Applying Natural and K-Enriched Zeolite Before Struvite Precipitation Improved the Recovery of NH4+ from Liquid Digestate and the Reagent Use Efficiency

Implementing nutrient recycling in wastewater treatment plants is essential for sustainable agriculture. In this study, we investigated a biphasic treatment system for anaerobic liquid digestate, which involved natural and K-enriched zeolite for NH4+ recovery (phase 1), followed by struvite crystallization under two conditions: NH4+ excess and Mg2+ excess (phase 2). The adsorption of NH4+ by natural zeolite enabled saving Mg and P reagents, used to achieve target Mg:NH4:PO4 ratios. The reagent use efficiency of struvite precipitation was highest with natural zeolite under NH4+ excess conditions (96%), whereas the other treatments exhibited lower yields. In this condition, the digestate enriched in Ca2+ released by zeolite; however, no P interferences occurred (Ca2+/Mg2+ < 0.5). Fractions of Ca2+ precipitated as CaCO3. Both the isomorphic NH4- and K-struvite occurred, distinguished by calibrating XRPD data (total struvite) with N contents (indicative of NH4+-struvite). The precipitates comprised NH4- and K-struvite at 60% and 30% (calcite at 9%) in the treatment that involved natural zeolite, 65% and 35% with the K-exchanged zeolite, due to higher presence of K+. Concerning the chemical evolution of the treated digestate, fewer alterations occurred for inorganic ions in the treatment that involved natural zeolite (phase 1) with NH4+ excess condition (phase 2), besides for unreacted SO42- derived from the Mg reagent. The recovered zeolite was enriched in N at 0.5%. Struvite precipitates met the EU regulations regarding permissible levels of organic C, P content, and heavy metal impurities, thereby potentially enabling its use as a fertilizer.