Doctor of Philosophy
Civil and Environmental Engineering
Biological nitrogen removal (BNR) processes have been widely used for removing wastewater nitrogen because of effectiveness, economics, and environmental friendliness. Considering the energy requirements for nitrification and need for external carbon for denitrification in conventional BNR processes, anaerobic ammonia-oxidizing bacterial (anammox) processes facilitate energy-neutral or net energy positive biological treatment as well as reduce operational costs because of reduction in aeration energy, which constitutes 50%-60% of the overall energy consumption at municipal wastewater treatment plants, elimination of external organic carbon requirements, and lower sludge production. However, one of the major challenges in the practical application of the anammox process is the slow growth rate of anammox bacteria (a doubling time range of 7 – 14 days), resulting in long start-up times and biomass washout. As biomass immobilization technologies offer several advantages over convnetional suspended growth systems, including enhanced higher cell densities and biomass retention, reusability, and biomass protection from extreme environmental conditions, the main objective of this study is to develop sustainable anammox gel beads to retain the biomass in reactor efficiently.
To develop sustainable anammox gel beads, four different types of anammox gel beads were prepared using sodium alginate (SA) and support materials, including distilled water (DW) as control SA bead (B1), sodium silicate as SA-SS bead (B2), polyvinyl alcohol as SA-PVA bead (B3), and colloidal silica SA-CS bead (B4). Detailed characterization and comparison of the anammox gel beads was conducted using four lab-scale semi-continuous stirred tank reactors (CSTRs) and revealed that the SA-SS beads showed superior durability with better reactor performance. SA-SS (B2) beads showed the minimum reduction in the effective diameter of 24% compared to 98%, 57%, and 96% for SA (B1), SA-PVA (B3), and SA -CS (B4), respectively indicating higher integrity of B2 beads. Moreover, the reactor containing SA-SS beads not only showed the maximum biomass retention after 30 days of operation, relative to the initial mass of 72% compared to 3%, 44%, and 5%, for SA, SA-PVA, and SA-CS, respectively but also the highest total nitrogen removal efficiencies of 80% . The diffusion coefficients (De) of ammonium in anammox gel beads was maximum for B2 beads (26.2 µm2/s) compared to B1 (18.8 µm2/s), B3 (22.4 µm2/s), and B4 (13.9 µm2/s) beads, indicating enhanced internal mass transfer.
Due to the extremely slow anammox growth rate, the minimum initial concentration of anammox biomass in gel beads for rapid reactor start up was assessed at, four different initial biomass concentrations of 208 mg/L, 310 mg/L, 416 mg/L, and 540 mg/L immobilized in SA-SS beads and packed at 20% (vbead/vtotal) in four 100-ml CSTRs, and was determined to be 311 mg/L . To evaluate the effect of bead volume on nitrogen removal performance, four identical anaerobic fluidized bed reactors (AFBRs) with 0.5L working volume were inoculated with immobilized beads containing 311 mg/L of anammox biomass at packing ratios of 20%, 30%, 45%, and 60%. The optimum packing ratio (on a volumetric basis) of the AFBR was 30% (v/v) with a nitrogen removal rate (NRR) of 0.40 kg N/m3-d at volumetric nitrogen loading rate (NLR) of 0.51 kg N/m3-d, corresponding to nitrogen removal efficiencies (NRE) of 77%. A higher anammox detachment rate was observed in AFBR1 (packing ratio 20%) compared to AFBR2 (packing ratio 30%) and AFBR3 (packing ratio 45%).
Finally, the impact of holed anammox gel beads on nitrogen removal performance was investigated using two lab-scale FBRs, one as control with immobilized non-holed anammox beads (CFBR) and the other with holed immobilized anammox beads (PFBR). The PFBR achieved a maximum NRR of 0.81 kg N/m3-d at NLR of 1.01 kg N/m3-d with NRE of 80% after 35 days without operational problems, whereas the CFBR with non-holed anammox gel beads failed after 30 days due to excessive biomass loss of 78% of the initial biomass from day 30 to day 36. The hindrance to diffusion of the generated nitrogen gas was the main mechanism of beads breakup and biomass washout.
Summary for Lay Audience
Nitrogen release from wastewater treatment plants causes eutrophication of rivers and deterioration of water sources. USEPA notes approximately 25% of water body impairments because of excess nutrient release in water (USEPA 2007). To protect the lakes and other natural water bodies from eutrophication, stringent nitrogen levels are set for the effluents from wastewater treatment plants. The current research aims at developing sustainable nitrogen removal technologies to meet the stringent regulations by optimizing the removal process without additional operational cost or energy demand. Biological nitrogen removal (BNR) processes have widely been used for removing wastewater nitrogen because of effectiveness, economics, and environmental friendliness. Considering the requirements of energy for nitrification and the need for external carbon for denitrification in conventional BNR processes, anaerobic ammonia-oxidizing bacterial (anammox) processes facilitate energy-neutral or net energy positive as well as reduce operational costs because of reduction in aeration energy. However, one of the major challenges in the anammox process's practical application is the slow growth rate of anammox bacteria, resulting in long start-up times and biomass washout.
Therefore, current research developed a novel approach of anammox immobilization in gel beads, whereas the durability of gel beads with nitrogen removal performance was investigated in FBRs. Second, nitrogen removal performance was investigated using non-porous and porous immobilized anammox gel beads in a fluidized bed bioreactor. The experimental results revealed that anammox immobilized in sodium alginate sodium silicate (SA-SS) shows the minimum reduction of the effective diameter of 24% and maximum biomass retention after 30 days of reactor operation compared to three other immobilized beads. The experimental results also revealed that anammox immobilized in porous SA-SS beads achieved 80% of nitrogen removal in a fluidized bed reactor.
Chowdhury, Mohammad M I, "Anammox process for nitrogen removal: development of sustainable gel beads and its application in fluidized bed reactors (FBRs)" (2021). Electronic Thesis and Dissertation Repository. 7749.