Fate of micronutrients and heavy metals in digestate processing using vibrating reversed osmosis as resource recovery technology

This study aims to evaluate the full-scale performance of vibrating membrane filtration (VSEP) technology in resource recovery from the liquid fraction of digestates, while reducing macronutrient concentrations down to dischargeable water. Although increasing attention is paid to mass flow assessment of macronutrients, to date little is known about the fate of micronutrients and heavy metals upon digestate processing. In this research, process streams were characterized and mass balances for micronutrients and heavy metals were performed throughout a complete digestate processing train. The VSEP system operated with reversed osmosis membranes and followed by a lagoon was capable of producing dischargeable water according to Flemish regulatory standards. Concentrates produced by one VSEP filtration of the liquid fraction of digestate and dried thick fractions resulting from solid-liquid separation were rich in macro- and micronutrients, while heavy metal concentrations did not exceed regulatory standards. Hence, these products showed high potential for reuse in agriculture

Nutrient recycling from bio-digestion waste as green fertilizers

In the transition from a fossil to a bio-based economy, it has become an important challenge to maximally recuperate valuable nutrients coming from waste streams. Nutrient resources are rapidly depleting, significant amounts of fossil energy are used for the production of chemical fertilizers, whereas costs for energy and fertilizers are increasing. In the meantime, biogas production through anaerobic digestion produces nutrient-rich digestates. In high-nutrient regions, these products cannot or only sparingly be returned to agricultural land in its crude unprocessed form. The consequent processing of this digestate requires a variety of technologies producing a lot of different derivatives, which could potentially be re-used as green fertilizers in agriculture. As such, a sustainable alternative for fossil-based mineral fertilizers could be provided. The aim of this study is to characterize the physicochemical properties of digestates and derivatives, in order to identify the fertilizer value and potential bottlenecks for agricultural re-use of these products, in line with European legislative constraints. In addition, the economic and ecological benefits of substituting conventional fertilizers by digestates and derivatives are quantified and evaluated. Waste water from acidic air scrubbers for ammonia removal shows potential as N-S-fertilizer, whereas concentrates resulting from membrane filtrated liquid fraction of digestate show promise as N-K-fertilizer. Substituting artificial fertilizers by air scrubber water or membrane filtration concentrates theoretically always results in significant economic and ecological benefits for the agriculturist. Field research is now on-going in order to evaluate the impact on soil and crop production by application of these new green fertilizers

Economic viability of phytoremediation of a cadmium contaminated agricultural area using energy maize: part II: economics of anaerobic digestion of metal contaminated maize in Belgium

This paper deals with remediation of the Campine soil, an agricultural area diffusely contaminated with metals where most farmers raise dairy cattle and grow fodder maize. In a previous study, we calculated the effect of switching from fodder to energy maize on the farmer's income. Selling this energy maize as feedstock for anaerobic digestion to produce renewable energy could lead to a significant increase in his income. This paper explores the economic opportunities for the farmer of digesting the harvested contaminated biomass himself, by performing a Net Present Value (NPV) analysis on the digestion activity and by calculating the probability of a positive NPV of income resulting from the digestion installation. We investigate the trade off between the maximum price for energy maize that can be paid by the digestion activity and the minimum price that the farming activity needs to compensate for covering its production costs. Integrating the previous study in the current analysis results in an increase of total extra income for the farmer (i.e., from both growing energy maize and performing digestion)

The nitrogen and phosphorus budget of Flanders: a tool for efficient waste management and nutrient recovery

The region of Flanders in Belgium is, due to its high population density, intensive industry and livestock production, a nutrient-rich region. This results in important anthropogenic emissions to the environment, but also a large potential for the recovery and reuse of nitrogen (N ) and phosphorus (P) from waste streams. In this study, a substance flow analysis study for N and P is presented, in which the anthropogenic fluxes, stocks and hot spots of these two nutrients are quantified throughout the Flemish economy and environment. The environmental impact of the different economic sectors is addressed through the determination of the N and P footprint. The importance of food production in the nutrient cycle is thereby demonstrated through the large contribution of agriculture to the nutrient footprint (49% of N and 36% of P). Further focus is placed on the nutrient use efficiencies across the different sectors of the food supply nexus to target key nutrient losses and inefficiencies. This leads to an overall fertilizer-to-consumer efficiency of 14% for N and P, with the main nutrient losses originating from livestock production and food processing. At the end of the production and consumption chain, important nutrient quantities are embedded in concentrated waste streams such as excess manure, food processing waste streams and activated sludge. This demonstrates the large potential for nutrient recovery as a tool to improve nutrient use efficiencies and reduce the dependency of inorganic fertilizers. Several nutrient recovery strategies, both physicochemical and microbial, were evaluated for their economic feasibility and their impact on the primary energy demand of the total food supply chain