Development of efficient ammonia removal methods from wastewater and waste seawater

Abstract

Food security is a pressing global issue. Aquaculture has emerged as a solution to meet the increasing food demand. However, environmental concerns remain a significant challenge for aquaculture. In particular, ammonia toxicity in aquatic systems is a critical environmental issue as ammonia is highly toxic with its lethal concentration to many aquatic species at less than 1 mg/L. There is a critical need to develop effective and economic processes to reduce ammonia concentration in wastewater to be as low as possible. Chapter 1 of this thesis introduces the background and motivation for addressing ammonia pollution inaquaculture and general environmental protection. The problem statements and research objectives are highlighted in this chapter. Chapter 2 offers a comprehensive review of existing ammonium removal technologies including biological, physical and chemical processes. The review critically examines the advantages and limitations of each method, particularly in terms of their applicability to complex wastewater compositions and varying environmental conditions. For example, biological treatments, while effective on a large scale, face challenges related to operational complexity and sensitivity to environmental factors. Adsorption and chemical precipitation emerge as promising technologies, particularly due to their economic viability and scalability. However, the review identifies significant gaps in the current understanding of these methods, particularly in optimising them for use in complex water matrices, such as those found in marine environments. Chapters 3, 4 and 5 present the main research conducted and findings obtained in this thesis work in developing efficient adsorption materials and struvite precipitation method for ammonium removal. In Chapter 3, TEMPO-oxidised cellulose was investigated as an adsorbent for ammonium removal from freshwater. The study found that TEMPO-oxidised cellulose exhibits rapid adsorption kinetics, achieving the equilibrium within 5 min and final ammonium concentration of ~4 mg/L with a starting ammonium concentration of 10 mg/L. A moderate maximum adsorption capacity (qmax) of 8.21 mg NH4+/g is achieved. However, the relatively high cost of TEMPO and microcrystalline cellulose and the complexity of the oxidation process pose challenges for large-scale application. Alternative and low-cost materials need to be further explored. The research in Chapter 4 is focused on the generation of bio-carbon adsorbent by pyrolysing textile waste as an alternative cellulose source. The research demonstrates that the pyrolysed carbon materials, especially when pre-oxidised and treated with dilute NaOH, can effectively remove ammonium in freshwater from 10 mg/L to 2.6 mg/L.The adsorbent performance was significantly enhanced by these modifications, resulting in higher adsorption capacities (qmax = 25.3 mg/g) without sacrificing rapid kinetics. This chapter highlights the potential for using waste materials in wastewater treatment, aligning with principles of sustainability and resource efficiency. Furthermore, the reusability of the the adsorbent was confirmed by using a simple cation-exchange method by treating the spent adsorbent in NaCl solution. While the adsorption method can only be applied for ammonium removal from freshwater, Chapter 5 addresses the challenge of ammonium removal from seawater through the use of struvite (MgNH4PO4·6H2O) precipitation method. Our research demonstrates that this method can effectively reduce ammonium to less than 1 mg/L, even at low initial concentrations (10 mg/L) in the presence of competing cations (Na+, Ca2+, Mg2+) of much higher concentrations. Furthermore, charcoal ash, a waste material, was applied to remove excess phosphate introduced during the precipitation. A simplified economic analysis further underscores the feasibility of this method, highlighting potential cost savings and sustainability benefits if the solid products including calcium and magnesium phosphates and used charcoal ash from the process can be upcycled to useful products including fire retardant, anti-corrosion coating materials and fertilisers. Overall, this thesis contributes to the development of innovative and sustainable methods for ammonium removal from wastewater. The findings highlight the importance of material selection, process optimisation and economic feasibility in advancing wastewater treatment technologies. This research offers potentially practical solutions that can be adopted and scaled up for ammonium removal in aquaculture industry.