Recovery of concentrated saccharide solution from microalgal residue via combined hot compressed water treatment and membrane separation

As fuel production from microalgae continues to attract attention, development of a technology to recover value-added compounds from its by-product, namely lipid-extracted microalgae residue, is highly desired. This study investigated a process combining hot compressed water (HCW) treatment and membrane separation for the recovery of concentrated saccharides solution from low-lipid Chlorella vulgaris as a model for lipid-extracted microalgae residue. By HCW treatment at 200 °C for 30 min, a solution containing polysaccharides with molecular weight of ∼1000–20000 and saccharides with lower molecular weight was obtained. Components were consecutively separated by ultrafiltration (UF) to recover polysaccharides, nanofiltration (NF) for the recovery of saccharides with lower molecular weight, and reverse osmosis for water purification. The UF membrane with molecular weight cut off (MWCO) = 1000 successfully retained polysaccharides with molecular weights 1000–20000, whereas the contained low molecular weight saccharides were retained by the subsequent NF membrane. Contrary to the MWCO of 1000, the UF permeate also contained some saccharides with molecular weight >1000, suggesting another factor such as hydrophobic/hydrophilic difference between the membrane and the molecules also present. In addition, saccharide concentrations of both UF and NF concentrates were higher than that of feed solution, suggesting that saccharides could be recovered as concentrated solutions. In the current process, the effects of the concentration of microalgae during HCW treatment and the operating pressure in membrane separation were also investigated.