Redesign of an Aerated Lagoon to a Long-Term Aerated Activated Sludge System with Anoxic Selector and Very Low Phosphorus Discharge, 1999 Environmental Conference Proceedings

Tomas Ericsson & Olle Simon

ÅF-IPK AB

Bengt Lundin

AssiDomän Frövi

This case study presents a redesign of an aerated lagoon system at AssiDomän Frövi, Sweden. The mill produces unbleached and bleached pulp integrated with the production of liquid and packaging board. The mill is situated at a very sensitive inland receiving water. One of the main reasons for the redesign was to decrease phosphorous discharge.

The inlet to the biological treatment is 11 – 16 tonne COD/day and approx. 30 000 m 3 /day The old aerated lagoon had a retention time of 5 days and a removal of 35 – 40% COD, 85% BOD and 0-5 % phosphorous. Different concepts were tested in pilot-plants during one year. The selected system has also been tested in pilot plants at several other Swedish mills.

The main principle of the system is the combination of an anoxic selector and an aeration stage. 1/3 of the return sludge is pumped to the anoxic zone and 2/3 to the aeration zone.

The hydraulic retention time of the new system is 35 hours. The important design parameters are the F/M-ratio (0.15 kg COD/kg DS, day) and the sludge age (25 days). The COD removal is 65%. The removal of phosphorous is approx. 70 % that correspond to a phosphorous level of 0.15 mg/l in the effluent. The nitrogen discharge has decreased with 50%. The sludge production is low, 0.2 kg DS/CODremoved.

The plant has been operating since October 1997 and is running efficiently since July 1998. The pilot plant results have been verified in the full plant operation.