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Staufener Bucht

Phosphorus recovery from sewage sludge 

WEHRLE supplies a fluidized bed incineration plant for the energy recovery of sewage sludge with subsequent phosphorus recovery for the sewage treatment plant of the Staufener Bucht Wastewater Association in Bad Krozingen.

Together with WEHRLE and the cooperation partners of the ERDF funding program, the University of Freiburg and the Center for Solar Energy and Hydrogen Research BW), the Staufener Bucht Wastewater Association has been pursuing the recovery of phosphorus from sewage sludge since 2020. The aim of the project was to enable the recovery of phosphorus from sewage sludge from local sewage treatment plants with the help of the thermal utilization of sewage sludge and to generate a marketable sewage sludge fertilizer with high plant availability. In the future, the plant will be able to treat 11,000 tons of mechanically dewatered sewage sludge and achieve a phosphorus recovery rate of approx. 80%.

Customer benefits at a glance

Compact system design with furnace integrated into the boiler saves valuable space for the waste processor

Low Nox exhaust gas values thanks to controlled combustion chamber temperature control, staged combustion and locally adapted asphyxiation system

Local waste recycling reduces transportation and reduces the footprint of waste incineration.

No costs for CO2 certificates and reliable compliance with emission values in accordance with the 17th BImSchV and the BAT Directive.

By adding the P-XTRACT process, the plant availability of phosphorus can be maintained and a higher separation of pollutants can be achieved. This favours the popularity of phosphorus ash among fertilizer producers.

The phosphorus recovered from the sewage sludge ash can be processed and used regionally as fertilizer. It has a high plant availability resulting from fuel conditioning, additive addition and combustion management. Thanks to the clever combination of processes, P-XTRACT satisfies a considerable proportion of the long-term phosphorus requirement.

The admixture of additives produces volatile heavy metal compounds in the ash, which pass into the gas phase due to the high process temperatures. The ash is then separated in the hot gas cyclone at over 700 °C so that heavy metal compounds cannot precipitate again. The volatile heavy metals are safely removed in the downstream flue gas cleaning system.

The reduction in disposal costs is achieved on the one hand by minimizing the amount of waste to be landfilled and on the other hand by producing a saleable recyclable material that can be recovered from incineration. The prerequisite for this is that the recovered recyclable material is accepted by potential fertilizer manufacturers, or that legal changes to the Fertilizer Ordinance increase the acceptance of the generated recyclable material.

The local recycling approach minimizes emissions and the traffic load caused by sewage sludge transport in relation to the overall performance of the plant technology. The simple, local processing option promotes a regional fertilizer production concept.

The production and further processing of the ashes into a direct phosphorus fertilizer requires fewer process steps compared to other methods such as acid digestion, and therefore less use of chemicals and energy.

The project's starting point

With the introduction of the Sewage Sludge Ordinance a few years ago, the problem of sewage sludge has come to the fore. Meanwhile, the Sewage Sludge Ordinance and the Fertilizer Ordinance set strict limits on the application of contaminated municipal sewage sludge on agricultural land. This means that there is not enough agricultural land available for spreading and an alternative utilization strategy must be developed. Many municipalities and investors are aware of this problem and are striving for thermal utilization of sewage sludge. As a GreenTech company, WEHRLE attaches great importance to sustainability and resource conservation and has been working on the topic of sewage sludge recycling for several years. Together with the ERDF funding program (supported by the European Union and the state of Baden-Württemberg) and the Staufener Bucht Wastewater Association as well as the research partners University of Freiburg and ZSW (Center for Solar Energy and Hydrogen Research Baden-Württemberg), WEHRLE combines the energy recovery of sewage sludge with the recovery of phosphorus from sewage sludge from wastewater treatment plants. The associated project is called the P-XTRACT process.

Solution approach and concept description

The practical application of the P-XTRACT® process is used within the pilot project of the Staufener Bucht Wastewater Association. The P-XTRACT® process is used to create a marketable sewage sludge fertilizer with high plant availability that enables the valuable resource of phosphorus to be recovered. The P-XTRACT® process is based on the proven technology of fluidized bed incineration of sewage sludge with the addition of certain (earth) alkali additives and special fluidized bed material. The two-stage combustion in a reducing and oxidizing atmosphere increases the transfer of heavy metals into the gas phase, so that the P-containing ash in the flue gas is entrained as fly ash and separated from the flue gas at the highest possible temperatures > 700 °C. Heavy metals remain mainly in the gas phase and are separated at lower temperatures in the subsequent flue gas cleaning process.

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Scope of delivery

WEHRLE is acting as general contractor for the construction of the fluidized bed incineration plant for the energy recovery of sewage sludge with phosphorus recovery in Bad Krozingen. The scope of services includes structural engineering (steel construction and façade), process engineering for the saturated steam boiler and purchased parts as well as the implementation of a phosphorus recovery system. In addition, WEHRLE supports the project in the approval planning, approval implementation and commissioning of the future-oriented project.

    Information on the Staufener Bucht Wastewater Association

    The Abwasserzweckverband Staufener Bucht is dedicated to treating wastewater to protect the environment and people. For more than 50 years, municipal representatives have been working together to purify wastewater and recover the raw materials it contains. 

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