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 (PO 4 ‐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 (NaHCO 3 ‐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 Ca 2+ 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).