Simultaneous biological nutrient removal: a state-of-the-art review.

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.