Brief Project Description
The H.L Mooney Waste Water Treatment Plant (WWTP) is owned by the Prince William County Service Authority and provides treatment services to the County of Prince William and Towns of Occoquan and Woodbridge. The existing plant has a design flow capacity of 18 MGD and is capable of nutrient reduction. The liquid treatment processes includes an influent mechanical screen and grit removal chamber with coagulant feed, followed by four primary clarifiers, five biological/aeration basins (nitrification and partial denitrification), five secondary clarifiers, ten effluent filters, methanol storage and feed facilities, disinfection and post aeration, prior to discharge to Neabsco Creek. The biosolids are gravity-thickened, followed by centrifuge dewatering and incineration.
Under a proposal for nutrient removal cost share, the design flow will be expanded to 24 MGD and the treatment processes will be upgraded to achieve State of the Art (SOA) nutrient removal. As described in the PER, the expansion and upgrade to the Mooney WWTP include the following process facilities:
- Replacement of two influent mechanical bar screens (not grant eligible)
- Expansion of the grit removal facilities with two new grit removal systems (not grant eligible)
- Expansion of the primary clarifiers with one new primary clarifier (partially grant eligible)
- Expansion and modification of the aeration basins to double their existing volume and result in five, 4-pass basins (partially grant eligible)
- Expansion of the secondary clarification with four new secondary clarifiers (partially grant eligible)
- Expansion of the denitrification filters with fourteen new filters (grant eligible)
- Expansion of the thickening facilities with two new gravity thickeners (partially grant eligible)
- Expansion of the dewatering system with one new dewatering centrifuge (partially grant eligible)
Primary Clarification- A total of five clarifiers (four existing clarifiers and one new clarifier) will be included once the project is complete. The clarifier expansion will include two new progressive cavity sludge pumps.
Aeration Tanks - The aeration tanks are being designed with four operational modes, maximizing flexibility with variability in loading and seasonal operations while ensuring the capability to maintain nitrification during extended colder periods. Normal operation is expected to be in a 4 zone Modified Ludzack-Ettinger (MLE) mode with maximized anoxic volume in the MLE configuration.
The aeration basins are provided with 3 permanent anoxic zones, 7 permanent aerobic zones, and 2 swing zones which can be operated in either an aerobic or anoxic mode. Aeration needs will be satisfied by the existing blowers which have sufficient capacity to satisfy maximum day air requirements. Air will be delivered to the basins via membrane diffusers. Mechanical mixers will be provided in the permanent anoxic zones, in the swing zones, and in Zone 10 (aerobic zone). The mixers in Zone 10 will allow plant operations to control the internal recycle dissolved oxygen without regard to mixing limitations of the diffusers (internal recycle will be from the end of Zone 10). Aeration basin 3 will have two mixers in zone 9 for the purposes of testing various modes of operation. The existing mixers will be relocated and retained.
Nitrified Recycle - One internal recycle pump will be provided per tank. Internal recycle capacity with all 5 aeration basins operating will range from 3 to 6.4 times the design flow. The expected primary effluent feed point is into the first anoxic zone, where it will mix with the RAS flow and the internal recycle flow; however, the basins are also being provided with a secondary feed point, located just upstream of the post-anoxic zone.
Waste Activated Sludge (WAS) Pumping - In lieu of the present WAS pumping scheme, WAS will be pumped from the RAS splitter box to the gravity thickeners. Two existing WAS pumps in the blower building will be removed and replaced with larger pumps; the other existing WAS pumps will remain as clarifier drain pumps.
Secondary Clarification - The existing secondary clarifiers will remain in service, four new secondary clarifiers (two sizes) will be added. At the conclusion of construction, the following will be in place: six 95-foot diameter clarifiers (5 existing, 1 new) and three new 125-foot diameter clarifiers. One new pump will be installed with same capacity as the existing pump in RAS Pump Station No. 1 to serve new Secondary Clarifier No. 6. In addition, 5 new pumps will be installed in new RAS Pump Station No. 2 dedicated to new Secondary Clarifiers No. 7 to 9.
Effluent Filtration - The upgrade will include the installation of 14 new filters in addition to the existing 10 filters. The filter upgrade is designed for an average hydraulic loading rate equaling 2 GPM/ft2 with a peak hydraulic capacity not to exceed 10 GPM/ft2.
Gravity Thickening - Based on the mass balance, it will be necessary to expand the gravity thickening facilities. The existing facilities will be retained and two additional thickeners will be constructed. Due to the low surface overflow rate of the gravity thickeners a source of water will be added to increase the overflow rate to prevent septicity of the solids.
Centrifuges - The centrifuge facilities will be expanded to include one new and 2 existing centrifuges. The design criteria are intended for a maximum of two centrifuges to be operating at average day maximum month loading criteria. A fourth centrifuge will be installed under Phase 2 construction, which is not grant eligible.
Chemical Pumping- Following the upgrade, there will be seven polymer feed points, as follows:
- Influent to Primary Clarifier No. 1-4 (existing)
- Primary Clarifier No. 5 (new - influent mixing box)
- Secondary Clarifier Influent Splitter Box (new - to serve all 9 secondary clarifiers)
- Gravity Thickeners Nos. 1 (new)
- Gravity Thickeners Nos. 2 (new)
- Gravity Thickeners Nos. 3 (new)
- Gravity Thickeners Nos. 4 (new)
After expansion the plant will have three chemical feed pumps to provide ferric chloride to the aeration basin discharge, two pumps to provide ferric chloride to the primary clarifiers and one back-up pump capable of pumping to any of these locations.
Three pumps will be required to pump lime slurry to the aeration basin splitter box out of which one will be a standby pump.
The facility will have ten methanol feed pumps. Feed pumps will be provided to each aeration basin influent splitter box (total 2 pumps), each aeration basin second anoxic zone (total 5 pumps) and to the final filter influent diversion chamber (1 pump). Two additional pumps are provided for standby service.