EXPERIMENTAL AND MODEL-BASED EVALUATION OF THE MAXIMUM ALLOWABLE FLOWS IN ACTIVATED SLUDGE SYSTEMS UNDER STORM CONDITIONS

The Hanover-Gümmerwald full-scale pilot wastewater treatment plant (WWTP), consisting of primary clarifiers and two lines of activated sludge systems, was subjected to a series of controlled, short-term hydraulic shock loading experiments reflecting storm events. The experimental results revealed that it was possible to treat biologically stormwater up to quadruple dry weather flow rate while still meeting the German effluent standards. The aim of the simulation study was to provide a validated model as a decision tool for estimating the maximum allowable peak flow rates. The ASM2d and ASM3P predictions confirmed the observations concerning the capabilities of the system for handling increased flows during storm events. The predicted peaks of N-NH4+ at the line with the quadruple dry weather flow rate were normally exceeding 8 gN/m3 (similar to the observations), whereas no (or minor) peaks of N-NH4+ were predicted for the line with the double dry weather flow rate. This finding was later confirmed by the full-scale data from the municipal WWTP in the city of Hildesheim. The relationships between ASM2d and ASM3P predictions for N-NH4+ and N-NO3− were highly correlated (r2=0.83−0.99) with the slopes remaining close to 1.0. Further investigations revealed that the effluent Ninorg. concentrations were primarily correlated with the influent concentrations of N-NH4+ measured during the hydraulic stress experiments.