Phosphorus and potassium recovery from anaerobically digested olive mill wastewater using modified zeolite, fly ash and zeolitic fly ash: a comparative study

Abstract

BACKGROUND Nutrient recovery from wastewater is gaining attention in the frame of circular economy. In this study, Ca(OH)2‐treated zeolite (CaT‐Z), lignite fly ash (FA) and zeolitic fly ash (ZFA) were primarily used to adsorb phosphate phosphorus (PO4‐P) from anaerobically digested olive mill wastewater (ADOMW). The simultaneous recovery of potassium (K) was also examined based on adsorption and desorption data. The fractionation of adsorbed P was determined as an important parameter for its plant availability. RESULTS More P was adsorbed from ADOMW on ZFA and FA (up to 4.35 and 5.21 mg g−1, respectively) than on CaT‐Z (2.62 mg g−1). An increased P adsorption on ZFA and FA was observed between incubation times of 7 and 14 days. The sequential desorption procedure verified the trend of P adsorption capacities and showed that the plant‐available P (NaHCO3‐P) amounted to 1.34, 2.34 and 1.69 mg g−1 CaT‐Z, ZFA and FA, respectively. After 14 days, CaT‐Z and ZFA adsorbed much more K (19.2 and 20.5 mg g−1, respectively) than FA (4.3 mg g−1). The desorption of exchangeable K confirmed this difference. Scanning electron microscopy–energy dispersive spectrometry analyses indicated P adsorption and surface precipitation as Ca–P phases as well as K+ for Ca2+ ion exchange on the loaded adsorbents. CONCLUSIONS ZFA was more efficient for dual adsorption combining the properties of the zeolitic fraction (enhanced K adsorption) and pristine FA (P adsorption on Ca‐bearing phases). The sufficient concentrations of plant‐available P and K on the three adsorbents suggest their potential use as soil amendments. © 2022 Society of Chemical Industry (SCI).