the structure was irreparable and needed to be replaced as soon as possible.
“We realized that we were continuing to make repairs to a leak at the base of this structure, and that there was the potential for creating a larger problem if we kept trying to make repairs to it,” said Ernesto Leal, superintendent for SAWS. “Obviously, we wanted to proactively avoid a catastrophic event.”
A Reliable Solution
SAWS elected to replace its failing high-flow siphon structure with newer technology that depends on buried fiberglass pipe. When the pipe is in place, the fiberglass makeup will deliver the flow of sewage through the line more quickly than the previous concrete structure. In addition, having the pipe buried underground presents less surface exposure to sewage overflow.
The SAWS management team—including Leal, Mike Sellers and Gordon Mahan—developed a plan to dig and replace the structure. At the same time, they called on Godwin Pumps—on contract with the organization—to provide a bypass solution that would divert the maximum peak flow of 40 mgd, with ability to handle wet peak flows of 70 to 92 mgd.
A Large Bypass
“I have been working with SAWS since 1992,” said Godwin Pumps San Antonio Branch Manager Greg White. “However, this is by far the largest bypass we have designed for them, and it involved crossing the San Antonio River, which presented a challenge.”
Within 20 days of receiving the call from SAWS, Godwin had designed, delivered and installed a complete system that included 12 Critically Silenced Dri-Prime CD300M (12-by-12-in.) pumps and four open-unit CD300M pumps for standby. Each pump is capable of delivering flows to 6,000 gpm and heads to 200 ft.
Powered by Caterpillar C-9 engines, the pumps were equipped with PrimeGuard controllers. The PrimeGuard is a digital control system that can be programmed to vary pump and diesel-engine speed based on input from level floats, allowing the pumps to start and stop automatically without an operator. These controllers came in handy on Feb. 1, 2009, when the use of more pumps was required to keep up with the extraordinary flow that was generated during the Superbowl.
In addition to the diesel-driven pumpsets, Godwin installed five 18-in. and two 24-in. high-density polyethylene (HDPE) pipes, each running 3,800 ft for a total of five miles of pipe. While a majority of the HDPE pipe setup was routine, the pipe did eventually have to cross the San Antonio River.
The pumps were situated upstream of the leaking siphon, intercepting the flow from the two 54-in. lines through a custom-designed and -built concrete suction pit. Bypass pumping began on Nov. 5, 2008, and ended on March 6, 2009, after the new 54-in. fiberglass siphon structure installation was completed.
“Our short-term goal was to bypass the leak at the old siphon structure and to prevent sewer from spilling into the San Antonio River,” Leal said. “In the long run, we are also looking to increase sewer flow with the new siphon.”
Making use of a custom-built suction pit and automatic self-priming pumps with digital controllers, and installing the latest in fiberglass pipe technology, SAWS has taken this Historic Mission Reach project to test the effectiveness of “new technology” and is happy to report success.