Recovery of micronutrients and heavy metals from digestate

Significance 

The core goals of strategic environmental policies are to significantly reduce the use of fossil fuels and the emission of greenhouse gases through development of renewable energy technologies. The agricultural sector, for example, is among the fields that have witnessed tremendous growth of innovative technologies for the recovery of valuable resources such as energy, metals, and nutrients from bio-wastes and wastewaters. Unfortunately, several minerals and natural resources have been observed to deteriorate at an alarming rate. This calls for an urgent resolution to avoid future food unrest and distribution interruptions considering the critical role played by the macronutrients and micronutrients in ensuring socio-economic stability.

Previously, the research teams of Prof. Vaneeckhaute (Canada) and Prof. Meers (Belgium) have identified anaerobic digestion as a promising technology for the conversion of biodegradable wastes into biogas and nutrient-rich digestate. The digestates can often not be directly utilized for agricultural purposes due to the existing stringent regulations. As such, the valuable nutrients must be extracted from the digestates. However, most of the present studies have focused on the recovery of macronutrients from the digestate with limited focus on the fate of micronutrients and heavy metals. This requires an understanding of the digestate treatment process and its influence on the micronutrient and heavy metal contents at the various processing phases.

To this note, Laval University scientists in Quebec, Canada: Professor Céline Vaneeckhaute and Olivier Darveau together with Professor Erik Meers from Ghent University in Belgium evaluated the feasibility of a new vibrating membrane filtration technology in the recovery of valuable micronutrients and heavy metals from the liquid fraction of digestates, all while reducing the macronutrient concentrations to dischargeable water. Their work is currently published in the journal, Separation and Purification Technology.

The two-step vibrating membrane filtration system was operated with a reversed osmosis membrane and was followed by a lagoon. First, the system was applied for digestate processing at a full-scale biogas plant treating animal manure, energy maize, and residues from the food industry through anaerobic co-digestion. Next, a physicochemical characterization of the process streams and mass balance analyses for macronutrients, micronutrients and heavy metals throughout the treatment process were performed. Finally, the authors investigated the possibility of recycling the resulting macro- and micronutrient concentrates as substitutes for synthetic fertilizers taking into consideration their potential pollution with heavy metals.

Dischargeable water was produced in agreement with the regional (Flemish) regulatory standards using the vibrating membrane filtration system followed by a lagoon. The lagoon supplemented the performance of the vibrating membrane filtration system by allowing for further biological and natural purification. The concentrates produced by the first vibrating membrane filtration step exhibited great potential for reuse as inorganic fertilizers in place of synthetic fertilizers because they were typically rich in nitrogen and potassium nutrients, along with micronutrients such as Cu and Zn. Moreover, none of the regulatory standards for heavy metal concentrations in soil enhancers were exceeded according to the Flemish regulation. On the other hand, the concentrates produced by the second vibrating membrane filtration step showed little potential for reuse as fertilizers due to poor macronutrient contents.

Micronutrients provide additional value to the potential fertilizer products since they play a crucial role in various metabolic activities and their absence may lead to retarded plant growth and low soil quality. Synthetic fertilizers are often poor in micronutrients as opposed to the recovered nutrient products in this study. It was worth noting that a huge portion of the micronutrient and heavy metals end up in the solid fraction of digestate due to solid-liquid separation. Dr. Céline Vaneeckhaute, corresponding author in a statement to Advances in Engineering, mentioned that their further research will target the recovery of heavy metals from solid fractions of digestate, along with field experiments to confirm the micronutrient value of recovered fertilizer products.

Recovery of micronutrients and heavy metals from digestate - Advances in Engineering

About the author

Dr. Céline Vaneeckhaute is Professor at the chemical engineering department of Laval University in Québec, Canada, where she leads the BioEngine research team on green process engineering and biorefineries. Her team’s research focuses on resource recovery from waste and wastewater streams. She received more than 10 prestigious research awards, such as the Quebec Emerging Talent Award in Municipal Infrastructures and the Flemish Water Technology Award. Céline’s key contributions include the development of a commercial mathematical model library for nutrient recovery, field trial studies leading to revisions of bio-based fertilizer regulations, as well as the development of a new technology for phosphorus recovery.

Céline leads a Québec stakeholder platform on nutrient recovery and is vice-president of Québec’s regional environmental network. She is steering board member of the International Water Association Cluster on Resource Recovery from Water and of the Phosphorus modelling group of the Sustainable Phosphorus Alliance. She is also scientific expert in the steering board of the Canadian nutrient recovery and reuse stakeholder platform, and affiliated as international advisor to the Biorefine Cluster Europe, among other appointments.

Before her occupation as a Professor, Céline worked for the Québec City Government as assistant director of the biomethanation processes. She has also been active as independent consultant in the field of resource recovery. Céline holds a double PhD degree (2015): PhD in water engineering from Laval University and PhD in applied bioscience engineering from Ghent University, Belgium. She obtained a Master (2010) and Bachelor (2008) degree in Bioscience Engineering at Ghent University with a specialization in Environmental Technologies.

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About the author

Olivier Darveau   obtained a bachelor degree in chemical engineering at Laval University, Québec, Canada in 2019. At the time of the present study (2018-2019), he was research assistant in the BioEngine research team led by Dr. Vaneeckhaute, during which period he was involved in various resource recovery projects. Olivier now works as a process engineer at Arcelor Mittal, Canada.

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About the author

The research group of Prof. Erik Meers (Faculty Bioscience Engineering, Ghent University, Belgium) consists of  >30 researchers and supporting staff (including 5 post-docs and over 20 PhD). The research group focuses on bioresource recovery from agro-industry, with distinct spearpoints on nutrient recovery processes, production of industrial crops on derelict (contaminated) soils, biogas as new generation biorefineries, new biomass production on waste(water)streams. Prof. Meers founded the Biorefine Cluster Europe and is promoter of the industrial EBA chair together funded by the European Biogas Association. He is also the promotor of the Business Development Platform End-of-Waste and the International Thematic Network RE-SOURCE.BIO. For the European Commission, he was the coordinating expert in the EIP-AGRI Focus Group on Nutrient Recycling.

In framework of his research, he is currently (at start of 2020) involved in 14 running European projects, several of which as coordinator (H2020-NUTRI2CYCLE, INTERREG-2SEAS-GRASSIFICATION, INTERREG-F-W-FL-NEW-C-LAND). As of 2020, he will also be the coordinator of the Flemish Nutrient Platform (NUTRICYCLE Vlaanderen), funded by the Flemish Government.

References

Vaneeckhaute, C., Darveau, O., & Meers, E. (2019). Fate of micronutrients and heavy metals in digestate processing using vibrating reversed osmosis as resource recovery technology. Separation and Purification Technology, 223, 81-87.

Go To Separation and Purification Technology

C. Vaneeckhaute, E. Meers, E. Michels, P. Christiaens & F. M. G. Tack . Fate of Macronutrients in Water Treatment of Digestate Using Vibrating Reversed Osmosis. Water, Air, & Soil Pollution volume 223, pages1593–1603(2012).

Go To Water, Air, & Soil Pollution

Céline Vaneeckhaute, Viooltje Lebuf, Evi Michels, Evangelina Belia, Peter A. Vanrolleghem, Filip M. G. Tack & Erik Meers. Nutrient Recovery from Digestate: Systematic Technology Review and Product Classification.
Waste and Biomass Valorization volume 8, pages21–40(2017).

Go To Waste and Biomass Valorization

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