BoFi+ - Advanced phosphorus removal using resource-saving (planted) constructed wetlands as a downstream stage of small and medium-sized wastewater treatment plants

Project duration: 10/2023 - 10/2025

Project work by:

• Verena Hilgenfeldt M.Sc.
• Carlo Morandi
• Prof. Dr. Heidrun Steinmetz

Funded by:
DBU - Deutsche Bundsstiftung Umwelt

Project Partners:

• Bundesanstalt für Materialforschung und -prüfung
• UVR-FIA GmbH
• Dr. Bruch und Partner
• ZetA Partikelanalytik GmbH

Project Description:

Despite the elimination of phosphorus (P) in municipal wastewater treatment plants in Germany, currently averaging around 93% (cf. DWA, 2021), the cleaning performance in many watershed areas is insufficient to achieve the good ecological status required by the European Water Framework Directive (2000). For most water bodies, a reference value of typically 0.1 mg/L total phosphorus (Ptotal) applies (OGewV 2016). While larger treatment plants with a capacity of > 100,000 population equivalents (PE) already face strict emission-based minimum requirements of 1.0 mg/L Ptotal in the effluent, additional technical and space-saving methods such as flocculation filtration are often necessary to further reduce P inputs into water bodies. On a per capita basis, there is significant potential to reduce P emissions for smaller and medium-sized plants up to approximately 50,000 PE, with low-maintenance technologies being advantageous. This includes modified constructed wetlands, which, especially considering the current shortage of precipitants, can contribute to reducing P inputs into water bodies. The use of constructed wetlands with recycling filter materials with high P adsorption capacity provides an opportunity to sustainably achieve a lower P limit of < 0.1 mg/L Ptotal in the long term.

The "BoFi+" project aims to investigate the design features and operational strategies of modified constructed wetlands and evaluate their suitability for further P elimination from effluents of municipal wastewater treatment plants. This involves improving the understanding of the chemical-physical and microbiological processes (adsorption, precipitation, filtration, biological uptake) of P elimination in constructed wetlands using various materials and material combinations. The suitability of different filter materials for P elimination is also tested. To save natural resources and produce a standardized product, the project examines suitable recycled building materials, lightweight aggregates, and hydrothermal granulates produced from mineral construction and demolition waste through a chemical transformation process. Material properties essential for P elimination are identified, enabling faster assessment of such filter materials for advanced P elimination in soil filter systems in the future.

Work Steps:

The project is divided into two phases, differing in the scale of experimentation:

• In Phase 1, the identification or definition of suitable filter materials begins with literature research and practical experience. For intensive screening, the adsorption capacities and loading limits of various filter materials, including hydrothermal granulates and different material combinations, are tested through laboratory experiments. Based on this, the materials are examined in laboratory filter columns in multi-week trials with synthetic phosphate solution to assess their phosphorus adsorption capacity. Detailed material analyses (e.g., particle size distribution, porosity, pore size distribution, specific BET surface area) complement these investigations. Simultaneously, research is conducted on the manufacturing process and composition of hydrothermal granulates to improve their properties for constructed wetland operation. Green granulates are formed from mixtures of masonry rubble and paper ash, which are hardened into artificial lightweight aggregates through hydrothermal autoclaving. Different process steps are modified and optimized (e.g., composition , forming processes, porosity of granulates).

• In Phase 2, the tested materials with the most promising properties are evaluated in small-scale constructed wetland columns over several months. Operational aspects such as the type, duration, and frequency of feeding, as well as increasing hydraulic load compared to the requirements of the existing DWA-A 262 regulations, are the main focus. Additionally, investigations into the scale-up of the manufacturing process of hydrothermal granulates are carried out. The knowledge gained about filter properties is integrated into iterative optimization steps. The research results form the basis for developing a requirement list for constructed wetland materials, recommendations for construction, operation, dimensioning, and service life, as well as concepts for integrating constructed wetlands for further P elimination into the operation of municipal wastewater treatment plants. In this context, potential environmental impacts of the production, regeneration, and disposal of hydrothermal granulates are also estimated.