Optimization of a periodic biological process for nitrogen removal from wastewater

The Bio-Denipho® process, a “phased isolation ditch” technology, varies both aeration pattern and flow path in a continuous flow multi-reactor system to force oscillation of organic and nutrient concentrations in process reactors. Using a six-phase cycle, desired biochemical transformations (e.g. nitrification, denitrification), are accomplished at different times in the same reactor. We used an industry-standard biokinetic model (IAWPRC) to develop and test three control strategies of increasing sophistication: (a) fixed phase lengths, (b) use of constant set points to switch between phases thus resulting in variable phase lengths, and (c) use of switching set points which are a function of on-line measurements (criteria functions). These strategies were optimized for nominal diurnal operating conditions, with the objective of minimizing effluent soluble nitrogen, subject to an upper bound on the oxygen transfer coefficient. In addition, performance of the strategies was simulated against 1-, 2-, and 5-day sustained peak loads. Under nominal diurnal conditions, the performance obtained with each strategy was comparable. Under peak loading conditions, both of the switching set point strategies gave substantially better performance than the fixed phase length strategy. Criteria functions were marginally better than constant switch points.