Investigation of nitrogen recovery from concentrated wastewater

Sammanfattning: Nitrogen recovery from wastewater treatment for fertilizers is a research topic that exists at the intersection of multiple topics important to the future of sustainable society. First, nitrogen recovery from wastewater implies a departure from the current methods of nitrogen mitigation, which involve nitrogen removal by conversion of various aqueous species to inert nitrogen gas. Secondly, by recovering nitrogen from wastewater specifically, there is the opportunity to begin a circular economy where value added products can be obtained from material that has historically been seen as a “waste”. Current wastewater treatment involves nitrogen removal through the biological transformation of aqueous nitrogen species to inert nitrogen gas. This process is energy intensive and risks the production of air pollutants such as N2O as intermediates in the biological transformation. If this nitrogen can be captured in a form that can be reused, a valuable product can be achieved with the potential reduction of both the energy required at the wastewater treatment plant as well as the carbon footprint. Finally, by recovering nitrogen in a form that can be used in agriculture as a fertilizer, additional environmental benefits can be realized by reducing reliance on Haber-Bosch based ammonia production, which is also energy intensive and contributes harmful emissions to the atmosphere.The work described in the following licentiate aims to consider the current status of nitrogen recovery from wastewater for fertilizers as a research topic. Literature was analytically examined to compare different techniques in terms of energy requirements, cost for fertilizer production, market for final fertilizer product, and technological readiness. The most interesting findings from this review were that there seems to be a disconnect between the fertilizer product produced by nitrogen recovery techniques and the market, which will become a challenge if these techniques are implemented at a large scale. The attitude of the farmers with regards to fertilizers from waste was overall positive, with their concerns mainly focused on the performance ability and cost of the product. Additionally, many techniques such as microbial fuel cells and microbial electrolysis cells have been unable to move past the laboratory phase despite being researched for many years. This indicates there are cost and technological barriers that are preventing the further scale up and implementation of these techniques. Energy and cost analyses will be crucial to motivate investment into these processes, and these are missing for many of the techniques found around this topic.To contribute to this field, experimental work was also included to assess the potential for ammonium adsorption from concentrated wastewater for fertilizer production. The experimental work focused on the utilization of the solid product of pyrolysis of sewage sludge (biochar) for adsorption and explored the potential enhancement of the char with various chemical treatments. The char with the best ammonium adsorption performance was found to be using a treatment of HNO3 followed by and NaOH, with an adsorption capacity of 4 mg NH4/g biochar. This char was compared with commercially activated carbon and clinoptilolite for full scale applications. It was found that even with this increased adsorption capacity, the use of chemically enhanced sewage sludge biochar for full scale applications is not realistic. The amount of raw material required for the complete recovery of ammonium from reject water at the municipal wastewater treatment plants exceeds the total amount of sewage sludge generated. Therefore it is recommended that the goal of incorporating sewage sludge biochar with wastewater treatment is to produce a solid fertilizer product loaded with ammonium (which would provide ammonium-N and phosphorus for plant growth, as well as carbon and other minerals for soil amendment) rather than having the goal be complete ammonium recovery from the wastewater stream.

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