(Bio)electrochemical recovery of ammonia from urine

Abstract

Results and discussion 5.4 Supporting information CHAPTER 6. General discussion and future perspectives 6.1 TAN recovery in (B)ES: State-of-the-art and limitations of the load ratio concept 6.2 Urine in (B)ES: opportunities and obstacles 6.3 Future perspectives and recommendations References Summary List of publications Acknowledgements CHAPTER 1 General introduction General introduction | 11 1 The invention of the Haber-Bosch process was a collaboration between Fritz Haber and Carl Bosch. Fritz Haber successfully synthesized ammonia from N 2 and hydrogen (H 2) in 1908, and only four years later, his laboratory setup had been upscaled to an industrial level by Carl Bosch [3, 4]. Around 80% of the ammonia produced globally by the Haber-Bosch process is used for the production of synthetic nitrogen fertilizers [4-6]. The role of the Haber-Bosch process in our modern agriculture is so crucial that, by 2008, almost half of the world's population was sustained by food produced with synthetic nitrogen fertilizers [3]. The production of ammonia via the Haber-Bosch process is energetically very demanding: it consumes 1-2% of the global energy supply [3, 6-9] and accounts for 3-5% of the global natural gas consumption [10]. N 2 is widely available in the atmosphere, where it accounts for 78% of all gases, but it is inert and very stable due to the strong triple bond shared by the two nitrogen atoms [9, 11, 12]. The nitrogen bond needs to be split up in order to reduce N 2 to bioavailable NH 3 , which requires a catalyst, high temperatures and pressures (α-iron or ruthenium catalyst, 350-550 °C, 150-350 atm [13]). Furthermore, the H 2 used in the Haber-Bosch process is mostly obtained from fossil fuels (mainly natural gas (72%) and coal (22%) [6]), which makes the process energy-intensive and results in emissions of large amounts of CO 2. In the Netherlands, for example, the CO 2 emissions from ammonia production account for 2% of all greenhouse gas emissions [14]. Even though the Haber-Bosch process has been optimized by using novel catalysts [11] or hydrogen produced by more sustainable processes (such as by water electrolysis [7]), its use of fossil fuels is currently the most economical and mature option [6, 15]. Consequences of our intervention on the nitrogen cycle The introduction of the Haber-Bosch process significantly increased the amount of biologically available nitrogen on Earth, causing an imbalance in the nitrogen cycle (Figure 1.1). Nowadays, over half of the nitrogen received by the world's crops comes from synthetic nitrogen fertilizers, so the amount of global nitrogen fixation has doubled [4]. This has led to an accumulation of reactive nitrogen in natural ecosystems, resulting in severe environmental consequences. General introduction | 13 1 1.2 Ammonia removal from wastewater 1.2.1 Why do we need to remove it? Apart from volatilization, denitrification, leaching, and runoff from agriculture, nitrogen also ends up in the environment via the discharge of wastewater. As plants are consumed by animals and...