In Activated Sludge (AS) systems Nitrite (NO 2 ) develops as an intermediate product of nitrification and denitrification (1). Nitrite concentration in WWTP's is very low under normal conditions (around 0.1 mg/l). Increased concentrations are usually an indication of a disturbance of microbiological processes, of an overloaded plant or insufficient aeration capacity. Microbiological inhibition can be caused by toxic substances, seasonal variations in temperature or generally unfavourable conditions for Nitrite Oxidizing Bacteria (NOB). Nitrite is a strong fish poison and is considered as a possible cause of fishes populations decrease in Swiss waters ("network fish decrease Switzerland ", fischnetz, 2004). Significant increases of the Nitrite concentration were determined by on-line measuring campaigns in the effluents of different WWTP's. Most of the increases were brief peak values at times temporarily exceeding the official maximum concentration of 0.3 mg NO 2 -N/l (GSchV, 2003) (cf. figure).
Both laboratory scale and half-scale tests (batch tests, pilot AS- WWTP's, ~75 IE) are being conducted at the EAWAG on Nitrite problematic. Future plans include a second full-scale investigation of the St-Gallen-Hofen WWTP's. Current results lead to different hypotheses, which are to be discussed and validated within the project. In particular the mechanisms, which lead to high Nitrite releases in WWTP's effluent, are to be examined with more details. The conclusions of this work will help the planning engineer, respectively the plant operator, to adopt adequate strategies to respect legal requirements.
Oxygen concentration and Temperature seem to be of particular significance in this thematic, therefore the following points will be further examined:
- Relation between oxygen concentration and temperature and the production of Nitrite in EAWAG pilot WWTP's.
- Impact of different automated controls of aeration cycles (especially regulation through online ammonium signal) on Nitrite build-up.
- In parallel to smaller scale investigation full-scale test will be implemented in an existing WWTP's that exerted high Nitrite peak (St-Gallen-Hofen). In particular the influence of oxygen and temperature will be investigated.
- In addition to these practical aspects an Activated Sludge Model (ASM3) extended at the EAWAG for a two steps nitrification will be calibrated and validated.