Pilot plant evaluation of membrane distillation for desalination of high-salinity brines

Membrane distillation (MD) is a hybrid thermal-membrane desalination process that can use either low-grade waste heat and/or solar energy with hydrophobic membranes to desalinate high-salinity brines and produce high quality distillate. A research consortium was launched to investigate the application of the MD process, at lab and pilot scale, for desalination of concentrated brines. Bench scale results showed the presence of antiscalants in the concentrated brines minimized the scale precipitation potential and offered stable membrane permeability performance. Various MD technologies were screened, and two suitable technologies were selected for field-testing. Pilot unit A was based on multi-effect vacuum showed a stable flux of 6.2 LMH with excellent salt rejection (> 99.9%) from the concentrated brine discharged from thermal desalination plant in Qatar. That pilot unit was also field tested on hypersaline groundwater in Texas (USA) to generate fresh water for reservoir fracking in unconventional oil production operations. The MD unit was coupled with humidification/dehumidification (HDH) unit to achieve zero liquid discharge (ZLD) for inland applications. The MD unit was operated at 40% recovery producing distillate of < 20 mg/L total dissolved solids (TDS) and observed a stable flux of 5 LMH. Key challenges that are critical for large-scale deployment of MD technology were identified at the end of the field-testing program. Finally, a review of active MD technologies was conducted to highlight recent promising developments for full-scale applications.


Introduction
Membrane distillation (MD) is a desalination process in which vapor pressure difference act as driving force across a hydrophobic membrane (Fig. 1) to produce high quality distilled water (Adham et al. 2018).
• Direct Contact MD • Air Gap MD • Sweeping Gas MD • Vacuum MD Thermal seawater desalination is dominant technology in Qatar and usually co-located with power generation plants.This leads to availability of abundant concentrated streams and possible low quality waste heat.If properly harnessed, MD technology has potential to desalinate the concentrate brines from both thermal desalination and seawater reverse osmosis (RO) membrane process.The concentrated brine from thermal desalination plants possesses higher temperature which significantly reduces the energy consumption for the MD process.The existing thermal desalination plants can be retrofitted with MD plants to increase water resources for industrial use without incurring major capital investments.
The major advantage of MD includes (Drioli et al. 2015): • Treatment of high salinity streams The main challenges for MD implementation are limited knowledge on large-scale field deployment, process design and pretreatment challenges of feed sources.

MD application
The proposed application of MD is illustrated in Fig. 2. The concentrated hot brine from the existing thermal desalination plants is taken to MD plant operated by low grade waste heat.For full-scale deployment of MD, following were the main knowledge gaps identified: • Suitability to Qatari environmental conditions • Availability of low-grade waste heat • Membrane/module/system configurations • Process optimization A research consortium was launched to investigate the application of the MD process, at lab and pilot scale, for desalination of concentrated brine.An Industries/Academic consortium consists of ConocoPhillips GWSC, Qatar Electricity & Water Company (QEWC) and Qatar University (QU) was established to develop the MD process in Qatar.

MD evaluation program
A multi-phase research program was carried out over several years.In phase 1, bench scale studies and the availability of low quality heat energy source was investigated.In phase 2, MD technologies were screened, and two suitable units were field tested at pilot size for desalination of brine discharged from a local plant to enhance product water recovery.In phase 3, MD was combined with humidification/dehumidification (HDH) unit and pilot tested for inland desalination of hypersaline groundwater to achieve zero liquid discharge (ZLD).In the last phase, active MD technologies were reviewed to identify promising developments.All the phases are briefly illustrated as follows: Phase 1: bench scale studies/ low grade waste heat assessment A state-of-the-art MD bench scale unit as illustrated in Fig. 3 was built, commissioned and the performance of different MD membranes were compared under various operating conditions using synthetic saline solutions, brine collected from local thermal desalination plant and seawater from the Arabian Gulf.
Below were the main conclusions from the bench scale studies (Adham et al. 2013): • MD produced consistently a good quality distillate (conductivity < 10 μS/cm) • The feed temperature and temperature gradient have direct impact on flux and productivity.• Seawater requires pH adjustment and antiscalant addition to minimize the precipitation of calcium carbonate on the membrane which was shown to reduce permeability.• A stable membrane flux was observed and not affected by salt concentration with no membrane fouling.Figure 4 illustrates performance of various membrane chemistries applied on the concentrated brine of thermal desalination plant • The presence of commercial antiscalants (Belgard® EV2035) in the concentrated brines minimized the precipitation potential and offered stable membrane performance.• The scaling phenomenon is confirmed by surface analysis of the membrane by scanning electron microscope as shown in Fig. 5. MD membrane applied to seawater without pH adjustment nor antiscalant showed scale on membranes while MD membranes applied to brine dosed with antiscalant inhibited scale deposition on the membrane surface.
A heat audit was also carried out to identify the possible source of low-grade waste heat from thermal desalination plants.The following locations were identified: • Steam ejectors blow down • Boiler blow down • Dump condenser • Flue gas chambers The main challenge identified was how to obtain the required latent heat of vaporization without affecting the performance of existing full-scale power/desalination process.Based on field studies, dump condensers intermittently release thermal energy in the power plant showed potential for recovering the low-grade waste heat can be coupled to MD plant.We also observed that recently built power/desalination plants are typically designed and operated at high efficiency which made it more challenging to capture low grade waste heat.It should be noted that the MD process may also be integrated with renewable solar energy as an alternative option to lower the energy consumption costs.

Phase 2: MD pilot unit selection & field testing
Based on the knowledge captured from phase 1, request for proposal (RFP) for 1000 l/d was prepared and shared with various MD technology vendors for implementing a field testing program.After evaluation of the received technical and commercial proposals, two MD technologies   Both units were installed with advanced data acquisition software to monitor performance remotely and control the process operating parameters.The pilot units specifications are illustrated in Table 1 (Minier-Matar et al. 2014).Figure 6 shows the picture of the pilot units tested adjacent to each other in the local desalination plant in Qatar.
Pilot unit A operates on a continuous flow, one pass mode.The feed water enters the system, preheated, and allowed to flow across the different effects under vacuum in series.In the last effect, it is rejected as a concentrated stream into an external tank.Distillate is produced at each of the system's four effects and is collected in a separate external tank.Pilot unit B operates under air gap mode with feed and bleed mode, i.e., the feed water enters the module, and the concentrated feed is recirculated back into the feed tank while the distillate is collected separately in an external tank.
The thermal brine was pretreated using a two-step filtration system [1 μm + Granular Activated Carbon (GAC)] to remove particulates and organic contaminants present in the feed stream for restoring the membrane integrity.As illustrated in Fig. 7, pilot unit A was able to consistently generate high quality product water (TDS < 10 mg/L) and maintained stable flux of 4.5 LMH.During the first 3 days of operation, the distillate conductivity was averaged at 2.5 μS/cm, after which a steady slight increase of conductivity was observed until it stabilized at ≈10 μS/cm.The slight increase in conductivity can be attributed to potential volatile and/or carbonate species that may pass to the permeate side.The overall product water recovery of the system was 35% and the concentrated MD brine to be discharged had a TDS of 112 g/L.The feed and distillate water qualities are illustrated in Table 2.
Pilot unit B generated poorer product water quality (2530 μS/cm) and significant decrease in flux (from 4 to 2.5 LMH) was observed throughout the testing period.Therefore, the system was stopped after few days of operation.
To evaluate the impact of pretreatment, tests were also conducted by removing the GAC filter from the upstream of the MD process.During the first 4 days of operation, the unit generated a distilled quality < 1 μS/cm.The water quality started to deteriorate on the 5th day with a decrease in distillate flux; the distillate conductivity increased exponentially to ≈700 μS/cm as shown in Fig. 8.This phenomenon  was attributed to membrane wetting due to the presence of chemical agents present in thermal brine.To confirm the hypothesis, a series of wetting tests as shown in Fig. 9 was carried out by dropping sufficient volume of concentrated brine, scale inhibitor and antifoam to the membrane surface.
It was concluded that antifoam chemical was responsible for the wetting phenomenon over a period of time on the membrane surface.Thus, GAC was effective in removing that problematic chemical via pretreatment which resulted in stable flux and salt rejection as was shown in Fig. 7. Finally, Pilot unit A was further optimized by variation of operating parameters.The feed water temperature was increased from 32 to 50 C and vacuum set point was adjusted to increase the system recovery from 35 to 52%.At these optimized conditions, a higher stable flux of 6.2 LMH (compared to 4.5 LMH) was achieved as shown in Fig. 10.

Phase 3: MD inland desalination of hyper-saline groundwater
Hydraulic fracturing (HF) is a widely used technique to recover oil/gas resources from shale reservoirs.During HF, a mixture of water, sand and feed chemicals were injected at high pressure into the geological formation to create tiny fissures which will allow oil/gas resources to flow smoothly without any friction and collected in the tank.The presence of boron and scale forming ions in the water can create challenges when utilizing the hypersaline groundwater due to its interference and reduces the efficacy of feed chemicals.Thus, removal of these salts was initially required to utilize saline groundwater for HF applications to ensure compatibility with field chemicals.
MD Pilot unit A was transferred from Qatar to the US to evaluate inland desalination of hypersaline hot groundwater for HF applications.Given this will be an inland desalination, the MD unit was combined with humidification-dehumidification (HDH) to achieve zero liquid discharge (ZLD) via salt production as shown in Fig. 11.A comprehensive energy analysis was also conducted during this field evaluation study (Minier-Matar et al. 2016).
A feed groundwater with TDS of 6.2% was concentrated to 10.2% with MD and the pilot unit achieved 38% recovery with stable flux of 5 LMH as shown in Fig. 12.The distillate effluent TDS was < 5 mg/L with salt rejection > 99.9% as shown in Table 3.The MD distillate was suitable to be used for HF applications using the early version of friction reducers used for HF operations.However, the chemical vendors introduced new generation of salt tolerant feed chemicals/ friction reducers that can be directly mixed with saline water for HF operations.Thus, the application of MD for inland desalination of hypersaline groundwater was no longer required for the HF applications but can be considered in the future for other possible needs including potable water production.More details on the HDH performance can be found elsewhere (Minier-Matar et al. 2016).
Overall, based on the pilot investigations in Phase 2 and 3, the research consortium identified major challenges that are critical for large scale deployment of MD technology for desalination of concentrated brines from thermal plants.
The key challenges, which were shared with the industry, are summarized as follows: • Optimized process energy efficiency with multi-effect channels

Phase 4: MD technology developments
After a few years, the research team decided to review recent developments of MD technology to assess if the above challenges were addressed for full-scale process deployment for our proposed application.Thus, several active MD vendors were contacted and requested to provide updates on their latest products and technology developments.MD vendors with innovative module designs, some of which promising a step change in performance, have recently emerged on the market.The current MD vendors leading technology developments are listed in Table 4 (Hussain et al. 2022).
Key MD technology developments are briefly summarized as follows: • Aquastill focused on the development of a spiral wound configuration (low-density polyethylene membrane (LDPE)) to improve energy efficiency (Hussain et al. 2022).The system can be integrated either with lowgrade waste heat or solar energy to lower the operating costs.The system has achieved performance ratio in the range of 10-14 due to a multi-effect design which signifi-  The key direction for future MD research lies in the development of multi-effect membrane configuration that can address the scale-up limitations listed above.MD vendors are continuously looking for niche applications where MD technology can be cost-effectively applied for removing target high-priced dissolved chemicals regardless of operating costs and energy requirements Advancements in membrane chemistry research target the addition of nanomaterials (e.g., carbon nanotubes, graphene, silicon dioxide, fluorinated compounds) to membranes (Aljumaily et al. 2022;Criscuoli et al. 2023) can increase hydrophobicity (to reduce wetting) and/or increase mass transfer rates (to increase flux and lower cost).

Conclusions
A multi-phase research program was implemented by the authors to advance MD technology application to desalinate high-salinity brines.The research program included bench and pilot-scale testing under relevant field conditions.Based on the overall investigations, the following are the key conclusions: • MD produced consistently a good quality distillate (conductivity < 10 μS/cm) • The presence of antiscalants in the concentrated brines minimized the precipitation potential and offered stable membrane performance.• Pilot unit A was optimized by varying the feed water temperature from 32 to 50 C, and the vacuum set point was adjusted resulting in increase of system recovery from 35 to 52% with a higher stable flux of 6.2 LMH.• Pretreatment to remove anti-foaming agents (surfactants) is critical to minimize membrane wetting.• The groundwater was concentrated from 6.2 to 10.2%, achieved 38% recovery with stable flux of 5 LMH & cooling water was required for inland desalination.• For inland application, cooling water requirement must be considered.• MD vendors with innovative module designs are being introduced to the market.• To improve MD's energy efficiency, future developments should focus on innovative multi-effect membrane module designs..5-3.0 99.9 2.0 -6.0 Memsift (Zuo et al. 2022) 45 2.5-5.5 99.9 1.5-2.5 Scarab (Minier-Matar et al. 2014) 35-70 5.0-6.299.0 0.7 Solarspring (Schwantes et al. 2019) 35-70 1.6 -1.4 99.9 1.6-1.4

Fig. 1
Fig. 1 Description of MD process

Fig. 4
Fig. 4 Performance of different MD membranes on actual brine from a local desalination plant

Fig. 7
Fig. 7 Performance Pilot Unit A on thermal brine with GAC

Fig. 8
Fig. 8 Performance Pilot Unit A on thermal brine without GAC

Fig. 10
Fig. 10 Performance Pilot Unit A on thermal brine (optimized vs. standard conditions)

Fig. 12
Fig. 12 Performance Pilot Unit A on saline groundwater

Table 1
MD plant specifications Fig. 6 Picture of MD pilot units in local desalination plant in Qatar

Table 2
Thermal brine feed and distillate water quality (Pilot unit A)

Table 3
Groundwater feed and distillate water quality

Table 5
Comparison of performance parameters of different MD technologies