In 2006, one of the largest water reclamation facilities in northern Virginia needed to expand the facility from 18 to 24 million gallons per day (mgd) to support future growth in Prince William County, Va. At the same time, new regulations necessitated an upgrade to improve the
nutrient removal capabilities of the plant. The new waste load allocations for total nitrogen (TN) were based on permitted discharge flows on 31 December 2010 with a 3 mg/L TN concentration. The Prince William County Service Authority (PWCSA) recognized the need to simultaneously increase flow and nutrient removal capabilities. At the same time, they needed to replace their plantwide data acquisition and control system (DACS) with a new modern supervisory control and data acquisition (SCADA) system.
This design-build project enhanced nutrient removal and increased capacity, doubling the existing aeration basin volume and reconfiguring to allow operation in either four-stage Bardenpho or modified Ludzack–Ettinger (MLE) modes. It implemented 14 new deep bed denitrification filters for a total of 24, and methanol feed to the filters was automated to be nitrate load paced and controlled by a proprietary software calculation algorithm. Furthermore, PWCSA installed an additional online analyzer for controlling the methanol feed to the filters for redundancy.
The proprietary software calculation algorithm for control is feedforward/feedback based upon flow and influent and effluent nitrate concentrations. This enhanced the operation and reliability of the process and also reduced the risk of methanol overdose by more closely matching the methanol feed to the actual demand. Consistent methanol dose control is challenging when trying to meet low effluent TN and simultaneously maintain a low effluent carbonaceous biochemical oxygen demand (CBOD).
This plant is currently in full operation and in compliance with the effluent requirements.
Before the expansion, the plant operated with 10 denitrification filters that had insufficient surface area to process the full plant capacity of 18 mgd in denitrification mode. When the filters were operated in the denitrifying mode, flows beyond 12 mgd bypassed the filters to prevent hydraulic overloading. Even though the filters were capable of hydraulically passing the full plant flow, denitrification could not be achieved at higher flows. Based on the processing limitations and operational cost savings, the filters were often operated seasonally, with methanol added only during the winter for the additional denitrification needed to meet effluent TN requirements. During summer, the plant had sufficient denitrification capability in the secondary treatment (aeration basins) to meet effluent TN requirements, and the filters operated in a polishing mode without methanol addition to remove suspended solids.
During the design phase, the MLE and four-stage Bardenpho processes were selected for implementation based on a wide range of criteria, including capital cost, overall cost, net present value, land requirements, effluent quality, operability, maintainability, and schedule.
The plant was also required to reduce effluent total phosphorus to 0.18 mg/L. Phosphoric acid addition capability is provided in the filter’s area if the filters become phosphorus limited. It was anticipated that any phosphorus allowed to bleed through to the filters or added to the secondary effluent will be removed by the denitrification filters and permit limits will not be exceeded. However, provisions were provided for future implementation of phosphoric acid feed.
The number of denitrification filters was increased from 10 to 24 to meet the new projected demands and to be able to handle when filters are offline for backwashing, bumping, or maintenance/repair.
Plant-wide control system replacement
As part of this design-build project, the existing plantwide DACS was replaced. It was obsolete, with key components of the system no longer available from the manufacturer.
The facility transitioned the existing DACS to a modern SCADA system as part of the overall implementation, including designing a system with both new process area control panels and upgraded existing control panels. The final system has about 5,000 I/O points, 25 programmable logic controllers (PLCs) with a self-healing fiber-optic ring, an object-oriented human-machine interface (HMI) system, and a historian interfaced with reporting software that integrates the SCADA and laboratory databases.
The engineer of record managed construction and did quality assurance/quality control (QA/QC) for the new SCADA system and the field instrumentation portion of the project. Some of the activities included QA/QC for the new instruments, startup coordination between PWCSA and the subcontractor, onsite response to design/implementation questions and clarifications, development of maintenance of plant operations (MOPO) plans for transitioning existing and in-service systems to the new SCADA system (with the objective of minimizing the effect on plant operations), and developing and continually updating the SCADA project schedule.
Furthermore, toward the end of the project, the engineer of record also guided, witnessed, and approved the testing procedures and results for the SCADA system as a whole. This activity included network testing, uninterruptible power supply (UPS) testing, software testing, and PLC programming testing. The subcontractor performed loop testing (operational readiness test) and the functional demonstration test with coordination from PWCSA, witnessed and approved by the engineer of record.