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The Henrico County Water Reclamation Facility recently underwent a treatment plant upgrade and expansion from 45 to 75 mgd. The existing solids train included dissolved air floatation thickeners (DAFs) for waste activated sludge (WAS) thickening, anaerobic digesters with fixed and floating covers for solids stabilization, high solids centrifuges for solids dewatering, and enclosed vessel composting facilities to produce a Class A end product.

There were numerous problems with the original facilities which needed attention during the recent upgrade. The in-vessel composting facilities never performed as intended. As a result, most of these existing facilities were never used and became dormant. These dormant facilities were enclosed in a large building with numerous ancillary equipment including mixers, conveyors, blowers, and odor control facilities. One of the County's primary goals was to reuse as much of the original facilities as possible. The existing composting equipment was demolished and new equipment was installed. Among the new equipment that was installed were high solids centrifuges, gravity belt thickeners, dry polymer feed systems, and a dewatered sludge cake conveyor system to discharge to a short-term sludge storage area.

The three existing digesters were significantly modified and a fourth digester was added as a part of this construction project. The existing digesters experienced chronic foaming problems that lead to subsequent failure of the floating covers tipping and the fixed cover blowing off. These problems were addressed in a two part response. The existing cannon mix system was replaced with a pump mix system utilizing jet nozzles, and the existing floating and fixed covers were replaced with membrane covers which also allowed the abandonment of a pressurized gas sphere. The design of the anaerobic digesters included the flexibility to operate the digesters in parallel or in series.

The existing DAFs were replaced with gravity belt thickeners as a part of this project. The old DAF units were also reused. One of the DAF units was converted to a sludge blend tank to blend primary sludge with thickened waste activated sludge to strip any residual dissolved oxygen prior to anaerobic digestion.

This project serves as a success story by salvaging and reusing as much of the existing infrastructure as practically feasible. This required input from plant staff during detailed design and flexibility during construction. The new facilities have been in operation since 2004.
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Document Type: Research Article

Publication date: 01 January 2006

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