No Carbon, No Worries: A Pilot-Scale Evaluation of Nitrogen Management Without Biodegradable Organic Carbon

Abstract:
The main objectives of this pilot testing were to (1) demonstrate the long-term efficacy of partial nitritation at pilot scale in low C:N (< 5 mg-sCOD/mg-NHx-N) temperate mainstream wastewater conditions, and (2) evaluate available sensor technologies to reliably monitor and control the process. Preliminary results suggest that high nitrite accumulation rates (NAR = 90% ± 9%) can be achieved using a simple SRT control strategy (3.0 ± 0.5 days) and a fixed DO setpoint (1mg/L) at relatively low loading rates (0.14 ± 0.05 kgN.m-3.d-1), and low C:N ratio (2.5 ± 0.9 mg-N/mg-sCOD). However, this research also points to key process and technology challenges associated with mainstream partial nitritation such as high nitrous oxide production, high sensitivity of the process, and the need for sensor technologies that are accurate in a wide range of Nitrite and Nitrate concentrations.
Introduction
Partial nitritation (PN) is a promising approach for sustainable nitrogen management at water resource recovery facilities (WRRFs). When integrated with a deammonification (AMX) process, a coupled PN-AMX process is an integrated process strategy for total nitrogen removal that is decoupled from the need for biodegradable organic carbon for denitrification. This is particularly relevant for utilities that are seeking to achieve significant diversion of incoming biodegradable organic material (i.e., carbon diversion) prior to the biological treatment process. Under such conditions conventional nitrogen removal through nitrification denitification is not viable to achieve low total nitrogen (TN) limits due to the lack of biodegradable organic carbon to support denitrification. Other salient benefits of the PN-AMX process relative to conventional nitrogen management processes include low energy consumption, reduced alkalinity requirements, and low carbon footprint. Recent research and most of the full-scale application of PN-AMX has focused on single-stage systems where NOBs are selectively washed out by limiting electron donor (nitrite) and/or electron acceptor (oxygen) availability (Lotti et. al., 2015; Laureni et al. 2019). However, the recent discovery of Commamox Nitrospira (Daims et al., 2015) which might thrive in these substrate limiting conditions could limit the applicability and robustness of these control strategies. This project aims to fill key knowledge gaps related to the practicability of PN-AMX systems in a two-stage process in mainstream conditions and answer unaddressed questions related to the reliability of and N2O emissions from PN systems (Kampschreur et al., 2009). The two specific objectives of our research are: 1. Demonstrating the long-term efficacy of a suspended growth PN system treating an influent with a low soluble COD to ammonia N ratio (C:N) ratio. 2. Evaluating the links between N2O production and PN performance.
Materials and Methods
The pilot system consisted of a 380 L sequencing batch reactor (SBR). The reactor was equipped with online pH, ORP, dissolved oxygen (DO), ammonia, nitrate, TSS (Hach Company, Loveland, CO), and nitrous oxide (Unisense, Aarhus, Denmark) sensors (Figure 1-1). The reactor was operated at a 5 h aerobic HRT and fed trickling filter effluent (Table 1-1). Aeration was controlled using a PID controller with a fixed DO setpoint. A summary of the operating cycle structure for the reactor is provided in Table 1-2. SRT control was achieved by wasting a fraction of the solids during the react phase (hydraulic wasting) and the remainder during the settle phase (selective wasting). In situ rate experiments were conducted weekly. In addition to the online data, samples were regularly collected to measure ammonia, nitrite, nitrate, alkalinity, phosphorous, COD (Hach Company), TSS, and VSS (Standard Methods).
Preliminary Results and Discussion
Partial nitritation was achieved after an initial start-up period of 96 days and stabilized when the SBR was operated at an aerobic SRT of 3.0 ± 0.4 d (total SRT 5.0 ± 0.6 d) and a DO setpoint (DO.SP) of 1.0 (react phase DO = 1.0 ± 0.1 mg/L) (Figure 1-2). Nitritation resulted in an effluent nitrite concentration of 13.7 ± 3.5mg/L and a nitrite accumulation ratio of 90% ± 9%. After a stable operation period of 6 weeks, the SBR DO.SP was reduced from 1.0 mg/L to 0.7 mg/L on day 151. Within four days, the in-situ NHx-N concentration decreased with a corresponding increase in nitrate concentration. We hypothesize that the enhancement in the nitration rate resulted from an increase in the aerobic SRT which happened concurrently with the DO.SP change (Figure 1-2, bottom). The SRT increase resulted from an improvement in the mixed liquor settling and reduction of solids selectively wasted during the settling phase and in the SBR effluent. The aerobic SRT was stabilized to 3.4 ± 0.4 d and this operation resulted in similarly high ammonia oxidation rates, but little nitrite accumulation (Figure 1-2, bottom). Minimal N2O emissions were recorded in the 96-day start-up period. The onset of nitrite accumulation resulted in a concurrent increase in N2O production (Figure 1-2). A decrease in N2O emissions was noted when the DO SP was reduced to 0.7 mg/L and nitrite accumulation reduced significantly. Our data suggest that when the SBR was operated with a DO.SP of 1 mg/L and the nitritation rate was higher than ~75 mg N/L/d, the rate of N2O emissions was in the range of 0.5-0.7 mg-N/L/d or less than 1% of the nitrite accumulation rate (Figure 1-3). At the lower DO.SP (0.7 mg/L), the rate of N2O emissions was significantly reduced. Our results demonstrate the efficacy of managing nitrogen in wastewater with a low C:N ratio (<5mg-sCOD/mg-NHx-N) using a nitritation process through control of the SRT and DO concentration. Although N2O, a potent greenhouse gas, was produced as a result, it was produced in small quantities relative to the nitrite accumulated. Overall, the value of a nitrogen management scheme built on nitritation followed by deamonification has significant value for utilities seeking to enhance carbon diversion, maximize resource utilization, and meet stringent nutrient removal targets.