Production of highly concentrated deep seawater by membrane distillation and its application for microalga Dunaliella salina cultivation

In Japan, deep seawater has been a valuable resource for food industries; in particular, deionized deep seawater is widely used for bottled mineral water. However, concentrated deep seawater, a byproduct of the deionization process, has no current applications. In this study, we produced highly concentrated deep seawater using a novel membrane distillation system and cultivated the well-known halotolerant microalga Dunaliella salina, which is a rich source for beta-carotene, using the distillation products. The proposed membrane distillation system consisted of two glass compartments (feed and permeate sides) and a polytetrafluoroethylene (PTFE) membrane. NaCl concentration from 5.2 to 12.3% was effectively achieved by maintaining the feed side at 36degC and the permeate side at 15degC; the water flux through the PTFE membrane was 0.4 t/day/square m. The highly concentrated deep seawater produced from the distillation system was used to cultivate D. salina by adding only nitrogen and phosphate sources. These results suggest that this membrane distillation system is useful in manufacturing highly concentrated deep seawater that can be used as a medium for D. salina cultivation.