Europe's Mediterranean countries have a serious problem with respect to supply for effective and safe fertilizers as in the case of Spain. In this way, since 1990 Spanish count with regulations to allow the safe use of bio-solids produced in the WWTP in agriculture: they are a valuable by-product, which concentrate the pollution present in urban wastewater. Production of bio-solids in Spain achieves more than 700.000 T/year (matter dried) and around 80% of these ones are sent to be used in agriculture as alternative fertilizer. This practice can be carried out by the fact that bio-solids have an important content in organic matter, N, P and, in minor level, K. To preserve soils, all bio-solids used in agriculture must not contain more quantity of heavy metals than those established in the Spanish regulations for Cd, Cu, Ni, Pb, Zn, Hg and Cr. In the case of La Golondrina´s WWTP, our study has been extended for 2000-2019: in this period the facility has treated a mean flow of 26.55x106 m3/year and has produced 38,000 T/year of bio-solids with 74.9% of organic matter over dried mass. On the other hand, concentrations of the seven metals limited in the Spanish normative have been respected in time. Moreover, from viewpoint to potential use for agriculture, major components of bio-solids were N, P (as P2 O5 ) and Ca (as CaO) which levels were 6.6%, 3.5% and 3.7%, respectively, as well as K (as K2 O) with 0.6%, over dried matter. Finally, our biosolids have always in accordance with the Spanish regulations and have been used for agriculture without problems.