This research was conducted in the context of an overall research project directed at better understanding and controlling simultaneous biological nutrient removal (SBNR) in full‐scale wastewater treatment plants. The basic hypothesis of the overall research is that three general mechanisms are responsible for SBNR and that all three can operate, to different degrees, within any biological nutrient removal (BNR) system. The three mechanisms are (1) biological reactor (bioreactor) mixing patterns that allow the anoxic and anaerobic zones necessary for BNR to develop, referred to as the bioreactor macroenvironment, (2) the development of anoxic and anaerobic zones within the floc, referred to as the floc microenvironment, and (3) the presence of novel microorganisms. System design and operating parameters will determine the relative importance of each mechanism. The objective of the overall research project is to identify the factors affecting the relative contributions of the three mechanisms to SBNR, thereby allowing SBNR to be implemented and controlled more effectively in full‐scale plants.
In the research reported in this paper the operating characteristics of seven full‐scale wastewater treatment plants using the staged, closed‐loop bioreactor process known as Orbal were evaluated to determine the degree of SBNR occurring. Low effluent total nitrogen concentrations from a process without distinct anoxic zones indicate that simultaneous nitrification and denitrification occur reliably in these facilities. Nitrogen removal is encouraged at these facilities by use of solids retention times sufficiently long to allow nitrifiers to grow under the low dissolved oxygen concentrations in channel 1 of the process. Environmental conditions are relatively uniform throughout each channel, and distinct anoxic and aerobic zones do not seem to form. This suggests that the anoxic conditions necessary for denitrification may develop principally within the biological flocs. Biological nitrogen removal was characterized using the International Association on Water Quality Activated Sludge Model Number 1. Influent and effluent total phosphate and biochemical oxygen demand data and microbiological observations suggest that biological phosphorus removal occurs. Distinct anaerobic zones were not observed, but anaerobic conditions may develop within the lower portion of channel 1. These systems are good candidates for further study of SBNR.

A full-scale, closed-loop bioreactor (Orbal oxidation ditch, Envirex brand technologies, Siemens, Waukesha, Wisconsin), previously examined for simultaneous biological nutrient removal (SBNR), was further evaluated using computational fluid dynamics (CFD). A CFD model was developed first by imparting the known momentum (calculated by tank fluid velocity and mass flowrate) to the fluid at the aeration disc region. Oxygen source (aeration) and sink (consumption) terms were introduced, and statistical analysis was applied to the CFD simulation results. The CFD model was validated with field data obtained from a test tank and a full-scale tank. The results indicated that CFD could predict the mixing pattern in closed-loop bioreactors. This enables visualization of the flow pattern, both with regard to flow velocity and dissolved-oxygen-distribution profiles. The velocity and oxygen-distribution gradients suggested that the flow patterns produced by directional aeration in closed-loop bioreactors created a heterogeneous environment that can result in dissolved oxygen variations throughout the bioreactor. Distinct anaerobic zones on a macroenvironment scale were not observed, but it is clear that, when flow passed around curves, a secondary spiral flow was generated. This second current, along with the main recirculation flow, could create alternating anaerobic and aerobic conditions vertically and horizontally, which would allow SBNR to occur. Reliable SBNR performance in Orbal oxidation ditches may be a result, at least in part, of such a spatially varying environment. Water Environ. Res., 79, 600 (2007).

Simultaneous biological nutrient removal (SBNR) is the occurrence of biological nutrient removal (BNR) in systems that do not possess defined anaerobic and/or anoxic zones. A review of the relevant literature demonstrates that two mechanisms are primarily responsible for SBNR: (1) the bioreactor macro-environment and (2) the floc microenvironment. Complex hydraulic flow patterns exist in full-scale bioreactors that can result in the cycling of mixed liquor through the different environments needed for BNR. Diffusion resistance further allows oxygen-sufficient and oxygen-deficient zones to develop in activated sludge flocs if the external dissolved oxygen concentration is properly controlled. The diffusion of substrates between these zones allows BNR to occur. Long-term acclimation to the unique environmental conditions occurring in these systems results in the selection of microorganisms well adapted to the low dissolved oxygen concentrations occurring in them. The experience base for the design and operation of SBNR systems is expanding, thereby allowing their more widespread application, especially coupled with conventional mathematical modeling approaches. Computational fluid dynamics is an evolving tool to assist with the design and optimization of SBNR.

Simultaneous biological nutrient removal (SBNR) is the biological removal of nitrogen and phosphorus in excess of that required for biomass synthesis in a biological wastewater treatment system without defined anaerobic or anoxic zones. Evidence is growing that significant SBNR can occur in many systems, including the aerobic zone of systems already configured for biological nutrient removal. Although SBNR systems offer several potential advantages, they cannot be fully realized until the mechanisms responsible for SBNR are better understood. Consequently, a research program was initiated with the basic hypothesis that three mechanisms might be responsible for SBNR: the reactor macroenvironment, the floc microenvironment, and novel microorganisms.Previously, the nutrient removal capabilities of seven full-scale, staged, closed-loop bioreactors known as Orbal oxidation ditches were evaluated. Chemical analysis and microbiological observations suggested that SBNR occurred in these systems. Three of these plants were further examined in this research to evaluate the importance of novel microorganisms, especially for nitrogen removal. A screening tool was developed to determine the relative significance of the activities of microorganisms capable of autotrophic denitrification and heterotrophic nitrification-aerobic denitrification in biological nutrient removal systems. The results indicated that novel microorganisms were not substantial contributors to SBNR in the plants studied. Phosphorus metabolism (anaerobic release, aerobic uptake) was also tested in one of the plants. Activity within the mixed liquor that was consistent with current theories for phosphorus-accumulating organisms (PAOs) was observed. Along with other observations, this suggests the presence of PAOs in the facilities studied. Water Environ. Res., 75, 138 (2003).

Based on the International Water Association's (London) Activated Sludge Model No. 2 (ASM2), biochemistry rate expressions for general heterotrophs and phosphorus-accumulating organisms (PAOs) were introduced to a previously developed, three-dimensional computational fluid dynamics (CFD) activated sludge model that characterized the mixing pattern within the outer channel of a full-scale, closed-loop bioreactor. Using acetate as the sole carbon and energy source, CFD simulations for general heterotrophs or PAOs individually agreed well with those of ASM2 for a chemostat with the same operating conditions. Competition between and selection of heterotrophs and PAOs was verified using conventional completely mixed and tanks-in-series models. Then, competition was studied in the CFD model. These results demonstrated that PAOs and heterotrophs can theoretically coexist in a single bioreactor when the oxygen input is appropriate to allow sufficient low-dissolved-oxygen zones to develop. Water Environ. Res., 79, 613 (2007).