The European-style A/B process utilizes a very high rate activated sludge (HRAS) A-stage operated at a low sludge age and low DO. The aim of the HRAS process is to provide a cost effective means of removing carbon while decreasing aeration demand and volume required for the subsequent
B-stage process, which usually consists of biological nutrient removal (BNR). A pilot study to evaluate the performance of an A/B process is currently underway at the Hampton Roads Sanitation District (HRSD) Chesapeake Elizabeth Wastewater Treatment Facility (CETP) in Norfolk, VA. To date,
the A-stage pilot process consistently removes approximately 60% of the influent particulate and soluble COD. Carbon removal in the A-stage results in B-stage denitrification operating in carbon-limited conditions. Thus it is essential to operate the system to take advantage of simultaneous
nitrification-denitrification (SND) as well as nitritation-denitritation (nitrite shunt) to avoid external carbon supplementation. This is accomplished by ammonia-based cyclic aeration control. This method of control allows the reactors to maintain DO levels low enough to support SND and nitrite
shunt, yet high enough to still achieve nearly complete nitrification with effluent total inorganic nitrogen (TIN) values in the range of 6-8 mg N/L.
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