Brief Project Description
The Town of Tappahannock proposes to retrofit their existing 0.8 MGD oxidation ditch treatment process with enhanced nutrient reduction (ENR) technology. There is no design flow expansion involved in this project and the ENR modifications are generally described as follows.
- In-line Equalization Basin: A new mixing system will be installed so the equalization basin will be capable of being mixed without the use of oxygen. This will promote anoxic and anaerobic conditions which will make the influent flow and recycle streams more amenable to ENR treatment.
Activated Sludge Treatment: A new anoxic basin will be constructed to include anoxic mixing and will serve as the first stage of a 4-Stage Bardenpho process. The new anoxic basin will be split into two (2) trains. The existing oxidation ditch basins (two existing trains) will each be converted into the other three stages of the 4-Stage Bardenpho process (i.e., aerobic zone 1, anoxic zone 2, and aerobic zone 2). The activated sludge treatment process will consist of two separate trains, which will utilize the existing four (4) secondary clarifiers as currently practiced. Each “converted” oxidation ditch will have nitrate recycle pumps, anoxic mixers, and fine bubble diffusers. Three new concrete block walls will be constructed in each “converted” oxidation ditch to promote separation between the aerated and non-aerated zones. New activated sludge blowers will be installed to provide aeration via the new fine bubble diffusers.
Recycle Pump Station: A new recycle pump station will be constructed to facilitate the return of nutrient laden recycle streams to the equalization basin for bio-augmentation.
Tertiary Filtration: Deep-bed tertiary filtration will be provided to achieve total suspended solids and particulate phosphorus removal. A filter feed pump station will be constructed to feed the tertiary filters. The four (4) cell filtration system will have a total of 185 square feet of filtration area and are designed for a peak hour loading rate of 6 GPM/FT2. The filter system will also contain a clear-well tank and backwash waste holding tank. Other ancillary equipment include: backwash water feed pumps, backwash air scour blowers and electric actuated valves and controls necessary for filter operation.
Aerobic Digestion System (partially eligible): To accommodate additional solids production resulting from ENR, a new aerobic digester with associated blowers and coarse bubble diffusers will be constructed to produce class B biosolids. Because the existing digester is adequate for disposal, the new digester is more a function of disposal method (operations) and not a necessity for nutrient reduction technology.
Sludge Dewatering (partially eligible): To accommodate additional solids production resulting from ENR, existing dewatering capacity will be augmented with an additional (identically sized) belt filter press. Dewatering needs are based on design flow; since the facility is only operating at about 50% of the design flow, installation of this unit could be delayed for 10 or so years. Thus, the project will include this item as an additive/deductible bid item with a determination as to whether or not to include the process, once bids are tabulated.
- Additional emergency/back up power will be provided for the new equipment being installed as part of the upgrade project.
- The existing Supervisory Control and Data Acquisition (SCADA) system will require modifications to facilitate the new equipment and unit processes being installed as part of the upgrade project.
Chemical Feed Systems:
- One new Sodium/Magnesium Hydroxide (supplemental alkalinity) storage will be constructed to ensure optimum pH for complete nitrification is maintained.
- A new supplemental Carbon storage system and feed to the second anoxic zone will be constructed to ensure the total nitrogen limit of 4.0 mg/l is achieved.
- Existing alum storage and feed systems will be improved to meet greater storage and feed demands. Aluminum Salt will be added just prior to the secondary clarifiers for phosphorus removal.
- A new polymer system to aid sludge settling during periods of poor sludge settle-ability, which is a direct result of long solids residence times to achieve low effluent total nitrogen, will be included in the upgrade project.