The anthropogenic nitrogen input into coastal waters is becoming increasingly problematic due to eutrophication of our waters. During digestion of municipal wastewater treatment sludge up to 50% of the organic nitrogen is released as soluble ammonium, whereby concentrations between 600-1000 g NH4-N/m3 in the sludge digester supernatant result. With the recirculation of this supernatant to the primary settling tank the total nitrogen input load is increased by up to 20%. Therefore this recirculation significantly contributes to the up to 70% of the input nitrogen load discharged into the receiving waters. To prevent this nitrogen recycle, a separate biological treatment of the supernatant can be integrated in the recirculation loop.
Upscaling a stable process
Conventionally ammonia is removed via nitrification and heterotrophic denitrification, ammonium being oxidized to nitrate and then reduced with an external carbon source (often methanol) to molecular nitrogen. Since August 2006 Eawag is researching on the combined single stage nitritation/anammox process. In this process a part of the ammonium is oxidized with oxygen into nitrite (partial nitritation), while the remaining ammonium is oxidized biologically to molecular nitrogen using the produced nitrite as electron acceptor (see figure).