Doctor of Philosophy
Civil and Environmental Engineering
The University of Western Ontario
Aeration is a critical component of the two-step nitrification, the biological conversion of ammonia (NH3) to nitrite (NO2-) and then to nitrate (NO3-), in municipal wastewater treatment plants. Aeration provides the necessary dissolved oxygen for the growth and activity of nitrifying bacteria to Partial nitrification-Anammox (PN-Anammox) is a two-step biological wastewater treatment process, used to remove nitrogen compounds from wastewater efficiently without the need for external carbon. It combines partial nitrification, where ammonia is partially oxidized to nitrite, with anaerobic ammonium oxidation (Anammox) to remove both ammonia and nitrite. About a 60% to 80% of the nitrification energy and 100% of the carbon sources can be saved by PN-Annamox process.
Mainstream partial nitrification, which relies on the inhibition of nitrite oxidizing bacteria (NOB) has drawn considerable attention from researchers and wastewater treatment professionals for several compelling reasons: energy efficiency, enhanced nitrogen removal, reduced carbon footprint and cost savings. 60-80℃ to achieve mainstream partial nitrification. In this study, heat treatment was explored as a new approach in continuous and intermittent for achieving mainstream PN.
Sustained continuous heating of 100% MLSS and offline heating of 20% of biomass at 37℃-42℃ at solids retention time (SRT) of 7 days, DO of 4 mg/L did not achieve any stable nitrite accumulation. In intermittent heating in sequencing batch reactors (SBR), heating once every 10 days at 45℃-2hr, 50℃-1hr and 55℃-0.5 hr with SRT of 7 days, DO of 4 mg/L showed 12% to 17% of nitrification energy savings with stable nitrite accumulation ratio (NAR) of 60%-80%. Similar NAR of 79% was found in continuous-flow systems using membrane bioreactors (MBRs), however with heating once in 4 days. A modified model, incorporating two Arrhenius temperature correction factors for positive and negative growths of nitrifiers: θ1=1.05 and 1.03, and θ2=1.02 and 1.01 respectively for ammonia oxidizing bacteria AOBs and NOBs as a pose to θ=1.072 and θ=1.065 for (AOBs) and NOBs respectively was developed for intermittent heating. This model, which deviated only 6%-20% from experimental data, is an impactful scientific contribution in PN modeling.
Optimization of temperature and contact time for NOB suppression using batches is not appropriate as NOBs are revived in 10-12 days in long-term studies. The impact of heating on the increase of COD of 45-90 mg/L at 45℃-60℃ compared to control ( 30 mg/L) and the reduced NOx of 17 mg/L compared to control ( 25 mg/L) which potentially occurred due to the lysis of heterotrophs and reduced ammonification, needs to be explored further. Any heat treatment higher than 45℃-2hr will solubilize microbial community and any heating higher than 60℃, will need external reactivation of AOBs before application for PN.
Summary for Lay Audience
Nitrogen is an essential part of environment, comprised of ammonia, nitrate, nitrite-nitrogen, and organically bonded nitrogen. Although nitrogen has several forms, fresh wastewater mainly contains ammonia and organic nitrogen. Ammonia causes direct toxic effect on the aquatic life. Due to high concentration of ammonia, oxygen is reduced, causes eutrophication which leads to excessive growth of blue green algae. Consequently, aquatic lives that are forced to store toxicants in their tissue and blood; eventually die.
Conventionally Ammonia is removed by biological nitrification and denitrification processes. Ammonia oxidation happens in two steps: In first step, ammonia is oxidized to nitrite by a bacteria called, ammonia oxidizing bacteria (AOB) and in second step nitrite is oxidized to nitrates by nitrite oxidizing bacteria (NOB). In denitrification process, nitrate that produced in nitrification process used by denitrification bacteria that are heterotrophic and hence requires external carbon source. However, an alternative process that can reduce energy from nitrification process and carbon source process is called partial nitrification and annamox process. In this process, NOBs in are supressed by the controlled application of the operating conditions and produces nitrite that is oxidized by Annamox bacteria without the requirement of carbon source.
In this Ph.D. thesis, the production of nitrate after the suppression of NOBs was investigated by the direct application of heat to the activated sludge. Batch reactors, sequencing batch reactors (SBRs) and membrane bioreactors (MBRs) were used for heating the activated sludge to achieve mainstream PN. The heat shock at 45℃-2hr,50℃-1hr, 55℃-0.5hr produced nitrite of 10-14 mg/L in SBRs whereas 55℃-1hr produces same level of nitrite concentration in MBRs. About 80% of stable nitrite accumulation ratio (NAR) was achieved for 30-70 days in SBRs. Heat optimization in SBRs showed that about 20% less energy is required for heating every 10 days compared to complete nitrification. For both systems, SRT of 7 days was optimum for achieving mainstream PN. Heating SBRs every 10 days and heating MBRs every 4 days produce effluent nitrite of 10-14 mg/L with maximum NAR of 80%.
Afroze, Niema, "Temperature-controlled partial nitrification for mainstream wastewater deammonification using batch, semi-batch and continuous systems" (2023). Electronic Thesis and Dissertation Repository. 9870.
Available for download on Tuesday, April 30, 2024