Brief Project Description
Grantee: ChesterfieldCounty (Falling Creek)
ChesterfieldCounty’s Falling Creek WWTP provides secondary treatment for a permitted design flow of 10.1 MGD. The existing plant consists of: screening, grit removal, communition, flow equalization, primary sedimentation, activated sludge with seasonal denitrification, secondary clarification, chemical coagulation and sedimentation, chlorination, post-aeration, and dechlorination. Sludge is anaerobically digested and land applied.
Under new nutrient discharge control regulations for the Chesapeake Bay and its tributaries, the County received a combined, bubble waste load allocation for nitrogen and phosphorus for its two treatment plants discharging within the James watershed.
As described in a Preliminary Engineering Report (PER) revised in September 2007, the County will upgrade the existing secondary treatment process to an enhanced nutrient removal system designed to achieve a 5.0 mg/l annual average nitrogen concentration.
This project consists of various facilities included in the PER which are intended to achieve nutrient compliance. Nutrient reduction technology (NRT) processes being funded for nitrogen removal are described as follows:
Headworks and Primary Treatment Areas: Fine screens are proposed in the plant upgrade. The screens provide protection to the Integrated Fixed Film Activated Sludge (IFAS) system and will also improve downstream processes (such as clarification and sludge handling). Due to lower maintenance requirements and owner preference, center flow band screens - installed upstream of the primary clarifiers – are anticipated for the final design. Influent flow to the screen facilities will be pumped from the existing pump station, and a new building will be required to house the center flow band screens and screenings handling equipment
Secondary Treatment: Secondary treatment facilities and upgrades for a 4-stage activated sludge, biological nutrient removal process includes the following:
A baffle wall will be placed in the initial chamber of each basin to provide aerobic and anoxic cells. The IFAS system will operate using an initial anoxic zone, followed by aerobic conditions, a second anoxic zone, and then a small aeration zone at the end. The first anoxic zone would receive a recycle of nitrates from the tail-end of the first aerobic section, as well as the influent and the RAS flows. Mixers will be installed in the anoxic zones. The first aerobic compartment will contain the suspended IFAS media and retention screens. The existing aeration grid will require modification to maintain the media screen which keeps the media from traveling forward and to supply air via fine bubble, membrane diffusers for the increased biological activity. The second anoxic zone receives the forward flow and the supplemental carbon, associated with denitrification.
The existing internal nitrate recycle pumps push flow through the wall. These pumps cannot remain in service, because the changing location of the aerobic environment will require similar pumps and additional piping to direct the flow.
The aeration system will also require better control than is currently available at the plant. The existing blower building includes five multi-stage centrifugal blowers with space and piping connections for a sixth blower unit. The total blower capacity for the existing system is 16,000 scfm, with a firm capacity of 12,000 scfm (with largest unit out of service). Existing blowers No. 1, 2, and 7 were installed in the late 1970’s; while existing blowers No. 4 and 5 were installed in the mid-1990’s. Although the existing blowers have a total design capacity of 16,000 scfm, new blowers are included due to concerns regarding age and reliable capacity of the existing blowers.
Per SCAT Regulations, the blower system is designed for worst case (peak hourly) air demand. Five new multi-stage centrifugal blowers are proposed to meet the air demands for the IFAS system (4 duty and 1 standby). New above-grade process air piping is anticipated to avoid underground pipe conflicts. Each aeration tank lateral will be provided with an isolation butterfly valve, and will feed air drop legs with flow meters and control valves at each aeration zone.
Clarifiers: The secondary clarifiers perform well and no modification is proposed. The tertiary clarifiers also perform well and no modification is proposed.
Chemical Feed Systems: Carbon for denitrification will require chemical storage/feed and final design will identify the carbon source(s) for use. While methanol is the traditional chemical, the facility will be constructed so other formulations (acetic acid or sugar solutions) may be used.
Process Piping Improvements: An existing 30-inch pipe line between two junction boxes at the west primary clarifiers may be utilized to send flow from the new screening facilities to the west primary clarifiers. However, a new routing on the south side of the primary clarifiers will be required to provide screened wastewater to the east clarifiers.
Flow distribution from the screen facilities to both east and west clarifiers will be evaluated. A new flow distribution chamber will likely be included in the final design.