Circular lake restoration: transforming lake sediments into valuable products (RePair) 

State of the art

We are currently facing a “Global Phosphorus (P) Challenge (GPC)” (PhosphorusFutures 2021, Winthers et al. 2019). The three main reasons are:

1. Phosphorus is a limited resource mined from P reservoirs and processed into P fertilizers to boost global food production. However, depletion of the known high-quality P rock reserves are expected within the next centuries (PhosphorusFutures 2021, Cordell et al. 2009, Koppelaar and Weikard 2013). Thus P is considered a critical resource by the EU (Eur-lex 2021a).

2. Phosphorus is unevenly distributed: five non-EU countries control more than 85% of the global P reservoirs (Cordell et al. 2015).

3. Phosphorus causes pollution: excess P from soil and insufficiently treated wastewater is discharged into aquatic ecosystems, causing pollution by algae blooms, biodiversity loss, and drinking water contamination (Elser and Bennett 2011). Our current, almost linear economic system exploits P as it is extracted for food production and returned as either waste or a deposit in the hydrosphere. Thus, a resource belonging to future generations is overexploited while polluting the environment (Figure 1). In other words, there is too little P for food production, yet too much in the environment.

In 2016, the EU adopted a Circular Economy Action Plan (CEAP) that will enable a nutrient circular economy in Europe (EC 2021a). The CEAP will provide the regulatory framework to develop a circular economy, where the value of products, materials, and resources is maintained in the economy for as long as possible, and the generation of waste is minimized by, for example, facilitating development, commercialization of recycled nutrient products and opening the European market for new recycling technologies. 

A circular economy can be achieved by driving investments and removing obstacles caused by European legislation, ensuring favorable conditions for innovation, and involving all stakeholders. The EU Water Framework Directive (WFD) (EC 2021b) covers legal and management aspects relevant to the GPC, by setting up the obligation to the Member States to bring water bodies to a good ecological state. In addition, regulatory frameworks such as the Urban Waste Water Treatment Directive (EC 2021c), the EU Fertilizer Regulation (Eur-lex 2021b,) and the Ground Water Directive (Eea 2021) alongside UN Sustainable Development Goals (SDG) provide new opportunities to address the GPC and eliminate the threats it represents both for the environment and society (Sustainable Development 2021). However, the success of these regulatory frameworks requires solutions to the global need for P recovery, environmental protection, and sustainability in an economically efficient and socially acceptable way. Such solutions will only be fostered through new expertise, knowledge and innovation. Consequently, the CEAP urgently requests the establishment of new interdisciplinary and transdisciplinary collaborations to produce solutions that will enable a circular economy for P.

The EU currently imports over 90% of its P fertilizers, most of which is lost to the agricultural system, polluting the environment. A large part of the P ends up in lakes, most of it in the sediment. The historical build-up of P in lake sediments is the main reason why roughly 70% of the Danish lakes do not fulfill the WFD. P within lake sediments are released into the lake water, where the P stimulates excess algae growth making the lake water turbid. This prevents the establishments of submerged macrophytes and causes anoxic conditions in the bottom water, which leads to further P release and emission of Greenhouse gases (GHGs) such as methane. To date, combating the harmful effects of excess P in lakes is purely linear; with either immobilization of P in the sediment, usually by addition of P binding agents such as aluminum, or with Phoslock, oxygenating of the lake to keep iron (Fe) oxidized, so it can bind P. Issues however arise when you consider that such intervention which merely buries the P in sediment and does not remove it. Therefore, in most cases, there is only a temporary improvement in water quality, and the lakes often fall back to a more turbid state after a few years (Reitzel et al., 2021). Thus, the pollution problem is not solved permanently, and the P resources are not utilized. 

Another method is sediment dredging, where the upper nutrient-rich sediment is removed by pumping or excavation. There are however handling and utilization issues with this option, as heavy metals present in the sediment can prevent its application to land (Zhang et al 2021), therefore the sediment requires incineration or landfill, leaving the P unavailable for crop production. When dredging, removal of water from sediments is also required, which is typically done using synthetic polymer flocculants such as polyacrylamide. Polyacrylamide monomers can be extremely toxic (Bolto and Gregory 2007, Bratby, Dao et al 2016, Duggan et al 2019) and potentially cause environmental hazards, either by the flocculated and separated water being returned to the lake or when the heavy metal polluted sediment is applied to land. 

To enable the recovery and reuse of P from sediment, new technology and techniques are required, whereby P can be recovered from the sediment, heavy metals can be removed to enable recycling to land. Synthetic polymer flocculants used in dewatering should be substituted or even replaced via new methods for dewatering requiring no chemicals, and the water content in the dredged sediment has to be reduced to lower transportation cost of P. Other components in sediment can also be reused (e.g. carbon materials) and their use have to be considered. 

The road to a circular P economy is long, with barriers such as limited societal awareness, relatively low costs of fertilizers, and a legal framework still not fully adapted to co-think innovative solutions for the GPC. Therefore, the “RePair project” will initiate the process of transforming the present linear lake restoration into a circular economy. RePair will ensure a clean aquatic environment while recovering P from the lake sediments and raising awareness of the added societal benefits (reduced GHG emission, improved recreational value etc.) associated with lake restoration. 

RePair will deliver the required holistic and societally viable solutions through engagement with stakeholders from the agri-food sector, industry, technology providers, policy makers, and managers. This approach will ensure intense collaboration within the project. We will have full access to state-of-the-art technologies, methodologies, and training facilities provided by the wide range of disciplines involved. Researchers will closely collaborate within and among the different WPs to promote sustainable P management, and through existing networks we secure interactions with our world leading international partners.

Project goal

RePair will directly address seven of the 17 UN SDGs, by promoting sustainable management of resources, protecting the environment, using applied research to assist good education, and mitigating GHG emissions. Our goal is to promote a paradigm shift in lake management by restoring freshwater bodies by transforming lake sediment and P polluted lake water into a valuable resource without any harmful effects on the environment. Our vision will be achieved through a unique and well-aligned constellation of interdisciplinary research between four Danish universities and relevant stakeholders from industry, agriculture, and lake managers. An integral part of the RePair project will be the interdisciplinary training of students by teams of academic supervisors providing cross-sectoral research collaborations among the students.

The major initiatives taken to ensure a successful outcome of the project are:

1: Develop a dredging technique, which results in only minimal disturbances of the lake ecosystem.

2: Reduce sludge handling volumes by mechanical dewatering, thus reducing sediment water content.

3: Remove dissolved P and nitrogen from the reject water so that it can be immediately returned to the lake without the need for further treatment while recovering nutrients for reuse.

4: Remove heavy metals from the sludge to ensure a sludge fraction suitable for disposal on farmland.

5: Further dewater the sludge for ease of transportation and storage, with fractional separation (P, C, inorganics) where necessary.

6: Improve the soil amendment properties of the sediment.

7: Verify the suitability of the products as environmentally safe fertilizers and soil improvers.

8: Increase public awareness of the GPC and the potential of restoration and recovering of P from lake sediment.

9: Provide research led education at both lab and pilot scale for the respective universities BSc, MSc and PhD students for engineers of the future

10. Train the next generation of interdisciplinary P professionals with the capacity to address complex socioecological problems and promote the circular economy for P.

These initiatives will be conducted in Lake Ormstrup (12 ha), which will serve as our demonstration lake during the project period. The project will be a showcase for how lake restorations of the future should be undertaken, develop the new future technologies required and successfully develop a “tool box” of technologies, which can be combined based on lake specific requirements, while demonstrating how P in lake sediment and lake water can be recovered and used as a fertilizer product. Furthermore, unwanted compounds in the sediment such as heavy metals and problematic organic compounds can be selectively removed from the sediment. The project will demonstrate dewatering and handling of the sediment in ways, which require no harmful chemicals or synthetic polymers. It is expected that the technologies developed will increase sediment dry matter to over 60 % and in a handling time of less than 24 hours. The technologies will ensure that even P and nitrogen in low concentrations can be removed from lake water and recovered in a manner that enables their reuse as a fertilizer. It is expected that this highly interdisciplinary project will not only impact future best practices in lake restoration but will also be applied to manure management and sludge treatment in wastewater treatment plants (WWTPs), where ever-increasing volumes of sludge and manure are proving an insurmountable global issue. 

Project description

The project naturally divides into eight scientific work packages (WPs), one for each goal (previous section), and a project integration WP:

1. Dredging and pumping of sediment
2. Mechanical dewatering 
3. Water treatment
4. Electrodialysis for removal of heavy metals
5. Hydrothermal Dewatering and Carbonization (HTC)
6. Electroosmotic dewatering and phosphorus recovery
7. Lake sediment as fertilizers (laboratory and field test of various phosphorus products) 
1. Effect of polymers from lake and from dewatering (e.g. micro/nanoplastic, fertilizer value)
2. Effect of treatments on e.g. GHG, heavy metals, soil improvements
8. Test of novel and innovative solution for a circular lake restoration 

The first seven work packages and the suggested process for lake restoration and P recovery is shown in Figure 2. The colors illustrate the different work packages.

As we accept only a minimum of waste streams (and this is a research project first and foremost), we have chosen a palette of technologies that are not all on the market but which show great potential, individually and in combination. 

In WP 1, we will develop a method to gently dredge sediment with minimal disturbance of the lake i.e. minimum release of nutrients, climate gasses and other pollutants. The sediment will be pumped on land for further treatment. 

In WP 2, sediment will be dewatered and water returned to the lake either directly or after post-treatment (WP 3). Flocculation is usually required before dewatering, and biopolymers will be tested as a substitute for synthetic polymers. Alternative methods for dewatering will be studied to avoid addition of chemicals. 

In WP 3, electrochemically based water treatment technologies will be used to concentrate P in a small stream, from which P can be recovered as an inorganic or organic fertilizer product. 

In WP 4, electrodialysis will be used to extract more P from the dewatered sediment. In case of sediment polluted with heavy metals (such as cadmium, copper etc.) they will be extracted for disposal.

In WP 5, hydrothermal dewatering and carbonisation (HTC) will be used to dehydrate the high moisture sediment, liberating more water which can be removed prior transportation, stabilizing the carbon and mobilizing metals, nitrogen and P for recovery in WP 3 

In WP6, electroosmotic dewatering will be used to dewater sediments from WP2, WP4 and WP5, demonstrating a new low-cost dewatering and demineralizing technology as opposed to more energy-intensive mechanical dewatering strategies.

In WP 7, we will test the P availability of the sediment from the previous WPs regarding fertilizer value and soil improvement in laboratory and field experiments. We will also focus on side effects of recycling sediments, such as nanoplastic contamination through synthetic polymers as well as GHG emissions from the recovered sediment.

In WP 8, we will look for new promising solutions for lake sediment dredging and handling. WP 8 can potentially cover all parts of the process from sediment dredging, handling of the reject water, treatment of the sediment, development of P products and reuse of the sediment. 

Figure 2: Overview of the suggested process (a detailed version of the diagram can be found in appendix A).

The project includes technologies at different maturation levels, especially considering its use on lake sediment restoration. In Figure 3, an assessment of each technology's Technology Readiness Level (TRL) at the start and end of the project is presented to illustrate the technological leap forward this project will facilitate. Some of the technologies are higher on TRL, considering other applications and contexts.  

Figure 3: The TRL bars for the technologies (in blue) indicate the TRL at the start and end of the project. The green bar indicates the societal readiness level (SRL) of circular lake restoration at the project start and the increased level of readiness the project will facilitate. The TRL and SRL scales used for the assessments are taken from Innovation Fund Denmark.  

Dredging and dewatering are established methods with high TRL levels, which can be established on-site and used to restore the lake. A gentle method for dredging will be identified and developed for lake restoration with an expected change of TRL level from 5 to 7. Biopolymers will be used for pre-treatment before dewatering. Biopolymers are not yet used for lake sediment, and the polymers are not specifically targeted for sediment treatment. Biopolymers will be used before dewatering and tested at the site, increasing TRL from 6 to 7. Ideally, no chemicals are required. Mechanical dewatering without polymers and other chemicals has not yet been tested for sediment, and the TRL is therefore markedly lower than for flocculation and dewatering (TRL 3-4). It is expected that the method can be tested on lake sediment, increasing TRL to 6. 

Electrodialysis is a mature technology within some water treatment applications but has been scarcely tested on research level on lake water for P removal. The project is expected to bring electrodialysis on this application from TRL 2 to 6. Capacitive deionization is a technology with only a few full-scale installations, with the main focus to date being desalination and wastewater treatment. Few lab studies focus on P and the project is expected to increase the TRL from 2 to 4.

Electroosmotic dewatering and electrodialysis are technologies, which are presently being explored for WWTP  and soil remediation, respectively. They have not yet been used for lake sediment treatment. 

Hydrothermal Carbonization (HTC) is an experimental technique that is currently being trialed in several applications requiring high carbon-containing materials. The process treats water-based slurries. The process liberates both free and bound water present within bio-slurries and therefore has substantial potential for the technology to be used as a slurry dewatering technique for industrial sludge’s presently being explored for WWTP use. It has not yet been used for sediment treatment. 

The project is open to new ideas, concepts, and technologies that may be relevant to test. Such ideas will be at a low TRL level (TRL 0-1). It is expected that initial research experiments can be set up and performed to explore the potential of the method and thereby lift the TRL to 2. 

The project is a cooperation between Dansk Ingeniør Service (DIS), Aalborg University, Aalborg (AAU-Aal), Aalborg University, Esbjerg (AAU-Esb), University of Southern Denmark (SDU), Aarhus University (AU) and Technical University of Denmark (DTU). In order to ensure a successful project outcome, close cooperation between all partners and the work packages is necessary. Figure 4 shows the major overlap between the participants in the different work packages, 

Figure 4: Collaboration between institutes and companies in work packages.

Dredging and dewatering are closely related, as the dredging method determines the water content of the dredged sediment and the sediment structure. DIS is therefore involved in both WP 1 and 2. Dewatering is vital for the following treatment of the filtrate and sediment, and AAU-Aal is involved in WP 2-5 to ensure the integration of the dewatering process and the following treatment method.

All the WP’s are linked to one important goal that is to reuse the recovered P as a fertilizer. All university partners are therefore involved in agricultural field tests of the final product (WP 7). New ideas and technology will come up during the project and will be discussed by all partners. The best ideas will be tested in WP 8, which involves all the university partners. 

The project is divided into four phases with gates and seven overall technical milestones. Detailed plans can be found under the description of the 8 work package and in appendix B.