The Three-sludge sewage treatment system proved to be a stable process for the treatment of wastewater. This new concept consists of three separated bioreactors. Biological removal of phosphorus and COD takes place in the first high loaded activated sludge bioreactor with an intermediate settler. The second reactor is a nitrifying biofilm reactor. Nitrate is removed in the third denitrifying biofilm reactor. Pilot plant research (scale 2–3 p.e.) showed that at a constant flow rate during night and day a minimal hydraulic retention time of 10.4 hours, inclusive, the settler, could be obtained and yet still the stringent effluent standards (10 mg N/l and 1 mg P/l) were met. At a dynamic flow rate with day/night rhythm and at a storm-weather peak flow rate of 2.5 times the dry weather flow rate the standards were still met. However, at a flow rate of three times the dry-weather flow rate the concentration of Ptotal increased which indicates instability of the dephosphatating bioreactor under the set process conditions. The nitrification and the denitrification capacity remained excellent. The economical evaluation showed that the Three-sludge sewage treatment system can compete with conventional low loaded activated sludge plants in The Netherlands. Moreover, this advanced technology only occupies 40% of the space of conventional plants.

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