Who Stole My Nitrification?


  • Wes Kucera, Frederick R. Holmes, P.E., Tina Hanson, P.E., Ron Latimer, P.E.

The City of Garland owns and operates the Duck Creek Wastewater Treatment Plant (WWTP), which is permitted to treat 40 million gallons per day (mgd) on an annual average basis. The Duck Creek WWTP has to comply with seasonal ammonia nitrogen limits, but has periodically experienced periods of high ammonia in the effluent. City staff had long suspected nitrification-inhibitory compounds in the wastewater and had developed operating procedures to minimize the impact to the process.

Operating procedures developed by the plant are reactionary in nature and require close monitoring by plant staff of the biological facility’s dissolved oxygen contents. When oxygen levels are determined to be following a trend that is consistent with nitrification inhibitory events, plant staff immediately institute a series of procedures that have been developed through the experience of key personnel.

In order to develop a long-term approach with a process that is less reactionary, the City undertook a project in coordination with the engineering firm, Hazen and Sawyer, to confirm the existence of nitrification inhibitors in the facility’s wastewater influent and to analyze the appropriate course of action for minimizing the facility’s risks associated with these events of reduced nitrification potential.

The team approach involved an analysis of historical data, which confirmed the consistent success of the facility for the removal of BOD and TSS. It also confirmed the presence of high ammonia events appearing to be consistent with nitrification inhibition. Next, a series of samples were collected by facility and engineering team staff to establish a nutrient profile across the facility. A nutrient profile was collected before and then during a suspect nitrification inhibition event. Bench-scale nitrification studies of the wastewater collected under the separate events allowed the team to demonstrate a baseline condition with successful nitrification versus a condition occurring during nitrification inhibition. Using the nutrient profile, BioWin model simulations were conducted to simulate historical data and identify further options for evaluation at the plant.

Now that the presence of nitrification inhibition has been clearly confirmed in the facility’s raw influent, the next steps are to develop an intense sampling protocol to study the facility’s response to the inhibition and identify potential capital expenditures or operating modifications, which may assist the facility in their response. Additionally, the City of Garland is working on methods for determining the source of the inhibition as this may offer a long-term means of removing the issue.

This presentation will focus on how to recognize nitrification inhibition, the steps that an operator can take to mitigate the risks for prolonged and sever impacts on their effluent and the City of Garland.

For more information, please contact the author at fholmes@hazenandsawyer.com.

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