## 1 Introduction  
The leachates generated as result of water infiltration and decomposition of organic matter in landfills of urban wastes are wastewaters with one of the highest polluting potentials [1; 2; 3; 4; 5]. Their uncontrolled disposal can produce severe deterioration of quality of soil and water bodies [4]. The characteristics of these wastewaters depend mainly on the type of landfill site and the kind and degradation degree of wastes. Methanogenic leachates (produced in mature landfills) show particularly high content of ammonium nitrogen and low biodegradable compounds, as well as a great amount of dissolved salts [3]. These characteristics make the treatment of methanogenic leachates, in conventional biological plants [6; 7], hardly applicable. In recent years, different physicochemical techniques have been proposed for the treatment of these wastewaters. Specifically, for the removal of low biodegradable compounds, advanced oxidation processes represent a suitable option [8; 9; 10; 11; 12; 13]. However, the development of effective and economic methods for the abatement of the high nitrogen loads of wastewaters still represents a big issue [14; 15]. In this regard, the precipitation of ammonium as struvite is a promising approach, due to its high process efficiency and the ease of management [16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27]. Struvite crystals are composed of Mg\({}^{+2}\), NH\({}_{4}\)\({}^{+}\), and PO\({}_{4}\)\({}^{3-}\) in equimolar amounts (MgNH\({}_{4}\)PO\({}_{4}\)-6H\({}_{2}\)O) [16; 17]. The formation of magnesium ammonium phosphate occurswhen the concentration of its elements exceeds the solubility product under basic conditions [28]. Despite its high efficiency, struvite precipitation for ammonia removal from landfill leachate is generally a costly treatment. Indeed, due to the lack of Mg\({}^{2+}\) and PO\({}_{4}\)\({}^{3-}\) ions with respect to the ammonium concentrations, great quantities of chemicals must be provided. With the aim to reduce the overall treatment cost, various alternative magnesium sources--such as the byproducts of magnesium oxide [29; 30], magnesite mineral [31], and seawater [32]--were efficiently exploited in the treatment of highly concentrated wastewaters. However, the phosphate reagents, being scarce and very expensive, generally represent a large part of the overall outlay. Nevertheless, few researches have paid attention to this aspect [4]. In recent years, the authors have proposed the bone meal of meat waste as an unconventional phosphorus reagent [33; 34; 35]. The use of this byproduct--in combination with seawater bittern, a residue of marine salt manufacturing--results in an effective and low-expense method for the ammonia removal and recovery from high-strength wastewaters [33; 34]. Furthermore, the struvite produced through this method is characterized by a remarkable fertilizer potential [35]. Despite these positive aspects, the supply of unconventional reagents may be difficult and could make the process barely applicable in industrial treatment plants. Moreover, the disposal of struvite could present a problem if its exploitation as fertilizer were not possible. To overcome this issue, the recycling of struvite decomposition products can represent a valid solution to reduce the operation costs. In fact, struvite decomposition causes the release of ammonium, and the resulting residues can be exploited as P and Mg sources for the cyclical treatment of wastewaters. Several decomposition methods, including struvite pyrogenation and acidosis, have been recently proposed [14; 36; 37; 38; 39; 40; 41; 42; 43; 44]. The pyrogenation in alkali solutions has widely proved its effectiveness for ammonium release. However, in this process, temperatures higher than 90 \({}^{\circ}\)C are generally required [36]. Moreover, the possible dispersion of ammonia gas in the atmosphere could hinder the process application in field conditions [14].