Water. desalination + reuse

RESEARCH RESULTS Phase II-A of the testing program has been completed. The main experimental conditions tested in Phase II-A were: Feed water TDS: up to 90,000 ppm Feed temperature: 40-60°C Cooling temperature: 15-40°C The effect of feed salinity on distillate flux was initially investigated. Table 1 shows the salinity level of the three different solutions (synthetic and actual brine) tested on the pilot unit. The flux was stable for all tested solutions and excellent salt rejection of >99.99% was observed. Figure 1 also shows the results of membrane productivity at these different salinity levels compared with tap water (TDS <100 mg/L). It was noted that, up to a feed salinity of 50,000 ppm, the flux was similar to tap water. However, when the feed salinity was increased to 90,000 ppm, the flux decreased by 20 %. The feed water temperature was also varied from 40°C to 60°C. As shown in Figure 2, higher feed water temperature and ∆T across the membrane yielded higher flux due to increase in vapor pressure difference across the membrane. In Phase II-B, which is currently ongoing, the two MD pilot units were relocated to a local thermal desalination plant for further long term testing with brine under real field conditions. Figure 3 provides an illustration of where Table 1. Rejections for the saline feed solutions tested (TDS = total dissolved solids) Feed Solution Synthetic NaCl Synthetic NaCl Brine from thermal plant Feed TDS (ppm) ≈50,000 ≈90,000 ≈70,000 Distillate TDS (ppm) <2 <2 <3 TDS Rejection (%) 99.99 99.99 99.99 10 Average distillate flux, LMH Tap water 8 NaCI - 50,000 mg/L NaCI - 90,000 mg/L Thermal desal brine 6 4 2 0 0 2 4 Time, hours 6 8 Figure 1. Impact of feed salinity on flux Figure 1. Impact of feed salinity on flux 10 Average distillate flux, LMH MD has inherent advantages over the RO and thermal desalination processes: Superior product water quality: This is achieved because the process is essentially a distillation process with 100% theoretical rejection of ions, including boron, and other non volatiles. Can treat high salinity brines: The distillate flux and water quality are not significantly affected by high feed salinity (up to 100,000 ppm TDS), since the water does not have to overcome osmotic pressure like in a RO system. Lower greenhouse-gas emission: The process can use waste heat and/or renewable energies as the primary energy source. Potentially lower capital cost: Because the process operates at ambient pressure and relatively low temperatures, the construction materials used for the system can be inexpensive plastics. Potentially lower operating costs: The process can use low-grade waste heat and/or renewable energies which potentially can reduce the energy costs. Tf=59oC ; ∆Tavg=6oC Tap water 8 Tf=59oC ; ∆Tavg=5oC Tf=53oC ; ∆Tavg=5oC Figure 1. Impact of feed salinity on flux 6 Tf=46oC ; ∆Tavg=6oC 4 2 0 0 2 4 Time, hours 6 8 Figure 2. Impact of feed temperature and ∆T on flux Fresh water Fresh water Brine Sea water Thermal desalination plant Brine Brine Membrane distillation Sea water Figure 3. Proposed MD location in a thermal desalination plant August-September 2013 | Desalination & Water Reuse | 41 |

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