WQIF - Onancock

 

 Project Grant Amount Grant Percentage
Onancock $2,790,07253 90%
 Revolving Loan Fund Project DEQ Regional Area Date Agreement Signed
Yes Tidewater Regional Office, Virginia Beach 3/13/07

Brief Project Description

The Town’s existing 0.25 MGD plant, capable of partial nitrification, will be replaced with Membrane Bioreactor (MBR) technology at an expanded design flow of 0.75 MGD..  The existing treatment process consists of grit screening and flow equalization followed by two steel package plants, which use an extended aeration activated sludge process and secondary clarification prior to disinfection.  The engineer felt the existing flow equalization tank (0.5 MGD) is sufficient for the proposed upgraded/expanded facility; thus, it will be retained for use ahead of the new MBR system.

Wastewater Treatment: The MBR configuration will employ 5 stages for treatment: (1) Return activated sludge de-aeration/primary anoxic denitrification; (2) Anaerobic; (3) Oxic nitrification; (4) Secondary anoxic denitrification; and (5) Membrane filtration (ultra-filtration), followed be reaeration.  The design incorporates flexible swing zones between most stages with a controlled dissolved oxygen level that allows simultaneous nitrification/denitrification.  Swing zones provide a way to modify the treatment process in response to seasonal variations.  A fine-bubble diffused aeration system will be provided in the aerobic and aerobic swing zones of each basin. 

The de-oxygenation, anaerobic, and anoxic zones will be provided with mixers to keep the mixed liquor in suspension.  Swing zones that may be aerobic or anoxic will also have mixers. To achieve denitrification in the process, nitrified mixed liquor will be recycled from the end of the aerobic reaction to the primary anoxic zone where denitrification will occur.  Since the aerobic reaction may end in either Zone 3 or Zone 4, each of these zones will have a recycle pump.  The pumps will be able to discharge to Zone 1 or Zone 2. The internal recycle capacity will be 400% of the maximum month flows.  If internal recycle should not be sufficient to achieve low total nitrogen (TN) concentrations, supplemental carbon may added to the process at the first secondary anoxic zone (Zone 5).  Methanol is included in the design for use as the supplemental carbon.

The use of membrane filtration for solids separation allows higher mixed liquor concentrations in the aeration tanks with smaller reactor volume; however, it requires fine screening to protect the membrane filters.  The MBR filtration system will be immersed in an aeration tank, with the membranes, air diffusers and associated water and air piping contained in a factory-assembled cassette.  For the proposed design flow and at the preliminary design stage, the plant will have 16 cassettes arranged in four trains which include two separate tanks of 8 cassettes each.  A total of 32 modules will be provided; however, this configuration could change based on the final design and receipt of bids.  The MBR system with fine screening will allow for biological phosphorus removal, which will be further enhanced through chemical addition of ferric chloride and polymer to the treatment process.

The components of the facility upgrade/expansion and their estimated eligible percentage for cost share include the following:

  • Influent fine screens suitable for MBR (~50%)
  • Construction of concrete bioreactor tanks (~57%)
  • Construction of a membrane tank, permeate pumping, and filtration equipment (~50%)
  • Supplemental carbon (methanol) feed system for TN polishing (100%)
  • Coagulant (ferric chloride) and polymer feed systems for TP removal (100%)
  • Thickening of waste sludge, with associated blowers and WAS conveyance system (25%)
  • Prorated costs of associated site work and yard piping
  • Prorated costs of electrical, process control, and HVAC
  • Prorated costs of contingency, testing, and general requirements
  • Prorated cost of engineering design and construction management services
  • Prorated Preliminary Engineering costs
  • Prorated cost of engineering design and construction management services

Items that are not eligible for cost share include the following:

  • Aeration components (due to ammonia control requirements).
  • UV disinfection process.
  • Laboratory building / furnishings.
  • Plant Utility Water System.

Solids Handling:  At the design flow of 0.75 MGD, the BNR process design calculations predicted waste activated sludge (WAS) to produce ~1,300 dry lbs/day.  The WAS solids concentration is expected to be about 1% (10,000 mg/L) after secondary clarification resulting in a daily liquid sludge production of about 16,000 GPD at design flow.  The sludge handling and digestion processes were each sized based on this predicted sludge production rate.   The sludge production rate for mechanical dewatering (dry lb/day) was determined using the following assumptions:  WAS volatile fraction = 80 percent and digester volatile solids reduction = 40 percent of volatile fraction.  Based on these assumptions the resulting daily solids production at the plant design capacity is expected to be ~ 900 dry lb/day.  The sludge handling and dewatering process evaluation was based on this sludge production estimate.

The Town of Onancock WWTP currently has a 30-ft diameter by 26-ft deep concrete tank (nominal volume of approximately 132,180 gallons) that is used for aerobic digestion.  Aerobic digester design calculations were performed to determine if the existing aerobic digester had enough storage volume for sludge stabilization at the upgraded plant capacity of 0.75 MGD.   Based on the design parameters, the required aerobic digester volume for WAS stabilization at design flow is estimated to be 125,000 gallons.  The existing concrete digester tank (132, 000 gallons) was therefore deemed to have adequate capacity for continued use in the new sludge management system.

The capacity of the existing belt filter press (BFP) was evaluated under the assumption that the system would be operated four hours per day, five days per week and that the feed solids concentration applied to the BFP is about 2%.  Using these assumptions, the existing BFP would need to be operated for up to 6 hours per day, five days per week to process sludge produced at design flow.  The PER evaluation assumed the BFP loading rate is about 220 dry lb/hr with an average hydraulic feed rate of about 22 GPM.  Based on this preliminary evaluation, it was decided the capacity of the existing BFP should be sufficient for sludge dewatering at the expanded design flow. 

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Virginia Department of
Environmental Quality
P.O. Box 1105
Richmond, VA 23218
(804) 698-4000


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