Abstract
Wastewater treatment plants combining bioreactors and natural processes, designed to achievecost efficient treatment, are described and evaluated. The plants have a common generallayout: an anaerobic pretreatment, an aerated bioreactor, sedimentation with sludge return anda final sub-surface flow wetland treatment. Variations in this design, adaptations to variousapplications, process control strategies and sludge handling are discussed. Removalefficiencies obtained varies in the range: 96-99 % BOO7, 72-88 % COD, 92-96 % SS, 80-99% P, 37-91% N, where more advanced control yield higher efficiency. Thermo-tolerant fecalcoliform bacteria are typically removed by 99.9 %. Most of the nitrogen is removed in thebioreactors. Computer controlled aeration and sludge handling is required to obtain the hightotal nitrogen removal (> 80 %) Phosphorus can also be removed in the bioreactors andexported as sludge, or, more cost effectively, mainly removed in the wetland part of the plants.The cost efficiency of such treatment plants is good compared to alternative solutions.
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