Enhanced nutrient removal with bio-filter at Rostock WWTP, Germany

Wastewater treatment plant are designed to remove nutrient from wastewater in order to limit the impact of effluent discharge on the environment.

After Helsinki convention 1974, about the protection of the Baltic Sea Area, there was the need for many cities to improve their wastewater treatment plant in order to achieve high removal of organic matter, phosphorus and nitrogen, but also pharmaceutical and pesticides. It is important to remove those substances from the treated effluents before their discharge in order to reduce their impact on the environment and protect water from contamination.

Bio-filters are aerobic or anoxic process biological reactors that use attached growth technology for application in municipal or industrial wastewater treatment. The bio-filter units allow treatment of soluble pollution and solids separation in one compact reactor. 

Components installed in the solution

Biofiltration system are typically robust, simple to construct and have low energy requirements. This innovative system can react flexibly and quickly to the conditions, even if there are great variations in the volumes of wastewater. Moreover this technology requires very little space in the sewage treatment plant.

In case of Rostock WWTP the two-step biological treatment system consist of 2 aeration tanks (30 000 m3) and a third clarifier, a downstream bio-filter and a new sludge treatment (including flotation, digestion 2 x 5 000 m3 and sludge dewatering).

In the first biological step were installed 2 basins for biological phosphorus elimination, 2 activated sludge basins with preceded denitrification, 3 turbine compressors and 3 final clarification basins.  

In the second biological step were installed 2 superfine screen and the biofiltration system that consist in 2 filter steps each with 12 filter chambers. 

Solution provider

Operational mode

The central sewage treatment plant in Rostock has the capacity to treat wastewater from 320 000 inhabitants. It was completely rebuilt in the middle of 90s (from March 1994). The current catchment area is around 175 km2 and it served 13 surrounding communities around Rostock. The plant treats both municipal (2/3) and industrial (1/3) wastewater.

As a result of the biofiltration, with nitrification and denitrification, solids, phosphorus compounds and the greatly amounts of nitrogen that occur during 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.

The new biofiltration facility has an overall filter area of 876 m2, distributed among a total of 12 nitrification and denitrification filters, with 73 m2 filter area each. The filter speeds vary between 5.2 and 8.7 m/h. If the filter speed exceeds 8 m/h another filter is activated, while if the speed slows down by 3 m/h a filter is turned off, although two filters are always working at the same time. The filter flow time vary between 2-3 days.

The biofiltration solution installed in Rostock WWTP works as follow: the effluent to be treated enters continuously from the bottom of the reactor and it is distributed over the entire filter surface area by the nozzle underdrain. The water then passes through the Biolite filter media which retain the suspended solids. Carbonaceous and nitrogenous pollution is eliminated through the high concentration of fixed-film biomass which is retained on the filter media during the filtration cycle.

Expanded clay is used as the filter material and as the carrier for the micro-organisms. When the aerobic biofiltration phase has been completed it is possible to add iron (III) chloride to achieve phosphate precipitation. In future phosphate will be able to be recovered from the wastewater.

The plant in Rostock was the first large sewage treatment plant in Germany to use the space-saving process that combined aeration and BIOFOR (biological fixed film oxygen reactor).

Thanks to counter-current biofiltration process the performance of the plant is stable and of a high quality, the treated water remains at a stable high level and only clean water flows into the Baltic Sea.

After installing the biofiltration treatment system, the efficiency of central wastewater treatment plant in Rostock was really improved. The nitrogen load in the effluent was reduced from 47 mg/l (1991) to 10.2 mg/l (2009), while the phosphorus load in the effluent was reduced from 5.6 mg/l (1991) to 0.21 mg/l (2009).

As a result of putting the central sewage treatment plant into operation, Rostock has been removed from Helcom’s list of “Hot Spots”. 

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