Floating treatment wetland for agri-food effluent tertiary treatment – FloWAT

A Life Cycle Assessment was performed taking into account the FTW’s material, manufacturing processes, usage and end of life, in order to identify possible improvements to reduce the FTW’s environmental footprint. In a context of decreasing phosphorus world resources, a specific material to recover this nutrient for future usage (e.g. soil fertilizer) appears of interest. Several material meeting the circular economy principle and/or bio-based products have been studied at laboratory scale in order to select the most appropriate material to be tested at pilot scale. An experimental site of approximately 100 m2 comprising 4 pilot lagoons equipped with various sizes FTWs and fed with lagoon effluent was built on an agri-food wastewater treatment plant. A minimum one year monitoring phase will provide relevant design guidelines to optimize FTWs’ size based on incoming pollutant loads and expected performances. A sizing tool and maintenance guidelines will be developed, which will ensure optimal FTWs’ implementation and operation to meet environmental standards. Furthermore, an assessment of the most suitable energy route for the harvested vegetation and treated water reuse for various applications (e.g. irrigation and/or non-sensitive industrial cleaning) will be performed to close the loop in a circular economy perspective

During the first phase of the FloWAT project, 4 materials types were developed and/or tested at laboratory scale: 1) oyster shell calcined at 4 temperatures, 2) oyster shell pyrolized at 4 temperatures, 3) expanded cellular concrete aggregate (ECC-waste / manufacturing co-product), 4) filtralite. The pyrolized oyster shells showed promising phosphorus removal performances. However, this efficiency decreased when the material was stored for a few weeks / months before use, limiting its application in a real context. ECC showed equivalent dissolved phosphorus removal efficiencies, and was stable over time, therefore it was selected to be tested at pilot scale. This material comes from the ECC manufacturing process and is considered as a waste. Non-toxic, it therefore appears to be an interesting candidate meeting the circular economy challenges.
The LCA ‘s preliminary results obtained with Open LCA software suggest that the FTW’s buoyant frame is the most impacting element from an environmental point of view. This is due to the nature of the raw material which is petro-sourced and the fact that it is the main FTW composing element in weight. Although 60% of the buoyant frame is made from recycled plastic, thus minimizing its environmental footprint, it appears necessary to investigate other possible raw materials which would be more sustainable from an environmental point of view. Bio-polymer research and development activities are underway and could answer this need in the future. The LCA will be updated during the project with the pilot scale experiment’s results.

During the second phase of the FloWAT project, data on the overall water treatment performance as well as P reduction and recovery under near real conditions will be collected. Design and maintenance guidelines will be developed to facilitate optimal implementation and operation. This will help reducing the use of precipitation co-product (e.g. FeCl3) usually added during the secondary treatment step and provide a phosphorus-laden by-product. Nationally, nearly 7,000 agri-food industries have been identified as potential users of this technology, which warrant such a research project.

1. Abi Hanna R., Borne K., Gétente C., “Enhanced Floating Treatment Wetland for Phosphorous Removal from Secondary Meat Processing Effluents” IWA World Water Congress & Exhibition Programme (9-14 mai2021).
2. Abi Hanna R., Borne K., Villot A., Gétente C., “Recovery of cellular concrete waste for phosphorous removal from agri-food wastewater”, WasteEng, Engineering for waste and biomass valorization, (31 Mai-4 Juin 2021)

The agri-food sector especially the meat processing industry is one of the biggest global water consumer and effluent producer. This trend is expected to increase as the world’s global population will increase by 2.2 billion people by 2050 requiring a 50 percent increase in the production of food. Many agri-food industries already operate their own wastewater treatment plants which can exceed the environmental standards due to the strengthening of regulation and threaten the receiving ecosystems’ good ecological status. During dry seasons, many industries actually have to store their secondary effluent in large lagoons as the rivers which usually receive this secondary effluent are not able to cope with the discharged pollutant load. An innovative way to improve effluent treatment is to install Floating Treatment Wetlands (FTW) on these existing lagoons as a complementary tertiary treatment.

This innovative nature-based solution, can easily be integrated into existing lagoons without the need for heavy civil or levelling work. A FTW is a hydroponic vegetated device typically installed on the surface of a pond without the need for structural changes. It is comprised of a floating mat planted with emergent macrophytes. Plant roots grow through the mat and hang into the water column to act as a physical and biological filter for dissolved and particulate pollutants. FTWs recently showed promising treatment performances for nutrients, metals, suspended solids and emerging pollutants like pharmaceuticals. However most of the reported studies were small scale experiments, mainly using synthetic effluent not representative of the agi-food sector and did not provide any design guidelines which slows down its implementation.

The overall aim of the project is to assess in a pilot scale experiment, representative of a real application (real effluent, continuous feeding), the effectiveness of a customized FTW to remove nutrients (N, P) for the agri-food sector. Moreover, in a context of decreasing phosphorus world resources, a specific material will be integrated into the FTW to recover this nutrient for future usage (i.e soil fertilizer). A particular attention will be paid to the product’s end of life favouring materials easy to be recycled or reused. A life cycle assessment will help selecting the adequate materials and manufacturing process to guarantee the low environmental foot print of the FTW. The pilots will be big enough to extrapolate results and provide research-based design guidelines to optimize FTWs’ size with respect to input load and expected treatment target. Design (sizing tool) and maintenance guidelines will be developed which will guarantee adequate implementation and operation of FTWs to meet environmental standards. Furthermore, a deep assessment of the most suitable energy route for the harvested vegetation and treated water reuse for various applications (e.g. irrigation and/or low quality cleaning) will be performed to close the loop in a circular economy perspective.

The outcome of the project will be to develop a robust and marketable product. It will be the first 100% recyclable FTW that can, on top of the phytoremediation processes involved (nutrient removal), allow easy P recovery for subsequent usage (e.g. soil fertilizer). There is no such product to date available on the market (neither in France nor abroad). At a national level, nearly 7000 agri-food industries have been identified as potential end-users which warrant the need for such a research project.