Integration of liquid-liquid membrane contactors and electrodialysis for ammonium recovery and concentration as a liquid fertilizer

Abstract

The accumulation of ammonia in water bodies can cause eutrophication and reduce water quality. Furthermore, 80% of the ammonia in the world is consumed as fertilizer, which makes it a resource that can be recovered under the circular economy concept. Then, ammonia from wastewater can be valorised for agricultural applications. Liquid-liquid membrane contactors (LLMCs) have been postulated as a novel and eco-friendly technology for ammonia recovery, because they can convert dissolved ammonia into ammonium salts by an acid stripping solution. The concentration of the ammonium salt produced is limited by the co-transport of water in LLMC. Further concentration by electrodialysis (ED) is presented as a solution to overcome this problem. In this work, ammonia streams with different initial ammonia concentrations (1.7-4.0 g/L) were treated by LLMCs to produce liquid ammonium salt fertilizers (as NH₄NO₃ and NH₄H₂PO₄). Then, these ammonium solutions were concentrated by ED in order to achieve the nitrogen content required for direct application in agriculture for fertigation. After the LLMC process, the fertilizer obtained was composed of approximately 5.1% or 10.1% (w/w) nitrogen, depending on the initial ammonia concentration. After that, it was possible to concentrate these ammonium salts by a factor of 1.6 ± 0.3 using ED with an optimal energy consumption of 0.21 ± 0.08 kWh/kg ammonium salt and 93.1 ± 4.2% of faradaic yield. This gave a liquid fertilizer composed of 15.6% (w/w) nitrogen as NH₄NO₃. Overall, it was possible to integrate two innovative membrane technologies for the valorisation and concentration of nutrients from ammonia wastewater streams.

Keywords: Ammonia valorisation; Ammonium salts; ED; LLMC; Nutrient recovery.