A series of technologies have been employed in pilot-scale to process digestate, i.e. the byproduct remaining after the anaerobic digestion of agricultural and other wastes, with the aim of recovering nutrients and reducing the load of solids and organics from it, hence improving the quality of digestate for potential subsequent reuse. In this case the digestate originated from a mixture of dairy and animal wastes and a small amount of agricultural wastes. It was processed by the application of several treatments, applied in series, i.e. microfiltration, ultrafiltration, reverse osmosis, selective electrodialysis and combined UV/ozonation. The initially applied membrane filtration methods (micro- and ultra-filtration) removed most of the suspended solids and macromolecules with a combined efficiency of more than 80%, while the reverse osmosis (at the end) removed almost all the remaining solutes (85-100%), producing sufficiently clarified water, appropriate for potential reuse. In the selective electrodialysis unit over 95% of ammonium and potassium were recovered from the feed, along with 55% of the phosphates. Of the latter, 75% was retrieved in the form of struvite.

A combination of membrane processes was applied to treat the digestate produced after the anaerobic treatment of pig manure in a biogas plant, aiming towards the recovery of nutrients and effective water treatment for potential reuse. Initially, coarse filtration (sieving and microfiltration) was used to remove particles larger than 1 µm, followed by ultrafiltration, to reduce the suspended solids concentrations below 1 g/L. Subsequently, selective electrodialysis is employed to recover the main nutrient ions, primarily ammonium and potassium. The ion-depleted digestate is then fed to a reverse osmosis unit, where clean water was recovered, yielding a by-product (concentrate) stream enriched in phosphates and organics content. The presence of antibiotics and the concentrations of heavy metals were monitored during all treatment stages to assess their behavior/removal in the various membrane processes. The results indicate that almost 51% of the digestate could be recovered as water free from ions and antibiotics, suitable for reuse in the biogas plant for process needs and irrigation purposes. The selective electrodialysis process can recover 51% of initial NH ₄ ⁺ content (corresponding to 96% of the electrodialysis feed), while the remainder largely ended up in the ultrafiltration concentrate. A similar behavior was observed for the case of K ⁺ , while approximately 68% of the phosphates content was retained by the coarse filtration process, with another 24% remaining in the ultrafiltration concentrate and the remaining 8% in the reverse osmosis concentrate. Most of the antibiotics and heavy metals were retained by the coarse and ultrafiltration steps, with smaller amounts detected in the reverse osmosis concentrate.

Digestate management is essential to both environmental protection and sustainable agriculture. However, its safe and effective agricultural use is increasingly challenged by strong variability in composition and the presence of persistent contaminants, including heavy metals, pharmaceuticals, microplastics and per- and polyfluoroalkyl substances. While numerous treatment and recovery technologies have been developed, existing research remains fragmented, often addressing individual contaminants or processes without a unified framework linking environmental risk, technological performance and system-level decision-making. This review systematically synthesizes current knowledge on digestate characteristics, contaminant profiles and treatment technologies, with particular emphasis on nutrient recovery, contaminant control and circular valorisation pathways. Conventional, advanced and emerging technologies are critically compared across Technology Readiness Levels, highlighting trade-offs between recovery efficiency, contaminant fate, energy demand and the generation of secondary waste streams. The analysis reveals that many widely applied technologies primarily redistribute contaminants into concentrated side streams, underscoring the need for integrated treatment trains that combine separation, recovery and destructive steps. Building on this synthesis, the review proposes a structured decision framework that links digestate composition and management objectives with appropriate technology combinations, explicitly integrating techno-economic feasibility, environmental performance and technology maturity. This framework provides a practical tool for selecting and designing digestate valorisation strategies tailored to feedstock characteristics and regulatory constraints. Finally, critical research gaps and policy needs are identified to support the transition toward environmentally safe, economically viable and circular digestate management systems.

The reuse of nutrient-rich waste in order to create green fertilizers in a sustainable and efficient way is a new strategy for the transition to a circular economy. Digestate is produced as a by-product from anaerobic digestion plants in large amounts creating a major management issue both for storage, utilization or disposal. The current work focuses on the development of an innovative, small-scale technology for the processing of digestate from anaerobic digestion plants that use food, animal, agricultural and sludge waste. The technology targets the recovery of fibres and nutrients for the production of high-performance bio-fertiliser products and clean water from digestate.