Mechanism for Simultaneous Nitrification/Denitrification

The operating characteristics of six full-scale wastewater treatment plants using the staged, closed loop bioreactor process known as orbal were evaluated to determine the degree of simultaneous biological nutrient removal (SBNR) occurring. Low effluent total nitrogen concentrations indicate that simultaneous nitrification and denitrification occurs reliably in these facilities. The use of SRT values sufficient long to allow nitrifiers to grow and the maintenance of low DO concentrations in channel 1 of the process encourage nitrogen removal. Environmental conditions are relatively uniform throughout each channel, and distinct anoxic and aerobic zones do not appear to form. This suggests that the anoxic conditions necessary for denitrification may be developing within the biological flocs. Biological nitrogen removal in these facilities could be characterized using the International Association on Water Quality (IAWQ) Activated Sludge Model number 1 (ASM1). Effluent total phosphate data, and observed phosphorus removal relative to process BOD5 loadings, suggest that biological phosphorus removal may be occurring. Distinct anaerobic zones were not observed. It is possible that the anaerobic zones needed for enhanced biological phosphorus removal may have developed inside the biological flocs. These systems are good candidates for further study of SBNR. This research was conducted in the context of an overall research project directed at better understanding and controlling SBNR in full-scale wastewater treatment plants. The basic hypothesis of this 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) bioreactor mixing patterns that allow the anoxic and/or anaerobic zones necessary for BNR to develop, referred to as the bioreactor macro-environment, (2) the development of anoxic and/or anaerobic zones within the floc, referred to as the micro-environment, 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 identified above to SBNR, thereby allowing SBNR to be implemented and controlled more effectively in full-scale plants.