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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.

This work describes the development of an intermittently aerated pilot-scale process (V = 0.45 m(3) ) operated for optimized efficient nitrogen removal in terms of volume, supplemental carbon and alkalinity requirements. The intermittent aeration pattern was controlled using a strategy based on effluent ammonia concentration set-points. The unique feature of the ammonia-based aeration control was that a fixed dissolved oxygen (DO) set-point was used and the length of the aerobic and anoxic time (anoxic time ≥25% of total cycle time) were changed based on the effluent ammonia concentration. Unlike continuously aerated ammonia-based aeration control strategies, this approach offered control over the aerobic solids retention time (SRT) to deal with fluctuating ammonia loading without solely relying on changes to the total SRT. This approach allowed the system to be operated at a total SRT with a small safety factor. The benefits of operating at an aggressive SRT were reduced hydraulic retention time (HRT) for nitrogen removal. As a result of such an operation, nitrite oxidizing bacteria (NOB) out-selection was also obtained (ammonia oxidizing bacteria [AOB] maximum activity: 400 ± 79 mgN/L/d, NOB maximum activity: 257 ± 133 mgN/L/d, P < 0.001) expanding opportunities for short-cut nitrogen removal. The pilot demonstrated a total inorganic nitrogen (TIN) removal rate of 95 ± 30 mgN/L/d at an influent chemical oxygen demand: ammonia (COD/NH4 (+) -N) ratio of 10.2 ± 2.2 at 25°C within the hydraulic retention time (HRT) of 4 h and within a total SRT of 5-10 days. The TIN removal efficiency up to 91% was observed during the study, while effluent TIN was 9.6 ± 4.4 mgN/L. Therefore, this pilot-scale study demonstrates that application of the proposed on-line aeration control is capable of relatively high nitrogen removal without supplemental carbon and alkalinity addition at a low HRT.

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Highlights1) Short-cut nitrogen removal was demonstrated in a two-sludge NO2 --shunt process followed by AMX polishing at 25 ºC 2) An aeration control strategy was developed and implemented in the NO2 --shunt phase to produce an effluent with equal NOx-N and NH4 + -N 3) NOB suppression was realized in a single CSTR and maintained without using any external inhibitors at 25 ºC 4) Mainstream AMX polishing was demonstrated in an un-aerated MBBR 5) Mainstream NOB out-selection strategies and mechanisms have been suggested AbstractMainstream deammonification is regarded as a paradigm shift from the conventional aerobic wastewater treatment that is capable of promoting more sustainable nitrogen removal. It is understood that for deammonification to be successful in mainstream wastewater treatment, the suppression of NOB in the cold and dilute conditions associated with mainstream applications needs to be resolved. While NOB suppression in low C/N sidestreams is generally well understood, the mainstream application remains elusive. Since known factors that favor NOB suppression in sidestream such as high free NH3, high temperature, and free HNO2 are not available in mainstream, it has been recognized that operational strategies based on a different set of parameters are needed to suppress NOB. The prospect of mainstream deammonification in treatment plants that lack anaerobic digestion (e.g., incinerator plants) and therefore the potential for sidestream-generated ammonia oxidizing bacteria (AOB) bioaugmentation (without NOB) beckons an entirely different approach that might include separating the nitritation and anaerobic ammonia oxidation (AMX) steps. To prove the viability of mainstream NOB suppression and deammonification, a pilot study was conducted. The pilot study incorporates two unique stages operated in series, known as an A/B process. The A-stage included a low solids retention time (SRT), high-rate activated sludge (HRAS) process targeted at 30-60% influent chemical oxygen demand (COD) removal. The B-stage included a two-sludge deammonification process incorporating a Nitritation-Denitritation through Modulating Aeration (NiDeMA) CSTR with a clarifier followed by a fully-anoxic AMX moving bed bioreactor (MBBR). This paper considers the B-Stage, which includes the NiDeMA CSTR operated under cyclical dissolved oxygen (DO) conditions based on a control strategy applying on-line in-situ NH4 + , NO2 -, and NO3 -1959 WEFTEC 2013 measurements and an unaerated AMX MBBR. This aeration scheme combined with aggressive SRT limitation provided NOB suppression which was confirmed through weekly nitrogen processing rate measurements coupled with targeted qPCR of the bacterial community. The fully anoxic AMX MBBR has demonstrated near complete removal of NO2 -. Consequently, NOB suppression mechanisms have been postulated, and an operational framework has been put forward paving the way for mainstream deammonification in treatment plants that do not have anaerobic digestion.