Brief Project Description
The existing Woodstock wastewater facility is a 0.8 MGD average discharge flow plant, with a 1.0 MGD-rated design capacity that is capable of secondary treatment with partial nitrification. The existing treatment process consists of grit screening, two oxidation ditches, two secondary clarifiers, a sludge pumping station, a polymer addition system, aerobic digestion of waste sludge with sand drying beds, and post-chlorination before discharge into the North Fork of the Shenandoah River.
The project funding involves two phases as described below.
Contract 1 entails the prepurchase of the permanent Membrane Bioreactor equipment system as well as a mobile MBR system for use during construction. The pre-purchase contract includes the furnish and delivery of: Membrane cassettes for ENR biological treatment, membrane cassettes for mechanical sludge thickening, Permeate pumps for both biological treatment and sludge thickening, membrane aeration blowers, thickening aeration blowers, process blowers, and ancillary equipment including process and air piping and valves, controls, staging tank pump, membrane cleaning system and air compressors and air drying system. This facilitates plant retrofit to ENR and maintains wastewater treatment through construction.
Contract 2 entails the general construction of the 2.0-MGD Expansion and ENR Upgrade Project, including the installation of the mobile MBR system for construction sequencing and the permanent MBR filtration system furnished and delivered under Contract 1. Contract 2 also includes the construction of: new fine screens and building, deoxygenation tank (associated with MBR), ENR bioreactor retrofit of existing oxidation ditches; construction of the Membrane Filtration Building housing process equipment, sludge holding, new UV disinfection, and O&M facilities; construction of a new Solids Handling Building including centrifuge dewatering and Class A lime-heat pasteurization equipment; as well as post aeration cascade, plant drain pump station, and site improvements.
Under the proposal submitted for nutrient removal cost share, the plant will be expanded to a permitted capacity of 2.0 MGD with State of the Art nutrient removal capability. The items that are eligible for cost share include the following:
- Influent fine screens suitable for MBR (~50%)
- Retrofit of both existing oxidation ditches with modified baffling, mixing, aeration, and internal recycle (70%)
- Construction of a membrane tank, permeate pumping, and blowers (75%)
- Supplemental carbon (acetic acid) feed system for TN polishing (100%)
- Coagulant (alum or ferric chloride) and polymer feed systems for TP removal (100%)
- Post-aeration cascade steps, needed due to low DO driven by nutrient removal (100%)
- Membrane thickening of waste sludge/scum, with associated blowers and WAS conveyance system (53%)
- Installation of two sludge dewatering centrifuges, with liquid sludge and sludge cake conveyance systems, and polymer feed system (53%)
- Installation of heat-lime stabilization process for Class A biosolids (53%)
- 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 Preliminary Engineering costs
- Prorated cost of engineering design and construction management services
Items that are not eligible for cost share include the following:
- UV disinfection process
- Laboratory building / furnishings
- Plant Utility Water System
Each of the existing oxidation ditches will be converted into one treatment train. The total volume of biological reaction tanks required is 1.70 MG. There is only 1.36 MG of volume available in the existing oxidation ditches; hence, some new reaction tanks will have to be built. Each treatment train will have the following reaction steps:
- RAS Deaeration
- Anaerobic Selection
- Primary Anoxic Denitrification
- Oxic Nitrification
- Secondary Anoxic Denitrification
Each reaction step will be divided into multiple completely mixed zones to enhance treatment efficiency. The zones in the existing oxidation ditches will be created by installing fiberglass baffles. A fine-bubble diffused aeration system will be provided in the oxic and oxic swing zones of each basin. The diffusers will be flexible membrane discs. The diffused aeration system will be designed to satisfy the BOD and fully nitrify the influent wastewater.
In order to provide operational flexibility, swing zones will be created between most steps. Swing zones provide a way to modify the treatment process in response to seasonal variations. The deaeration, anaerobic, and anoxic zones will be provided with mixers to keep the mixed liquor in suspension. Swing zones that may be oxic or anoxic will also have mixers. To achieve denitrification in the process, nitrified mixed liquor will be recycled from the end of the oxic reaction to the primary anoxic zone where denitrification will occur. Since the oxic reaction may end in either Zone 11 or Zone 12, each of these zones will have a recycle pump. The pumps will be able to discharge to Zone 4 or Zone 5. The internal recycle capacity will be 300% 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 (either Zone 12 or Zone 13). Acetic acid is included in the design for use as the supplemental carbon source due to its easier handling requirements, versus methanol, which has explosion risks and may require fire protection.
Hollow-fiber ultrafiltration membranes will be immersed in an aeration tank, in direct contact with mixed liquor. Through the use of a permeate pump, a vacuum is applied to a header connected to the membranes. The vacuum draws the treated water through the membrane wall and into the hollow core of the membrane. Permeate then passes through the permeate pump and is discharged to the disinfection facilities. Intermittent airflow is introduced to the bottom of the membrane module, producing turbulence that scours the external surface of the hollow fibers. This scouring action transfers rejected solids away from the membrane surface. The membranes are also back-pulsed to remove solids from the membrane surface. The membranes, course-bubble air diffusers and associated water and air piping are all contained in a factory-assembled cassette. For the design flows, the WWTF will have 24 cassettes arranged in three separate tanks of 8 cassettes each.
The MBR system with ultra-filtration will allow for biological phosphorus removal, which will be further enhanced through chemical addition of ferric chloride and polymer to the treatment process.