Tanja Kurbus,* Janez Vrtovšek and Milenko Roš

National Institute of Chemistry, PO BOX 660, Hajdrihova 19, SI-1001 Ljubljana, Slovenia
* Corresponding author: E-mail: tanja.kurbus@ki.si
Tel.: +386(0)14760243, Fax: +386(0)4760300

Abstract
Nitrification/denitrification processes at high flocculent biomass concentrations (11.6–13.4 g L^–1) were studied in pilot Sequencing Batch Reactor (SBR). Optimal operating conditions for maximal nitrogen elimination efficiency were as follows: organic loading of 0.9 g COD L^–1 d^–1, nitrogen loading of 0.11 gN L^–1 d^–1, MLSS of 12 g L^–1 and SRT of 52 days. Medium sized sludge flocs (0.1–0.4 mm) with excellent settle ability (SVI = 27 mL g^–1) were formed during the experiment. Volumetric nitrogen removal rate of 90 mg NL^–1 d^–1 was achieved, which is twice as much as in SBR operating at low MLSS of 3.5 gL^–1 and low SRT of 21 days. This process offers an attractive option for the treatment of highly polluted wastewaters with lower investment and operating costs.

Keywords: High MLSS, denitrification, nitrification, SBR.