Resumen de Nutrient recovery from waste water treatment plant by sorption processes: technical and economic analysis
In the last years has been performed a huge number of research related to nutrients (mainly N and P) recovery from waste water in order to promote their reuse and also to avoid eutrophication. Recovering nutrient from waste water can promote the circular economy, minimize the environmental footprint of waste water treatment and reduce anthropogenic alteration of nitrogen and phosphorus natural cycle.
Among the different techniques studied, sorption is nowadays one of the most interesting alternatives as requires less energy than electrochemical or membrane technologies and sorbents can be regenerated for being reused. Thus, the overall sorption performance of different ion exchangers was characterized for nutrient recovery from waste water treatment plant effluents.
Two commercial polymeric resins doped with Hydrated Ferrous Oxide (HFO) (Lewatit FO36 and Fiban-As) were assessed for ammonium and phosphate uptake from waste water streams. Equilibrium and kinetics studies have been performed through batch experiments and the dynamic studies were carried out by fixed-bed column. The experimental study covered from the simplest monocomponent synthetic water (ammonium, phosphate solutions) to the most complex real water from waste water treatment plant (WWTP) which contains ions, TSS and organic matter.
Sorption experiments reported for both sorbents high selectivity towards phosphate ions as sorption capacity did not significantly decrease in presence of competing ions. Loaded sorbents were regenerated through alkaline and acid desorption in order to reuse resin and obtain a high concentrated phosphate solution, which could be used for high quality fertilizer production. Results in dynamic flow experiments reported up to 80% of loaded phosphate extraction, although during desorption process was observed sorption capacity decrease up to 30% after three sorption-desorption working cycles.
Besides, salt modified synthetic zeolites were assessed for being used both as one charge (di-valent forms) and reusable (mono-valent forms) sorbent. Raw zeolites provided in sodium form as well as it modification in potassium form showed high sorption capacity for ammonium uptake but poor performance for phosphate recovery from waste water streams as the sorption mechanism is mainly ion exchange. Regeneration experiments showed that both Ze-Na and Ze-K can be reused several sorption-desorption working cycles without significant sorption capacity decrease (< 5%).
Nevertheless, after salt modification to calcium and magnesium forms, the precipitation of phosphate salts was favoured obtaining hydroxyapatite and struvite in loaded Ze-Ca and Ze-Mg, respectively. As hydroxyapatite’s solubility and phosphorus availability is similar to commercial fertilizers, loaded Ze-Ca could be potentially recycled for agricultural uses.
Finally, an economic assessment simulation was performed by applying Ze-Ca in the Baix Llobregat Waste Water Treatment Plant, located in the Metropolitan Area of Barcelona. The study reported that the implantation Ze-Ca filtration for nutrient recovery would be economically feasible, despite that more research is required in order to fit technical issues as the loaded sorbent disposal or the capability of loaded zeolites to be used as fertilizer.
