Characterization of simultaneous nutrient removal in staged, closed-loop bioreactors

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.