1. Introduction

Nitrogen is an important element in the structure of proteins and Deoxyribonucleic acid (DNA) and this makes it an essential building block for living cells ^[1]. With the increase in world population, both the demands on nitrogen in the form of fertilizers and the discharge of it in wastewater rise. Nitrogen fertilizers are currently produced through the Haber Bosch process, while reactive nitrogen is removed from wastewater via biological techniques. The production and removal techniques are resources intensive and have negative environmental impacts. It has been documented in the literature that Haber Bosch requires approximately 35-50 MJ/kg N (accounting for 1-2% of total world energy ^[2]) and emits about 1.6 tons of CO₂ per each ton of ammonia fertilizer produced ^[3]. The biological removal of nitrogen requires high energy for aeration that accounts for about 50% of the total energy consumption of the wastewater industry, which is estimated to be nearly 2% of total world energy consumption ^[4]. Although the concept of green ammonia exists where the energy required for deriving the Haber Bosch process is extracted from renewable resources. This concept is worth acknowledgment, but it still does not reduce the energy requirements of the process but rather presents alternative sources for it ^[5]. Nitrogen recovery from wastewater affords great opportunities to address the challenges of increasing nitrogen fertilizers demands and high energy requirements for nitrogen removal and conventional fertilizers production. Given the complex nature of wastewater and the high-quality standards of fertilizers, an efficient separation process is required for successful nitrogen recovery. Membrane technologies offer the best separation techniques that can utilized for this purpose. This entry critically discusses the application of pressure-driven membrane for nitrogen recovery from wastewater based on the literature analysis presented in a recent article in the Membranes journal 10.3390/membranes13010015

2. Application of Pressure-Driven Membrane for Nitrogen Recovery from Wastewater

The pressure-driven membranes are common separation techniques in many essential industries such as desalination, pharmaceuticals, and food processing. It is important to note here that the most precise way to describe nitrogen recovery with pressure-driven membranes is the process of concentrating nitrogen in the treated stream rather than producing high-quality nitrogen products. Pressure-driven membrane technologies include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). The separation capacity of these membranes varies depending on their porosity profile, the chemical structure of the active layer, and the operating pressure limits. The nominal pore sizes of the first three membranes are 0.1 μm, 0.01 μm, and 0.001μm, respectively, whereas RO is considered a nonporous membrane ^[6]. The larger the pore size the smaller the operating pressure required. In the context of nitrogen recovery, ammonium ion (NH₄⁺) is the target structure for recovery from wastewater. The separation mechanism of MF and UF is straining, constituents with a larger diameter than the pore size of membranes are retained. In the case of ammonium ions, the hydrated radius is reported to be around 0.25 nm ^[7]. This is smaller than the theoretical pore size of MF and UF. This suggests that MF and UF cannot retain NH₄⁺, but this does not mean that they are not useful in the recovery process. They have a different function and that is the pre-treatment of wastewater to prepare it for subsequent recovery with NF, RO, or their combination. MF and UF have the capacity of removing a wide range of contaminants such as solids, large organic molecules, microbes, and viruses. This in turn reduces fouling formation in the subsequent concentrating membrane processes and improves the quality of the concentrated stream. Moreover, if circulation is applied, the formed fouling layer on MF and UF surfaces may reduce their pore sizes resulting in higher NH₄⁺ rejection ^[8]. However, circulation can only be feasible in treating small waste streams where retention time can be prolonged to achieve the desired level of concentration. The case is different for NF and RO where the separation mechanisms are affected by the chemistry of the feed solution and membrane surface as well as the conformation of the target compounds. NF and RO separation mechanisms include steric effects, size exclusion and Dannon and membrane potentials ^[9]. This means that molecules with a neutral charge and small size such as urea are hard to be efficiently retained by NF and RO. This signifies the importance of understanding the chemical speciation of nitrogen in the waste stream before feeding it to the membrane recovery process. For instance, urea should be converted to NH₄⁺ and this can be achieved through abundantly naturally occurring bacterial and fungal enzyme, Urease ^[10]. However, special attention should be paid to the accompanying pH rise of this process. Increasing pH beyond the dissociation of NH₄⁺ (pK_a = 9.4) leads to the conversion of NH₄⁺ to free ammonia that can be lost to the atmosphere. However, the increase in pH might be very small in streams with large volumes. Customizing the surface chemistry of membranes can also improve NH₄⁺ through its interaction with the surface functional groups either directly or indirectly (being involved in maintaining electrical neutrality of the feed) ^[11].

Polymeric and ceramic membranes have been utilized for nitrogen recovery. However, due to the costly manufacturing of the latter, the former has commonly been more used. Several polymers have been used for nitrogen recoveries such as polytetrafluoroethylene (PTFE), polypropylene (PP), polyethersulfone (PES), polysulfone (PSU), polyvinylidene fluoride (PVDF) and polyamide (PA). The last two types of polymers are the most commonly used for the nitrogen recovery process. PVDF is mostly used in MF and UF, while PA is used in NF and RO. PVDF and PA are characterized by their low fouling propensity, however, cleaning them is harder compared to other polymers ^[12]. Despite the theoretically low NH₄⁺ rejection with MF and UF, some studies reported rejection levels up to 50% and this all depends on the chemistry and the quality of the nitrogen-rich stream. NH₄⁺ rejection can reach up to 95-100% with NF and RO. The main challenges that face the pressure-driven membrane recovery processes are fouling and the quality of the concentrated feed. The presence of harmful materials such as heavy metals and micropollutants in the waste stream can significantly affect the value of the produced nitrogen-concentrated stream. Although the production of pure or high-quality water in the case of RO and NF may justify the application of these processes.