June 2010, Vol. 22, No.6

Problem Solvers

California Water Agency Searches for OzoneContactor Installation Option

Problem: A water treatment plant needed to expand its capacity and convert an alternative filtration treatment process to an ozone contactor while minimizing costs and downtime.
Solution: Installation of a sidestream ozone contactor using venturi injectors.

 

As with many other water providers, the Castaic Lake Water Agency (CLWA; Santa Clarita, Calif.) was confronted with a growing population that required additional water resources. CLWA, a public water wholesaler, operates two treatment plants, two pump stations, two storage facilities, and more than 27 km (17 mi) of transmission pipelines to deliver water to the Santa Clarita Valley, according to the agency’s Web site.

To increase capacity, both its Rio Vista Water Treatment Plant and Earl Schmidt Filtration Plant recently underwent major expansions, according to a Mazzei Injector Co. (Bakersfield, Calif.) news release. The 114,000-m3/d (30-mgd) Rio Vista plant was a conventional facility utilizing pre-ozonation for primary disinfection, followed by chemical coagulation, upflow contact clarification, and media filtration. To meet the growing community’s water needs, the plant needed to expand to 227,000-m3/d (60 mgd). As part of the plant’s expansion, an alternative filtration treatment process was converted to an ozone contactor, the news release says.

“CLWA wished to avoid the construction costs and down-time associated with traditional ozone contact basins,” said Jim Jackson, manager of water and wastewater municipal systems at Mazzei Injector.

CLWA hired Black & Veatch (Overland Park, Kan.) to conduct a feasibility study on the use of a pipeline contactor for the plant expansion. The engineers considered two different sidestream gas-injection-pipeline contacting designs and selected the design that incorporated a Pipeline Flash Reactor™.

CLWA chose this design because of previous success with this system at the Earl Schmidt plant, Jackson said. The Earl Schmidt sidestream injection system had an ozone dosage range of 1.0 to 9.6 mg/L and achieved ozone transfer efficiency greater than or equal to 99%, Jackson said.

The system installed at the Rio Vista plant includes four sidestream injection systems, with three operating regularly and one on standby, Jackson said. Venturi injectors utilize a high-velocity water flow to aspirate and mix ozone gas into the pressurized sidestream. The sidestream flow then discharges into an 1829-mm (72-in.) reactor, where a high-velocity nozzle provides ozone transfer. After passing the nozzles, the flow reaches a multi-element 2565-mm (101-in.) static mixer that is being used as a tertiary bulk-water-mixing device. “The typical municipal Pipeline Flash Reactor installation does not utilize a tertiary bulk-water-mixing device,” Jackson said. “However, the design engineer for CLWA required the placement of static mixer elements following the high-velocity nozzles to ensure that he would get a stable dissolved ozone reading at the discharge point.”

The reactors are constructed of ozone-resistant 316 L stainless steel and designed to provide rapid dissolution of gas to solution while eliminating the downtime and confined-space maintenance associated with fine-bubble diffusion basins, Jackson said.

“In new plant designs, moving the ozone-gas mixing out of the basin and into the pipeline allows for a more innovative contactor design,” Jackson said. The inclusion of a reactor in front of an existing ozone contacting basin eliminates the gas-mixing zones in the basin and increases the hydraulic detention time, resulting in an increase in CT credits — CT units are a product of residual disinfectant concentration and contact time — and microbial inactivation. “Contactors may be smaller, dedicated to providing the contact time needed for microbial inactivation,” Jackson said. “This reduces the installation footprint and allows for the utilization of serpentine-type contact basins, which are easier to clean.”

In some installations, such as the Rio Vista plant, the basin can be eliminated because ozone offgas removal utilizing a pipeline offgas device allows the post-injection pipeline to become a plugflow ozone contactor, where all the water moves at the same velocity, Jackson said.

 

©2010 Water Environment Federation. All rights reserved.