Methane (CH 4 ) and nutrients (N and P) are valuable resources that can be recovered to improve the overall cost-competitiveness of the anaerobic wastewater treatment processes. In particular, achieving net positive energy production from anaerobic wastewater treatment processes is a longstanding vision,22 which can be potentially realized by recovering CH 4 via a membrane contacting process, for example. As for N and P recovery, a number of membrane-based processes have also been utilized. However, due to their low concentrations, it is economically less feasible to recover nutrients using membrane-based processes. Our discussion in this section is therefore galvanized toward the strategic use of membrane-based processes for concentrating nutrients.

Recovery of dissolved CH 4

Produced CH 4 from anaerobic processes can be used to generate electrical and thermal energies, which can be channeled back to the anaerobic digestion processes to realize energy self-sufficient wastewater treatment plants (Fig. 3). At present, conventional physical processes such as spray aeration, jet tower, packed column, or diffused aeration have been applied for the recovery/removal of dissolved CH 4 from the anaerobic effluents.12 A critical drawback of these techniques is that the stripping gas (usually air), which directly contacts the anaerobic effluents, can cause foaming and flooding. To prevent such operational problems, the use of gas/liquid “membrane contactor” process was proposed.12 The gas/liquid membrane contacting process uses a stripping gas (or vacuum) to establish a mass transfer driving force between the gas and liquid phases, which are isolated using polymeric membranes as phase barriers.12,23 To reduce the mass transfer resistance, membranes are commonly designed to have high porosity and low wettability. Typically, small-diameter hollow fiber membranes are used in membrane contactors to provide a much higher mass transfer area per unit volume (Fig. 3). As a result, it is possible to lower the energy consumption of gas/liquid membrane contactor process as compared to that of the conventional separation processes.12,23

Fig. 3 Illustration showing retrofitting of the membrane contactor unit within the anaerobic digestion and membrane separation processes Full size image

Several groups have studied the influence of membranes and operation modes on the CH 4 recovery from anaerobic effluents using membrane contactors. In 2011 and 2012, Bandara et al.24,25 reported two consecutive studies on the performance of degassing membranes (porous polyurethane layer sandwiched between dense polyethylene layers) for the treatment of a bench-scale upflow anaerobic sludge blanket (UASB) reactor effluent. In their first study, they operated the UASB reactor for 170 days and recovered dissolved CH 4 from the effluents intermittently.24 As they have studied different temperatures, hydraulic retention times (HRTs), and transmembrane pressure (TMP) conditions, their CH 4 recovery efficiencies fluctuated widely, but typically reached around 70 to 90%. In their follow-up work, Bandara et al. operated the UASB reactor for 18 months under ambient conditions and found that the CH 4 recovery reached “57% ± 7%” and “66% ± 8%” at warmer (20–31 °C) and colder (6–10 °C) seasons, respectively.25 In another study published in 2012, Cookney and coworkers have tested the potential of dense polydimethylsiloxane (PDMS) membrane contactors for CH 4 recovery from expanded granular sludge bed (EGSB) anaerobic reactors.26 They found that ~45% of the produced CH 4 remained in the effluents, and 72% of this can be recovered by using a low liquid velocity (0.0025 m s−1) during the membrane contacting process.

Membrane pore wetting is a severe limitation of the membrane contactors. Pore wetting phenomenon occurs when the liquid phase enters the pore of the membranes, leading to a tremendous increase in mass transfer resistance. Henares et al.27 recently investigated the performances of porous polypropylene (PP) and dense PDMS membranes for dissolved CH 4 recovery and found that the fluxes of porous PP membranes were much higher than the dense PDMS membranes. Despite this, porous PP membranes suffered from wetting problems with increased liquid flow rate. As such, Cookney et al.29 suggested that dense membranes are more suitable as they have a higher capacity to obviate the pore wetting phenomenon. Pore wetting can also be prevented by enhancing the surface hydrophobicity of the membranes. For instance, Wongchitphimon et al.30 incorporated fluorinated silica nanoparticles on the surface of porous Matrimid® membranes. The resulting composite membranes demonstrated a high contact angle of ~126°, which resulted in at least 1.75 times higher performance than a commercial PP membrane for the CH 4 recovery application. Later, Dilhara et al.31 evaluated the performance of poly(vinylidene difluoride) (PVDF) membranes that were surface-functionalized with a solution comprising 3 wt% commercial perfluoropolyether, Fluorolink S10, and 2 wt% tetraethoxysilane (TEOS). The surface-modified PVDF membrane exhibited almost two times higher CH 4 flux and had a stronger resistance toward pore wetting (more than 10 days) as compared to the commercial PP membrane.

Apart from the membrane aspects, the operation mode of membrane contactors is also critical for achieving a high recovery of dissolved CH 4 . Comparing the two operation modes, i.e., feeding the liquid phase in lumen or shell side of the membrane module, researchers have found that feeding the liquid in lumen side gives better results, owing to more effective cross-flow hydrodynamics which suppress the formation of dead zones.27,29 However, using the lumen side of the membrane modules increases the risk of clogging when the effluents contain a high solid content. In view of this potential disadvantage, processing the effluents in the shell side of the membrane modules is still the preferred operation mode unless the total soluble solids (TSS) is sufficiently low.

To improve the quality of outlet gas from the membrane contactors, careful optimization of the operating parameters is needed. For example, a trade-off between %CH 4 removal efficiency and concentration of CH 4 in the gas outlet of the membrane contactor can occur due to the additional stripping gas supplied to increase the mass transfer driving force. While this approach can increase the %CH 4 removal efficiency, it dilutes the CH 4 concentration in the outlet gas inevitably.32 To address this problem, McLeod et al.32 deliberately lowered the stripping gas flow rate to obtain a gas that possessed a CH 4 concentration higher than the upper flammable limit and in a readily usable form. Besides, as a highly versatile unit, the membrane contactors allow easy retrofitting to other sources of biogas to mix with the outlet gas and afford a product gas that has on-demand CH 4 concentration.12

Membrane fouling is another critical issue that results in compromised recovery of membrane contactors but not actively explored hitherto. Among the limited number of studies reported on fouling of membrane contactors so far, only Bandara et al.24,25 noticed an insignificant level of membrane fouling after a ~5.5- and 18-month long investigation using one type of anaerobic effluent. However, when another type of anaerobic effluent was used, Henares et al.33 observed membrane fouling on PDMS membrane contactors. The removal efficiency remained constant for 120 h before suffering from a fouling-driven flux decline, which lowered the initial flux to ~40% after 175 h. Although the fouling was reversible (the performance was restored after physical cleaning by water), we believe that a deeper understanding of membrane fouling and its mechanisms are essential for the future application of membrane contactors in dissolved CH 4 recovery.

Nitrogen (N) and phosphorous (P) recovery

Ammonium nitrogen (NH 4 +-N) and phosphate phosphorus (PO 4 3−-P) do not break down completely during the anaerobic processes and remain in the anaerobic effluents with average values of 36 and 6 mg L−1, respectively.18 In this age of depleting natural resources, it is strategic to recycle valuable resources from the effluents, especially in recovering N and P from anaerobic effluents as high-value products such as fertilizers. To date, direct N and P recovery using membrane-based processes are economically less feasible due to low concentrations of nutrients in the effluents.34 As such, recent focus is on concentrating nutrients prior to their recovery. Several membrane-based processes have been utilized for this purpose and shown promising results. Their basic principles, reported performances, advantages/disadvantages as well as other membrane processes for direct recovery of N and P are summarized in Table 1.

Table 1 Membrane-based processes for the recovery of N and P species Full size table

Forward osmosis (FO),35 reverse osmosis (RO),36 nanofiltration (NF), and membrane distillation (MD) are competitive candidates for concentrating N and P compounds in the effluents while producing clean water as a permeate. Among them, FO emerges owing to its energy-efficiency since it operates under ambient pressure, unlike RO.37,38 It was experimentally determined that a seawater-driven FO process effectively concentrate PO 4 3−-P (2.3-fold) and NH 4 +-N (2.1-fold) from treated municipal wastewater. Furthermore, the concentrated nutrients can theoretically reach more than tenfold concentration by using a draw-to-feed solution-volume ratio of 2:1.8 Although clean water is not the final product in this case, the FO process can be coupled with another membrane-based process to realize hybrid FO-based process that has the capacity to produce clean water. For instance, an FO-MD hybrid process demonstrated the feasibility of recovering clean water from a draw solution that was diluted by water extracted from the digested sludge centrate.39 In addition, the FO process is strategically appropriate for P recovery from the effluents of digested sludge centrates which have high fouling propensity. A study by Ansari et al.21 revealed that although a 30% decline in water flux occurred as the digested sludge was concentrated by threefold, the water flux was almost recovered to the initial flux by simply flushing the membrane with deionized water owing to reversible fouling observed in the FO process. As a result of the concentration of the effluent, P recovery by precipitation was able to reach 92%. Such a high enrichment also offers the advantage of reducing the incidental expense in terms of storage and transport of the effluent. Moreover, the enriched solution suggests the possibility of using the FO process to facilitate existing methods to recover nutrients, such as struvite (MgNH 4 PO 4 .6H 2 O) precipitation, which are less viable when implemented alone. At present, struvite precipitation is known to be the most effective among current methods for P removal when the P concentration is over 100 mg L−1 (higher than average value in anaerobic effluents).20 In this regard, membrane-based processes can be a cost-effective solution to reduce the water content of the effluents for an efficient nutrient recovery.