Ammonia removal can generate agricultural fertilizers
Urban centers generate more and more solid waste (garbage), the management of which challenges public sectors. Brazilian legislation requires municipalities to dispose of urban solid waste (MSW) in landfills, as a way of minimizing environmental impacts resulting from the use of controlled dumps and landfills.
During the biodegradation of organic compounds from MSW, a turbid, dark liquid effluent with a strong nauseating odor called leachate is formed, with a high organic content and variable composition, which depends on the stage of formation, the location, the soil, the rainfall regime, climate. Leachate, as leachate is also called, is highly contaminating, and must be collected and treated appropriately throughout the useful life of the landfill and after its closure. Its main pollutants are organic compounds and ammonia, a composition that makes its treatment difficult, which in Brazil is still very incipient in the vast majority of municipalities and does not comply with current legislation.
The tendency in the country is to dump leachate into the sewer, whose treatment does not eliminate certain toxic components that, as a result, reach water bodies, as ETEs are not designed to process leachate. It is therefore necessary to eliminate certain components in advance, which make treatment difficult. To this end, it is necessary to carry out research that contributes to the understanding of the characteristics of leachate in tropical climates.
This was the motivation of environmental sanitation technologist Jorge Luiz da Paixão Filho when he proposed to study alternatives for treating landfill leachate (LAS), in research carried out at the Faculty of Civil Engineering, Architecture and Urbanism (FEC) at Unicamp. , guided and co-supervised, respectively, by professors Adriano Luiz Tonetti and José Roberto Guimarães, and with the collaboration of researcher Jerusa Schneider and several units of the University.
Although the researcher focused on determining the composition of LAS at various stages of its formation, on the processes that enabled the removal of its organic matter and on its joint treatment with sanitary sewage, a practice in force in Brazil, this article will address the removal of ammonia by chemical precipitation and the agricultural use of the solid product obtained (struvite), with fertilizer characteristics, as it consists of nitrogen (N), phosphorus (P) and magnesium (Mg).
Ammonia removal
All phases of the research focused on leachate from a large experimental cell (80x60x5m), built in 2012 at the Delta A landfill in Campinas, SP. Stabilized leachate, formed more than a year after landfill establishment, was collected at predetermined intervals for approximately 950 days. The concentration of ammonia nitrogen in it was found to be around 2200 mg per liter, much higher than the maximum of 20 mg per liter that can be released into water courses, according to current legislation.
The high concentration of ammonia in effluents can cause death of fish and living organisms and, associated with the presence of phosphates, eliminates dissolved oxygen in the liquid body leading to anaerobiosis, that is, the extermination of organisms, as occurs in stretches of the Tietê River , in Sao Paulo. The structuring of methods that enable the drastic reduction of ammonia in leachate before subjecting it to treatment in WWTPs was the researcher's object of study.
To do so, he treated the leachate separately with two sets of reagents. First with magnesium chloride (MgCl2) and sodium hydrogen phosphate (Na2HPO4), later replaced by magnesium oxide (MgO) and phosphoric acid (H3PO4). The first treatment resulted in the mineral struvite (MgNH4PO4.6H2O) and the second a mixture of struvite and carnallite (KMgCl3.6H2O), products with agricultural fertilizer potential as they contain the macronutrients N and P and also Mg, important in photosynthesis, and which also have the advantage of containing a low concentration of heavy metals. Both combinations of reagents were 99% efficient in removing ammonia, proving the efficiency of this technique. To study the feasibility of using the two solids obtained as fertilizer, it was then necessary to test their efficiency, which was done in the cultivation of lettuce (lactuca sativa).
Lettuce cultivation
To evaluate the influence of struvite on the production of flat lettuce, the mineral was used as fertilizer in three different situations: one in which the added quantity contained the phosphate concentration recommended in cultivation and in the other two with, respectively, half and double of the prescribed dose so that the results could be compared. The same criteria were used in relation to cultivation with the struvite and carnalite mixture. As a control, crops were used only in soil and also in soil with NPK, a commercial fertilizer.
The use of struvite in this crop proved to be more efficient than commercial fertilizer, enabling greater plant growth – aerial part and roots – and a greater concentration of nutrients in the leaves (N, P and Mg) and in the soil, keeping it still fertile for a second planting, as long as double the recommended dosage in relation to phosphate is used. This efficiency may be linked to the slow release of nutrients, meeting the plant's demand needs at each growth stage.
The use of a mixture of struvite and carnallite, which would be the perfect fertilizer as it contains NPK, had the disadvantage of salinizing the soil due to the presence of sodium chloride. But future studies may contribute to the feasibility of its use.
Jorge considers that the future use as fertilizer, both of the structive and its mixture with carnallite, could make the extraction of ammonia from leachate economically viable, promoting integration between agricultural production and soil and water conservation.
