Transitioning from Plate and Frame to High Solids Centrifuge Dewatering
- Jennifer Whitaker and Robert Wichser - Rivanna Water and Sewer Authority
- Laurissa Cubbage, Michael Bullard, and Hunter Long - Hazen and Sawyer
The Rivanna Water and Sewer Authority (RWSA) Moores Creek Wastewater Treatment Plant (MCWWTP) is currently undergoing upgrades for enhanced nutrient removal at a flow of 15 mgd with provisions for a future expansion to 20 mgd. As a part of the upgrade project the existing residuals dewatering unit process was upgraded to replace a plate and frame filter press with a high solids centrifuge dewatering unit. The focus of this paper compares the two dewatering technologies from bench scale testing and onsite pilot testing to final construction and full scale performance testing of the high solids dewatering centrifuge.
The MCWWTP’s solids processing train includes anaerobic digestion of primary and thickened secondary treatment solids followed by dewatering prior to disposal at an off-site contract composting facility. The existing dewatering facility, prior to upgrade, includes two horizontal recessed plate and frame (P/F) style filter presses. The older of the two P/F presses was installed approximately 25 years ago and has been out of service for the last several years requiring extensive repairs to return it to service. Currently, the dewatering service is limited to the output from the single operating P/F press which has relatively stable performance.
High Solids Centrifuge dewatering equipment was added to the dewatering unit processes to meet additional dewatering demands associated with the 15-mgd enhanced nutrient removal upgrade project at MCWWTP. Selection of equipment was based on bench-scale performance testing and on-site performance testing for prequalification. Ultimately, Westfalia Separator was selected during the competitive bidding for the project. The newly installed HSC equipment was started and performance tested in mid-June 2011.
The testing consisted of four runs of six hours each. During the testing period, the feed solids decreased in total solids and thus this run was omitted from the data set. The performance specifications only required three six-hour runs to meet all requirements.
The HSC dewatering equipment provides a “win-win” for the MCWWTP. Based on performance testing and the same throughput per technology, the HSC delivers approximately 1200 wet pounds less cake per dry ton processed than the existing P/F press due to the avoidance of ferric and lime additions to condition solids. The cake is ultimately trucked off-site to a contract composting facility generating trucking and disposal fees; therefore the production of less wet dewatered cake results in operating savings to the utility. Lastly, the facility will see a reduction in chemical cost by avoiding ferric and lime for sludge conditioning. Detailed economic comparisons for capital and operating costs will be presented in the presentation.
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