Mechanisms of simultaneous biological nutrient (nitrogen and phosphorus) removal in closed loop bioreactors

Simultaneous biological nutrient removal (SBNR) is the removal of nitrogen and/or phosphorus in excess of that required for biomass synthesis in biological wastewater treatment systems where there are no defined anoxic and/or anaerobic zones. This phenomenon has been observed numerous times in activated sludge systems, but the mechanisms are not well understood. This research hypothesized that SBNR is the result of one or more of three mechanisms within individual systems: variations in the bioreactor macro-environment created by the mixing pattern, gradients within the floc micro-environment , and/or novel microorganism activity. Understanding of the mechanisms of SBNR can be expected to lead to improved efficiency and reliability in its application.; Preliminary work documented the operating characteristics and degree of simultaneous nitrogen and phosphorus removal occurring in seven full-scale closed loop bioreactors employing the Orbal™ oxidation ditch process. Biological nitrogen removal was evaluated by using Activated Sludge Model No. 1 (ASM1), which incorporated the floc micro-environment under oxygen limited conditions. While the floc micro-environment likely plays an important role in nitrogen removal in such plants, it cannot explain phosphorus removal.; A simple batch assay was developed that screens for novel microorganism activities in nitrogen removal. It demonstrated that novel microorganism activity was of little importance in SBNR at the 3 plants tested. Biological phosphate anaerobic release and aerobic uptake were also observed in bench-scale experiments, consistent with conventional stoichiometry and current understanding of phosphate accumulating organism (PAO) metabolism. The estimated PAO population was 13% of the mixed liquor biomass.; A computational fluid dynamics (CFD) model was developed to elucidate the role of the bioreactor macro-environment in SBNR. This is the first reported application of CFD to activated sludge biological wastewater treatment. It allowed the flow pattern within the outer channel of a closed-loop bioreactor to be visualized, and incorporated biochemical reactions from ASM2. CFD analysis revealed that informal anoxic/anaerobic and aerobic zones exist within the Orbal™ system, allowing the growth of PAOs. Although the software and computational requirements limited model complexity, it still simulated the creation of dissolved oxygen gradients within the system, demonstrating that the anaerobic zones required for SBNR could occur.