The purification requirements for the Stockholm sewage treatment plants will become more stringent in the future. The expected limit values for the effluent, expressed as annual mean values, are for BOD7, Tot-P, and Tot-N, 10, 0.3 and 15 mg/l respectively.

If these contents are multiplied by the design flow values for the three plants, we obtain the maximum quantities which may be released. If the relevant authorities permit the municipality to distribute these total quantities as desired between the three plants, future necessary extensions can be optimized. The following main principles apply to an extension of the three plants:

Loudden sewage treatment plant: This comparatively small treatment plant could, if the requirements are lower than in the other two plants, continue in operation with no other extensions than the inclusion of anoxic zones. It would, however, be necessary to refurbish the plant after a number of years of neglected maintenance.

Bromma sewage treatment plant: The biological stage was extended during the 1982-84 period. For this reason, the municipality suggests that no further extensions of the aeration tanks be required, before 1995 at the earliest. A nitrogen removal with outgoing contents of Tot-N of 15-17 mg/l is expected to be achieved by measures taken to reduce the load on the biological stage instead. These measures consist of centrifuging the excess sludge and pumping it directly to the digesters instead of returning it to the inlet. Furthermore, separate treatment of the reject water from the sludge centrifuges is planned. A third measure could be changing over to a more efficient precipitation chemical to permit a further reduction of the load on the biological stage with regard to, inter alia, BOD7, Tot-N etc. To meet the requirements for phosphorus removal (0.3 mg/l), the plant will be extended with a filter stage after the existing biological stage.

Henriksdal sewage treatment plant: At this plant, which is the largest of the three, the largest extensions are planned. To meet the requirements for nitrogen removal, the present volumes in the aeration tanks will be tripled and will be utilized as anoxic and aerated zones as required. Three new lines with aeration tanks and secondary sedimentation tanks will be constructed. The existing aeration tanks will also be deepened from 5 to 12 m.

The requirements for low phosphorus contents in the effluent will be met by installing a filter stage, as in the Bromma plant.

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