## 3 Results and Discussion
### Struvite Decomposition  
The decomposition of struvite aims to maximize the dissolution of ammonium and, on the contrary, to minimize the release of phosphorus. In this way, the solid residues could be properly used as reactants for the NH4+ removal in the subsequent treatments of raw leachate. During the tests, the influence of pH, temperature, and acid type on release of ammonium and phosphorus from wet struvite was investigated. The results of conducted experiments showed that, at room temperature, for both the tested values of pH, the amount of dissolved ammonium was around to 0.7 mmol (N)/g (millimoles per gram of decomposed solid) when using hydrochloric and acetic acid as solubilizing  
Figure 4: Phosphorus recovery during the tests conducted with the simultaneous dosage of reagents. Figure 5: Diffractogram of solid produced from the leachate treatment using industrial-grade reagents, simultaneously dosed. Figure 3: Ammonium removal during the tests conducted with the simultaneous dosage of reagents. agents (Figure 6). Lower releases, between 0.4-0.5 mmol (N)/g, were observed with sulfuric and nitric acid. The effects of reactant used on struvite decomposition were much more pronounced at the temperature of 50 \({}^{\circ}\)C. In fact, small rises in dissolved NH\({}_{4}\)\({}^{+}\) amounts were observed by increasing the temperature in the tests with H\({}_{2}\)SO\({}_{4}\) and HNO\({}_{3}\) (Figure 6). On the contrary, a notable growth in ammonium release was detected, respect the values observed at 20 \({}^{\circ}\)C, by exploiting hydrochloric acid or acetic acid. Moreover, with these reagents, at 50 \({}^{\circ}\)C the solubilization performance was positively affected by the pH reduction. In fact, for a process pH of 5.5, the monitored amount of dissolved ammonium was of about 1.5 mmol (N)/g, which increased, at pH 3.5, up to about 1.9 and 2.18 for HCl and CH\({}_{3}\)COOH, respectively (Figure 6). The use of these two acids was advantageous also in terms of phosphorus release. In fact, in these cases, the lowest amounts of dissolved phosphate were observed (Figure 7). This is clearly a positive aspect because the phosphorus remains as an insoluble form, and the residual solid can be efficiently reused for the treatment of raw leachate. Also the use of nitric acid produced low solubilization of phosphates, while the detected dissolved amounts were much higher in the tests carried out with sulfuric acid. This high phosphorus release denotes a poor applicability of H\({}_{2}\)SO\({}_{4}\) for struvite decomposition. For every reactant tested, the experimental results showed a reduction of phosphorus solubilization in response of pH and temperature increase (Figure 7). Figure 6: N-NH\({}_{4}\)\({}^{+}\) release (millimoles per gram of decomposed solids) detected during the struvite decomposition tests. Figure 7: P-PO\({}_{4}\)\({}^{3-}\) release (millimoles per gram of decomposed solid) detected during the struvite decomposition tests. This causes a corresponding increase of molar ratio between the dissolved ammonium and phosphates _n_(N):_n_(P), as it can be noticed by the analysis of trends plotted in Figure 8. Clearly, this behavior is much more pronounced when using HCl and CH3 COOH. The increase of the _n_(N):_n_(P) ratio indicates that the increase of temperature to 50 \({}^{\circ}\)C and pH to 5.5 promotes a more selective release of NH4\({}^{+}\) with respect to phosphorus. Indeed, despite that the maximum ammonium dissolution occurred at pH 3.5, the corresponding amount of solubilized P limits the _n_(N):_n_(P) values. These findings are in agreement with [PERSON] et al. [14], who observed maximum _n_(N):_n_(P) molar ratios similar to those detected in this study. The increase of the _n_(N):_n_(P) ratio with the process pH from 3.5 to 5.5 is clearly a positive aspect because it allows reduction of the acid amount required during the decomposition phase. Furthermore, as above mentioned, the detected results showed a great effect of the acid type on struvite decomposition.