Report
Teaser, summary, work performed and final results
Periodic Reporting for period 1 - N2OPNA (Understanding Nitrous Oxide Production from The Mainstream Partial Nitritation and Anammox Process)
Teaser
Mainstream partial nitritation/anammox (PNA) is a cost-effective technology for biological nitrogen removal from wastewater. However, to achieve high nitrogen removal rates and efficiencies in mainstream PNA process is of high complexity considering dynamic characteristic of...
Summary
Mainstream partial nitritation/anammox (PNA) is a cost-effective technology for biological nitrogen removal from wastewater. However, to achieve high nitrogen removal rates and efficiencies in mainstream PNA process is of high complexity considering dynamic characteristic of municipal wastewater (regarding composition, quantity, pH, temperature, etc.) and the competition among different microbes for substrates and space. The key point for PNA process is to suppress nitrite oxidizing bacteria (NOB), which can be achieved by using inhibitors. Two internal inhibitors, free nitrous acid (FNA) and free ammonia (FA) have great advantages over others, attributing to their readily availability and sustainability in WWTPs. The FNA and FA from sidestream can be applied for NOB suppression in the sludge returning line. However, relevant research on how AerAOB, NOB and AnAOB respond to FNA and FA stress in one-stage PN/A process remains scarce.
Another challenge of the PNA process is the emission of nitrous oxide (N2O). The destruction of stratospheric ozone layer has become a significant environmental issue in 21st century, which is contributed largely by N2O. N2O is also a potent greenhouse gas with a global warming potential (GWP) of approximately 265 times stronger than carbon dioxide. N2O production from PNA process remains far from fully understood, and a fundamental understanding of the mechanisms is highly desired to further optimize the process.
In this project, the hybrid reactor technology and the NOB-inhibiting return-sludge treatment with realistic FNA/FA levels in full-scale sidestream were combined to achieve mainstream PN/A process. The immediate stress impact of FNA/FA levels and contact time on the activities of AerAOB, NOB and AnAOB were investigated in batch reactor, while long-term recovery was assessed in an integrated film activated sludge (IFAS) reactor, with sludge as flocs and biofilm on carriers during a period of 10 months. The shift of microbial community in flocs and carriers and N2O emissions were closely followed under different stress conditions. Economic potential of the combined technology was assessed. The final goal was to optimize the stress treatment conditions to maximize N removal efficiency and minimize N2O emissions in the IFAS reactors.
Work performed
Work Package 1: Reactor set-up and culture enrichment
Two 4.5-L integrated fixed film activated sludge reactors were operated in sequencing batch mode for over 300 days. In first one-month operation, two operational strategies were implemented: residual ammonium and low DO set-points (0-0.75 mg O2/L). Ammonia removal reached close to 80%, but TN removal was poor (around 20%) due to nitrate accumulation. This demonstrated that FNA/FA treatment on return sludge is essential for NOB suppression. Besides, we observed that AerAOB and NOB preferred to grow on flocs, while most of AnAOB grew on carriers, which make separate SRT control possible.
From one month on, FNA and FA were used to treat the two IFAS reactors once per week, which were termed as IFAS-FNA and IFAS-FA respectively. We tested different combinations of FNA levels and contact times and concluded that optimal stress conditions was FNA 2.0 mg N/L and contact time of 4 hours. This is because low FNA level (such as 0.5 mg N/L) resulted in NOB adaptation and high FNA level (3.0 mg N/L) with high contact time did not promote the N removal, but increased cost (see details in work package 4). In terms of IFAS-FA, it was revealed that FA at level of 30 mg N/L combined with 1 hour contact time has reversibly inhibitory impact of AnAOB. Besides, the tested FA stress condition has very limited effect on NOB suppression. Switching from FA treatment to FNA treatment immediately deactivated NOB. Further investigation demonstrated that release from FNA stress would immediately reactivate NOB, leading to poor N removal of the IFAS system again.
We used an on-line infrared gas analyser to monitor N2O emissions from two IFAS reactors. Nitrite accumulation has been identified as the most important factor affecting N2O production. The N2O emission factor, defined as the ratio of emitted N2O nitrogen to nitrogen loading, was linearly correlated to nitrite accumulation, indicating that AOB denitrification pathway served to be the main N2O production mechanism. Moreover, we demonstrated that at similar nitrite level, FNA treatment at higher level (2.0 and 3.0 mg N/L) gave rise to lower N2O emission compared to FNA treatment at lower level 0.5 mg N/L), FA treatment and benchmark situation without inhibiting treatment. This observation revealed potential shifted mechanism caused by variation of stress conditions and pave the way for future mitigation implementation.
We performed impact extrapolation for proposed FNA return sludge technology in full-scale WWTP. The idea is to use anaerobic digester to produce ammonium-rich effluent. The digester liquor would be directed to a partial nitritation (PN) unit to produce FNA-rich liquor. A extra contact tank will be used to mix the thickened sludge and the FNA-rich liquor. The treated sludge will be redirected to mainstream water line (refer to attached image). On this basis, the extra cost for the proposed technology consists of CAPEX and OPEX for thickener, PN unit and contact tank. Four scenarios has been investigated based on assumptions and realistic FNA level boundaries in full scale, those are FNA 3.0 + 16h + 100% nitritation without thickener (S1), FNA 3.0 + 16h + 65% nitritation with thickener (S2), FNA 3.0 + 4h + 65% nitritation with thickener (S3) and FNA 2.0 + 4h + 65% nitritation without thickener (S4). It is concluded that the S4 was cheapest and feasible for stable PNA process. In terms of environmental assessment, we consider carbon footprint from electricity consumption (major by aeration), carbon sequestration from anerobic digestion, carbon footprint from N2O emissions from mainstream and sidestream PNA process. It is concluded that in order for a neutral carbon emission, N2O emission factor for mainstream PNA can never exceed 0.52%, which is lower than our observation in lab-scale reactors in most cases. This indicates that future mitigation strategy is essential to minimize carbon emission caused by mainstream N2O produ
Final results
The proposed project has provided important clues for a promising technology and future N2O mitigation development
Our experimental observation proved that stable, efficient and robust PNA process is possible under mainstream conditions. The combination of advanced hybrid reactor technology and novel sidestream FNA stressor technology achieved much better treatment performance than conventional operational strategies. The lab-scale demonstration paved the way for successful implementation of mainstream PNA in full scale. The comprehensive knowledge of N2O emission from the PNA process is indispensable, and will facilitate the reactor optimization and strategy development for minium N2O emission. The experimental data along with our impact extrapolation is very significant for designing, constructing and operating sustainable wastewater treatment plants and thus for assisting to fulfil the EUROPE 2020 objectives of smart, sustainable and inclusive growth.