Precipitação de estruvita: recuperação de nitrogênio e fósforo utilizando fontes alternativas de reagentes

Abstract

The physico-chemical process of struvite precipitation is a technique that shows a high potential in recovering nutrients from different kinds of wastewaters. The present work aimed the evaluation of alternative sources of magnesium and phosphate to turn the process economically viable. The alternative reagents performance was evaluated in function of nutrients recovery and quality of the obtained crystals with possible identification of struvite. Industrial magnesia compounds in different MgO contents were subjected to hydration reactions as a prior step to the chemical precipitation. The Mg(OH)2 slurries obtained were used as magnesium and alkalinity source necessary to the precipitation reactions. As alternative source of phosphate, bone meal was also evaluated; a residue generated by food industry and subjected to characterization and acid dissolution steps. In a first stage performed in an jar-tests apparatus, the optimum conditions were set out through an experimental design and used in the evaluation of the alternative sources. In operational conditions of pH equal to 8.5 and Mg2+:NH4+:PO43- stoichiometric ratio equal to 1.5:1.0:1.25, the Mg(OH)2 slurry promoted a NH4+-N removal equal to 72.9%. On the other hand, bone meal promoted a higher NH4+-N removal, equal to 82.1% at the same operational conditions. This was probably due to the smaller amount of interfering inserted into the reaction medium. In a second stage, the chemical precipitation was evaluated under two different conditions and reaction systems operated in semi-continuous: a conic reactor and a fluidized bed reactor. The conic reactor was operated in laboratory scale by using Mg(OH)2 slurries as alternative source of reagent. It promoted an average PO43--P removal equal to 93.5%, however, this reactor has been shown ineffective regarding the crystals growth possibly due to shear forces generated in the medium by mechanical stirring. The fluidized bed reactor was operated in pilot scale by using the same alternative source of reagent. It promoted an average PO43--P removal equal to 74.6% and a discrete crystals growth. Analysis of the obtained solids in this last stage allowed the majority identification of struvite with a purity degree over 90%. Finally, it highlights the effectiveness and the feasibility of the chemical precipitation process as a technique of effluents treatment using alternative and low-cost reagent sources such as those evaluated in this study.