The central sewage treatment plant in Rostock is on the Unterwarnow estuary not far from the Baltic Sea and has the capacity to treat wastewater from 320,000 inhabitants (or rather 220,000 inhabitants plus industrial and commercial businesses). In the middle of the 90s, it was almost completely rebuilt at a cost of around 82 million euros. State-of-the-art technology is now in place, such as the biological counter-current filtration process, BIOFOR, which is used in the biological treatment stage. BIOFOR stands for “biological fixed film oxygen reactor”. This innovative process is making an important contribution to protecting the Baltic Sea and the River Warne which flows through the City of Rostock into the sea. Thanks to the modernisation of the plant, levels of phosphorus in the Baltic Sea have been reduced by 150t each year and nitrogen by 800t.
Before the sewage treatment plant was extended and modernised, it had operated using just one single treatment stage. As part of the modernisation project, the facility was expanded to include a second and third treatment stage. The result: the biological counter-current filtration process (BIOFOR) was used here for the very first time in Germany. This innovative system can react flexibly and quickly to the conditions, even if there are great variations in the volumes of wastewater. Moreover, this highly efficient technology requires very little space in the sewage treatment plant so that it was not necessary to move the plant from its location in the Rostock-Bramow district.
The central sewage treatment plant at the mouth of the Warne in Rostock is one of the most modern wastewater treatment facilities in Germany
The two-step biological treatment system, consisting of an aeration basin followed by a two-stage bio-filtration process, is one of the special features of the Rostock sewage treatment plant. The BIOFOR facility carries out the final cleaning process before the wastewater is discharged into the Warnow. As a result of the bio-filtration (with nitrification and de-nitrification), solids, phosphorus compounds and, above all, the greatly varying amounts of nitrogen that occur during heavy rainfall are also eliminated. In the past, these varying volumes of wastewater – caused by the weather – were a considerable problem for the sewage treatment plant. This is no longer the case for the new bio-filtration facility. It has an overall filter area of 876m2, distributed among a total of 12 nitrification and de-nitrification filters. The filter speeds vary between 5.2m/h in dry weather and 8.7m/h when it is raining. If the filter speed exceeds 8m/h, then another filter is activated. In contrast, if the speed slows down by 3m/h then a filter is turned off, although at least two filters are always working at the same time. Expanded clay is used as the filter material and as the carrier for the micro-organisms. Once the aerobic bio-filtration phase has been completed, it is possible to add iron(III) chloride to achieve phosphate precipitation. This means that, in the future, the valuable raw material, phosphate, will be able to be recovered from the wastewater.
Thanks to EURAWASSER and BIOFOR, the quality of the treated water remains at a stable high level and only clean water flows into the Baltic Sea – no matter what the weather.
EURAWASSER has received support from the University of Rostock throughout its BIOFOR project. The plant in Rostock was the first large sewage treatment plant in Germany to use the space-saving process that combined aeration and BIOFOR. The authorities, therefore, ruled that the process should be accompanied and observed by scientists over a three-year period. To this effect, a trial sewage treatment plant was operated that had been made available by the German state of Mecklenburg-Vorpommern. Thanks to the BIOFOR biological counter-current filtration process, the performance of the plant is both stable and of a high quality and the discharge values are excellent. No matter whether the weather is dry or wet, the plant clearly lies below the ceiling limits for both nitrogen and phosphorus. This also helps to greatly improve the quality of the water in the Warnow and Baltic Sea. Consequently, it is making a considerable and sustainable contribution towards protecting the environment and to fulfilling the international treaties reached at the 1985 Helsinki Conference. As a result of putting the central sewage treatment plant into operation, Rostock has been removed from HELCOM’s list of “Hot Spots“. HELCOM is the international commission set up in 1974 to protect the Baltic Sea.