Combined application of induced MAP (Magnesium Ammonium Phosphate) precipitation and adsorption through natural zeolites for reducing NH4+ and PO43- in swine wastewaters

Combined application of induced MAP (Magnesium Ammonium Phosphate) precipitation and adsorption through natural zeolites for reducing NH₄⁺ and PO₄^3- in swine wastewaters

Nicola Tescaro*1, Giulio Galamini1, Giacomo Ferretti1, Barbara Faccini, Negar Eftekhari, Massimo Coltorti

* Corresponding author

1) 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 PO₄^3- 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, NO_x, NH₃) and eutrophication of water bodies.

In this work, a new method for reducing NH₄⁺ and PO₄^3- 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⁺ : PO₄^3- 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 PO₄^3-.  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⁺ : PO₄^3- 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 PO₄^3- 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.