(506c) Multiple-Effect Membrane Distillation for Desalination of Seawater and Deep Concentration of the Brine Drained From Conventional Desalination Plants

Many countries in the world suffer from a shortage of fresh water. Water shortage limits the socioeconomic sustainable development of the world. Desalination of seawater is an effect way to obtain fresh water. At present, three important commercially available desalination technologies are multi-stage flash (MSF), multiple-effect distillation (MED) and reverse osmosis(RO). However water recovery ratio of those technologies is usually 40-55%. The concentrated brine by-produced in the routine desalination plant by using RO, MSF, or MED is usually discharged into the sea. After pretreatment and desalination, the brine by-produced has a rise in salinity and temperature while contains many harmful chemicals such as scale inhibitor and biocide. The discharge of the brine by-produced was actually a salt, heat and chemical pollution to the sea which has negative impacts on marine environment.

Membrane distillation is a separation method in which volatile species diffuse through a porous hydrophobic membrane. The driving force for transport is the partial pressure difference across the membrane. Depending on the method employed to impose the vapor pressure across the membrane, membrane distillation system can be classified broadly into four categories: direct-contact membrane distillation (DCMD), vacuum membrane distillation (VMD), sweeping gas membrane distillation and air gap membrane distillation (AGMD). Potential advantages of membrane distillation over traditional evaporation processes include operation at low pressures, lower temperature as well as ease of process scale-up, avoid of corrosion as a result of inertness of hydrophobic polymer membrane material, and so on.

However, traditional membrane distillation process is with extremely low thermal efficiency. It is a reason that MD is not currently use in commercial scale. The performance ratio (PR) is usually used to determine the thermal efficiency of evaporation-based process, which is defined as the amount of latent heat needed for evaporation of the produced water and the amount of heat provided to the system from an external energy source. The PR of conventional MD process typically ranged from 0.2 to 1 while the PR of MSF and MED usually ranged from 7 to 15. Another problem with MD is wetting or fouling of microporous membrane surface, which leads to the decrease of permeate flux and leakage; leakage further leads to the contamination of the permeate product by the impurities in the feed. Wetting or fouling is a more grievous problem in a DCMD or VMD process, since the intimate contact of the permeate with the membrane in DCMD or the large trans-membrane pressure difference in VMD. On the other hand, the AGMD process with mild operation condition and thus with potentially longer operation stability was rarely reported in literature, it might be attributed the difficulty to fabricate large AGMD modules.

In the present study, multiple-effect membrane distillation (MEMD) based on AGMD module with function of internal heat recovery has been developed to desalinate seawater and to deep concentrate the brine drained from the conventional desalination plant. This kind of MEMD process combines the advantages of MD process and conventional MSF process, avoids the disadvantages of MSF such as evacuation operation, and can provide a high PR value. The water vapor flux (J), perform ratio (PR) and thermal efficiency (©¯) are the most important indicators for evaluation of MEMD modules. Experimental were conducted to investigate the performance of MEMD module by using fresh seawater after preliminary pretreatment such as ultrafiltration, using the brine from a MED desalination plant without further pretreatment until the salt concentration in the retentate went up to 15°Bé, and by using more concentrated brine of >15°Bé as a feed after deep de-calcium step. Under optimized operational conditions, when the concentration of salt varied between 3.5 - 27 °Bé, the value of J decreased from 6.5 to 2.3 kg/m²h; the value of ƞ decreased from 0.91 to 0.45; and the value of PR decreased from 15 to 2.8. The value of PR is the highest value experimentally obtained for MD process. The long-term stability test was continuously performed for two than months. In the late stage of the test, when a brine of 25°Bé was used as the feed, the conductivity of the permeate was still less than that of tap water. All these results demonstrated that MEMD process can be efficiently used to produce drinkable water and highly concentrated brine when fresh water or brine from conventional desalination plants was used as the feed to this MEMD process.