Continuous phosphorus discharges in bodies of water, generated by human activities, such as agriculture, domestic effluences or wastewater from industrial processes, produce contaminated water and eutrophication. For this reason, efficient and low-cost systems that can remove phosphorus from contaminated water are necessary. In addition, it is important to generate renewable energy such as the energy produced in biomass power plants, taking advantage of the available biomass waste in each place. When producing this renewable energy, the resulting ash is a residue that can be used for phosphorus removal by adsorption processes. Moreover, according to the concept of the circular economy, the ash waste generated in this bio energy process should be reduced as much as possible. One of the advantages of this research being that surplus phosphorus-laden ash can be reused as fertilizer in agricultural fields. Considering this, the efficiency of reed ash (RA) (Phragmites australis) has been analysed in batch experiments, as well as the effect of several parameters on the removal of phosphate, such as contact time, phosphate-ash ratio, ash dose and temperature. Significant results obtained show that RA can be used to improve water quality.