To meet increased load or requirements on effluent standards the upgrading of a WWTP may be carried out with a large biological stage meaning higher investment costs but lower operation and maintenance (O&M) costs, or with a compact biological stage meaning lower investment costs but higher O&M costs.

Henriksdal and Käppala WWTPs in Sweden, both built in rock with large biological reactors operated with pre-denitrification, show lower specific O&M costs than Himmerfjärden WWTP, operated with post-denitrification in a fluidised bed. It is the other way round for the specific capital costs. The total specific cost, which is the sum of the specific O&M and capital cost, is a useful tool when choosing an alternative for upgrading. The experiences from these three plants show that a “large volume alternative” is competitive with a “compact alternative” if the volumes can be available at a reasonable cost. With proper technology rock chamber plants may be comparatively cheap to build. The economical and environmental advantages are discussed in this paper.

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