Challenge

The King George County Wastewater Treatment Plant in Dahlgren, Virgina discharges into a tributary of the Potomac River that eventually empties into the Chesapeake Bay. In anticipation of ever-increasing ENR discharge limits, the King George Service Authority recognized the need to further optimize its total nitrogen (TN) and total phosphorus (TP) removal to meet "significant discharge" nutrient poundage caps. Its annual nutrient discharge poundage allocation is 12 mg/L TN and 0.3 mg/L TP in effluent at current flow. While the plant was meeting the acceptable 12 mg/L limit, lower discharge levels were desired. Additionally, the plant was losing process efficiency due to over-aerating the reactor.

Solution

In the early 1990s, Siemens Water Technologies provided the WWTP with a three-channel Orbal® process for effective biological nutrient removal (BNR). In 2004, the plant added an MLSS recycle pump and a fourth Orbal channel to assist with discharge requirements, as well as a SmartBNR™ control system for increased process efficiency.

Although the plant was meeting its discharge requirements of 12 mg/L, the design engineer wanted to further increase plant performance without requiring a major capital-intensive upgrade. In 2007, Siemens Water Technologies, in conjuntion with Timmons Group, created a process optimization program for the Dahlgren plant.

Key process recommendations included:

• Reducing oxygen delivered to the first two channels
• Automating aerators in the same two channels and allowing the controls to determine appropriate oxygen delivery based on oxidation reduction potential (ORP) measurements
• Running the MLSS recycle pump at twice the influent flow rate
• Reducing holding and aeration time in the flow equalization basin to increase influent BOD

Results

One month into the program, the plant experienced cost and energy savings, as well as a considerable reduction in total nitrogen. The upgraded Orbal multichannel oxidation ditch significantly reduced effluent TN from 12 mg/L to less than 4 mg/L. The SmartBNR controls allowed for improved process performance and lower operating costs.  Power usage for aeration was reduced by approximately 25% and chemical costs for phosphorus precipitation and supplemental alkalinity were cut in half.

Download Full Case Study