## 3 Results and Discussion
### Reuse of Struvite Decomposition Residues
Consequently, the availability of reactants in the subsequent cycles for leachate treatment progressively diminished. This effect was more marked in the tests with air insufflation  
Figure 11: Diffractogram of precipitate produced by reusing the struvite decomposition residues for the treatment of raw leachate. operating modality, the NH4+ abatements decreased more quickly to an efficiency of only about 23% in the 5 th cycle. Thus, the detected trends prove that the pH correction through chemical addition is more favorable for the treatment of leachate using struvite decomposition residues. Indeed, in the tests conducted by means of air insufflation, a lower settleability of solids formed was observed in each cycle. This, clearly, makes the phases of recovery and reuse more difficult. The process with the supplementation of proper amounts of magnesium and phosphorus source was much more effective. In fact, by dosing the reactants at molar ratios of only \(n\)(N):\(n\)(Mg):\(n\)(P) = 1:0.05:0.05, the ammonium removal was constantly maintained at approximately 82% (Figure 12). Moreover, the residual phosphorus and magnesium amounts, in the treated leachate, were steadily around to 37 mg(P)/L and 69 mg(Mg)/L. The performance stability is attributable to the amount of produced precipitate, that, in the subsequent treatments, did not show substantial reductions (Table 2). Consequently, after each acid dissolution phase, the solubilized NH4+ and PO43- and the amount of decomposition residues remained pretty steady (Table 2). This ensured adequate availability of Mg and P in the sequential treatments of raw leachate. Moreover, the XRD analyses of struvite residues was always in line with those showed in Figure 9, which indicates that the formation of newbeyite repeatedly occurred during the sequential decomposition steps. Therefore, the generation of other adverse compounds, such as Mg2P2O7 ([PERSON] et al., 2016), able to hinder the ammonia removal in leachate treatment, was prevented. Efficiencies even greater than those detected in the present study have been achieved by [PERSON] et al. (2014) by reusing struvite residues obtained by acid dipping, after a washing and drying phase. These purification treatments clearly accentuate the effectiveness of decomposition products, however, they are very expensive and hardly applicable in field conditions. On the contrary, the proposed method makes the process management easier and reduces the overall costs and, at the same time, ensures satisfactory performances. Figure 12: Ammonium removal during the repeated cycles conducted by using the struvite decomposition residues. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Cycle**} & \multicolumn{6}{c}{**Without Supplement**} & \multicolumn{6}{c}{**With Air Insufflation**} & \multicolumn{6}{c}{**With Supplement**} \\ \cline{2-13}  & **1** & **2** & **3** & **4** & **5** & **1** & **2** & **3** & **4** & **5** & **1** & **2** & **3** & **4** & **5** & **1** & **2** & **3** & **4** & **5** \\ \hline Amount of wet precipitate (g) & 29.4 & 27.3 & 26.7 & 25.3 & 20.9 & 30.1 & 27.9 & 25.2 & 21.20 & 16.9 & 29.9 & 28.2 & 29.4 & 28.6 & 28.5 \\ Amount of wet composition residues (g) & 20.1 & 19.5 & 19.2 & 14.1 & 10.3 & 20.7 & 19.2 & 17.7 & 12.5 & 8.8 & 20.9 & 19.5 & 19.6 & 19.0 & 19.1 \\ Dissolved N-NH\({}_{4}\) (mmol (N)/g) & 1.51 & 1.48 & 1.45 & 12.4 & 1.07 & 1.49 & 1.46 & 1.35 & 1.08 & 0.87 & 1.52 & 1.50 & 1.48 & 1.48 & 1.45 \\ Dissolved P-PO\({}_{4}\)\({}^{-}\) (mmol (P)/g) & 0.041 & 0.038 & 0.036 & 0.035 & 0.030 & 0.04 & 0.039 & 0.038 & 0.031 & 0.029 & 0.04 & 0.038 & 0.038 & 0.041 & 0.039 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Amounts of produced precipitate, decompositions residues, and solubilized N-NH\({}_{4}\)\({}^{*}\) and P-PO\({}_{4}\)\({}^{3-}\) detected during the treatment cycles.