A critical review on ammonium recovery from wastewater for sustainable wastewater management

The growing global population's demand for ammonium has triggered an increase in its supply, given that ammonium plays a crucial role in fertilizer production for the purpose of food security. Currently, ammonia used in fertilizer production is put through what is known as the industrial Haber Bosch process, but this approach is substantially expensive and requires much energy. For this reason, looking for effective methods to recover ammonium is important for environmental sustainability. One of the greatest opportunities for ammonium recovery occurs in wastewater treatment plants due to wastewater containing a large quantity of ammonium ions. The comprehensively and critically review studies on ammonium recovery conducted, have the potential to be applied in current wastewater treatment operations. Technologies and their ammonium recovery mechanisms are included in this review. Furthermore the economic feasibility of such processes is analysed. Possible future directions for ammonium recovery from wastewater are suggested.


Introduction
The combination of wastewater, concerns for people's health and environmental hazard, is one where basic engineering seeks to remove contaminants from wastewater treatment plants, so that a satisfactory effluent is generated (Taddeo et al., 2018). However, substantial energy and resources such as land and infrastructure are important aspects of wastewater treatment. A high quantity of sludge is generated during this process, one which may pose a risk to the environment. Fortunately, remediation methods have changed and more attention is being paid to make wastewater treatment facilities more sustainable ( Nitrogen (N) as the renewable resource is of great importance for organisms' growth (Smith & Smith, 2015). The natural nitrogen cycle is present in Figure 1 and it can be seen 4 is called nitrification. In contrast, denitrification is referred to as the reduction of nitrate to molecular nitrogen by denitrifying microorganisms. Ammonia with nitrite as an electron acceptor can be oxidized to nitrogen gas through anaerobic ammonium oxidation bacteria (Ye et al., 2018). It is worth noting that the ammonium derived from the biological nitrogen fixation is not enough to support the world's ammonia demand for crops and plants.
Therefore, an approach known as the Haber-Bosch process is applied at the industrial scale to produce ammonia with molecular nitrogen used as the raw material, which is further employed in fertilizer production. The equation of this particular process is presented in Eq. (1): other than the Haber-Bosch process to sustainably produce ammonia for fertilizer production is a problem that must be urgently solved.
Ammonia-based fertilizers eventually enter the aquatic environment by runoff while nitrogenous compounds consumed by human and animals through crops will also reach water bodies in the major forms of sewage and manure, respectively. This may increase the concentration of ammonium (nitrogenous compounds exist in the main form of ammonium ions in water) in the aquatic environment. Once the ammonium concentration cannot be purified by the water itself, several environmental issues such as eutrophication will arise (Ye . Moreover, a substantial chemical input is necessary since various chemicals must serve as the electron donor in the nitrification-denitrification process. By-products such as nitrate resulting from the nitrification-denitrification process do not have significant market values and often need further purification prior to their emission.
As discussed above, a sustainable supply of ammonia and efficient ammonium removal mechanism is important. Hence, ammonium recovery in wastewater treatment seems more valuable than ammonia removal, especially given that high energy and costs beset the classical ammonium removal processes. Apart from this, ammonium recovery can not only supplement fertilizer production, but also lead to sustainable and better resource management.
A combination of wastewater sources such as municipal wastewater, piggery wastewater, landfill leachate and urine are ammonium-dense, as shown in Table 1.
A focus on reducing the environmental footprint that is part of classical ammonium removal, yet at the same time it must increase the amount of ammonium for fertilizer production. This has triggered much research on recovering ammonium from wastewater. improvements. We assert that this review can provide some recommendations for future work on diversified technologies that can recover ammonium efficiently and effectively.

Mechanism of ammonium recovery in the wastewater treatment
It is important to comprehend the mechanisms of ammonium recovery in wastewater treatment because they provide useful information on optimizing the recovery process and

Struvite precipitation
The first mechanism is most commonly used to recover ammonium from wastewater. In this process, the ammonium is recovered in the form of struvite with simultaneous phosphate recovery at alkaline pH. It should be noted here that the struvite formation needs stoichiometric amounts of ammonium, phosphate and magnesium as described in Equation (2) ( Abbona et al., 1982).

Ammonium stripping
As well, ammonium recovery through the stripping-adsorption process is mainly attributed to the fact that at high reaction temperature and/or pH, the ammonium can be converted to volatile ammonia. This reaction can be described in Eq. (7).
Thus, one method to recover ammonium from wastewater is to shift the equilibrium toward the gaseous phase, followed by ammonia stripping from the solution. In this scenario, A cation-exchange membrane (CEM) is used in the ED process. The ammonium ions are driven by an electrical current to diffuse the CEM towards the cathode chamber, which causes the ammonium retention in an individual compartment. High current density certainly improves the ammonium concentration, but also contributes to large energy consumption in the ED system.

Membrane technology for ammonium recovery from wastewater
Generally, wastewater contains a mix of substances such as organics, heavy metals and toxic substances, which may seriously affect the ammonium recovery process. The biological process is the most widely used mechanism for treating wastewater because it can reduce the amount of foreign matter. As a result, ammonium ions with high purity can be achieved within the reactor and this facilitates the ammonium recovery. Nevertheless, more effort should be made to separate the ammonium from foreign substances to enhance ammonium recovery. For this reason, efficient membrane technology is proposed because it can enrich the ammonium ions within the reactor and separate the foreign matter from ammonium without energy input. Using membrane technology to concentrate ammonium ions is a lowcost exercise. In addition, integration of membrane technology with biological process can enhance the organic removal and thereby reduce the membrane fouling which seriously influences the membrane application. For example, the MD membrane is easily subjected to high organic fouling while applying it to recover ammonium in the wastewater treatment. In this scenario, membrane wetting will be caused, which results in the diminished amount of (9)-(10).
Cathode reaction in MFC: Cathode reaction in MEC: Once the aeration was supplied, 0.77 M of ammonia was recovered after being removed from the cathode chamber. MFC can be deemed a positive energy balance system for recovering ammonium from wastewater because the electrons generated by itself drive the ammonium transport for ammonium enrichment and pH elevation for ammonium transformation.
Nevertheless, the current density should be increased if the ammonium recovery via MFC is expected to be improved.
Another feature is that stripped ammonia can be utilized as the draw solute in the AEMs to recover ammonium and purify wastewater. In their study, wastewater was circulated between the anode and cathode chambers, which resulted in the concentrations of ammonium condensing to 1.5 times bigger than the initial concentrations. Thus, ammonium was recovered as struvite and 96% of NH 4 + -N was removed from the wastewater. When a phosphate buffer served as the catholyte, the ammonium recovery could actually improve.
The possible explanation for this is that the phosphate buffer solution could: (i) be used as the phosphate sources for the struvite precipitation; and (ii) increase the ammonium transport from the anode chamber to the cathode chamber as a result of maintaining the ion balance (Sotres et al., 2015).

Current density
High current density can significantly affect the ammonium recovery process in the BES.
This is because high current density can increase the ammonium migration across the CEM from the anolyte to the catholyte and facilitate the pH increase of the latter due to providing which improved the ammonium transport from the anode chamber to cathode chamber. When the current was reduced to 5 mA, however, the catholyte's pH fell to 8.6, which may negatively influence the formation of volatile ammonia and the later ammonia adsorption.

Coexisting cations
Cations other than ammonium ions are also driven by the current field to migrate across the CEM from the anolyte to the catholyte so that the charge neutrality of the BES can be maintained. So the coexisting cations such as K + , Na + and Ca 2+ in the wastewater may affect the ammonium migration and further recovery. For the most common cations, Kim et al.  Table 2.

Osmotic membrane bioreactor
Compared to RO, MD and ED membranes, using FO membrane requires lower energy input and is involved in less membrane fouling. Hence, the recovery of ammonium from wastewater can also occur through the osmotic membrane bioreactor (OMBR) consisting of the FO membrane and biological process. The advantage of this method is that: (i) more ammonium ions could be accumulated within the bioreactor as well as the mineral salts; (ii) less energy input is needed; (iii) decrease in membrane fouling potential could be observed; For using BES and OMBR to recover ammonium from wastewater, membrane fouling is still a challenge. However, the membrane fouling potential can be reduced to some degree in the BES when electricity generation is brought into play. Since the anaerobic sludge particles are negatively charged, the current field generated between the anode and cathode may inhibit sludge accumulating on the membrane surface due to electrostatic repulsion (Wang et al., 2013). Consequently, the membrane fouling rate decreases and moreover, the membrane fouling in the OMBR is relatively small because the FO membrane is used. Applying the FO membrane could contribute to low membrane fouling and chemical cleaning methods are conducted regularly to effectively minimize the risks of membrane fouling (Achilli et al., 2009; Yap et al., 2012). This is despite the fact that using the OMBR for ammonium recovery could be more expensive. The summarization of ammonium recovery in the BES/membrane hybrid system is shown in Table 3.

Costs and energy consumption associated with ammonium recovery
The economic feasibility of ammonium recovery is determined by both operational costs and the benefits of recovered ammonium in future commercial undertakings. Ammonium recovery via struvite precipitation presents its obvious advantages: firstly, it can simultaneously recover phosphate which is a non-renewable and limited source from wastewater; and secondly, struvite is a safe and effective slow release fertilizer that be directly applied to land. In Japan, struvite was reportedly sold at a value of US$250 per tonne Furthermore substantial magnesium materials for struvite formation may be needed due to most wastewater sources lacking a sufficient magnesium source.
With reference to the process of stripping coupled with adsorption for ammonium recovery, this approach is insensitive to feed concentration. It indicates the method can be applied to a wider range of wastewater sources. Furthermore, the selection of the acid solutions for ammonia adsorption also affects the economics of ammonium recovery.
Generally, sulphuric, hydrochloric and nitric acid are mainly utilized to produce their associated ammonium salts. One study found that the resulting ammonium sulphate as the In addition, the energy generated from the anaerobic digestion biogas can be used to heat the liquid water while using stripping coupled with adsorption to recover ammonium. can be generated, which can be subsequently utilized for direct land application as a fertilizer.
Using the BES to recover ammonium is advantageous because there is no need to increase pH to convert ammonium into gas ammonium. Indeed, the possible energy balance associated with the ammonium recovery by the BES may include aeration in the cathode chamber, ammonia adsorption by sulphuric acid, additional power (only for the MEC), and energy generation (only for the MFC). Based on this, an analysis of energy balance for the ammonium recovery is presented in As shown in Table 4, the MFC shows a positive energy balance for recovering ammonium while the conventional ammonia stripping requires the highest energy input.

Recovered ammonium
It is necessary to evaluate the performance of recovered ammonium in agriculture Another issue involved in ammonium recovery through struvite precipitation is that most wastewaters contain more ammonium and phosphate than magnesium, so additional magnesium is always needed in this process (Rahman et al., 2014). If the concentration of ammonium and phosphate can satisfy the chemical requirements of struvite formation, the magnesium material utilized in this method may account for 75% of the overall costs of struvite production (Dockhorn, 2009). For this reason, researchers are currently studying inexpensive magnesium sources in struvite precipitation, but the solution is still a long way off.

Conclusion
Recovering ammonium from wastewater not only reduces the costs, energy and environmental footprint associated with this removal process. Another benefit is that the material can be used to supplement fertilizer production and save the expense required in the industrial Haber-Bosch process. Although the trade-off of BES between ammonium recovery and energy recovery significantly affects the amount of ammonium recovered and the ammonium-based precipitate influences how well the BES performs, the great potential of BES for recovering ammonium is very evident, despite the current challenges that need to be dealt with.      Ammonium recovery from wastewater for sustainable wastewater management Table captions   Table 1 Ammonium content in the main types of wastewater sources Table 2 Effects of influencing parameters on ammonium recovery in the BES Table 3 Ammonium recovery in the BES/membrane hybrid system Table 4 Comparison of energy balance involved in the ammonium recovery processes  (1) High current density increases the ammonium migration.
(2) High current density facilitates the pH elevation of catholyte. (1) Influence the ammonium migration.