What are Distributed States and when are they Important? New Strategies to Improve EBPR Performance

Computer simulation of activated sludge processes is a critical tool for design, operation, and troubleshooting, but simulating enhanced biological phosphorus removal (EBPR) systems has proven to be particularly challenging. This may be due in part to uncertainties in biokinetic models, but new research suggests it may also be due to deficiencies in conventional “lumped state” approaches to simulation, which model bulk concentrations of the polyphosphate accumulating organisms (PAOs) responsible for EBPR, as well as their microbial storage product (polyphosphate, glycogen, and polyhydroxyalkanoates) contents. A recently developed, alternative method for modeling biological treatment systems is the “distributed state” approach, which models individual bacteria as they move through a biological reactor system, rather than bulk concentrations. This approach predicts that PAO states (their microbial storage product contents) tend to diverge when reactors are completely mixed, and this can produce very different outcomes than those predicted by the conventional lumped approach. A MATLAB-based distributed state program (DisSimulator 2.0) was applied to an A2O system and it was determined that distributed states tend to become more important with (1) shorter internal recycle ratios and (2) longer reactor hydraulic residence times. Consequently errors resulting from relying on lumped simulations are greatest in these conditions. This work illustrates that there appear to be interesting process phenomena related to changing state distributions that apparently cannot be accounted for by lumped simulations. These insights suggest that the continued advancement of the distributed simulator approach has the potential to improve design and operation of biological nutrient removal systems.