Stainless steel membrane UF coupled with NF process for the recovery of sodium hydroxide from alkaline wastewater in chitin processing

The recovery of sodium hydroxide from alkali wastewater in chitin processing was investigated using stainless steel ultrafiltration membrane (SSM) and HDS-04 nanofiltration (NF) membranes with membrane area of 0.35 m2 and 1.4 m2, respectively. Flux behaviors were observed with respect to filtration time, volumetric concentration ratio (VCR), operating pressure, temperature, and cleaning. As the VCR increased, the permeate flux declined while almost the same concentration of NaOH was permeable. The SSM and NF operations end with a concentrated protein solution that needed a small amount of waste acid for neutralization and easy spray drying and the permeate of the NaOH solution can be reused. Concentrations of NaOH that govern reusability of permeate were measured to be independent on VCR. The most suitable VCRs for SSM and NF in terms of maintaining relatively good membrane productivity and high rejection of protein and chemical oxygen demand (COD) were approximately 50. At a VCR of 50, the total rejections of protein, COD and suspended solid (SS) were 82.5%, 94% and 100%, respectively, while total NaOH recovery was 96% with SSM average flux 270 LMH and NF average flux 25 LMH. SSM filtration was essential for the pretreatment of the alkali wastewater before it was fed into the NF system.