The Tjustvik SBR-plant outside Stockholm, Sweden has been in operation for four years. The plant has to meet stringent effluent standards, BOD7<10 PPM, total N < 15 PPM and total P < 0.3 PPM. The plant is a typical two reactor SBR-plant, sized for about 15 000 inhabitants. During the first year of operation there were difficulties in meeting the P consent level. The difficulties were linked to a deficit of available organic carbon and a secondary phosphorus release. The problem was solved with the addition of septic sludge, in an amount equivalent to about 10,000 to 15,000 inhabitants with respect to the BOD-load.

The altered operation resulted in a very stable and good effluent quality from the plant that has been maintained ever since, giving typical discharge levels as follows: BOD7, < 3 mg/l; Total-P, < 0.15 mg/l; Total-N, < 7 mg/l; NH4-N, < 1 mg/l.

The change of process saved the community from a major investment in a separate treatment facility for the septic sludge. The stabilisation degree of the waste activated sludge is sufficient to by pass the anaerobic digestion for the time being. In the beginning, the SBR-process stability played an unwanted role during start up as it maintained a secondary phosphorus release for a considerable time. Later the process stability became an asset as the varying loads from the septic sludge addition were handled with very good results.

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