Nutrients (N and P) recovery from thickening and dewatering sludge centrates of a wastewater treatment plant

ABSTRACT: Wastewater treatment plants (WWTPs) have been conceived as a method to remove polluting compounds from water, especially organic matter. However, a recent change of paradigm conceives wastewater as a valuable resource to recover energy and nutrients, mainly nitrogen (N) and phosphorus (P),...

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Autores:: González Morales, Carolina

Tipo de recurso:: Doctoral thesis

Fecha de publicación:: 2021

Institución:: Universidad de Antioquia

Repositorio:: Repositorio UdeA

Idioma:: eng

Description
Summary:	ABSTRACT: Wastewater treatment plants (WWTPs) have been conceived as a method to remove polluting compounds from water, especially organic matter. However, a recent change of paradigm conceives wastewater as a valuable resource to recover energy and nutrients, mainly nitrogen (N) and phosphorus (P), which is a finite natural substance from phosphate rock that is predicted to be in short supply within the next 100 to 150 years. Both N and P are critical nutrients for crop growth and agronomic productivity. Still, their uncontrolled discharge into water bodies causes environmental deterioration due to eutrophication, so their recovery from wastewater will reduce the level of eutrophication and the risk of a food security emergency in the future. Although there are different physical, chemical and biological techniques for the removal and recovery of nutrients from wastewater, struvite crystallisation is one of the most investigated worldwide, because it allows the joint recovery of N and P as effective fertilisers with low accumulation of heavy metals. However, research in this field has been scarce in Latin America, possibly due to the low number of existing WWTP. Therefore, it is necessary to conduct further research in nutrient recovery strategies as an opportunity to increase the level of sanitation in the region. In this regard, this doctoral research evaluated the possibility of recovering nutrients in the form of struvite from the sludge dewatering and thickening supernatants (centrates) of a WWTP located in the south of the Aburrá valley, close to Medellin (Colombia) that treats domestic and industrial wastewater. The thesis is divided into seven chapters, which are described below: Chapter 1 provides an overview of the context in which the thesis was developed. The chapter discusses the importance of recovering nutrients within a circular economy, the associated legislation, presents the main techniques used for the removal and recovery of nutrients from wastewater and explains the main characteristics of the mineral struvite. The research question, hypotheses and objectives are also posed. Chapters 2 to 6, presented in the form of articles, address the following aspects: Chapter 2 presents the mass balance of N and P developed in the San Fernando WWTP. The mass balance was conducted over seven composite samplings (lasting 7 hours with a frequency of 1.5 h) to address different climatic conditions. The objective of this chapter was to evaluate the behaviour of the nutrients in the different 15 identified flows of the WWTP to establish their possible recovery. In addition, the characteristics of the centrate from the San Fernando WWTP were compared with those of other two WWTPs that use chemical precipitation for the removal of P. Chapter 3. evaluates the effect of stirring speed on the quantity, size and quality of the struvite precipitates obtained. For this, 6 agitation speeds were evaluated using the sludge dewatering centrate of the La Llagosta WWTP (Catalonia-Spain). Chapter 4 examines the effect of pH and temperature on struvite formation; for these experiments, the optimal velocity gradient obtained in chapter 3 and the centrate of the Esholt WWTP (Bradford-England) was used. Chapter 5 assesses the effect of 3 variables (initial concentration of phosphorus, aeration and seed material) on reactor efficiency and the quantity and quality of the precipitated product, based on experiments developed in two types of designed reactors: the 5L full-mix reactor (CSTR) and the 6.5L fluidised bed reactor (FBR). For this evaluation, the actual sludge dewatering centrate of the San Fernando WWTP (Colombia) was used with a 3x22 factorial design. Chapter 6 evaluates the quality of the struvite obtained in chapter 5 as a fertiliser, considering the Colombian and European regulations. The evaluation considered two struvite samples (one for each reactor), and a one-factor design, using the biomass produced and the amounts of N and P in this biomass as the response variables. The fertilisers evaluated in the selected crop (Pasto Brachiaria Brizantha Marandú) were the struvite obtained from the two reactors, biosolids from the same San Fernando WWTP and two fertilisers used commercially (Urea and Triple 15). A leaching test was also included in this chapter. Chapter 7 presents the general conclusions and recommendations of this research. Finally, the ANNEXES section introduces the general materials and methods used in the research, starting with the analytical methods at the laboratory level, the main calculations used in the different chapters of this thesis, following with the design of the reactors, the preliminary design tests, the equations used for the design of continuous stirred tank reactor (CSTR) and a fluidised bed reactor (FBR) and the residence times distribution tests (RTD) for the hydraulic evaluation of the reactors designed. Finally, annex 4 presents additional information of chapter 5.

Nutrients (N and P) recovery from thickening and dewatering sludge centrates of a wastewater treatment plant

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