A method of removing silicon dioxide from a waste liquid is described. A solution containing fluoride ions is added into the waste liquid to form a reaction product. A method of cleaning a membrane tube for processing waste water is also described, wherein the membrane tube contains residues of waste water that include a first residue and a second residue containing silicon dioxide. A first cleaning step is conducted to reduce the concentration of the first residue in the membrane tube to a first concentration. A solution containing fluoride ions is added into the tube to react with the second residue to form a reaction product. A second cleaning step is then conducted to reduce the concentration of the residual fluorine ions in the membrane tube to a second concentration.