Brief Project Description
The Rivanna Water and Sewer Authority (RWSA) owns and operates an advanced wastewater treatment plant, VPDES Permit #VA0025518, currently rated for a design flow of 15.0 million gallons per day (MGD) with a discharge to Moores Creek. The existing Moores Creek Advanced Wastewater Treatment Plant (AWTP) was originally constructed in two phases.
The first phase was constructed in the late 1950’s and included a grit basin, grit decanting bed, pre-aeration basins, intermediate, primary and final clarifiers, primary and secondary trickling filters, sludge drying beds, and primary and secondary digesters. These treatment units were located on the north side of Moores Creek.
The Moores Creek AWTP was upgraded in 1981 and incorporated preliminary screening and grit removal, daily flow equalization, primary clarification, biological treatment, secondary clarification, chlorination and tertiary settling in flocculation and settling basins, as well as in final clarifiers. Dechlorination was added during a later improvement. These treatment facilities were constructed on the south side of Moores Creek. Some of the old treatment units remained in operation while others where taken out of operation. In 1988, the aeration basins, which provide biological treatment, were retrofitted with limited modifications in an attempt to create an initial anoxic zone. Due to the fact that the anoxic zone is mixed using diffused aeration, the existing facility configuration can not reliably provide significant biological nutrient removal. Solids are thickened, anaerobically digested, and compressed through filter press equipment before disposal.
New facilities will be designed for an average design flow of 15 MGD and a peak flow of 37.5 MGD with provisions for possible future expansion to a design capacity of 20 MGD and a peak flow of 50 MGD. No expansion of the average design flow will result from this upgrade project.
The Preliminary Engineering Report (PER) prepared for the Grantee by Hazen and Sawyer, P. C., recommended the following conceptual modifications, which could be further refined on the basis of findings during final design or the receipt of bids.
- The Rivanna Pump Station is not currently provided with bar screens to protect the pump. Mechanical bar screens may be provided at the option of the Grantee for pump protection and to improve the pump station reliability (not grant eligible).
- Refurbishment of existing grit removal facilities may be provided at the option for the Grantee (not grant eligible).
- Conversion of the four existing aeration basins to a five stage nutrient removal process and construction of a fifth basin. Internal baffling of the reactors would provide dedicated anaerobic, pre-anoxic, aerobic, post-anoxic and reaeration zones. The anaerobic and anoxic cells should also be capable of being operated aerobically. It is recommended that the proposed fifth aeration basin be configured such that it can be isolated to provide for dedicated side stream recycle treatment. It is also recommended that the basins be configured to allow wet weather step feed operation to reduce solids loading to the clarifiers during sustained wet weather flows.
- Nitrified Recycle (NRCY) pumping capacity between 200% and 400% of the plant influent flow is recommended to return nitrate from the end of the aerobic zone upstream to the anoxic zone. The basins would be configured to provide either four or five stage treatment by allowing distribution of NRCY to either the anaerobic or pre-anoxic zone. The recommended NRCY pumping range for the 15 MGD design flow is 30 to 60 MGD. High flow, low head pumps are recommended for this application. It is recommended that individual pumps be provided for each aeration basin to maximize process flexibility; two 6.7-mgd pumps per aeration basin would provide 400% NRCY pumping capacity. The need for spare NRCY pumps will be evaluated during final design.
- Construction of supplemental carbon storage and feed facilities is necessary to provide adequate substrate for denitrification. Carbon feed to the post-anoxic zone will be provided for this purpose.
- Construction of chemical precipitant feed facilities to provide for chemical phosphorus removal is required to meet the proposed TP requirements. In addition to providing chemical feed upstream of the tertiary flocculation/sedimentation basins, feed points are recommended at the primary and secondary clarifier influent to allow for multi-point chemical addition. Chemical feed is also suggested upstream of the in-plant clarifiers to reduce the effect of solids recycle streams on influent phosphorus concentrations.
- Construction of alkalinity feed facilities to prevent pH depression, which can reduce nitrification and denitrification. Alkalinity should be provided upstream of the aeration basins.
- The existing medium bubble tube diffusers should be replaced with fine bubble membrane disc diffusers to provide greater aeration efficiency. Automated dissolved oxygen control was recommended to optimize energy usage and reduce the introduction of oxygen to the anaerobic and anoxic basins. Replacement or rehabilitation of the existing blowers will be evaluated during detailed design (not grant eligible).
- Construction of one additional 160-foot diameter secondary clarifier for adequate solids settling capacity and biomass retention at the increased mixed liquor suspended solids (MLSS) concentrations and peak hydraulic flows proposed for the nutrient removal improvements. Polymer feed facilities to enhance solids separation under stressed or wet-weather conditions are also recommended.
- Construction of a new return activated sludge (RAS) pump station to serve the new clarifier was recommended. Two 4,500 GPM pumps will provide sufficient firm capacity at the 15 MGD average day design flow. Each clarifier would be provided with a dedicated pump for operational flexibility and the existing RAS pumps would be modified to provide for dedicated sludge withdrawal.
- Construction of deep bed polishing tertiary filters is required to provide adequate solids removal to meet the low TP requirement. The filters, which consist of about half the surface area needed for complete denitrification of the effluent, are designed for a loading rate of 4 GPM/FT2. In addition to constructing the filter structure and installing media, backwash blowers and pumps are proposed for installation along with backwash storage.
- In order to provide adequate detention time for disinfection, the existing chlorine residual monitoring point should be moved downstream of its present location at the tertiary sedimentation basins. Relocation of the chlorine monitoring point should provide for 20 minutes of detention time at the peak hydraulic flow to provide adequate contact time. Relocation of the dechlorination facilities is also necessary to allow/maintain chlorine residual through the tertiary filters (necessary and eligible due to hydraulics associated with installation of nutrient reduction technology [NRT]).
- Various hydraulic improvements were recommended in order to provide sufficient hydraulic capacity, based on design peaking factors, for the current peak flow of 37.5 MGD, up through the future peak flow of 50 MGD. Because there is an economy of scale to install the piping now, it is being undertaken; however, the resulting capacity of the peak flow is about 25% higher than the eligibility criteria in DEQ Guidance Memo 06-2012, and therefore will receive prorated grant eligibility.
- Solids handling improvements are based on the disposal method selected by RWSA to replace the existing compost yard for the Moores Creek AWTP. The proposed improvements include: installation of redundant dewatering equipment and appurtenant equipment (polymer storage and feed, etc.) is necessary at the 15 MGD design flow.
Electrical and Control
- A programmable logic controller (PLC) process control system including a plant-wide computerized monitoring and control system is recommended to provide real-time monitoring of plant processes and equipment; remote manual and automatic control of these processes and equipment; and data gathering, storage, retrieval and reporting functions (largely an eligible component ~ 77%).
- Install a new larger emergency generator at the facility (preliminary estimate of eligibility is 80%; final eligibility to be determined by kilowatt usage associated with the NRT component).