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
