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 nitritationdenitritation (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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To evaluate continuous and sporadic nitrification inhibition at the HRSD Nansemond Wastewater Treatment Plant, which has a history of nitrification upsets, continuous sequencing batch reactors (SBRs) were operated to simulate the full-scale plant. Four reactors were operated in this study. One reactor was fed with raw influent (RWI) from the Nansemond Wastewater Treatment Plant (NP). Another was fed with NP primary clarifier influent (PCI), which includes the raw influent, as well as plant recycle streams and truck delivered septage, grease, and chemical toilet waste. The remaining two SBRs were fed with RWI from the VIP Wastewater Treatment Plant, which achieves reliable nitrification year-round. One of these VIP SBRs would remain a control at all times, while the other would be used to evaluate suspected inhibitors to nitrification.The first phase of this project was to determine whether NP was inhibited when compared to VIP, which would be ascertained through a comparison of nitrification performance. The next step was to determine whether the source of inhibition was an industry within the collection system or plant recycles and delivered wastes, which would be ascertained based on comparison of the NR RWI and NP PCI reactor performance. If nitrification performance was comparable between the two SBRs, then it would indicate that the source of inhibition is somewhere within the collection system, whereas if the NP PCI reactor was iii inhibited when compared to the NP RWI reactor, it would mean that the inhibition is a result of plant recycles or delivered wastes. The next phase would be to determine the specific source by either working back up the collection system or by testing the plant recycles and delivered wastes.After approximately 27 weeks of SBR sampling and monitoring, there was no statistical difference between nitrification rates in reactors A and B, and no signs of nitrification inhibition in either reactor when compared to the VIP control.Simulation modeling of reactors A, B, and D (control) was performed with BioWin 3.1 (EnviroSim, Ltd.) as a means for comparison and to ensure reactors were performing as intended. Results suggest that there was some level of continuous inhibition for both NP RWI and PCI reactors, however no sporadic inhibition events were observed. It also appeared that the VIP RWI control reactor experienced some level of continuous nitrification inhibition, although BioWin modeling results indicated that both NP RWI and NP PCI were more inhibitory than VIP RWI. Conclusions drawn from modeling results conflict with those drawn from nitrification rate comparisons. Since solids retention time (SRT) was maintained at exactly 15 days for all reactors, it was assumed that a direct comparison of corrected maximum nitrification rates could be used to compare nitrification performance between SBRs, however the significantly higher influent COD, TKN, and TSS loading to the NP reactors resulted in higher nitrification rates when compared to the VIP RWI control reactors. This wa...

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