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Combined application of induced MAP (Magnesium Ammonium Phosphate) precipitation and adsorption through natural zeolites for reducing NH4+ and PO43- in swine wastewatersNicola Tescaro*1, Giulio Galamini1, Giacomo Ferretti1, Barbara Faccini, Negar Eftekhari, Massimo Coltorti * Corresponding author1) Department of Physics and Earth Science, University of Ferrara, Ferrara Via Saragat 1 44122, Italy. Modern agriculture, in response to the constantly increasing need of high crop production, requires application of high levels of N and P fertilizers to soil. These substances are mainly composed by nutrients such as NH4+ and PO43- and are often applied in strong excess in order to assure high crop yield. The nutrients applied through fertilizers and not exploited by crops (estimated on average around 50%) can considerably impairs environmental quality through nitrogen losses in atmosphere (N2O, NOx, NH3) and eutrophication of water bodies.In this work, a new method for reducing NH4+ and PO43- in swine wastewaters (commonly used as organic fertilizer) was studied. The aim is to reduce their environmental impact and concomitantly create a new slow-release fertilizer.Two techniques have been combined: the induced MAP precipitation (magnesium ammonium phosphate) and natural zeolite ammonium adsorption for removing the NH4+ excess that generally remains in solution after MAP precipitation. Given the complexity of working with real wastewaters, in this preliminary phase a synthetic analogue was used in order to better evaluate the efficiency of this method. Two synthetic wastewaters with different Mg2+ : NH4+ : PO43- molar ratio were tested: MR1 (1:1,5:1) and MR2 (2:1:1), which according to the literature give the best reductions of NH4+ and PO43-. Since swine wastewater are naturally rich of both NH4+ and K+, isotherm studies were conducted on natural zeolites for evaluating their adsorption capacity of NH4+ under different levels of K+ competition. Results showed that the potential in NH4+ adsorption decreased while competition with K+ increased. The combination of MAP precipitation and NH4+ adsorption by natural zeolite has been tested in 2 ways: 1) zeolite was added before inducing MAP precipitation 2) zeolite was added after inducing MAP precipitation. These two treatments were compared to a blank in which only MAP precipitation technique was used.The amount of NH4+ and PO4+ was monitored in various steps during the experiments as well as SEM observations were conducted on precipitated obtained. Results showed that adding zeolites before MAP precipitation induce a variation in the Mg2+ : NH4+ : PO43- ratio due to cation exchange processes before MAP precipitation which introduce interfering ions such as Ca2+ favoring calcium phosphates precipitation instead of MAP. The best test conditions, which produced the 75,1% of NH4+ and 99,9% of PO43- reductions, occurred when zeolites have been added after MAP precipitation using the MR2. This new material obtained combines good N and P concentration and have therefore potentialities to be a high-quality slow-release fertilizer. |
