Solution:
The STP had 3 identical treatment trains, each with 3 zones. During reconstruction, the nitrification zone in each train was upgraded with Mutag BioChip™ high-performance biofilm carriers, whereas 27 m³ Mutag BioChip™ were used per train, which corresponds to a filling rate of 13.5% of the tank volume dedicated to nitrification. To accommodate the increased pollutant load due to the plant upgrade, the oxygen supply was boosted by upgrading the blowers. The old aeration systems were also replaced with high-performance plate-type membrane diffusers. The reconstruction included supplying electro-mechanical equipment, return sludge pumps, separation walls and a blower station by Techfina SA. Calculation and imensioning related to the biological stage, functional description, and retention screen design (for retaining the carriers in the tanks) were provided by Mutag.

Results:
The STP Oberes Surbtal, which originally served 16,000 population equivalents (PE) using a classical activated sludge process, was upgraded by Techfina SA, in cooperation with Mutag, in 2012-13, whereas the STP was expanded to handle 20,750 PE with a hybrid process (combining activated sludge and MBBR). The upgrade was possible by using the existing tanks, whereas modifications on some plant components, such as the aeration system, were required besides of the addition of a retention screen for retaining the carrier media. After completion, the plant met all discharge requirements during performance testing and is still showing the same good performance, using the same amount of carrier media, until today.