title: Operation and Modification of a B-Stage for Efficient Nitrogen Removal in an A/B Process Pilot Study
Creators: name: Bunce, Ryder
Publisher: name: Old Dominion University Libraries
Year: 2013

description: Hampton Roads Sanitation District's Chesapeake-Elizabeth Wastewater Treatment Plant is a High-Rate Activated Sludge (HRAS) plant with no primary clarifiers. The plant operates at an SRT of 1.5 - 2 days and does not perform biological nitrogen removal (BNR). By 2021 HRSD anticipates the plant will be included in a bubble permit, resulting in a total nitrogen (TN) effluent target of approximately 5-8 mg/L. Converting the plant to a conventional BNR process would require a significant increase in reactor volume. Capital costs for the upgrade are estimated at 125 -150 million dollars, with a dramatic increase in operating costs as well. As an alternative HRSD is pilot testing an A/B process. The very high rate, low DO A-stage would replace construction of primary clarifiers. The reduced organic load from the A-stage would allow the B-stage to perform BNR within the existing reactor volume at Chesapeake-Elizabeth. Ammonia-based aeration control would allow the B-stage to achieve its effluent nitrogen goals in carbon-limited conditions without supplemental carbon addition. To meet the effluent goal of 5-8 mg/L TN, Chesapeake-Elizabeth will set a process goal of 8-12 mg/L TN, followed by dentrification filters to reduce effluent TN to 5 mg/L or below.

The pilot treatment process is a continuous flow process designed to simulate the available infrastructure at Chesapeake-Elizabeth. The pilbt plant's A-stage is a 12 foot tall, 10 inch cylindrical reactor followed by a cone-bottom clarifier. A-stage effluent feeds the B-stage, which consists of 3 equal volume tanks in series followed by a cone-bottom clarifier. A flowrate of 0.5 gallons per minute (gpm) gives the B-stage a hydraulic retention time (HRT) of 4 hours, identical to that of the main plant at design flow. The B-stage was originally configured as a Modified Ludzack-Ettinger (MLE), with an anoxic tank followed by two aerobic tanks and an internal mixed liquor recycle (IMLR) to return nitrified sludge to the anoxic zone for denitrification. The reactors were aerated intermittently based on DO setpoints that were controlled by the effluent ammonia level, allowing some simultaneous nitrification-denitrification (SND) to occur in the aerobic zone.

Operating as a conventional MLE under ammonia-based aeration control, the B-stage was able to consistently achieve effluent TN levels of 6-10 mg/L. During the course of operation, it became evident that the B-stage was capable of nitrite-shunt as a result of transient anoxia in the aerobic zone. Recognizing that nitrite-shunt would reduce the carbon requirements and increase the nitrogen removal capacity of the system, the system was re-designed to capitalize on this. The new B-stage, called NiDeMA (Nitritation-Dentitritation through Modulating Aeration), was operated without a dedicated anoxic zone or internal recycle, with the entire process volume operated under intermittent aeration. The system also featured a cyclic ammonia-based aeration control scheme designed to maximize nitrite oxidizing bacteria (NOB) suppression. The NiDeMA system, operating without a formal anoxic zone under the cyclic aeration scheme, was able to improve on the nitrogen removal of the B-stage MLE with sustained effluent TN values of 5-7 mg/L.

To improve sludge settle ability, an innovative cyclone-selective wasting system for the B-stage was conceived and implemented. The system utilized a hydrocyclone to separate mixed liquor based on density and settleability. The theory was that dense, better settling floe would be retained while lighter, poor-settling floe would be wasted. Over time this would create a better settling sludge. Implementation and initial results are discussed in this thesis but there are currently not enough data to draw significant conclusions about the system.