Brief Project Description
The Maury Service Authority owns and operates an advanced wastewater treatment plant, VPDES Permit #VA0088161, currently rated for 3.0 million gallons per day (MGD) with a discharge to the Maury River. The existing major unit processes include head works with screening and grit removal, Orbal oxidation ditch, secondary clarifiers, UV disinfection, and cascade post aeration. Sludge is aerobically digested and land applied. No design flow expansion will result from this upgrade project
As a result of the Water Quality Management Planning Regulation requirements for nutrient discharge control, technology-based concentration limits for total nitrogen and total phosphorus were developed. The Preliminary Engineering Report (PER) concluded that the existing Orbal ® ditch is hydraulically designed to meet the design flow; however, in order to consistently meet effluent nutrient limit concentrations, modifications to the wastewater treatment process are necessary. The PER recommended the following conceptual modifications, which could change based on the final design and receipt of bids, to the treatment plant processes.
Existing Oxidation Ditch
- Demolish the existing brush aerators.
- Install Orbal ® disc aerators in strategic locations in the oxidation ditch with new drive systems which include Variable Frequency Drives.
- Add a pump in the inner ditch to recycle nitrate to the outer ditch.
Chemical Feed Systems
- Construct a new chemical feed storage area for the ferric chloride system.
- The chemical feed storage area would be constructed to include a second containment area for any future chemical requirements at the facility.
- Construct deep bed effluent filters sized to handle 11.2 MGD (peak hourly condition) with an upstream rapid mix tank for coagulant addition. The effluent filters would be loaded at 4 GPM and would be located near the electrical/generator building. If denitrification filters were needed in the future, the surface area could be expanded so the loading rate was 2 GPM.
Solids Handling (1)
- Construct two additional digesters with mixing equipment (similar in size to the existing digesters).
- Replace the existing digester mixing and aeration equipment with new equipment (coarse bubble diffused aeration system) that can handle up to a 2% solids concentration.
- Construct a gravity thickener to pre-thicken WAS sludge before being fed to the aerobic digesters.
- New thickened sludge feed pumps to convey thickened sludge from the gravity thickener to the aerobic digesters.
The recommended plan does not include provisions for a second sludge storage tank. Based on the solids generation rates for the facility at design conditions (average day and max. month), the existing sludge storage tank will only be capable of providing approximately 10 weeks (2.5 months) worth of storage to the MSA. The additional digester volume will assist with solids volume, but as plant capacity continues to increase towards the design capacity in the future, the Authority will need to contemplate either increasing contract operations towards 100% or adding solids dewatering facilities to eliminate bottlenecks with solids handling and disposal at the facility.
Electrical and Control
- Install a PLC process control system similar to the existing PLC structure that would perform monitoring of processes including the chemical feed systems, dissolved oxygen control in the oxidation ditch, effluent filter operations, and other additional monitoring and control necessary for nutrients.
- Install a new larger emergency generator at the facility.
As an amendment to the PER, an alternative sludge biodegradation Evaluation is to be completed. The results of the Evaluation (i.e. feasibility study) may alter the process for sludge handling and/or the performance expectations. The amendment to the PER will be submitted as part of the final engineering in conjunction with plans and specifications.