Process optimization for ammonium removal from municipal food waste treatment plant liquid digestate using natural rocks and volcanic ash

This study investigates the adsorption of ammonium (NH₄⁺) from the liquid fraction of food waste digestate. Six natural rocks and volcanic ash were used as adsorbents. X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) were employed to analyze mineral composition and surface morphology before and after adsorption. Results indicated a significant ammonium reduction of up to 64 %, with an adsorption capacity of 93 mg L⁻¹ at equilibrium within 50 min. After adsorption, changes in the XRD patterns and mineral composition confirmed transformations in the adsorbents, SEM revealed rough and irregular surface morphologies, likely attributed to the uptake of NH₄⁺, and EDS analysis showed a decrease in the concentration of exchangeable cations, which was associated with increased nitrogen levels in each rock sample. These findings suggest that the primary mechanism of these materials for NH₄⁺ adsorption is cation exchange. Additionally, generated models for nitrogen removal efficiency (NRE) and adsorption capacity (AC) concluded that factors such as pH, adsorbent dosage, and temperature also influenced adsorption efficiency. Triplicate experimental runs and ANOVA confirmed the models' predictive accuracy, yielding an R² of 0.9955, a standard deviation of 1.02, and a minimal variability of 2.63 % for NRE. Optimal conditions were identified at a pH of 6, a dosage of 0.32 g, and a temperature of 23 °C, achieving an NRE of 62 % and an AC of 159 mg g⁻¹. Efficient ammonium adsorption by these natural adsorbents shows promise for practical wastewater treatment and environmental remediation.