More than 90% of soils in Europe have been declared unhealthy, largely as a result of injudicious use of synthetic fertilizers and overexploitation of soil. This calls for an increased use of more sustainable types of fertilizers and soil amenders rich in carbon.
The Anaerobic digestion (AD), a process that is used to produce renewable energy with low CO2 footprint, can produce a valuable by-product, the digestate, that is used in agricultural activities as a soil improver or for fertilization of the fields. The digestate is actually the by-product of the valorization of waste materials. A wide variety of organic wastes-manure, food processing residues, municipal solid waste, and agricultural residues, are used in AD plants and the produced digestate is returned back to the fields, closing the loop in a circular economy approach.
However, it has been identified that the direct application of digestate is associated to several environmental issues. It is therefore required that the digestate is pre-processed in order that the nutrients, the carbon and water can be recovered and efficiently utilized, while potential pollutants are removed and not released to the environment, optimizing in this way the products’ composition according to particular agricultural needs with the adoption of an adequate separation system.
This work executes a Life Cycle Assessment on a membrane-based treatment process including a solid-liquid separation system with a screw press, microfiltration (MF), and ultrafiltration (UF) to produce a phosphorus- and potassium-rich soil amender and a Selective ElectroDialysis unit (SED) for nutrient recovery to produce fertilizers such as ammonium sulfate and struvite. Furthermore, reverse osmosis (RO) is applied for the recovery of clean water.
The environmental impacts that were considered and studied, were the greenhouse gas emissions, terrestrial acidification and eutrophication, ecotoxicity in freshwater, and marine environments, using input from experimental testing in the pilot scale and in real environment. The results showed that an integrated approach could achieve significant environmental improvements and enhance soil health, compared to conventional applications.