ReviewA critical review on membrane hybrid system for nutrient recovery from wastewater
Introduction
Phosphorus (P) and nitrogen (N) are essential nutrients for the growth of organisms. However, such nutrients can also cause eutrophication which may seriously impair the quality of water and even cause aquatic life to die [1]. Therefore, the discharge of nutrient should be strictly controlled. For this reason, nutrient removal from wastewater is necessary and the discharge concentration of nutrient is required to be below 1–3 mg·N/L and 0.1 mg·P/L, respectively [2]. In wastewater, ammonium and phosphate ions are the main present forms of N and P respectively. The former ions are always removed by ammonia stripping and nitrification-denitrification while the removal of the latter is mainly achieved by chemical precipitation and biological uptake [3], [4], [5], [6]. However, nutrient removal does consume a large amount of energy and chemicals. It is reported that an extra 4% electricity is needed for removing N in wastewater treatment plants in the United States [7], [8]. Similarly, Xie et al. [9] recently stated that a sustainable energy supply of 45 MJ/N·kg is used to remove N from wastewater. Nutrient removal may also aggravate global warming due to the emission of 0.9 kg CO2/m3 as a consequence of this process [10], [11].
The explosive growth in the world’s population has in turn sparked an increase in the demand for nutrient-based fertilizers, approximately 1.8% per year for food production [12]. However, the current production of fertilizers faces many challenges. Firstly, around 90% of global phosphate demand is utilized for food production [13], but the remaining accessible deposits of phosphate rock will be completely consumed in 30–300 years [14]. Secondly, no materials can substitute the role of P in the fertilizers production [15]. Furthermore, the industrial Haber-Bosch process is always employed to produce ammonia for fertilizer production, through which the atmospheric N can be converted into NH3-N [2]. However, the anthropogenic production of NH3-N exceeds the amount of NH3-N converted by the natural process [2] and hence disturbs the nature N-cycle. Consequently the environment is subjected to great risks. More importantly, the generation of ammonia from air depletes around 35–50 MJ/kg·N of energy, which accounts for 2% of global energy [16].
As discussed above, nutrient recovery is more valuable than nutrient removal because: i) it can produce nutrient-based fertilizers to ensure food security; ii) it can minimize the environmental footprint of wastewater treatment such as production of much less excess sludge and reduced eutrophication; and iii) N recovery can decrease the consumption of natural resources and save costs associated with N fixation. A wide range of wastewater sources containing rich nutrient such as sewage, urine and leachate has been utilized for the purposes of nutrient recovery [17], [18], [19], [20].
Of all the methods employed in nutrient recovery from wastewater, chemical precipitation is the most widely used due to its high efficiency and stability while the biological nutrient recovery is more attractive [21], [22]. Chemical precipitation seeks to choose an appropriate precipitator to react with nutrient for precipitates’ formation at pH > 8 [21], [23], [24]. Mg and Ca materials are usually employed as the precipitator to form MgNH4PO4·6H2O (struvite) and Ca5(OH)(PO4)3 (HAP), respectively. However, Fe and Al materials may not be suitably applied as precipitators because the recovered product tightly binds with phosphate ions, making it difficult for plants to take up phosphate [25]. Moreover, the nutrient recovery via chemical precipitation can be economically feasible only if the P concentration in wastewater is over 100 mg/L [26], [27].
On the other hand, a biological process such as an enhanced biological phosphorus removal (EBPR) system was developed to recover phosphate [28]. This scenario does highly depend on the phosphate accumulating organisms (PAOs) because phosphate is released from cells to wastewater in an anaerobic state while PAOs can accumulate excessive amounts of polyphosphate in aerobic/anoxic environments [29]. In this scenario, the surplus sludge containing rich P is achieved. However, the recovered product (i.e. excess sludge) cannot be applied in agriculture due to the fact it contains heavy metals, toxic matter and pathogens [30]. As for N, the ammonia is normally stripped via air/N2 out of wastewater at high temperature and pH, after which the volatile ammonia is adsorbed to form liquid ammonia or ammonium salts such as ammonium sulphate and ammonium carbonate [31], [32], [33].
In the nutrient recovery process, the coexisting heavy metals and toxic substances exert serious effects on nutrient recovery such as impairing the quality of recovered nutrient. For example, Xie et al. [9] concluded that the recovered struvite crystals were found to have toxic heavy metals contents, in which the arsenic concentration was even greater than 570 mg/kg. As a result of this, the application of such recovered struvite may be forbidden in agriculture due to its low quality and purity. Therefore, it is essential to separate nutrient from the foreign matter to enhance the application potential of recovered nutrient. Since membranes have selective high-rejection for ions, the nutrient can be enriched and separated from foreign substances in the streams [34]. Fig. 1 illustrates the membrane separation process for ions from feed solution [35].
From Fig. 1, it can be seen that a membrane filtration process mainly relies on the pore size of a membrane and water in the feed solution could penetrate the membrane under pressure [36]. The membrane separation technique is a simple physical process, so no chemicals and energy are included in it. For this reason the membrane separation process is more economical than other separation processes in terms of operational and maintenance costs. Currently, some membrane techniques have been integrated with chemical precipitation and biological processes as the membrane hybrid systems for nutrient recovery from wastewater [15], [36], [37], [38], [39], [40], [41]. In such cases the technical and economic feasibility of the nutrient recovery system can be improved.
According to the process of membrane separation, Forward osmosis (FO), membrane distillation (MD) and electrodialysis (ED) are the main membrane technologies that concentrate nutrient in the wastewater treatment [9], [37], [38], [39], [42]. A bioelectrochemical system (BES) containing a proton-exchange membrane has also attracted attention for its ability to recycle nutrient through a combination of chemical and bioelectrochemical reactions [31], [32], [43]. In addition, membrane bioreactor (MBR) is also a promising membrane technique for recovering nutrient [40]. The possible reason for this is the nutrient can also be rejected within the bioreactor and the low level of membrane fouling is observed due to the removal of organics. Furthermore the traditional MBRs can: a) reduce the production of surplus sludge, environmental (carbon) footprint and size of equipment; b) improve the quality of effluent due to having a high concentration of suspended solids and long sludge retention time; and c) enhance safety due to no biological sedimentation units [44]. The main types of membrane hybrid systems for nutrient recovery from wastewater are summarized in Table 1.
Currently, more researchers have endorsed the idea that nutrient recovery from wastewater can replenish resources for fertilizer production and overcome the negative effects of treated wastewater on the environment [15], [40]. The membrane hybrid system for nutrient recovery is attractive since it can concentrate the nutrient within the reactor with few foreign substances. Several reviews have reviewed the membrane hybrid systems for recovering nutrient from wastewater [9], [31], [32], [49], [50], [51], [52], [53], [54]. However, most of them lack comprehensiveness, where either membrane-based hybrid systems or MBR-based hybrid systems are overlooked while reviewing the nutrient recovery. Moreover, such papers lack a comparison of the membrane hybrid systems with reference to technical and economic feasibility which are essential for readers to understand the subject. To improve our knowledge of this topic, this paper aims to present a critical and comprehensive review on the current state of the membrane hybrid system for nutrient recovery from wastewater. The mechanisms and processes of membrane hybrid systems for nutrient recovery are introduced. These systems are compared in terms of their technical and economic feasibility. Simultaneously, possible challenges and future direction associated with the application of the membrane hybrid systems for recovering nutrient are discussed.
Section snippets
Forward osmosis
In the FO process, a semipermeable membrane is utilized while the driving force is derived from the osmotic pressure gradient between the feed solution and draw solution. In this scenario, water in the feed side can be transported across the FO membrane to the draw side. Simultaneously, some draw solute may also transfer to the feed side due to reverse salt flux [55].
Fig. 2 presents the schematic representation of the FO-based hybrid system for the nutrient recovery. It is worth noting that the
Technical feasibility
As shown in Table 2, membrane fouling is a big challenge while applying the membrane hybrid systems for nutrient recovery from wastewater (especially from urine and digested sludge) [50]. The problems encountered are that it may deteriorate the membrane performance, increase energy consumption and consequently reduce economic feasibility.
The membrane fouling resulting from the FO process is readily reversible and thereby easier to clean [53]. Another challenge associated with the FO process is
Future perspectives
Wastewater is considered to be the greatest potential source for nutrient recovery because it contains rich nutrient and in large quantities. As nutrient is transferred from a decentralized industry and household to a centralized wastewater treatment plant, the concentration is decreased along this route. The use of membrane technology can enhance the nutrient enrichment, which substantially improves the economic feasibility of nutrient recovery. In addition, membrane process can also separate
Conclusion
Currently, recovering nutrient from wastewater is more valuable than nutrient removal. The possible explanation is that this recovery can provide a supplementary source for fertilizer production, thus easing the burden of increasing demand for food production. Another advantage associated with nutrient recovery is that it can reduce the serious impacts of wastewater treatment on the environment and save costs. Membrane technology can enrich nutrient with high purity, which can improve the
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These authors contributed equally to this work.