Brief Project Description
Design Package No.1
The significant improvements to be designed for the Arlington County Water Pollution Control Plant (ACWPCP) under DP-l will provide equalization to minimize wet weather bypasses, provide chemical storage and feed for phosphorus removal, minimize odors from the primary side of the plant; provide treated effluent water for on-site use, prepare for implementation of DP-2, upgrade of associated electrical equipment, and provide for effluent filtration.
The proposed improvements will provide two equalization tanks with volumes of 4.6 million gallons and 6.7 million gallons, for a total of 11.3 million gallons. A new odor control system will be provided at the following areas to treat foul air on the primary side of the treatment works:
- Existing 4.9 MG Equalization Tank;
- Two (2) new Equalization tanks;
- Existing Primary Clarifier Effluent Channel; and
- Existing Potomac Interceptor (raw wastewater).
The odor control system will have a minimum capacity of approximately 30,000 cfm. A new building will be provided to house the chemical storage tanks, feed equipment and metering pumps.
The existing effluent filters will be upgraded to allow for more consistent effluent polishing, phosphorous removal, and denitrification to meet more stringent nutrient limits anticipated to occur in the near future.
A methanol system will be designed to provide supplemental carbon to the primary effluent channel, last pass of each of the aeration tanks, and/or the filters, in order to facilitate denitrification.
A multi-point ferric chloride feed systems will be provided to achieve lower total phosphorous levels. The four locations for ferric chloride addition are:
- Raw wastewater;
- Primary effluent;
- Activated Sludge upstream of the secondary clarifiers; and
- Activated Sludge Effluent (ASE), upstream of the Lime Reaction Tanks (in the interim), or the mono-media filters (in the future).
The ACWPCP Plant Effluent Water (PEW) System is the non-potable water system at the plant used for wash-down, foam control, etc. The PEW system will be upgraded to meet new demands associated with the improvements, as well as improve the overall performance of the system.
Design Package No.2
DP-2 will expand the biological treatment capacity of the ACWPCP from 30 MGD up to 40 MGD and provide for future reduced levels of effluent total nitrogen less than 8 mg/L. . It will include the following major elements:
- Demolition of existing lime handling facilities (to provide room for construction of aeration tanks and secondary clarifiers);
- Construction of 1 or 2 new aeration tanks and blower replacements;
- Mixed liquor flow distribution improvements;
- Existing secondary clarifier renovation;
- Construction of 2 or 3 new secondary clarifiers and related pump station;
- Activated sludge effluent pumping system improvements;
- Waste backwash system modifications;
- Security and aesthetic improvements; and
- Supervisory Control and Data Acquisition (SCADA) system.
The ACWPCP currently has facilities that were originally provided to allow lime-clarification to be used for phosphorus removal. These facilities have been modified and are currently used to provide tertiary clarification upstream of the effluent filters (using ferric chloride as a coagulant). As indicated above the structural condition of the Lime Reaction Tanks (LRTs) is very poor and the tanks will need to be demolished. The demolition of the LRTs (spell out) will facilitate the construction of the new aeration tanks and secondary clarifiers. The following facilities will be demolished:
- Three First Stage LRTs each 120 feet square;
- Three Second Stage LRTs each 120 feet square;
- One multi-story masonry Lime Solids Processing Building which has been modified to provide storage and feed equipment related to the ferric chloride system; and
- Two Lime Sludge Thickeners each approximately 60 feet in diameter that are no longer used.
The ACWPCP currently utilizes four aeration tanks to provide BOD removal and Biological Nutrient Removal (BNR). The tanks are configured to operate in the step feed BNR mode. Each tank has four separate passes. The new aeration tank(s) will be constructed for BNR, redundancy and reliability, and increased capacity. The new aeration tank(s) will be four-pass, step feed biological treatment reactors, similar in arrangement to the existing tanks.
The mixed liquor flow distribution system will be upgraded to provide improved effluent quality and extended to serve the new aeration tanks. Two new flow distribution structures will be provided at each end of the new mixed liquor channel to provide even flow distribution to all of the existing and new clarifiers. At this time it is not clear whether the upgrade and expansion at ACWPCP will require 1 or 2 new aeration tanks. Studies are ongoing to determine the quantity of aeration tanks required to effectively improve treatment capabilities, increase redundancy and reliability, and meet projected flow requirements.
DP-2 will provide two or three new circular secondary clarifiers. Each unit will be 140 feet in diameter, with a sidewater depth of 16 feet. In conjunction with the ongoing studies described above to determine the number the new aeration tanks, the number of new secondary clarifiers is also being evaluated. In addition to the new clarifiers, the six existing clarifiers will be modified as needed to replace old mechanisms and improve sludge withdrawal and scum removal.
A new ASE pump station will be provided with the new clarifiers.