2011 Grand Prize - Design

Arlington County Water Pollution Control Plant Upgrade and Expansion

Location: Arlington, Virginia
Entrant: Malcolm Pirnie, the Water Division of ARCADIS
Engineer in Charge: Nicholas O. Rodzianko, P.E., BCEE
Media Contact: Nicholas Rodzianko, P.E., BCEE; 703-465-4228; Nick.Rodzianko@arcadis-us.com

The Arlington County WPCP was plagued with aging infrastructure and hydraulics, severe space constraints and insufficient treatment to handle large wet weather flows. Critically important, the County wanted to practively achieve Chesapeake Bay Enhanced Nutrient Removal (ENR) goals to protect public health and the environment in advance of new regulatory requirements. Aerial view of the completed project with insets showing the plant before construction.	Virtually every process and structure in the plant was renovated, expanded or upgraded. As a result of these improvements, the plant now provides *enhanced biological nutrient removal* for both total nitrogen and total phosphorus. Aerial view of the completed project, showing the Washington DC area in the background. Arlington County is now a major contributor to the Chesapeake Bay's environmental quality goals.	Taking a leadership role to protect Chesapeake Bay water quality, the County upgraded treatment processes to provide "limit of technology" nutrient removal - upgrading the plant's BNR (8 mg/l TN) capability to meet an anticipated TN limit of ∼ 3 mg/l. One of the construction challenges was its location on tight site bisected by South Globe Road and the proximity of the surrounding neighborhoods.
Expanding the plant's treatment capacity form 30 to 40 mgd provides enhanced capacity to handle wet weather treatment flows. Photo shows construction of two new banks of aeration basins (foreground). An internal recycle system and multiple anoxic sections were installed in each aeration tank to reduce effluent nitrate concentration to the effluent DeNite filters, decreasing methanol demand and reducing the facility's overall carbon footprint.	Two new aeration tanks and improvements to four existing tanks will enable the plant to meet reliability and redundancy requirements for enhanced nutrient removal. Since June 2010, with the upgrade completed, the Arlington plant has consistently achieved enhanced BNR - one of the first major plants in the Chesapeake Bay watershed to achieve ENR treatment limits for both total nitrogen and total phosphorus.	Construction of equalization tanks. Highly creative logistics and staging, including this round-the-clock concrete pour, helped maintain uninterrupted treatment plant operations during construction. To maintain flow while existing facilities were demolished to make room for new facilities, the engineers constructed two temporary 72-in-dia pipes under the new aeration basins.

Entrant Profile

The upgrade and expansion of the Arlington County Water Pollution Control Plant (WPCP) represents a major contribution to the environmental quality goals of the Chesapeake Bay - it is one of the first major wastewater treatment plants in the watershed to achieve Enhanced Nutrient Removal (ENR) treatment quality, protecting this valuable resource for future generations.

In accordance with Chesapeake Bay Initiative goals to protect the Bay from impacts of excess nutrient discharges, the WPCP was providing biological nutrient removal (BNR) to 30 million gallons per day (mgd) of wastewater flows from Arlington County and other communities. In a forward-looking approach, the County decided to upgrade the treatment process to "limit of technology" nutrient removal, achieving Chesapeake Bay ENR goals to comply with future, more stringent regulatory effluent nutrient limits. Overcoming severe site space constraints, they also increased the plant's treatment capacity from 30 to 40 mgd to accommodate wet weather loadings up to fourfold dry-weather flows.

Malcolm Pirnie, the Water Division of ARCADIS, provided complete design and engineering services for this project, including upgrade of the plant's aging mechanical, electrical and instrumentation and control infrastructure, provisions to handle significant wet weather flows, and pilot testing of two denitrification filter technologies. Virtually every process and structure was renovated, expanded or upgraded, from primary treatment and flow equalization improvements to effluent denitrification filter upgrades and a new plant-wide SCADA system.

Consultants who played significant roles were: AECOM (Program Manager); Hailey & Aldrich (Geotechnical Engineering); and William H. Gordon & Associates (Surveying and Mapping).

Project Description

The expansion and upgrade of the Arlington County Water Pollution Control Plant (WPCP) reflects a decade of effort since the County, in its 2001 Master Plan, recognized critical water quality challenges. Treating flows from Arlington County and parts of Alexandria, Fairfax, and Falls Church, VA, the WPCP was plagued with aging infrastructure and hydraulics, severe site space constraints, and insufficient treatment capacity to handle wet weather flows that could increase nearly fourfold. Of paramount importance, the County sought a plan to proactively achieve Chesapeake Bay Enhanced Nutrient Removal (ENR) goals to protect public health and the environment in advance of recently changed regulatory requirements.

The County selected Malcolm Pirnie, the Water Division of ARCADIS, to engineer this complex project, where virtually every process and structure was renovated, expanded or upgraded, from primary treatment and flow equalization through effluent filtration and new automated control system. The plant's aging mechanical, electrical and instrumentation and control infrastructure was upgraded, and Pirnie/ARCASIS engineers also conducted pilot testing to assess the ability of two denitrification filter technologies to achieve "limit of technology" total nitrogen effluent limits at low phosphorus concentrations.

Integrated Approach

Design Integrates Myriad Elements - In a project of this magnitude, the most effective original thinking focuses on how the project's myriad elements are put together, from design through construction, to achieve the client's goals. Here, every wet stream unit process within the plant was upgraded and/or replaced, including much of its aging infrastructure. To better manage wet weather peaks, an effective strategy was devised to balance out diurnal flows to ensure optimum treatment ability and be able to reliably meet ENR treatment levels; maintaining uninterrupted plant operations during construction was critical.

Originality

Proactive Approach to Compliance - In a forward-looking approach, the County took a leadership role to protect Chesapeake Bay water quality. As an integral part of the plant expansion design, they decided to proactively upgrade the plant's treatment process to provide "limit of technology" nutrient removal in advance of the facility's next National Pollutant Discharge Elimination System (NPDES) permit -- upgrading the plant's BNR (8 mg/l TN) capability to meet an anticipated TN limit of ∼3 mg/l.
Innovative Technology - Pirnie/ARCADIS's design replaced existing polishing filters with innovative deep-bed filter technology for denitrification and removal of effluent total suspended solids and phosphorus. This created one of the first large systems to achieve limit of technology nutrient limits for both total nitrogen and total phosphorus. The design was based on side-by-side pilot testing of two filter technologies, conducted under various flows and loads to test performance at average and peak conditions and influent phosphorus concentrations to determine each technology's ability to achieve effluent limits.
Model for Enhanced Nutrient Removal - With regulatory drivers continuing to reduce limits on nutrients in plant discharges, designers face a dilemma - how to achieve low effluent phosphorus concentrations while providing sufficient phosphorus to enable denitrification microorganisms to reduce effluent oxidized nitrogen. The Arlington plant is the first major effluent denitrification facility achieving effluent TN ⟨ 3.0 mg/l and TP ⟨ 0.18 mg/l, accomplished by not only replacing effluent filters but also modifying all upstream processes. For example, to protect the treatment process against wide fluctuations in flow during wet weather, Pirnie/ARCADIS's design included increased equalization basin tankage and other primary treatment modifications.

Quality/User Satisfaction

Since June 2010, with the treatment process for nutrient removal upgraded to "limit of technology," the Arlington plant has consistently achieved ⟨ 2.5 mg/l TN and 0.1 mg/l TP- one of the first major plants in the Chesapeake Bay watershed to achieve ENR treatment quality limits for both total nitrogen and total phosphorus.
The plant's treatment capacity, expanded from 30 to 40 mgd, provides enhanced capacity to handle wet weather treatment flows. The new process control system has resulted in fully automated plant operations, and the plant's aging infrastructure has been upgraded.
Improvements were implemented through multiple construction packages, each designed to allow the treatment process to stay in continuous operation. Design and construction were completed within budget and major components were completed approximately 18 months ahead of the overall Consent Order decree deadline.

Complexity

Complex Construction Project - Achieving extremely low values for nutrients on a very tight site, surrounded by residential development, roadways, overhead power lines and waterway, demanded a very compact treatment process. Even more challenging, highly creative approaches to logistics were required to plan, design and construct this massive project while maintaining the plant in service. Examples are numerous - to maintain the required steady concrete flow during the equalization tank pour despite heavy traffic, work was scheduled for 32 straight hours over a weekend. To maintain flow while existing facilities were demolished to make room for new facilities, Pirnie/ARCADIS engineers constructed two temporary 72-in-dia pipes under the new aeration basins.
Complex Management Challenge - Coordinating numerous subcontractors and contracts over a four-year timeframe was extremely complex, requiring creative construction staging and design of temporary facilities. Detailed construction staging plan included provisions for temporary power generation facilities to provide continuous power during construction while the entire electrical system was being rebuilt.

Social/Economic/Sustainable Design

Sustainable reuse of existing structures - Design strategy incorporated retrofitting existing tanks and systems for needed new facilities. Existing aeration tanks were modified by adding baffles, mixers, internal recycle, and foam control; sludge collection equipment and skimming devices were added to secondary clarifiers; chlorine contact tanks and re-aeration tanks were converted into post-aeration tanks.
Sustainable Approach reduced methanol demand - The treatment process was designed to reduce effluent nitrate concentration to the effluent denitrification filters, decreasing methanol demand and reducing the facility's overall carbon footprint. This was accomplished by installing an internal recycle system and multiple anoxic sections in each aeration tank.
Major Milestone in Chesapeake Quality Improvement - As one of the first major wastewater treatment plants in the Chesapeake Bay watershed to achieve ENR treatment quality, the Arlington plant is a major contributor to the Bay's environmental quality goals, protecting this valuable resource for future generations.

Construction of flow equalization tank FEQ2. Flow equalization improvements included two new concrete tanks which added 11.6 million gallons of storage capacity for a total of 14 million gallons of storage plus new pumps to manage wet weather peak flows that can be fourfold higher than average plant flows.	The new equalization tanks and related improvements will allow operators to balance out diurnal flows to ensure optimum wastewater treatment.	The North Ferric Feed facility was constructed between two new flow equalization tanks. It providenhanced primary settling.
Construction of three new 140-foot-diameter clarifiers next to an active roadway without disrupting traffic or treatment operations was challenging.	New secondary clarifier #8 in operation. As part of the design strategy, existing tanks and systems were retrofitted for new facilities. Sludge collection equipment and skimming devices were added to secondary clarifiers, and existing aeration tanks were modified by adding baffles, mixers, internal recycle and foam control.	De-Nite Filter Facility under construction. De-Nite filters are innovative deep bed denitrification filters that are producing effluent total nitrogen to ⟨ 2.5 mg/l and total phosphorus to ⟨ 0.1 mg/l -- a milestone in the Chesapeake Bay watershed. Because of site constraints, filters were constructed above the new chlorine contact tanks, while existing chlorine tanks were converted to post-aeration tanks.
De-Nite filters in operation. Malcolm Pirnie's design replaced existing polishing filters with innovative deep-bed filter technology for denitrification and removal of effluent total suspended solids and phosphorus. This created the first major effluent denitrification facility achieving effluent TP ⟨ 0.18 mg/l, accomplished by not only replacing effluent filters but also modifying all upstream processes.	De-Nite Filter pipe gallery. Improvements at the WPCP were implemented through multiple construction packages, each designed to allow the treatment process to stay in continuous operation.	New Return Activated Sludge Pumping Station. Highly creative approaches to logistics were required to plan, design and construct this massive project while maintaining the plant in service.
Maintenance of Plant Operations during construction was a major challenge of this project. This view shows how two 72-in-dia lines were constructed below new aeration tanks in order to maintain flow to the second step lime reaction tanks. Design and construction were completed within budget and major components were completed approximately 18 months ahead of the overall Consent Order decree.	New process control system (PCS) was constructed within the existing operations control building. The plant is fully automated and is monitored and control from this control room.	PCS in operation.
New Plant Security System is monitored in the PCS Control Room. Cameras throught the plant can be monitored from this location.	Plant effluent discharges into Four Mile run. This view from the top of the De-Nite Filters shows how this project preserves the environment. A bike path runs adjacent to the plant and Four Mile Run.