The invention is a process for manufacturing acid-resistant biopolymer-lignin-membranes from the liquor from cellulose extraction processes. Black liquor is dried in the first stage of the process and is finally pressed into form. Through treatment with acid the lignin membranes acquire their porous structure and become insoluble in water and resistant to acid. According to use the membranes can be cleaned with water to remove the acid portion. The resulting membranes let filtration processes be carried out effectively and inexpensively and can be used as encapsulating media.