Proposição e avaliação de processos físico-químicos para a recuperação de nutrientes do digestato de resíduos orgânicos

Abstract

Nitrogen and phosphorus are essential raw materials for fertilizers production. These nutrients are obtained through non-renewable sources (phosphorus – phosphate rock) and/or high energy-demanding processes (ammonia – Haber-Bosch process). Furthermore, they are wasted on soil and water bodies through inadequate effluents and waste disposal, which represents a waste of valuable resources. In this context, anaerobic digestion (AD) is an alternative for waste treatment, due to the production of biogas (that can be used for energy) and a nutrient-rich digestate. Physical-chemical processes like stripping-absorption for nitrogen recovery and chemical precipitation for phosphorus recovery can be used for nutrient recovery from digestate since they are most viable technical-operational options. However, the high operational costs related to chemical addition (chemical precipitation) and heating (stripping-absorption), hinder a wider application of these technologies, mainly in developing countries. Therefore, the aims of this work were to propose and to evaluate potential alternatives to reduce these operational costs. For this, two different organic-based digestates were used: food waste (DIG1) and agro industrial waste (DIG2). Initially, a simplified stripping-absorption process for ammonia recovery as ammonium sulfate from DIG1 was proposed and evaluated. This system used no chemicals for pH adjustment and solar energy for heating (vacuum tubes). Results showed that solar heating was able to keep temperature steady on 45ºC throughout experiments. Besides, ammonia removal higher than 91% was achieved for all evaluated experimental conditions. In the second part, phosphorus recovery from DIG1 by calcium phosphate and struvite precipitation using conventional precipitants (Ca(OH)2, hydrated lime, Mg(OH)2) was proposed and evaluated. Results showed phosphorus removal higher than 95%. However, recovered precipitated have a little phosphorus content due to the low concentration of this element in DIG1. Finally, phosphorus recovery from DIG2 by precipitation of calcium phosphates and struvite using low-cost precipitants (seawater and limestone powder) was proposed and evaluated. Results showed that phosphorus removal were 44% e 66%, respectively for limestone powder and seawater. Higher phosphorus removals were inhibited by calcite formation in experiments using limestone powder. On the other hand, phosphorus was removed by struvite and K-struvite precipitation in experiments using seawater. The results of this thesis may contribute to the implementation of alternative methods for managing agro-industrial organic waste and recovering nutrients from waste, especially in tropical countries, contributing to the sustainability of agriculture and food production.