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.
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.
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).
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).