Use of nitrogen-and phosphorus-based synthetic fertilizers shows an increasing trend, but this has led to largescale influx of reactive nitrogen in the environment, with serious implications on human health and the environment. On the other hand, phosphorus, a non-renewable resource, faces a serious risk of depletion. Therefore, recovery and reuse of nitrogen and phosphorus is highly desirable. For nitrogen recovery, an ion exchange/adsorption-based process provides concentrated streams of reactive nitrogen. Bioelectrochemical systems efficiently and effectively recover nitrogen as NH 3 (g) or (NH 4 ) 2 SO 4 . Air stripping of ammonia from anaerobic digestate has been reported to recover 70-92 % of nitrogen. Membrane separation provides recovery in the order of 99-100 % with no secondary pollutant in the permeate.With regard to phosphorus (P) removal, physical filtration and membrane processes have the potential to reduce suspended P to trace amounts but provide minimal dissolved P removal. Chemical precipitation can remove 80-99 % P in wastewater streams and recover it in the form of fertilizer (struvite). Acid hydrolysis can convert recovered P into usable phosphoric acid and phosphate fertilizers. Physical-chemical adsorption and ion exchange media can reduce P to trace or non-detect concentrations, with minimal waste production and high reusability. Biological assimilation through constructed wetlands removes both N (83-87 %) and P (70-85 %) from wastewaters, with recovery in the form of fish/animal feeds and biofuel. The paper discusses methods and important results on recovery of nitrogen and phosphorus from wastewater.
Inputs of P into receiving water bodies are attracting increasing attention due to the negative effects of eutrophication. Presently available P treatment technologies are unable to achieve strict P discharge limits from wastewater treatment plants (WWTPs) that may be as low as 10 µg/L as P. Moreover, P is a nonrenewable resource and needs to be recycled in a closed‐loop process for environmental sustainability. This article provides details of a process where a pyridine‐based polymeric ion exchanger is modified with a combination of impregnated hydrated ferric oxide (HFO) nanoparticles and a preloaded Lewis acid (Cu2+) to effectuate selective P removal from wastewater and its recovery as a solid‐phase fertilizer. Three such ion exchangers were studied: DOW‐HFO, DOW‐Cu, and DOW‐HFO‐Cu. Each of these materials displays selective phosphate affinity over competing anions chloride and sulfate, and also has the ability to be regenerated upon exhaustion to strip off the P in a concentrated solution. The P in concentrated regenerant can be recovered as struvite, MgNH4PO4, a slow‐release fertilizer, after addition of MgCl2 and NH4Cl. Results of equilibrium and kinetic studies and column experiments with synthetic solutions and a real WWTP effluent are discussed. Practitioner points Fixed‐bed columns with DOW‐HFO, DOW‐Cu, or DOW‐HFO‐Cu—can selectively remove phosphorus over competing anions. Fixed‐bed columns of above‐listed ion exchangers can produce an effluent P < 6 μg/L. DOW‐Cu fixed‐bed column ran for ≈500 Bed Volumes before breakthrough when fed Dartmouth WWTP secondary effluent. Regeneration of the exhausted DOW‐Cu column resulted in ≈90% recovery of the phosphorus. Regenerant solution was used to generate high‐purity crystals of magnesium ammonium phosphate, MgNH4PO4 (struvite), a slow‐release fertilizer.
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