Potential of denitrification and solids removal in the rectangular clarifier

In full scale experiments, the denitrification capacity of secondary clarifiers and measures to increase it were evaluated. Due to a higher activated-sludge concentration and a lower scraper velocity, the sludge mass and thus the denitrification capacity in the secondary clarifier at the WWTP Zürich–Werdhölzli was significantly increased. During the investigated period, the denitrification in the clarifier represented 37% of the total denitrification, corresponding to 19% of the total inlet nitrogen. The main part was denitrified in the inlet channel of the clarifier, so that doubling the scraper interval increased the denitrification in the clarifier by only about 14%. An approach for modelling the sludge-blanket volume was developed on the basis of measurements of the sludge-blanket height along the rectangular clarifier with transverse flow and vacuum-removal scrapers. The limiting sludge-volume load was calculated from sludge settling tests and compared with observed loads on the full-scale clarifier. Activated sludge model No. 2 was verified with the nitrate and ammonium profiles in the activated sludge tanks, return sludge and clarifier effluent.