Nitrogen Reduction on a Small Budget and Big Ingenuity

Last Modified: Aug 10, 2018


  • Holly Anne Matel, Colin Beck - Hazen and Sawyer
  • Mike Parsons - HRSD

The Williamsburg plant has a unique technology consisting of oxidation towers, primarily intended to remove organic loading. The primary effluent flow required for optimal nitrification is pumped to the BNR basins. Remaining flow is treated through oxidation towers and intermediate clarifiers before entering the BNR basins. Flow to the towers varies seasonally with more in the winter and less in the summer. Nitrification is observed during warmer temperatures and lighter load. An equation was developed to predict the mass of TKN in the intermediate clarifier effluent (ICE) as a function of COD load to towers and temperature.

Mass ICE TKN =304+0.04*(COD load to towers, lb/d)–19*(temp, C)

A calibrated process model was developed to assess various nitrogen and phosphorus alternatives. A hydraulic model was also developed to ensure process changes were hydraulically feasible. The selected configuration converts the existing MLE process to a 5-stage process with a swing zone in a portion of the last cell to further enhance denitrification. Baffle walls were required and constructed for approximately $4,000 each using fiberglass and PVC roofing panels. Other improvements include extending the influent feed piping to split flow between Cell 1 and the second anoxic cell. This allows some additional carbon to enter the second anoxic zone for denitrification. However, the amount of ammonia load sent to this cell must be limited since there is only one additional aerobic cell downstream.

Plant staff handbuilt six large bubble mixers out of PVC for each swing zone. An air header was tapped to provide an air drip into the PVC chamber, erupting when the chamber fills. An actuator was added to automatically turn air on/off as needed to achieve the desired effluent nitrogen.

Historical effluent TN decreased on average from 6.2 to 4.2 mg/L. The improvements cost $185,000 total.

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