The city of Grass Valley owns and operates the Grass Valley Wastewater Treatment Plant that provides service for a population of approximately 12,000 people. Situated in California’s Gold Country, the area has some challenges that include mine drainage and heavy flows during the winter storm season in the scenic Sierra Nevada foothills. On average, the City discharges 2.1 million gpd of treated wastewater effluent to Wolf Creek, a tributary of the Bear River.
The Challenge: Equipment End of Life
The plant’s rectangular clarifiers use chain and scraper systems for sludge collection, scum pipes for scum collection, and a cross collector system to consolidate sludge across the three clarifier basins into a single sump. The clarifiers were approaching the end of their service life in 2013, placing a burden on the system and its operators. Matters came to a head when the collector system in two of the tanks became inoperable due to a breakdown of the triple-output shaft drive system. The outage left just one of the plant’s three secondary clarifier tanks operational, and threatened to shut down the entire system.
The Solution: Envirex Chain and Scraper Systems
In response to a proposal from MISCOwater, Evoqua Water Technologies Sales Partner, Grass Valley commissioned an upgrade project to replace the chain and scraper systems, the drive system and the scum pipes with new equipment. With the plant’s secondary treatment system operating with limited capacity, a fast response and efficient completion of the project was a Priority.
Work began in December 2013, with installation performed by Sacramento-based contractor NMI Industrial Holdings Inc. The project was planned and executed as a complete turn-key solution through MISCOwater in order to get the secondary treatment systems back online as soon as possible.
The chain and scraper systems were replaced with Envirex Chain and Scraper sludge collection systems from Evoqua. These systems feature non-metallic chain components with individual drives and simple lever operated scum pipes. The non-metallic chain technology was pioneered by Envirex, and is now considered the standard in the industry. The system’s lightweight design made installation easy and simplifies maintenance.
Triple Trouble: Cost-Effective Drive Replacement
The project included replacing the existing triple-output gearbox used to drive each chain and scraper, as well as the cross collector. The failure of this gearbox had been partially responsible for disabling the equipment and was the triggering event for the upgrade project. However, replacing this gearbox with a similar triple-output system would prove costly, with a lead time that was too long to meet Grass Valley’s requirements. Evoqua replaced the triple-output system with three individual drives to reduce costs, improve maintainability, and keep the project on track.
Scum Pipes: Longer Operating Life
Scum pipes were replaced with the latest Evoqua design, featuring UHMW polyethylene liners on the end supports for easy operation, corrosion resistance, and longer operating life. In addition, at Evoqua’s recommendation, Grass Valley elected to replace the existing worm gear positioners, which had reached the end of their operating life, with lever positioners. The lever positioners are easier to operate and maintain and promise more reliability and longer life.
Results: Downtime Eliminated, Efficiency Improved
The Grass Valley project had a high-risk, compressed schedule that required a rolling shipment of components to coordinate with the contractor’s time on site. The execution was highly interactive and characterized by frequent customer contact. As the OEM for the chain and scraper equipment, Evoqua was able to move quickly in providing new equipment. Although the purchase order was received with only 10 weeks to deliver the equipment, the project team facilitated installation by early October 2014, preceding the region’s winter storm season by about 3 weeks.
With the new equipment in place, downtime and maintenance related issues have been eliminated. In addition, the new Envirex chain and scraper systems have increased the efficiency of solids removal at the plant and operating life has been extended for another 15 to 20 years.
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