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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.Highlights: Zeolites allowed less reagents for reaching optimum struvite precipitation. N loads were reduced with zeolite–struvite treatment up to 86%. Struvite from digestate is usable in agriculture and complied with EU regulations. The precipitation of K-struvite limited the recovery of N from digestate. To discern (K) and (NH4)-struvite, XRD must be combined with chemical information. [ABSTRACT FROM AUTHOR]

Volcanic tuffs rich in chabazite zeolites have been extensively examined for their potential to enhance soil properties and increase fertilizer efficiency, both in their natural state and when enriched with nitrogen (N). However, there is a scarcity of data regarding their utilization in acidic sandy soil, particularly when used alongside organic fertilizers. This paper presents the findings of a 50-day laboratory incubation study that investigated the dynamics of N pools in an acidic sandy-loam agricultural soil treated with various N sources. These sources included urea, N-enriched chabazite zeolite tuff, and pelleted composted manure applied at a rate of 170 kg N/ha. Additionally, the N sources were tested in combination with chabazite zeolite tuff mixed into the soil to assess its role as a soil conditioner. The results revealed distinct behaviours among the tested N sources, primarily impacting soil pH and N dynamics. Soil fertilized with manure exhibited slow N mineralization, whereas N-enriched zeolite displayed a more balanced behaviour concerning net NO3−-N production and NH4+-N consumption. Both N-enriched zeolite and urea temporarily altered the soil pH, resembling a "liming" effect, while pelleted manure facilitated a prolonged shift towards neutral pH values. Considering the water adsorption capacity of zeolite minerals, caution is advised when adjusting water content and employing combustion methods to measure soil organic matter in zeolite-treated soil to avoid potential inaccuracies. In summary, N-enriched chabazite zeolite tuff emerged as a valuable N source in acidic sandy-loam soil, offering a promising alternative to synthetic fertilizers and showcasing a sustainable means of N recycling. [ABSTRACT FROM AUTHOR]

The large use and the bad management of fertilizers that are applied to soil for improving crop production have dramatically impaired soil, water, and air quality. To meet the requirements to reduce nitrogen (N) losses and all the related negative impacts on the environment and food production, it is mandatory to substitute or at least partially substitute the use of inefficient and unsustainable fertilizers with more efficient alternatives. The aim of this paper was to address the amount and speciation of the N released by a sandy soil fertilized with "slow‐release fertilizers" and traditional fertilizers (urea and liquid digestate) by means of a series of column leaching experiments. The slow‐release alternatives were represented by NH4‐enriched zeolitic tuff and struvite, both obtained by recovering the N from liquid digestate. The treatments consisted of sandy soil fertilized with (i) urea (U) (ii) liquid digestate (LD), (iii) NH4‐enriched zeolitic tuff (N‐CHA) and (iv) struvite (STRV). Eight different flushing events were performed over 38 days, leachates were collected and analysed for total Kjeldahl N, organic‐N, NH4+‐N, NO3−‐N, NO2−‐N and pH. U and LD lost the majority of N within the first 2 flushing events as organic N and NH4+‐N, respectively. On the other hand, STRV and N‐CHA lost less N over the whole course of the experiment and with more balanced speciation. The mass balance outlined that after the experiment, native soil N was mined in U and LD treatments while in N‐CHA and STRV a fraction of N from the fertilizers was still present. The results showed a slow release of N which can be used more efficiently in agricultural applications, minimizing the N losses. [ABSTRACT FROM AUTHOR]