Participants:  AAU (JM and MLC) ; DTU (LMO); SDU (KR), AU (AMS)

Description 

Recovery of P and nitrogen from the flocculated and dewatered lake sediment reject water (WP2) presents a challenge with respect to the expected low concentrations (< 100 µg/L orthophosphate ; < 200 µg/L TP ; < 10 mg/L N-NH₄⁺) that puts high requirements on the efficiencies of the individual and combined treatment processes to achieve an overall cost-beneficial solution. These low nutrient and P concentrations maybe challenging for P removal technologies, these low levels of P can still stimulate excess algae growth and therefore it’s likely the nutrient content is too high in the reject water from the dewatering process to be discharged directly back to the lake. In that case, technologies are needed to generate a concentrated P-rich stream of reduced flow. P can be efficiently precipitated or adsorbed to a solid material to generate a product with fertilizer properties. In WP3, two electric potential-driven technologies, electrodialysis (ED) and capacitive deionization (CDI) will be studied and used for this purpose. ED and CDI has been chosen due to the possibility to achieve sufficient high process efficiencies at low concentrations and since the driving force of the separation is electric potential driven transport through a membrane or electro-adsorption on a surface, and as such may match a treatment concept driven by sustainable electricity production. The clean water effluents of the treated water will be returned to the lake. ED is a mature technology considering other wastewater treatment and brackish water desalination processes (Gurreri et al. 2020) and has also on lab-scale been demonstrated on P removal, but within wastewater applications (Rotta et al. 2019; Kedwell et al. 2019). It has not been demonstrated on lake restoration. The same applies to CDI, even though only few full-scale installations exist (Huang et al. 2014). The main attractive feature of CDI towards lake restoration applications is the enhanced mass transport at low concentration facilitated by the electrostatic attraction that improves the adsorption efficiency compared to pollutant capture and removal by traditional adsorbents (Suss et al. 2015). Selective electrostatic adsorption of P is a major challenge as the process will be in competition with the adsorption of chloride and other naturally present anions. A major factor is the adsorption affinity of the specific electrode material used. A main scientific contribution of this WP is to generate increased fundamental knowledge on how electrode material characteristics and properties influence adsorption mechanisms and affinities.

The experiences obtained in WP3 will in parallel be used to explore a robust treatment concept aimed at low flow continuous treatment of lake bottom water. This application is considered part of potential long-term management of lakes after the sediment dredging and treatment operation and to be used on lakes where sediment removal is not strictly needed.

The concentration technologies produce a P-rich concentrate, from where P is to be recovered. The same applies from the reject water of the electrodialytic sediment treatment (WP4), the hydrothermal treatment of the dewatered sediment in WP5 that generates an aqueous effluent rich in P, and the electroosmotic dewatering (WP6). In WP3, research on precipitation and adsorption technologies will consider the performance of different cheap and sustainable precipitation reagents and adsorbent materials to selectively recover P from these concentrates with the aim of generating a product with fertilizer properties, when adsorption capacity is exhausted. The suggested fertilizer products will be tested in WP7. In focus will be the production of calcium phosphate and struvite products that will provide the benchmark on which more novel minerals, such as vivanite that can be magnetically separated, can be compared. A task in this WP is also to search the literature and scientific networks for state-of-the-art adsorbent materials that may be relevant to test in WP8. Literature review on all technologies explored in the WP is embedded in the tasks presented below to ensure that choices and plans are taken and developed on the current state-of–the-art.  

Objectives

The objectives of this WP is three-fold:

 1) to develop a treatment process that allows recovery of the residual orthophosphate and return (> 95%) of the reject water back to the lake,

2) to explore a robust treatment process to be applied for low flow continuous long term treatment of lake bottom water, and

3) to develop a recovery process that produces a solid fertilizer product for further use and clean reject water that can be safely recycled to the lake

Activity

Task 3.1: Test of electrodialytic reject water polishing and concentrating

Electrodialysis is a commercially available technology that, considering the low ionic strength of the water, may provide sufficient efficiencies for concentrating P in a retentate and allow return of the reject water to the lake. The research focus for this particular application will be studying the influence of electrode and membrane selection, electric potential, and feed composition on solid/reject water flow ratio and compositions and scaling and subsequent cleaning strategy. 

Task 3.2: Test of capacitive deionization for reject water polishing and concentrating

Capacitive deionization (CDI) is a technology that can remove ions from a water stream using direct current by electro-adsorption of negatively charged anions to the positive electrode (anode) or positively charged cations to the negative electrode (cathode). The technology is promising with respect to process and energy efficiency, but less commercial even though commercial applications do exist. The research focus will be on the influence of electrode materials, current densities and feed composition and properties on the selective rejection of phosphorous and the combination of the CDI principle with ion-exchange membranes in the so-called membrane CDI (MCDI) process that enhance the selective separation efficiency but increase complexity. When sorption capacity is reached, desorption by discharge is needed that generates a concentrate and another research focus will be to elucidate the reversibility of the adsorption/desorption process.

On the CDI/MCDI technology, collaboration with a profiled international research lab will be engaged and external stays of the PhD student will be planned early in the project to facilitate fast progress within this subject. Identification of the right research environment to approach has been initiated.

Task 3.3: Test of phosphorus recovery by precipitation 

Precipitation of P by calcium, ammonium and magnesium to produce solid calcium phosphate and struvite are benchmark technologies that will be tested on the generated concentrates and the mix with reject waters. 

A second approach that will be researched is the application of the ViViMAG technology. The EU-funded ViViMAG project develops a magnetic separation technology to recover the iron phosphate mineral vivianite from digested sewage sludge. By the end of 2020, the project should be completed and the technology will be ready for market introduction. We will engage with the vendors and research the use of the technology for this application. Selected mineral products will be tested for fertilizer properties in WP7.

Task 3.4: Test of phosphorus recovery by adsorption

Selective adsorption on filter materials of inorganic and organic nature will be researched. One approach that will be investigated is to use Mg_xFe layered double hydroxides (LDH). LDHs have demonstrated fast adsorption kinetics and high adsorption efficiency of P in higher concentrations and compose a promising environmentally friendly material for P removal and is a candidate for further investigations. Research focus may be the influence on the feed composition on the adsorption mechanisms of LDHs, the adsorption/desorption selectivity and reversibility, dissolution and reusability of LDHs and possibility of synthesis of LDHs with magnetic properties that ease recovery of the P rich spent sorbent after use. 

Task 3.5: Provide state-of-the-art of organic sorbents for p-recovery 

We will search the scientific literature and review the most novel research on organic-based sorbents to recover phosphorous. Interesting project ideas generated from this work will be added to the project catalog of WP8 and offered as student projects, thesis projects etc., for further testing on this specific application.   

Project structure

JM will be the leader of WP3. The research in the project will be completed by a PhD student (PhD2) hosted at AAU supervised by JM and MLC (AAU) with LMO (DTU) as co-supervisor. The PhD student will focus on the electric potential driven separation processes (electrodialysis and CDI/MCDI) and the use of these technologies for this particular application. Main workplace will be AAU Esbjerg with periods at AAU Aalborg and DTU Lyngby. On the CDI/MCDI technology, collaboration with a profiled international research lab will be engaged and external stay of the PhD student will be planned early in the project to facilitate fast progress within this subject. Identification of the right research environment to approach has been initiated.

Postdoc1 (shared with WP2) will be the lead investigator on the P adsorption and precipitation studies supported by PhD2 with the main workplace at AAU Aalborg. Postdoc supervisor will be MLC,  JM from AAU and AMS (AU) as co-supervisor. Collaboration will be engaged with Ulla Gro Nielsen (SDU) on the LDH technology as continuation of already established research activities between AAU and SDU. 

Path to commercialization 

For lake restoration applications, WP3 will bring ED to TRL6 and CDI to TRL 4. Engagement and collaboration with technology providers are needed to bring the proposed water treatment concept to TRL9 and ready for market. Technology providers do exist, primarily for ED and few for CDI.   

Risks associated with work package 

A risk associated with WP3 is that the feed water quality from WP2 is insufficient to direct treatment and that further pre-treatment is needed. In this case, filtration will be used to mitigate suspended solids. Another risk is that the P concentration is too low to allow efficient treatment by ED. In this case, more resources will be devoted to the development of the CDI technology.

The Results