Plant nutrients recovery from agro-food wastewaters using microbial electrochemical technologies based on porous biocompatible materials

Microbial recycling cells (MRCs) are a category of microbial electrochemical technologies aimed at recovering plant nutrients from agro-food wastewater and recycling them as fertilizers. Here, we used terracotta as a biocompatible and low-cost material to fabricate porous air-water separators. At the interface, access to oxygen induces a red-ox gradients, facilitating nutrients deposition on the material itself. We tested: a) raw swine manure (rich in suspended solids, rich in mineral-N) and b) an artificial wastewater (suspended-solid-free, rich in mineral-P). Bioanodes (carbon cloths enriched in electroactive biofilm) were connected to air-exposed cathodes, generating currents of ≈ 2 mA and an electric field of ≈ 400 mV. Over 250 days, consistent amounts (up to 69%) of the main nutrients (N, P, K, Mg, Mn, Fe) were removed from the wastewater and deposited on the terracotta separators at significantly higher concentrations in presence of the electrochemical circuit (MRC), as compared to open-circuit controls. Cation and water electro-driven migration towards the cathode and high cathodic pH (up to 10.7) were favouring inorganic salts accumulation and precipitation on the porous surface. Accordingly, halophilic microbial species were populating the biofilm grown on the terracotta. In the case of SM, suspended-solids were also deposited on the surface (as revealed by microscope imaging), driving to recover organic forms of nutrients. Porous biocompatible materials (such as terracotta) can be enriched of nutrients from wastewaters, to be later re-used in agriculture as soil conditioners. Future studies on this concept should look at optimizing surface/volume ratios, to maximize nutrients recovery efficiencies.