Biomass ashes: are they suitable to remove phosphorus from wastewaters?

Batch assays with biomass ashes - fly and bottom ashes - were performed with the aim to study the removal of phosphates from a synthetic wastewater and from a wastewater of a pulp and paper mill obtained after a secondary treatment. Supernatants resulting from the treatment with biomass ashes were characterized for different chemical species and for ecotoxicity levels. The bacterium Vibrio fischeri and the microcrustacean Artemia franciscana were used as biological indicators of the ecotoxicity levels. The addition of fly ashes in a Solid/Liquid (S/L) ratio of 3.35 g L^-1 and of bottom ashes in a S/L ratio of 9.05 g L^-1 to the synthetic wastewater allowed to remove phosphates in percentages above 99% and 97%, respectively. In what deals with the pulp and paper mill wastewater, the addition of fly ashes in a S/L ratio of 34.45 g L^-1 and of bottom ashes in a S/L ratio of 46.59 g L^-1 achieved removal percentages of phosphates >97% and >90%, respectively. Additionally, reduction percentages of 45.5%, 71.8%, 73.5%, 87.2% for BOD₅, COD, color, and TSS, respectively, were determined in the supernatant resulting from the addition of fly ashes to the pulp and paper mill wastewater at a S/L ratio of 34.45 g L^-1. The addition of bottom ashes in a S/L ratio of 46.59 g L^-1 to the pulp and paper mill wastewater had conducted to reduction percentages of 46.7%, >90.9%, 46.0% and 79.6% for BOD₅, COD, color, and TSS, respectively. The addition of ashes to the synthetic wastewater and to the pulp and paper mill wastewater had not caused ecotoxicity in the treated effluents (EC₅₀-30 min for V. fischeri ≥ 99% v/v and EC₅₀-24 h for A. franciscana ≥ 90% v/v for all supernatants).